Ellen Hubin

Transient dynamics of Aβ contribute to toxicity in Alzheimer's disease.

The aggregation and deposition of the amyloid-β peptide (Aβ) in the brain has been linked with neuronal death, which progresses in the diagnostic and pathological signs of Alzheimer’s disease (AD). The transition of an unstructured monomeric peptide into self-assembled and more structured aggregates is the crucial conversion from what appears to be a harmless polypeptide into a malignant form that causes synaptotoxicity and neuronal cell death. Despite efforts to identify the toxic form of Aβ, the development of effective treatments for AD is still limited by the highly transient and dynamic nature of interconverting forms of Aβ. The variability within the in vivo “pool” of different Aβ peptides is another complicating factor. Here we review the dynamical interplay between various components that influence the heterogeneous Aβ system, from intramolecular Aβ flexibility to intermolecular dynamics between various Aβ alloforms and external factors. The complex dynamics of Aβ contributes to the causative role of Aβ in the pathogenesis of AD.

A comparative analysis of the aggregation behavior of amyloid-β peptide variants

Amyloid beta and Amyloid beta bind by filter binding (View Interaction: 1, 2, 3)

Chemical Cross-Linking/Mass Spectrometry Maps the Amyloid β Peptide Binding Region on Both Apolipoprotein E Domains

Apolipoprotein E (apoE) binds the amyloid β peptide (Aβ), one of the major culprits in Alzheimers disease development. The formation of apoE:Aβ complexes is implicated in both Aβ clearance and fibrillization. However, the binding interface between apoE and Aβ is poorly defined despite substantial previous research efforts, and the exact role of apoE in the pathology of Alzheimers disease remains largely elusive. Here, we compared the three main isoforms of apoE (E2, E3, and E4) for their interaction with Aβ1-42 in an early stage of aggregation and at near physiological conditions. Using electron microscopy and Western blots, we showed that all three isoforms are able to prevent Aβ fibrillization and form a noncovalent complex, with one molecule of Aβ bound per apoE. Using chemical cross-linking coupled to mass spectrometry, we further examined the interface of interaction between apoE2/3/4 and Aβ. Multiple high-confidence intermolecular apoE2/3/4:Aβ cross-links confirmed that Lys16 is located in the region of Aβ binding to apoE2/3/4. Further, we demonstrated that both N- and C-terminal domains of apoE2/3/4 are interacting with Aβ. The cross-linked sites were mapped onto and evaluated in light of a recent structure of apoE. Our results support binding of the hydrophobic Aβ at the apoE domain-domain interaction interface, which would explain how apoE is able to stabilize Aβ and thereby prevent its subsequent aggregation.

New mimetic peptides inhibitors of Αβ aggregation. Molecular guidance for rational drug design

A new series of mimetic peptides possessing a significant Aβ aggregation modulating effect was reported here. These compounds were obtained based on a molecular modelling study which allowed us to perform a structural-based virtual selection. Monitoring Aβ aggregation by thioflavin T fluorescence and transmission electron microscopy revealed that fibril formation was significantly decreased upon prolonged incubation in presence of the active compounds. Dot blot analysis suggested a decrease of soluble oligomers strongly associated with cognitive decline in Alzheimers disease. For the molecular dynamics simulations, we used an Aβ42 pentameric model where the compounds were docked using a blind docking technique. To analyze the dynamic behaviour of the complexes, extensive molecular dynamics simulations were carried out in explicit water. We also measured parameters or descriptors that allowed us to quantify the effect of these compounds as potential inhibitors of Aβ aggregation. Thus, significant alterations in the structure of our Aβ42 protofibril model were identified. Among others we observed the destruction of the regular helical twist, the loss of a stabilizing salt bridge and the loss of a stabilizing hydrophobic interaction in the β1 region. Our results may be helpful in the structural identification and understanding of the minimum structural requirements for these molecules and might provide a guide in the design of new aggregation modulating ligands.

Two distinct β-sheet structures in Italian-mutant amyloid-beta fibrils: A potential link to different clinical phenotypes

Most Alzheimer’s disease (AD) cases are late-onset and characterized by the aggregation and deposition of the amyloid-beta (Aβ) peptide in extracellular plaques in the brain. However, a few rare and hereditary Aβ mutations, such as the Italian Glu22-to-Lys (E22K) mutation, guarantee the development of early-onset familial AD. This type of AD is associated with a younger age at disease onset, increased β-amyloid accumulation, and Aβ deposition in cerebral blood vessel walls, giving rise to cerebral amyloid angiopathy (CAA). It remains largely unknown how the Italian mutation results in the clinical phenotype that is characteristic of CAA. We therefore investigated how this single point mutation may affect the aggregation of Aβ1–42 in vitro and structurally characterized the resulting fibrils using a biophysical approach. This paper reports that wild-type and Italian-mutant Aβ both form fibrils characterized by the cross-β architecture, but with distinct β-sheet organizations, resulting in differences in thioflavin T fluorescence and solvent accessibility. E22K Aβ1–42 oligomers and fibrils both display an antiparallel β-sheet structure, in comparison with the parallel β-sheet structure of wild-type fibrils, characteristic of most amyloid fibrils described in the literature. Moreover, we demonstrate structural plasticity for Italian-mutant Aβ fibrils in a pH-dependent manner, in terms of their underlying β-sheet arrangement. These findings are of interest in the ongoing debate that (1) antiparallel β-sheet structure might represent a signature for toxicity, which could explain the higher toxicity reported for the Italian mutant, and that (2) fibril polymorphism might underlie differences in disease pathology and clinical manifestation.

Characterization of insulin-degrading enzyme-mediated cleavage of Aβ in distinct aggregation states

To enhance our understanding of the potential therapeutic utility of insulin-degrading enzyme (IDE) in Alzheimers disease (AD), we studied in vitro IDE-mediated degradation of different amyloid-beta (Aβ) peptide aggregation states. Our findings show that IDE activity is driven by the dynamic equilibrium between Aβ monomers and higher ordered aggregates. We identify Met(35)-Val(36) as a novel IDE cleavage site in the Aβ sequence and show that Aβ fragments resulting from IDE cleavage form non-toxic amorphous aggregates. These findings need to be taken into account in therapeutic strategies designed to increase Aβ clearance in AD patients by modulating IDE activity.

Could ecosystem management provide a new framework for Alzheimer's disease?

Alzheimers disease (AD) is a progressive neurodegenerative brain disorder that involves a plethora of molecular pathways. In the context of therapeutic treatment and biomarker profiling, the amyloid‐beta (Aβ) peptide constitutes an interesting research avenue that involves interactions within a complex mixture of Aβ alloforms and other disease‐modifying factors. Here, we explore the potential of an ecosystem paradigm as a novel way to consider AD and Aβ dynamics in particular. We discuss the example that the complexity of the Aβ network not only exhibits interesting parallels with the functioning of complex systems such as ecosystems but that this analogy can also provide novel insights into the neurobiological phenomena in AD and serve as a communication tool. We propose that combining network medicine with general ecosystem management principles could be a new and holistic approach to understand AD pathology and design novel therapies.

Lipidated apolipoprotein E4 structure and its receptor binding mechanism determined by a combined cross-linking coupled to mass spectrometry and molecular dynamics approach

Apolipoprotein E (apoE) is a forefront actor in the transport of lipids and the maintenance of cholesterol homeostasis, and is also strongly implicated in Alzheimer’s disease. Upon lipid-binding apoE adopts a conformational state that mediates the receptor-induced internalization of lipoproteins. Due to its inherent structural dynamics and the presence of lipids, the structure of the biologically active apoE remains so far poorly described. To address this issue, we developed an innovative hybrid method combining experimental data with molecular modeling and dynamics to generate comprehensive models of the lipidated apoE4 isoform. Chemical cross-linking combined with mass spectrometry provided distance restraints, characterizing the three-dimensional organization of apoE4 molecules at the surface of lipidic nanoparticles. The ensemble of spatial restraints was then rationalized in an original molecular modeling approach to generate monomeric models of apoE4 that advocated the existence of two alternative conformations. These two models point towards an activation mechanism of apoE4 relying on a regulation of the accessibility of its receptor binding region. Further, molecular dynamics simulations of the dimerized and lipidated apoE4 monomeric conformations revealed an elongation of the apoE N-terminal domain, whereby helix 4 is rearranged, together with Arg172, into a proper orientation essential for lipoprotein receptor association. Overall, our results show how apoE4 adapts its conformation for the recognition of the low density lipoprotein receptor and we propose a novel mechanism of activation for apoE4 that is based on accessibility and remodeling of the receptor binding region.