Autocatalytic amplification of Alzheimer-associated Aβ42 peptide aggregation in human cerebrospinal fluid

Funder: Knut och Alice Wallenbergs Stiftelse (Knut and Alice Wallenberg Foundation); doi: https://doi.org/10.13039/501100004063Funder: Alzheimerfonden; doi: https://doi.org/10.13039/501100008599Abstract: Alzheimer’s disease is linked to amyloid β (Aβ) peptide aggregation in the brain, and a detailed understanding of the molecular mechanism of Aβ aggregation may lead to improved diagnostics and therapeutics. While previous studies have been performed in pure buffer, we approach the mechanism in vivo using cerebrospinal fluid (CSF). We investigated the aggregation mechanism of Aβ42 in human CSF through kinetic experiments at several Aβ42 monomer concentrations (0.8–10 µM). The data were subjected to global kinetic analysis and found consistent with an aggregation mechanism involving secondary nucleation of monomers on the fibril surface. A mechanism only including primary nucleation was ruled out. We find that the aggregation process is composed of the same microscopic steps in CSF as in pure buffer, but the rate constant of secondary nucleation is decreased. Most importantly, the autocatalytic amplification of aggregate number through catalysis on the fibril surface is prevalent also in CSF

Treatment and analysis of spectroscopic infrared data from observations of Saturn's and Titan's atmospheres by Cassini

Since July 2004, the spacecraft Cassini has been in orbit around Saturn on a multiple year mission to investigate the Saturnian system. The instrument CIRS on the orbiter scans for infrared light that is emitted from the atmospheres on Saturn and Titan, in order to find the neccessary information to retrieve temperature and mixing ratio profiles. The timelines for the observations are carefully planned since there are different scan modes and a trade-off between the spatial and the spectral resolutions. Retrieving the temperature and mixing ratio profiles is done by performing an inverse solution of the radiative-transfer equation. The parameters may be retrieved at specific altitudes in the atmosphere, due to a correlation between frequency and pressure level for each molecule. An iteration method is used to merge the calculated data with the observed data, where the correct parameters are retrieved. A part of this thesis was to create an interface written in IDL, which could quickly display data from several locations at the same time.With the help partly of this program, early observations from the first months of the mission have been analysed, and discussed in this report.Validerat; 20101217 (root

Secondary Nucleation in Amyloid Formation

Research into Alzheimer's disease is still hampered by a lack of fundamental understanding of the underlying mechanisms. While the aggregation of the amyloid β peptide (Aβ) into amyloid fibrils is highly implicated as a key factor in the disease, the molecular nature of its involvement has proven complex and elusive. This thesis and the work herein is part of an ongoing effort to map out the aggregation mechanism of Aβ in vitro in as much detail as possible, in the hope to provide a better basis for understanding its role in disease. In particular, the mechanism of secondary nucleation, whereby the fibril surface catalyses the formation of new fibrils is of interest due to its capacity to generate large numbers of toxic oligomers. In this work, we probe the determinants of secondary nucleation by studying its influence in different temperatures and pH and we confirm that it remains an important factor in aggregation in human cerebrospinal fluid. We also report a transient accumulation of pre-fibrillar aggregates, likely to be a result of heavily saturated secondary nucleation, which can form a basis for further structural studies of this phenomenon

Chiral Selectivity of Secondary Nucleation in Amyloid Fibril Propagation

Chirality is a fundamental feature of asymmetric molecules and of critical importance for intermolecular interactions. The growth of amyloid fibrils displays a strong enantioselectivity, which is manifested as elongation through the addition of monomers of the same, but not opposite, chirality as the parent aggregate. Here we ask whether also secondary nucleation on the surface of amyloid fibrils, of relevance for toxicity, is governed by the chirality of the nucleating monomers. We use short amyloid peptides (Aβ20-34 and IAPP20-29) with all residues as L- or all D-enantiomer in self and cross-seeding experiments with low enough seed concentration that any acceleration of fibril formation is dominated by secondary nucleation. We find a strong enantio-specificity of this auto-catalytic process with secondary nucleation being observed in the self-seeding experiments only. The results highlight a role of secondary nucleation in strain propagation

Autocatalytic amplification of Alzheimer-associated Aβ42 peptide aggregation in human cerebrospinal fluid

Funder: Knut och Alice Wallenbergs Stiftelse (Knut and Alice Wallenberg Foundation); doi: https://doi.org/10.13039/501100004063Funder: Alzheimerfonden; doi: https://doi.org/10.13039/501100008599Abstract: Alzheimer’s disease is linked to amyloid β (Aβ) peptide aggregation in the brain, and a detailed understanding of the molecular mechanism of Aβ aggregation may lead to improved diagnostics and therapeutics. While previous studies have been performed in pure buffer, we approach the mechanism in vivo using cerebrospinal fluid (CSF). We investigated the aggregation mechanism of Aβ42 in human CSF through kinetic experiments at several Aβ42 monomer concentrations (0.8–10 µM). The data were subjected to global kinetic analysis and found consistent with an aggregation mechanism involving secondary nucleation of monomers on the fibril surface. A mechanism only including primary nucleation was ruled out. We find that the aggregation process is composed of the same microscopic steps in CSF as in pure buffer, but the rate constant of secondary nucleation is decreased. Most importantly, the autocatalytic amplification of aggregate number through catalysis on the fibril surface is prevalent also in CSF

Secondary nucleation in amyloid formation

Nucleation of new peptide and protein aggregates on the surfaces of amyloid fibrils of the same peptide or protein has emerged in the past two decades as a major pathway for both the generation of molecular species responsible for cellular toxicity and for the autocatalytic proliferation of peptide and protein aggregates. A key question in current research is the molecular mechanism and driving forces governing such processes, known as secondary nucleation. In this context, the analogies with other self-assembling systems for which monomer-dependent secondary nucleation has been studied for more than a century provide a valuable source of inspiration. Here, we present a short overview of this background and then review recent results regarding secondary nucleation of amyloid-forming peptides and proteins, focusing in particular on the amyloid β peptide (Aβ) from Alzheimer's disease, with some examples regarding α-synuclein from Parkinson's disease. Monomer-dependent secondary nucleation of Aβ was discovered using a combination of kinetic experiments, global analysis, seeding experiments and selective isotope-enrichment, which pinpoint the monomer as the origin of new aggregates in a fibril-catalyzed reaction. Insights into driving forces are gained from variations of solution conditions, temperature and peptide sequence. Selective inhibition of secondary nucleation is explored as an effective means to limit oligomer production and toxicity. We also review experiments aimed at finding interaction partners of oligomers generated by secondary nucleation in an ongoing aggregation process. At the end of this feature article we bring forward outstanding questions and testable mechanistic hypotheses regarding monomer-dependent secondary nucleation in amyloid formation

Distinct thermodynamic signatures of oligomer generation in the aggregation of the amyloid-β peptide

Mapping free-energy landscapes has proved to be a powerful tool for studying reaction mechanisms. Many complex biomolecular assembly processes, however, have remained challenging to access using this approach, including the aggregation of peptides and proteins into amyloid fibrils implicated in a range of disorders. Here, we generalize the strategy used to probe free-energy landscapes in protein folding to determine the activation energies and entropies that characterize each of the molecular steps in the aggregation of the amyloid-β peptide (Aβ42), which is associated with Alzheimer's disease. Our results reveal that interactions between monomeric Aβ42 and amyloid fibrils during fibril-dependent secondary nucleation fundamentally reverse the thermodynamic signature of this process relative to primary nucleation, even though both processes generate aggregates from soluble peptides. By mapping the energetic and entropic contributions along the reaction trajectories, we show that the catalytic efficiency of Aβ42 fibril surfaces results from the enthalpic stabilization of adsorbing peptides in conformations amenable to nucleation, resulting in a dramatic lowering of the activation energy for nucleation