On the stability of the soluble amyloid aggregates

Electrostatic interactions mediate the nucleation and growth of a bacterial functional amyloid

Bacterial biofilm formation can have severe impacts on human and environmental health. Enteric bacteria produce functional amyloid fibers called curli that aid in biofilm formation and host colonization. CsgA is the major proteinaceous component of curli amyloid fibers and is conserved in many gram-negative enteric bacteria. The CsgA amyloid core consists of five imperfect repeats (R1-R5). R2, R3, and R4 have aspartic acid (D) and glycine (G) residues that serve as "gatekeeper" residues by modulating the intrinsic aggregation propensity of CsgA. Here, using mutagenesis, salt-mediated charge screening, and by varying pH conditions, we show that the ability of CsgA variants to nucleate and form amyloid fibers is dictated by the charge state of the gatekeeper residues. We report that in Citrobacter youngae CsgA, certain arginine (R) and lysine (K) residues also act as gatekeeper residues. A mechanism of gatekeeping is proposed wherein R and K residues electrostatically interact with negatively charged D residues, tempering CsgA fiber formation.

Unveiling the Potential of Polyphenols as Anti-Amyloid Molecules in Alzheimer's Disease

Alzheimer's disease (AD) is a devastating neurodegenerative disease that mostly affects the elderly population. Mechanisms underlying AD pathogenesis are yet to be fully revealed, but there are several hypotheses regarding AD. Even though free radicals and inflammation are likely to be linked with AD pathogenesis, still amyloid-beta (Aβ) cascade is the dominant hypothesis. According to the Aβ hypothesis, a progressive buildup of extracellular and intracellular Aβ aggregates has a significant contribution to the AD-linked neurodegeneration process. Since Aβ plays an important role in the etiology of AD, therefore Aβ-linked pathways are mainly targeted in order to develop potential AD therapies. Accumulation of Aβ plaques in the brains of AD individuals is an important hallmark of AD. These plaques are mainly composed of Aβ (a peptide of 39-42 amino acids) aggregates produced via the proteolytic cleavage of the amyloid precursor protein. Numerous studies have demonstrated that various polyphenols (PPHs), including cyanidins, anthocyanins, curcumin, catechins and their gallate esters were found to markedly suppress Aβ aggregation and prevent the formation of Aβ oligomers and toxicity, which is further suggesting that these PPHs might be regarded as effective therapeutic agents for the AD treatment. This review summarizes the roles of Aβ in AD pathogenesis, the Aβ aggregation pathway, types of PPHs, and distribution of PPHs in dietary sources. Furthermore, we have predominantly focused on the potential of food-derived PPHs as putative anti-amyloid drugs.

Determining the Stoichiometry of Amyloid Oligomers by Single-Molecule Photobleaching

Small oligomers are the initial intermediates in the pathway to amyloid fibril formation. They have a distinct identity from the monomers as well as from the protofibrils and the fibrils, both in their structure and in their properties. In many cases, they play a crucial biological role. However, due to their transient nature, they are difficult to characterize. "Oligomer" is a diffuse definition, encompassing aggregates of many different sizes, and this lack of precise definition causes much confusion and disagreement between different research groups. Here, we define the small oligomers as "n"-mers with n < 10, which is the size range in which the amyloid proteins typically exist at the initial phase of the aggregation process. Since the oligomers dynamically interconvert into each other, a solution of aggregating amyloid proteins will contain a distribution of sizes. A precise characterization of an oligomeric solution will, therefore, require quantification of the relative population of each size. Size-based separation methods, such as size-exclusion chromatography, are typically used to characterize this distribution. However, if the interconversion between oligomers of different sizes is fast, this would not yield reliable results. Single-molecule photobleaching (smPB) is a direct method to evaluate this size distribution in a heterogeneous solution without separation. In addition, understanding the mechanism of action of amyloid oligomers requires knowing the affinity of each oligomer type to different cellular components, such as the cell membrane. These measurements are also amenable to smPB. Here we show how to perform smPB, both for oligomers in solution and for oligomers attached to the membrane.

Antenna Protein Clustering In Vitro Unveiled by Fluorescence Correlation Spectroscopy

Antenna protein aggregation is one of the principal mechanisms considered effective in protecting phototrophs against high light damage. Commonly, it is induced, in vitro, by decreasing detergent concentration and pH of a solution of purified antennas; the resulting reduction in fluorescence emission is considered to be representative of non-photochemical quenching in vivo. However, little is known about the actual size and organization of antenna particles formed by this means, and hence the physiological relevance of this experimental approach is questionable. Here, a quasi-single molecule method, fluorescence correlation spectroscopy (FCS), was applied during in vitro quenching of LHCII trimers from higher plants for a parallel estimation of particle size, fluorescence, and antenna cluster homogeneity in a single measurement. FCS revealed that, below detergent critical micelle concentration, low pH promoted the formation of large protein oligomers of sizes up to micrometers, and therefore is apparently incompatible with thylakoid membranes. In contrast, LHCII clusters formed at high pH were smaller and homogenous, and yet still capable of efficient quenching. The results altogether set the physiological validity limits of in vitro quenching experiments. Our data also support the idea that the small, moderately quenching LHCII oligomers found at high pH could be relevant with respect to non-photochemical quenching in vivo.

Evidence of the existence of micellar-like aggregates for α-synuclein

We have been investigating the early stages of α-synuclein (Syn) aggregation, a small presynaptic protein implicated in Parkinson's disease. We previously reported that for pH jumps (1000 s) from pH 7 to pH 2 the variation of the Syn intrinsic fluorescence intensity did not change in the concentration range of ca. 10-50 μM (ref. 16). Additionally, I reported dynamic light scattering (DLS) experiments revealing the formation of early large Syn aggregates (ref. 7). These reported results mean that some molecular entity is being early formed. Herein, it was decided to investigate in detail these early Syn aggregates by using light scattering. By DLS analysis, these aggregates exhibited a hydrodynamic diameter of ca. 420 nm along with a high scattering intensity, characteristic of micellar-like aggregates formation. The critical micelle concentration (CMC) at which the Syn micellar-like aggregates are formed was ca. 10 μM. DLS analysis has also revealed that the micellar-like aggregates for Syn evolved, for protein concentrations >100 μM, to the formation of smaller aggregates (hydrodynamic diameter of ca. 165 nm), possibly Syn oligomers. The Syn micellar-like aggregates formed at pH 7 solutions seem to be active species and to have a role in this protein aggregation mechanism.

Correction of Systematic Bias in Single Molecule Photobleaching Measurements

Single molecule photobleaching is a powerful technique to measure the number of fluorescent units in subresolution molecular complexes, such as in toxic protein oligomers associated with amyloid diseases. However, photobleaching can occur before the sample is appropriately placed and focused. Such "prebleaching" can introduce a strong systematic bias toward smaller oligomers. Quantitative correction of prebleaching is known to be an ill-posed problem, limiting the utility of the technique. Here, we provide an experimental solution to improve its reliability. We chemically construct multimeric standards to estimate the prebleaching probability, B. We show that B can be used as a constraint to reliably correct the statistics obtained from a known distribution of standard oligomers. Finally, we apply this method to the data obtained from a heterogeneous oligomeric solution of human islet amyloid polypeptide. Our results show that photobleaching can critically skew the estimation of oligomeric distributions, so that low abundance monomers display a much higher apparent abundance. In summary, any inference from photobleaching experiments with B > 0.1 is likely to be unreliable, but our method can be used to quantitatively correct possible errors.

Amphiphilic ionic liquid induced fusion of phospholipid liposomes

The impact of increasing concentration of imidazolium-based ionic liquids ([C_nMIM]⁺[Br]⁻) on the structural integrity of large unilamellar vesicles (LUVs) made of pure phosphatidylcholine (PC) and phosphatidylglycerol (PG) lipids.

Self-assembly in amphiphilic macromolecules with solvent exposed hydrophobic moieties

Self-assembly by amphiphilic molecules with solvent exposed hydrophobic groups are relevant in biomolecular systems as well as in technological applications. Here we study such self-assembly in these systems using a model system of spherical particles having charge at core but solvent repelling surface, using Monte-Carlo simulations and mean field treatment. We find that solvophobicity mediated attraction leads aggregation, while electrostatic repulsions control stability of finite clusters. The aggregation threshold relates the parameters of two interactions through an algebraic dependence. The study also qualitatively explains experimental observations on aggregation of misfolded proteins and can be useful guide to tune stability of nm sized self-assembly in systems with exposed hydrophobic groups.

Sintered electrospun polycaprolactone for controlled model drug delivery

Electrospinning has been used widely for drug delivery applications due to its versatility and ease of modification of spun fiber properties. Net drug loading and release is typically limited by the inherent surface-area of the sample. In a relatively novel approach, sintering of electrospun fiber was used to create a capsule favoring long-term delivery. We showed that electrospun polycaprolactone (PCL) retained its initial morphology out to 1042 days of in vitro exposure, illustrating its potential for extended performance. Sintering decreased the electrospun pore size by 10- and 28-fold following 56 and 57 °C exposures, respectively. At 58 and 59 °C, the PCL capsules lost all apparent surface porosity, but entrapped pores were observed in the 58 °C cross-section. The use of Rhodamine B (RhB, 479.02 g mol^-1), Rose Bengal (RB, 1017.64 g mol^-1) and albumin-fluorescein isothiocyanate conjugate from bovine serum (BSA-FITC, ~66,000 g mol^-1) as model compounds demonstrated that release (RhB > RB ≫ BSA-FITC) is controlled both by molecular weight and available porosity. Interestingly, the ranking of release following sintering was 57 > 56 > 59 > 58 °C; COMSOL simulations explored the effects of capsule wall thickness and porosity on release rate. It was hypothesized that model drug adsorption on the available fiber surface-area (57 versus 56 °C) and entrapped porosity (59 versus 58 °C) could have also attributed to the observed ranking of release rates. While the 56 and 57 °C exposures allowed the bulk of the release to occur in <1 day, the capsules sintered at 58 and 59 °C exhibited release that continued after 12 days of exposure.

How Fluorescent Tags Modify Oligomer Size Distributions of the Alzheimer Peptide

Within the complex aggregation process of amyloidogenic peptides into fibrils, early stages of aggregation play a central role and reveal fundamental properties of the underlying mechanism of aggregation. In particular, low-molecular-weight aggregates of the Alzheimer amyloid-β peptide (Aβ) have attracted increasing interest because of their role in cytotoxicity and neuronal apoptosis, typical of aggregation-related diseases. One of the main techniques used to characterize oligomeric stages is fluorescence spectroscopy. To this end, Aβ peptide chains are functionalized with fluorescent tags, often covalently bound to the disordered N-terminus region of the peptide, with the assumption that functionalization and presence of the fluorophore will not modify the process of self-assembly nor the final fibrillar structure. In this investigation, we systematically study the effects of four of the most commonly used fluorophores on the aggregation of Aβ (1–40). Time-resolved and single-molecule fluorescence spectroscopy have been chosen to monitor the oligomer populations at different fibrillation times, and transmission electron microscopy, atomic force microscopy and x-ray diffraction to investigate the structure of mature fibrils. Although the structures of the fibrils were only slightly affected by the fluorescent tags, the sizes of the detected oligomeric species varied significantly depending on the chosen fluorophore. In particular, we relate the presence of high-molecular-weight oligomers of Aβ (1–40) (as found for the fluorophores HiLyte 647 and Atto 655) to net-attractive, hydrophobic fluorophore-peptide interactions, which are weak in the case of HiLyte 488 and Atto 488. The latter leads for Aβ (1–40) to low-molecular-weight oligomers only, which is in contrast to Aβ (1–42). The disease-relevant peptide Aβ (1–42) displays high-molecular-weight oligomers even in the absence of significant attractive fluorophore-peptide interactions. Hence, our findings reveal the potentially high impact of the properties of fluorophores on transient aggregates, which needs to be included in the interpretation of experimental data of oligomers of fluorescently labeled peptides.

Single-molecule photobleaching: Instrumentation and applications

Single-molecule photobleaching (smPB) technique is a powerful tool for characterizing molecular assemblies. It can provide a direct measure of the number of monomers constituting a given oligomeric particle and generate the oligomer size distribution in a specimen. A major current application of this technique is in understanding protein aggregation, which is linked to many incurable diseases. Quantitative measurement of the size distribution of an aggregating protein in a physiological solution remains a difficult task, since techniques such as dynamic light scattering or fluorescence correlation spectroscopy (FCS) can provide an average size, but cannot accurately resolve the underlying size distribution. Here we describe the smPB method as implemented on a home-built total internal reflection fluorescence microscope (TIRF). We first describe the construction of a TIRF microscope, and then demonstrate the power of smPB by characterizing a solution of Amylin (hIAPP) oligomers, a 37-residue peptide whose aggregation is associated with Type II diabetes. We compare our results with FCS data obtained from the same specimen, and discuss the advantages and disadvantages of the two techniques.

Amyloid β (1-40) Toxicity Depends on the Molecular Contact between Phenylalanine 19 and Leucine 34

The formation of the hydrophobic contact between phenylalanine 19 (F19) and leucine 34 (L34) of amyloid β (1-40) (Aβ(1-40)) is known to be an important step in the fibrillation of Aβ(1-40) peptides. Mutations of this putatively early molecular contact were shown to strongly influence the toxicity of Aβ(1-40) ( Das et al. ( 2015 ) ACS Chem. Neurosci. 6 , 1290 - 1295 ). Any mutation of residue F19 completely abolished the toxicity of Aβ(1-40), suggesting that a proper F19-L34 contact is crucial also for the formation of transient oligomers. In this work, we investigate a series of isomeric substitutions of L34, namely, d-leucine, isoleucine, and valine, to study further details of this molecular contact. These replacements represent very minor alterations in the Aβ(1-40) structure posing the question how these alterations challenge the fibrillation kinetics, structure, dynamics, and toxicity of the Aβ(1-40) aggregates. Our work involves kinetic studies using thioflavin T, transmission electron microscopy, X-ray diffraction for the analysis of the fibril morphology, and nuclear magnetic resonance experiments for local structure and molecular dynamics investigations. Combined with cell toxicity assays of the mutated Aβ(1-40) peptides, the physicochemical and biological importance of the early folding contact between F19 and L34 in Aβ(1-40) is underlined. This implies that the F19-L34 contact influences a broad range of different processes including the initiation of fibrillation, oligomer stability, fibril elongation, local fibril structure, and dynamics and cellular toxicity. These processes do not only cover a broad range of diverse mechanisms, but also proved to be highly sensitive to minor modulations of this crucial contact. Furthermore, our work shows that the contact is not simply mediated by general hydrophobic interactions, but also depends on stereospecific mechanisms.

Major Reaction Coordinates Linking Transient Amyloid-β Oligomers to Fibrils Measured at Atomic Level

The structural underpinnings for the higher toxicity of the oligomeric intermediates of amyloidogenic peptides, compared to the mature fibrils, remain unknown at present. The transient nature and heterogeneity of the oligomers make it difficult to follow their structure. Here, using vibrational and solid-state nuclear magnetic resonance spectroscopy, and molecular dynamics simulations, we show that freely aggregating Aβ₄₀ oligomers in physiological solutions have an intramolecular antiparallel configuration that is distinct from the intermolecular parallel β-sheet structure observed in mature fibrils. The intramolecular hydrogen-bonding network flips nearly 90°, and the two β-strands of each monomeric unit move apart, to give rise to the well-known intermolecular in-register parallel β-sheet structure in the mature fibrils. Solid-state nuclear magnetic resonance distance measurements capture the interstrand separation within monomer units during the transition from the oligomer to the fibril form. We further find that the D23-K28 salt-bridge, a major feature of the Aβ₄₀ fibrils and a focal point of mutations linked to early onset Alzheimer's disease, is not detectable in the small oligomers. Molecular dynamics simulations capture the correlation between changes in the D23-K28 distance and the flipping of the monomer secondary structure between antiparallel and parallel β-sheet architectures. Overall, we propose interstrand separation and salt-bridge formation as key reaction coordinates describing the structural transition of the small Aβ₄₀ oligomers to fibrils.

Stereoisomers Probe Steric Zippers in Amyloid-β

Shape complementarity between close-packed residues plays a critical role in the amyloid aggregation process. Here, we probe such "steric zipper" interactions in amyloid-β (Aβ₄₀), whose aggregation is linked to Alzheimer's disease, by replacing natural residues by their stereoisomers. Such mutations are expected to specifically destabilize the shape sensitive "packing" interactions, which may potentially increase their solubility and change other properties. We study the stereomutants DF19 and DL34 and also the DA2/DF4/DH6/DS8 mutant of Aβ₄₀. F19-L34 is a critical contact in a tightly packed region of Aβ, while residues 1-9 are known to be disordered. While both DF19 and DL34 slow down the kinetics of aggregation and form amyloid fibrils efficiently, only DL34 increases the final solubility. DF19 gives rise to additional off-pathway aggregation which results in large, kinetically stable aggregates, and has lower net solubility. DA2/DF4/DH6/DS8 does not have an effect on the kinetics or the solubility. Notably, both DF19 and DL34 oligomers have a significantly lower level of interactions with lipid vesicles and live cells. We conclude that stereoisomers can cause complex site dependent changes in amyloid properties, and provide an effective tool to determine the role of individual residues in shaping the packed interiors of amyloid aggregates.

Highly Sensitive FRET-FCS Detects Amyloid β-Peptide Oligomers in Solution at Physiological Concentrations

Oligomers formed by the amyloid β-peptide (Aβ) are pathogens in Alzheimer's disease. Increased knowledge on the oligomerization process is crucial for understanding the disease and for finding treatments. Ideally, Aβ oligomerization should be studied in solution and at physiologically relevant concentrations, but most popular techniques of today are not capable of such analyses. We demonstrate here that the combination of Förster Resonance Energy Transfer and Fluorescence Correlation Spectroscopy (FRET-FCS) has a unique ability to detect small subpopulations of FRET-active molecules and oligomers. FRET-FCS could readily detect a FRET-active oligonucleotide present at levels as low as 0.5% compared to FRET-inactive dye molecules. In contrast, three established fluorescence fluctuation techniques (FCS, FCCS, and PCH) required fractions between 7 and 11%. When applied to the analysis of Aβ, FRET-FCS detected oligomers consisting of less than 10 Aβ molecules, which coexisted with the monomers at fractions as low as 2 ± 2%. Thus, we demonstrate for the first time direct detection of small fractions of Aβ oligomers in solution at physiological concentrations. This ability of FRET-FCS could be an indispensable tool for studying biological oligomerization processes, in general, and for finding therapeutically useful oligomerization inhibitors.

Cell-Membrane-Mimicking Lipid-Coated Nanoparticles Confer Raman Enhancement to Membrane Proteins and Reveal Membrane-Attached Amyloid-β Conformation

Identifying the structures of membrane bound proteins is critical to understanding their function in healthy and diseased states. We introduce a surface enhanced Raman spectroscopy technique which can determine the conformation of membrane-bound proteins, at low micromolar concentrations, and also in the presence of a substantial membrane-free fraction. Unlike conventional surface enhanced Raman spectroscopy, our approach does not require immobilization of molecules, as it uses spontaneous binding of proteins to lipid bilayer-encapsulated Ag nanoparticles. We apply this technique to probe membrane-attached oligomers of Amyloid-β40 (Aβ40), whose conformation is keenly sought in the context of Alzheimer's disease. Isotope-shifts in the Raman spectra help us obtain secondary structure information at the level of individual residues. Our results show the presence of a β-turn, flanked by two β-sheet regions. We use solid-state NMR data to confirm the presence of the β-sheets in these regions. In the membrane-attached oligomer, we find a strongly contrasting and near-orthogonal orientation of the backbone H-bonds compared to what is found in the mature, less-toxic Aβ fibrils. Significantly, this allows a "porin" like β-barrel structure, providing a structural basis for proposed mechanisms of Aβ oligomer toxicity.

An early folding contact between Phe19 and Leu34 is critical for amyloid-β oligomer toxicity

Small hydrophobic oligomers of aggregation-prone proteins are thought to be generically toxic. Here we examine this view by perturbing an early folding contact between Phe19 and Leu34 formed during the aggregation of Alzheimer's amyloid-β (Aβ40) peptide. We find that even conservative single mutations altering this interaction can abolish Aβ40 toxicity. Significantly, the mutants are not distinguishable either by the oligomers size or by the end-state fibrillar structure from the wild type Aβ40. We trace the change in their toxicity to a drastic lowering of membrane affinity. Therefore, nonlocal folding contacts play a key role in steering the oligomeric intermediates through specific conformations with very different properties and toxicity levels. Our results suggest that engineering the folding energy landscape may provide an alternative route to Alzheimer therapeutics.

Mapping amyloid-β(16-22) nucleation pathways using fluorescence lifetime imaging microscopy

The cross-β peptide architecture is associated with numerous functional biomaterials and deleterious disease related aggregates. While these diverse and ubiquitous paracrystalline assemblies have been widely studied, a fundamental understanding of the nucleation and aggregation pathways to these structures remains elusive. Here we highlight a novel application of fluorescence lifetime imaging microscopy in characterising the critical stages of peptide aggregation. Using the central nucleating core of the amyloid-β (Aβ), Aβ(16-22), as a model cross-β system, and utilising a small fraction of rhodamine labelled peptide (Rh110-Aβ(17-22)), we map out a folding pathway from monomer to paracrystalline nanotube. Using this intrinsic fluorescence reporter, we demonstrate the effects of interfaces and evaporation on the nucleation of sub-critical concentration solutions, providing access to previously uncharacterised intermediate morphologies. Using fluorescence lifetime we follow the local peptide environment through the stages of nucleation and hydrophobic collapse, ending in a stable final structure. This work provides a metric for future implementations of measuring fluorescence lifetimes of intrinsic fluorescence reporters during the very dynamic processes relating to peptide nucleation and maturation.

pH changes the aggregation propensity of amyloid-β without altering the monomer conformation

Decoupling conformational changes from aggregation will help us understand amyloids better. Here we attach Alzheimer's amyloid-β(1-40) monomers to silver nanoparticles, preventing their aggregation, and study their conformation under aggregation-favoring conditions using SERS. Surprisingly, the α-helical character of the peptide remains unchanged between pH 10.5 and 5.5, while the solubility changes >100×. Amyloid aggregation can therefore start without significant conformational changes.

Ammonium hydroxide treatment of Aβ produces an aggregate free solution suitable for biophysical and cell culture characterization

Alzheimer's disease is the leading cause of dementia in the elderly. Pathologically it is characterized by the presence of amyloid plaques and neuronal loss within the brain tissue of affected individuals. It is now widely hypothesised that fibrillar structures represent an inert structure. Biophysical and toxicity assays attempting to characterize the formation of both the fibrillar and the intermediate oligomeric structures of Aβ typically involves preparing samples which are largely monomeric; the most common method by which this is achieved is to use the fluorinated organic solvent 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). Recent evidence has suggested that this method is not 100% effective in producing an aggregate free solution. We show, using dynamic light scattering, size exclusion chromatography and small angle X-ray scattering that this is indeed the case, with HFIP pretreated Aβ peptide solutions displaying an increased proportion of oligomeric and aggregated material and an increased propensity to aggregate. Furthermore we show that an alternative technique, involving treatment with strong alkali results in a much more homogenous solution that is largely monomeric. These techniques for solubilising and controlling the oligomeric state of Aβ are valuable starting points for future biophysical and toxicity assays.

Selective molecular recognition in amyloid growth and transmission and cross-species barriers

Mutual conformational selection and population shift followed by minor induced-fit optimization is the key mechanism in biomolecular recognition, and monomers and small oligomers binding to amyloid seeds in fibril growth is a molecular recognition event. Here, we describe amyloid aggregation, preferred species, cross-species barriers and transmission within the broad framework of molecular recognition. Cross-seeding of amyloid species is governed by conformational selection of compatible (complementary) states. If the dominant conformations of two species are similar, they can cross-seed each other; on the other hand, if they are sufficiently different, they will grow into different fibrils, reflecting species barriers. Such a scenario has recently been observed for the tau protein, which has four repeats. While a construct consisting of repeats 1, 3 and 4 can serve as a seed for the entire four-repeat tau segment, the inverse does not hold. On the other hand, the tau protein repeats with the characteristic U-turn shape can cross-seed Alzheimer's amyloid β and, similarly, the islet amyloid polypeptide. Within this framework, we suggest that the so-called "central dogma" of amyloid formation, where aggregation takes place through nonspecific backbone hydrogen bonding interactions, which are common to all peptides and proteins, is a simple reflection of the heterogeneous, polymorphic free-energy landscape of amyloid species. Here, we review available data and make some propositions addressing this key problem. In particular, we argue that recent theoretical and experimental observations support the key role of selective molecular recognition in amyloidosis and in determining cross-species barriers and transmission.

Biochemistry of amyloid β-protein and amyloid deposits in Alzheimer disease

Progressive cerebral deposition of the amyloid β-protein (Aβ) in brain regions serving memory and cognition is an invariant and defining feature of Alzheimer disease. A highly similar but less robust process accompanies brain aging in many nondemented humans, lower primates, and some other mammals. The discovery of Aβ as the subunit of the amyloid fibrils in meningocerebral blood vessels and parenchymal plaques has led to innumerable studies of its biochemistry and potential cytotoxic properties. Here we will review the discovery of Aβ, numerous aspects of its complex biochemistry, and current attempts to understand how a range of Aβ assemblies, including soluble oligomers and insoluble fibrils, may precipitate and promote neuronal and glial alterations that underlie the development of dementia. Although the role of Aβ as a key molecular factor in the etiology of Alzheimer disease remains controversial, clinical trials of amyloid-lowering agents, reviewed elsewhere in this book, are poised to resolve the question of its pathogenic primacy.

Ion-specific weak adsorption of salts and water/octanol transfer free energy of a model amphiphilic hexapeptide

An amphiphilic hexapeptide has been used as a model to quantify how specific ion effects induced by addition of four salts tune the hydrophilic/hydrophobic balance and induce temperature-dependant coacervate formation from aqueous solution. The hexapeptide chosen is present as a dimer with low transfer energy from water to octanol. Taking sodium chloride as the reference state in the Hofmeister scale, we identify water activity effects and therefore measure the free energy of transfer from water to octanol and separately the free energy associated to the adsorption of chaotropic ions or the desorption of kosmotropic ions for the same amphiphilic peptide. These effects have the same order of magnitude: therefore, both energies of solvation as well as transfer into octanol strongly depend on the nature of the electrolytes used to formulate any buffer. Model peptides could be used on separation processes based on criteria linked to "Hofmeister" but different from volume and valency.

Nature of the amyloid-beta monomer and the monomer-oligomer equilibrium

The monomer to oligomer transition initiates the aggregation and pathogenic transformation of Alzheimer amyloid-β (Aβ) peptide. However, the monomeric state of this aggregation-prone peptide has remained beyond the reach of most experimental techniques, and a quantitative understanding of this transition is yet to emerge. Here, we employ single-molecule level fluorescence tools to characterize the monomeric state and the monomer-oligomer transition at physiological concentrations in buffers mimicking the cerebrospinal fluid (CSF). Our measurements show that the monomer has a hydrodynamic radius of 0.9 ± 0.1 nm, which confirms the prediction made by some of the in silico studies. Surprisingly, at equilibrium, both Aβ(40) and Aβ(42) remain predominantly monomeric up to 3 μm, above which it forms large aggregates. This concentration is much higher than the estimated concentrations in the CSF of either normal or diseased brains. If Aβ oligomers are present in the CSF and are the key agents in Alzheimer pathology, as is generally believed, then these must be released in the CSF as preformed entities. Although the oligomers are thermodynamically unstable, we find that a large kinetic barrier, which is mostly entropic in origin, strongly impedes their dissociation. Thermodynamic principles therefore allow the development of a pharmacological agent that can catalytically convert metastable oligomers into nontoxic monomers.

Histone-based self-assembly into DNA-wrapped meso-clusters

The recent discovery of meso-cluster phase includes not only colloidal molecules of synthetic polymer particles but also equilibrium protein clusters. Here we report self-assembly of histone protein into stable submicron clusters that can be generated even in centrifuged supernatants containing no initial aggregates. Furthermore, dark-field microscopy of the electrophoresis has verified charge reversal of individual histone clusters by adding DNA. We have determined the critical nucleotide concentration at which the electrophoretic mobility vanishes in three types of DNA, revealing the coexistence of nucleosomes with DNA-wrapped meso-clusters.

Measurement of the attachment and assembly of small amyloid-β oligomers on live cell membranes at physiological concentrations using single-molecule tools

It is thought that the pathological cascade in Alzheimer's disease is initiated by the formation of amyloid-β (Aβ) peptide complexes on cell membranes. However, there is considerable debate about the nature of these complexes and the type of solution-phase Aβ aggregates that may contribute to their formation. Also, it is yet to be shown that Aβ attaches strongly to living cell membranes, and that this can happen at low, physiologically relevant Aβ concentrations. Here, we simultaneously measure the aggregate size and fluorescence lifetime of fluorescently labeled Aβ(1-40) on and above the membrane of cultured PC12 cells at near-physiological concentrations. We find that at 350 nM Aβ concentration, large (>>10 nm average hydrodynamic radius) assemblies of codiffusing, membrane-attached Aβ molecules appear on the cell membrane together with a near-monomeric species. When the extracellular concentration is 150 nM, the membrane contains only the smaller species, but with a similar degree of attachment. At both concentrations, the extracellular solution contains only small (∼2.3 nm average hydrodynamic radius) Aβ oligomers or monomers. We conclude that at near-physiological concentrations only the small oligomeric Aβ species are relevant, they are capable of attaching to the cell membrane, and they assemble in situ to form much larger complexes.