Light scattering analysis of fibril growth from the amino-terminal fragment beta(1-28) of beta-amyloid peptide

Thermodynamic characterization of amyloid polymorphism by microfluidic transient incomplete separation

Amyloid fibrils of proteins such as α-synuclein are a hallmark of neurodegenerative diseases and much research has focused on their kinetics and mechanisms of formation. The question as to the thermodynamic stability of such structures has received much less attention. Here, we newly utilize the principle of transient incomplete separation of species in laminar flow in combination with chemical depolymerization for the quantification of amyloid fibril stability. The relative concentrations of fibrils and monomer at equilibrium are determined through an in situ separation of these species based on their different diffusivity inside a microfluidic capillary. The method is highly sample economical, using much less than a microliter of sample per data point and its only requirement is the presence of aromatic residues (W, Y) because of its label-free nature, which makes it widely applicable. Using this method, we investigate the differences in thermodynamic stability between different fibril polymorphs of α-synuclein and quantify these differences for the first time. Importantly, we show that fibril formation can be under kinetic or thermodynamic control and that a change in solution conditions can both stabilise and destabilise amyloid fibrils. Taken together, our results establish the thermodynamic stability as a well-defined and key parameter that can contribute towards a better understanding of the physiological roles of amyloid fibril polymorphism.

Human cystatin C induces the disaggregation process of selected amyloid beta peptides: a structural and kinetic view

Neurodegenerative diseases, such as Alzheimer's disease (AD) and various types of amyloidosis, are incurable; therefore, understanding the mechanisms of amyloid decomposition is crucial to develop an effective drug against them for future therapies. It has been reported that one out of three people over the age of 85 are suffering from dementia as a comorbidity to AD. Amyloid beta (Aβ), the hallmark of AD, transforms structurally from monomers into β-stranded aggregates (fibrils) via multiple oligomeric states. Astrocytes in the central nervous system secrete the human cystatin C protein (HCC) in response to various proteases and cytokines. The codeposition of Aβ and HCC in the brains of patients with AD led to the hypothesis that cystatin C is implicated in the disease process. In this study, we investigate the intermolecular interactions between different atomic structures of fibrils formed by Aβ peptides and HCC to understand the pathological aggregation of these polypeptides into neurotoxic oligomers and then amyloid plaques. To characterize the interactions between Aβ and HCC, we used a complementary approach based on the combination of small-angle neutron scattering analysis, atomic force microscopy and computational modelling, allowing the exploration of the structures of multicomponent protein complexes. We report here an optimized protocol to study that interaction. The results show a dependency of the sequence length of the Aβ peptide on the ability of the associated HCC to disaggregate it.

Deconstructing Alzheimer's Disease: How to Bridge the Gap between Experimental Models and the Human Pathology?

Discovered more than a century ago, Alzheimer's disease (AD) is not only still present in our societies but has also become the most common dementia, with 50 million people worldwide affected by the disease. This number is expected to double in the next generation, and no cure is currently available to slow down or stop the disease progression. Recently, some advances were made due to the approval of the aducanumab treatment by the American Food and Drug Administration. The etiology of this human-specific disease remains poorly understood, and the mechanisms of its development have not been completely clarified. Several hypotheses concerning the molecular mechanisms of AD have been proposed, but the existing studies focus primarily on the two main markers of the disease: the amyloid β peptides, whose aggregation in the brain generates amyloid plaques, and the abnormally phosphorylated tau proteins, which are responsible for neurofibrillary tangles. These protein aggregates induce neuroinflammation and neurodegeneration, which, in turn, lead to cognitive and behavioral deficits. The challenge is, therefore, to create models that best reproduce this pathology. This review aims at gathering the different existing AD models developed in vitro, in cellulo, and in vivo. Many models have already been set up, but it is necessary to identify the most relevant ones for our investigations. The purpose of the review is to help researchers to identify the most pertinent disease models, from the most often used to the most recently generated and from simple to complex, explaining their specificities and giving concrete examples.

Peptide Nanomaterials for Drug Delivery Applications

Self-assembled peptides have been shown to form well-defined nanostructures which display outstanding characteristics for many biomedical applications and especially in controlled drug delivery. Such biomaterials are becoming increasingly popular due to routine, standardized methods of synthesis, high biocompatibility, biodegradability and ease of upscale. Moreover, one can modify the structure at the molecular level to form various nanostructures with a wide range of applications in the field of medicine. Through environmental modifications such as changes in pH and ionic strength and the introduction of enzymes or light, it is possible to trigger self-assembly and design a host of different self-assembled nanostructures. The resulting nanostructures include nanotubes, nanofibers, hydrogels and nanovesicles which all display a diverse range of physico-chemical and mechanical properties. Depending on their design, peptide self-assembling nanostructures can be manufactured with improved biocompatibility and in vivo stability and the ability to encapsulate drugs with the capacity for sustained drug delivery. These molecules can act as carriers for drug molecules to ferry cargo intracellularly and respond to stimuli changes for both hydrophilic and hydrophobic drugs. This review explores the types of self-assembling nanostructures, the effects of external stimuli on and the mechanisms behind the assembly process, and applications for such technology in drug delivery.

The Clustering of mApoE Anti-Amyloidogenic Peptide on Nanoparticle Surface Does Not Alter Its Performance in Controlling Beta-Amyloid Aggregation

The deposition of amyloid-β (Aβ) plaques in the brain is a significant pathological signature of Alzheimer’s disease, correlating with synaptic dysfunction and neurodegeneration. Several compounds, peptides, or drugs have been designed to redirect or stop Aβ aggregation. Among them, the trideca-peptide CWG-LRKLRKRLLR (mApoE), which is derived from the receptor binding sequence of apolipoprotein E, is effectively able to inhibit Aβ aggregation and to promote fibril disaggregation. Taking advantage of Atomic Force Microscopy (AFM) imaging and fluorescence techniques, we investigate if the clustering of mApoE on gold nanoparticles (AuNP) surface may affect its performance in controlling Aβ aggregation/disaggregation processes. The results showed that the ability of free mApoE to destroy preformed Aβ fibrils or to hinder the Aβ aggregation process is preserved after its clustering on AuNP. This allows the possibility to design multifunctional drug delivery systems with clustering of anti-amyloidogenic molecules on any NP surface without affecting their performance in controlling Aβ aggregation processes.

Examination of Adsorption Orientation of Amyloidogenic Peptides Over Nano-Gold Colloidal Particle Surfaces

The adsorption of amyloidogenic peptides, amyloid beta 1–40 (Aβ_1–40), alpha-synuclein (α-syn), and beta 2 microglobulin (β2m), was attempted over the surface of nano-gold colloidal particles, ranging from d = 10 to 100 nm in diameter (d). The spectroscopic inspection between pH 2 and pH 12 successfully extracted the critical pH point (pH_o) at which the color change of the amyloidogenic peptide-coated nano-gold colloids occurred due to aggregation of the nano-gold colloids. The change in surface property caused by the degree of peptide coverage was hypothesized to reflect the ΔpH_o, which is the difference in pH_o between bare gold colloids and peptide coated gold colloids. The coverage ratio (Θ) for all amyloidogenic peptides over gold colloid of different sizes was extracted by assuming Θ = 0 at ΔpH_o = 0. Remarkably, Θ was found to have a nano-gold colloidal size dependence, however, this nano-size dependence was not simply correlated with d. The geometric analysis and simulation of reproducing Θ was conducted by assuming a prolate shape of all amyloidogenic peptides. The simulation concluded that a spiking-out orientation of a prolate was required in order to reproduce the extracted Θ. The involvement of a secondary layer was suggested; this secondary layer was considered to be due to the networking of the peptides. An extracted average distance of networking between adjacent gold colloids supports the binding of peptides as if they are “entangled” and enclosed in an interfacial distance that was found to be approximately 2 nm. The complex nano-size dependence of Θ was explained by available spacing between adjacent prolates. When the secondary layer was formed, Aβ_1–40 and α-syn possessed a higher affinity to a partially negative nano-gold colloidal surface. However, β2m peptides tend to interact with each other. This difference was explained by the difference in partial charge distribution over a monomer. Both Aβ_1–40 and α-syn are considered to have a partial charge (especially δ+) distribution centering around the prolate axis. The β2m, however, possesses a distorted charge distribution. For a lower Θ (i.e., Θ <0.5), a prolate was assumed to conduct a gyration motion, maintaining the spiking-out orientation to fill in the unoccupied space with a tilting angle ranging between 5° and 58° depending on the nano-scale and peptide coated to the gold colloid.

Dynamics of proteins in solution

The dynamics of proteins in solution includes a variety of processes, such as backbone and side-chain fluctuations, interdomain motions, as well as global rotational and translational (i.e. center of mass) diffusion. Since protein dynamics is related to protein function and essential transport processes, a detailed mechanistic understanding and monitoring of protein dynamics in solution is highly desirable. The hierarchical character of protein dynamics requires experimental tools addressing a broad range of time- and length scales. We discuss how different techniques contribute to a comprehensive picture of protein dynamics, and focus in particular on results from neutron spectroscopy. We outline the underlying principles and review available instrumentation as well as related analysis frameworks.

Basified Human Lysozyme: A Potent Inhibitor against Amyloid β-Protein Fibrillogenesis

The aggregation of amyloid β-proteins (Aβ) has been recognized as a key process in the pathogenesis of Alzheimer's disease (AD), so inhibiting Aβ aggregation is an important strategy to prevent the onset and treatment of AD. Our recent work indicated that decreasing the positive charges (or introducing negative charges) on human lysozyme (hLys) was unfavorable in keeping the inhibiting capability of hLys on Aβ aggregation. Therefore, we have herein proposed to basify hLys by conversion of the carboxyl groups into amino groups by modification with ethylene diamine. Basified hLys (Lys-B) preparations of three modification degrees (MDs), denoted as hLys-B1 (MD, 1.5), hLys-B2 (MD, 3.3), and hLys-B3 (MD, 4.4), were synthesized for modulating Aβ fibrillogenesis. The hLys-B preparations kept the stability and biocompatibility as native hLys did, whereas the inhibitory potency of hLys-B on Aβ fibrillogenesis increased with increasing MD. Cytotoxicity analysis showed that cell viability with 2.5 μM hLys-B3 increased from 62.5% (with 25 μM Aβ only) to 76.1%, similar to the case with 12.5 μM hLys (75.5%); cell viability with 6.25 μM hLys-B3 increased to 82.0%, similar to the case with 25 μM hLys (80.9%). The results indicate about four- to fivefold increase in the inhibition efficiency of hLys by the amino modification. Mechanistic analysis suggests that such a superior inhibitory capability of hLys-B was attributed to its more widely distributed positive charges, which promoted broad electrostatic interactions between Aβ and hLys-B. Thus, hLys-B suppressed the conformational transition of Aβ to β-sheet structures at low concentrations (e.g., 2.5 μM hLys-B3), leading to changes in the aggregation pathway and the formation of Aβ species with less cytotoxicity. The findings provided new insights into the development of more potent protein-based inhibitors against Aβ fibrillogenesis.

Analysis of phase behavior and morphology during freeze-thaw applications of lysozyme

Knowledge of protein behavior/stability during freeze/thaw (FT) operations is essential for storage and production processes in the biopharmaceutical industry. FT stress involves freeze concentration, cold denaturation, and ice crystals formation which can result in protein aggregation. Therefore, it is important to understand the ongoing FT processes, and the influence of different solution parameters. In order to evaluate the ongoing processes during FT (up to -80°C), phase diagrams with lysozyme from chicken egg white and sodium chloride were generated. Thereby, three different buffer systems with varying buffer substances and ionic strengths at pH 3 and pH 5 were investigated. As indicators for the ongoing FT processes, the phase behavior, crystal morphology and solubility were used. An increased number of cycles led, for example, to the formation of micro crystals, sea urchin crystals - indicating LLPS and/or high supersaturation - and precipitate. Furthermore, the buffer substances had a more distinct influence on the phase behavior and morphology compared to the ionic strength differences. The solubility line itself was only shifted when distinct changes in the phase behavior could be observed. In summary, a tool was developed for using the phase behavior and especially the crystal morphology as indicator for underlying processes during FT operations.

Cu(II) binding to various forms of amyloid-β peptides. Are they friends or foes?

In the present micro-review, we describe the Cu(II) binding to several forms of amyloid-β peptides, the peptides involved in Alzheimer's disease. It has indeed been shown that in addition to the "full-length" peptide originating from the precursor protein after cleavage at position 1, several other shorter peptides do exist in large proportion and may be involved in the disease as well. Cu(II) binding to amyloid-β peptides is one of the key interactions that impact both the aggregating properties of the amyloid peptides and the Reactive Oxygen Species (ROS) production, two events linked to the etiology of the disease. Binding sites and affinity are described in correlation with Cu(II) induced ROS formation and Cu(II) altered aggregation, for amyloid peptides starting at position 1, 3, 4, 11 and for the corresponding pyroglutamate forms when they could be obtained (i.e. for peptides cleaved at positions 3 and 11). It appears that the current paradigm which points out a toxic role of the Cu(II) - amyloid-β interaction might well be shifted towards a possible protective role when the peptides considered are the N-terminally truncated ones.

Modulating protein amyloid aggregation with nanomaterials

Direct exposure or intake of nanopaticles (NPs) to the human body can invoke a series of biological responses, some of which are deleterious, and as such the role of NPs in vivo requires thorough examination. Over the past decade, it has been established that biomolecules such as proteins can bind NPs to form a 'corona', where the structures and dynamics of NP-associated proteins can assign new functionality, systemic distribution and toxicity. However, the behavior and fate of NPs in biological systems are still far from being fully understood. Growing evidence has shown that some natural or artificial NPs could either up- or down-regulate protein amyloid aggregation, which is associated with neurodegenerative diseases like Alzheimer's and Parkinson's diseases, as well as metabolic diseases such as type 2 diabetes. These effects can be either indirect (e.g., through a crowding effect) or direct, depending on the NP composition, size, shape and surface chemistry. However, efforts to design anti-amyloid NPs for biomedical applications have been largely hindered by insufficient understanding of the complex processes, even though proof-of-concept experiments have been conducted. Therefore, exploring the general mechanisms of NP-meditated protein aggregation marks an emerging field in bio-nano research and a new stage of handling nanotechnology that not only aids in elucidating the origin of nanotoxicity, but also provides a foundation for engineering de novo anti-amyloid nanomedicines. In this review, we summarize research on NP-mediated protein amyloid aggregation, with the goal of contributing to sustained nanotechnology and safe nanomedicine against amyloid diseases.

Activity of Zn and Mg phthalocyanines and porphyrazines in amyloid aggregation of insulin

Formation of the deposits of protein aggregates—amyloid fibrils in an intracellular and intercellular space—is common to a large group of amyloid‐associated disorders. Among the approaches to develop of therapy of such disorders is the use of agents preventing protein fibrillization. Polyaromatic complexes—porphyrins and phthalocyanines—are known as compounds possessing anti‐fibrillogenic activity.

Here, we explore the impact of related macrocyclic complexes—phthalocyanines (Pc) and octaphenyl porphyrazines (Pz) of Mg and Zn—on aggregation of amyloidogenic protein insulin. Pz complexes are firstly reported as compounds able to affect protein fibrillization.

The effect of Pc and Pz complexes on the kinetics and intensity of insulin aggregation was studied by the fluorescent assay using amyloid sensitive cyanine dye. This has shown the impact of metal ion on the anti‐fibrillogenic properties of macrocyclic complexes—the effect on the fibrillization kinetics of Mg‐containing compounds is much more pronounced comparing to that of Zn analogues.

Scanning electron microscopy experiments have demonstrated that filamentous fibrils are the main product of aggregation both for free insulin and in the presence of macrocyclic complexes. However, those fibrils are distinct by their length and proneness to lateral aggregation. The Pc complexes cause the increase in variation of fibrils length 0.9 to 2.7 nm in opposite to 1.4 to 2.0 nm for free insulin, whereas Pz complexes cause certain shortening of the fibrils to 0.8 to 1.6 nm.

The averaged size of the fibrils population was estimated by dynamic light scattering; it correlates with the size of single fibrils detected by scanning electron microscopy.

Protein fibrillation and nanoparticle interactions: opportunities and challenges

Due to their ultra-small size, nanoparticles (NPs) have distinct properties compared with the bulk form of the same materials. These properties are rapidly revolutionizing many areas of medicine and technology. NPs are recognized as promising and powerful tools to fight against the human brain diseases such as multiple sclerosis or Alzheimer's disease. In this review, after an introductory part on the nature of protein fibrillation and the existing approaches for its investigations, the effects of NPs on the fibrillation process have been considered. More specifically, the role of biophysicochemical properties of NPs, which define their affinity for protein monomers, unfolded monomers, oligomers, critical nuclei, and other prefibrillar states, together with their influence on protein fibrillation kinetics has been described in detail. In addition, current and possible-future strategies for controlling the desired effect of NPs and their corresponding effects on the conformational changes of the proteins, which have significant roles in the fibrillation process, have been presented.

Cu(II) mediates kinetically distinct, non-amyloidogenic aggregation of amyloid-beta peptides

Cu(II) ions are implicated in the pathogenesis of Alzheimer disease by influencing the aggregation of the amyloid-β (Aβ) peptide. Elucidating the underlying Cu(II)-induced Aβ aggregation is paramount for understanding the role of Cu(II) in the pathology of Alzheimer disease. The aim of this study was to characterize the qualitative and quantitative influence of Cu(II) on the extracellular aggregation mechanism and aggregate morphology of Aβ(1-40) using spectroscopic, microelectrophoretic, mass spectrometric, and ultrastructural techniques. We found that the Cu(II):Aβ ratio in solution has a major influence on (i) the aggregation kinetics/mechanism of Aβ, because three different kinetic scenarios were observed depending on the Cu(II):Aβ ratio, (ii) the metal:peptide stoichiometry in the aggregates, which increased to 1.4 at supra-equimolar Cu(II):Aβ ratio; and (iii) the morphology of the aggregates, which shifted from fibrillar to non-fibrillar at increasing Cu(II):Aβ ratios. We observed dynamic morphological changes of the aggregates, and that the formation of spherical aggregates appeared to be a common morphological end point independent on the Cu(II) concentration. Experiments with Aβ(1-42) were compatible with the conclusions for Aβ(1-40) even though the low solubility of Aβ(1-42) precluded examination under the same conditions as for the Aβ(1-40). Experiments with Aβ(1-16) and Aβ(1-28) showed that other parts than the Cu(II)-binding His residues were important for Cu(II)-induced Aβ aggregation. Based on this study we propose three mechanistic models for the Cu(II)-induced aggregation of Aβ(1-40) depending on the Cu(II):Aβ ratio, and identify key reaction steps that may be feasible targets for preventing Cu(II)-associated aggregation or toxicity in Alzheimer disease.

Aggregation properties of the peptide fragments derived from the 17-29 region of the human and rat IAPP: a comparative study with two PEG-conjugated variants of the human sequence

The amyloidogenic amino acid sequence Ac-VHSSNNFGAILSS-NH(2), corresponding to the 17-29 peptide region of human amylin (hIAPP17-29), was modified by grafting a hydrophilic PEG chain in order to obtain a novel class of peptides to be used as models to study the aggregation process of the full-length IAPP. The amphiphilic feature of the pegylated peptide fragment at the N-terminus (PEG-N-hIAPP17-29) drives the aggregation process toward stable micellar clusters without fibrillogenesis, despite the presence of beta-sheet interaction between peptides at pH values higher than 4.0. The hIAPP17-29-C-PEG, in which the PEG moiety is linked to the C-terminus, does not possess analogous amphiphilic character and the ability of PEG in forming H-bonds with the solvent overcomes that of the peptide chain, thereby causing peptide flocculation. The comparison with the unmodified hIAPP17-29 and the rat's peptide sequence Ac-VRSSNNLGPGLPP-NH(2)(rIAPP17-29) revealed the crucial role of hydrogen bonding between peptide and solvent in determining the aggregate structure and preventing fibril formation, as well as the non-negligible effect of a small amount of organic solvent in the aqueous solution which affects the aggregation process and rate.

Influence of the solvent on the self-assembly of a modified amyloid beta peptide fragment. I. Morphological investigation

The solvent-induced transition between self-assembled structures formed by the peptide AAKLVFF is studied via electron microscopy, light scattering, and spectroscopic techniques. The peptide is based on a core fragment of the amyloid beta-peptide, KLVFF, extended by two alanine residues. AAKLVFF exhibits distinct structures of twisted fibrils in water or nanotubes in methanol. For intermediate water/methanol compositions, these structures are disrupted and replaced by wide filamentous tapes that appear to be lateral aggregates of thin protofilaments. The orientation of the beta-strands in the twisted tapes or nanotubes can be deduced from X-ray diffraction on aligned stalks, as well as FT-IR experiments in transmission compared to attenuated total reflection. Strands are aligned perpendicular to the axis of the twisted fibrils or the nanotubes. The results are interpreted in light of recent results on the effect of competitive hydrogen bonding upon self-assembly in soft materials in water/methanol mixtures.

Hybrid peptides attenuate cytotoxicity of beta-amyloid by inhibiting its oligomerization: implication from solvent effects

Abnormal assembly of monomeric beta-amyloid (Abeta) in Alzheimer's disease leads to the formation of most neurotoxic oligomers in vivo. In this study, we explored a linking strategy to design hybrid peptides, by combining the Abeta recognition motif and the solvent disruptive sequences. We found that in vitro all synthetic peptides with the recognition motif can affect Abeta fibrillization and alter the morphology of Abeta aggregates variously, different from those without the recognition motif. The effects of peptides containing recognition motif on Abeta aggregation correlate with their abilities to change the surface tension of solutions. In addition, compounds with the recognition motif, not those without such motif, can inhibit cytotoxicity of Abeta in cell culture probably by decreasing the amount of toxic Abeta oligomers. These results indicate that recognition domain and solvent effect should be considered as important factors when designing molecules to target Abeta aggregation.

Mechanism of suppression of protein aggregation by α-crystallin

This review summarizes experimental data illuminating the mechanism of suppression of heat-induced protein aggregation by alpha-crystallin, one of the small heat shock proteins. The dynamic light scattering data show that the initial stage of thermal aggregation of proteins is the formation of the initial aggregates involving hundreds of molecules of the denatured protein. Further sticking of the starting aggregates proceeds in a regime of diffusion-limited cluster-cluster aggregation. The protective effect of alpha-crystallin is due to transition of the aggregation process to the regime of reaction-limited cluster-cluster aggregation, wherein the sticking probability for the colliding particles becomes lower than unity.

Structure-function relationships of pre-fibrillar protein assemblies in Alzheimer's disease and related disorders

Several neurodegenerative diseases, including Alzheimer's, Parkinson's, Huntington's and prion diseases, are characterized pathognomonically by the presence of intra- and/or extracellular lesions containing proteinaceous aggregates, and by extensive neuronal loss in selective brain regions. Related non-neuropathic systemic diseases, e.g., light-chain and senile systemic amyloidoses, and other organ-specific diseases, such as dialysis-related amyloidosis and type-2 diabetes mellitus, also are characterized by deposition of aberrantly folded, insoluble proteins. It is debated whether the hallmark pathologic lesions are causative. Substantial evidence suggests that these aggregates are the end state of aberrant protein folding whereas the actual culprits likely are transient, pre-fibrillar assemblies preceding the aggregates. In the context of neurodegenerative amyloidoses, the proteinaceous aggregates may eventuate as potentially neuroprotective sinks for the neurotoxic, oligomeric protein assemblies. The pre-fibrillar, oligomeric assemblies are believed to initiate the pathogenic mechanisms that lead to synaptic dysfunction, neuronal loss, and disease-specific regional brain atrophy. The amyloid beta-protein (Abeta), which is believed to cause Alzheimer's disease (AD), is considered an archetypal amyloidogenic protein. Intense studies have led to nominal, functional, and structural descriptions of oligomeric Abeta assemblies. However, the dynamic and metastable nature of Abeta oligomers renders their study difficult. Different results generated using different methodologies under different experimental settings further complicate this complex area of research and identification of the exact pathogenic assemblies in vivo seems daunting. Here we review structural, functional, and biological experiments used to produce and study pre-fibrillar Abeta assemblies, and highlight similar studies of proteins involved in related diseases. We discuss challenges that contemporary researchers are facing and future research prospects in this demanding yet highly important field.

Soluble amyloid beta1-28-copper(I)/copper(II)/Iron(II) complexes are potent antioxidants in cell-free systems

Amyloid beta (Abeta) is a central characteristic of Alzheimer's disease (AD). Currently, there is a long-standing dispute regarding the role of Abeta-metal ion (Zn, Cu, and Fe) complexes in AD pathogenesis. Here, we aim to decipher the connection between oxidative damage implicated in AD and Abeta-metal ion complexes. For this purpose we study, using ESR, the modulation of Cu/Fe-induced H 2O 2 decomposition by Abeta 1-28 (Abeta 28), a soluble model of Abeta 40/42. The addition of H 2O 2 to 0.6 nM-360 microM Abeta 28 solutions containing 100 microM Cu(II)/Cu(I)/Fe(II) at pH 6.6 results in a concentration-dependent sigmoidal decay of [*OH] with IC 50 values of 61, 59, and 84 microM, respectively. Furthermore, Abeta 28 reduces 90% of *OH production rate in the Cu(I)-H 2O 2 system in 5 min. Unlike soluble Abeta 28, Abeta 28-Cu aggregates exhibit poor antioxidant activity. The mode of antioxidant activity of soluble Abeta 28 is twofold. The primary (rapid) mechanism involves metal chelation, whereas the secondary (slow) mechanism involves (*)OH scavenging and oxidation of Cu(Fe)-coordinating ligands. On the basis of our findings, we propose that soluble Abeta may play a protective role in the early stages of AD, but not in healthy individuals, where Abeta's concentration is nanomolar. Yet, when Abeta-metal ion complexes undergo aggregation, they significantly lose their protective function and allow oxidative damage to occur.

Energy landscapes of the monomer and dimer of the Alzheimer's peptide Abeta(1-28)

The cytotoxicity of Alzheimer's disease has been linked to the self-assembly of the 4042 amino acid of the amyloid-beta (Abeta) peptide into oligomers. To understand the assembly process, it is important to characterize the very first steps of aggregation at an atomic level of detail. Here, we focus on the N-terminal fragment 1-28, known to form fibrils in vitro. Circular dichroism and NMR experiments indicate that the monomer of Abeta(1-28) is alpha-helical in a membranelike environment and random coil in aqueous solution. Using the activation-relaxation technique coupled with the OPEP coarse grained force field, we determine the structures of the monomer and of the dimer of Abeta(1-28). In agreement with experiments, we find that the monomer is predominantly random coil in character, but displays a non-negligible beta-strand probability in the N-terminal region. Dimerization impacts the structure of each chain and leads to an ensemble of intertwined conformations with little beta-strand content in the region Leu17-Ala21. All these structural characteristics are inconsistent with the amyloid fibril structure and indicate that the dimer has to undergo significant rearrangement en route to fibril formation.

Formation of amyloid fibrils in vitro by human gammaD-crystallin and its isolated domains

PURPOSE

Amyloid fibrils are associated with a variety of human protein misfolding and protein deposition diseases. Previous studies have shown that bovine crystallins form amyloid fibers under denaturing conditions and amyloid fibers accumulate in the lens of mice carrying mutations in crystallin genes. Within differentiating lens fiber cells, crystallins may be exposed to low pH lysosome compartments. We have investigated whether human gammaD-crystallin forms amyloid fibrils in vitro, when exposed to low pH partially denaturing conditions.

METHODS

Human gammaD-crystallin expressed and purified from E. coli, is stable and soluble at 37 degrees C, pH7, and refolds from the fully denatured state back to the native state under these conditions. Purified Human gammaD-crystallin as well as its isolated NH2- and COOH-terminal domains were incubated at acid pH and subsequently examined by transmission electron microscopy, absorption spectroscopy in the presence of Congo red, FTIR, and low-angle X-ray scattering.

RESULTS

Incubation of the intact protein at 37 degrees C in 50 mM acetate buffer pH 3 at 50 mg/ml for 2 days, led to formation of a viscous, gel-like solution. Examination of negatively stained samples by transmission electron microscopy revealed linear, non-branching fibrils of variable lengths, with widths ranging from 15 to 35 nm. Incubation with the dye Congo red generated the spectral red shift associated with dye binding to amyloid. Low-angle X-ray scattering from samples showed clear meridional reflection at 4.7 A and a more diffuse reflection on the equator between 10 and 11 A which is the typical "cross-beta" X-ray fiber diffraction pattern for amyloid fibers. FTIR was used to follow the evolution of the secondary structure of gammaD-crystallin with time during incubation of the protein at pH 3. The native protein displayed a major band at 1640 cm-1 that converted during incubation at 37 degrees C to a band at 1616 cm-1. An additional band at 1689 cm-1 also appeared with time. The presence of bands in the regions about 1620 cm-1 and about 1680 cm-1 has been attributed to the formation of intermolecular beta-sheet structure that characterizes the fibrillar amyloid motif. The isolated NH2-terminal 1-82 and COOH-terminal 86-174 domains of HgammaD-crystallin also formed amyloid fibrils after incubation under the same conditions, but to a lesser extent than the full length.

CONCLUSIONS

HgammaD-crystallin, as well as its isolated NH2-terminal 1-82 and COOH-terminal 86-174 domains of HgammaD-crystallin formed amyloid fibrils upon incubation at acid pH. Investigations of early stages in cataract formation within the lens will be required to assess whether amyloid fibrils play a role in the initiation of cataract in vivo.

The role of lipid–protein interactions in amyloid-type protein fibril formation

Structural transition of polypeptide chains into the beta-sheet state followed by amyloid fibril formation is the key characteristic of a number of the so-called conformational diseases. The multistep process of protein fibrillization can be modulated by a variety of factors, in particular by lipid-protein interactions. A wealth of experimental evidence provides support to the notion that amyloid fibril assembly and the toxicity of pre-fibrillar aggregates are closely related and are both intimately membrane associated phenomena. The present review summarizes the principal factors responsible for the enhancement of fibril formation in a membrane environment, viz. (i) structural transformation of polypeptide chain into a partially folded conformation, (ii) increase of the local concentration of a protein upon its membrane binding, (iii) aggregation-favoring orientation of the bound protein, and (iv) variation in the depth of bilayer penetration affecting the nucleation propensity of the membrane associated protein. The molecular mechanisms of membrane-mediated protein fibrillization are discussed. Importantly, the toxicity of lipid-induced pre-fibrillar aggregates is likely to have presented a very strong negative selection pressure in the evolution of amino acid sequences.

Pharmaceutical liquid crystals: the relevance of partially ordered systems

Pharmaceutical solids have generally been characterized as either three-dimensional crystals or amorphous solids based on X-ray powder diffraction and modulated temperature differential scanning calorimetry. In contrast, fewer examples of thermotropic and lyotropic liquid crystals, or mesophases, appear in the pharmaceutical literature, and that literature teaches that the aforementioned analytical techniques should be complemented with polarized light microscopy and small-angle X-ray scattering in order to effectively identify potential liquid crystalline states. Lyotropic liquid crystals are induced by the presence of solvent, and have been extensively described elsewhere in the context of emulsion technology; however, other pharmaceutical examples are emerging. Thermotropic liquid crystals are induced by a change in temperature and are essentially free of solvent, where more pharmaceutical applications appear in the literature. In the present review the general structural characteristics that favor the formation of liquid crystalline mesophases are categorized by therapeutic target and molecular size, and the analytical means of their identification are presented.

Protein aggregation and its inhibition in biopharmaceutics

Protein aggregation is arguably the most common and troubling manifestation of protein instability, encountered in almost all stages of protein drug development. Protein aggregation, along with other physical and/or chemical instabilities of proteins, remains to be one of the major road barriers hindering rapid commercialization of potential protein drug candidates. Although a variety of methods have been used/designed to prevent/inhibit protein aggregation, the end results are often unsatisfactory for many proteins. The limited success is partly due to our lack of a clear understanding of the protein aggregation process. This article intends to discuss protein aggregation and its related mechanisms, methods characterizing protein aggregation, factors affecting protein aggregation, and possible venues in aggregation prevention/inhibition in various stages of protein drug development.

Interactions of lysozyme in guanidinium chloride solutions from static and dynamic light-scattering measurements

The interactions of partially unfolded proteins provide insight into protein folding and protein aggregation. In this work, we studied partially unfolded hen egg lysozyme interactions in solutions containing up to 7 M guanidinium chloride (GdnHCl). The osmotic second virial coefficient (B(22)) of lysozyme was measured using static light scattering in GdnHCl aqueous solutions at 20 degrees C and pH 4.5. B(22) is positive in all solutions, indicating repulsive protein-protein interactions. At low GdnHCl concentrations, B(22) decreases with rising ionic strength: in the absence of GdnHCl, B(22) is 1.1 x 10(-3) mLmol/g(2), decreasing to 3.0 x 10(-5) mLmol/g(2) in the presence of 1 M GdnHCl. Lysozyme unfolds in solutions at GdnHCl concentrations higher than 3 M. Under such conditions, B(22) increases with ionic strength, reaching 8.0 x 10(-4) mLmol/g(2) at 6.5 M GdnHCl. Protein-protein hydrodynamic interactions were evaluated from concentration-dependent diffusivity measurements, obtained from dynamic light scattering. At moderate GdnHCl concentrations, lysozyme interparticle interactions are least repulsive and hydrodynamic interactions are least attractive. The lysozyme hydrodynamic radius was calculated from infinite-dilution diffusivity and did not change significantly during protein unfolding. Our results contribute toward better understanding of protein interactions of partially unfolded states in the presence of a denaturant; they may be helpful for the design of protein refolding processes that avoid protein aggregation.

Mechanism of accelerated assembly of beta-amyloid filaments into fibrils by KLVFFK(6)

Extracellular senile plaques are a central pathological feature of Alzheimer's disease. At the core of these plaques are fibrillar deposits of beta-amyloid peptide (Abeta). In vitro, Abeta spontaneously assembles into amyloid fibrils of cross-beta sheet structure. Although it was once believed that the fibrils themselves were toxic, more recent data supports the hypothesis that aggregation intermediates, rather than fully formed fibrils, are the most damaging to neuronal tissue. In previously published work, we identified several small peptides that interact with Abeta and increase its aggregation rate while decreasing its toxicity. In this work, we examined in detail the interaction between Abeta and one of these peptides. Using a mathematical model of Abeta aggregation kinetics, we show that the dominant effect of the peptide is to accelerate lateral association of Abeta filaments into fibrils.

Urea modulation of beta-amyloid fibril growth: experimental studies and kinetic models

Aggregation of beta-amyloid (Abeta) into fibrillar deposits is widely believed to initiate a cascade of adverse biological responses associated with Alzheimer's disease. Although it was once assumed that the mature fibril was the toxic form of Abeta, recent evidence supports the hypothesis that Abeta oligomers, intermediates in the fibrillogenic pathway, are the dominant toxic species. In this work we used urea to reduce the driving force for Abeta aggregation, in an effort to isolate stable intermediate species. The effect of urea on secondary structure, size distribution, aggregation kinetics, and aggregate morphology was examined. With increasing urea concentration, beta-sheet content and the fraction of aggregated peptide decreased, the average size of aggregates was reduced, and the morphology of aggregates changed from linear to a globular/linear mixture and then to globular. The data were analyzed using a previously published model of Abeta aggregation kinetics. The model and data were consistent with the hypothesis that the globular aggregates were intermediates in the amyloidogenesis pathway rather than alternatively aggregated species. Increasing the urea concentration from 0.4 M to 2 M decreased the rate of filament initiation the most; between 2 M and 4 M urea the largest change was in partitioning between the nonamyloid and amyloid pathways, and between 4 M and 6 M urea, the most significant change was a reduction in the rate of filament elongation.

Contact regions in the dimer of Alzheimer beta-amyloid domain [1-28] studied by mass spectrometry

Information is provided about the amino acid residues in the [1-28] domain of the Alzheimer b- amyloid protein, which participate in interstrand pairing and initiate fibillogenesis. The study was carried out using electrospray ionization on a four sector mass spectrometer, measuring kinetic energy release for a fragmentation process, and modeling the transition state with molecular dynamics calculations. The results eliminate the sequence [11-24] proposed earlier as the central core, and are consistent with, but do not distinguish between, residues [17-28] and [17-23] proposed by others based on biochemical studies.

Conformational restriction via cyclization in beta-amyloid peptide Abeta(1-28) leads to an inhibitor of Abeta(1-28) amyloidogenesis and cytotoxicity

The aggregation process of beta-amyloid peptide Abeta into amyloid is strongly associated with the pathology of Alzheimer's disease (AD). Aggregation may involve a transition of an alpha helix in Abeta(1-28) into beta sheets and interactions between residues 18-20 of the "Abeta amyloid core." We applied an i, i+4 cyclic conformational constraint to the Abeta amyloid core and devised side chain-to-side chain lactam-bridged cyclo(17, 21)-[Lys(17), Asp(21)]Abeta(1-28). In contrast to Abeta(1-28) and [Lys(17), Asp(21)]Abeta(1-28), cyclo(17, 21)-[Lys(17), Asp(21)]Abeta(1-28) was not able to form beta sheets and cytotoxic amyloid aggregates. Cyclo(17, 21)-[Lys(17), Asp(21)]Abeta(1-28) was able to interact with Abeta(1-28) and to inhibit amyloid formation and cytotoxicity. Cyclo(17, 21)-[Lys(17), Asp(21)]Abeta(1-28) also interacted with Abeta(1-40) and interfered with its amyloidogenesis. Cyclo(17, 21)-[Lys(17), Asp(21)]Abeta(1-28) or similarly constrained Abeta sequences may find therapeutic and diagnostic applications in AD.

4,4(')-Dianilino-1,1(')-binaphthyl-5,5(')-disulfonate: report on non-beta-sheet conformers of Alzheimer's peptide beta(1-40)

The venerable fluorescent probe of protein hydrophobic regions, 4,4(')-dianilino-1,1(')-binaphthyl-5,5(')-disulfonate (bis-ANS), unexpectedly increases in fluorescence with soluble beta(1-40) in acidic buffer solutions but reacts weakly with amyloid fibrils while other hydrophobic probes react with the fibrils. CD analysis correlates reaction with the probe with random coil/mixed conformations and alpha-helical forms of beta(1-40) in buffer solutions but less so with soluble beta-sheet forms or amyloid fibrils. The kinetics of the fluoroalcohol-induced interconversion of conformers can be followed by changes in bis-ANS fluorescence. Formation of the beta-sheet form in aqueous buffer is limited by a slow component (minutes) while fluoroalcohol-promoted changes between beta-sheet and alpha-helix occur over seconds. Variants of beta(1-40) such as beta(1-42) or the Dutch E22Q mutation of beta(1-40) and fragments beta(1-28), beta(12-28), beta(10-20 amide), and beta(10-35 amide) react with bis-ANS under conditions that do not support fibril formation. Primary amino acid sequence is important as beta(1-11) does not cause bis-ANS fluorescence while beta(1-16) does, but hydrophobicity is not as beta(25-35) and beta(15-20 amide) are unreactive. bis-ANS is a useful biophysical tool for characterizing particular, but not all, soluble Abeta conformations distinct from the fibrillar form of amyloid peptides detected by Thioflavin T.

Low levels of asparagine deamidation can have a dramatic effect on aggregation of amyloidogenic peptides: implications for the study of amyloid formation

The polypeptide hormone amylin forms amyloid deposits in Type 2 diabetes mellitus and a 10-residue fragment of amylin (amylin(20-29)) is commonly used as a model system to study this process. Studies of amylin(20-29) and several variant peptides revealed that low levels of deamidation can have a significant effect on the secondary structure and aggregation behavior of these molecules. Results obtained with a variant of amylin(20-29), which has the primary sequence SNNFPAILSS, are highlighted. This peptide is particularly interesting from a technical standpoint. In the absence of impurities the peptide does not spontaneously aggregate and is not amyloidogenic. This peptide can spontaneously deamidate, and the presence of less than 5% of deamidation impurities leads to the formation of aggregates that have the hallmarks of amyloid. In addition, small amounts of deamidated material can induce amyloid formation by the purified peptide. These results have fundamental implications for the definition of an amyloidogenic sequence and for the standards of purity of peptides and proteins used for studies of amyloid formation.

Models of amyloid seeding in Alzheimer's disease and scrapie: mechanistic truths and physiological consequences of the time-dependent solubility of amyloid proteins

Ordered protein aggregation in the brain is a hallmark of Alzheimer's disease and scrapie. The disease-specific amyloid fibrils comprise primarily a single protein, amyloid beta, in Alzheimer's disease, and the prion protein in scrapie. These proteins can be induced to form aggregates in vitro that are indistinguishable from brain-derived fibrils. Consequently, much effort has been invested in the development of in vitro model systems to study the details of the aggregation processes and the effects of endogenous molecules that have been implicated in disease. Selected studies of this type are reviewed herein. A simple mechanistic model has emerged for both processes that involves a nucleation-dependent polymerization. This mechanism dictates that aggregation is dependent on protein concentration and time. Furthermore, amyloid formation can be seeded by a preformed fibril. The physiological consequences of this mechanism are discussed.