Alzheimer's amyloid fibrils: structure and assembly

Analysis of the secondary structure of beta-amyloid (Abeta42) fibrils by systematic proline replacement

Amyloid fibrils in Alzheimer's disease mainly consist of 40- and 42-mer beta-amyloid peptides (Abeta40 and Abeta42) that exhibit aggregative ability and neurotoxicity. Although the aggregates of Abeta peptides are rich in intermolecular beta-sheet, the precise secondary structure of Abeta in the aggregates remains unclear. To identify the amino acid residues involved in the beta-sheet formation, 34 proline-substituted mutants of Abeta42 were synthesized and their aggregative ability and neurotoxicity on PC12 cells were examined. Prolines are rarely present in beta-sheet, whereas they are easily accommodated in beta-turn as a Pro-X corner. Among the mutants at positions 15-32, only E22P-Abeta42 extensively aggregated with stronger neurotoxicity than wild-type Abeta42, suggesting that the residues at positions 15-21 and 24-32 are involved in the beta-sheet and that the turn at positions 22 and 23 plays a crucial role in the aggregation and neurotoxicity of Abeta42. The C-terminal proline mutants (A42P-, I41P-, and V40P-Abeta42) hardly aggregated with extremely weak cytotoxicity, whereas the C-terminal threonine mutants (A42T- and I41T-Abeta42) aggregated potently with significant cytotoxicity. These results indicate that the hydrophobicity of the C-terminal two residues of Abeta42 is not related to its aggregative ability and neurotoxicity, rather the C-terminal three residues adopt the beta-sheet. These results demonstrate well the large difference in aggregative ability and neurotoxicity between Abeta42 and Abeta40. In contrast, the proline mutants at the N-terminal 13 residues showed potent aggregative ability and neurotoxicity similar to those of wild-type Abeta42. The identification of the beta-sheet region of Abeta42 is a basis for designing new aggregation inhibitors of Abeta peptides.

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.

Protein fiber linear dichroism for structure determination and kinetics in a low-volume, low-wavelength couette flow cell

High-resolution structure determination of soluble globular proteins relies heavily on x-ray crystallography techniques. Such an approach is often ineffective for investigations into the structure of fibrous proteins as these proteins generally do not crystallize. Thus investigations into fibrous protein structure have relied on less direct methods such as x-ray fiber diffraction and circular dichroism. Ultraviolet linear dichroism has the potential to provide additional information on the structure of such biomolecular systems. However, existing systems are not optimized for the requirements of fibrous proteins. We have designed and built a low-volume (200 microL), low-wavelength (down to 180 nm), low-pathlength (100 microm), high-alignment flow-alignment system (couette) to perform ultraviolet linear dichroism studies on the fibers formed by a range of biomolecules. The apparatus has been tested using a number of proteins for which longer wavelength linear dichroism spectra had already been measured. The new couette cell has also been used to obtain data on two medically important protein fibers, the all-beta-sheet amyloid fibers of the Alzheimer's derived protein Abeta and the long-chain assemblies of alpha1-antitrypsin polymers.

Zinc binding agonist effect on the recognition of the β-amyloid (4–10) epitope by anti-β-amyloid antibodies

Amyloid plaques associated to Alzheimer's disease present a high content of zinc ions. We previously showed that the N-terminal region of the amyloid peptide Abeta constitutes an autonomous zinc-binding domain. This region encompasses the previously identified epitope Abeta(4-10) targeted by antibodies capable to reduce amyloid deposition, but the influence of Abeta/Zn binding on the epitope recognition remains unknown. We demonstrate here the effect of Zn2+ ions on the recognition of peptides sharing the sequence of the Abeta N-terminal domain, by two monoclonal antibodies recognizing the beta-amyloid(4-10) epitope. The presence of Zn2+, but not of other cations, increased the recognition of the (1-16) peptide, while it was without effect on the recognition of the (1-10) peptide. These findings show a zinc-induced conformational change of the (1-16)-N-terminal region of AP3, which results in a better accessibility of the Abeta(4-10) epitope to the anti-Abeta antibodies, and suggest a role of zinc in epitope-based vaccination approaches.

The circularization of amyloid fibrils formed by apolipoprotein C-II

Amyloid fibrils have historically been characterized by diagnostic dye-binding assays, their fibrillar morphology, and a "cross-beta" x-ray diffraction pattern. Whereas the latter demonstrates that amyloid fibrils have a common beta-sheet core structure, they display a substantial degree of morphological variation. One striking example is the remarkable ability of human apolipoprotein C-II amyloid fibrils to circularize and form closed rings. Here we explore in detail the structure of apoC-II amyloid fibrils using electron microscopy, atomic force microscopy, and x-ray diffraction studies. Our results suggest a model for apoC-II fibrils as ribbons approximately 2.1-nm thick and 13-nm wide with a helical repeat distance of 53 nm +/- 12 nm. We propose that the ribbons are highly flexible with a persistence length of 36 nm. We use these observed biophysical properties to model the apoC-II amyloid fibrils either as wormlike chains or using a random-walk approach, and confirm that the probability of ring formation is critically dependent on the fibril flexibility. More generally, the ability of apoC-II fibrils to form rings also highlights the degree to which the common cross-beta superstructure can, as a function of the protein constituent, give rise to great variation in the physical properties of amyloid fibrils.

The stability and dynamics of the human calcitonin amyloid peptide DFNKF

The stability and dynamics of the human calcitonin-derived peptide DFNKF (hCT(15-19)) are studied using molecular dynamics (MD) simulations. Experimentally, this peptide is highly amyloidogenic and forms fibrils similar to the full length calcitonin. Previous comparative MD studies have found that the parallel beta-stranded sheet is a stable organization of the DFNKF protofibril. Here, we probe the stability and dynamics of the small parallel DFNKF oligomers. The results show that even small DFNKF oligomers, such as trimers and tetramers, are stable for a sufficient time in the MD simulations, indicating that the crucial nucleus seed size for amyloid formation can be quite small. The simulations also show that the stability of DFNKF oligomers increases with their sizes. The small but stable seed may reflect the experimental rapid formation of the DFNKF fibrils. Further, a noncooperative process of parallel beta-sheet formation from the out-of-register trimer is observed in the simulations. In general, the residues of DFNKF peptides near the N-/C-termini are more flexible, whereas the interior residues are more stable. Simulations of mutants and capped peptides show that both interstrand hydrophobic and electrostatic interactions play important roles in stabilizing the DFNKF parallel oligomers. This study provides insights into amyloid formation.

Monitoring the process of HypF fibrillization and liposome permeabilization by protofibrils

Much information has appeared in the last few years on the low resolution structure of amyloid fibrils and on their non-fibrillar precursors formed by a number of proteins and peptides associated with amyloid diseases. The fine structure and the dynamics of the process leading misfolded molecules to aggregate into amyloid assemblies are far from being fully understood. Evidence has been provided in the last five years that protein aggregation and aggregate toxicity are rather generic processes, possibly affecting all polypeptide chains under suitable experimental conditions. This evidence extends the number of model proteins one can investigate to assess the molecular bases and general features of protein aggregation and aggregate toxicity. We have used tapping mode atomic force microscopy to investigate the morphological features of the pre-fibrillar aggregates and of the mature fibrils produced by the aggregation of the hydrogenase maturation factor HypF N-terminal domain (HypF-N), a protein not associated to any amyloid disease. We have also studied the aggregate-induced permeabilization of liposomes by fluorescence techniques. Our results show that HypF-N aggregation follows a hierarchical path whereby initial globules assemble into crescents; these generate large rings, which evolve into ribbons, further organizing into differently supercoiled fibrils. The early pre-fibrillar aggregates were shown to be able to permeabilize synthetic phospholipid membranes, thus showing that this disease-unrelated protein displays the same amyloidogenic behaviour found for the aggregates of most pathological proteins and peptides. These data complement previously reported findings, and support the idea that protein aggregation, aggregate structure and toxicity are generic properties of polypeptide chains.

Irreversible formation of intermediate BSA oligomers requires and induces conformational changes

Understanding the relation between protein conformational changes and aggregation, and the physical mechanisms leading to such processes, is of primary importance, due to its direct relation to a vast class of severe pathologies. Growing evidence also suggests that oligomeric intermediates, which may occur early in the aggregation pathway, can be themselves pathogenic. The possible cytotoxicity of oligomers of non-disease-associated proteins adds generality to such suggestion and to the interest of studies of oligomer formation. Here we study the early stages of aggregation of Bovine Serum Albumin (BSA), a non pathogenic protein which has proved to be a useful model system. Dynamic light scattering and circular dichroism measurements in kinetic experiments following step-wise temperature rises, show that the "intermediate" form, which initiates large-scale aggregation, is the result of structural and conformational changes and concurrent formation of oligomers, of average size in the range of 100-200 A. Two distinct thresholds are observed. Beyond the first one oligomerization starts and causes partial irreversibility of conformational changes. Beyond the second threshold, additional secondary structural changes occurring in proteins being recruited progress on the same time scale of oligomerization. The concurrent behavior causes a mutual stabilization of oligomerization, and of structural and conformational changes, evidenced by a progressive increase of their irreversibility. This process interaction appears to be pivotal in producing irreversible oligomers.

Self-association process of a peptide in solution: from beta-sheet filaments to large embedded nanotubes

Lanreotide is a synthetic octapeptide used in the therapy against acromegaly. When mixed with pure water at 10% (w/w), Lanreotide (acetate salt) forms liquid crystalline and monodisperse nanotubes with a radius of 120 A. The molecular and supramolecular organization of these structures has been determined in a previous work as relying on the lateral association of 26 beta-sheet filaments made of peptide noncovalent dimers, the basic building blocks. The work presented here has been devoted to the corresponding self-association mechanisms, through the characterization of the Lanreotide structures formed in water, as a function of peptide (acetate salt) concentration (from 2% to 70% (w/w)) and temperature (from 15 degrees C to 70 degrees C). The corresponding states of water were also identified and quantified from the thermal behavior of water in the Lanreotide mixtures. At room temperature and below 3% (w/w) Lanreotide acetate in water, soluble aggregates were detected. From 3% to 20% (w/w) long individual and monodisperse nanotubes crystallized in a hexagonal lattice were evidenced. Their molecular and supramolecular organizations are identical to the ones characterized for the 10% (w/w) sample. Heating induces the dissolution of the nanotubes into soluble aggregates of the same structural characteristics as the room temperature ones. The solubilization temperature increases from 20 degrees C to 70 degrees C with the peptide concentration and reaches a plateau between 15% and 25% (w/w) in peptide. These aggregates are proposed to be the beta-sheet filaments that self-associate to build the walls of the nanotubes. Above 20% (w/w) of Lanreotide acetate in water, polydisperse embedded nanotubes are formed and the hexagonal lattice is lost. These embedded nanotubes exhibit the same molecular and supramolecular organizations as the individual monodisperse nanotubes formed at lower peptide concentration. The embedded nanotubes do not melt in the range of temperature studied indicating a higher thermodynamic stability than individual nanotubes. In parallel, the thermal behaviors of water in mixtures containing 2-80% (w/w) in peptide have been studied by differential scanning calorimetry, and three different types of water were characterized: 1), bulk water melting at 0 degrees C, 2), nonfreezing water, and 3), interfacial water melting below 0 degrees C. The domains of existence and coexistence of these different water states are related to the different Lanreotide supramolecular structures. All these results were compiled into a binary Lanreotide-water phase diagram and allowed to propose a self-association mechanism of Lanreotide filaments into monodisperse individual nanotubes and embedded nanotubes.

Discrete molecular dynamics simulations of peptide aggregation

We study the aggregation of peptides using the discrete molecular dynamics simulations. Specifically, at temperatures above the alpha-helix melting temperature of a single peptide, the model peptides aggregate into a multilayer parallel beta-sheet structure. This structure has an interstrand distance of 4.8 A and an intersheet distance of 10 A, which agree with experimental observations. Our model explains these results as follows: hydrogen-bond interactions give rise to the interstrand spacing in beta sheets, while Gō interactions between side chains make beta strands parallel to each other and allow beta sheets to pack into layers. An important feature of our results is that the aggregates contain free edges, which may allow for further aggregation of model peptides to form elongated fibrils.

Integrating folding kinetics and protein function: biphasic kinetics and dual binding specificity in a WW domain

Because of the association of beta-sheet formation with the initiation and propagation of amyloid diseases, model systems have been sought to further our understanding of this process. WW domains have been proposed as one such model system. Whereas the folding of the WW domains from human Yes-associated protein (YAP) and Pin have been shown to obey single-exponential kinetics, the folding of the WW domain from formin-binding protein (FBP) 28 has been shown to proceed via biphasic kinetics. From an analysis of free-energy landscapes from atomic-level molecular dynamics simulations, the biphasic folding kinetics observed in the FBP WW domain may be traced to the ability of this WW domain to adopt two slightly different forms of packing in its hydrophobic core. This conformational change is propagated along the peptide backbone and affects the position of a tryptophan residue shown in other WW domains to play a key role in binding. The WW domains of Pin and YAP do not support more than one type of packing each, leading to monophasic folding kinetics. The ability of the FBP WW domain to assume two different types of packing may, in turn, explain the capacity of this WW domain to bind two classes of ligand, a property that is not shared by other WW domains. These findings lead to the hypothesis that lability with respect to conformations separated by an observable barrier as a requirement for function is incompatible with the ability of a protein to fold via single-exponential kinetics.

Copper binding to the amyloid-beta (Abeta) peptide associated with Alzheimer's disease: folding, coordination geometry, pH dependence, stoichiometry, and affinity of Abeta-(1-28): insights from a range of complementary spectroscopic techniques

There is now direct evidence that copper is bound to amyloid-beta peptide (Abeta) in senile plaque of Alzheimer's disease. Copper is also linked with the neurotoxicity of Abeta and free radical damage, and Cu(2+) chelators represent a possible therapy for Alzheimer's disease. We have therefore used a range of complementary spectroscopies to characterize the coordination of Cu(2+) to Abeta in solution. The mode of copper binding is highly pH-dependent. EPR spectroscopy indicates that both coppers have axial, Type II coordination geometry, square-planar or square-pyramidal, with nitrogen and oxygen ligands. Circular dichroism studies indicate that copper chelation causes a structural transition of Abeta. Competition studies with glycine and l-histidine indicate that copper binds to Abeta-(1-28) at pH 7.4 with an affinity of K(a) approximately 10(7) m(-1). (1)H NMR indicates that histidine residues are involved in Cu(2+) coordination but that Tyr(10) is not. Studies using analogues of Abeta-(1-28) in which each of the histidine residues have been replaced by alanine or in which the N terminus is acetylated suggest that the N terminus and His(13) are crucial for Cu(2+) binding and that His(6) and His(14) are also implicated. Evidence for the link between Alzheimer's disease and Cu(2+) is growing, and our studies have made a significant contribution to understanding the mode of Cu(2+) binding to Abeta in solution.

Engineering amyloidogenicity towards the development of nanofibrillar materials

When folded into their native structures, proteins in biological systems function as nanostructured machines. By contrast, some polypeptides tend to aggregate into other well-ordered structures, namely amyloid fibrils. Such well-ordered protein fibrils are attractive materials for nanobiotechnology because they self-associate through noncovalent bonds under controlled conditions - a property that is shared with small organic molecules called organogelators. Recently, the use of amyloid fibrils as structural templates for constructing nanowires has been demonstrated. Such applications will potentially become one of the next trends in protein engineering and nanobiotechnology.

Generation of Amyloid A Protein by the Cell Lines from Amyloid-Susceptible and -Resistant Mice

It has been proposed that impaired degradation is the cause of amyloid A (AA) formation in reactive amyloidosis (Ham et al., Scand J Immunol 1997; 45: 354-60). The current SDS-PAGE of the culture medium showed that the macrophage cell line from the amyloid-susceptible mouse strain (ANA1) degraded amyloid precursor protein serum amyloid A into the AA-like amyloidogenic product of approximately 8.6 kDa but went no further, whereas cells from the resistant strain (A/J10) cleared the AA-like derivates proceeding to approximately 7.7 kDa products within the incubation period. Degradation occurred in the chemically defined medium at a slower rate than in the medium with serum. This may imply that a lack of the serum components as well as impaired degradation could contribute to the development of amyloidosis.

Chain-length dependence of ?-helix to ?-sheet transition in polylysine: Model of protein aggregation studied by temperature-tuned FTIR spectroscopy

The chain-length dependence of the alpha-helix to beta-sheet transition in poly(L-lysine) is studied by temperature-tuned FTIR spectroscopy. This study shows that heterogeneous samples of poly(L-lysine), comprising polypeptide chains with various lengths, undergo the alpha-beta transition at an intermediate temperature compared to homogeneous ingredients. This holds true as long as each individual fraction of the polypeptide is capable of adopting an antiparallel beta-sheet structure. The tendency is that the longer chain is, the lower the alpha-beta transition temperature is, which has been linked to the presence of distorted or solvated helices with turns or beta sheets in elongating chains of poly(L-lysine). As such helical structures are apparently conducive to the alpha-beta transition, this draws a comparison to the hypothesis of metastable protein conformational states being a common stage in amyloid-formation pathways. The antiparallel architecture of the beta sheet is likely to reflect the pretransition interhelical interactions in poly(L-lysine). Namely, the chains are arranged in an antiparallel manner because of energetically favored antiparallel pre-assembly of dipolar alpha helices.

Assembly and kinetic folding pathways of a tetrameric beta-sheet complex: molecular dynamics simulations on simplified off-lattice protein models

We have performed discontinuous molecular dynamic simulations of the assembly and folding kinetics of a tetrameric beta-sheet complex that contains four identical four-stranded antiparallel beta-sheet peptides. The potential used in the simulation is a hybrid Go-type potential characterized by the bias gap parameter g, an artificial measure of a model protein's preference for its native state, and the intermolecular contact parameter eta, which measures the ratio of intermolecular to intramolecular native attractions. The formation of the beta-sheet complex and its equilibrium properties strongly depend on the size of the intermolecular contact parameter eta. The ordered beta-sheet complex in the folded state and nonaligned beta-sheets or tangled chains in the misfolded state are distinguished by measuring the squared radius of gyration Rg2 and the fraction of native contacts Q. The folding yield for the folded state is high at intermediate values of eta, but is low at both small and large values of eta. The folded state at small eta is liquid-like, but is solid-like at both intermediate and large eta. The misfolded state at small eta contains nonaligned beta-sheets and tangled chains with poor secondary structure at large eta. Various folding pathways via dimeric and trimeric intermediates are observed, depending on eta. Comparison with experimental results on protein aggregation indicates that intermediate eta values are most appropriate for modeling fibril formation and small eta values are most appropriate for modeling the formation of amorphous aggregates.

Thermodynamics and stability of a beta-sheet complex: molecular dynamics simulations on simplified off-lattice protein models

We have performed discontinuous molecular dynamics simulations of the thermodynamics and stability of a tetrameric beta-sheet complex that contains four identical four-stranded antiparallel beta-sheet peptides. The potential used in the simulation is a hybrid Go-type potential characterized by the bias gap parameter g, an artificial measure of the preference of a model protein for its native state, and the intermolecular contact parameter eta, which measures the ratio of intermolecular to intramolecular native attractions. Despite the simplicity of the model, a complex set of thermodynamic transitions for the beta-sheet complex is revealed that shows there are three distinct oligomer (partially ordered, ordered, and highly ordered beta-sheet complex) states and four noninteracting monomers phases. The thermodynamic properties of the three oligomer states strongly depend on both the size of the intermolecular contact parameter eta and the temperature. The partially ordered beta-sheet complex is made up of four ordered globules and is observed at intermediate to large eta at high temperatures. The ordered beta-sheet complex contains four native beta-sheets and is located at small to intermediate eta at low temperatures in the phase diagram. The highly ordered beta-sheet complex has fully-stiff beta-sheet strands, the same as the global energy minimum structure, and is observed for all eta at low temperatures.

Protein aggregation/folding: the role of deterministic singularities of sequence hydrophobicity as determined by nonlinear signal analysis of acylphosphatase and Abeta(1-40)

The problem of protein folding vs. aggregation was investigated in acylphosphatase and the amyloid protein Abeta(1-40) by means of nonlinear signal analysis of their chain hydrophobicity. Numerical descriptors of recurrence patterns provided the basis for statistical evaluation of folding/aggregation distinctive features. Static and dynamic approaches were used to elucidate conditions coincident with folding vs. aggregation using comparisons with known protein secondary structure classifications, site-directed mutagenesis studies of acylphosphatase, and molecular dynamics simulations of amyloid protein, Abeta(1-40). The results suggest that a feature derived from principal component space characterized by the smoothness of singular, deterministic hydrophobicity patches plays a significant role in the conditions governing protein aggregation.

Glutamate and amyloid beta-protein rapidly inhibit fast axonal transport in cultured rat hippocampal neurons by different mechanisms

Impairment of axonal transport leads to neurodegeneration and synapse loss. Glutamate and amyloid beta-protein (Abeta) have critical roles in the pathogenesis of Alzheimer's disease (AD). Here we show that both agents rapidly inhibit fast axonal transport in cultured rat hippocampal neurons. The effect of glutamate (100 microm), but not of Abeta25-35 (20 microm), was reversible, was mimicked by NMDA or AMPA, and was blocked by NMDA and AMPA antagonists and by removal of extracellular Ca2+. The effect of Abeta25-35 was progressive and irreversible, was prevented by the actin-depolymerizing agent latrunculin B, and was mimicked by the actin-polymerizing agent jasplakinolide. Abeta25-35 induced intracellular actin aggregation, which was prevented by latrunculin B. Abeta31-35 but not Abeta15-20 exerted effects similar to those of Abeta25-35. Full-length Abeta1-42 incubated for 7 d, which specifically contained 30-100 kDa molecular weight assemblies, also caused an inhibition of axonal transport associated with intracellular actin aggregation, whereas freshly dissolved Abeta1-40, incubated Abeta1-40, and fresh Abeta1-42 had no effect. These results suggest that glutamate inhibits axonal transport via activation of NMDA and AMPA receptors and Ca2+ influx, whereas Abeta exerts its inhibitory effect via actin polymerization and aggregation. The ability of Abeta to inhibit axonal transport seems to require active amino acid residues, which is probably present in the 31-35 sequence. Full-length Abeta may be effective when it represents a structure in which these active residues can access the cell membrane. Our results may provide insight into the early pathogenetic mechanisms of AD.

Protein aggregation and aggregate toxicity: new insights into protein folding, misfolding diseases and biological evolution

The deposition of proteins in the form of amyloid fibrils and plaques is the characteristic feature of more than 20 degenerative conditions affecting either the central nervous system or a variety of peripheral tissues. As these conditions include Alzheimer's, Parkinson's and the prion diseases, several forms of fatal systemic amyloidosis, and at least one condition associated with medical intervention (haemodialysis), they are of enormous importance in the context of present-day human health and welfare. Much remains to be learned about the mechanism by which the proteins associated with these diseases aggregate and form amyloid structures, and how the latter affect the functions of the organs with which they are associated. A great deal of information concerning these diseases has emerged, however, during the past 5 years, much of it causing a number of fundamental assumptions about the amyloid diseases to be re-examined. For example, it is now apparent that the ability to form amyloid structures is not an unusual feature of the small number of proteins associated with these diseases but is instead a general property of polypeptide chains. It has also been found recently that aggregates of proteins not associated with amyloid diseases can impair the ability of cells to function to a similar extent as aggregates of proteins linked with specific neurodegenerative conditions. Moreover, the mature amyloid fibrils or plaques appear to be substantially less toxic than the pre-fibrillar aggregates that are their precursors. The toxicity of these early aggregates appears to result from an intrinsic ability to impair fundamental cellular processes by interacting with cellular membranes, causing oxidative stress and increases in free Ca2+ that eventually lead to apoptotic or necrotic cell death. The 'new view' of these diseases also suggests that other degenerative conditions could have similar underlying origins to those of the amyloidoses. In addition, cellular protection mechanisms, such as molecular chaperones and the protein degradation machinery, appear to be crucial in the prevention of disease in normally functioning living organisms. It also suggests some intriguing new factors that could be of great significance in the evolution of biological molecules and the mechanisms that regulate their behaviour.

Influence of Hydrophobic Teflon Particles on the Structure of Amyloid β-Peptide

The amyloid beta-protein (Abeta) constitutes the major peptide component of the amyloid plaque deposits of Alzheimer's disease in humans. The Abeta changes from a nonpathogenic to a pathogenic conformation resulting in self-aggregation and deposition of the peptide. It has been established that denaturing factors (such as the interaction with membranes) are involved in the structural transition. This work is aimed at determining the effect of hydrophobic Teflon on the conformation of the Abeta (1-40). Prior to adsorption, the secondary structure and self-aggregation state of the Abeta in solution were established as a function of pH. Three different species coexist: unordered monomers/dimers, small oligomers in mainly a regular beta-sheet structure, and bigger aggregates having a twisted beta-sheet conformation. Transferring the Abeta from the solution to the Teflon surface strongly promotes alpha-helix formation. Furthermore, increasing the degree of coverage of the Teflon by the Alphabeta protein leads to a conformational change toward a more enriched beta-sheet structure.

AFM and STM study of beta-amyloid aggregation on graphite

Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) have been employed in situ and ex situ to directly study the aggregation of beta-amyloid(1-42) (Abeta42) peptide on hydrophobic graphite. From in situ AFM images, Abeta42 peptides were seen to aggregate into the sheets that preferred to three orientations with characteristic 3-fold symmetry (Proc. Natl. Acad. Sci. USA 96 (1999) 3688). The sheets were formed by parallel narrow lines with a height of 0.8-1.0nm and a width of 12-14nm. The narrow lines looked like beaded chains and have a right-handed axial periodicity. The high-resolution ex situ AFM and STM images showed that some fibrils of beta-amyloid had a characteristic domain texture, indicating they were formed through the association of protofibrils and monomers. The fibril containing lateral associated filaments that exhibited right-handed twist was clearly observed in the STM image. These results provide important clues to study the detailed structure of beta-amyloid aggregates and the mechanism of the Abeta fibrils formation on hydrophobic surface.

Structure and texture of fibrous crystals formed by Alzheimer's abeta(11-25) peptide fragment

Amyloid fibril deposition is central to the pathology of Alzheimer's disease. X-ray diffraction from amyloid fibrils formed from full-length Abeta(1-40) and from a shorter fragment, Abeta(11-25), have revealed cross-beta diffraction fingerprints. Magnetic alignment of Abeta(11-25) amyloid fibrils gave a distinctive X-ray diffraction texture, allowing interpretation of the diffraction data and a model of the arrangement of the peptides within the amyloid fiber specimen to be constructed. An intriguing feature of the structure of fibrillar Abeta(11-25) is that the beta sheets, of width 5.2 nm, stack by slipping relative to each other by the length of two amino acid units (0.70 nm) to form beta ribbons 4.42 nm in thickness. Abeta(1-40) amyloid fibrils likely consist of once-folded hairpins, consistent with the size of the fibers obtained using electron microscopy and X-ray diffraction.

Optical spectroscopic investigations of model beta-sheet hairpins in aqueous solution

In this contribution we report optical spectroscopic data on a series of designed beta hairpins previously shown by NMR to contain a substantial population of beta-sheet structure. These models contain a designed hydrophobic cluster and a (D)Pro-Gly sequence to promote formation of a turn geometry. FTIR, electronic and vibrational CD (ECD and VCD) spectra for these small peptides are comparable to expected bandshapes for peptides of high beta-sheet content. The (D)Pro-Gly sequence provides a better turn motif than Asn-Gly as measured by its beta-sheet spectral characteristics. IR and VCD spectra are in qualitative agreement with theoretical simulations based on transfer of parameters from ab initio quantum mechanical force field and intensity computations for the turn and strands. These calculations provide assignments for some distinguishing modes in both IR and VCD spectra. Increased sheet structure can be induced in these hairpins by use of mixed solvent conditions. Thermal denaturation studies reveal that these hairpins undergo very broad unfolding transitions. Guanidine hydrochloride unfolding transitions for the selected hairpin models are similarly broad. However, the "end-states" of temperature and chaotropic denaturation are spectroscopically differentiable.

Activation of muscarinic receptors inhibits beta-amyloid peptide-induced signaling in cortical slices

Deposition of fibrillar aggregates of the beta-amyloid peptide (Abeta) is a key pathologic feature during the early stage of Alzheimer's disease. The initial neuronal responses to Abeta in cortical circuits and the regulation of Abeta-induced signaling remain unclear. In this study, we found that exposure of cortical slices to Abeta(1-42) or Abeta(25-35) induced a marked increase in the activation of protein kinase C (PKC) and Ca(2+)/calmodulin-dependent kinase II (CaMKII), two enzymes critically involved in a variety of cellular functions. Activation of M1 muscarinic receptors, but not nicotinic receptors, significantly inhibited the Abeta activation of PKC and CaMKII. Increasing inhibitory transmission mimicked the M1 effect on Abeta, whereas blocking GABA(A) receptors eliminated the M1 action. Moreover, electrophysiological evidence shows that application of Abeta to cortical slices induced action potential firing and enhanced excitatory postsynaptic currents, whereas muscarinic agonists potently increased inhibitory postsynaptic currents. These results suggest that Abeta activates PKC and CaMKII through enhancing excitatory activity in glutamatergic synaptic networks. Activation of M1 receptors inhibits Abeta signaling by enhancing the counteracting GABA(ergic) inhibitory transmission. Thus the muscarinic reversal of the Abeta-induced biochemical and physiological changes provides a potential mechanism for the treatment of Alzheimer's disease with cholinergic enhancers.

The structural basis for biphasic kinetics in the folding of the WW domain from a formin-binding protein: lessons for protein design?

The mechanism of formation of beta-sheets is of great importance because of the significant role of such structures in the initiation and propagation of amyloid diseases. In this study we examine the folding of a series of three-stranded antiparallel beta-sheets known as WW domains. Whereas other WW domains have been shown to fold with single-exponential kinetics, the WW domain from murine formin-binding protein 28 has recently been shown to fold with biphasic kinetics. By using a combination of kinetics and thermodynamics to characterize a simple model for this protein, the origins of the biphasic kinetics is found to lie in the fact that most of the protein is able to fold without requiring one of the beta-hairpins to be correctly registered. The correct register of this hairpin is enforced by a surface-exposed hydrophobic contact, which is not present in other WW domains. This finding suggests the use of judiciously chosen surface-exposed hydrophobic pairs as a protein design strategy for enforcing the desired strand registry.

Enhanced correction methods for hydrogen exchange-mass spectrometric studies of amyloid fibrils

We describe methods for minimization of and correction for artifactual forward and backward exchange occurring during hydrogen exchange-mass spectrometric (HX-MS) studies of amyloid fibrils of the Abeta(1-40) peptide. The quality of the corrected data obtained using published and new correction algorithms is evaluated quantitatively. Using the new correction methods, we have determined that 20.1 +/- 1.4 of the 39 backbone amide hydrogens in Abeta(1-40) exchange with deuteriums in 100 h when amyloid fibrils of this peptide are suspended in D(2)O. These data reinforce our previous conclusions based on uncorrected data that amyloid fibrils contain a rigid protective core structure that involves only about half of the Abeta backbone amides. The methods developed here should be of general value for HX-MS studies of amyloid fibrils and other protein aggregates.

In vitro fibrillogenesis of the amyloid beta 1-42 peptide: cholesterol potentiation and aspirin inhibition

Understanding the formation of extracellular amyloid neurofibrillar bundles/senile plaques and their role in the development of Alzheimer's disease is of considerable interest to neuroscientists and clinicians. Major components of the extracellular neurofibrillar bundles are polymerized amyloid beta (Abeta) peptides (1-40), (1-42) and (1-43), derived in vivo from the soluble amyloid precursor protein (sAPP) by proteolytic (beta- and gamma-secretase) cleavage. The Abeta(1-42) peptide is widely considered to be of greatest significance in relation to the pathogenesis of Alzheimer's disease. A well-defined ultrastructural characteristic within Alzheimer dense plaques is the presence of helical fibrils that are believed to consist of polymerized amyloid beta, together with other associated proteins such as the serum amyloid P protein, apolipoprotein E isoform epsilon 4, alpha1-anti-chymotrypsin, catalase, glycoproteins, proteoglycans, cholesterol and other lipids. The spontaneous in vitro fibrillogenesis of chemically synthesized Abeta(1-42) peptide (rat sequence), following 20h incubation at 37 degrees C, has been assessed from uranyl acetate negatively stained specimens studied by transmission electron microscopy (TEM). Amyloid beta(1-42) peptide fibrillogenesis in the presence of cholesterol has been investigated using aqueous suspensions of microcrystalline cholesterol and cholesteryl acetate, globular particles of cholesteryl oleate, a soluble (micellar) cholesterol derivative (polyoxyethyl cholesteryl sebacate/cholesteryl PEG 600 sebacate), cholesterol-sphingomyelin liposomes and sphingomyelin liposomes. In all these cases, with the exception of cholesteryl oleate, considerable potentiation of long smooth helical fibril formation occurred, compared to 20h 37 degrees C control samples containing the Abeta(1-42) peptide alone. The binding of polyoxyethyl cholesteryl sebacate micelles to helical Abeta fibrils/filaments and the binding of fibrils to the surface of cholesterol and cholesteryl acetate microcrystals, and to a lesser extent on cholesteryl oleate globules, indicates an affinity of the Abeta peptide for cholesterol. This potentiation of Abeta(1-42) polymerization is likely to be mediated at the molecular level via hydrophobic interaction between the amino acid side chains of the peptide and the tetracyclic sterol nucleus. Addition of cupric sulphate (0.1mM) to the Abeta solution produced large disorganized fibril aggregates. Inclusion of 1mM aspirin (sodium acetylsalicylate) in the Abeta peptide alone and as an addition to Abeta peptide solution containing cholesterol, cholesteryl acetate, soluble cholesterol, sphingomyelin and sphingomyelin-cholesterol liposomes, and to 0.1mM cupric sulphate solution, completely inhibited fibrillogenesis. Instead, only non-crystalline diffuse, non-filamentous microaggregates of insoluble Abeta particles were found, free and attached to the sterol particles. The in vitro system presented here provides a way to rapidly monitor at the structural/TEM level other compounds (e.g. chelating agents, drugs, beta-sheet breaking peptides and anti-oxidants) for their effects on amyloid beta peptide fibrillogenesis (and on preformed fibril disassembly) in parallel with in vitro biochemical studies and in vivo studies using animal models of Alzheimer's disease as well as studies on man.

From Alzheimer to Huntington: why is a structural understanding so difficult?

An increasing family of neurodegenerative disorders such as Alzheimer's, Parkinson's and Huntington's diseases, prion encephalopathies and cystic fibrosis is associated with aggregation of misfolded polypeptide chains which are toxic to the cell. Knowledge of the three-dimensional structure of the proteins implicated is essential for understanding why and how endogenous proteins may adopt a non-native fold. Yet, structural work has been hampered by the difficulty of handling proteins insoluble or prone to aggregation, and at the same time that is why it is interesting to study these molecules. In this review, we compare the structural knowledge accumulated for two paradigmatic misfolding disorders, Alzheimer's disease (AD) and the family of poly-glutamine diseases (poly-Q) and discuss some of the hypotheses suggested for explaining aggregate formation. While a common mechanism between these pathologies remains to be proven, a direct comparison may help in designing new strategies for approaching their study.

Cellular polyamines promote the aggregation of alpha-synuclein

The cellular polyamines putrescine, spermidine, and spermine accelerate the aggregation and fibrillization of alpha-synuclein, the major protein component of Lewy bodies associated with Parkinson's disease. Circular dichroism and fluorometric thioflavin T kinetic studies showed a transition of alpha-synuclein from unaggregated to highly aggregated states, characterized by lag and transition phases. In the presence of polyamines, both the lag and transition times were significantly shorter. All three polyamines accelerated the aggregation and fibrillization of alpha-synuclein to a degree that increased with the total charge, length, and concentration of the polyamine. Electron and scanning force microscopy of the reaction products after the lag phase revealed the presence of aggregated particles (protofibrils) and small fibrils. At the end of the transition phase, alpha-synuclein formed long fibrils in all cases, although some morphological variations were apparent. In the presence of polyamines, fibrils formed large networks leading ultimately to condensed aggregates. In the absence of polyamines, fibrils were mostly isolated. We conclude that the polyamines at physiological concentrations can modulate the propensity of alpha-synuclein to form fibrils and may hence play a role in the formation of cytosolic alpha-synuclein aggregates.

Amyloid beta -protein (Abeta) assembly: Abeta 40 and Abeta 42 oligomerize through distinct pathways

Amyloid beta-protein (Abeta) is linked to neuronal injury and death in Alzheimer's disease (AD). Of particular relevance for elucidating the role of Abeta in AD is new evidence that oligomeric forms of Abeta are potent neurotoxins that play a major role in neurodegeneration and the strong association of the 42-residue form of Abeta, Abeta42, with the disease. Detailed knowledge of the structure and assembly dynamics of Abeta thus is important for the development of properly targeted AD therapeutics. Recently, we have shown that Abeta oligomers can be cross-linked efficiently, and their relative abundances quantified, by using the technique of photo-induced cross-linking of unmodified proteins (PICUP). Here, PICUP, size-exclusion chromatography, dynamic light scattering, circular dichroism spectroscopy, and electron microscopy have been combined to elucidate fundamental features of the early assembly of Abeta40 and Abeta42. Carefully prepared aggregate-free Abeta40 existed as monomers, dimers, trimers, and tetramers, in rapid equilibrium. In contrast, Abeta42 preferentially formed pentamerhexamer units (paranuclei) that assembled further to form beaded superstructures similar to early protofibrils. Addition of Ile-41 to Abeta40 was sufficient to induce formation of paranuclei, but the presence of Ala-42 was required for their further association. These data demonstrate that Abeta42 assembly involves formation of several distinct transient structures that gradually rearrange into protofibrils. The strong etiologic association of Abeta42 with AD may thus be a result of assemblies formed at the earliest stages of peptide oligomerization.

Practical assay and molecular mechanism of aggregation inhibitors of beta-amyloid

Beta-Amyloid peptide (Abeta) is the main protein component of neuritic plaques in the brain of patients of Alzheimer's disease (AD), and its neurotoxicity would be exposed by the formation of aggregates. The aggregation inhibitors composed of an Abeta recognition element (KLVFF) and a hydrophilic moiety are evaluated by a novel fluorescence assay. These compounds inhibit growth of the model aggregates on the KLVFF immobilized surface. In addition, some compounds also possess disrupting activities of preformed aggregates. These compounds could be a key candidate for therapeutic drugs for AD by their novel molecular mechanisms.

Amyloid formation of native folded protein induced by peptide-based graft copolymer

We report here that a native folded holo-myoglobin, when incubated with a synthetic amyloidogenic peptide in aqueous solutions, forms fibrils. These fibrils took a cross-beta form (inter-strand spacing: 4.65 A and inter-sheet spacing: 10.65 A) and bound the amyloidophilic dye Congo red as did the authentic amyloid fibrils. In contrast such fibril formation of myoglobin did not occur in the absence of the peptide. These results suggest the possibility that inter-molecular interaction of native protein with the amyloidogenic peptide trigger the amyloid formation even for the non-pathogenic native protein like myoglobin, which itself exists as a globular form, under certain conditions.

Aqueous gel formation of a synthetic peptide derived from the beta-sheet domain of platelet factor-4

We observed gelation of a 23-residue peptide derived from the beta-sheet domain of platelet factor-4 (PF4(24)(-)(46)). The gels were primarily heterogeneous mixtures of 50-200 microm spherical aggregates in a less-dense gel matrix. Infrared and circular dichroism spectroscopies showed gelation involving the conversion of PF4(24)(-)(46) from random coil to beta-sheet. We used aggregation-induced NMR resonance broadening to show that temperature, pH, and ionic strength influenced PF4(24)(-)(46) gelation rates. Under identical solution conditions, gel formation took days at T </= 20 degrees C but only 30 min at T >/= 50 degrees C. Gelation was most rapid at pH values near the pK(a) of the central His35 residue. Increases in solution ionic strength reduced the critical gelation concentration of PF4(24)(-)(46). Our results suggest that PF4(24)(-)(46) gels by a process combining aspects of both heat-set and beta-fibril gelation mechanisms.

Stabilities and conformations of Alzheimer's beta -amyloid peptide oligomers (Abeta 16-22, Abeta 16-35, and Abeta 10-35): Sequence effects

Previously, we have studied the minimal oligomer size of an aggregate amyloid seed and the mechanism of seed growth with a multilayer beta-sheet model. Under high temperature simulation conditions, our approach can test the stability of possible amyloid forms. Here, we report our study of oligomers of Alzheimer's amyloid beta-peptide (Abeta) fragments 16-22, 16-35, and 10-35 (abbreviated Abeta(16-22), Abeta(16-35), and Abeta(10-35), respectively). Our simulations indicate that an antiparallel beta-sheet orientation is the most stable for the Abeta(16-22), in agreement with a solid state NMR-based model [Balbach, J. J., Ishii, Y., Antzutkin, O. N., Leapman, R. D., Rizzo, N. W., et al. (2000) Biochemistry 39, 13748-13759]. A model with twenty-four Abeta(16-22) strands indicates a highly twisted fibril. Whereas the short Abeta(16-22) and Abeta(24-36) may exist in fully extended form, the linear parallel beta-sheets for Abeta(16-35) appear impossible, mainly because of the polar region in the middle of the 16-35 sequence. However, a bent double-layered hairpin-like structure (called hook) with the polar region at the turn forms parallel beta-sheets with higher stability. An intra-strand salt-bridge (D23-K28) stabilizes the bent hairpin-like hook structure. The bent double-beta-sheet model for the Abeta(10-35) similarly offers oligomer stability.

Stimulation of enveloped virus infection by beta-amyloid fibrils

Alzheimer's disease is characterized by deposition of beta-amyloid peptide (Abeta) into plaques in the brain, leading to neuronal toxicity and dementia. Human immunodeficiency virus type 1 (HIV-1) infection of the central nervous system can also cause a dementia, and amyloid deposition in the central nervous system is significantly higher in HIV-1-infected individuals compared with uninfected controls. Here we report that Abeta fibrils stimulated, by 5-20-fold, infection of target cells expressing CD4 and an appropriate coreceptor by multiple HIV-1 isolates but did not permit infection of cells lacking these receptors. Abeta enhanced infection at the stage of virus attachment or entry into the cell. Abeta fibrils also stimulated infection by amphotrophic Moloney leukemia virus, herpes simplex virus, and viruses pseudotyped with the envelope glycoprotein of vesicular stomatitis virus. Other synthetic fibril-forming peptides similarly enhanced viral infection and may be useful in gene delivery applications utilizing retroviral vectors. These data suggest that Abeta deposition may increase the vulnerability of the central nervous system to enveloped viral infection and that amyloidogenic peptides could be useful in enhancing gene transfer by enveloped viral vectors.

Pharmacotherapy of agitation and psychotic symptoms in Alzheimer's disease

Behavioral disorders, such as agitation and psychosis, are the main reasons for nursing home placement of Alzheimer's disease patients. This review article discusses the efficacy and adverse effects rates of antipsychotics, the most effective and widely-used medications for psychosis in elderly patients and the most commonly prescribed class of psychotropic medicines in long-term care institutions. In addition, the efficacy of selective serotonin reuptake inhibitors is discussed. Recommendations are provided regarding these existing and potential new clinical strategies. A future perspective is provided, taking into consideration prospective developments in preventative treatments of Alzheimer's disease over the next 5 years.

Paradigm shifts in Alzheimer's disease and other neurodegenerative disorders: the emerging role of oligomeric assemblies

Alzheimer's disease (AD) is a progressive, neurodegenerative disorder characterized by amyloid deposition in the cerebral neuropil and vasculature. These amyloid deposits comprise predominantly fragments and full-length (40 or 42 residue) forms of the amyloid beta-protein (Abeta) organized into fibrillar assemblies. Compelling evidence indicates that factors that increase overall Abeta production or the ratio of longer to shorter forms, or which facilitate deposition or inhibit elimination of amyloid deposits, cause AD or are risk factors for the disease. In vitro studies have demonstrated that fibrillar Abeta has potent neurotoxic effects on cultured neurons. In vivo experiments in non-human primates have demonstrated that Abeta fibrils directly cause pathologic changes, including tau hyperphosphorylation. In concert with histologic studies revealing a lack of tissue injury in areas of the neuropil in which non-fibrillar deposits were found, these data suggested that fibril assembly was a prerequisite for Abeta-mediated neurotoxicity in vivo. Recently, however, both in vitro and in vivo studies have revealed that soluble, oligomeric forms of Abeta also have potent neurotoxic activities, and in fact, may be the proximate effectors of the neuronal injury and death occurring in AD. A paradigm shift is thus emerging that necessitates the reevaluation of the relative importance of polymeric (fibrillar) vs. oligomeric assemblies in the pathobiology of AD. In addition to AD, an increasing number of neurodegenerative disorders, including Parkinson's disease, familial British dementia, familial amyloid polyneuropathy, amyotrophic lateral sclerosis, and prion diseases, are associated with abnormal protein assembly processes. The archetypal features of the assembly-dependent neuropathogenetic effects of Abeta may thus be of relevance not only to AD but to these other disorders as well.

Aggregation and neurotoxicity of mutant amyloid beta (A beta) peptides with proline replacement: importance of turn formation at positions 22 and 23

Aggregation of the amyloid beta peptides (A beta 1-42 and A beta 1-40) plays a pivotal role in pathogenesis of Alzheimer's disease. Although it is widely accepted that the aggregates of A betas mainly consist of beta-sheet structure, the precise aggregation mechanism remains unclear. To identify amino acid residues that are important for the beta-sheet formation, a series of proline-substituted mutants of A beta 1-42 peptides at positions 19-26 was synthesized in a highly pure form and their aggregation ability and neurotoxicity on PC12 cells were investigated. All proline-substituted A beta 1-42 mutants except for 22P- and 23P-A beta 1-42 were hard to aggregate and showed weaker cytotoxicity than wild-type A beta 1-42, suggesting that the residues at positions 19-21 and 24-26 are important for the beta-sheet formation. In contrast, 22P-A beta 1-42 extensively aggregated with stronger cytotoxicity than wild-type A beta 1-42. Since proline has a propensity for beta-turn structure as a Pro-X corner, these data implicate that beta-turn formation at positions 22 and 23 plays a crucial role in the aggregation and neurotoxicity of A beta peptides.

Familial conformational diseases and dementias

Familial conformational diseases occur when a mutation alters the conformation of a protein resulting in abnormal intermolecular interactions, protein aggregation, and consequent tissue damage. The molecular mechanisms of conformational disease are best understood for the serine protease inhibitor (serpin) superfamily of proteins. The serpinopathies include alpha(1)-antitrypsin (SERPINA1) deficiency and the newly characterized familial encephalopathy with neuroserpin inclusion bodies (FENIB) resulting from mutations in the neuroserpin (SERPINI1) gene. This review discusses how insights gained from the study of the serpins may be used to guide our research into other common diseases such as Alzheimer disease, Huntington disease, and Parkinson disease.

Amyloid-fibril formation. Proposed mechanisms and relevance to conformational disease

The phenomenon of the transformation of proteins into amyloid-fibrils is of interest, firstly, because it is closely connected to the so-called conformational diseases, many of which are hitherto incurable, and secondly, because it remains to be explained in physical terms (energetically and structurally). The process leads to fibrous aggregates in the form of extracellular amyloid plaques, neuro-fibrillary tangles and other intracytoplasmic or intranuclear inclusions. In this review, basic principles common to the field of amyloid fibril formation and conformational disease are underlined. Existing models for the mechanism need to be tested by experiment. The kinetic and energetic bases of the process are reviewed. The main controversial issue remains the coexistence of more than one protein conformation. The possible role of oligomeric intermediates, and of domain-swapping is also discussed. Mechanisms for cellular defence and novel therapies are considered.

Mutations that reduce aggregation of the Alzheimer's Abeta42 peptide: an unbiased search for the sequence determinants of Abeta amyloidogenesis

The primary component of amyloid plaque in the brains of Alzheimer's patients is the 42 residue amyloid-beta-peptide (Abeta42). Although the amino acid residue sequence of Abeta42 is known, the molecular determinants of Abeta amyloidogenesis have not been elucidated. To facilitate an unbiased search for the sequence determinants of Abeta aggregation, we developed a genetic screen that couples a readily observable phenotype in E. coli to the ability of a mutation in Abeta42 to reduce aggregation. The screen is based on our finding that fusions of the wild-type Abeta42 sequence to green fluorescent protein (GFP) form insoluble aggregates in which GFP is inactive. Cells expressing such fusions do not fluoresce. To isolate variants of Abeta42 with reduced tendencies to aggregate, we constructed and screened libraries of Abeta42-GFP fusions in which the sequence of Abeta42 was mutated randomly. Cells expressing GFP fusions to soluble (non-aggregating) variants of Abeta42 exhibit green fluorescence. Implementation of this screen enabled the isolation of 36 variants of Abeta42 with reduced tendencies to aggregate. The sequences of most of these variants are consistent with previous models implicating hydrophobic regions as determinants of Abeta42 aggregation. Some of the variants, however, contain amino acid substitutions not implicated in pre-existing models of Abeta amyloidogenesis.

Synthesis, aggregation, neurotoxicity, and secondary structure of various A beta 1-42 mutants of familial Alzheimer's disease at positions 21-23

Cerebral amyloid angiopathy (CAA) due to amyloid beta (A beta) deposition is a key pathological feature of Alzheimer's disease (AD), especially in some form of familial Alzheimer's disease (FAD) including hereditary cerebral hemorrhage with amyloidosis-Dutch type. A beta mainly consists of 40- and 42-mer peptides (Abeta 1-40 and A beta 1-42), which accumulate in senile plaques of AD brains and show neurotoxicity for cultured nerve cells. We synthesized all variant forms of A beta 1-42 associated with reported FAD, such as A21G (Flemish), E22Q (Dutch), E22K (Italian), E22G (Arctic), and D23N (Iowa) along with three potential mutants by one point missense mutation (E22A, E22D, and E22V) in a highly pure form, and examined their ability to aggregate and their neurotoxicity in PC12 cells. The mutants at positions 22 and 23 showed potent aggregative ability and neurotoxicity whereas the potential mutants did not, indicating that A beta 1-42 mutants at positions 22 and 23 play a critical role in FAD of Dutch-, Italian-, Arctic-, and Iowa-types. However, Flemish-type FAD needs alternative explanation except the aggregation and neurotoxicity of the corresponding A beta 1-42 mutant.

Mapping of possible binding sequences of two beta-sheet breaker peptides on beta amyloid peptide of Alzheimer's disease

Aggregation of amyloid peptide (Abeta) has been identified as a major feature of the pathogenesis of Alzheimer's disease. Increased risk for disease is associated with increased formation of polymerized Abeta. Inhibition of formation of toxic (aggregated) form of Abeta is one of the therapeutic possibilities. Beta sheet breaker peptides (BSBs) fulfill the requirements of an effective inhibitor. After having attached to the Abeta molecules, BSBs can prevent aggregation of Abeta to polymeric forms (aggregates). In the present study, we performed molecular modelling of complex formation between Abeta and two BSB peptides. Our aim was to find proper binding sequences for the BSB peptides on Abeta and characterize them. A dimeric model of Abeta was also used to study the interaction of BSBs with the aggregated forms of Abeta and find the sequences responsible for the polymerization process. A fast and efficient computational method: molecular docking was used for the afore-mentioned purposes.

Amyloid fibril formation by human stefin B in vitro: immunogold labelling and comparison to stefin A

The mechanism by which proteins form amyloid fibrils is of high interest to the scientific community as its understanding could resolve questions relevant to conformational diseases. The structural and energetic basis of the process is still largely unknown. The main controversial issue is the co-existence of several protein conformations. Three models for the mechanism of protein fibrillogenesis have been proposed which need to be tested by experiments. In this report, amyloid fibrils grown from human stefin B (type I cystatin) are described. This physiologically relevant protein readily forms fibrils in vitro, in contrast to the homologue--human stefin A--which forms fibrils under extreme conditions only. In order to specifically label stefin B fibrils in vitro, rabbit polyclonal antibody and mouse monoclonal antibody A6/2 against human stefin B were used for immunogold labelling. Samples were examined by transmission electron microscopy. Fibrils of stefin B were strongly labelled using polyclonal antibody and Protein A gold, whereas no positive reaction was observed with monoclonal antibody A6/2.

Pentapeptide amides interfere with the aggregation of beta-amyloid peptide of Alzheimer's disease

Amyloid peptides (Abeta) play a central role in the pathogenesis of Alzheimer's disease (AD). The aggregation of Abeta molecules leads to fibril and plaque formation. Fibrillogenesis is at the same time a marker and an indirect cause of AD. Inhibition of the aggregation of Abeta could be a realistic therapy for the illness. Beta sheet breakers (BSBs) are one type of fibrillogenesis inhibitors. The first BSB peptides were designed by Tjernberg et al. (1996) and Soto et al. (1998). These pentapeptides have proved their efficiency in vitro and in vivo. In the present study, the effects of two pentapeptide amides are reported. These compounds were designed by using the C-terminal sequence of the amyloid peptide as a template. Biological assays were applied to demonstrate efficiency. Modes of action were studied by FT-IR spectroscopy and molecular modeling methods.