Intrinsic linear heterogeneity of amyloid β protein fibrils revealed by higher resolution mass-per-length determinations

Structure of alpha-synuclein fibrils derived from human Lewy body dementia tissue

The defining feature of Parkinson disease (PD) and Lewy body dementia (LBD) is the accumulation of alpha-synuclein (Asyn) fibrils in Lewy bodies and Lewy neurites. Here we develop and validate a method to amplify Asyn fibrils extracted from LBD postmortem tissue samples and use solid state nuclear magnetic resonance (SSNMR) studies to determine atomic resolution structure. Amplified LBD Asyn fibrils comprise a mixture of single protofilament and two protofilament fibrils with very low twist. The protofilament fold is highly similar to the fold determined by a recent cryo-electron microscopy study for a minority population of twisted single protofilament fibrils extracted from LBD tissue. These results expand the structural characterization of LBD Asyn fibrils and approaches for studying disease mechanisms, imaging agents and therapeutics targeting Asyn.

Structure of alpha-synuclein fibrils derived from human Lewy body dementia tissue

The defining feature of Parkinson disease (PD) and Lewy body dementia (LBD) is the accumulation of alpha-synuclein (Asyn) fibrils in Lewy bodies and Lewy neurites. We developed and validated a novel method to amplify Asyn fibrils extracted from LBD postmortem tissue samples and used solid state nuclear magnetic resonance (SSNMR) studies to determine atomic resolution structure. Amplified LBD Asyn fibrils comprise two protofilaments with pseudo-21 helical screw symmetry, very low twist and an interface formed by antiparallel beta strands of residues 85-93. The fold is highly similar to the fold determined by a recent cryo-electron microscopy study for a minority population of twisted single protofilament fibrils extracted from LBD tissue. These results expand the structural landscape of LBD Asyn fibrils and inform further studies of disease mechanisms, imaging agents and therapeutics targeting Asyn.

Post-Translational Chemical Modification of Amyloid-β Peptides by 4-Hydroxy-2-Nonenal

BACKGROUND

The extraction and quantification of amyloid-β (Aβ) peptides in brain tissue commonly uses formic acid (FA) to disaggregate Aβ fibrils. However, it is not clear whether FA can disaggregate post-translationally modified Aβ peptides, or whether it induces artifact by covalent modification during disaggregation. Of particular interest are Aβ peptides that have been covalently modified by 4-hydroxy-2-nonenal (HNE), an oxidative lipid degradation product produced in the vicinity of amyloid plaques that dramatically accelerates the aggregation of Aβ peptides.

OBJECTIVE

Test the ability of FA to disaggregate Aβ peptides modified by HNE and to induce covalent artifacts.

METHODS

Quantitative liquid-chromatography-tandem-mass spectrometry of monomeric Aβ peptides and identify covalently modified forms.

RESULTS

FA disaggregated ordinary Aβ fibrils but also induced the time-dependent formylation of at least 2 residue side chains in Aβ peptides, as well as oxidation of its methionine side chain. FA was unable to disaggregate Aβ peptides that had been covalently modified by HNE.

CONCLUSION

The inability of FA to disaggregate Aβ peptides modified by HNE prevents FA-based approaches from quantifying a pool of HNE-modified Aβ peptides in brain tissue that may have pathological significance.

Multi-eGO: An in silico lens to look into protein aggregation kinetics at atomic resolution

Protein aggregation into amyloid fibrils is the archetype of aberrant biomolecular self-assembly processes, with more than 50 associated diseases that are mostly uncurable. Understanding aggregation mechanisms is thus of fundamental importance and goes in parallel with the structural characterization of the transient oligomers formed during the process. Oligomers have been proven elusive to high-resolution structural techniques, while the large sizes and long time scales, typical of aggregation processes, have limited the use of computational methods to date. To surmount these limitations, we here present multi-eGO, an atomistic, hybrid structure-based model which, leveraging the knowledge of monomers conformational dynamics and of fibril structures, efficiently captures the essential structural and kinetics aspects of protein aggregation. Multi-eGO molecular dynamics simulations can describe the aggregation kinetics of thousands of monomers. The concentration dependence of the simulated kinetics, as well as the structural features of the resulting fibrils, are in qualitative agreement with in vitro experiments carried out on an amyloidogenic peptide from Transthyretin, a protein responsible for one of the most common cardiac amyloidoses. Multi-eGO simulations allow the formation of primary nuclei in a sea of transient lower-order oligomers to be observed over time and at atomic resolution, following their growth and the subsequent secondary nucleation events, until the maturation of multiple fibrils is achieved. Multi-eGO, combined with the many experimental techniques deployed to study protein aggregation, can provide the structural basis needed to advance the design of molecules targeting amyloidogenic diseases.

Copper-Mediated β-Amyloid Toxicity and its Chelation Therapy in Alzheimer's Disease

The link between bio-metals, Alzheimer's disease (AD), and its associated protein, amyloid-β (Aβ) is very complex and one of the most studied aspects currently. Alzheimer's disease, a progressive neurodegenerative disease, is proposed to occurs due to the misfolding and aggregation of Aβ. Dyshomeostasis of metal ions and their interaction with Aβ has largely been implicated in AD. Copper plays a crucial role in amyloid-β toxicity and AD development potentially occurs through direct interaction with the copper-binding motif of APP and different amino acid residues of Aβ. Previous reports suggest that high levels of copper accumulation in the AD brain result in modulation of toxic Aβ peptide levels, implicating the role of copper in the pathophysiology of AD. In this review, we explore the possible mode of copper ion interaction with Aβ which accelerates the kinetics of fibril formation and promote amyloid-β mediated cell toxicity in Alzheimer's disease and the potential use of various copper chelators in the prevention of copper-mediated Aβ toxicity.

Morphological analysis of Apolipoprotein E binding to Aβ Amyloid using a combination of Surface Plasmon Resonance, Immunogold Labeling and Scanning Electron Microscopy

Background

Immunogold labeling in combination with transmission electron microscopy analysis is a technique frequently used to correlate high-resolution morphology studies with detailed information regarding localization of specific antigens. Although powerful, the methodology has limitations and it is frequently difficult to acquire a stringent system where unspecific low-affinity interactions are removed prior to analysis.

Results

We here describe a combinatorial strategy where surface plasmon resonance and immunogold labeling are used followed by a direct analysis of the sensor-chip surface by scanning electron microscopy. Using this approach, we have probed the interaction between amyloid-β fibrils, associated to Alzheimer’s disease, and apolipoprotein E, a well-known ligand frequently found co-deposited to the fibrillar form of Aβ in vivo. The results display a lateral binding of ApoE along the amyloid fibrils and illustrates how the gold-beads represent a good reporter of the binding.

Conclusions

This approach exposes a technique with generic features which enables both a quantitative and a morphological evaluation of a ligand-receptor based system. The methodology mediates an advantage compared to traditional immunogold labeling since all washing steps can be monitored and where a high stringency can be maintained throughout the experiment.

Identifying Polymorphs of Amyloid-β (1-40) Fibrils Using High-Resolution Atomic Force Microscopy

Many amyloid-β fibril preparations are highly polymorphic, and the conditions under which they are formed determine their morphology. This report describes the application of high-resolution atomic force microscopy (HR-AFM), combined with volume-per-length analysis, to define, identify, and quantify the structural components of polymorphic Aβ fibril preparations. Volume-per-length analysis confirms that they are composed of discrete cross-β filaments, and the analysis of HR-AFM images yields the number of striations in each fibril. Compared to mass-per-length analysis by electron microscopy, HR-AFM analysis yields narrower distributions, facilitating rapid and label-free quantitative morphological characterization of Aβ fibril preparations.

Amyloid fibrils embodying distinctive yeast prion phenotypes exhibit diverse morphologies

Yeast prions are self-templating protein-based mechanisms of inheritance whose conformational changes lead to the acquisition of diverse new phenotypes. The best studied of these is the prion domain (NM) of Sup35, which forms an amyloid that can adopt several distinct conformations (strains) that confer distinct phenotypes when introduced into cells that do not carry the prion. Here, we investigate the structure of NM fibrils templated into the prion conformation with cellular lysates. Our electron microscopy studies reveal that NM fibrils that confer either a strong or a weak prion phenotype are both mixtures of thin and thick fibrils that result from differences in packing of the M domain. Strong NM fibrils have more thin fibrils and weak NM fibrils have more thick fibrils. Interestingly, both mass per length and solid state NMR reveal that the thin and thick fibrils have different underlying molecular structures in the prion strain variants that do not interconvert.

Quantitative Characterization of Metastability and Heterogeneity of Amyloid Aggregates

Amyloids are heterogeneous assemblies of extremely stable fibrillar aggregates of proteins. Although biological activities of the amyloids are dependent on its conformation, quantitative evaluation of heterogeneity of amyloids has been difficult. Here we use disaggregation of the amyloids of tetramethylrhodamine-labeled Aβ (TMR-Aβ) to characterize its stability and heterogeneity. Disaggregation of TMR-Aβ amyloids, monitored by fluorescence recovery of TMR, was negligible in native buffer even at low nanomolar concentrations but the kinetics increased exponentially with addition of denaturants such as urea or GdnCl. However, dissolution of TMR-Aβ amyloids is different from what is expected in the case of thermodynamic solubility. For example, the fraction of soluble amyloids is found to be independent of total concentration of the peptide at all concentrations of the denaturants. Additionally, soluble fraction is dependent on growth conditions such as temperature, pH, and aging of the amyloids. Furthermore, amyloids undissolved in a certain concentration of the denaturant do not show any further dissolution after dilution in the same solvent; instead, these require higher concentrations of the denaturant. Taken together, our results indicate that amyloids are a heterogeneous ensemble of metastable states. Furthermore, dissolution of each structurally homogeneous member requires a unique threshold concentration of denaturant. Fraction of soluble amyloids as a function of concentration of denaturants is found to be sigmoidal. The sigmoidal curve becomes progressively steeper with progressive seeding of the amyloids, although the midpoint remains unchanged. Therefore, heterogeneity of the amyloids is a major determinant of the steepness of the sigmoidal curve. The sigmoidal curve can be fit assuming a normal distribution for the population of the amyloids of various kinetic stabilities. We propose that the mean and the standard deviation of the normal distribution provide quantitative estimates of mean kinetic stability and heterogeneity, respectively, of the amyloids in a certain preparation.

Identification of the primary peptide contaminant that inhibits fibrillation and toxicity in synthetic amyloid-β42

Understanding the pathophysiology of Alzheimer disease has relied upon the use of amyloid peptides from a variety of sources, but most predominantly synthetic peptides produced using t-butyloxycarbonyl (Boc) or 9-fluorenylmethoxycarbonyl (Fmoc) chemistry. These synthetic methods can lead to minor impurities which can have profound effects on the biological activity of amyloid peptides. Here we used a combination of cytotoxicity assays, fibrillation assays and high resolution mass spectrometry (MS) to identify impurities in synthetic amyloid preparations that inhibit both cytotoxicity and aggregation. We identify the Aβ42Δ39 species as the major peptide contaminant responsible for limiting both cytotoxicity and fibrillation of the amyloid peptide. In addition, we demonstrate that the presence of this minor impurity inhibits the formation of a stable Aβ42 dimer observable by MS in very pure peptide samples. These results highlight the critical importance of purity and provenance of amyloid peptides in Alzheimer’s research in particular, and biological research in general.

Quenched hydrogen-deuterium exchange NMR of a disease-relevant Aβ(1-42) amyloid polymorph

Alzheimer’s disease is associated with the aggregation into amyloid fibrils of Aβ(1–42) and Aβ(1–40) peptides. Interestingly, these fibrils often do not obtain one single structure but rather show different morphologies, so-called polymorphs. Here, we compare quenched hydrogen-deuterium (H/D) exchange of a disease-relevant Aβ(1–42) fibril for which the 3D structure has been determined by solid-state NMR with H/D exchange previously determined on another structural polymorph. This comparison reveals secondary structural differences between the two polymorphs suggesting that the two polymorphisms can be classified as segmental polymorphs.

Determination of Size of Folding Nuclei of Fibrils Formed from Recombinant Aβ(1-40) Peptide

We have developed a highly efficient method for purification of the recombinant product Aβ(1-40) peptide. The concentration dependence of amyloid formation by recombinant Aβ(1-40) peptide was studied using fluorescence spectroscopy and electron microscopy. We found that the process of amyloid formation is preceded by lag time, which indicates that the process is nucleation-dependent. Further exponential growth of amyloid fibrils is followed by branching scenarios. Based on the experimental data on the concentration dependence, the sizes of the folding nuclei of fibrils were calculated. It turned out that the size of the primary nucleus is one "monomer" and the size of the secondary nucleus is zero. This means that the nucleus for new aggregates can be a surface of the fibrils themselves. Using electron microscopy, we have demonstrated that fibrils of these peptides are formed by the association of rounded ring structures.

Amyloid structure exhibits polymorphism on multiple length scales in human brain tissue

Aggregation of Aβ amyloid fibrils into plaques in the brain is a universal hallmark of Alzheimer's Disease (AD), but whether plaques in different individuals are equivalent is unknown. One possibility is that amyloid fibrils exhibit different structures and different structures may contribute differentially to disease, either within an individual brain or between individuals. However, the occurrence and distribution of structural polymorphisms of amyloid in human brain is poorly documented. Here we use X-ray microdiffraction of histological sections of human tissue to map the abundance, orientation and structural heterogeneities of amyloid. Our observations indicate that (i) tissue derived from subjects with different clinical histories may contain different ensembles of fibrillar structures; (ii) plaques harboring distinct amyloid structures can coexist within a single tissue section and (iii) within individual plaques there is a gradient of fibrillar structure from core to margins. These observations have immediate implications for existing theories on the inception and progression of AD.

Distinct Membrane Disruption Pathways Are Induced by 40-Residue β-Amyloid Peptides

Cellular membrane disruption induced by β-amyloid (Aβ) peptides has been considered one of the major pathological mechanisms for Alzheimer disease. Mechanistic studies of the membrane disruption process at a high-resolution level, on the other hand, are hindered by the co-existence of multiple possible pathways, even in simplified model systems such as the phospholipid liposome. Therefore, separation of these pathways is crucial to achieve an in-depth understanding of the Aβ-induced membrane disruption process. This study, which utilized a combination of multiple biophysical techniques, shows that the peptide-to-lipid (P:L) molar ratio is an important factor that regulates the selection of dominant membrane disruption pathways in the presence of 40-residue Aβ peptides in liposomes. Three distinct pathways (fibrillation with membrane content leakage, vesicle fusion, and lipid uptake through a temporarily stable ionic channel) become dominant in model liposome systems under specific conditions. These individual systems are characterized by both the initial states of Aβ peptides and the P:L molar ratio. Our results demonstrated the possibility to generate simplified Aβ-membrane model systems with a homogeneous membrane disruption pathway, which will benefit high-resolution mechanistic studies in the future. Fundamentally, the possibility of pathway selection controlled by P:L suggests that the driving forces for Aβ aggregation and Aβ-membrane interactions may be similar at the molecular level.

Quantitative analysis of amyloid polymorphism using height histograms to correct for tip convolution effects in atomic force microscopy imaging

Development of a statistical height analysis method to study amyloid polymorphism.

A synchrotron-based hydroxyl radical footprinting analysis of amyloid fibrils and prefibrillar intermediates with residue-specific resolution

Structural models of the fibrils formed by the 40-residue amyloid-β (Aβ40) peptide in Alzheimer’s disease typically consist of linear polypeptide segments, oriented approximately perpendicular to the long axis of the fibril, and joined together as parallel in-register β-sheets to form filaments. However, various models differ in the number of filaments that run the length of a fibril, and in the topological arrangement of these filaments. In addition to questions about the structure of Aβ40 monomers in fibrils, there are important unanswered questions about their structure in prefibrillar intermediates, which are of interest because they may represent the most neurotoxic form of Aβ40. To assess different models of fibril structure and to gain insight into the structure of prefibrillar intermediates, the relative solvent accessibility of amino acid residue side chains in fibrillar and prefibrillar Aβ40 preparations was characterized in solution by hydroxyl radical footprinting and structural mass spectrometry. A key to the application of this technology was the development of hydroxyl radical reactivity measures for individual side chains of Aβ40. Combined with mass-per-length measurements performed by dark-field electron microscopy, the results of this study are consistent with the core filament structure represented by two- and three-filament solid state nuclear magnetic resonance-based models of the Aβ40 fibril (such as 2LMN, 2LMO, 2LMP, and 2LMQ), with minor refinements, but they are inconsistent with the more recently proposed 2M4J model. The results also demonstrate that individual Aβ40 fibrils exhibit structural heterogeneity or polymorphism, where regions of two-filament structure alternate with regions of three-filament structure. The footprinting approach utilized in this study will be valuable for characterizing various fibrillar and nonfibrillar forms of the Aβ peptide.

Intrinsic structural heterogeneity and long-term maturation of amyloid β peptide fibrils

Amyloid β peptides form fibrils that are commonly assumed to have a dry, homogeneous, and static internal structure. To examine these assumptions, fibrils under various conditions and different ages have been examined with multidimensional infrared spectroscopy. Each peptide in the fibril had a ¹³C═¹⁸O label in the backbone of one residue to disinguish the amide I' absorption due to that residue from the amide I' absorption of other residues. Fibrils examined soon after they formed, and reexamined after 1 year in the dry state, exhibited spectral changes confirming that structurally significant water molecules were present in the freshly formed fibrils. Results from fibrils incubated in solution for 4 years indicate that water molecules remained trapped within fibrils and mobile over the 4 year time span. These water molecules are structurally significant because they perturb the parallel β-sheet hydrogen bonding pattern at frequent intervals and at multiple points within individual fibrils, creating structural heterogeneity along the length of a fibril. These results show that the interface between β-sheets in an amyloid fibril is not a "dry zipper", and that the internal structure of a fibril evolves while it remains in a fibrillar state. These features, water trapping, structural heterogeneity, and structural evolution within a fibril over time, must be accommodated in models of amyloid fibril structure and formation.

Polymorph-specific kinetics and thermodynamics of β-amyloid fibril growth

Amyloid fibrils formed by the 40-residue β-amyloid peptide (Aβ(1-40)) are highly polymorphic, with molecular structures that depend on the details of growth conditions. Underlying differences in physical properties are not well understood. Here, we investigate differences in growth kinetics and thermodynamic stabilities of two Aβ(1-40) fibril polymorphs for which detailed structural models are available from solid-state nuclear magnetic resonance (NMR) studies. Rates of seeded fibril elongation in the presence of excess soluble Aβ(1-40) and shrinkage in the absence of soluble Aβ(1-40) are determined with atomic force microscopy (AFM). From these rates, we derive polymorph-specific values for the soluble Aβ(1-40) concentration at quasi-equilibrium, from which relative stabilities can be derived. The AFM results are supported by direct measurements by ultraviolet absorbance, using a novel dialysis system to establish quasi-equilibrium. At 24 °C, the two polymorphs have significantly different elongation and shrinkage kinetics but similar thermodynamic stabilities. At 37 °C, differences in kinetics are reduced, and thermodynamic stabilities are increased significantly. Fibril length distributions in AFM images provide support for an intermittent growth model, in which fibrils switch randomly between an "on" state (capable of elongation) and an "off" state (incapable of elongation). We also monitor interconversion between polymorphs at 24 °C by solid-state NMR, showing that the two-fold symmetric "agitated" (A) polymorph is more stable than the three-fold symmetric "quiescent" (Q) polymorph. Finally, we show that the two polymorphs have significantly different rates of fragmentation in the presence of shear forces, a difference that helps explain the observed predominance of the A structure when fibrils are grown in agitated solutions.

Atomic structure and hierarchical assembly of a cross-β amyloid fibril

The cross-β amyloid form of peptides and proteins represents an archetypal and widely accessible structure consisting of ordered arrays of β-sheet filaments. These complex aggregates have remarkable chemical and physical properties, and the conversion of normally soluble functional forms of proteins into amyloid structures is linked to many debilitating human diseases, including several common forms of age-related dementia. Despite their importance, however, cross-β amyloid fibrils have proved to be recalcitrant to detailed structural analysis. By combining structural constraints from a series of experimental techniques spanning five orders of magnitude in length scale--including magic angle spinning nuclear magnetic resonance spectroscopy, X-ray fiber diffraction, cryoelectron microscopy, scanning transmission electron microscopy, and atomic force microscopy--we report the atomic-resolution (0.5 Å) structures of three amyloid polymorphs formed by an 11-residue peptide. These structures reveal the details of the packing interactions by which the constituent β-strands are assembled hierarchically into protofilaments, filaments, and mature fibrils.

Novel mechanistic insight into the molecular basis of amyloid polymorphism and secondary nucleation during amyloid formation

The formation of amyloid β (Aβ) fibrils is crucial in initiating the cascade of pathological events that culminates in Alzheimer's disease. In this study, we investigated the mechanism of Aβ fibril formation from hydrodynamically well defined species under controlled aggregation conditions. We present a detailed mechanistic model that furnishes a novel insight into the process of Aβ42 fibril formation and the molecular basis for the different structural transitions in the amyloid pathway. Our data reveal the structure and polymorphism of Aβ fibrils to be critically influenced by the oligomeric state of the starting materials, the ratio of monomeric-to-aggregated forms of Aβ42 (oligomers and protofibrils), and the occurrence of secondary nucleation. We demonstrate that monomeric Aβ42 plays an important role in mediating structural transitions in the amyloid pathway, and for the first time, we provide evidences that Aβ42 fibrillization occurs via a combined mechanism of nucleated polymerization and secondary nucleation. These findings will have significant implications to our understanding of the molecular basis of amyloid formation in vivo, of the heterogeneity of Aβ pathology (e.g., diffuse versus amyloid plaques), and of the structural basis of Aβ toxicity.

Size distribution of amyloid nanofibrils

We consider the size distribution of amyloid nanofibrils (protofilaments) in nucleating protein solutions when the nucleation process occurs by the mechanism of direct polymerization of β-strands (extended peptides or protein segments) into β-sheets. Employing the atomistic nucleation theory, we derive a general expression for the stationary size distribution of amyloid nanofibrils constituted of successively layered β-sheets. The application of this expression to amyloid β(1-40) (Aβ(40)) fibrils allows us to determine the nanofibril size distribution as a function of the protein concentration and temperature. The distribution is most remarkable with its exhibiting a series of peaks positioned at "magic" nanofibril sizes (or lengths), which are due to deep local minima in the work for fibril formation. This finding of magic sizes or lengths is consistent with experimental results for the size distribution of aggregates in solutions of Aβ(40) proteins. Also, our approach makes it possible to gain insight into the effect of point mutations on the nanofibril size distribution, an effect that may play a role in experimentally observed substantial differences in the fibrillation lag-time of wild-type and point-mutated amyloid-β proteins.

Xanthene food dye, as a modulator of Alzheimer's disease amyloid-beta peptide aggregation and the associated impaired neuronal cell function

Background

Alzheimer's disease (AD) is the most common form of dementia. AD is a degenerative brain disorder that causes problems with memory, thinking and behavior. It has been suggested that aggregation of amyloid-beta peptide (Aβ) is closely linked to the development of AD pathology. In the search for safe, effective modulators, we evaluated the modulating capabilities of erythrosine B (ER), a Food and Drug Administration (FDA)-approved red food dye, on Aβ aggregation and Aβ-associated impaired neuronal cell function.

Methodology/Principal Findings

In order to evaluate the modulating ability of ER on Aβ aggregation, we employed transmission electron microscopy (TEM), thioflavin T (ThT) fluorescence assay, and immunoassays using Aβ-specific antibodies. TEM images and ThT fluorescence of Aβ samples indicate that protofibrils are predominantly generated and persist for at least 3 days. The average length of the ER-induced protofibrils is inversely proportional to the concentration of ER above the stoichiometric concentration of Aβ monomers. Immunoassay results using Aβ-specific antibodies suggest that ER binds to the N-terminus of Aβ and inhibits amyloid fibril formation. In order to evaluate Aβ-associated toxicity we determined the reducing activity of SH-SY5Y neuroblastoma cells treated with Aβ aggregates formed in the absence or in the presence of ER. As the concentration of ER increased above the stoichiometric concentration of Aβ, cellular reducing activity increased and Aβ-associated reducing activity loss was negligible at 500 µM ER.

Conclusions/Significance

Our findings show that ER is a novel modulator of Aβ aggregation and reduces Aβ-associated impaired cell function. Our findings also suggest that xanthene dye can be a new type of small molecule modulator of Aβ aggregation. With demonstrated safety profiles and blood-brain permeability, ER represents a particularly attractive aggregation modulator for amyloidogenic proteins associated with neurodegenerative diseases.

Molecular basis for amyloid-beta polymorphism

Amyloid-beta (Aβ) aggregates are the main constituent of senile plaques, the histological hallmark of Alzheimer's disease. Aβ molecules form β-sheet containing structures that assemble into a variety of polymorphic oligomers, protofibers, and fibers that exhibit a range of lifetimes and cellular toxicities. This polymorphic nature of Aβ has frustrated its biophysical characterization, its structural determination, and our understanding of its pathological mechanism. To elucidate Aβ polymorphism in atomic detail, we determined eight new microcrystal structures of fiber-forming segments of Aβ. These structures, all of short, self-complementing pairs of β-sheets termed steric zippers, reveal a variety of modes of self-association of Aβ. Combining these atomic structures with previous NMR studies allows us to propose several fiber models, offering molecular models for some of the repertoire of polydisperse structures accessible to Aβ. These structures and molecular models contribute fundamental information for understanding Aβ polymorphic nature and pathogenesis.