Amyloid structure and assembly: insights from scanning transmission electron microscopy

Structural Mapping of Protein Aggregates in Live Cells Modeling Huntington's Disease

While protein aggregation is a hallmark of many neurodegenerative diseases, acquiring structural information on protein aggregates inside live cells remains challenging. Traditional microscopy does not provide structural information on protein systems. Routinely used fluorescent protein tags, such as Green Fluorescent Protein (GFP), might perturb native structures. Here, we report a counter-propagating mid-infrared photothermal imaging approach enabling mapping of secondary structure of protein aggregates in live cells modeling Huntington's disease. By comparing mid-infrared photothermal spectra of label-free and GFP-tagged huntingtin inclusions, we demonstrate that GFP fusions indeed perturb the secondary structure of aggregates. By implementing spectra with small spatial step for dissecting spectral features within sub-micrometer distances, we reveal that huntingtin inclusions partition into a β-sheet-rich core and a ɑ-helix-rich shell. We further demonstrate that this structural partition exists only in cells with the [RNQ+] prion state, while [rnq-] cells only carry smaller β-rich non-toxic aggregates. Collectively, our methodology has the potential to unveil detailed structural information on protein assemblies in live cells, enabling high-throughput structural screenings of macromolecular assemblies.

Recent Advancements in Biosensors for the Detection and Characterization of Amyloids: A Review

Modern medicine has increased the human lifespan. However, with an increase in average lifespan risk of amyloidosis increases. Amyloidosis is a condition characterized by protein misfolding and aggregation. Early detection of amyloidosis is crucial, yet conventional diagnostic methods are costly and lack precision, necessitating innovative tools. This review explores recent advancements in diverse amyloid detection methodologies, highlighting the need for interdisciplinary research to develop a miniaturized electrochemical biosensor leveraging nanotechnology. However, the diagnostics industry faces obstacles such as skilled labor shortages, standardized selection processes, and concurrent multi-analyte identification challenges. Research efforts are focused on integrating electrochemical techniques into clinical applications and diagnostics, with the successful transition of miniaturized technologies from development to testing posing a significant hurdle. Label-free transduction techniques like voltammetry and electrochemical impedance spectroscopy (EIS) have gained traction due to their rapid, cost-effective, and user-friendly nature.

Nasopharyngeal amyloidoma: report of three cases and review of the literature

BACKGROUND

Nasopharyngeal amyloidoma is a rare, locally aggressive tumor that has been reported in the English literature in only 38 cases to date, most of which were in the form of case reports. The present study was aimed to summarize the characteristics of this rare tumor, with the goal of providing new insights for diagnosis and treatment.

MATERIALS AND METHODS

We report three cases of nasopharyngeal amyloidoma diagnosed in our hospital following comprehensive medical examination and review the current literature on all cases of nasopharyngeal amyloidoma from PubMed. The journey of nasopharyngeal amyloidoma, including presentation, diagnostics, surgeries, and follow-up was outlined.

RESULTS

None of the three patients had systemic amyloidosis. CT and nasal endoscopy showed irregular masses obstructing the nasopharyngeal cavity. Congo red staining confirmed the deposition of amyloid, and immunohistochemical analysis showed that the amyloid deposition was the AL light chain type. Through literature review, we found that nasopharyngeal amyloidoma most commonly occurred in individuals over the age of 40, patients usually had a good prognosis after complete tumor resection; however, there were still cases of recurrence, and unresected patients were at risk of progression to systemic amyloidosis. The efficacy of radiotherapy and chemotherapy was currently uncertain.

CONCLUSION

Early clinical and pathological diagnosis is crucial, and surgical intervention is the primary treatment option for this disease. Although patients usually have a favorable prognosis, long-term monitoring is necessary to detect potential relapses and initiate timely intervention.

Amyloids, amorphous aggregates and assemblies of peptides - Assessing aggregation

Amyloid and amorphous aggregates represent the two major categories of aggregates associated with diseases, and although exhibiting distinct features, researchers often treat them as equivalent, which demonstrates the need for more thorough characterization. Here, we compare amyloid and amorphous aggregates based on their biochemical properties, kinetics, and morphological features. To further decipher this issue, we propose the use of peptide self-assemblies as minimalistic models for understanding the aggregation process. Peptide building blocks are significantly smaller than proteins that participate in aggregation, however, they make a plausible means to bridge the gap in discerning the aggregation process at the more complex, protein level. Additionally, we explore the potential use of peptide-inspired models to research the liquid-liquid phase separation as a feasible mechanism preceding amyloid formation. Connecting these concepts can help clarify our understanding of aggregation-related disorders and potentially provide novel drug targets to impede and reverse these serious illnesses.

Experimental techniques for detecting and evaluating the amyloid fibrils

Amyloid fibrils are insoluble proteins with intricate β-sheet structures associated with various human diseases, including Parkinson's, Alzheimer's, and prion diseases. Proteins can form aggregates when their structure is misfolded, resulting in highly organized amyloid fibrils or amorphous aggregates. The formation of protein aggregates is a promising research field for mitigating diseases and the pharmaceutical and food industries. It is important to monitor and minimize the appearance of aggregates in these protein products. Several methods exist to assess protein aggregation, that includes from basic investigations to advanced biophysical techniques. Physicochemical parameters such as molecular weight, conformation, structure, and dimension are examined to study aggregation. There is an urgent need to develop methods for the detection of protein aggregation and amyloid fibril formation both in vitro and in vivo. This chapter focuses on a comprehensive discussion of the methods used to characterize and evaluate aggregates and amyloid fibrils.

Alzheimer's disease: The role of proteins in formation, mechanisms, and new therapeutic approaches

Alzheimer's disease (AD) is a progressive neurological disorder that affects the central nervous system (CNS), leading to memory and cognitive decline. In AD, the brain experiences three main structural changes: a significant decrease in the quantity of neurons, the development of neurofibrillary tangles (NFT) composed of hyperphosphorylated tau protein, and the formation of amyloid beta (Aβ) or senile plaques, which are protein deposits found outside cells and surrounded by dystrophic neurites. Genetic studies have identified four genes associated with autosomal dominant or familial early-onset AD (FAD): amyloid precursor protein (APP), presenilin 1 (PS1), presenilin 2 (PS2), and apolipoprotein E (ApoE). The formation of plaques primarily involves the accumulation of Aβ, which can be influenced by mutations in APP, PS1, PS2, or ApoE genes. Mutations in the APP and presenilin (PS) proteins can cause an increased amyloid β peptides production, especially the further form of amyloidogenic known as Aβ42. Apart from genetic factors, environmental factors such as cytokines and neurotoxins may also have a significant impact on the development and progression of AD by influencing the formation of amyloid plaques and intracellular tangles. Exploring the causes and implications of protein aggregation in the brain could lead to innovative therapeutic approaches. Some promising therapy strategies that have reached the clinical stage include using acetylcholinesterase inhibitors, estrogen, nonsteroidal anti-inflammatory drugs (NSAIDs), antioxidants, and antiapoptotic agents. The most hopeful therapeutic strategies involve inhibiting activity of secretase and preventing the β-amyloid oligomers and fibrils formation, which are associated with the β-amyloid fibrils accumulation in AD. Additionally, immunotherapy development holds promise as a progressive therapeutic approach for treatment of AD. Recently, the two primary categories of brain stimulation techniques that have been studied for the treatment of AD are invasive brain stimulation (IBS) and non-invasive brain stimulation (NIBS). In this article, the amyloid proteins that play a significant role in the AD formation, the mechanism of disease formation as well as new drugs utilized to treat of AD will be reviewed.

Transthyretin Stabilizers and Seeding Inhibitors as Therapies for Amyloid Transthyretin Cardiomyopathy

Transthyretin (TTR) amyloid cardiomyopathy (ATTR-CM) is a progressive and increasingly recognized cause of heart failure which is associated with high mortality and morbidity. ATTR-CM is characterized by the misfolding of TTR monomers and their deposition within the myocardium as amyloid fibrils. The standard of care for ATTR-CM consists of TTR-stabilizing ligands, such as tafamidis, which aim at maintaining the native structure of TTR tetramers, thus preventing amyloid aggregation. However, their efficacy in advanced-staged disease and after long-term treatment is still a source of concern, suggesting the existence of other pathogenetic factors. Indeed, pre-formed fibrils present in the tissue can further accelerate amyloid aggregation in a self-propagating process known as “amyloid seeding”. The inhibition of amyloidogenesis through TTR stabilizers combined with anti-seeding peptides may represent a novel strategy with additional benefits over current therapies. Finally, the role of stabilizing ligands needs to be reassessed in view of the promising results derived from trials which have evaluated alternative strategies, such as TTR silencers and immunological amyloid disruptors.

Hyperphosphorylated tau (p-tau) and drug discovery in the context of Alzheimer's disease and related tauopathies

Alzheimer's disease (AD) is the most common form of dementia, characterized by intracellular neurofibrillary tangles (NFTs) and extracellular β-amyloid (βA) plaques. No disease-modifying therapy is currently available to prevent the progression of, or cure, the disease. Misfolded hyperphosphorylated tau (p-tau) is considered a pivotal point in the pathogenesis of AD and other tauopathies. Compelling evidence suggests that it is a key driver of the accumulation of NFTs and can be directly correlated with the extent of dementia in patients with AD. Therefore, inhibiting tau hyperphosphorylation-induced aggregation could be a viable strategy to discover and develop therapeutics for patients with AD.

Monitoring α-synuclein aggregation

Synucleinopathies, including Parkinson's disease (PD), dementia with Lewy Bodies (DLB), and multiple system atrophy (MSA), are characterized by the misfolding and subsequent aggregation of alpha-synuclein (α-syn) that accumulates in cytoplasmic inclusions bodies in the cells of affected brain regions. Since the seminal report of likely-aggregated α-syn presence within the Lewy bodies by Spillantini et al. in 1997, the keyword "synuclein aggregation" has appeared in over 6000 papers (Source: PubMed October 2022). Studying, observing, describing, and quantifying α-syn aggregation is therefore of paramount importance, whether it happens in tubo, in vitro, in post-mortem samples, or in vivo. The past few years have witnessed tremendous progress in understanding aggregation mechanisms and identifying various polymorphs. In this context of growing complexity, it is of utmost importance to understand what tools we possess, what exact information they provide, and in what context they may be applied. Nonetheless, it is also crucial to rationalize the relevance of the information and the limitations of these methods for gauging the final result. In this review, we present the main techniques that have shaped the current views about α-syn structure and dynamics, with particular emphasis on the recent breakthroughs that may change our understanding of synucleinopathies.

Folding Mechanism and Aggregation Propensity of the KH0 Domain of FMRP and Its R138Q Pathological Variant

The K-homology (KH) domains are small, structurally conserved domains found in proteins of different origins characterized by a central conserved βααβ “core” and a GxxG motif in the loop between the two helices of the KH core. In the eukaryotic KHI type, additional αβ elements decorate the “core” at the C-terminus. Proteins containing KH domains perform different functions and several diseases have been associated with mutations in these domains, including those in the fragile X mental retardation protein (FMRP). FMRP is an RNA-binding protein crucial for the control of RNA metabolism whose lack or mutations lead to fragile X syndrome (FXS). Among missense mutations, the R138Q substitution is in the KH0 degenerated domain lacking the classical GxxG motif. By combining equilibrium and kinetic experiments, we present a characterization of the folding mechanism of the KH0 domain from the FMRP wild-type and of the R138Q variant showing that in both cases the folding mechanism implies the accumulation of an on-pathway transient intermediate. Moreover, by exploiting a battery of biophysical techniques, we show that the KH0 domain has the propensity to form amyloid-like aggregates in mild conditions in vitro and that the R138Q mutation leads to a general destabilization of the protein and to an increased fibrillogenesis propensity.

Amyloid seeding as a disease mechanism and treatment target in transthyretin cardiac amyloidosis

Transthyretin (TTR) is a tetrameric transport protein mainly synthesized by the liver and choroid plexus. ATTR amyloidosis is characterized by the misfolding of TTR monomers and their accumulation within tissues as amyloid fibres. Current therapeutic options rely on the blockade of TTR production, TTR stabilization to maintain the native structure of TTR, amyloid degradation, or induction of amyloid removal from tissues. “Amyloid seeds” are defined as small fibril fragments that induce amyloid precursors to assume a structure rich in β-sheets, thus promoting fibrillogenesis. Amyloid seeds are important to promote the amplification and spread of amyloid deposits. Further studies are needed to better understand the molecular structure of ATTR seeds (i.e. the characteristics of the most amyloidogenic species), and the conditions that promote the formation and multiplication of seeds in vivo. The pathological cascade may begin months to years before symptom onset, suggesting that seeds in tissues might potentially be used as biomarkers for the early disease stages. Inhibition of amyloid aggregation by anti-seeding peptides may represent a disease mechanism and treatment target in ATTR amyloidosis, with an additional benefit over current therapies.

Single-molecule Observation of Self-Propagating Amyloid Fibrils

The assembly of misfolded proteins into amyloid fibrils is associated with amyloidosis, including neurodegenerative diseases, such as Alzheimer’s, Parkinson’s, and prion diseases. The self-propagation of amyloid fibrils is widely observed in the aggregation pathways of numerous amyloidogenic proteins. This propensity with plasticity in primary nucleation allows amyloid fibril polymorphism, which is correlated with the pathology/phenotypes of patients. Because the interference with the nucleation and replication processes of amyloid fibrils can alter the amyloid structure and the outcome of the disease, these processes can be a target for developing clinical drugs. Single-molecule observation of amyloid fibril replication can be an experimental system to provide the kinetic parameters for simulation studies and confirm the effect of clinical drugs. Here, we review single-molecule observation of the amyloid fibril replication process using fluorescence microscopy and time-lapse atomic force microscopy, including high-speed atomic force microscopy. We discussed the amyloid fibril replication process and combined single-molecule observation results with molecular dynamics simulations.

Mini Abstract Structural dynamics in amyloid aggregation is related with various Alzheimer’s and Parkinson’s disease symptoms. Single-molecule observation using high-speed atomic force microscopy can directly visualize the structural dynamics of individual amyloid aggregate assemblies. Here, we review historical and recent studies of single-molecule observation of amyloid aggregation with supportive molecular dynamics simulation.

From Quantum Mechanics, Classical Mechanics, and Bioinformatics to Artificial Intelligence Studies in Neurodegenerative Diseases

The amyloid β-protein is an intrinsically disordered protein that has the potential to assemble into myriad structures, including oligomers and fibrils. These structures are neurotoxic and are thought to initiate a cascade of events leading to Alzheimer's disease. Understanding this pathogenetic process and elucidating targets for drug therapy depends on elucidation of the structural dynamics of Aβ assembly. In this chapter, we describe work packages required to determine the three-dimensional structures of Aβ and of smaller bioactive fragments thereof, which may be important in AD pathogenesis. These packages include density functional theory, Car-Parrinello molecular dynamics simulations, temperature-dependent replica exchange molecular dynamics simulations, disorder predictors based on bioinformatics, and neural network deep learning.

Challenges in Experimental Methods

Experimental studies of amyloids encounter many challenges. There are many methods available for studying proteins, which can be applied to amyloids: from basic staining techniques, allowing visualization of fibers, to complex methods, e.g., AFM-IR used to their detailed biochemical and structural characterization in nanoscale. Which method is appropriate depends on the goal of an experiment: verification of aggregational properties of a peptide, distinguishing oligomers from mature fibers, or kinetic studies. Insolubility, rapid aggregation, and the need of using a high-purity peptide may be a limiting factor in studies involving amyloids. Moreover, the results obtained by various experimental methods often differ significantly, which may lead to misclassification of amyloid peptides. Due to ambiguity of experimental results, laborious and time-consuming analysis, bioinformatical methods become more widely used for amyloids.

Modified Low-Temperature Extraction Method for Isolation of Bletilla striata Polysaccharide as Antioxidant for the Prevention of Alzheimer's Disease

Amyloid-β (Aβ) peptides play a key role in Alzheimer's disease (AD), the most common type of dementia. In this study, a polysaccharide from Bletilla striata (BSP), with strong antioxidant and anti-inflammatory properties, was extracted using a low-temperature method and tested for its efficacy against AD, in vitro using N2a and BV-2 cells, and in vivo using an AD rat model. The characterization of the extracted BSP for its molecular structure and functional groups demonstrated the effectiveness of the modified method for retaining its bioactivity. In vitro, BSP reduced by 20% reactive oxygen species (ROS) levels in N2a cells (p = 0.0082) and the expression levels of inflammation-related genes by 3-fold TNF-α (p = 0.0048), 4-fold IL-6 (p = 0.0019), and 2.5-fold IL-10 (p = 0.0212) in BV-2 cells treated with Aβ fibrils. In vivo, BSP recovered learning memory, ameliorated morphological damage in the hippocampus and cortex, and reduced the expression of the β-secretase protein in AlCl3-induced AD rats. Collectively, these findings demonstrated the efficacy of BSP for preventing and alleviating the effects of AD.

Multiple-State Monitoring of SOD1 Amyloid Formation at Single-Residue Resolution by Rheo-NMR Spectroscopy

Formation of protein aggregates or fibrils entails the conversion of soluble native protein monomers via multiple molecular states. No spectroscopic techniques have succeeded in capturing the transient molecular-scale events of fibrillation in situ. Here we report residue- and state-specific real-time monitoring of the fibrillation of amyotrophic lateral sclerosis-related SOD1 by rheology NMR (Rheo-NMR) spectroscopy. Under moderately denaturing conditions, where NMR signals of folded and unfolded monomeric SOD1 are simultaneously observable, the cross-peak intensities of folded monomeric SOD1 decreased faster than those of the unfolded species, and a 3₁₀-helix in folded SOD1 was deformed prior to global unfolding. Furthermore, real-time protein dynamics analysis identified residues involved in the core structure formation of SOD1 oligomers. Our findings provide insight into local and global unfolding events in SOD1 and fibril formation. This Rheo-NMR analysis will be applicable not only to atomic-level monitoring of other amyloidogenic proteins but also to quantification of shear-induced structural changes of non-amyloidogenic proteins and elucidation of shear-enhanced chemical phenomena such as viscosity increase and crystallization of various solution-state compounds.

Structures of Pathological and Functional Amyloids and Prions, a Solid-State NMR Perspective

Infectious proteins or prions are a remarkable class of pathogens, where pathogenicity and infectious state correspond to conformational transition of a protein fold. The conformational change translates into the formation by the protein of insoluble amyloid aggregates, associated in humans with various neurodegenerative disorders and systemic protein-deposition diseases. The prion principle, however, is not limited to pathogenicity. While pathological amyloids (and prions) emerge from protein misfolding, a class of functional amyloids has been defined, consisting of amyloid-forming domains under natural selection and with diverse biological roles. Although of great importance, prion amyloid structures remain challenging for conventional structural biology techniques. Solid-state nuclear magnetic resonance (SSNMR) has been preferentially used to investigate these insoluble, morphologically heterogeneous aggregates with poor crystallinity. SSNMR methods have yielded a wealth of knowledge regarding the fundamentals of prion biology and have helped to solve the structures of several prion and prion-like fibrils. Here, we will review pathological and functional amyloid structures and will discuss some of the obtained structural models. We will finish the review with a perspective on integrative approaches combining solid-state NMR, electron paramagnetic resonance and cryo-electron microscopy, which can complement and extend our toolkit to structurally explore various facets of prion biology.

Amyloidoma: a review and case report

Amyloidoma is a solitary mass of amyloid protein that arises in patients with or without evidence of systemic amyloidosis, and can be found in a variety of different organ systems. Herein, we describe three cases of localized biopsy-positive amyloidomas with no evidence of systemic involvement-primary amyloidoma. Our cases include a patient with a paraspinal soft tissue amyloidoma, a patient with multiple primary amyloidomas involving the thoracic cavity and flank, and a patient with insulin-injection induced amyloidoma of the left shoulder. We present these cases to provide further insights into the clinical presentation of this uncommon clinical entity. We review the pathophysiology of amyloidosis and discuss our cases in the context of previous reports of amyloidoma.

The folding and aggregation properties of a single KH-domain protein: Ribosome binding factor A (RbfA) from Pseudomonas aeruginosa

BACKGROUND

Ribosome-binding factor A from the pathogenic bacterium Pseudomonas aeruginosa (PaRbfA) is a small ribosome assembly factor, composed by a single KH domain, involved in the maturation of the 30S subunit. These domains are characterized by the ability to bind RNA or ssDNA and are often located in proteins involved in a variety of cellular functions. However, although the ability of proteins to fold properly, to misfold or to aggregate is of paramount importance for their cellular functions, limited information is available on these dynamic properties in the case of KH domains.

METHODS

PaRbfA thermodynamic stability and folding mechanism: Far-UV CD and fluorescence spectroscopy, stopped-flow kinetics and chevron plot analysis, site-directed mutagenesis. Fibrils characterization: FT-IR spectroscopy, Thioflavin T fluorescence, Transmission Electron Microscopy (TEM) and X-ray fibrils diffraction.

RESULTS

Quantitative analysis of the (un)folding kinetics of PaRbfA show that, in vitro, the protein folds via a 3-states mechanism involving a transiently populated folding intermediate. We also provide experimental evidences that PaRbfA can form ordered fibrils endowed with cross-β structure even in mild conditions.

CONCLUSION

These results lead to the hypothesis that the folding intermediate of PaRbfA may expose (some of) the predicted amyloidogenic regions, which could act as aggregation nuclei in the fibrillogenesis.

GENERAL SIGNIFICANCE

The methodological approach presented herein could be readily adapted to verify the ability of other KH domain proteins to form cross-β structured fibrils and to transiently populate a folding intermediate.

Strategies for the Molecular Imaging of Amyloid and the Value of a Multimodal Approach

Protein aggregation has been widely implicated in neurodegenerative diseases such as Alzheimer's disease, frontotemporal dementia, Parkinson's disease, and Huntington disease, as well as in systemic amyloidoses and conditions associated with localized amyloid deposits, such as type-II diabetes. The pressing need for a better understanding of the factors governing protein assembly has driven research for the development of molecular sensors for amyloidogenic proteins. To date, a number of sensors have been developed that report on the presence of protein aggregates utilizing various modalities, and their utility demonstrated for imaging protein aggregation in vitro and in vivo. Analysis of these sensors highlights the various advantages and disadvantages of the different imaging modalities and makes clear that multimodal sensors with properties amenable to more than one imaging technique need to be developed. This critical review highlights the key molecular scaffolds reported for molecular imaging modalities such as fluorescence, positron emission tomography, single photon emission computed tomography, and magnetic resonance imaging and includes discussion of the advantages and disadvantages of each modality, and future directions for the design of amyloid sensors. We also discuss the recent efforts focused on the design and development of multimodal sensors and the value of cross-validation across multiple modalities.

Amyloid-β oligomerization monitored by single-molecule stepwise photobleaching

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Method enables investigation of amyloid-β oligomer stoichiometry without requiring extrinsic fluorescent probes.

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Uses single-molecule stepwise photobleaching in vitro.

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Unveils heterogeneity within populations of oligomers.

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Assays oligomer-induced dysregulation of intracellular Ca²⁺ homeostasis in living cells.

A major hallmark of Alzheimer’s disease is the misfolding and aggregation of the amyloid- β peptide (Aβ). While early research pointed towards large fibrillar- and plaque-like aggregates as being the most toxic species, recent evidence now implicates small soluble Aβ oligomers as being orders of magnitude more harmful. Techniques capable of characterizing oligomer stoichiometry and assembly are thus critical for a deeper understanding of the earliest stages of neurodegeneration and for rationally testing next-generation oligomer inhibitors. While the fluorescence response of extrinsic fluorescent probes such as Thioflavin-T have become workhorse tools for characterizing large Aβ aggregates in solution, it is widely accepted that these methods suffer from many important drawbacks, including an insensitivity to oligomeric species. Here, we integrate several biophysics techniques to gain new insight into oligomer formation at the single-molecule level. We showcase single-molecule stepwise photobleaching of fluorescent dye molecules as a powerful method to bypass many of the traditional limitations, and provide a step-by-step guide to implementing the technique in vitro. By collecting fluorescence emission from single Aβ(1–42) peptides labelled at the N-terminal position with HiLyte Fluor 555 via wide-field total internal reflection fluorescence (TIRF) imaging, we demonstrate how to characterize the number of peptides per single immobile oligomer and reveal heterogeneity within sample populations. Importantly, fluorescence emerging from Aβ oligomers cannot be easily investigated using diffraction-limited optical microscopy tools. To assay oligomer activity, we also demonstrate the implementation of another biophysical method involving the ratiometric imaging of Fura-2-AM loaded cells which quantifies the rate of oligomer-induced dysregulation of intracellular Ca²⁺ homeostasis. We anticipate that the integrated single-molecule biophysics approaches highlighted here will develop further and in principle may be extended to the investigation of other protein aggregation systems under controlled experimental conditions.

Revealing the assembly of filamentous proteins with scanning transmission electron microscopy

Filamentous proteins are responsible for the superior mechanical strength of our cells and tissues. The remarkable mechanical properties of protein filaments are tied to their complex molecular packing structure. However, since these filaments have widths of several to tens of nanometers, it has remained challenging to quantitatively probe their molecular mass density and three-dimensional packing order. Scanning transmission electron microscopy (STEM) is a powerful tool to perform simultaneous mass and morphology measurements on filamentous proteins at high resolution, but its applicability has been greatly limited by the lack of automated image processing methods. Here, we demonstrate a semi-automated tracking algorithm that is capable of analyzing the molecular packing density of intra- and extracellular protein filaments over a broad mass range from STEM images. We prove the wide applicability of the technique by analyzing the mass densities of two cytoskeletal proteins (actin and microtubules) and of the main protein in the extracellular matrix, collagen. The high-throughput and spatial resolution of our approach allow us to quantify the internal packing of these filaments and their polymorphism by correlating mass and morphology information. Moreover, we are able to identify periodic mass variations in collagen fibrils that reveal details of their axially ordered longitudinal self-assembly. STEM-based mass mapping coupled with our tracking algorithm is therefore a powerful technique in the characterization of a wide range of biological and synthetic filaments.

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.

Cytotoxic species in amyloid-associated diseases: Oligomers or mature fibrils

Amyloid diseases especially, Alzheimer's disease (AD), is characterized by an imbalance between the production and clearance of amyloid-β (Aβ) species. Amyloidogenic proteins or peptides can transform structurally from monomers into β-stranded fibrils via multiple oligomeric states. Among various amyloid species, structured oligomers are proposed to be more toxic than fibrils; however, the identification of amyloid oligomers has been challenging due to their heterogeneous and metastable nature. Multiple techniques have recently helped in better understanding of oligomer's assembly details and structural properties. Moreover, some progress on elucidating the mechanisms of oligomer-triggered toxicity has been made. Based on the collection of current findings, there is growing consensus that control of toxic amyloid oligomers could be a valid approach to regulate amyloid-associated toxicity, which could advance development of new diagnostics and therapeutics for amyloid-related diseases. In this review, we have described the recent scenario of amyloid diseases with a great deal of information about the recent understanding of oligomers' assembly, structural properties, and toxicity. Also comprehensive details have been provided to differentiate the degree of toxicity associated with prefibrillar aggregates.

Amyloid Oligomers, Protofibrils and Fibrils

Amyloid diseases are of major concern all over the world due to a number of factors including: (i) aging population, (ii) increasing life span and (iii) lack of effective pharmacotherapy options. The past decade has seen intense research in discovering disease-modifying multi-targeting small molecules as therapeutic options. In recent years, targeting the amyloid cascade has emerged as an attractive strategy to discover novel neurotherapeutics. Formation of amyloid species, with different degrees of solubility and neurotoxicity is associated with the gradual decline in cognition leading to dementia/cell dysfunction. Here, in this chapter, we have described the recent scenario of amyloid diseases with a great deal of information about the structural features of oligomers, protofibrils and fibrils. Also, comprehensive details have been provided to differentiate the degree of toxicity associated with prefibrillar aggregates. Moreover, a review of the technologies that aid characterisation of oligomer, protofibrils and fibrils as well as various inhibition strategies to overcome protein fibrillation are also discussed.

Growth probability and formation time of the individual Oosawa-Kasai protein fibril

Protein fibrils are currently of great academic and practical interest because of their involvement in scores of severe human diseases and their promising use in various high-technology devices. The Oosawa-Kasai (OK) model of protein self-assembly into fibrils has been widely used to gain mechanistic insight into the process of fibril formation and growth. Here this model is employed to obtain exact and mathematically simple expressions for the probability P_{n} of an individual fibril of n protein monomers to grow to a macroscopically large size and for the mean time τ_{n} that such a fibril needs for its formation. These expressions quantify the increase of P_{n} and the decrease of τ_{n} with increasing the concentration of monomeric protein in the solution. When used for analysis of experimental P_{n} and τ_{n} data, they make it possible to determine the parameters characterizing fibril nucleation and growth in the framework of the OK model. Finally, an expression is found for the mean time of the first appearance of an n-sized fibril in the protein solution. The results obtained are applicable to the formation of other aggregates corresponding to the OK fibrils, such as the one-dimensional Kossel-Stranski crystals and Ising ferromagnets.

Recent Advances by In Silico and In Vitro Studies of Amyloid-β 1-42 Fibril Depicted a S-Shape Conformation

The amyloid-β 1-42 (Aβ1-42) peptide is produced by proteolytic cleavage of the amyloid precursor protein (APP) by sequential reactions that are catalyzed by γ and β secretases. Aβ1-42, together with the Tau protein are two principal hallmarks of Alzheimer's disease (AD) that are related to disease genesis and progression. Aβ1-42 possesses a higher aggregation propensity, and it is able to form fibrils via nucleated fibril formation. To date, there are compounds available that prevent Aβ1-42 aggregation, but none have been successful in clinical trials, possibly because the Aβ1-42 structure and aggregation mechanisms are not thoroughly understood. New molecules have been designed, employing knowledge of the Aβ1-42 structure and are based on preventing or breaking the ionic interactions that have been proposed for formation of the Aβ1-42 fibril U-shaped structure. Recently, a new Aβ1-42 fibril S-shaped structure was reported that, together with its aggregation and catalytic properties, could be helpful in the design of new inhibitor molecules. Therefore, in silico and in vitro methods have been employed to analyze the Aβ1-42 fibril S-shaped structure and its aggregation to obtain more accurate Aβ1-42 oligomerization data for the design and evaluation of new molecules that can prevent the fibrillation process.

Understanding amyloid fibril formation using protein fragments: structural investigations via vibrational spectroscopy and solid-state NMR

It is well established that amyloid proteins play a primary role in neurodegenerative diseases. Alzheimer's, Parkinson's, type II diabetes, and Creutzfeldt-Jakob's diseases are part of a wider family encompassing more than 50 human pathologies related to aggregation of proteins. Although this field of research is thoroughly investigated, several aspects of fibrillization remain misunderstood, which in turn slows down, or even impedes, advances in treating and curing amyloidoses. To solve this problem, several research groups have chosen to focus on short fragments of amyloid proteins, sequences that have been found to be of great importance for the amyloid formation process. Studying short peptides allows bypassing the complexity of working with full-length proteins and may provide important information relative to critical segments of amyloid proteins. To this end, efficient biophysical tools are required. In this review, we focus on two essential types of spectroscopic techniques, i.e., vibrational spectroscopy and its derivatives (conventional Raman scattering, deep-UV resonance Raman (DUVRR), Raman optical activity (ROA), surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS), infrared (IR) absorption spectroscopy, vibrational circular dichroism (VCD)) and solid-state nuclear magnetic resonance (ssNMR). These techniques revealed powerful to provide a better atomic and molecular comprehension of the amyloidogenic process and fibril structure. This review aims at underlining the information that these techniques can provide and at highlighting their strengths and weaknesses when studying amyloid fragments. Meaningful examples from the literature are provided for each technique, and their complementarity is stressed for the kinetic and structural characterization of amyloid fibril formation.

Multicomponent peptide assemblies

Self-assembled peptide nanostructures have been increasingly exploited as functional materials for applications in biomedicine and energy. The emergent properties of these nanomaterials determine the applications for which they can be exploited. It has recently been appreciated that nanomaterials composed of multicomponent coassembled peptides often display unique emergent properties that have the potential to dramatically expand the functional utility of peptide-based materials. This review presents recent efforts in the development of multicomponent peptide assemblies. The discussion includes multicomponent assemblies derived from short low molecular weight peptides, peptide amphiphiles, coiled coil peptides, collagen, and β-sheet peptides. The design, structure, emergent properties, and applications for these multicomponent assemblies are presented in order to illustrate the potential of these formulations as sophisticated next-generation bio-inspired materials.

Insights into the Molecular Mechanisms of Alzheimer's and Parkinson's Diseases with Molecular Simulations: Understanding the Roles of Artificial and Pathological Missense Mutations in Intrinsically Disordered Proteins Related to Pathology

Amyloid-β and α-synuclein are intrinsically disordered proteins (IDPs), which are at the center of Alzheimer's and Parkinson's disease pathologies, respectively. These IDPs are extremely flexible and do not adopt stable structures. Furthermore, both amyloid-β and α-synuclein can form toxic oligomers, amyloid fibrils and other type of aggregates in Alzheimer's and Parkinson's diseases. Experimentalists face challenges in investigating the structures and thermodynamic properties of these IDPs in their monomeric and oligomeric forms due to the rapid conformational changes, fast aggregation processes and strong solvent effects. Classical molecular dynamics simulations complement experiments and provide structural information at the atomic level with dynamics without facing the same experimental limitations. Artificial missense mutations are employed experimentally and computationally for providing insights into the structure-function relationships of amyloid-β and α-synuclein in relation to the pathologies of Alzheimer's and Parkinson's diseases. Furthermore, there are several natural genetic variations that play a role in the pathogenesis of familial cases of Alzheimer's and Parkinson's diseases, which are related to specific genetic defects inherited in dominant or recessive patterns. The present review summarizes the current understanding of monomeric and oligomeric forms of amyloid-β and α-synuclein, as well as the impacts of artificial and pathological missense mutations on the structural ensembles of these IDPs using molecular dynamics simulations. We also emphasize the recent investigations on residual secondary structure formation in dynamic conformational ensembles of amyloid-β and α-synuclein, such as β-structure linked to the oligomerization and fibrillation mechanisms related to the pathologies of Alzheimer's and Parkinson's diseases. This information represents an important foundation for the successful and efficient drug design studies.

Solid-State NMR Structural Characterization of Self-Assembled Peptides with Selective 13C and 15N Isotopic Labels

For the structural characterization methods discussed here, information on molecular conformation and intermolecular organization within nanostructured peptide assemblies is discerned through analysis of solid-state NMR spectral features. This chapter reviews general NMR methodologies, requirements for sample preparation, and specific descriptions of key experiments. An attempt is made to explain choices of solid-state NMR experiments and interpretation of results in a way that is approachable to a nonspecialist. Measurements are designed to determine precise NMR peak positions and line widths, which are correlated with secondary structures, and probe nuclear spin-spin interactions that report on three-dimensional organization of atoms. The formulation of molecular structural models requires rationalization of data sets obtained from multiple NMR experiments on samples with carefully chosen 13C and 15N isotopic labels. The information content of solid-state NMR data has been illustrated mostly through the use of simulated data sets and references to recent structural work on amyloid fibril-forming peptides and designer self-assembling peptides.

Inhibition of Aβ Amyloid Growth and Toxicity by Silybins: The Crucial Role of Stereochemistry

The self-assembling of the amyloid β (Aβ) peptide into neurotoxic aggregates is considered a central event in the pathogenesis of Alzheimer's disease (AD). Based on the "amyloid hypothesis", many efforts have been devoted to designing molecules able to halt disease progression by inhibiting Aβ self-assembly. Here, we combine biophysical (ThT assays, TEM and AFM imaging), biochemical (WB and ESI-MS), and computational (all-atom molecular dynamics) techniques to investigate the capacity of four optically pure components of the natural product silymarin (silybin A, silybin B, 2,3-dehydrosilybin A, 2,3-dehydrosilybin B) to inhibit Aβ aggregation. Despite TEM analysis demonstrated that all the four investigated flavonoids prevent the formation of mature fibrils, ThT assays, WB and AFM investigations showed that only silybin B was able to halt the growth of small-sized protofibrils thus promoting the formation of large, amorphous aggregates. Molecular dynamics (MD) simulations indicated that silybin B interacts mainly with the C-terminal hydrophobic segment ³⁵MVGGVV⁴⁰ of Aβ40. Consequently to silybin B binding, the peptide conformation remains predominantly unstructured along all the simulations. By contrast, silybin A interacts preferentially with the segments ¹⁷LVFF²⁰ and ²⁷NKGAII³² of Aβ40 which shows a high tendency to form bend, turn, and β-sheet conformation in and around these two domains. Both 2,3-dehydrosilybin enantiomers bind preferentially the segment ¹⁷LVFF²⁰ but lead to the formation of different small-sized, ThT-positive Aβ aggregates. Finally, in vivo studies in a transgenic Caenorhabditis elegans strain expressing human Aβ indicated that silybin B is the most effective of the four compounds in counteracting Aβ proteotoxicity. This study underscores the pivotal role of stereochemistry in determining the neuroprotective potential of silybins and points to silybin B as a promising lead compound for further development in anti-AD therapeutics.

Measurement of amyloid formation by turbidity assay-seeing through the cloud

Detection of amyloid growth is commonly carried out by measurement of solution turbidity, a low-cost assay procedure based on the intrinsic light scattering properties of the protein aggregate. Here, we review the biophysical chemistry associated with the turbidimetric assay methodology, exploring the reviewed literature using a series of pedagogical kinetic simulations. In turn, these simulations are used to interrogate the literature concerned with in vitro drug screening and the assessment of amyloid aggregation mechanisms.

Recognizing and analyzing variability in amyloid formation kinetics: Simulation and statistical methods

We examine the phenomenon of variability in the kinetics of amyloid formation and detail methods for its simulation, identification and analysis. Simulated data, reflecting intrinsic variability, were produced using rate constants, randomly sampled from a pre-defined distribution, as parameters in an irreversible nucleation-growth kinetic model. Simulated kinetic traces were reduced in complexity through description in terms of three characteristic parameters. Practical methods for assessing convergence of the reduced parameter distributions were introduced and a bootstrap procedure was applied to determine convergence for different levels of intrinsic variation. Statistical methods for assessing the significance of shifts in parameter distributions, relating to either change in parameter mean or distribution shape, were tested. Robust methods for analyzing and interpreting kinetic data possessing significant intrinsic variance will allow greater scrutiny of the effects of anti-amyloid compounds in drug trials.

STED imaging of tau filaments in Alzheimer's disease cortical grey matter

Alzheimer's disease (AD) involves the propagation of filaments of tau protein throughout the cerebral cortex. Imaging tau filaments and oligomers in human brain at high resolution would help contribute insight into the mechanism and progression of tauopathic diseases. STED microscopy is a nano-scale imaging technique and we aimed to test the abilities of this method for resolving tau structures within human brain. Using autopsied 50μm AD brain sections, we demonstrate that STED microscopy can resolve immunolabelled tau filaments at 77nm resolution. Ribbon-like tau filaments imaged by STED appeared smooth along their axis with limited axial undulations. STED also resolved 70-80nm wide tau puncta. Of the fluorophores tested, STAR635p was optimal for STED imaging in this tissue. This was in part due to brain tissue autofluorescence within the lower wavelength ranges (488-590nm). Further, the stability and minimal photobleaching of STAR635p allowed STED z-stacks of neurons packed with tau filaments (neurofibrillary tangles) to be collated. There was no loss of x-y image resolution of individual tau filaments through the 20μm z-stack. This demonstrates that STED can contribute to nano-scale analysis and characterisation of pathologies within banked human autopsied brain tissue. Resolving tau structures at this level of resolution provides promising avenues for understanding mechanisms of pathology propagation in the different tauopathies as well as illuminating what contributes to disease heterogeneity.

MpUL-multi: Software for Calculation of Amyloid Fibril Mass per Unit Length from TB-TEM Images

Structure determination for amyloid fibrils presents many challenges due to the high variability exhibited by fibrils and heterogeneous morphologies present, even in single samples. Mass per unit length (MPL) estimates can be used to differentiate amyloid fibril morphologies and provide orthogonal evidence for helical symmetry parameters determined by other methods. In addition, MPL data can provide insight on the arrangement of subunits in a fibril, especially for more complex fibrils assembled with multiple parallel copies of the asymmetric unit or multiple twisted protofilaments. By detecting only scattered electrons, which serve as a relative measure of total scattering, and therefore protein mass, dark field imaging gives an approximation of the total mass of protein present in any given length of fibril. When compared with a standard of known MPL, such as Tobacco Mosaic Virus (TMV), MPL of the fibrils in question can be determined. The program suite MpUL-multi was written for rapid semi-automated processing of TB-TEM dark field data acquired using this method. A graphical user interface allows for simple designation of fibrils and standards. A second program averages intensities from multiple TMV molecules for accurate standard determination, makes multiple measurements along a given fibril, and calculates the MPL.

The twisted tauopathies: surface interactions of helically patterned filaments seen in alzheimer's disease and elsewhere

This paper broadly examines the dynamics of helically patterned filaments interacting with a surface and focuses on the surface interaction of amyloid fibrils formed by tau protein. Two structures are addressed in detail: cylindrical filaments with periodic thinning (CF-PT) and paired helical filaments (PHFs). PHFs are observed in neural tissue affected by Alzheimer's disease and may aggregate to form the pathological neurofibrillary tangles associated with the illness. Work using electron microscopy has demonstrated the conversion of CF-PT into PHFs in vitro, suggesting CF-PT to be a PHF precursor in vivo. Here we model CF-PT as a patterned elastic rod placed on a flat surface (characteristic of the environment during microscopy) and examine the conformational changes resulting in stable surface bonding. Analysis of this conformational space reveals structures resembling PHFs and thus provides a mechanistic explanation of the CF-PT to PHF transition. We develop a general phase diagram of the filament conformation as a function of filament twist and bend rigidity. Results of this work also suggest that we can obtain desired filament conformations by patterning interactions of elastic filaments with a substrate, and therefore can be used as a method in microfabrication.

α-Synuclein Amyloid Fibrils with Two Entwined, Asymmetrically Associated Protofibrils

Parkinson disease and other progressive neurodegenerative conditions are characterized by the intracerebral presence of Lewy bodies, containing amyloid fibrils of α-synuclein. We used cryo-electron microscopy and scanning transmission electron microscopy (STEM) to study in vitro-assembled fibrils. These fibrils are highly polymorphic. Focusing on twisting fibrils with an inter-crossover spacing of 77 nm, our reconstructions showed them to consist of paired protofibrils. STEM mass per length data gave one subunit per 0.47 nm axial rise per protofibril, consistent with a superpleated β-structure. The STEM images show two thread-like densities running along each of these fibrils, which we interpret as ladders of metal ions. These threads confirmed the two-protofibril architecture of the 77-nm twisting fibrils and allowed us to identify this morphotype in STEM micrographs. Some other, but not all, fibril morphotypes also exhibit dense threads, implying that they also present a putative metal binding site. We propose a molecular model for the protofibril and suggest that polymorphic variant fibrils have different numbers of protofibrils that are associated differently.

Nanoscale Heterogeneity of the Molecular Structure of Individual hIAPP Amyloid Fibrils Revealed with Tip-Enhanced Raman Spectroscopy

Type 2 diabetes mellitus is characterized by the pathological deposition of fibrillized protein, known as amyloids. It is thought that oligomers and/or amyloid fibrils formed from human islet amyloid polypeptide (hIAPP or amylin) cause cell death by membrane damage. The molecular structure of hIAPP amyloid fibrils is dominated by β-sheet structure, as probed with conventional infrared and Raman vibrational spectroscopy. However, with these techniques it is not possible to distinguish between the core and the surface structure of the fibrils. Since the fibril surface crucially affects amyloid toxicity, it is essential to know its structure. Here the surface molecular structure and amino acid residue composition of hIAPP fibrils are specifically probed with nanoscale resolution using tip-enhanced Raman spectroscopy (TERS). The fibril surface mainly contains unordered or α-helical structures, in contrast to the β-sheet-rich core. This experimentally validates recent models of hIAPP amyloids based on NMR measurements. Spatial mapping of the surface structure reveals a highly heterogeneous surface structure. Finally, TERS can probe fibrils formed on a lipid interface, which is more representative of amyloids in vivo.

The Tubular Sheaths Encasing Methanosaeta thermophila Filaments Are Functional Amyloids

Archaea are renowned for their ability to thrive in extreme environments, although they can be found in virtually all habitats. Their adaptive success is linked to their unique cell envelopes that are extremely resistant to chemical and thermal denaturation and that resist proteolysis by common proteases. Here we employ amyloid-specific conformation antibodies and biophysical techniques to show that the extracellular cell wall sheaths encasing the methanogenic archaea Methanosaeta thermophila PT are functional amyloids. Depolymerization of sheaths and subsequent MS/MS analyses revealed that the sheaths are composed of a single major sheath protein (MspA). The amyloidogenic nature of MspA was confirmed by in vitro amyloid formation of recombinant MspA under a wide range of environmental conditions. This is the first report of a functional amyloid from the archaeal domain of life. The amyloid nature explains the extreme resistance of the sheath, the elastic properties that allow diffusible substrates to penetrate through expandable hoop boundaries, and how the sheaths are able to split and elongate outside the cell. The archaeal sheath amyloids do not share homology with any of the currently known functional amyloids and clearly represent a new function of the amyloid protein fold.

The structure of fibrils from 'misfolded' proteins

Recent developments in solid-state NMR have opened the way to the structural analysis of protein fibrils, with the power of studying them at atomic resolution. Solid-state NMR is a relatively new player in the field of structural biology, and reliable approaches to successfully tackle 3D structures have been developed and applied recently. Here we discuss a number of applications to selected fibrils, including prions, α-synuclein and Amyloid-β (Aβ). The latter is, as for its small monomer size, accessible to full 3D structure determination by solid-state NMR. In addition, chemical-shift assignments, from which secondary structure can be directly be determined, is possible for much larger proteins, and has provided important insight in the structural organization of prions and other amyloids playing a central role in disease.