Amyloid plaque structure and cell surface interactions of β-amyloid fibrils revealed by electron tomography

Evaluating the Combined Neurotoxicity of Amyloid Beta and Tau Oligomers in Alzheimer's Disease: A Novel Cellular-Level Criterion

A criterion characterizing the combined neurotoxicity of amyloid beta and tau oligomers is suggested. A mathematical model for calculating the value of this criterion during senile plaque and neurofibrillary tangle (NFT) formation is proposed. Computations show that for physiologically relevant parameter values, the value of the criterion increases approximately linearly with time. Once neurofibrillary tangles begin forming in addition to senile plaques, there is an increase in the slope characterizing the rate at which the criterion increases. The critical value of the criterion at which a neuron dies is estimated. Unless the production rates of amyloid beta and tau monomers are very large, computations predict that for the accumulated toxicity to reach the critical value, the neural machinery responsible for the degradation of amyloid beta and tau monomers and aggregates must become dysfunctional. The value of the criterion after 20 years of the aggregation process is strongly influenced by the deposition rates of amyloid beta and tau oligomers into senile plaques and NFTs. This suggests that deposition of amyloid beta and tau oligomers into senile plaques and NFTs may reduce accumulated toxicity by sequestering more toxic oligomeric species into less toxic insoluble aggregates.

Aβ40 Fibril Assembly on Human Cerebral Smooth Muscle Cells Impairs Cell Viability

Cerebral vascular deposition of the amyloid-β (Aβ) peptide, a condition known as cerebral amyloid angiopathy (CAA), is associated with intracerebral hemorrhaging and contributes to disease progression in Alzheimer's disease (AD) and vascular cognitive impairment and dementia (VCID). Familial mutations at positions 22 and 23 within the Aβ peptide lead to early onset and severe CAA pathology. Here, we evaluate the effects of fibrillar Aβ peptides on the viability of primary-cultured human cerebral smooth muscle (HCSM) cells, which are the major site of amyloid deposition in cerebral blood vessel walls. Comparisons are made of the familial E22Q (Dutch) mutant of Aβ40 with wild-type Aβ40 and Aβ42. In agreement with previous studies, we find that there is a significant reduction in cell viability when Aβ40-Dutch or Aβ42-WT peptides are added to HCSM cell cultures as monomeric Aβ, whereas Aβ40-WT is relatively nontoxic. The binding of Aβ fibrils derived from sporadic CAA or familial Dutch-type CAA brain tissue to the membrane surface of HCSM cells does not result in a significant loss of cell viability. In contrast, when Aβ40-WT monomers and sporadic CAA fibrils are coincubated in HCSM cell cultures, there is a significant reduction in HCSM cell viability that is accompanied by an increase in cell surface fibril formation. Lastly, intrathecal administration of Aβ40-Dutch fibrillar seeds promotes fibrillar amyloid accumulation in the smooth muscle of meningeal vessels in the rTg-D transgenic rat model of CAA. Together, the present findings suggest that fibrillar Aβ seeds propagate the expansion of new amyloid fibrils on cerebral vascular smooth muscle, leading to membrane disruption and cell death.

How is the Amyloid Fold Built? Polymorphism and the Microscopic Mechanisms of Fibril Assembly

For a given protein sequence, many, up to sometimes hundreds of different amyloid fibril folds, can be formed in vitro. Yet, fibrils extracted from, or found in, human tissue, usually at the end of a long disease process, are often structurally homogeneous. Through monitoring of amyloid assembly reactions in vitro, the scientific community has gained a detailed understanding of the kinetic mechanisms of fibril assembly and the rates at which the different processes involved occur. However, how this kinetic information relates to the structural changes as a protein transforms from its initial, native structure to the canonical cross-β structure of amyloid remain obscure. While cryoEM has yielded a plethora of high-resolution information that portray a vast variety of fibril structures, there remains little knowledge of how and why each particular structure resulted. Recent work has demonstrated that fibril structures can change over an assembly time course, despite the different fibril types having similar thermodynamic stability. This points to kinetic control of the fibrils formed, with structures that initiate or elongate faster becoming the dominant products of assembly. Annotating kinetic assembly mechanisms alongside structural analysis of the fibrils formed is required to truly understand the molecular mechanisms of amyloid formation. However, this is a complicated task. In this review, we discuss how embracing this challenge could open new frontiers in amyloid research and new opportunities for therapy.

Histopathological changes of nervous tissue in women over 60 years of age with Alzheimer's disease and their relationship with menopause

INTRODUCTION

Ageing is a natural and irreversible process that primarily manifests in older age, becoming more common after the age of 60. Currently, a significant increase has been observed in the elderly population, with forecasts indicating that this group will triple in size over the next 50 years. This phenomenon is evident in several countries, including Japan, Mexico, Brazil, and Colombia, where the growing population of older adults is accompanied by an increased risk of neurodegenerative diseases, such as Alzheimer's disease. Studies have shown differences in the onset and progression of the disease between men and women, highlighting menopause and hormonal factors as key determinants in women. An association has been identified between a lower exposure to endogenous oestrogens and a higher risk of dementia in women, linked to the action of the enzyme β-secretase (BACE1), which is involved in the formation of amyloid aggregates associated with Alzheimer's disease. These findings highlight the importance of thoroughly investigating and understanding the impact of ageing and related diseases on the current and future population.

OBJECTIVE

This study aims to describe the histopathological changes in nervous tissue in women over 60 years of age with Alzheimer's disease and their relationship to menopause.

METHODOLOGY

A comprehensive search was conducted in databases such as PubMed, ScienceDirect, Frontiers, Scopus, and Springer.

RESULTS

Two hundred thirteen articles were selected for review and 45 full articles were chosen.

CONCLUSIONS

Alzheimer's disease is characterised by a progressive loss of cognitive function due to brain lesions, including the accumulation of amyloid-beta plaques and neuronal apoptosis. Hormonal changes during menopause may contribute to the onset of the disease.

The growth rate of senile plaques is determined by the competition between the rate of deposition of free Aβ aggregates into plaques and the autocatalytic production of free Aβ aggregates

The formation of amyloid beta (Aβ) deposits (senile plaques) is one of the hallmarks of Alzheimer's disease (AD). This study investigates what processes are primarily responsible for their formation. A model is developed to simulate the diffusion of amyloid beta (Aβ) monomers, the production of free Aβ aggregates through nucleation and autocatalytic processes, and the deposition of these aggregates into senile plaques. The model suggests that efficient degradation of Aβ monomers alone may suffice to prevent the growth of senile plaques, even without degrading Aβ aggregates and existing plaques. This is because the degradation of Aβ monomers interrupts the supply of reactants needed for plaque formation. The impact of Aβ monomer diffusivity is demonstrated to be small, enabling the application of the lumped capacitance approximation and the derivation of approximate analytical solutions for limiting cases with both small and large rates of Aβ aggregate deposition into plaques. It is found that the rate of plaque growth is governed by two competing processes. One is the deposition rate of free Aβ aggregates into senile plaques. If this rate is small, the plaque grows slowly. However, if the rate of deposition of Aβ aggregates into senile plaques is very large, the free Aβ aggregates are removed from the intracellular fluid by deposition into the plaques, leaving insufficient free Aβ aggregates to catalyze the production of new aggregates. This suggests that under certain conditions, Aβ plaques may offer neuroprotection and impede their own growth. Additionally, it indicates that there exists an optimal rate of deposition of free Aβ aggregates into the plaques, at which the plaques attain their maximum size.

CryoET of β-amyloid and tau within postmortem Alzheimer's disease brain

A defining pathological feature of most neurodegenerative diseases is the assembly of proteins into amyloid that form disease-specific structures1. In Alzheimer's disease, this is characterized by the deposition of β-amyloid and tau with disease-specific conformations. The in situ structure of amyloid in the human brain is unknown. Here, using cryo-fluorescence microscopy-targeted cryo-sectioning, cryo-focused ion beam-scanning electron microscopy lift-out and cryo-electron tomography, we determined in-tissue architectures of β-amyloid and tau pathology in a postmortem Alzheimer's disease donor brain. β-amyloid plaques contained a mixture of fibrils, some of which were branched, and protofilaments, arranged in parallel arrays and lattice-like structures. Extracellular vesicles and cuboidal particles defined the non-amyloid constituents of β-amyloid plaques. By contrast, tau inclusions formed parallel clusters of unbranched filaments. Subtomogram averaging a cluster of 136 tau filaments in a single tomogram revealed the polypeptide backbone conformation and filament polarity orientation of paired helical filaments within tissue. Filaments within most clusters were similar to each other, but were different between clusters, showing amyloid heterogeneity that is spatially organized by subcellular location. The in situ structural approaches outlined here for human donor tissues have applications to a broad range of neurodegenerative diseases.

Silencing Apoe with divalent-siRNAs improves amyloid burden and activates immune response pathways in Alzheimer's disease

INTRODUCTION

The most significant genetic risk factor for late-onset Alzheimer's disease (AD) is APOE4, with evidence for gain- and loss-of-function mechanisms. A clinical need remains for therapeutically relevant tools that potently modulate APOE expression.

METHODS

We optimized small interfering RNAs (di-siRNA, GalNAc) to potently silence brain or liver Apoe and evaluated the impact of each pool of Apoe on pathology.

RESULTS

In adult 5xFAD mice, siRNAs targeting CNS Apoe efficiently silenced Apoe expression and reduced amyloid burden without affecting systemic cholesterol, confirming that potent silencing of brain Apoe is sufficient to slow disease progression. Mechanistically, silencing Apoe reduced APOE-rich amyloid cores and activated immune system responses.

DISCUSSION

These results establish siRNA-based modulation of Apoe as a viable therapeutic approach, highlight immune activation as a key pathway affected by Apoe modulation, and provide the technology to further evaluate the impact of APOE silencing on neurodegeneration.

Role of mechanoregulation in mast cell-mediated immune inflammation of the smooth muscle in the pathophysiology of esophageal motility disorders

Major esophageal disorders involve obstructive transport of bolus to the stomach, causing symptoms of dysphagia and impaired clearing of the refluxed gastric contents. These may occur due to mechanical constriction of the esophageal lumen or loss of relaxation associated with deglutitive inhibition, as in achalasia-like disorders. Recently, immune inflammation has been identified as an important cause of esophageal strictures and the loss of inhibitory neurotransmission. These disorders are also associated with smooth muscle hypertrophy and hypercontractility, whose cause is unknown. This review investigated immune inflammation in the causation of smooth muscle changes in obstructive esophageal bolus transport. Findings suggest that smooth muscle hypertrophy occurs above the obstruction and is due to mechanical stress on the smooth muscles. The mechanostressed smooth muscles release cytokines and other molecules that may recruit and microlocalize mast cells to smooth muscle bundles, so that their products may have a close bidirectional effect on each other. Acting in a paracrine fashion, the inflammatory cytokines induce genetic and epigenetic changes in the smooth muscles, leading to smooth muscle hypercontractility, hypertrophy, and impaired relaxation. These changes may worsen difficulty in the esophageal transport. Immune processes differ in the first phase of obstructive bolus transport, and the second phase of muscle hypertrophy and hypercontractility. Moreover, changes in the type of mechanical stress may change immune response and effect on smooth muscles. Understanding immune signaling in causes of obstructive bolus transport, type of mechanical stress, and associated smooth muscle changes may help pathophysiology-based prevention and targeted treatment of esophageal motility disorders.NEW & NOTEWORTHY Esophageal disorders such as esophageal stricture or achalasia, and diffuse esophageal spasm are associated with smooth muscle hypertrophy and hypercontractility, above the obstruction, yet the cause of such changes is unknown. This review suggests that smooth muscle obstructive disorders may cause mechanical stress on smooth muscle, which then secretes chemicals that recruit, microlocalize, and activate mast cells to initiate immune inflammation, producing functional and structural changes in smooth muscles. Understanding the immune signaling in these changes may help pathophysiology-based prevention and targeted treatment of esophageal motility disorders.

The influence of zwitterionic and anionic phospholipids on protein aggregation

The progressive aggregation of misfolded proteins is the underlying molecular cause of numerous pathologies including Parkinson's disease and injection and transthyretin amyloidosis. A growing body of evidence indicates that protein deposits detected in organs and tissues of patients diagnosed with such pathologies contain fragments of lipid membranes. In vitro experiments also showed that lipid membranes could strongly change the aggregation rate of amyloidogenic proteins, as well as alter the secondary structure and toxicity of oligomers and fibrils formed in their presence. In this review, the effect of large unilamellar vesicles (LUVs) composed of zwitterionic and anionic phospholipids on the aggregation rate of insulin, lysozyme, transthyretin (TTR) and α- synuclein (α-syn) will be discussed. The manuscript will also critically review the most recent findings on the lipid-induced changes in the secondary structure of protein oligomers and fibrils, as well as reveal the extent to which lipids could alter the toxicity of protein aggregates formed in their presence.

Secondary structure and toxicity of lysozyme fibrils are determined by the length and unsaturation of phosphatidic acid

A progressive aggregation of misfolded proteins is a hallmark of numerous pathologies including diabetes Type 2, Alzheimer's disease, and Parkinson's disease. As a result, highly toxic protein aggregates, which are known as amyloid fibrils, are formed. A growing body of evidence suggests that phospholipids can uniquely alter the secondary structure and toxicity of amyloid aggregates. However, the role of phosphatidic acid (PA), a unique lipid that is responsible for cell signaling and activation of lipid-gated ion channels, in the aggregation of amyloidogenic proteins remains unclear. In this study, we investigate the role of the length and degree of unsaturation of fatty acids (FAs) in PA in the structure and toxicity of lysozyme fibrils formed in the presence of this lipid. We found that both the length and saturation of FAs in PA uniquely altered the secondary structure of lysozyme fibrils. However, these structural differences in PA caused very little if any changes in the morphology of lysozyme fibrils. We also utilized cell toxicity assays to determine the extent to which the length and degree of unsaturation of FAs in PA altered the toxicity of lysozyme fibrils. We found that amyloid fibrils formed in the presence of PA with C18:0 FAs exerted significantly higher cell toxicity compared to the aggregates formed in the presence of PA with C16:0 and C18:1 FAs. These results demonstrated that PA can be an important player in the onset and spread of amyloidogenic diseases.

Intermediate Antiparallel β Structure in Amyloid β Plaques Revealed by Infrared Spectroscopic Imaging

Aggregation of amyloid β (Aβ) peptides into extracellular plaques is a hallmark of the molecular pathology of Alzheimer's disease (AD). Amyloid aggregates have been extensively studied in vitro, and it is well-known that mature amyloid fibrils contain an ordered parallel β structure. The structural evolution from unaggregated peptide to fibrils can be mediated through intermediate structures that deviate significantly from mature fibrils, such as antiparallel β-sheets. However, it is currently unknown if these intermediate structures exist in plaques, which limits the translation of findings from in vitro structural characterizations of amyloid aggregates to AD. This arises from the inability to extend common structural biology techniques to ex vivo tissue measurements. Here we report the use of infrared (IR) imaging, wherein we can spatially localize plaques and probe their protein structural distributions with the molecular sensitivity of IR spectroscopy. Analyzing individual plaques in AD tissues, we demonstrate that fibrillar amyloid plaques exhibit antiparallel β-sheet signatures, thus providing a direct connection between in vitro structures and amyloid aggregates in the AD brain. We further validate results with IR imaging of in vitro aggregates and show that the antiparallel β-sheet structure is a distinct structural facet of amyloid fibrils.

Length and saturation of fatty acids in phosphatidylserine determine the rate of lysozyme aggregation simultaneously altering the structure and toxicity of amyloid oligomers and fibrils

Abrupt aggregation of misfolded proteins is the underlying molecular cause of numerous severe pathologies including Alzheimer's and Parkinson's diseases. Protein aggregation yields small oligomers that can later propagate into amyloid fibrils, β-sheet-rich structures with a variety of topologies. A growing body of evidence suggests that lipids play an important role in abrupt aggregation of misfolded proteins. In this study, we investigate the roles of length and saturation of fatty acids (FAs) in phosphatidylserine (PS), an anionic lipid that is responsible for the recognition of apoptotic cells by macrophages, in lysozyme aggregation. We found that both the length and saturation of FAs in PS contribute to the aggregation rate of insulin. PS with 14-carbon-long FAs (14:0) enabled a much stronger acceleration of protein aggregation compared to PS with 18-carbon-long FAs (18:0). Our results demonstrate that the presence of double bonds in FAs accelerated the rate of insulin aggregation relative to PS with fully saturated FAs. Biophysical methods revealed morphological and structural differences in lysozyme aggregates grown in the presence of PS with varying lengths and FA saturation. We also found that such aggregates exerted diverse cell toxicities. These results demonstrate that the length and saturation of FAs in PS can uniquely alter the stability of misfolded proteins on lipid membranes.

Gold nanoparticle-coated apoferritin conductive nanowires

Gold-metallic nanofibrils were prepared from three different iso-apoferritin (APO) proteins with different Light/Heavy (L/H) subunit ratios (from 0% up to 100% L-subunits). We show that APO protein fibrils have the ability to in situ nucleate and grow gold nanoparticles (AuNPs) simultaneously assembled on opposite strands of the fibrils, forming hybrid inorganic-organic metallic nanowires. The AuNPs are arranged following the pitch of the helical APO protein fiber. The mean size of the AuNPs was similar in the three different APO protein fibrils studied in this work. The AuNPs retained their optical properties in these hybrid systems. Conductivity measurements showed ohmic behavior like that of a continuous metallic structure.

Imaging Amyloid-β Membrane Interactions: Ion-Channel Pores and Lipid-Bilayer Permeability in Alzheimer's Disease

The accumulation of the amyloid-β peptides (Aβ) is central to the development of Alzheimer's disease. The mechanism by which Aβ triggers a cascade of events that leads to dementia is a topic of intense investigation. Aβ self-associates into a series of complex assemblies with different structural and biophysical properties. It is the interaction of these oligomeric, protofibril and fibrillar assemblies with lipid membranes, or with membrane receptors, that results in membrane permeability and loss of cellular homeostasis, a key event in Alzheimer's disease pathology. Aβ can have an array of impacts on lipid membranes, reports have included: a carpeting effect; a detergent effect; and Aβ ion-channel pore formation. Recent advances imaging these interactions are providing a clearer picture of Aβ induced membrane disruption. Understanding the relationship between different Aβ structures and membrane permeability will inform therapeutics targeting Aβ cytotoxicity.

Imaging Amyloid-β Membrane Interactions: Ion-Channel Pores and Lipid-Bilayer Permeability in Alzheimer's Disease

The accumulation of the amyloid-β peptides (Aβ) is central to the development of Alzheimer's disease. The mechanism by which Aβ triggers a cascade of events that leads to dementia is a topic of intense investigation. Aβ self-associates into a series of complex assemblies with different structural and biophysical properties. It is the interaction of these oligomeric, protofibril and fibrillar assemblies with lipid membranes, or with membrane receptors, that results in membrane permeability and loss of cellular homeostasis, a key event in Alzheimer's disease pathology. Aβ can have an array of impacts on lipid membranes, reports have included: a carpeting effect; a detergent effect; and Aβ ion-channel pore formation. Recent advances imaging these interactions are providing a clearer picture of Aβ induced membrane disruption. Understanding the relationship between different Aβ structures and membrane permeability will inform therapeutics targeting Aβ cytotoxicity.

The in-tissue molecular architecture of β-amyloid pathology in the mammalian brain

Amyloid plaques composed of Aβ fibrils are a hallmark of Alzheimer's disease (AD). However, the molecular architecture of amyloid plaques in the context of fresh mammalian brain tissue is unknown. Here, using cryogenic correlated light and electron tomography we report the in situ molecular architecture of Aβ fibrils in the AppNL-G-F familial AD mouse model containing the Arctic mutation and an atomic model of ex vivo purified Arctic Aβ fibrils. We show that in-tissue Aβ fibrils are arranged in a lattice or parallel bundles, and are interdigitated by subcellular compartments, extracellular vesicles, extracellular droplets and extracellular multilamellar bodies. The Arctic Aβ fibril differs significantly from an earlier AppNL-F fibril structure, indicating a striking effect of the Arctic mutation. These structural data also revealed an ensemble of additional fibrillar species, including thin protofilament-like rods and branched fibrils. Together, these results provide a structural model for the dense network architecture that characterises β-amyloid plaque pathology.

Intermediate antiparallel beta structure in amyloid plaques revealed by infrared spectroscopic imaging

Aggregation of amyloid beta (Aβ) peptides into extracellular plaques is a hallmark of the molecular pathology of Alzheimer's disease (AD). Amyloid aggregates have been extensively studied in-vitro, and it is well known that mature amyloid fibrils contain an ordered parallel β structure. The structural evolution from unaggregated peptide to fibrils can be mediated through intermediate structures that deviate significantly from mature fibrils, such as antiparallel β-sheets. However, it is currently unknown if these intermediate structures exist in plaques, which limits the translation of findings from in-vitro structural characterizations of amyloid aggregates to AD. This arises from the inability to extend common structural biology techniques to ex-vivo tissue measurements. Here we report the use of infrared (IR) imaging, wherein we can spatially localize plaques and probe their protein structural distributions with the molecular sensitivity of IR spectroscopy. Analyzing individual plaques in AD tissues, we demonstrate that fibrillar amyloid plaques exhibit antiparallel β-sheet signatures, thus providing a direct connection between in-vitro structures and amyloid aggregates in AD brain. We further validate results with IR imaging of in-vitro aggregates and show that antiparallel β-sheet structure is a distinct structural facet of amyloid fibrils.

A coarse-grained molecular dynamics investigation on spontaneous binding of Aβ1-40 fibrils with cholesterol-mixed DPPC bilayers

Alzheimer's disease is the most common form of dementia. Its aetiology is characterized by the misfolding and aggregation of amyloid-β (Aβ) peptides into β-sheet-rich Aβ oligomers/fibrils. Although multiple experimental studies have suggested that Aβ oligomers/fibrils interact with the cell membranes and perturb their structures and dynamics, the molecular mechanism of this interaction is still not fully understood. In the present work, we have performed a total of 120 μs-long simulations to investigate the interaction between trimeric or hexameric Aβ1-40 fibrils with either a 100% DPPC bilayer, a 70% DPPC-30% cholesterol bilayer or a 50% DPPC-50% cholesterol bilayer. Our simulation data capture the spontaneous binding of the aqueous Aβ1-40 fibrils with the membranes and show that the central hydrophobic amino acid cluster, the lysine residue adjacent to it and the C-terminal hydrophobic residues are all involved in the process. Moreover, our data show that while the Aβ1-40 fibril does not bind to the 100% DPPC bilayer, its binding affinity for the membrane increases with the amount of cholesterol. Overall, our data suggest that two clusters of hydrophobic residues and one lysine help Aβ1-40 fibrils establish stable interactions with a cholesterol-rich DPPC bilayer. These residues are likely to represent potential target regions for the design of inhibitors, thus opening new avenues in structure-based drug design against Aβ oligomer/fibril-membrane interaction.

The interactions of amyloid β aggregates with phospholipid membranes and the implications for neurodegeneration

Misfolding, aggregation and accumulation of Amyloid-β peptides (Aβ) in neuronal tissue and extracellular matrix are hallmark features of Alzheimer's disease (AD) pathology. Soluble Aβ oligomers are involved in neuronal toxicity by interacting with the lipid membrane, compromising its integrity, and affecting the function of receptors. These facts indicate that the interaction between Aβ oligomers and cell membranes may be one of the central molecular level factors responsible for the onset of neurodegeneration. The present review provides a structural understanding of Aβ neurotoxicity via membrane interactions and contributes to understanding early events in Alzheimer's disease.

The Potential Roles of Exosomes Carrying APP and Tau Cleavage Products in Alzheimer's Disease

Alzheimer's disease (AD) is the leading cause of dementia throughout the world. It is characterized by major amyloid plaques and neurofibrillary tangles (NFTs), which are composed of amyloid-β (Aβ) peptide and hyperphosphorylated Tau (p-Tau), respectively. Exosomes, which are secreted by cells, are single-membrane lipid bilayer vesicles found in bodily fluids and they have a diameter of 30-150 nm. Recently, they have been considered as critical carriers and biomarkers in AD, as they facilitate communication between cells and tissues by delivering proteins, lipids, and nucleic acids. This review demonstrates that exosomes are natural nanocontainers that carry APP as well as Tau cleavage products secreted by neuronal cells and that their formation is associated with the endosomal-lysosomal pathway. Moreover, these exosomes can transfer AD pathological molecules and participate in the pathophysiological process of AD; therefore, they have potential diagnostic and therapeutic value for AD and might also provide novel insights for screening and prevention of the disease.

Self-Assembly of Amyloid Fibrils into 3D Gel Clusters versus 2D Sheets

The deposition of dense fibril plaques represents the pathological hallmark for a multitude of human disorders, including many neurodegenerative diseases. Fibril plaques are predominately composed of amyloid fibrils, characterized by their underlying cross beta-sheet architecture. Research into the mechanisms of amyloid formation has mostly focused on characterizing and modeling the growth of individual fibrils and associated oligomers from their monomeric precursors. Much less is known about the mechanisms causing individual fibrils to assemble into ordered fibrillar suprastructures. Elucidating the mechanisms regulating this "secondary" self-assembly into distinct suprastructures is important for understanding how individual protein fibrils form the prominent macroscopic plaques observed in disease. Whether and how amyloid fibrils assemble into either 2D or 3D supramolecular structures also relates to ongoing efforts on using amyloid fibrils as substrates or scaffolds for self-assembling functional biomaterials. Here, we investigated the conditions under which preformed amyloid fibrils of a lysozyme assemble into larger superstructures as a function of charge screening or pH. Fibrils either assembled into three-dimensional gel clusters or two-dimensional fibril sheets. The latter displayed optical birefringence, diagnostic of amyloid plaques. We presume that pH and salt modulate fibril charge repulsion, which allows anisotropic fibril-fibril attraction to emerge and drive the transition from 3D to 2D fibril self-assembly.

Secondary Metabolites with Biomedical Applications from Plants of the Sarraceniaceae Family

Carnivorous plants have fascinated researchers and hobbyists for centuries because of their mode of nutrition which is unlike that of other plants. They are able to produce bioactive compounds used to attract, capture and digest prey but also as a defense mechanism against microorganisms and free radicals. The main purpose of this review is to provide an overview of the secondary metabolites with significant biological activity found in the Sarraceniaceae family. The review also underlines the necessity of future studies for the biochemical characterization of the less investigated species. Darlingtonia, Heliamphora and Sarracenia plants are rich in compounds with potential pharmaceutical and medical uses. These belong to several classes such as flavonoids, with flavonol glycosides being the most abundant, monoterpenes, triterpenes, sesquiterpenes, fatty acids, alkaloids and others. Some of them are well characterized in terms of chemical properties and biological activity and have widespread commercial applications. The review also discusses biological activity of whole extracts and commercially available products derived from Sarraceniaceae plants. In conclusion, this review underscores that Sarraceniaceae species contain numerous substances with the potential to advance health. Future perspectives should focus on the discovery of new molecules and increasing the production of known compounds using biotechnological methods.

Faulty autolysosome acidification in Alzheimer's disease mouse models induces autophagic build-up of Aβ in neurons, yielding senile plaques

Autophagy is markedly impaired in Alzheimer's disease (AD). Here we reveal unique autophagy dysregulation within neurons in five AD mouse models in vivo and identify its basis using a neuron-specific transgenic mRFP-eGFP-LC3 probe of autophagy and pH, multiplex confocal imaging and correlative light electron microscopy. Autolysosome acidification declines in neurons well before extracellular amyloid deposition, associated with markedly lowered vATPase activity and build-up of Aβ/APP-βCTF selectively within enlarged de-acidified autolysosomes. In more compromised yet still intact neurons, profuse Aβ-positive autophagic vacuoles (AVs) pack into large membrane blebs forming flower-like perikaryal rosettes. This unique pattern, termed PANTHOS (poisonous anthos (flower)), is also present in AD brains. Additional AVs coalesce into peri-nuclear networks of membrane tubules where fibrillar β-amyloid accumulates intraluminally. Lysosomal membrane permeabilization, cathepsin release and lysosomal cell death ensue, accompanied by microglial invasion. Quantitative analyses confirm that individual neurons exhibiting PANTHOS are the principal source of senile plaques in amyloid precursor protein AD models.

Poloxamer-188 Exacerbates Brain Amyloidosis, Presynaptic Dystrophies, and Pathogenic Microglial Activation in 5XFAD Mice

Background:

Alzheimer’s disease (AD) is initiated by aberrant accumulation of amyloid beta (Aβ) protein in the brain parenchyma. The microenvironment surrounding amyloid plaques is characterized by the swelling of presynaptic terminals (dystrophic neurites) associated with lysosomal dysfunction, microtubule disruption and impaired axonal transport. Aβ-induced plasma membrane damage and calcium influx could be potential mechanisms underlying dystrophic neurite formation.

Objective:

We tested whether promoting membrane integrity by brain administration of a safe FDA approved surfactant molecule poloxamer-188 (P188) could attenuate AD pathology in vivo.

Methods:

Three-month-old 5XFAD male mice were administered several concentrations of P188 in the brain for 42 days with mini-osmotic pumps. After 42 days, mice were euthanized and assessed for amyloid pathology, dystrophic neurites, pathogenic microglia activation, tau phosphorylation and lysosomal / vesicular trafficking markers in the brain.

Results:

P188 was lethal at the highest concentration of 10mM. Lower concentrations of P188 (1.2, 12 and 120μM) were well tolerated. P188 increased brain Aβ burden, potentially through activation of the γ-secretase pathway. Dystrophic neurite pathology was exacerbated in P188 treated mice as indicated by increased LAMP1 accumulation around Aβ deposits. Pathogenic microglial activation was increased by P188. Total tau levels were decreased by P188. Lysosomal enzyme cathepsin D and calcium-dependent vesicular trafficking regulator synaptotagmin-7 (SYT7) were dysregulated upon P188 administration.

Conclusion:

P188 brain delivery exacerbated amyloid pathology, dystrophic neurites and pathogenic microglial activation in 5XFAD mice. These effects correlated with lysosomal dysfunction and dysregulation of plasma membrane vesicular trafficking. P188 is not a promising therapeutic strategy against AD pathogenesis.

A unifying framework for amyloid-mediated membrane damage: The lipid-chaperone hypothesis

Over the past thirty years, researchers have highlighted the role played by a class of proteins or polypeptides that forms pathogenic amyloid aggregates in vivo, including i) the amyloid Aβ peptide, which is known to form senile plaques in Alzheimer's disease; ii) α-synuclein, responsible for Lewy body formation in Parkinson's disease and iii) IAPP, which is the protein component of type 2 diabetes-associated islet amyloids. These proteins, known as intrinsically disordered proteins (IDPs), are present as highly dynamic conformational ensembles. IDPs can partially (mis) fold into (dys) functional conformations and accumulate as amyloid aggregates upon interaction with other cytosolic partners such as proteins or lipid membranes. In addition, an increasing number of reports link the toxicity of amyloid proteins to their harmful effects on membrane integrity. Still, the molecular mechanism underlying the amyloidogenic proteins transfer from the aqueous environment to the hydrocarbon core of the membrane is poorly understood. This review starts with a historical overview of the toxicity models of amyloidogenic proteins to contextualize the more recent lipid-chaperone hypothesis. Then, we report the early molecular-level events in the aggregation and ion-channel pore formation of Aβ, IAPP, and α-synuclein interacting with model membranes, emphasizing the complexity of these processes due to their different spatial-temporal resolutions. Next, we underline the need for a combined experimental and computational approach, focusing on the strengths and weaknesses of the most commonly used techniques. Finally, the last two chapters highlight the crucial role of lipid-protein complexes as molecular switches among ion-channel-like formation, detergent-like, and fibril formation mechanisms and their implication in fighting amyloidogenic diseases.

Origin, toxicity and characteristics of two amyloid oligomer polymorphs

There is compelling evidence that small oligomeric aggregates, emerging during the assembly of amyloid fibrils and plaques, are important molecular pathogens in many amyloid diseases. While significant progress has been made in revealing the mechanisms underlying fibril growth, understanding how amyloid oligomers fit into the fibril assembly process, and how they contribute to the pathogenesis of amyloid diseases, has remained elusive. Commonly, amyloid oligomers are considered to be metastable, early-stage precursors to fibril formation that are either on- or off-pathway from fibril growth. In addition, amyloid oligomers have been reported to colocalize with late-stage fibrils and plaques. Whether these early and late-stage oligomer species are identical or distinct, and whether both are relevant to pathogenesis remains unclear. Here we report on the formation of two distinct oligomer species of lysozyme, formed either during the early or late-stages of in vitro fibril growth. We further observe that the pH change from in vitro growth conditions to cell media used for toxicity studies induced distinct mesoscopic precipitates, two of which resemble either diffuse or neuritic plaques seen in Alzheimer's histology. Our biophysical characterization indicates that both oligomer species share morphological and tinctorial features considered characteristic for amyloid oligomers. At the same time, their sizes, morphologies, their immunostaining, detailed tinctorial profiles and, most prominently, their biological activity are clearly distinct from each other. Probing the conditions promoting the formation of these two distinct oligomer species suggests distinct roles of charge interactions, hydrophobicity and monomer flexibility in directing oligomer assembly.

Antimicrobial α-defensins as multi-target inhibitors against amyloid formation and microbial infection

Amyloid aggregation and microbial infection are considered as pathological risk factors for developing amyloid diseases, including Alzheimer's disease (AD), type II diabetes (T2D), Parkinson's disease (PD), and medullary thyroid carcinoma (MTC). Due to the multifactorial nature of amyloid diseases, single-target drugs and treatments have mostly failed to inhibit amyloid aggregation and microbial infection simultaneously, thus leading to marginal benefits for amyloid inhibition and medical treatments. Herein, we proposed and demonstrated a new "anti-amyloid and antimicrobial hypothesis" to discover two host-defense antimicrobial peptides of α-defensins containing β-rich structures (human neutrophil peptide of HNP-1 and rabbit neutrophil peptide of NP-3A), which have demonstrated multi-target, sequence-independent functions to (i) prevent the aggregation and misfolding of different amyloid proteins of amyloid-β (Aβ, associated with AD), human islet amyloid polypeptide (hIAPP, associated with T2D), and human calcitonin (hCT, associated with MTC) at sub-stoichiometric concentrations, (ii) reduce amyloid-induced cell toxicity, and (iii) retain their original antimicrobial activity upon the formation of complexes with amyloid peptides. Further structural analysis showed that the sequence-independent amyloid inhibition function of α-defensins mainly stems from their cross-interactions with amyloid proteins via β-structure interactions. The discovery of antimicrobial peptides containing β-structures to inhibit both microbial infection and amyloid aggregation greatly expands the new therapeutic potential of antimicrobial peptides as multi-target amyloid inhibitors for better understanding pathological causes and treatments of amyloid diseases.

3D-visualization of amyloid-β oligomer interactions with lipid membranes by cryo-electron tomography

Amyloid-β (Aβ) assemblies have been shown to bind to lipid bilayers. This can disrupt membrane integrity and cause a loss of cellular homeostasis, that triggers a cascade of events leading to Alzheimer's disease. However, molecular mechanisms of Aβ cytotoxicity and how the different assembly forms interact with the membrane remain enigmatic. Here we use cryo-electron tomography (cryoET) to obtain three-dimensional nano-scale images of various Aβ assembly types and their interaction with liposomes. Aβ oligomers and curvilinear protofibrils bind extensively to the lipid vesicles, inserting and carpeting the upper-leaflet of the bilayer. Aβ oligomers concentrate at the interface of vesicles and form a network of Aβ-linked liposomes, while crucially, monomeric and fibrillar Aβ have relatively little impact on the membrane. Changes to lipid membrane composition highlight a significant role for GM1-ganglioside in promoting Aβ-membrane interactions. The different effects of Aβ assembly forms observed align with the highlighted cytotoxicity reported for Aβ oligomers. The wide-scale incorporation of Aβ oligomers and curvilinear protofibrils into the lipid bilayer suggests a mechanism by which membrane integrity is lost.

Cryo-electron tomography 3D imaging of amyloid-β oligomers carpeting the surface of lipid bilayers in near native conditions.

Melatonin Rescues the Dendrite Collapse Induced by the Pro-Oxidant Toxin Okadaic Acid in Organotypic Cultures of Rat Hilar Hippocampus

The pro-oxidant compound okadaic acid (OKA) mimics alterations found in Alzheimer's disease (AD) as oxidative stress and tau hyperphosphorylation, leading to neurodegeneration and cognitive decline. Although loss of dendrite complexity occurs in AD, the study of this post-synaptic domain in chemical-induced models remains unexplored. Moreover, there is a growing expectation for therapeutic adjuvants to counteract these brain dysfunctions. Melatonin, a free-radical scavenger, inhibits tau hyperphosphorylation, modulates phosphatases, and strengthens dendritic arbors. Thus, we determined if OKA alters the dendritic arbors of hilar hippocampal neurons and whether melatonin prevents, counteracts, or reverses these damages. Rat organotypic cultures were incubated with vehicle, OKA, melatonin, and combined treatments with melatonin either before, simultaneously, or after OKA. DNA breaks were assessed by TUNEL assay and nuclei were counterstained with DAPI. Additionally, MAP2 was immunostained to assess the dendritic arbor properties by the Sholl method. In hippocampal hilus, OKA increased DNA fragmentation and reduced the number of MAP2(+) cells, whereas melatonin protected against oxidation and apoptosis. Additionally, OKA decreased the dendritic arbor complexity and melatonin not only counteracted, but also prevented and reversed the dendritic arbor retraction, highlighting its role in post-synaptic domain integrity preservation against neurodegenerative events in hippocampal neurons.

Aβ Plaques

Aβ plaques are one of the two lesions in the brain that define the neuropathological diagnosis of Alzheimer's disease. Plaques are highly diverse structures; many of them include massed, fibrillar polymers of the Aβ protein referred to as Aβ-amyloid, but some lack the defining features of amyloid. Cellular elements in 'classical' plaques include abnormal neuronal processes and reactive glial cells, but these are not present in all plaques. Plaques have been given various names since their discovery in 1892, including senile plaques, amyloid plaques, and neuritic plaques. However, with the identification in the 1980s of Aβ as the obligatory and universal component of plaques, the term 'Aβ plaques' has become a unifying term for these heterogeneous formations. Tauopathy, the second essential lesion of the Alzheimer's disease diagnostic dyad, is downstream of Aβ-proteopathy, but it is critically important for the manifestation of dementia. The etiologic link between Aβ-proteopathy and tauopathy in Alzheimer's disease remains largely undefined. Aβ plaques develop and propagate via the misfolding, self-assembly and spread of Aβ by the prion-like mechanism of seeded protein aggregation. Partially overlapping sets of risk factors and sequelae, including inflammation, genetic variations, and various environmental triggers have been linked to plaque development and idiopathic Alzheimer's disease, but no single factor has emerged as a requisite cause. The value of Aβ plaques per se as therapeutic targets is uncertain; although some plaques are sites of focal gliosis and inflammation, the complexity of inflammatory biology presents challenges to glia-directed intervention. Small, soluble, oligomeric assemblies of Aβ are enriched in the vicinity of plaques, and these probably contribute to the toxic impact of Aβ aggregation on the brain. Measures designed to reduce the production or seeded self-assembly of Aβ can impede the formation of Aβ plaques and oligomers, along with their accompanying abnormalities; given the apparent long timecourse of the emergence, maturation and proliferation of Aβ plaques in humans, such therapies are likely to be most effective when begun early in the pathogenic process, before significant damage has been done to the brain. Since their discovery in the late 19th century, Aβ plaques have, time and again, illuminated fundamental mechanisms driving neurodegeneration, and they should remain at the forefront of efforts to understand, and therefore treat, Alzheimer's disease.

Polypeptides derived from α-Synuclein binding partners to prevent α-Synuclein fibrils interaction with and take-up by cells

α-Synuclein (αSyn) fibrils spread from one neuronal cell to another. This prion-like phenomenon is believed to contribute to the progression of the pathology in Parkinson’s disease and other synucleinopathies. The binding of αSyn fibrils originating from affected cells to the plasma membrane of naïve cells is key in their prion-like propagation propensity. To interfere with this process, we designed polypeptides derived from proteins we previously showed to interact with αSyn fibrils, namely the molecular chaperone Hsc70 and the sodium/potassium pump NaK-ATPase and assessed their capacity to bind αSyn fibrils and/or interfere with their take-up by cells of neuronal origin. We demonstrate here that polypeptides that coat αSyn fibrils surfaces in such a way that they are changed affect αSyn fibrils binding to the plasma membrane components and/or their take-up by cells. Altogether our observations suggest that the rationale design of αSyn fibrils polypeptide binders that interfere with their propagation between neuronal cells holds therapeutic potential.

The crosstalk: exosomes and lipid metabolism

Exosomes have been considered as novel and potent vehicles of intercellular communication, instead of "cell dust". Exosomes are consistent with anucleate cells, and organelles with lipid bilayer consisting of the proteins and abundant lipid, enhancing their "rigidity" and "flexibility". Neighboring cells or distant cells are capable of exchanging genetic or metabolic information via exosomes binding to recipient cell and releasing bioactive molecules, such as lipids, proteins, and nucleic acids. Of note, exosomes exert the remarkable effects on lipid metabolism, including the synthesis, transportation and degradation of the lipid. The disorder of lipid metabolism mediated by exosomes leads to the occurrence and progression of diseases, such as atherosclerosis, cancer, non-alcoholic fatty liver disease (NAFLD), obesity and Alzheimer's diseases and so on. More importantly, lipid metabolism can also affect the production and secretion of exosomes, as well as interactions with the recipient cells. Therefore, exosomes may be applied as effective targets for diagnosis and treatment of diseases. Video abstract.

Neuropathological assessment of the Alzheimer spectrum

Alzheimer disease (AD), the most common form of dementia globally, classically defined a clinicopathological entity, is a heterogenous disorder with various pathobiological subtypes, currently referred to as Alzheimer continuum. Its morphological hallmarks are extracellular parenchymal β-amyloid (amyloid plaques) and intraneuronal (tau aggregates forming neurofibrillary tangles) lesions accompanied by synaptic loss and vascular amyloid deposits, that are essential for the pathological diagnosis of AD. In addition to "classical" AD, several subtypes with characteristic regional patterns of tau pathology have been described that show distinct clinical features, differences in age, sex distribution, biomarker levels, and patterns of key network destructions responsible for cognitive decline. AD is a mixed proteinopathy (amyloid and tau), frequently associated with other age-related co-pathologies, such as cerebrovascular lesions, Lewy and TDP-43 pathologies, hippocampal sclerosis, or argyrophilic grain disease. These and other co-pathologies essentially influence the clinical picture of AD and may accelerate disease progression. The purpose of this review is to provide a critical overview of AD pathology, its defining pathological substrates, and the heterogeneity among the Alzheimer spectrum entities that may provide a broader diagnostic coverage of this devastating disorder as a basis for implementing precision medicine approaches and for ultimate development of successful disease-modifying drugs for AD.

Functionalized Mesoporous Silicas Direct Structural Polymorphism of Amyloid-β Fibrils

The aggregation of amyloid-β (Aβ) is associated with the onset of Alzheimer's disease (AD) and involves a complex kinetic pathway as monomers self-assemble into fibrils. A central feature of amyloid fibrils is the existence of multiple structural polymorphs, which complicates the development of disease-relevant structure-function relationships. Developing these relationships requires new methods to control fibril structure. In this work, we evaluated the effect that mesoporous silicas (SBA-15) functionalized with hydrophobic (SBA-PFDTS) and hydrophilic groups (SBA-PEG) have on the aggregation kinetics and resulting structure of Aβ_1-40 fibrils. The hydrophilic SBA-PEG had little effect on amyloid kinetics, while as-synthesized and hydrophobic SBA-PFDTS accelerated aggregation kinetics. Subsequently, we quantified the relative population of fibril structures formed in the presence of each material using electron microscopy. Fibrils formed from Aβ_1-40 exposed to SBA-PEG were structurally similar to control fibrils. In contrast, Aβ_1-40 incubated with SBA-15 or SBA-PFDTS formed fibrils with shorter crossover distances that were more structurally representative of fibrils found in AD patient derived samples. Overall, our results suggest that mesoporous silicas and other exogenous materials are promising scaffolds for the de novo production of specific fibril polymorphs of Aβ_1-40 and other amyloidogenic proteins.

Three-dimensional real time imaging of amyloid β aggregation on living cells

Alzheimer’s disease (AD) is a progressive disorder of the brain that gradually decreases thinking, memory, and language abilities. The aggregation process of amyloid β (Aβ) is a key step in the expression of its neurocytotoxicity and development of AD because Aβ aggregation and accumulation around neuronal cells induces cell death. However, the molecular mechanism underlying the neurocytotoxicity and cell death by Aβ aggregation has not been clearly elucidated. In this study, we successfully visualized real-time process of Aβ₄₂ aggregation around living cells by applying our established QD imaging method. 3D observations using confocal laser microscopy revealed that Aβ₄₂ preferentially started to aggregate at the region where membrane protrusions frequently formed. Furthermore, we found that inhibition of actin polymerization using cytochalasin D reduced aggregation of Aβ₄₂ on the cell surface. These results indicate that actin polymerization-dependent cell motility is responsible for the promotion of Aβ₄₂ aggregation at the cell periphery. 3D observation also revealed that the aggregates around the cell remained in that location even if cell death occurred, implying that amyloid plaques found in the AD brain grew from the debris of dead cells that accumulated Aβ₄₂ aggregates.

The Structural Basis of Amyloid Strains in Alzheimer's Disease

Amyloid fibrils represent one of the defining features of Alzheimer's disease (AD). They are made up of protofilaments composed of amyloid β (Aβ) peptides that are held together with extraordinary stability by a network of tight steric zippers and axial hydrogen bonds. This review explores the hypothesis that the peptide conformation within a protofilament represents the physical embodiment of a "strain" of AD. Evidence suggests that within a single strain the fold of individual peptides is invariant. However, the fibrils are capable of structural polymorphism that includes variation in the arrangement of protofilaments into fibrils, the pitch of the resultant fibrils, and the higher-order organization of the plaques into which they aggregate. These intrastrain polymorphisms are separated by low energy barriers, allowing multiple configurations to coexist within a single preparation or tissue. Clinical presentation of different strains may be determined by variation in the way different protofilament structures generate the relevant toxic species, be they monomers, oligomers, or higher-order structures. Evidence reviewed here is consistent with a model in which disease progression is concomitant with a gradual, progressive annealing of amyloid fibrils from benign, loosely packed structures into dense neurotoxic aggregates. This model challenges the commonly held hypothesis that oligomers of Aβ peptides are the only active proximate species in neurodegeneration. However, the data do not implicate fibrils themselves. Rather, they cast suspicion on larger-scale supramolecular aggregates as toxic agents. Electron tomography of amyloid plaques in situ strongly suggests that the formation of amyloid aggregates results in perturbation of the cellular membrane integrity, warranting further investigation of this as a potential mode of neurotoxicity. If dense supramolecular amyloid aggregates prove to be important agents of neurodegeneration in AD, this model may also have relevance to other forms of amyloidoses.

Label-Free SERS Strategy for In Situ Monitoring and Real-Time Imaging of Aβ Aggregation Process in Live Neurons and Brain Tissues

The aggregation of Aβ has been reported to closely correlate with Alzheimer's disease (AD). However, clear monitoring of the entire aggregation process of Aβ from monomer to fibril has been scarcely reported until now. Herein, we developed a label-free ratiometric surface enhanced Raman spectroscopic (SERS) platform for real-time monitoring of the entire process of Aβ aggregation in neurons and brain tissues. Different gold nanoparticles, generated in situ with Aβ monomer and fibril as templates separately, were served as effective SERS substrates to achieve a high sensitivity with a limit of detection (LOD) down to 70 ± 4 pM and 3.0 ± 0.5 pM for Aβ₄₀ monomer and fibrils, respectively. Besides, the introduction of ratiometric determination of Aβ monomer and fibril (I₁₂₄₄/I₁₂₆₈) realized real-time monitoring of the entire aggregation process of Aβ monomer with high accuracy and selectivity against other proteins and amino acids. The significant analytical performance of the developed platform, together with good biocompatibility, long-term stability, and remarkable spatial resolution, enabled the present SERS platform imaging and real-time monitoring and imaging of Aβ aggregation influenced by different metal ions (Cu²⁺, Zn²⁺, and Fe³⁺) in neurons and brain tissues at the single cell level. Our results suggested that Cu²⁺ and Zn²⁺ ion of low concentration (10 μM) promoted fibril formation, while Fe³⁺ and Zn²⁺ of high concentration (100 μM) showed inhibition of fibrosis.

Stability Analysis of a Bulk-Surface Reaction Model for Membrane Protein Clustering

Protein aggregation on the plasma membrane (PM) is of critical importance to many cellular processes such as cell adhesion, endocytosis, fibrillar conformation, and vesicle transport. Lateral diffusion of protein aggregates or clusters on the surface of the PM plays an important role in governing their heterogeneous surface distribution. However, the stability behavior of the surface distribution of protein aggregates remains poorly understood. Therefore, understanding the spatial patterns that can emerge on the PM solely through protein-protein interaction, lateral diffusion, and feedback is an important step toward a complete description of the mechanisms behind protein clustering on the cell surface. In this work, we investigate the pattern formation of a reaction-diffusion model that describes the dynamics of a system of ligand-receptor complexes. The purely diffusive ligand in the cytosol can bind receptors in the PM and the resultant ligand-receptor complexes not only diffuse laterally but can also form clusters resulting in different oligomers. Finally, the largest oligomers recruit ligands from the cytosol using positive feedback. From a methodological viewpoint, we provide theoretical estimates for diffusion-driven instabilities of the protein aggregates based on the Turing mechanism. Our main result is a threshold phenomenon, in which a sufficiently high recruitment of ligands promotes the input of new monomeric components and consequently drives the formation of a single-patch spatially heterogeneous steady state.

Neuropathology of the Alzheimer's continuum: an update

Alzheimer's disease (AD), the most common form of dementia worldwide, is a mixed proteinopathy (amyloid and tau). Originally defined as a clinicopathological entity, it is a heterogenous, multifactorial disorder, currently referred to as the Alzheimer's continuum. Its cardinal pathological features are extracellular β-amyloid (amyloid plaques) and intraneuronal tau aggregates forming neurofibrillary tangles, which are accompanied by vascular amyloid deposits (cerebral amyloid angiopathy), synapse and neuronal loss, as well as neuroinflammation and reactive astrogliosis. In addition to "typical" AD, various subtypes with characteristic regional patterns of tau pathology have been described that show distinct clinical features, biomarker levels, and patterns of key network destructions responsible for cognitive decline. AD is frequently associated with other age-related changes including Lewy and TDP-43 pathologies, hippocampal sclerosis, argyrophilic grain disease, cerebrovascular lesions, and others. These additional pathologies influence the clinical picture of AD, may accelerate disease progression, and can cause a number of challenges in our understanding of the disease including the threshold of each individual pathology to cause dementia and the possibility of underlying common etiologies. This article provides an up-to-date overview of AD neuropathology, its heterogeneity, and additional pathologies in order to explain the difficulties in the diagnosis and the failure of clinical trials in AD patients.

Unobstructed Multiscale Imaging of Tissue Sections for Ultrastructural Pathology Analysis by Backscattered Electron Scanning Microscopy

Ultrastructural analysis of healthy, diseased, or experimental tissues is essential in diagnostic and investigative pathology. Evaluation of large tissue areas with suborganelle resolution is challenging because biological structures ranging from several millimeters to nanometers in size need to be identified and imaged while maintaining context over multiple scales. Imaging with field emission scanning electron microscopes (FE-SEMs) is uniquely suited for this task. We describe an efficient workflow for the preparation and unobstructed multiscale imaging of tissue sections with backscattered electron scanning electron microscopy (BSE-SEM) for applications in ultrastructural pathology. We demonstrate that a diverse range of tissues, processed by conventional electron microscopy protocols and avoiding the use of mordanting agents, can be imaged on standard glass slides over multiple scales, from the histological to the ultrastructural level, without any visual obstructions. Our workflow takes advantage of the very large scan fields possible with modern FE-SEMs that allow for the acquisition of wide-field overview images which can be explored at the ultrastructural level by digitally zooming into the images. Examples from applications in pulmonary research and neuropathology demonstrate the versatility and efficiency of this method. This BSE-SEM-based multiscale imaging procedure promises to substantially simplify and accelerate ultrastructural tissue analysis in pathology.

The neuropathological diagnosis of Alzheimer's disease

Alzheimer's disease is a progressive neurodegenerative disease most often associated with memory deficits and cognitive decline, although less common clinical presentations are increasingly recognized. The cardinal pathological features of the disease have been known for more than one hundred years, and today the presence of these amyloid plaques and neurofibrillary tangles are still required for a pathological diagnosis. Alzheimer's disease is the most common cause of dementia globally. There remain no effective treatment options for the great majority of patients, and the primary causes of the disease are unknown except in a small number of familial cases driven by genetic mutations. Confounding efforts to develop effective diagnostic tools and disease-modifying therapies is the realization that Alzheimer's disease is a mixed proteinopathy (amyloid and tau) frequently associated with other age-related processes such as cerebrovascular disease and Lewy body disease. Defining the relationships between and interdependence of various co-pathologies remains an active area of investigation. This review outlines etiologically-linked pathologic features of Alzheimer's disease, as well as those that are inevitable findings of uncertain significance, such as granulovacuolar degeneration and Hirano bodies. Other disease processes that are frequent, but not inevitable, are also discussed, including pathologic processes that can clinically mimic Alzheimer's disease. These include cerebrovascular disease, Lewy body disease, TDP-43 proteinopathies and argyrophilic grain disease. The purpose of this review is to provide an overview of Alzheimer's disease pathology, its defining pathologic substrates and the related pathologies that can affect diagnosis and treatment.

Supervised learning to quantify amyloidosis in whole brains of an Alzheimer's disease mouse model acquired with optical projection tomography

Alzheimer's disease (AD) is characterized by amyloidosis of brain tissues. This phenomenon is studied with genetically-modified mouse models. We propose a method to quantify amyloidosis in whole 5xFAD mouse brains, a model of AD. We use optical projection tomography (OPT) and a random forest voxel classifier to segment and measure amyloid plaques. We validate our method in a preliminary cross-sectional study, where we measure 6136 ± 1637, 8477 ± 3438, and 17267 ± 4241 plaques (AVG ± SD) at 11, 17, and 31 weeks. Overall, this method can be used in the evaluation of new treatments against AD.

Repeat domain-associated O-glycans govern PMEL fibrillar sheet architecture

PMEL is a pigment cell-specific protein that forms a functional amyloid matrix in melanosomes. The matrix consists of well-separated fibrillar sheets on which the pigment melanin is deposited. Using electron tomography, we demonstrate that this sheet architecture is governed by the PMEL repeat (RPT) domain, which associates with the amyloid as an accessory proteolytic fragment. Thus, the RPT domain is dispensable for amyloid formation as such but shapes the morphology of the matrix, probably in order to maximize the surface area available for pigment adsorption. Although the primary amino acid sequence of the RPT domain differs vastly among various vertebrates, we show that it is a functionally conserved, interchangeable module. RPT domains of all species are predicted to be very highly O-glycosylated, which is likely the common defining feature of this domain. O-glycosylation is indeed essential for RPT domain function and the establishment of the PMEL sheet architecture. Thus, O-glycosylation, not amino acid sequence, appears to be the major factor governing the characteristic PMEL amyloid morphology.

Amyloid-β oligomers have a profound detergent-like effect on lipid membrane bilayers, imaged by atomic force and electron microscopy

The ability of amyloid-β peptide (Aβ) to disrupt membrane integrity and cellular homeostasis is believed to be central to Alzheimer's disease pathology. Aβ is reported to have various impacts on the lipid bilayer, but a clearer picture of Aβ influence on membranes is required. Here, we use atomic force and transmission electron microscopies to image the impact of different isolated Aβ assembly types on lipid bilayers. We show that only oligomeric Aβ can profoundly disrupt the bilayer, visualized as widespread lipid extraction and subsequent deposition, which can be likened to an effect expected from the action of a detergent. We further show that Aβ oligomers cause widespread curvature and discontinuities within lipid vesicle membranes. In contrast, this detergent-like effect was not observed for Aβ monomers and fibers, although Aβ fibers did laterally associate and embed into the upper leaflet of the bilayer. The marked impact of Aβ oligomers on membrane integrity identified here reveals a mechanism by which these oligomers may be cytotoxic.

Cell assay for the identification of amyloid inhibitors in systemic AA amyloidosis

Systemic AA amyloidosis is still, up to this day, a life-threatening complication of chronic inflammatory diseases. Despite the success of anti-inflammatory treatment, the prognosis of some AA patients is still poor, which is why therapies directed at the amyloidogenic pathway in AA amyloidosis are being sought after. The cell culture model of amyloid formation from serum amyloid A1 (SAA1) protein remodels crucial features of AA amyloid deposit formation in vivo. We here demonstrate how the cell model can be utilized for the identification of compounds with amyloid inhibitory activity. Out of five compounds previously reported to inhibit self-assembly of various amyloidogenic proteins, we found that epigallocatechin gallate (EGCG) inhibited the formation of SAA1-derived fibrils in cell culture. From a series of compounds targeting the protein quality control machinery, the autophagy inhibitor wortmannin reduced amyloid formation, while the other tested compounds did not lead to a substantial reduction of the amyloid load. These data suggest that amyloid formation can be targeted not only via the protein self-assembly pathway directly, but also by treatment with compounds that impact the cellular protein machinery.

Congo Red and amyloids: history and relationship

Staining with Congo Red (CR) is a qualitative method used for the identification of amyloids in vitro and in tissue sections. However, the drawbacks and artefacts obtained when using this dye can be found both in vitro and in vivo. Analysis of scientific data from previous studies shows that CR staining alone is not sufficient for confirmation of the amyloid nature of protein aggregates in vitro or for diagnosis of amyloidosis in tissue sections. In the present paper, we describe the characteristics and limitations of other methods used for amyloid studies. Our historical review on the use of CR staining for amyloid studies may provide insight into the pitfalls and caveats related to this technique for researchers considering using this dye.