Pore-forming proteins share structural and functional homology with amyloid oligomers

Amyloid oligomers and their membrane toxicity - A perspective study

Amyloidosis is a condition involving a disparate group of pathologies characterized by the extracellular deposition of insoluble fibrils composed of broken-down proteins. These proteins can accumulate locally, causing peculiar symptoms, or in a widespread way, involving many organs and. causing severe systemic failure. The damage that is created is related not only to the accumulation of. amyloid fibrils but above all to the precursor oligomers of the fibrils that manage to enter the cell in a very particular way. This article analyzes the current state of research related to the entry of these oligomers into the cell membrane and the theories related to their toxicity. The paper proposed here not only aims to review the contents in the literature but also proposes a new vision of amyloid toxicity. that could occur in a multiphase process catalyzed by the cell membrane itself. In this process, the denaturation of the lipid bilayer is followed by the stabilization of a pore through energetically favorable self-assembly processes which are achieved through particular oligomeric structures.

Roles of Apigenin and Nepetin in the Assembly Behavior and Cytotoxicity of Prion Neuropeptide PrP106-126

Development of potential inhibitors to prevent prion protein (PrP) fibrillation is a therapeutic strategy for prion diseases. The prion neuropeptide PrP106-126, a research model of abnormal PrP (PrPSc), presents similar physicochemical and biochemical characters to PrPSc, which is also a target of potential inhibitors against prion deposition. Many flavones have antioxidant, anti-inflammatory, and antibacterial properties, and they are applied in treating prion disorder and other amyloidosis as well. However, the inhibition mechanism of flavones on PrP106-126 fibrillation is still unclear. In the current work, apigenin and nepetin were used to suppress the aggregation of PrP106-126 and to alleviate the peptide-induced cytotoxicity. The results showed that apigenin and nepetin impeded the fibril formation of PrP106-126 and depolymerized the preformed fibrils. They were bound to PrP106-126 predominantly by hydrophobic and hydrogen bonding interactions. In addition, both flavones upregulated cell viability and decreased membrane leakage through reducing peptide oligomerization. The differences in inhibition and cell protection between the two small molecules were presumably attributed to the substitution of hydroxyl and methoxy groups in nepetin, which demonstrated the significant structure-function relationship of flavones with prion neuropeptide and the prospect of flavonoids as drug candidates against prion diseases.

P. mirabilis-derived pore-forming haemolysin, HpmA drives intestinal alpha-synuclein aggregation in a mouse model of neurodegeneration

BACKGROUND

Recent studies suggesting the importance of the gut-microbiome in intestinal aggregated alpha synuclein (α-syn) have led to the exploration of the possible role of the gut-brain axis in central nervous system degeneration. Proteus mirabilis (P. mirabilis), a gram-negative facultative anaerobic bacterium, has been linked to brain neurodegeneration in animal studies. We hypothesised that P. mirabilis-derived virulence factors aggregate intestinal α-synuclein and could prompt the pathogenesis of dopaminergic neurodegeneration in the brain.

METHODS

We used vagotomised- and antibiotic-treated male murine models to determine the pathogenesis of P. mirabilis during brain neurodegeneration. The neurodegenerative factor that is driven by P. mirabilis was determined using genetically mutated P. mirabilis. The pathological functions and interactions of the virulence factors were determined in vitro.

FINDINGS

The results showed that P. mirabilis-induced motor dysfunction and neurodegeneration are regulated by intestinal α-syn aggregation in vagotomised- or antibiotic-treated murine models. We deduced that the specific virulence factor, haemolysin A (HpmA), plays a role in the pathogenesis of P. mirabilis. HpmA is involved in α-synuclein oligomerisation and membrane pore formation, resulting in the activation of mTOR-mediated autophagy signalling in intestinal neuroendocrine cells.

INTERPRETATION

Taken together, the results of the present study suggest that HpmA can interact with α-syn and act as a possible indicator of brain neurodegenerative diseases that are induced by P. mirabilis.

FUNDING

This study was supported by a grant from the National Research Foundation of Korea.

Putative Structures of Membrane-Embedded Amyloid β Oligomers

Perturbation of cell membranes by amyloid β (Ab) peptide oligomers is one possible mechanism of cytotoxicity in Alzheimer's disease, but the structure of such Ab-membrane complexes is unknown. Here we examine the stability of several putative structures by implicit membrane and all-atom molecular dynamics simulations. The structures include (a) a variety of models proposed by other researchers in the past, (b) a heptameric β barrel determined by grafting the Ab sequence onto α-hemolysin, (c) a similar structure with modified strand orientation and turn location based on an experimental β-hairpin structure, (d) oligomers inserting C-terminal β hairpins into one leaflet of the bilayer, (e) oligomers forming parallel C-terminal β barrels, and (f) a helical hexamer made of C-terminal fragments. The α-hemolysin-grafted structure and its alternately oriented variant are stable in the membrane and form an aqueous pore. In contrast, the C-terminal parallel barrels are not stable, presumably due to excessive hydrophobicity of their inner surface. The helical hexamer also failed to stabilize an aqueous pore for the same reason. The C-terminal hairpin-inserting structures remain stably inserted but, again, do not form an aqueous pore. Our results suggest that only β-barrels inserting a combination of C-terminal and other residues can form stable aqueous pores.

Conformation-dependent anti-Aβ monoclonal antibody signatures of disease status and severity in urine of women with preeclampsia

Prior research has shown that urine of women with preeclampsia (PE) contains amyloid-like aggregates that are congophilic (exhibit affinity for the amyloidophilic dye Congo red) and immunoreactive with A11, a polyclonal serum against prefibrillar β-amyloid oligomers, thereby supporting pathogenic similarity between PE and protein conformational disorders such as Alzheimer's and prion disease. The objective of this study was to interrogate PE urine using monoclonal antibodies with previously characterized A11-like epitopes. Over 100 conformation-dependent monoclonals were screened and three (mA11-09, mA11-89, and mA11-205) selected for further confirmation in 196 urine samples grouped as follows: severe features PE (sPE, n = 114), PE without severe features (mPE, n = 30), chronic hypertension (crHTN, n = 14) and normotensive pregnant control (P-CRL, n = 38). We showed that the selected conformation-specific monoclonals distinguished among patients with varying severities of PE from P-CRL and patients with crHTN. By use of latent class analysis (LCA) we identified three classes of subjects: Class 1 (n = 94) comprised patients whose urine was both congophilic and reactive with the monoclonals. These women were more likely diagnosed with early-onset sPE and had severe hypertension and proteinuria; Class 2 patients (n = 55) were negative for congophilia and against the antibodies. These were predominantly P-CRL and crHTN patients. Lastly, Class 3 patients (n = 48) were positive for urine congophilia, albeit at lower intensity, but negative for monoclonal immunoreactivities. These women were diagnosed primarily as mPE or late-onset sPE. Collectively, our study validates conformation-dependent Aβ imunoreactivity of PE urine which in conjunction to urine congophilia may represent an additional indicator of disease severity.

Synaptic dysregulation and hyperexcitability induced by intracellular amyloid beta oligomers

Intracellular amyloid beta oligomer (iAβo) accumulation and neuronal hyperexcitability are two crucial events at early stages of Alzheimer's disease (AD). However, to date, no mechanism linking iAβo with an increase in neuronal excitability has been reported. Here, the effects of human AD brain-derived (h-iAβo) and synthetic (iAβo) peptides on synaptic currents and action potential firing were investigated in hippocampal neurons. Starting from 500 pM, iAβo rapidly increased the frequency of synaptic currents and higher concentrations potentiated the AMPA receptor-mediated current. Both effects were PKC-dependent. Parallel recordings of synaptic currents and nitric oxide (NO)-associated fluorescence showed that the increased frequency, related to pre-synaptic release, was dependent on a NO-mediated retrograde signaling. Moreover, increased synchronization in NO production was also observed in neurons neighboring those dialyzed with iAβo, indicating that iAβo can increase network excitability at a distance. Current-clamp recordings suggested that iAβo increased neuronal excitability via AMPA-driven synaptic activity without altering membrane intrinsic properties. These results strongly indicate that iAβo causes functional spreading of hyperexcitability through a synaptic-driven mechanism and offers an important neuropathological significance to intracellular species in the initial stages of AD, which include brain hyperexcitability and seizures.

Cryo-electron Microscopy Imaging of Alzheimer's Amyloid-beta 42 Oligomer Displayed on a Functionally and Structurally Relevant Scaffold

Amyloid-β peptide (Aβ) oligomers are pathogenic species of amyloid aggregates in Alzheimer's disease. Like certain protein toxins, Aβ oligomers permeabilize cellular membranes, presumably through a pore formation mechanism. Owing to their structural and stoichiometric heterogeneity, the structure of these pores remains to be characterized. We studied a functional Aβ42-pore equivalent, created by fusing Aβ42 to the oligomerizing, soluble domain of the α-hemolysin (αHL) toxin. Our data reveal Aβ42-αHL oligomers to share major structural, functional, and biological properties with wild-type Aβ42-pores. Single-particle cryo-EM analysis of Aβ42-αHL oligomers (with an overall 3.3 Å resolution) reveals the Aβ42-pore region to be intrinsically flexible. The Aβ42-αHL oligomers will allow many of the features of the wild-type amyloid oligomers to be studied that cannot be otherwise, and may be a highly specific antigen for the development of immuno-base diagnostics and therapies.

An "epitomic" analysis of the specificity of conformation-dependent, anti-Aß amyloid monoclonal antibodies

Antibodies against Aß amyloid are indispensable research tools and potential therapeutics for Alzheimer's disease. They display several unusual properties, such as specificity for aggregated forms of the peptide, the ability to distinguish polymorphic aggregate structures, and the ability to recognize generic aggregation-related epitopes formed by unrelated amyloid sequences. Understanding the mechanisms underlying these unusual properties and the structures of their corresponding epitopes is crucial for the understanding why antibodies display different therapeutic activities and for the development of more effective therapeutic agents. Here we employed a novel "epitomic" approach to map the fine structure of the epitopes of 28 monoclonal antibodies against amyloid-beta using immunoselection of random sequences from a phage display library, deep sequencing, and pattern analysis to define the critical sequence elements recognized by the antibodies. Although most of the antibodies map to major linear epitopes in the amino terminal 1 to 14 residues of Aß, the antibodies display differences in the target sequence residues that are critical for binding and in their individual preferences for nontarget residues, indicating that the antibodies bind to alternative conformations of the sequence by different mechanisms. Epitomic analysis also identifies discontinuous, nonoverlapping sequence Aß segments that may constitute the conformational epitopes that underlie the aggregation specificity of antibodies. Aggregation-specific antibodies recognize sequences that display a significantly higher predicted propensity for forming amyloid than antibodies that recognize the monomer, indicating that the ability of random sequences to aggregate into amyloid is a critical element of their binding mechanism.

The human islet amyloid polypeptide in protein misfolding disorders: Mechanisms of aggregation and interaction with biomembranes

Human islet amyloid polypeptide (hIAPP), otherwise known as amylin, is a 37-residue peptide hormone which is reported to be a common factor in protein misfolding disorders such as type-2 diabetes mellitus, Alzheimer's disease and Parkinson's disease, due to deposition of insoluble hIAPP amyloid in the pancreas and brain. Multiple studies point to the importance of the peptide's interaction with biological membranes and the cytotoxicity of hIAPP species. Here, we discuss the aggregation pathways of hIAPP amyloid fibril formation and focus on the complex interplay between membrane-mediated assembly of hIAPP and the associated mechanisms of membrane damage caused by the peptide species. Mitochondrial membranes, which are unique in their lipid composition, are proposed as prime targets for the early intracellular formation of hIAPP toxic entities. We suggest that future studies should include more physiologically-relevant and in-cell studies to allow a more accurate model of in vivo interactions. Finally, we underscore an urgent need for developing effective therapeutic strategies aimed at hindering hIAPP-phospholipid interactions.

The Erythrocytic Hypothesis of Brain Energy Crisis in Sporadic Alzheimer Disease: Possible Consequences and Supporting Evidence

Alzheimer’s disease (AD) is a fatal form of dementia of unknown etiology. Although amyloid plaque accumulation in the brain has been the subject of intensive research in disease pathogenesis and anti-amyloid drug development; the continued failures of the clinical trials suggest that amyloids are not a key cause of AD and new approaches to AD investigation and treatment are needed. We propose a new hypothesis of AD development based on metabolic abnormalities in circulating red blood cells (RBCs) that slow down oxygen release from RBCs into brain tissue which in turn leads to hypoxia-induced brain energy crisis; loss of neurons; and progressive atrophy preceding cognitive dysfunction. This review summarizes current evidence for the erythrocytic hypothesis of AD development and provides new insights into the causes of neurodegeneration offering an innovative way to diagnose and treat this systemic disease.

Protein Misfolding during Pregnancy: New Approaches to Preeclampsia Diagnostics

Preeclampsia (PE) is a multisystem heterogeneous complication of pregnancy remaining a leading cause of maternal and perinatal morbidity and mortality over the world. PE has a large spectrum of clinical features and symptoms, which make diagnosis challenging. Despite a long period of studying, PE etiology is still unclear and there are no reliable rapid tests for early diagnosis of this disease. During the last decade, it was shown that proteins misfolding and aggregation are associated with PE. Several proteins, including amyloid beta peptide, transthyretin, alpha-1 antitrypsin, albumin, IgG k-free light chains, and ceruloplasmin are dysregulated in PE, resulting in toxic deposition of amyloid-like aggregates in the placenta and body fluids. It is also possible that aggregated proteins induce defective trophoblast invasion, placental ischemia, ER stress, and promote PE manifestation. The fact that protein aggregation is an emerging biomarker of PE provides an opportunity to develop new diagnostic approaches based on amyloids special features, such as Congo red (CR) staining and thioflavin T (ThT) enhanced fluorescence.

New Perspectives on Roles of Alpha-Synuclein in Parkinson's Disease

Parkinson’s disease (PD) is one of the synucleinopathies spectrum of disorders typified by the presence of intraneuronal protein inclusions. It is primarily composed of misfolded and aggregated forms of alpha-synuclein (α-syn), the toxicity of which has been attributed to the transition from an α-helical conformation to a β-sheetrich structure that polymerizes to form toxic oligomers. This could spread and initiate the formation of “LB-like aggregates,” by transcellular mechanisms with seeding and subsequent permissive templating. This hypothesis postulates that α-syn is a prion-like pathological agent and responsible for the progression of Parkinson’s pathology. Moreover, the involvement of the inflammatory response in PD pathogenesis has been reported on the excessive microglial activation and production of pro-inflammatory cytokines. At last, we describe several treatment approaches that target the pathogenic α-syn protein, especially the oligomers, which are currently being tested in advanced animal experiments or are already in clinical trials. However, there are current challenges with therapies that target α-syn, for example, difficulties in identifying varying α-syn conformations within different individuals as well as both the cost and need of long-duration large trials.

Infection of Fungi and Bacteria in Brain Tissue From Elderly Persons and Patients With Alzheimer's Disease

Alzheimer's disease (AD) is the leading cause of dementia in elderly people. The etiology of this disease remains a matter of intensive research in many laboratories. We have advanced the idea that disseminated fungal infection contributes to the etiology of AD. Thus, we have demonstrated that fungal proteins and DNA are present in nervous tissue from AD patients. More recently, we have reported that bacterial infections can accompany these mycoses, suggesting that polymicrobial infections exist in AD brains. In the present study, we have examined fungal and bacterial infection in brain tissue from AD patients and control subjects by immunohistochemistry. In addition, we have documented the fungal and bacterial species in brain regions from AD patients and control subjects by next-generation sequencing (NGS). Our results from the analysis of ten AD patients reveal a variety of fungal and bacterial species, although some were more prominent than others. The fungal genera more prevalent in AD patients were Alternaria, Botrytis, Candida, and Malassezia. We also compared these genera with those found in elderly and younger subjects. One of the most prominent genera in control subjects was Fusarium. Principal component analysis clearly indicated that fungi from frontal cortex samples of AD brains clustered together and differed from those of equivalent control subjects. Regarding bacterial infection, the phylum Proteobacteria was the most prominent in both AD patients and controls, followed by Firmicutes, Actinobacteria, and Bacteroides. At the family level, Burkholderiaceae and Staphylococcaceae exhibited higher percentages in AD brains than in control brains. These findings could be of interest to guide targeted antimicrobial therapy for AD patients. Moreover, the variety of microbial species in each patient may constitute a basis for a better understanding of the evolution and severity of clinical symptoms in each patient.

Common mechanisms involved in Alzheimer's disease and type 2 diabetes: a key role of chronic bacterial infection and inflammation

Strong epidemiologic evidence and common molecular mechanisms support an association between Alzheimer's disease (AD) and type 2-diabetes. Local inflammation and amyloidosis occur in both diseases and are associated with periodontitis and various infectious agents. This article reviews the evidence for the presence of local inflammation and bacteria in type 2 diabetes and discusses host pathogen interactions in chronic inflammatory disorders. Chlamydophyla pneumoniae, Helicobacter pylori and spirochetes are demonstrated in association with dementia and brain lesions in AD and islet lesions in type 2 diabetes. The presence of pathogens in host tissues activates immune responses through Toll-like receptor signaling pathways. Evasion of pathogens from complement-mediated attack results in persistent infection, inflammation and amyloidosis. Amyloid beta and the pancreatic amyloid called amylin bind to lipid bilayers and produce Ca(2+) influx and bacteriolysis. Similarly to AD, accumulation of amylin deposits in type 2 diabetes may result from an innate immune response to chronic bacterial infections, which are known to be associated with amyloidosis. Further research based on an infectious origin of both AD and type 2 diabetes may lead to novel treatment strategies.

The contribution of biophysical and structural studies of protein self-assembly to the design of therapeutic strategies for amyloid diseases

Many neurodegenerative disorders, including Alzheimer's, Parkinson's and the prion diseases, are characterized by a conformational conversion of normally soluble proteins or peptides into pathological species, by a process of misfolding and self-assembly that leads ultimately to the formation of amyloid fibrils. Recent studies support the idea that multiple intermediate species with a wide variety of degrees of neuronal toxicity are generated during such processes. The development of a high level of knowledge of the nature and structure of the pathogenic amyloid species would significantly enhance efforts to underline the molecular origins of these disorders and also to develop both accurate diagnoses and effective therapeutic interventions for these types of conditions. In this review, we discuss recent biophysical and structural information concerning different types of amyloid aggregates and the way in which such information can guide rational therapeutic approaches designed to target specific pathogenic events that occur during the development of these highly debilitating and increasingly common diseases.

Drosophila melanogaster Models of Metal-Related Human Diseases and Metal Toxicity

Iron, copper and zinc are transition metals essential for life because they are required in a multitude of biological processes. Organisms have evolved to acquire metals from nutrition and to maintain adequate levels of each metal to avoid damaging effects associated with its deficiency, excess or misplacement. Interestingly, the main components of metal homeostatic pathways are conserved, with many orthologues of the human metal-related genes having been identified and characterized in Drosophila melanogaster. Drosophila has gained appreciation as a useful model for studying human diseases, including those caused by mutations in pathways controlling cellular metal homeostasis. Flies have many advantages in the laboratory, such as a short life cycle, easy handling and inexpensive maintenance. Furthermore, they can be raised in a large number. In addition, flies are greatly appreciated because they offer a considerable number of genetic tools to address some of the unresolved questions concerning disease pathology, which in turn could contribute to our understanding of the metal metabolism and homeostasis. This review recapitulates the metabolism of the principal transition metals, namely iron, zinc and copper, in Drosophila and the utility of this organism as an experimental model to explore the role of metal dyshomeostasis in different human diseases. Finally, a summary of the contribution of Drosophila as a model for testing metal toxicity is provided.

Iron is a specific cofactor for distinct oxidation- and aggregation-dependent Aβ toxicity mechanisms in a Drosophila model

Metals, including iron, are present at high concentrations in amyloid plaques in individuals with Alzheimer's disease, where they are also thought to be cofactors in generating oxidative stress and modulating amyloid formation. In this study, we present data from several Drosophila models of neurodegenerative proteinopathies indicating that the interaction between iron and amyloid beta peptide (Aβ) is specific and is not seen for other aggregation-prone polypeptides. The interaction with iron is likely to be important in the dimerisation of Aβ and is mediated by three N-terminal histidines. Transgenic fly lines systematically expressing all combinations of His>Ala substitutions in Aβ were generated and used to study the pathological role of these residues. Developmental eye phenotypes, longevity and histological examinations indicate that the N-terminal histidines have distinct position-dependent and -independent mechanisms. The former mediate the toxic effects of metals and Aβ aggregation under non-oxidising conditions and the latter are relevant under oxidising conditions. Understanding how Aβ mediates neurotoxic effects in vivo will help to better target pathological pathways using aggregation blockers and metal-modifying agents.

Seeking a mechanism for the toxicity of oligomeric α-synuclein

In a number of neurological diseases including Parkinson’s disease (PD), α‑synuclein is aberrantly folded, forming abnormal oligomers, and amyloid fibrils within nerve cells. Strong evidence exists for the toxicity of increased production and aggregation of α-synuclein in vivo. The toxicity of α-synuclein is popularly attributed to the formation of “toxic oligomers”: a heterogenous and poorly characterized group of conformers that may share common molecular features. This review presents the available evidence on the properties of α-synuclein oligomers and the potential molecular mechanisms of their cellular disruption. Toxic α-synuclein oligomers may impact cells in a number of ways, including the disruption of membranes, mitochondrial depolarization, cytoskeleton changes, impairment of protein clearance pathways, and enhanced oxidative stress. We also examine the relationship between α-synuclein toxic oligomers and amyloid fibrils, in the light of recent studies that paint a more complex picture of α-synuclein toxicity. Finally, methods of studying and manipulating oligomers within cells are described.

Monoclonal antibodies against Aβ42 fibrils distinguish multiple aggregation state polymorphisms in vitro and in Alzheimer disease brain

Amyloidogenic proteins generally form intermolecularly hydrogen-bonded β-sheet aggregates, including parallel, in-register β-sheets (recognized by antiserum OC) or antiparallel β-sheets, β-solenoids, β-barrels, and β-cylindrins (recognized by antiserum A11). Although these groups share many common properties, some amyloid sequences have been reported to form polymorphic structural variants or strains. We investigated the humoral immune response to Aβ42 fibrils and produced 23 OC-type monoclonal antibodies recognizing distinct epitopes differentially associated with polymorphic structural variants. These mOC antibodies define at least 18 different immunological profiles represented in aggregates of amyloid-β (Aβ). All of the antibodies strongly prefer amyloid aggregates over monomer, indicating that they recognize conformational epitopes. Most of the antibodies react with N-terminal linear segments of Aβ, although many recognize a discontinuous epitope consisting of an N-terminal domain and a central domain. Several of the antibodies that recognize linear Aβ segments also react with fibrils formed from unrelated amyloid sequences, indicating that reactivity with linear segments of Aβ does not mean the antibody is sequence-specific. The antibodies display strikingly different patterns of immunoreactivity in Alzheimer disease and transgenic mouse brain and identify spatially and temporally unique amyloid deposits. Our results indicate that the immune response to Aβ42 fibrils is diverse and reflects the structural polymorphisms in fibrillar amyloid structures. These polymorphisms may contribute to differences in toxicity and consequent effects on pathological processes. Thus, a single therapeutic monoclonal antibody may not be able to target all of the pathological aggregates necessary to make an impact on the overall disease process.

Antiparallel triple-strand architecture for prefibrillar Aβ42 oligomers

Aβ42 oligomers play key roles in the pathogenesis of Alzheimer disease, but their structures remain elusive partly due to their transient nature. Here, we show that Aβ42 in a fusion construct can be trapped in a stable oligomer state, which recapitulates characteristics of prefibrillar Aβ42 oligomers and enables us to establish their detailed structures. Site-directed spin labeling and electron paramagnetic resonance studies provide structural restraints in terms of side chain mobility and intermolecular distances at all 42 residue positions. Using these restraints and other biophysical data, we present a novel atomic-level oligomer model. In our model, each Aβ42 protein forms a single β-sheet with three β-strands in an antiparallel arrangement. Each β-sheet consists of four Aβ42 molecules in a head-to-tail arrangement. Four β-sheets are packed together in a face-to-back fashion. The stacking of identical segments between different β-sheets within an oligomer suggests that prefibrillar oligomers may interconvert with fibrils via strand rotation, wherein β-strands undergo an ∼90° rotation along the strand direction. This work provides insights into rational design of therapeutics targeting the process of interconversion between toxic oligomers and non-toxic fibrils.

Protein misfolding, congophilia, oligomerization, and defective amyloid processing in preeclampsia

Preeclampsia is a pregnancy-specific disorder of unknown etiology and a leading contributor to maternal and perinatal morbidity and mortality worldwide. Because there is no cure other than delivery, preeclampsia is the leading cause of iatrogenic preterm birth. We show that preeclampsia shares pathophysiologic features with recognized protein misfolding disorders. These features include urine congophilia (affinity for the amyloidophilic dye Congo red), affinity for conformational state–dependent antibodies, and dysregulation of prototype proteolytic enzymes involved in amyloid precursor protein (APP) processing. Assessment of global protein misfolding load in pregnancy based on urine congophilia (Congo red dot test) carries diagnostic and prognostic potential for preeclampsia. We used conformational state–dependent antibodies to demonstrate the presence of generic supramolecular assemblies (prefibrillar oligomers and annular protofibrils), which vary in quantitative and qualitative representation with preeclampsia severity. In the first attempt to characterize the preeclampsia misfoldome, we report that the urine congophilic material includes proteoforms of ceruloplasmin, immunoglobulin free light chains, SERPINA1, albumin, interferon-inducible protein 6-16, and Alzheimer’s β-amyloid. The human placenta abundantly expresses APP along with prototype APP-processing enzymes, of which the α-secretase ADAM10, the β-secretases BACE1 and BACE2, and the γ-secretase presenilin-1 were all up-regulated in preeclampsia. The presence of β-amyloid aggregates in placentas of women with preeclampsia and fetal growth restriction further supports the notion that this condition should join the growing list of protein conformational disorders. If these aggregates play a pathophysiologic role, our findings may lead to treatment for preeclampsia.

Nuclear translocation uncovers the amyloid peptide Aβ42 as a regulator of gene transcription

Although soluble species of the amyloid-β peptide Aβ42 correlate with disease symptoms in Alzheimer disease, little is known about the biological activities of amyloid-β (Aβ). Here, we show that Aβ peptides varying in lengths from 38 to 43 amino acids are internalized by cultured neuroblastoma cells and can be found in the nucleus. By three independent methods, we demonstrate direct detection of nuclear Aβ42 as follows: (i) biochemical analysis of nuclear fractions; (ii) detection of biotin-labeled Aβ in living cells by confocal laser scanning microscopy; and (iii) transmission electron microscopy of Aβ in cultured cells, as well as brain tissue of wild-type and transgenic APPPS1 mice (overexpression of amyloid precursor protein and presenilin 1 with Swedish and L166P mutations, respectively). Also, this study details a novel role for Aβ42 in nuclear signaling, distinct from the amyloid precursor protein intracellular domain. Chromatin immunoprecipitation showed that Aβ42 specifically interacts as a repressor of gene transcription with LRP1 and KAI1 promoters. By quantitative RT-PCR, we confirmed that mRNA levels of the examined candidate genes were exclusively decreased by the potentially neurotoxic Aβ42 wild-type peptide. Shorter peptides (Aβ38 or Aβ40) and other longer peptides (nontoxic Aβ42 G33A substitution or Aβ43) did not affect mRNA levels. Overall, our data indicate that the nuclear translocation of Aβ42 impacts gene regulation, and deleterious effects of Aβ42 in Alzheimer disease pathogenesis may be influenced by altering the expression profiles of disease-modifying genes.

Apoptosis induced by islet amyloid polypeptide soluble oligomers is neutralized by diabetes-associated specific antibodies

Soluble oligomeric assemblies of amyloidal proteins appear to act as major pathological agents in several degenerative disorders. Isolation and characterization of these oligomers is a pivotal step towards determination of their pathological relevance. Here we describe the isolation of Type 2 diabetes-associated islet amyloid polypeptide soluble cytotoxic oligomers; these oligomers induced apoptosis in cultured pancreatic cells, permeated model lipid vesicles and interacted with cell membranes following complete internalization. Moreover, antibodies which specifically recognized these assemblies, but not monomers or amyloid fibrils, were exclusively identified in diabetic patients and were shown to neutralize the apoptotic effect induced by these oligomers. Our findings support the notion that human IAPP peptide can form highly toxic oligomers. The presence of antibodies identified in the serum of diabetic patients confirms the pathological relevance of the oligomers. In addition, the newly identified structural epitopes may also provide new mechanistic insights and a molecular target for future therapy.

Localization of A11-reactive oligomeric species in prion diseases

AIMS

To investigate in prion diseases the in-situ localization of prion protein oligomers sharing a common epitope with amyloid oligomers involved in a range of neurodegenerative diseases.

METHODS AND RESULTS

We performed immunohistochemistry on sporadic Creutzfeldt-Jakob disease (sCJD) (n = 9) and hereditary Gerstmann-Sträussler-Scheinker disease (GSS) (n = 1) specimens with the anti-oligomer antibody A11 to determine the localization of reactive species. We found that A11 reactivity in the sCJD specimens was localized to the cerebral and cerebellar cortices both in spongiform and adjacent, non-spongiform areas, reminiscent of multicentric or diffuse plaques. In the GSS specimens, we found that staining was closely associated with kuru-like plaques, and that A11-reactive species colocalized with protease-resistant prion protein (Prp(Sc)). We also observed sporadic neuronal cytosolic staining in both types of specimen.

CONCLUSIONS

We confirm that intracellular and extracellular A11-reactive species are present in situ in sCJD cases and GSS, and that immunoreactivity for A11 and Prp(Sc) overlaps. We argue that the A11-reactive species are indeed composed of oligomeric Prp(Sc), and suggest that the toxic effects of Prp(Sc) oligomers could be related to the generic oligomeric conformation recognized by A11.

Toxic prefibrillar α-synuclein amyloid oligomers adopt a distinctive antiparallel β-sheet structure

Parkinson's disease is an age-related movement disorder characterized by the presence in the mid-brain of amyloid deposits of the 140-amino-acid protein AS (α-synuclein). AS fibrillation follows a nucleation polymerization pathway involving diverse transient prefibrillar species varying in size and morphology. Similar to other neurodegenerative diseases, cytotoxicity is currently attributed to these prefibrillar species rather than to the insoluble aggregates. Nevertheless, the underlying molecular mechanisms responsible for cytotoxicity remain elusive and structural studies may contribute to the understanding of both the amyloid aggregation mechanism and oligomer-induced toxicity. It is already recognized that soluble oligomeric AS species adopt β-sheet structures that differ from those characterizing the fibrillar structure. In the present study we used ATR (attenuated total reflection)-FTIR (Fourier-transform infrared) spectroscopy, a technique especially sensitive to β-sheet structure, to get a deeper insight into the β-sheet organization within oligomers and fibrils. Careful spectral analysis revealed that AS oligomers adopt an antiparallel β-sheet structure, whereas fibrils adopt a parallel arrangement. The results are discussed in terms of regions of the protein involved in the early β-sheet interactions and the implications of such conformational arrangement for the pathogenicity associated with AS oligomers.

Atomic view of a toxic amyloid small oligomer

Amyloid diseases, including Alzheimer's, Parkinson's, and the prion conditions, are each associated with a particular protein in fibrillar form. These amyloid fibrils were long suspected to be the disease agents, but evidence suggests that smaller, often transient and polymorphic oligomers are the toxic entities. Here, we identify a segment of the amyloid-forming protein αB crystallin, which forms an oligomeric complex exhibiting properties of other amyloid oligomers: β-sheet-rich structure, cytotoxicity, and recognition by an oligomer-specific antibody. The x-ray-derived atomic structure of the oligomer reveals a cylindrical barrel, formed from six antiparallel protein strands, that we term a cylindrin. The cylindrin structure is compatible with a sequence segment from the β-amyloid protein of Alzheimer's disease. Cylindrins offer models for the hitherto elusive structures of amyloid oligomers.

Aberrant action of amyloidogenic host defense peptides: a new paradigm to investigate neurodegenerative disorders?

Host defense peptides (HDPs) are components of the innate immune system with activity against a broad range of microbes. In some cases, it appears that this activity is mediated by the ability of these peptides to permeabilize microbial membranes via the formation of amyloid associated structures. Recent evidence suggests that the naturally occurring function of the Aβ40 and Aβ42 peptides, which are causative agents of Alzheimer's disease, may be to serve as amyloidogenic HDPs. Here, it is hypothesized that the neurotoxicity of these peptides is related to aberrant use of their amyloid-mediated antimicrobial mechanisms, which provides the as yet unexplored paradigm of a relationship among HDPs, neurodegenerative disorders, and other conditions that could contribute to their understanding and remediation.

Equal force recovery in dysferlin-deficient and wild-type muscles following saponin exposure

Dysferlin plays an important role in repairing membrane damage elicited by laser irradiation, and dysferlin deficiency causes muscular dystrophy and associated cardiomyopathy. Proteins such as perforin, complement component C9, and bacteria-derived cytolysins, as well as the natural detergent saponin, can form large pores on the cell membrane via complexation with cholesterol. However, it is not clear whether dysferlin plays a role in repairing membrane damage induced by pore-forming reagents. In this study, we observed that dysferlin-deficient muscles recovered the tetanic force production to the same extent as their WT counterparts following a 5-min saponin exposure (50 μg/mL). Interestingly, the slow soleus muscles recovered significantly better than the fast extensor digitorum longus (EDL) muscles. Our data suggest that dysferlin is unlikely involved in repairing saponin-induced membrane damage and that the slow muscle is more efficient than the fast muscle in repairing such damage.

A diversity of assembly mechanisms of a generic amyloid fold

Protein misfolding and amyloid assembly have long been recognized as being responsible for many devastating human diseases. Recent findings indicate that amyloid assemblies may facilitate crucial biological processes from bacteria to mammals. This review focuses on the mechanistic understanding of amyloid formation, including the transformation of initially innocuous proteins into oligomers and fibrils. The result is a competing folding and assembly energy landscape, which contains a number of routes by which the polypeptide chain can convert its primary sequence into functional structures, dysfunctional assemblies, or epigenetic entities that provide both threats and opportunities in the evolution of life.

Oxidative stress and cell membranes in the pathogenesis of Alzheimer's disease

Amyloid β proteins and oxidative stress are believed to have central roles in the development of Alzheimer's disease. Lipid membranes are among the most vulnerable cellular components to oxidative stress, and membranes in susceptible regions of the brain are compositionally distinct from those in other tissues. This review considers the evidence that membranes are either a source of neurotoxic lipid oxidation products or the target of pathogenic processes involving amyloid β proteins that cause permeability changes or ion channel formation. Progress toward a comprehensive theory of Alzheimer's disease pathogenesis is discussed in which lipid membranes assume both roles and promote the conversion of monomeric amyloid β proteins into fibrils, the pathognomonic histopathological lesion of the disease.

Suppression of amyloid beta A11 antibody immunoreactivity by vitamin C: possible role of heparan sulfate oligosaccharides derived from glypican-1 by ascorbate-induced, nitric oxide (NO)-catalyzed degradation

Amyloid β (Aβ) is generated from the copper- and heparan sulfate (HS)-binding amyloid precursor protein (APP) by proteolytic processing. APP supports S-nitrosylation of the HS proteoglycan glypican-1 (Gpc-1). In the presence of ascorbate, there is NO-catalyzed release of anhydromannose (anMan)-containing oligosaccharides from Gpc-1-nitrosothiol. We investigated whether these oligosaccharides interact with Aβ during APP processing and plaque formation. anMan immunoreactivity was detected in amyloid plaques of Alzheimer (AD) and APP transgenic (Tg2576) mouse brains by immunofluorescence microscopy. APP/APP degradation products detected by antibodies to the C terminus of APP, but not Aβ oligomers detected by the anti-Aβ A11 antibody, colocalized with anMan immunoreactivity in Tg2576 fibroblasts. A 50-55-kDa anionic, sodium dodecyl sulfate-stable, anMan- and Aβ-immunoreactive species was obtained from Tg2576 fibroblasts using immunoprecipitation with anti-APP (C terminus). anMan-containing HS oligo- and disaccharide preparations modulated or suppressed A11 immunoreactivity and oligomerization of Aβ42 peptide in an in vitro assay. A11 immunoreactivity increased in Tg2576 fibroblasts when Gpc-1 autoprocessing was inhibited by 3-β[2(diethylamino)ethoxy]androst-5-en-17-one (U18666A) and decreased when Gpc-1 autoprocessing was stimulated by ascorbate. Neither overexpression of Gpc-1 in Tg2576 fibroblasts nor addition of copper ion and NO donor to hippocampal slices from 3xTg-AD mice affected A11 immunoreactivity levels. However, A11 immunoreactivity was greatly suppressed by the subsequent addition of ascorbate. We speculate that temporary interaction between the Aβ domain and small, anMan-containing oligosaccharides may preclude formation of toxic Aβ oligomers. A portion of the oligosaccharides are co-secreted with the Aβ peptides and deposited in plaques. These results support the notion that an inadequate supply of vitamin C could contribute to late onset AD in humans.

Amyloidogenic protein-membrane interactions: mechanistic insight from model systems

The toxicity of amyloid-forming proteins is correlated with their interactions with cell membranes. Binding events between amyloidogenic proteins and membranes result in mutually disruptive structural perturbations, which are associated with toxicity. Membrane surfaces promote the conversion of amyloid-forming proteins into toxic aggregates, and amyloidogenic proteins, in turn, compromise the structural integrity of the cell membrane. Recent studies with artificial model membranes have highlighted the striking resemblance of the mechanisms of membrane permeabilization of amyloid-forming proteins to those of pore-forming toxins and antimicrobial peptides.

Limiting an antimicrobial peptide to the lipid-water interface enhances its bacterial membrane selectivity: a case study of MSI-367

In a minimalist design approach, a synthetic peptide MSI-367 [(KFAKKFA)(3)-NH(2)] was designed and synthesized with the objective of generating cell-selective nonlytic peptides, which have a significant bearing on cell targeting. The peptide exhibited potent activity against both bacteria and fungi, but no toxicity to human cells at micromolar concentrations. Bacterial versus human cell membrane selectivity of the peptide was determined via membrane permeabilization assays. Circular dichroism investigations revealed the intrinsic helix propensity of the peptide, β-turn structure in aqueous buffer and extended and turn conformations upon binding to lipid vesicles. Differential scanning calorimetry experiments with 1,2-dipalmitoleoyl-sn-glycero-3-phosphatidylethanolamine bilayers indicated the induction of positive curvature strain and repression of the fluid lamellar to inverted hexagonal phase transition by MSI-367. Results of isothermal titration calorimetry (ITC) experiments suggested the possibility of formation of specific lipid-peptide complexes leading to aggregation. (2)H nuclear magnetic resonance (NMR) of deuterated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) multilamellar vesicles confirmed the limited effect of the membrane-embedded peptide at the lipid-water interface. (31)P NMR data indicated changes in the lipid headgroup orientation of POPC, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylglycerol, and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine lipid bilayers upon peptide binding. Membrane-embedded and membrane-inserted states of the peptide were observed via sum frequency generation vibrational spectroscopy. Circular dichroism, ITC, and (31)P NMR data for Escherichia coli lipids agree with the hypothesis that strong electrostatic lipid-peptide interactions embrace the peptide at the lipid-water interface and provide the basis for bacterial cell selectivity.

Biochemical and biophysical features of both oligomer/fibril and cell membrane in amyloid cytotoxicity

A great deal must still be learnt on the structural features of amyloid assemblies, particularly prefibrillar aggregates, and the relationship of the latter with amyloid cytotoxicity. Presently, it is recognized that the population of unstable, heterogeneous amyloid oligomers and protofibrils is mainly responsible for amyloid cytotoxicity. Conversely, mature fibrils are considered stable, harmless reservoirs of molecular species devoid of toxicity in the polymerized state. This view has been modified by recent reports showing that mature fibrils grown at different conditions can display different structural features and stabilities, possibly leading them to undergo disassembly with the leak of toxic oligomers. Fibril polymorphism is paralleled by oligomer polymorphism and both can be traced back to amyloid growth from differently destabilized monomers with distinct structural features at differing conditions. Recent research has started to unravel oligomer structural and biophysical features and the relationship between the latter and oligomer cytotoxicity. These data have led to the proposal that, together, both oligomer and membrane physical features determine the extent of oligomer-membrane interaction with the resulting disruption of membrane integrity and cell impairment. Such a view can help to explain the variable vulnerability of different cell types to the same amyloids and the lack of relationship between amyloid load and the severity of clinical symptoms. It also stresses the importance, for cell/tissue impairment, of the presence, in tissue, in addition to toxic oligomers, of fibrils conformers of reduced stability as a possible source of toxic oligomers, whose leakage can be favoured upon interaction with suitable surfaces or by other environmental conditions.

The culprit behind amyloid beta peptide related neurotoxicity in Alzheimer's disease: oligomer size or conformation?

Since the reformulation of the amyloid cascade hypothesis to focus on oligomeric aggregates of amyloid beta as the prime toxic species causing Alzheimer's disease, many researchers refocused on detecting a specific molecular assembly of defined size thatis the main trigger of Alzheimer's disease. The result has been the identification of a host of molecular assemblies containing from two up to a hundred molecules of the amyloid beta peptide, which were all found to impair memory formation in mice. This clearly demonstrates that size is insufficient to define toxicity and peptide conformation has to be taken into account. In this review we discuss the interplay between oligomer size and peptide conformation as the key determinants of the neurotoxicity of the amyloid beta peptide.

Cholesterol specificity of some heptameric beta-barrel pore-forming bacterial toxins: structural and functional aspects

Apart from the thiol-specific/cholesterol-dependent cytolysin family of toxins (see Chapter 20) there are a number of other unrelated bacterial toxins that also have an affinity for plasma membrane cholesterol. Emphasis is given here on the Vibrio cholerae cytolysin (VCC) and the cytolysins from related Vibrio species. The inhibition of the cytolytic activity of these toxins by prior incubation with extracellular cholesterol or low density lipoprotein emerges as a unifying feature, as does plasma membrane cholesterol depletion. Incubation of VCC with cholesterol produces a heptameric oligomer, which is not equivalent to the pre-pore since it is unable to penetrate the plasma membrane. In structural terms, the precise sequence of VCC monomer binding to membrane, oligomer formation and pore insertion through the bilayer has yet to be fully defined. Several other bacterial toxins have a dependency for cholesterol, although the available data is limited in most cases.

Cholesterol in Alzheimer's disease and other amyloidogenic disorders

The complex association of cholesterol metabolism and Alzheimer's disease is presented in depth, including the possible benefits to be gained from cholesterol-lowering statin therapy. Then follows a survey of the role of neuronal membrane cholesterol in Abeta pore formation and Abeta fibrillogenesis, together with the link with membrane raft domains and gangliosides. The contribution of structural studies to Abeta fibrillogenesis, using TEM and AFM, is given some emphasis. The role of apolipoprotein E and its isoforms, in particular ApoE4, in cholesterol and Abeta binding is presented, in relation to genetic risk factors for Alzheimer's disease. Increasing evidence suggests that cholesterol oxidation products are of importance in generation of Alzheimer's disease, possibly induced by Abeta-produced hydrogen peroxide. The body of evidence for a link between cholesterol in atherosclerosis and Alzheimer's disease is increasing, along with an associated inflammatory response. The possible role of cholesterol in tau fibrillization, tauopathies and in some other non-Abeta amyloidogenic disorders is surveyed.

Synaptic depression and aberrant excitatory network activity in Alzheimer's disease: two faces of the same coin?

Neurodegenerative diseases, including Alzheimer's disease (AD), target specific and functionally connected neuronal networks, raising the possibility that neurodegeneration may spread through abnormal patterns of neural network activity. AD is associated with high levels of amyloid-beta (A beta) peptides in the brain, synaptic depression, aberrant excitatory neuronal activity, and cognitive decline. However, the relationships among these alterations and their underlying mechanisms are poorly understood. In experimental models of AD, high concentrations of pathogenic A beta assemblies reduce glutamatergic transmission and enhance long-term depression at the synaptic level. At the network level, they cause dysrhythmias, including neuronal synchronization, epileptiform activity, seizures, and postictal suppression. Both synaptic depression and aberrant network synchronization likely interfere with activity-dependent synaptic regulation, which is critical for learning and memory. Abnormal patterns of neuronal activity across functionally connected brain regions may also trigger and perpetuate trans-synaptic mechanisms of neurodegeneration. It remains to be determined if synaptic depression and network dysrhythmias are mechanistically related, which of them is primary or secondary, and whether normalization of one will prevent the other as well as cognitive dysfunction in AD.

Antiparallel beta-sheet: a signature structure of the oligomeric amyloid beta-peptide

AD (Alzheimer's disease) is linked to Abeta (amyloid beta-peptide) misfolding. Studies demonstrate that the level of soluble Abeta oligomeric forms correlates better with the progression of the disease than the level of fibrillar forms. Conformation-dependent antibodies have been developed to detect either Abeta oligomers or fibrils, suggesting that structural differences between these forms of Abeta exist. Using conditions which yield well-defined Abeta-(1-42) oligomers or fibrils, we studied the secondary structure of these species by ATR (attenuated total reflection)-FTIR (Fourier-transform infrared) spectroscopy. Whereas fibrillar Abeta was organized in a parallel beta-sheet conformation, oligomeric Abeta displayed distinct spectral features, which were attributed to an antiparallel beta-sheet structure. We also noted striking similarities between Abeta oligomers spectra and those of bacterial outer membrane porins. We discuss our results in terms of a possible organization of the antiparallel beta-sheets in Abeta oligomers, which may be related to reported effects of these highly toxic species in the amyloid pathogenesis associated with AD.

Amyloid Beta annular protofibrils in cell processes and synapses accumulate with aging and Alzheimer-associated genetic modification

Amyloid beta (Abeta) annular protofibrils (APFs) have been described where the structure is related to that of beta barrel pore-forming bacterial toxins and exhibits cellular toxicity. To investigate the relationship of Abeta APFs to disease and their ultrastructural localization in brain tissue, we conducted a pre-embedding immunoelectron microscopic study using anti-annular protofibril antiserum. We examined brain tissues of young- and old-aged amyloid precursor protein transgenic mice (APP23), neprilysin knockout APP23 mice, and nontransgenic littermates. alphaAPF-immunoreactions tended to be found (1) on plasma membranes and vesicles inside of cell processes, but not on amyloid fibrils, (2) with higher density due to aging, APP transgene, and neprilysin deficiency, and (3) with higher positive rate at synaptic compartments in aged APP23, especially in neprilysin knockout APP23 mice. These findings imply that APFs are distinct from amyloid fibrils, interact with biological membranes, and might be related to synaptic dysfunction in Alzheimer model mouse brains.

Poloxamer 188 copolymer membrane sealant rescues toxicity of amyloid oligomers in vitro

Amyloid oligomers and protofibrils increase cell membrane permeability, eventually leading to cell death. Here, we demonstrate that amyloid oligomer toxicity and membrane permeabilization can be reversed using the membrane sealant copolymer poloxamer 188. The data indicate that amyloid oligomer toxicity is caused by defects in the lipid bilayer of the type that are sealed by poloxamer 188. Our results also suggest the possibility of using polymer-based membrane sealants to prevent or reverse amyloid oligomer toxicity in vivo. Because the ability to permeabilize membranes is a generic property of amyloid oligomers, this therapeutic approach may be effective for the treatment of many degenerative diseases caused in part by the interaction of misfolded proteins with cell membranes, as in Alzheimer's disease, type II diabetes, and a host of others.