Does the Cytotoxic Effect of Transient Amyloid Oligomers from Common Equine Lysozyme in Vitro Imply Innate Amyloid Toxicity?

Effect of formulation and peptide folding on the fibrillar aggregation, gelation, and oxidation of a therapeutic peptide

The physical and chemical stability of therapeutic peptides presents challenges in developing robust formulations. The stability of the formulation affects product safety, efficacy and quality. Therefore, an understanding of the effects of formulation variables on the peptide's conformational structure and on its possible physical and chemical degradation is vital. To this end, computational and experimental analysis were employed to investigate the impact of formulation, peptide folding and product handling on oxidation, fibrillar aggregation and gelation of teriparatide. Teriparatide was used as a model drug due to the correlation of its conformation in solution with its pharmacological activity. Fibrillar aggregation and gelation were monitored using four orthogonal techniques. An innovative, automated platform coupled with ion mobility mass spectrometry was used for profiling chemical degradants. Increases in teriparatide concentration, pH, and ionic strength were found to increase the rate of fibrillar aggregation and gelation. Conversely, an increase in peptide folding and stabilization of the folded structures was found to decrease the rate of fibrillar aggregation and gelation. Moreover, the rate of oxidation was found to be inversely related to its solution concentration and extent of peptide folding. The present study provides an insight into formulation strategies designed to reduce the potential risk of physical and chemical degradation of peptides with a defined conformation.

Starting at the beginning: endoplasmic reticulum proteostasis and systemic amyloid disease

Systemic amyloid diseases are characterized by the deposition of an amyloidogenic protein as toxic oligomers and amyloid fibrils on tissues distal from the site of protein synthesis. Traditionally, these diseases have been viewed as disorders of peripheral target tissues where aggregates are deposited, and toxicity is observed. However, recent evidence highlights an important role for endoplasmic reticulum (ER) proteostasis pathways within tissues synthesizing and secreting amyloidogenic proteins, such as the liver, in the pathogenesis of these disorders. Here, we describe the pathologic implications of ER proteostasis and its regulation on the toxic extracellular aggregation of amyloidogenic proteins implicated in systemic amyloid disease pathogenesis. Furthermore, we discuss the therapeutic potential for targeting ER proteostasis to reduce the secretion and toxic aggregation of amyloidogenic proteins to mitigate peripheral amyloid-associated toxicity involved in the onset and progression of systemic amyloid diseases.

Surface-Directed Structural Transition of Amyloidogenic Aggregates and the Resulting Neurotoxicity

The transition of amyloidogenic species into ordered structures (i.e., prefibrillar oligomers, protofibrils, mature fibrils, and amyloidogenic aggregates) is closely associated with many neurodegenerative disease pathologies. It is increasingly appreciated that the liquid-solid interface contributes to peptide aggregation under physiological conditions. However, much remains to be explored on the molecular mechanism of surface-directed amyloid formation. We herein demonstrate that physical environmental conditions (i.e., negatively charged surface) affect amyloid formation. Nontoxic amyloid aggregates quickly develop into intertwisting fibrils on a negatively charged mica surface. These fibrillar structures show significant cytotoxicity on both neuroblastoma cell-lines (SH-SY5Y) and primary neural stem cells. Our results suggest an alternative amyloid development pathway, following which Aβ peptides form large amyloidogenic aggregates upon stimulation, and later transit into neurotoxic fibrillar structures while being trapped and aligned by a negatively charged surface. Conceivably, the interplay between chemical and physical environmental conditions plays important roles in the development of neurodegenerative diseases.

Amyloid-Beta Peptides Trigger Aggregation of Alpha-Synuclein In Vitro

Alzheimer's disease (AD) and Parkinson's disease (PD), including dementia with Lewy bodies (DLB), account for the majority of dementia cases worldwide. Interestingly, a significant number of patients have clinical and neuropathological features of both AD and PD, i.e., the presence of amyloid deposits and Lewy bodies in the neocortex. The identification of α-synuclein peptides in amyloid plaques in DLB brain led to the hypothesis that both peptides mutually interact with each other to facilitate neurodegeneration. In this article, we report the influence of Aβ(1-42) and pGlu-Aβ(3-42) on the aggregation of α-synuclein in vitro. The aggregation of human recombinant α-synuclein was investigated using thioflavin-T fluorescence assay. Fibrils were investigated by means of antibody conjugated immunogold followed by transmission electron microscopy (TEM). Our data demonstrate a significantly increased aggregation propensity of α-synuclein in the presence of minor concentrations of Aβ(1-42) and pGlu-Aβ(3-42) for the first time, but without effect on toxicity on mouse primary neurons. The analysis of the composition of the fibrils by TEM combined with immunogold labeling of the peptides revealed an interaction of α-synuclein and Aβ in vitro, leading to an accelerated fibril formation. The analysis of kinetic data suggests that significantly enhanced nucleus formation accounts for this effect. Additionally, co-occurrence of α-synuclein and Aβ and pGlu-Aβ, respectively, under pathological conditions was confirmed in vivo by double immunofluorescent labelings in brains of aged transgenic mice with amyloid pathology. These observations imply a cross-talk of the amyloid peptides α-synuclein and Aβ species in neurodegeneration. Such effects might be responsible for the co-occurrence of Lewy bodies and plaques in many dementia cases.

Serum amyloid P component promotes formation of distinct aggregated lysozyme morphologies and reduces toxicity in Drosophila flies expressing F57I lysozyme

Many conflicting reports about the involvement of serum amyloid P component (SAP) in amyloid diseases have been presented over the years; SAP is known to be a universal component of amyloid aggregates but it has been suggested that it can both induce and suppress amyloid formation. By using our Drosophila model of systemic lysozyme amyloidosis, SAP has previously been shown to reduce the toxicity induced by the expression of the disease-associated lysozyme variant, F57I, in the Drosophila central nervous system. This study further investigates the involvement of SAP in modulating lysozyme toxicity using histochemistry and spectral analyses on the double transgenic WT and F57I lysozyme flies to probe; i) formation of aggregates, ii) morphological differences of the aggregated lysozyme species formed in the presence or absence of SAP, iii) location of lysozyme and iv) co-localisation of lysozyme and SAP in the fly brain. We found that SAP can counteract the toxicity (measured by the reduction in the median survival time) induced by F57I lysozyme by converting toxic F57I species into less toxic amyloid-like structures, as reflected by the spectral changes that p-FTAA undergoes when bound to lysozyme deposits in F57I-F57I-SAP flies as compared to F57I-F57I flies. Indeed, when SAP was introduced to in vitro lysozyme fibril formation, the endpoint fibrils had enhanced ThT fluorescence intensity as compared to lysozyme fibrils alone. This suggests that a general mechanism for SAP's role in amyloid diseases may be to promote the formation of stable, amyloid-like fibrils, thus decreasing the impact of toxic species formed along the aggregation pathway.

Catechol-Containing Hydroxylated Biomimetic 4-Thiaflavanes as Inhibitors of Amyloid Aggregation

The study of compounds able to interfere in various ways with amyloid aggregation is of paramount importance in amyloid research. Molecules characterized by a 4-thiaflavane skeleton have received great attention in chemical, medicinal, and pharmaceutical research. Such molecules, especially polyhydroxylated 4-thiaflavanes, can be considered as structural mimickers of several natural polyphenols that have been previously demonstrated to bind and impair amyloid fibril formation. In this work, we tested five different 4-thiaflavanes on the hen egg-white lysozyme (HEWL) amyloid model for their potential anti-amyloid properties. By combining a thioflavin T assay, atomic force microscopy, and a cell toxicity assay, we demonstrated that such compounds can impair the formation of high-order amyloid aggregates and mature fibrils. Despite this, the tested 4-thiaflavanes, although non-toxic per se, are not able to prevent amyloid toxicity on human neuroblastoma cells. Rather, they proved to block early aggregates in a stable, toxic conformation. Accordingly, 4-thiaflavanes can be proposed for further studies aimed at identifying blocking agents for the study of toxicity mechanisms of amyloid aggregation.

Mechanisms for the inhibition of amyloid aggregation by small ligands

This work investigates by biochemical, biophysical and MD techniques the opposite anti-amyloid properties of resveratrol and rosmarinic acid on the aggregation of hen egg white lysozyme (HEWL). Differences in association energy and contact maps were found that explain the different behaviours.

The formation of amyloid aggregates is the hallmark of systemic and neurodegenerative disorders, also known as amyloidoses. Many proteins have been found to aggregate into amyloid-like fibrils and this process is recognized as a general tendency of polypeptides. Lysozyme, an antibacterial protein, is a well-studied model since it is associated in human with systemic amyloidosis and that is widely available from chicken eggs (HEWL, hen egg white lysozyme). In the present study we investigated the mechanism of interaction of aggregating HEWL with rosmarinic acid and resveratrol, that we verified to be effective and ineffective, respectively, in inhibiting aggregate formation. We used a multidisciplinary strategy to characterize such effects, combining biochemical and biophysical methods with molecular dynamics (MD) simulations on the HEWL peptide 49–64 to gain insights into the mechanisms and energy variations associated to amyloid formation and inhibition. MD revealed that neither resveratrol nor rosmarinic acid were able to compete with the initial formation of the β-sheet structure. We then tested the association of two β-sheets, representing the model of an amyloid core structure. MD showed that rosmarinic acid displayed an interaction energy and a contact map comparable to that of sheet pairings. On the contrary, resveratrol association energy was found to be much lower and its contact map largely different than that of sheet pairings. The overall characterization elucidated a possible mechanism explaining why, in this model, resveratrol is inactive in blocking fibril formation, whereas rosmarinic acid is instead a powerful inhibitor.

Attenuation of lysozyme amyloid cytotoxicity by SPION-mediated modulation of amyloid aggregation

The formation and deposition of proteinaceous aggregates of amyloid fibrils characterize diverse degenerative diseases, such as Alzheimer's, Parkinson's, and systemic amyloidosis. The presence of these aggregates is associated with clinical manifestations, and various forms of amyloid aggregates have been identified to be cytotoxic. Although the exact mechanism of amyloid toxicity remains to be elucidated, prevention of amyloid fibril formation and aggregation forms a possible therapeutic approach. Nanomaterials possess the potential for such a strategy. Using hen egg white lysozyme (HEWL) as a prototypic amyloid-forming protein, we found a reduction in the aggregation rate of HEWL in the presence of super-paramagnetic iron oxide nanoparticles (SPIONs) with slowing of nucleation and amyloid fibril elongation. HEWL-amyloid had a predominantly fibrillar structure and was toxic to various cells. A significant attenuation of cytotoxicity was observed when cells were treated with SPION-interacted HEWL-amyloid. Ultra-structural differences were observed between the native and SPION-interacted HEWL-amyloids by SEM and TEM imaging. Our findings confirm that SPIONs perturb amyloid fibrillation, thereby reducing the cytotoxicity of amyloid.

Interactions of Lipid Membranes with Fibrillar Protein Aggregates

Amyloid fibrils are an intriguing class of protein aggregates with distinct physicochemical, structural and morphological properties. They display peculiar membrane-binding behavior, thus adding complexity to the problem of protein-lipid interactions. The consensus that emerged during the past decade is that amyloid cytotoxicity arises from a continuum of cross-β-sheet assemblies including mature fibrils. Based on literature survey and our own data, in this chapter we address several aspects of fibril-lipid interactions, including (i) the effects of amyloid assemblies on molecular organization of lipid bilayer; (ii) competition between fibrillar and monomeric membrane-associating proteins for binding to the lipid surface; and (iii) the effects of lipids on the structural morphology of fibrillar aggregates. To illustrate some of the processes occurring in fibril-lipid systems, we present and analyze fluorescence data reporting on lipid bilayer interactions with fibrillar lysozyme and with the N-terminal 83-residue fragment of amyloidogenic mutant apolipoprotein A-I, 1-83/G26R/W@8. The results help understand possible mechanisms of interaction and mutual remodeling of amyloid fibers and lipid membranes, which may contribute to amyloid cytotoxicity.

Oligomers, protofibrils and amyloid fibrils from recombinant human lysozyme (rHL): fibrillation process and cytotoxicity evaluation for ARPE-19 cell line

Amyloid-associated diseases, such Alzheimer's, Huntington's, Parkinson's, and type II diabetes, are related to protein misfolding and aggregation. Herein, the time evolution of scattered light intensity, hydrophobic properties, and conformational changes during fibrillation processes of rHL solutions at 55 °C and pH 2.0 were used to monitor the aggregation process of recombinant human lysozyme (rHL). Dynamic light scattering (DLS), thioflavin T (ThT) fluorescence, and surface tension (ST) at the air-water interface were used to analyze the hydrophobic properties of pre-amyloid aggregates involved in the fibrillation process of rHL to find a correlation between the hydrophobic character of oligomers, protofibrils and amyloid aggregates with the gain in cross-β-sheet structure, depending on the increase in the incubation periods. The ability of the different aggregates of rHL isolated during the fibrillation process to be adsorbed at the air-water interface can provide important information about the hydrophobic properties of the protein, which can be related to changes in the secondary structure of rHL, resulting in cytotoxic or non-cytotoxic species. Thus, we evaluated the cytotoxic effect of oligomers, protofibrils and amyloid fibrils on the cell line ARPE-19 using the MTT reduction test. The more cytotoxic protein species arose after a 600-min incubation time, suggesting that the hydrophobic character of pre-amyloid fibrils, in addition to the high prevalence of the cross-β-sheet conformation, can become toxic for the cell line ARPE-19.

Preventing α-synuclein aggregation: the role of the small heat-shock molecular chaperone proteins

Protein homeostasis, or proteostasis, is the process of maintaining the conformational and functional integrity of the proteome. The failure of proteostasis can result in the accumulation of non-native proteins leading to their aggregation and deposition in cells and in tissues. The amyloid fibrillar aggregation of the protein α-synuclein into Lewy bodies and Lewy neuritis is associated with neurodegenerative diseases classified as α-synucleinopathies, which include Parkinson's disease and dementia with Lewy bodies. The small heat-shock proteins (sHsps) are molecular chaperones that are one of the cell's first lines of defence against protein aggregation. They act to stabilise partially folded protein intermediates, in an ATP-independent manner, to maintain cellular proteostasis under stress conditions. Thus, the sHsps appear ideally suited to protect against α-synuclein aggregation, yet these fail to do so in the context of the α-synucleinopathies. This review discusses how sHsps interact with α-synuclein to prevent its aggregation and, in doing so, highlights the multi-faceted nature of the mechanisms used by sHsps to prevent the fibrillar aggregation of proteins. It also examines what factors may contribute to α-synuclein escaping the sHsp chaperones in the context of the α-synucleinopathies.

Amyloid oligomers and protofibrils, but not filaments, self-replicate from native lysozyme

Self-assembly of amyloid fibrils is the molecular mechanism best known for its connection with debilitating human disorders such as Alzheimer’s disease but is also associated with various functional cellular responses. There is increasing evidence that amyloid formation proceeds along two distinct assembly pathways involving either globular oligomers and protofibrils or rigid monomeric filaments. Oligomers, in particular, have been implicated as the dominant molecular species responsible for pathogenesis. Yet the molecular mechanisms regulating their self-assembly have remained elusive. Here we show that oligomers/protofibrils and monomeric filaments, formed along distinct assembly pathways, display critical differences in their ability to template amyloid growth at physiological vs denaturing temperatures. At physiological temperatures, amyloid filaments remained stable but could not seed growth of native monomers. In contrast, oligomers and protofibrils not only remained intact but were capable of self-replication using native monomers as the substrate. Kinetic data further suggested that this prion-like growth mode of oligomers/protofibrils involved two distinct activities operating orthogonal from each other: autocatalytic self-replication of oligomers from native monomers and nucleated polymerization of oligomers into protofibrils. The environmental changes to stability and templating competence of these different amyloid species in different environments are likely to be important for understanding the molecular mechanisms underlying both pathogenic and functional amyloid self-assembly.

A complex of equine lysozyme and oleic acid with bactericidal activity against Streptococcus pneumoniae

HAMLET and ELOA are complexes consisting of oleic acid and two homologous, yet functionally different, proteins with cytotoxic activities against mammalian cells, with HAMLET showing higher tumor cells specificity, possibly due to the difference in propensity for oleic acid binding, as HAMLET binds 5-8 oleic acid molecules per protein molecule and ELOA binds 11-48 oleic acids. HAMLET has been shown to possess bactericidal activity against a number of bacterial species, particularly those with a respiratory tropism, with Streptococcus pneumoniae displaying the greatest degree of sensitivity. We show here that ELOA also displays bactericidal activity against pneumococci, which at lower concentrations shows mechanistic similarities to HAMLET’s bactericidal activity. ELOA binds to S. pneumoniae and causes perturbations of the plasma membrane, including depolarization and subsequent rupture, and activates an influx of calcium into the cells. Selective inhibition of calcium channels and sodium/calcium exchange activity significantly diminished ELOA’s bactericidal activity, similar to what we have observed with HAMLET. Finally, ELOA-induced death was also accompanied by DNA fragmentation into high molecular weight fragments – an apoptosis-like morphological phenotype that is seen during HAMLET-induced death. Thus, in contrast to different mechanisms of eukaryote cell death induced by ELOA and HAMLET, these complexes are characterized by rather similar activities towards bacteria. Although the majority of these events could be mimicked using oleic acid alone, the concentrations of oleic acid required were significantly higher than those present in the ELOA complex, and for some assays, the results were not identical between oleic acid alone and the ELOA complex. This indicates that the lipid, as a common denominator in both complexes, is an important component for the complexes’ bactericidal activities, while the proteins are required both to solubilize and/or present the lipid at the bacterial membrane and likely to confer other and separate functions during the bacterial death.

Characterization of the nucleation process of lysozyme at physiological pH: primary but not sole process

We report on a kinetic study of the heat-induced aggregation process of lysozyme at physiological pH. The time evolution of the aggregation extent and the conformational changes of the protein were followed by dynamic light scattering (DLS) and FTIR spectroscopy, respectively, whereas the morphology of the aggregates was observed by Atomic Force Microscopy (AFM). The conformational changes of the secondary and tertiary structures were simultaneous and distinct in time with respect to the formation of aggregates. Oligomer formation occurred through at least two different aggregation processes: a nucleation process and a homogeneous non-nucleative diffusion-controlled process. FTIR measurements showed that supramolecular aggregation proceeded without the formation of β-aggregated structures and AFM images revealed the presence of oligomers and amorphous aggregates; no fibrillar structures were observed.

Characterization of oligomers of heterogeneous size as precursors of amyloid fibril nucleation of an SH3 domain: an experimental kinetics study

Understanding the earliest molecular events during nucleation of the amyloid aggregation cascade is of fundamental significance to prevent amyloid related disorders. We report here an experimental kinetic analysis of the amyloid aggregation of the N47A mutant of the α-spectrin SH3 domain (N47A Spc-SH3) under mild acid conditions, where it is governed by rapid formation of amyloid nuclei. The initial rates of formation of amyloid structures, monitored by thioflavine T fluorescence at different protein concentrations, agree quantitatively with high-order kinetics, suggesting an oligomerization pre-equilibrium preceding the rate-limiting step of amyloid nucleation. The curves of native state depletion also follow high-order irreversible kinetics. The analysis is consistent with the existence of low-populated and heterogeneous oligomeric precursors of fibrillation that form by association of partially unfolded protein monomers. An increase in NaCl concentration accelerates fibrillation but reduces the apparent order of the nucleation kinetics; and a double mutant (K43A, N47A) Spc-SH3 domain, largely unfolded under native conditions and prone to oligomerize, fibrillates with apparent first order kinetics. On the light of these observations, we propose a simple kinetic model for the nucleation event, in which the monomer conformational unfolding and the oligomerization of an amyloidogenic intermediate are rapidly pre-equilibrated. A conformational change of the polypeptide chains within any of the oligomers, irrespective of their size, is the rate-limiting step leading to the amyloid nuclei. This model is able to explain quantitatively the initial rates of aggregation and the observed variations in the apparent order of the kinetics and, more importantly, provides crucial thermodynamic magnitudes of the processes preceding the nucleation. This kinetic approach is simple to use and may be of general applicability to characterize the amyloidogenic intermediates and oligomeric precursors of other disease-related proteins.

The C-terminal repeating units of CsgB direct bacterial functional amyloid nucleation

Curli are functional amyloids produced by enteric bacteria. The major curli fiber subunit, CsgA, self-assembles into an amyloid fiber in vitro. The minor curli subunit protein, CsgB, is required for CsgA polymerization on the cell surface. Both CsgA and CsgB are composed of five predicted β-strand-loop-β-strand-loop repeating units that feature conserved glutamine and asparagine residues. Because of this structural homology, we proposed that CsgB might form an amyloid template that initiates CsgA polymerization on the cell surface. To test this model, we purified wild-type CsgB and found that it self-assembled into amyloid fibers in vitro. Preformed CsgB fibers seeded CsgA polymerization as did soluble CsgB added to the surface of cells secreting soluble CsgA. To define the molecular basis of CsgB nucleation, we generated a series of mutants that removed each of the five repeating units. Each of these CsgB deletion mutants was capable of self-assembly in vitro. In vivo, membrane-localized mutants lacking the first, second, or third repeating units were able to convert CsgA into fibers. However, mutants missing either the fourth or fifth repeating units were unable to complement a csgB mutant. These mutant proteins were not localized to the outer membrane but were instead secreted into the extracellular milieu. Synthetic CsgB peptides corresponding to repeating units 1, 2, and 4 self-assembled into ordered amyloid polymers, while peptides corresponding to repeating units 3 and 5 did not, suggesting that there are redundant amyloidogenic domains in CsgB. Our results suggest a model where the rapid conversion of CsgB from unstructured protein to a β-sheet-rich amyloid template anchored to the cell surface is mediated by the C-terminal repeating units.

Neuroprotective and nootropic drug noopept rescues α-synuclein amyloid cytotoxicity

Parkinson's disease is a common neurodegenerative disorder characterized by α-synuclein (α-Syn)-containing Lewy body formation and selective loss of dopaminergic neurons in the substantia nigra. We have demonstrated the modulating effect of noopept, a novel proline-containing dipeptide drug with nootropic and neuroprotective properties, on α-Syn oligomerization and fibrillation by using thioflavin T fluorescence, far-UV CD, and atomic force microscopy techniques. Noopept does not bind to a sterically specific site in the α-Syn molecule as revealed by heteronuclear two-dimensional NMR analysis, but due to hydrophobic interactions with toxic amyloid oligomers, it prompts their rapid sequestration into larger fibrillar amyloid aggregates. Consequently, this process rescues the cytotoxic effect of amyloid oligomers on neuroblastoma SH-SY5Y cells as demonstrated by using cell viability assays and fluorescent staining of apoptotic and necrotic cells and by assessing the level of intracellular oxidative stress. The mitigating effect of noopept against amyloid oligomeric cytotoxicity may offer additional benefits to the already well-established therapeutic functions of this new pharmaceutical.

Antimicrobial peptides and proteins of the horse--insights into a well-armed organism

Antimicrobial peptides play a pivotal role as key effectors of the innate immune system in plants and animals and act as endogenous antibiotics. The molecules exhibit an antimicrobial activity against bacteria, viruses, and eukaryotic pathogens with different specificities and potencies depending on the structure and amino-acid composition of the peptides. Several antimicrobial peptides were comprehensively investigated in the last three decades and some molecules with remarkable antimicrobial properties have reached the third phase of clinical studies. Next to the peptides themselves, numerous organisms were examined and analyzed regarding their repertoire of antimicrobial peptides revealing a huge number of candidates with potencies and properties for future medical applications. One of these organisms is the horse, which possesses numerous peptides that are interesting candidates for therapeutical applications in veterinary medicine. Here we summarize investigations and knowledge on equine antimicrobial peptides, point to interesting candidates, and discuss prospects for therapeutical applications.

α-synuclein reactive antibodies as diagnostic biomarkers in blood sera of Parkinson's disease patients

Background

Auto-antibodies with specificity to self-antigens have been implicated in a wide variety of neurological diseases, including Parkinson's (PD) and Alzheimer's diseases, being sensitive indicators of neurodegeneration and focus for disease prevention. Of particular interest are the studies focused on the auto-immune responses to amyloidogenic proteins associated with diseases and their applications in therapeutic treatments such as vaccination with amyloid antigens and antibodies in PD, Alzheimer's disease and potentially other neurodegeneration ailments.

Methodology/Principal Findings

Generated auto-antibodies towards the major amyloidogenic protein involved in PD Lewy bodies – α-synuclein and its amyloid oligomers and fibrils were measured in the blood sera of early and late PD patients and controls by using ELISA, Western blot and Biacore surface plasmon resonance. We found significantly higher antibody levels towards monomeric α-synuclein in the blood sera of PD patients compared to controls, though the responses decreased with PD progression (P<0.0001). This indicates potential protective role of autoimmunity in maintaining the body homeostasis and clearing protein species whose disbalance may lead to amyloid assembly. There were no noticeable immune responses towards amyloid oligomers, but substantially increased levels of IgGs towards α-synuclein amyloid fibrils both in PD patients and controls, which subsided with the disease progression (P<0.0001). Pooled IgGs from PD patients and controls interacted also with the amyloid fibrils of Aβ (1–40) and hen lysozyme, however the latter were recognized with lower affinity. This suggests that IgGs bind to the generic amyloid conformational epitope, displaying higher specificity towards human amyloid species associated with neurodegeneration.

Conclusions/Significance

Our findings may suggest the protective role of autoimmunity in PD and therefore immune reactions towards PD major amyloid protein – α-synuclein can be of value in the development of treatment and diagnostic strategies, especially during the early disease stages.

Structure and function of human α-lactalbumin made lethal to tumor cells (HAMLET)-type complexes

Human α-lactalbumin made lethal to tumor cells (HAMLET) and equine lysozyme with oleic acid (ELOA) are complexes consisting of protein and fatty acid that exhibit cytotoxic activities, drastically differing from the activity of their respective proteinaceous compounds. Since the discovery of HAMLET in the 1990s, a wealth of information has been accumulated, illuminating the structural, functional and therapeutic properties of protein complexes with oleic acid, which is summarized in this review. In vitro, both HAMLET and ELOA are produced by using ion-exchange columns preconditioned with oleic acid. However, the complex of human α-lactalbumin with oleic acid with the antitumor activity of HAMLET was found to be naturally present in the acidic fraction of human milk, where it was discovered by serendipity. Structural studies have shown that α-lactalbumin in HAMLET and lysozyme in ELOA are partially unfolded, 'molten-globule'-like, thereby rendering the complexes dynamic and in conformational exchange. HAMLET exists in the monomeric form, whereas ELOA mostly exists as oligomers and the fatty acid stoichiometry varies, with HAMLET holding an average of approximately five oleic acid molecules, whereas ELOA contains a considerably larger number (11- 48). Potent tumoricidal activity is found in both HAMLET and ELOA, and HAMLET has also shown strong potential as an antitumor drug in different in vivo animal models and clinical studies. The gain of new, beneficial function upon partial protein unfolding and fatty acid binding is a remarkable phenomenon, and may reflect a significant generic route of functional diversification of proteins via varying their conformational states and associated ligands.

Lipoprotein complex of equine lysozyme with oleic acid (ELOA) interactions with the plasma membrane of live cells

Recent evidence supports the idea that early aggregates, protein, and lipoprotein oligomers but not large aggregates like fibrils that are formed at late stages of the aggregation process are responsible for cytotoxicity. Oligomers can interact with the cellular plasma membrane affecting its structure and/or dynamics or may be taken up by the cells. In either case, disparate cascades of molecular interactions are activated in the attempt to counteract the disturbance induced by the oligomers. If unsuccessful, cell death follows. Here, we study the molecular and cellular mechanisms underlying PC12 cell death caused by ELOA oligomers. ELOA, a lipoprotein complex formed by equine lysozyme (EL) and oleic acid (OA), induces cell death in all tested cell lines, but the actual mechanism of its action is not known. We have used methods with single-molecule sensitivity, fluorescence correlation spectroscopy (FCS), fluorescence cross-correlation spectroscopy (FCCS), and confocal laser scanning microscopy (CLSM) imaging by avalanche photodiodes (APD), so-called APD imaging, to study ELOA interactions with the plasma membrane in live PC12 cells. We detected ELOA accumulation in the cell surroundings, observed ELOA interactions with the plasma membrane, and local changes in plasma membrane lipid dynamics in the vicinity of ELOA complexes. These interactions resulted in plasma membrane rupture, followed by rapid influx and distribution of ELOA inside the already dead cell. In order to probe the ELOA-plasma membrane interaction sites at the molecular and atomic levels, the ELOA complexes were further studied by photochemically induced dynamic nuclear polarization (photo-CIDNP) spectroscopy, nuclear magnetic resonance (NMR) and atomic force microscopy (AFM). We observed a novel mechanism of oligomer toxicity-cell death induced by continuous disturbance of the plasma membrane, eventually causing permanent plasma membrane damage and identified the sites in ELOA that are potentially involved in the interactions with the plasma membrane.

Cytotoxic aggregation and amyloid formation by the myostatin precursor protein

Myostatin, a negative regulator of muscle growth, has been implicated in sporadic inclusion body myositis (sIBM). sIBM is the most common age-related muscle-wastage disease with a pathogenesis similar to that of amyloid disorders such as Alzheimer's and Parkinson's diseases. Myostatin precursor protein (MstnPP) has been shown to associate with large molecular weight filamentous inclusions containing the Alzheimer's amyloid beta peptide in sIBM tissue, and MstnPP is upregulated following ER stress. The mechanism for how MstnPP contributes to disease pathogenesis is unknown. Here, we show for the first time that MstnPP is capable of forming amyloid fibrils in vitro. When MstnPP-containing Escherichia coli inclusion bodies are refolded and purified, a proportion of MstnPP spontaneously misfolds into amyloid-like aggregates as characterised by electron microscopy and binding of the amyloid-specific dye thioflavin T. When subjected to a slightly acidic pH and elevated temperature, the aggregates form straight and unbranched amyloid fibrils 15 nm in diameter and also exhibit higher order amyloid structures. Circular dichroism spectroscopy reveals that the amyloid fibrils are dominated by beta-sheet and that their formation occurs via a conformational change that occurs at a physiologically relevant temperature. Importantly, MstnPP aggregates and protofibrils have a negative effect on the viability of myoblasts. These novel results show that the myostatin precursor protein is capable of forming amyloid structures in vitro with implications for a role in sIBM pathogenesis.

Lability landscape and protease resistance of human insulin amyloid: a new insight into its molecular properties

Amyloid formation is a universal behavior of proteins central to many important human pathologies and industrial processes. The extreme stability of amyloids towards chemical and proteolytic degradation is an acquired property compared to the precursor proteins and is a major prerequisite for their accumulation. Here, we report a study on the lability of human insulin amyloid as a function of pH and amyloid ageing. Using a range of methods such as atomic force microscopy, thioflavin T fluorescence, circular dichroism, and gas-phase electrophoretic mobility macromolecule analysis, we probed the propensity of human insulin amyloid to propagate or dissociate in a wide span of pH values and ageing in a low concentration regime. We generated a three-dimensional amyloid lability landscape in coordinates of pH and amyloid ageing, which displays three distinctive features: (i) a maximum propensity to grow near pH 3.8 and an age corresponding to the inflection point of the growth phase, (ii) an abrupt cutoff between growth and disaggregation at pH 8-10, and (iii) isoclines shifted towards older age during the amyloid growth phase at pH 4-9, reflecting the greater stability of aged amyloid. Thus, lability of amyloid strongly depends on the ionization state of insulin and on the structure and maturity of amyloid fibrils. The stability of insulin amyloid towards protease K was assessed by using real-time atomic force microscopy and thioflavin T fluorescence. We estimated that amyloid fibrils can be digested both from the free ends and within the length of the fibril with a rate of ca 4 nm/min. Our results highlight that amyloid structures, depending on solution conditions, can be less stable than commonly perceived. These results have wide implications for understanding the propagation of amyloids via a seeding mechanism as well as for understanding their natural clearance and dissociation under solution conditions unfavorable for amyloid formation in biological systems and industrial applications.

Polymorphism of Alzheimer's Abeta17-42 (p3) oligomers: the importance of the turn location and its conformation

Abeta(17-42) (so-called p3) amyloid is detected in vivo in the brains of individuals with Alzheimer's disease or Down's syndrome. We investigated the polymorphism of Abeta(17-42) oligomers based on experimental data from steady-state NMR measurements, electron microscopy, two-dimensional hydrogen exchange, and mutational studies, using all-atom molecular-dynamics simulation with explicit solvent. We assessed the structural stability and the populations. Our results suggest that conformational differences in the U-turn of Abeta(17-42) lead to polymorphism in beta-sheet registration and retention of an ordered beta-strand organization at the termini. Further, although the parallel Abeta(17-42) oligomer organization is the most stable of the conformers investigated here, different antiparallel Abeta(17-42) organizations are also stable and compete with the parallel architectures, presenting a polymorphic population. In this study we propose that 1), the U-turn conformation is the primary factor leading to polymorphism in the assembly of Abeta(17-42) oligomers, and is also coupled to oligomer growth; and 2), both parallel Abeta(17-42) oligomers and an assembly of Abeta(17-42) oligomers that includes both parallel and antiparallel organizations contribute to amyloid fibril formation. Finally, since a U-turn motif generally appears in amyloids formed by full proteins or long fragments, and since to date these have been shown to exist only in parallel architectures, our results apply to a broad range of oligomers and fibrils.

Prions: Protein Aggregation and Infectious Diseases

Transmissible spongiform encephalopathies (TSEs) are inevitably lethal neurodegenerative diseases that affect humans and a large variety of animals. The infectious agent responsible for TSEs is the prion, an abnormally folded and aggregated protein that propagates itself by imposing its conformation onto the cellular prion protein (PrPC) of the host. PrPCis necessary for prion replication and for prion-induced neurodegeneration, yet the proximal causes of neuronal injury and death are still poorly understood. Prion toxicity may arise from the interference with the normal function of PrPC, and therefore, understanding the physiological role of PrPCmay help to clarify the mechanism underlying prion diseases. Here we discuss the evolution of the prion concept and how prion-like mechanisms may apply to other protein aggregation diseases. We describe the clinical and the pathological features of the prion diseases in human and animals, the events occurring during neuroinvasion, and the possible scenarios underlying brain damage. Finally, we discuss potential antiprion therapies and current developments in the realm of prion diagnostics.

A model of amyloid's role in disease based on fibril fracture

Although the correlative evidence relating the presence of amyloid fibrils and certain disease states (e.g. Alzheimer's disease and Type 2 Diabetes) is overwhelming, a direct causative role for amyloid has proved harder to establish. Current thinking links a narrow region of the aggregate protein size distribution, the so called 'early aggregate' domain to cellular toxicity. A troubling feature of this theory however is that the nucleated reaction mechanism by which amyloid formation is believed to occur results in a very low number concentration of early aggregates which are rapidly extended to form amyloid fibrils. This situation means that the concentration of early aggregates is very low at the time when they are supposedly at their most toxic. Here we adopt a novel explicit simulation strategy to examine a kinetic regime involving nucleated growth combined with fibril fragmentation under which this situation can be reversed so as to produce a high number concentration of small on-pathway toxic aggregates. Dependent upon the rate of fragmentation, the time scale for generation of toxic early aggregates may be coupled, uncoupled or disassociated from the time scale for the appearance of amyloid fibrils. Furthermore the model presents itself as a biochemical 'switch' transitioning between modes of amyloid induced cell death dependent upon either specific amyloid toxicity or non-specific solid mass induced tissue damage.

Expression and purification of active recombinant equine lysozyme in Escherichia coli

Equine lysozyme (EL) is a calcium (Ca)-binding lysozyme and is an intermediary link between non-Ca-binding C-type lysozyme and alpha-lactalbumin. The feature of lysozymes to assemble into the fibrils has recently gained considerable attention for the investigation of the functional properties of these proteins. To study the structural and functional properties of EL, a synthetic gene was cloned and EL was overexpressed in Escherichia coli as a fused protein. The His-tagged recombinant EL was accumulated as inclusion bodies. Up to 50 mg/l of the recombinant EL could be achieved after purification by Ni(2+) affinity chromatography, refolding in the presence of arginine, CM-Sepharose column purification following TEV protease cleavage. The purified protein was functionally active, as determined by the lysozyme activity, proving the proper folding of protein. The purified lysozyme was used for the oligomerisation studies. The protein formed amyloid fibrils during incubation in acidic pH and elevated temperature. The recombinant EL forms two types of fibrils: ring shaped and linear, similar to the native EL.

Amyloid formation by the pro-inflammatory S100A8/A9 proteins in the ageing prostate

Background

The conversion of soluble peptides and proteins into polymeric amyloid structures is a hallmark of many age-related degenerative disorders, including Alzheimer's disease, type II diabetes and a variety of systemic amyloidoses. We report here that amyloid formation is linked to another major age-related phenomenon − prostate tissue remodelling in middle-aged and elderly men.

Methodology/Principal Findings

By using multidisciplinary analysis of corpora amylacea inclusions in prostate glands of patients diagnosed with prostate cancer we have revealed that their major components are the amyloid forms of S100A8 and S100A9 proteins associated with numerous inflammatory conditions and types of cancer. In prostate protease rich environment the amyloids are stabilized by dystrophic calcification and lateral thickening. We have demonstrated that material closely resembling CA can be produced from S100A8/A9 in vitro under native and acidic conditions and shows the characters of amyloids. This process is facilitated by calcium or zinc, both of which are abundant in ex vivo inclusions. These observations were supported by computational analysis of the S100A8/A9 calcium-dependent aggregation propensity profiles. We found DNA and proteins from Escherichia coli in CA bodies, suggesting that their formation is likely to be associated with bacterial infection. CA inclusions were also accompanied by the activation of macrophages and by an increase in the concentration of S100A8/A9 in the surrounding tissues, indicating inflammatory reactions.

Conclusions/Significance

These findings, taken together, suggest a link between bacterial infection, inflammation and amyloid deposition of pro-inflammatory proteins S100A8/A9 in the prostate gland, such that a self-perpetuating cycle can be triggered and may increase the risk of malignancy in the ageing prostate. The results provide strong support for the prediction that the generic ability of polypeptide chains to convert into amyloids could lead to their involvement in an increasing number of otherwise apparently unrelated diseases, particularly those associated with ageing.

Cytotoxic aggregates of alpha-lactalbumin induced by unsaturated fatty acid induce apoptosis in tumor cells

The effects of three fatty acids on cytotoxic aggregate formation of Ca(2+)-depleted bovine alpha-lactalbumin (apo-BLA) have been studied by UV absorbance spectroscopy and transmission electron microscopy. The experimental results demonstrate that two unsaturated fatty acids, oleic acid and linoleic acid, and one saturated fatty acid, stearic acid, induce the intermediate of apo-BLA at pH 4.0-4.5 to form amorphous aggregates in time- and concentration-dependent manners. These aggregates are dissolved under physiological conditions at 37 degrees C and further characterized by fluorescence spectroscopy, circular dichroism and time-of-flight mass spectrometry. Our data here indicate that the structural characteristics of these aggregates are similar to those of HAMLET/BAMLET (human/bovine alpha-lactalbumin made lethal to tumor cells), a complex of the partially unfolded alpha-lactalbumin with oleic acid. Cell viability experiments indicate the aggregates of apo-BLA induced by oleic acid and linoleic acid show significant dose-dependent cytotoxicity to human lung tumor cells of A549 but those induced by stearic acid have no toxicity to tumor cells. Furthermore, the cytotoxic aggregates of apo-BLA induced by both unsaturated fatty acids induce apoptosis of human lung cancer cell line A549, suggesting that such cytotoxic aggregates of apo-BLA could be potential antitumor drugs. The present study provides insight into the mechanism of fatty acid-dependent oligomerization and cytotoxicity of alpha-lactalbumin, and will be helpful in the understanding of the molecular mechanism of HAMLET/BAMLET formation.

Localized and efficient curli nucleation requires the chaperone-like amyloid assembly protein CsgF

Elucidation of the early events in amyloidogenesis is key to understanding the pathology of, and developing therapies for, amyloid diseases. Critical informants about these early events are amyloid assembly proteins that facilitate the transition from monomer to amyloid fiber. Curli are a functional amyloid whose in vivo polymerization requires a dedicated nucleator protein, CsgB, and an assembly protein, CsgF. Here we demonstrate that without CsgF, curli subunits are released from the cell into the media and are inefficiently polymerized, resulting in fewer and mislocalized curli fibers. CsgF is secreted to the cell surface, where it mediates the cell-association and protease-resistance of the CsgB nucleator, suggesting that CsgF is required for specific localization and/or chaperoning of CsgB for full nucleator activity. CsgF is thus critical to achieve localized and efficient nucleation of fiber subunits into functional, cell-associated amyloid.

Soluble oligomers from a non-disease related protein mimic Abeta-induced tau hyperphosphorylation and neurodegeneration

Protein aggregation and amyloid accumulation in different tissues are associated with cellular dysfunction and toxicity in important human pathologies, including Alzheimer's disease and various forms of systemic amyloidosis. Soluble oligomers formed at the early stages of protein aggregation have been increasingly recognized as the main toxic species in amyloid diseases. To gain insight into the mechanisms of toxicity instigated by soluble protein oligomers, we have investigated the aggregation of hen egg white lysozyme (HEWL), a normally harmless protein. HEWL initially aggregates into beta-sheet rich, roughly spherical oligomers which appear to convert with time into protofibrils and mature amyloid fibrils. HEWL oligomers are potently neurotoxic to rat cortical neurons in culture, while mature amyloid fibrils are little or non-toxic. Interestingly, when added to cortical neuronal cultures HEWL oligomers induce tau hyperphosphorylation at epitopes that are characteristically phosphorylated in neurons exposed to soluble oligomers of the amyloid-beta peptide. Furthermore, injection of HEWL oligomers in the cerebral cortices of adult rats induces extensive neurodegeneration in different brain areas. These results show that soluble oligomers from a non-disease related protein can mimic specific neuronal pathologies thought to be induced by soluble amyloid-beta peptide oligomers in Alzheimer's disease and support the notion that amyloid oligomers from different proteins may share common structural determinants that would explain their generic cytotoxicities.

Fluorescence depolarization studies of filamentous actin analyzed with a genetic algorithm

A new method, in which a genetic algorithm was combined with Brownian dynamics and Monte Carlo simulations, was developed to analyze fluorescence depolarization data collected by the time-correlated single photon-counting technique. It was applied to studies of BODIPY-labeled filamentous actin (F-actin). The technique registered the local order and reorienting motions of the fluorophores, which were covalently coupled to cysteine 374 (C374) in actin and interacted by electronic energy migration within the actin polymers. Analyses of F-actin samples composed of different fractions of labeled actin molecules revealed the known helical organization of F-actin, demonstrating the usefulness of this technique for structure determination of complex protein polymers. The distance from the filament axis to the fluorophore was found to be considerably less than expected from the proposed position of C374 at a high filament radius. In addition, polymerization experiments with BODIPY-actin suggest a 25-fold more efficient signal for filament formation than pyrene-actin.

The nootropic and neuroprotective proline-containing dipeptide noopept restores spatial memory and increases immunoreactivity to amyloid in an Alzheimer's disease model

The effects of the novel proline-containing nootropic and neuroprotective dipeptide, noopept (GVS-111, N-phenylacetyl-L-prolylglycine ethyl ester) were investigated in NMRI mice following olfactory bulbectomy. We have shown previously that these animals developed Alzheimer's disease (AD)-like behaviour, morphology and biochemistry including impairment of spatial memory, regional neuronal degeneration and elevated Abeta peptide brain levels. In the current investigation, spatial memory was assessed using the Morris water maze and serum antibodies to in vitro morphologically characterized amyloid structures of both Abeta((25-35)) peptide and equine lysozyme, as well as to neurotrophic glial factor S100b, were analyzed by enzyme-linked immunosorbent assay (ELISA). Noopept (administered at a dose of 0.01 mg/kg for a period of 21 days and during a further 5 days training) restored spatial memory and increased serum antibody levels to oligomers of Abeta((25-35)) peptide but not to equine lysozyme amyloid or S100b protein in bulbectomized animals. The positive immunotropic effect of noopept to Abeta((25-35)) peptide prefibrillar aggregates was more marked in sham-operated compared to the bulbectomized subjects which were characterized by an overall suppression of immunoreactivity. Enhancement of the immune response to Abeta((25-35)) peptide prefibrils caused by noopept may attenuate the neurotoxic consequences of amyloid fibrillization and also be associated with an improvement in spatial memory in bulbectomized mice. These actions of noopept, combined with its previously reported neuroprotective and cholinomimetic properties, suggests that this dipeptide may well be useful for improving cognitive deficits induced by neurodegenerative diseases.

The curli nucleator protein, CsgB, contains an amyloidogenic domain that directs CsgA polymerization

Curli are functional amyloid fibers assembled by enteric bacteria such as Escherichia coli and Salmonella spp. In E. coli, the polymerization of the major curli fiber subunit protein CsgA into an amyloid fiber depends on the minor curli subunit protein, CsgB. The outer membrane-localized CsgB protein shares approximately 30% sequence identity with the amyloid-forming protein CsgA, suggesting that CsgB might also have amyloidogenic properties. Here, we characterized the biochemical properties of CsgB and the molecular basis for CsgB-mediated nucleation of CsgA. Deletion analysis revealed that a CsgB molecule missing 19 amino acids from its C terminus (CsgB(trunc)) was not outer membrane-associated, but secreted away from the cell. CsgB(trunc) was overexpressed and purified from the extracellular milieu of cells as an SDS-soluble, nonaggregated protein. Soluble CsgB(trunc) assembled into fibers that bound to the amyloid-specific dyes Congo red and thioflavin-T. CsgB(trunc) fibers were able to seed soluble CsgA polymerization in vitro. CsgB(trunc) displayed modest nucleator activity in vivo, as demonstrated by its ability to convert extracellular CsgA into an amyloid fiber. Unlike WT CsgB, CsgB(trunc) was only able to act as a nucleator when cells were genetically manipulated to secrete higher concentrations of CsgA. This work represents a unique demonstration of functional amyloid nucleation and it suggests an elegant model for how E. coli guides efficient amyloid fiber formation on the cell surface.

Equine lysozyme: The molecular basis of folding, self-assembly and innate amyloid toxicity

Calcium-binding equine lysozyme (EL) combines the structural and folding properties of c-type lysozymes and alpha-lactalbumins, connecting these two most studied subfamilies. The structural insight into its native and partially folded states is particularly illuminating in revealing the general principles of protein folding, amyloid formation and its inhibition. Among lysozymes EL forms one of the most stable molten globules and shows the most uncooperative refolding kinetics. Its partially-folded states serve as precursors for calcium-dependent self-assembly into ring-shaped and linear amyloids. The innate amyloid cytotoxicity of the ubiquitous lysozyme highlights the universality of this phenomenon and necessitates stringent measures for its prevention.

Immune reactivity towards insulin, its amyloid and protein S100B in blood sera of Parkinson's disease patients

Peripheral immune responses can be sensitive indicators of disease pathology. We evaluated the autoimmune reactions to endocrine (insulin) and astrocytical (S100B) biomarkers in the blood sera of 26 Parkinson's disease (PD) patients compared with controls by using ELISA. We found a statistically significant increase of the autoimmune responses to both antigens in PD patients compared with controls with a mean increase of 70% and 50% in the autoimmune reactions towards insulin and S100B, respectively. Heterogeneity of the immune responses observed in patients may reflect the modulating effect of multiple variables associated with neurodegeneration and also changes in the basic mechanisms of individual autoimmune reactivity. We did not detect any pronounced immune reactions towards insulin amyloid fibrils and oligomers in PD patients, indicating that an amyloid-specific conformational epitope is not involved in immune recognition of this amyloid type, while sequential epitope of native insulin is hidden within the amyloid structures. Immune reactions towards S100B and insulin may reflect the neurodegenerative brain damaging processes and impaired insulin homeostasis occurring in PD.

Differential neuroimmune markers to the onset of Alzheimer's disease neurodegeneration and dementia: Autoantibodies to Aβ(25–35) oligomers, S100b and neurotransmitters

Alzheimer's disease (AD) autoimmunity is a focus for dementia prevention. Generated autoantibodies against major etiopathogenic molecular targets as neuroimmune markers of dementia were measured by ELISA in patient sera. Biphasic antibody levels to Abeta((25-35)) oligomers, S100b and DA were detected during distinctly diagnosed dementia stages. Abeta((25-35)) oligomer autoimmune responses reflected mild to moderate AD dementia, while those to S100b, DA and the S100b concentrations, matched moderate to severe dementia progression. 5-HT antibodies increased during mild dementia and plateaued thereafter. This autoimmunity pattern may be used as a differential biomarker profile in designing AD therapeutic strategies involving early vaccination.

Amyloid-linked cellular toxicity triggered by bacterial inclusion bodies

The aggregation of proteins in the form of amyloid fibrils and plaques is the characteristic feature of some pathological conditions ranging from neurodegenerative disorders to systemic amyloidoses. The mechanisms by which the aggregation processes result in cell damage are under intense investigation but recent data indicate that prefibrillar aggregates are the most proximate mediators of toxicity rather than mature fibrils. Since it has been shown that prefibrillar forms of the nondisease-related misfolded proteins are highly toxic to cultured mammalian cells we have studied the cytoxicity associated to bacterial inclusion bodies that have been recently described as protein deposits presenting amyloid-like structures. We have proved that bacterial inclusion bodies composed by a misfolding-prone beta-galactosidase fusion protein are clearly toxic for mammalian cells but the beta-galactosidase wild type enzyme forming more structured thermal aggregates does not impair cell viability, despite it also binds and enter into the cells. These results are in the line that the most cytotoxic aggregates are early prefibrilar assemblies but discard the hypothesis that the membrane destabilization is the key event to subsequent disruption of cellular processes, such as ion balance, oxidative state and the eventually cell death.

Protein misfolding, functional amyloid, and human disease

Peptides or proteins convert under some conditions from their soluble forms into highly ordered fibrillar aggregates. Such transitions can give rise to pathological conditions ranging from neurodegenerative disorders to systemic amyloidoses. In this review, we identify the diseases known to be associated with formation of fibrillar aggregates and the specific peptides and proteins involved in each case. We describe, in addition, that living organisms can take advantage of the inherent ability of proteins to form such structures to generate novel and diverse biological functions. We review recent advances toward the elucidation of the structures of amyloid fibrils and the mechanisms of their formation at a molecular level. Finally, we discuss the relative importance of the common main-chain and side-chain interactions in determining the propensities of proteins to aggregate and describe some of the evidence that the oligomeric fibril precursors are the primary origins of pathological behavior.