Monitoring the process of HypF fibrillization and liposome permeabilization by protofibrils

Structure-function relationships of pre-fibrillar protein assemblies in Alzheimer's disease and related disorders

Several neurodegenerative diseases, including Alzheimer's, Parkinson's, Huntington's and prion diseases, are characterized pathognomonically by the presence of intra- and/or extracellular lesions containing proteinaceous aggregates, and by extensive neuronal loss in selective brain regions. Related non-neuropathic systemic diseases, e.g., light-chain and senile systemic amyloidoses, and other organ-specific diseases, such as dialysis-related amyloidosis and type-2 diabetes mellitus, also are characterized by deposition of aberrantly folded, insoluble proteins. It is debated whether the hallmark pathologic lesions are causative. Substantial evidence suggests that these aggregates are the end state of aberrant protein folding whereas the actual culprits likely are transient, pre-fibrillar assemblies preceding the aggregates. In the context of neurodegenerative amyloidoses, the proteinaceous aggregates may eventuate as potentially neuroprotective sinks for the neurotoxic, oligomeric protein assemblies. The pre-fibrillar, oligomeric assemblies are believed to initiate the pathogenic mechanisms that lead to synaptic dysfunction, neuronal loss, and disease-specific regional brain atrophy. The amyloid beta-protein (Abeta), which is believed to cause Alzheimer's disease (AD), is considered an archetypal amyloidogenic protein. Intense studies have led to nominal, functional, and structural descriptions of oligomeric Abeta assemblies. However, the dynamic and metastable nature of Abeta oligomers renders their study difficult. Different results generated using different methodologies under different experimental settings further complicate this complex area of research and identification of the exact pathogenic assemblies in vivo seems daunting. Here we review structural, functional, and biological experiments used to produce and study pre-fibrillar Abeta assemblies, and highlight similar studies of proteins involved in related diseases. We discuss challenges that contemporary researchers are facing and future research prospects in this demanding yet highly important field.

Differentiation increases the resistance of neuronal cells to amyloid toxicity

A substantial lack of information is recognized on the features underlying the variable susceptibility to amyloid aggregate toxicity of cells with different phenotypes. Recently, we showed that different cell types are variously affected by early aggregates of a prokaryotic hydrogenase domain (HypF-N). In the present study we investigated whether differentiation affects cell susceptibility to amyloid injury using a human neurotypic SH-SY5Y cell differentiation model. We found that retinoic acid-differentiated cells were significantly more resistant against Abeta1-40, Abeta1-42 and HypF-N prefibrillar aggregate toxicity respect to undifferentiated cells treated similarly. Earlier and sharper increases in cytosolic Ca(2+) and ROS with marked lipid peroxidation and mitochondrial dysfunction were also detected in exposed undifferentiated cells resulting in apoptosis activation. The reduced vulnerability of differentiated cells matched a more efficient Ca(2+)-ATPase equipment and a higher total antioxidant capacity. Finally, increasing the content of membrane cholesterol resulted in a remarkable reduction of vulnerability and ability to bind the aggregates in either undifferentiated and differentiated cells.

New structures help the modeling of toxic amyloidbeta ion channels

The mechanism of amyloid toxicity is poorly understood and there are two schools of thought in this hotly debated field: the first favors membrane destabilization by intermediate-to-large amyloid oligomers, with consequent thinning and non-specific ion leakage; the second favors ion-specific permeable channels lined by small amyloid oligomers. Published results currently support both mechanisms. However, the amyloidbeta (Abeta) peptide has recently been shown to form a U-shaped 'beta-strand-turn-beta-strand' structure. This structure and the available physiological data present a challenge for computational biology--to provide candidate models consistent with the experimental data. Modeling based on small Abeta oligomers containing extramembranous N-termini predicts channels with shapes and dimensions consistent with experimentally derived channel structures. These results support the hypothesis that small Abeta oligomers can form ion channels. Molecular dynamics modeling can provide blueprints of 3D structural conformations for many other amyloids whose membrane association is key to their toxicity.

Analysis of Abeta interactions using ProteinChip technology

Abeta peptides are now acknowledged to play a central role in the pathogenesis of Alzheimer's disease. Their generation results from the sequential cleavage of amyloid precursor protein by beta and gamma secretases. The resulting peptide fragments impart toxicity via their ability to form soluble oligomers and bind to cell membranes. In this chapter we describe the use of ProteinChip technology to study the physicochemical behaviour of Abeta and its mechanisms of toxicity. These include analyzing (1) Abeta processing and quantitation of peptide fragments, (2) Abeta aggregation and the quantitation of oligomers, and (3) Abeta-lipid interactions.

Amyloid peptides and proteins in review

Amyloids are filamentous protein deposits ranging in size from nanometres to microns and composed of aggregated peptide beta-sheets formed from parallel or anti-parallel alignments of peptide beta-strands. Amyloid-forming proteins have attracted a great deal of recent attention because of their association with over 30 diseases, notably neurodegenerative conditions like Alzheimer's, Huntington's, Parkinson's, Creutzfeldt-Jacob and prion disorders, but also systemic diseases such as amyotrophic lateral sclerosis (Lou Gehrig's disease) and type II diabetes. These diseases are all thought to involve important conformational changes in proteins, sometimes termed misfolding, that usually produce beta-sheet structures with a strong tendency to aggregate into water-insoluble fibrous polymers. Reasons for such conformational changes in vivo are still unclear. Intermediate aggregated state(s), rather than precipitated insoluble polymeric aggregates, have recently been implicated in cellular toxicity and may be the source of aberrant pathology in amyloid diseases. Numerous in vitro studies of short and medium length peptides that form amyloids have provided some clues to amyloid formation, with an alpha-helix to beta-sheet folding transition sometimes implicated as an intermediary step leading to amyloid formation. More recently, quite a few non-pathological amyloidogenic proteins have also been identified and physiological properties have been ascribed, challenging previous implications that amyloids were always disease causing. This article summarises a great deal of current knowledge on the occurrence, structure, folding pathways, chemistry and biology associated with amyloidogenic peptides and proteins and highlights some key factors that have been found to influence amyloidogenesis.

Generic cell dysfunction in neurodegenerative disorders: role of surfaces in early protein misfolding, aggregation, and aggregate cytotoxicity

Recent knowledge supports the idea that early protein aggregates share basic structural features and are responsible for cytotoxicity underlying neurodegeneration; in most cases, early aggregate cytotoxicity apparently proceeds through similar molecular mechanisms and results in similar biochemical modifications. Data suggest that aggregate cytotoxicity may be considered a generic property of the oligomers preceding fibril appearance. Oligomers can interact with cell membranes, impairing their structural organization and destroying their selective ion permeability, eventually culminating with cell death. This process can be influenced by the physicochemical features and aggregation state of amyloids as well as by the physical and biochemical features of cell surfaces. The roles of synthetic and biological surfaces in affecting protein folding and misfolding, in speeding up aggregate nucleation, and as targets of aggregate toxicity is gaining consideration. Recent research has highlighted the involvement of surfaces as protein-misfolding chaperones and aggregation catalysts and their effects in these phenomena.

In vitro and in vivo neurotoxicity of prion protein oligomers

The mechanisms underlying prion-linked neurodegeneration remain to be elucidated, despite several recent advances in this field. Herein, we show that soluble, low molecular weight oligomers of the full-length prion protein (PrP), which possess characteristics of PrP to PrPsc conversion intermediates such as partial protease resistance, are neurotoxic in vitro on primary cultures of neurons and in vivo after subcortical stereotaxic injection. Monomeric PrP was not toxic. Insoluble, fibrillar forms of PrP exhibited no toxicity in vitro and were less toxic than their oligomeric counterparts in vivo. The toxicity was independent of PrP expression in the neurons both in vitro and in vivo for the PrP oligomers and in vivo for the PrP fibrils. Rescue experiments with antibodies showed that the exposure of the hydrophobic stretch of PrP at the oligomeric surface was necessary for toxicity. This study identifies toxic PrP species in vivo. It shows that PrP-induced neurodegeneration shares common mechanisms with other brain amyloidoses like Alzheimer disease and opens new avenues for neuroprotective intervention strategies of prion diseases targeting PrP oligomers.

Effects of succinylation on thermal induced amyloid formation in Concanavalin A

We have recently shown that upon slight thermal destabilization the legume lectin Concanavalin A may undergo two different aggregation processes, leading, respectively, to amyloid fibrils at high pH and amorphous aggregates at low pH. Here we present an experimental study on the amyloid aggregation of Succinyl Concanavalin A, which is a dimeric active variant of Concanavalin. The results show that, as for the native protein, the fibrillation process appears to be favoured by alkaline pH, far from the isoelectric point of the protein. Moreover, it strongly depends on temperature and requires large conformational changes both at secondary and tertiary structure level. With respect to the native protein, the succinyl derivative forms amyloid fibrils in considerably longer times and with a minor exposure of hydrophobic regions. At physiological conditions, Concanavalin A still displays a sizeable tendency to form amyloid fibril, while the succinyl variant does not. A close correlation was observed between the progress of amyloid formation and a narrowing of the tryptophans fluorescence emission band, indicating a reduction of protein conformational heterogeneity in amyloid fibrils.

Solid-state NMR as a method to reveal structure and membrane-interaction of amyloidogenic proteins and peptides

It is important to understand the Amyloid fibril formation in view of numerous medical and biochemical aspects. Structural determination of amyloid fibril has been extensively studied using electron microscopy. Subsequently, solid state NMR spectroscopy has been realized to be the most important means to determine not only microscopic molecular structure but also macroscopic molecular packing. Molecular structure of amyloid fibril was first predicted to be parallel beta-sheet structure, and subsequently, was further refined for Abeta(1-40) to be cross beta-sheet with double layered in register parallel beta-sheet structure by using solid state NMR spectroscopy. On the other hand, anti-parallel beta-sheet structure has been reported to short fragments of Abeta-amyloid and other amyloid forming peptides. Kinetic study of amyloid fibril formation has been studied using a variety of methods, and two-step autocatalytic reaction mechanism used to explain fibril formation. Recently, stable intermediates or proto-fibrils have been observed by electron microscope (EM) images. Some of the intermediates have the same microscopic structure as the matured fibril and subsequently change to matured fibrils. Another important study on amyloid fibril formation is determination of the interaction with lipid membranes, since amyloid peptide are cleaved from amyloid precursor proteins in the membrane interface, and it is reported that amyloid lipid interaction is related to the cytotoxicity. Finally it is discussed how amyloid fibril formation can be inhibited. Firstly, properly designed compounds are reported to have inhibition ability of amyloid fibril formation by interacting with amyloid peptide. Secondly, it is revealed that site directed mutation can inhibit amyloid fibril formation. These inhibitors were developed by knowing the fibril structure determined by solid state NMR.

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.

Kinetics of Different Processes in Human Insulin Amyloid Formation

Human insulin has long been known to form amyloid fibrils under given conditions. The molecular basis of insulin aggregation is relevant for modeling the amyloidogenesis process, which is involved in many pathologies, as well as for improving delivery systems, used for diabetes treatments. Insulin aggregation displays a wide variety of morphologies, from small oligomeric filaments to huge floccules, and therefore different specific processes are likely to be intertwined in the overall aggregation. In the present work, we studied the aggregation kinetics of human insulin at low pH and different temperatures and concentrations. The structure and the morphogenesis of aggregates on a wide range of length scales (from monomeric proteins to elongated fibrils and larger aggregates networks) have been monitored by using different experimental techniques: time-lapse atomic force microscopy (AFM), quasi-elastic light-scattering (QLS), small and large angle static light-scattering, thioflavin T fluorescence, and optical microscopy. Our experiments, along with the analysis of scattered intensity distribution, show that fibrillar aggregates grow following a thermally activated heterogeneous coagulation mechanism, which includes both tip-to-tip elongation and lateral thickening. Also, the association of fibrils into bundles and larger clusters (up to tens of microns) occurs simultaneously and is responsible for an effective lag-time.

Heme binding inhibits the fibrillization of amyloidogenic apomyoglobin and determines lack of aggregate cytotoxicity

Myoglobin is an alpha-helical globular protein containing two highly conserved tryptophanyl residues at positions 7 and 14 in the N-terminal region. The double W/F replacement renders apomyoglobin highly susceptible to aggregation and amyloid-like fibril formation under physiological conditions. In this work we analyze the early stage of W7FW14F apomyoglobin aggregation following the time dependence of the process by far-UV CD, Fourier-transform infrared (FTIR) spectroscopy, and heme-binding properties. The results show that the aggregation of W7FW14F apomyoglobin starts from a native-like globin state able to bind the prosthetic group with spectroscopic properties similar to those observed for wild-type apoprotein. Nevertheless, it rapidly aggregates, forming amyloid fibrils. However, when the prosthetic group is added before the beginning of aggregation, amyloid fibrillization is inhibited, although the aggregation process is not prevented. Moreover, the apomyoglobin aggregates formed in these conditions are not cytotoxic differently from what is observed for all amyloidogenic proteins. These results open new insights into the relationship between the structure adopted by the protein into the aggregates and their ability to trigger the impairment of cell viability.

Amyloid fibrils formation and amorphous aggregation in concanavalin A

We here report an experimental study on the thermal aggregation process of concanavalin A, a protein belonging to the legume lectins family. The aggregation process and the involved conformational changes of the protein molecules were followed by means of fluorescence techniques, light scattering, circular dichroism, zeta potential measurements and atomic force microscopy. Our results show that the aggregation process of concanavalin A may evolve through two distinct pathways leading, respectively, to the formation of amyloids or amorphous aggregates. The relative extent of the two pathways is determined by pH, as amyloid aggregation is favored at high pH values ( approximately 9), while the formation of amorphous aggregates is favored at low pH ( approximately 5). At difference from amorphous aggregation, the formation of amyloid fibrils requires significant conformational changes on the protein, both at secondary and tertiary structural level. To our knowledge, this is the first observation of amyloid fibrils from concanavalin A.

Natively folded HypF-N and its early amyloid aggregates interact with phospholipid monolayers and destabilize supported phospholipid bilayers

Recent data depict membranes as the main sites where proteins/peptides are recruited and concentrated, misfold, and nucleate amyloids; at the same time, membranes are considered key triggers of amyloid toxicity. The N-terminal domain of the prokaryotic hydrogenase maturation factor HypF (HypF-N) in 30% trifluoroethanol undergoes a complex path of fibrillation starting with initial 2-3-nm oligomers and culminating with the appearance of mature fibrils. Oligomers are highly cytotoxic and permeabilize lipid membranes, both biological and synthetic. In this article, we report an in-depth study aimed at providing information on the surface activity of HypF-N and its interaction with synthetic membranes of different lipid composition, either in the native conformation or as amyloid oligomers or fibrils. Like other amyloidogenic peptides, the natively folded HypF-N forms stable films at the air/water interface and inserts into synthetic phospholipid bilayers with efficiencies depending on the type of phospholipid. In addition, HypF-N prefibrillar aggregates interact with, insert into, and disassemble supported phospholipid bilayers similarly to other amyloidogenic peptides. These results support the idea that, at least in most cases, early amyloid aggregates of different peptides and proteins produce similar effects on the integrity of membrane assembly and hence on cell viability.

Prefibrillar amyloid aggregates could be generic toxins in higher organisms

More than 40 human diseases are associated with fibrillar deposits of specific peptides or proteins in tissue. Amyloid fibrils, or their precursors, can be highly toxic to cells, suggesting their key role in disease pathogenesis. Proteins not associated with any disease are able to form oligomers and amyloid assemblies in vitro displaying structures and cytotoxicity comparable with those of aggregates of disease-related polypeptides. In isolated cells, such toxicity has been shown to result from increased membrane permeability with disruption of ion homeostasis and oxidative stress. Here we microinjected into the nucleus basalis magnocellularis of rat brains aggregates of an Src homology 3 domain and the N-terminal domain of the prokaryotic HypF, neither of which is associated with amyloid disease. Prefibrillar aggregates of both proteins, but not their mature fibrils or soluble monomers, impaired cholinergic neuron viability in a dose-dependent manner similar to that seen in cell cultures. Contrary to the situation with cultured cells, however, under our experimental conditions, cell stress in tissue is not followed by a comparable level of cell death, a result that is very likely to reflect the presence of protective mechanisms reducing aggregate toxicity. These findings support the hypothesis that neurodegenerative disorders result primarily from a generic cell dysfunction caused by early misfolded species in the aggregation process.

Collagen Plays an Active Role in the Aggregation of β2-Microglobulin under Physiopathological Conditions of Dialysis-related Amyloidosis

Dialysis-related amyloidosis is characterized by the deposition of insoluble fibrils of beta(2)-microglobulin (beta(2)-m) in the musculoskeletal system. Atomic force microscopy inspection of ex vivo amyloid material reveals the presence of bundles of fibrils often associated to collagen fibrils. Aggregation experiments were undertaken in vitro with the aim of reproducing the physiopathological fibrillation process. To this purpose, atomic force microscopy, fluorescence techniques, and NMR were employed. We found that in temperature and pH conditions similar to those occurring in periarticular tissues in the presence of flogistic processes, beta(2)-m fibrillogenesis takes place in the presence of fibrillar collagen, whereas no fibrils are obtained without collagen. Moreover, the morphology of beta(2)-m fibrils obtained in vitro in the presence of collagen is extremely similar to that observed in the ex vivo sample. This result indicates that collagen plays a crucial role in beta(2)-m amyloid deposition under physiopathological conditions and suggests an explanation for the strict specificity of dialysis-related amyloidosis for the tissues of the skeletal system. We hypothesize that positively charged regions along the collagen fiber could play a direct role in beta(2)-m fibrillogenesis. This hypothesis is sustained by aggregation experiments performed by replacing collagen with a poly-L-lysine-coated mica surface. As shown by NMR measurements, no similar process occurs when poly-L-lysine is dissolved in solution with beta(2)-m. Overall, the findings are consistent with the estimates resulting from a simplified collagen model whereby electrostatic effects can lead to high local concentrations of oppositely charged species, such as beta(2)-m, that decay on moving away from the fiber surface.

Common mechanisms of amyloid oligomer pathogenesis in degenerative disease

Many age-related degenerative diseases, including Alzheimer's, Parkinson's, Huntington's diseases and type II diabetes, are associated with the accumulation of amyloid fibrils. The protein components of these amyloids vary widely and the mechanisms of pathogenesis remain an important subject of competing hypotheses and debate. Many different mechanisms have been postulated as significant causal events in pathogenesis, so understanding which events are primary and their causal relationships is critical for the development of more effective therapeutic agents that target the underlying disease mechanisms. Recent evidence indicates that amyloids share common structural properties that are largely determined by their generic polymer properties and that soluble amyloid oligomers may represent the primary pathogenic structure, rather than the mature amyloid fibrils. Since protein function is determined by the three-dimensional structure, the fact that amyloids share generic structures implies that they may also share a common pathological function. Amyloid oligomers from several different proteins share the ability to permeabilize cellular membranes and lipid bilayers, indicating that this may represent the primary toxic mechanism of amyloid pathogenesis. This suggests that membrane permeabilization may initiate a core sequence of common pathological events leading to cell dysfunction and death that is shared among degenerative diseases, whereas pathological events that are unique to one particular type of amyloid or disease may lie in up stream pathways leading to protein mis-folding. Although, these upstream events may be unique to a particular disease related protein, their effects can be rationalized as having a primary effect of increasing the amount of mis-folded, potentially amyloidogenic proteins.

Early events in insulin fibrillization studied by time-lapse atomic force microscopy

The importance of understanding the mechanism of protein aggregation into insoluble amyloid fibrils lies not only in its medical consequences, but also in its more basic properties of self-organization. The discovery that a large number of uncorrelated proteins can form, under proper conditions, structurally similar fibrils has suggested that the underlying mechanism is a general feature of polypeptide chains. In this work, we address the early events preceding amyloid fibril formation in solutions of zinc-free human insulin incubated at low pH and high temperature. Here, we show by time-lapse atomic force microscopy that a steady-state distribution of protein oligomers with a quasiexponential tail is reached within a few minutes after heating. This metastable phase lasts for a few hours, until fibrillar aggregates are observable. Although for such complex systems different aggregation mechanisms can occur simultaneously, our results indicate that the prefibrillar phase is mainly controlled by a simple coagulation-evaporation kinetic mechanism, in which concentration acts as a critical parameter. These experimental facts, along with the kinetic model used, suggest a critical role for thermal concentration fluctuations in the process of fibril nucleation.

From the Polymorphism of Amyloid Fibrils to their Assembly Mechanism and Cytotoxicity

Extracellular amyloid deposits are present in a variety of diseases. They contain amyloid fibrils that arise from the association of proteins or peptides. At the molecular level, all these fibrils share a common assembly principle based on a conformational change of the protein precursor leading to the formation of a cross-beta sheet structure. The smallest observed fibrils in vitro, often called protofibrils, are 4-5 nm in diameter. An amyloid fibril is generally composed of several of these protofibrils and may adopt different morphologies such as ribbons, sheets, or multistranded cables. This polymorphism was observed with many different amyloid-forming peptides and proteins using electron microscopy. The need to understand the molecular origin of this effect as well as the desire to find inhibitors of fibril formation has driven researchers toward the dissection of amyloid fibril assembly pathways. We review the current knowledge on amyloid polymorphism and discuss recent findings in the field concerning amyloid fibril assembly pathways and cytotoxicity mechanisms.

Amyloid formation of a protein in the absence of initial unfolding and destabilization of the native state

In 5% (v/v) trifluoroethanol, pH 5.5, 25 degrees C one of the acylphosphatases from Drosophila melanogaster (AcPDro2) forms fibrillar aggregates that bind thioflavin T and Congo red and have an extensive beta-sheet structure, as revealed by circular dichroism. Atomic force microscopy indicates that the fibrils and their constituent protofilaments have diameters compatible with those of natural amyloid fibrils. Spectroscopic and biochemical investigation, carried out using near- and far-UV circular dichroism, intrinsic and 1-anilino-8-naphthalenesulfonic acid-derived fluorescence, dynamic light scattering, and enzymatic activity assays, shows that AcPDro2 has, before aggregation, a secondary structure content packing around aromatic and hydrophobic residues, hydrodynamic diameter, and catalytic activity indistinguishable from those of the native protein. The native protein was found to have the same conformational stability under native and aggregating conditions, as determined from urea-induced unfolding. The kinetic analysis supports models in which AcPDro2 aggregates initially without need to unfold and subsequently undergoes a conformational change into amyloid-like structures. Although fully or partially unfolded states have a higher propensity to aggregate, the residual aggregation potential that proteins maintain upon complete folding can be physiologically relevant and be directly involved in the pathogenesis of some protein deposition diseases.

Evidence for a mechanism of amyloid formation involving molecular reorganisation within native-like precursor aggregates

The aggregation of the alpha/beta protein acylphosphatase from Sulfolobus solfataricus has been studied under conditions in which the protein maintains a native-like, although destabilised, conformation and that therefore bear resemblance to a physiological medium. Static and dynamic light-scattering measurements indicate that under these conditions the protein aggregates rapidly, within two minutes. The initial aggregates are enzymatically active and have a secondary structure that is not yet characterized by the high content of cross-beta structure typical of amyloid, as inferred from Fourier transform infra-red and circular dichroism measurements. These species then convert slowly into enzymatically inactive aggregates that bind thioflavin T and Congo red, characteristic of amyloid structures, and contain extensive beta-sheet structure. Transmission electron microscopy reveals the presence in the latter aggregates of spherical species and thin, elongated protofibrils, both with diameters of 3-5 nm. Kinetic tests reveal that this process occurs without the need for dissolution and re-nucleation of the aggregates. Formation of thioflavin T-binding and beta-structured aggregates is substantially more rapid than unfolding of the native state, indicating that the initial aggregation process promotes formation of amyloid structures. Taken together, these findings suggest a mechanism of amyloid formation that may have physiological relevance and in which the amyloid structures result from reorganisation of the molecular interactions within the initially formed non-amyloid aggregates.

beta-amyloid is different in normal aging and in Alzheimer disease

The mechanism of neurodegeneration caused by beta-amyloid in Alzheimer disease is controversial. Neuronal toxicity is exerted mostly by various species of soluble beta-amyloid oligomers that differ in their N- and C-terminal domains. However, abundant accumulation of beta-amyloid also occurs in the brains of cognitively normal elderly people, in the absence of obvious neuronal dysfunction. We postulated that neuronal toxicity depends on the molecular composition, rather than the amount, of the soluble beta-amyloid oligomers. Here we show that soluble beta-amyloid aggregates that accumulate in Alzheimer disease are different from those of normal aging in regard to the composition as well as the aggregation and toxicity properties.

Protofibril Formation of Amyloid β-Protein at Low pH via a Non-cooperative Elongation Mechanism

Deposition of the amyloid beta-protein (Abeta) in senile or diffuse plaques is a distinctive feature of Alzheimer's disease. The role of Abeta aggregates in the etiology of the disease is still controversial. The formation of linear aggregates, known as amyloid fibrils, has been proposed as the onset and the cause of pathological deposition. Yet, recent findings suggest that a more crucial role is played by prefibrillar oligomeric assemblies of Abeta that are highly toxic in the extracellular environment. In the present work, the mechanism of protofibril formation is studied at pH 3.1, starting from a solution of oligomeric precursors. By combining static light scattering and photon correlation spectroscopy, the growth of the mass and the size of aggregates are determined at different temperatures. Analysis and scaling of kinetic data reveal that under the studied conditions protofibrils are formed via a single non-cooperative elongation mechanism, not prompted by nucleation. This process is well described as a linear colloidal aggregation due to diffusion and coalescence of growing aggregates. The rate of elongation follows an Arrhenius law with an activation enthalpy of 15 kcal mol(-1). Such a value points to a conformational change of peptides or oligomers being involved in binding to protofibrils or in general to a local reorganization of each aggregate. These results contribute to establishing a clearer relation at the molecular level between the fibrillation mechanism and fibrillar precursors. The observation of a non-cooperative aggregation pathway supports the hypothesis that amyloid formation may represent an escape route from a dangerous condition, induced by the presence of toxic oligomeric species.

Insights into the molecular basis of the differing susceptibility of varying cell types to the toxicity of amyloid aggregates

It has been reported that different tissue or cultured cell types are variously affected by the exposure to toxic protein aggregates, however a substantial lack of information exists about the biochemical basis of cell resistance or susceptibility to the aggregates. We investigated the extent of the cytotoxic effects elicited by supplementing the media of a panel of cultured cell lines with aggregates of HypF-N, a prokaryotic domain not associated with any amyloid disease. The cell types exposed to early, pre-fibrillar aggregates (not mature fibrils) displayed variable susceptibility to damage and to apoptotic death with a significant inverse relation to membrane content in cholesterol. Susceptibility to damage by the aggregates was also found to be significantly related to the ability of cells to counteract early modifications of the intracellular free Ca2+ and redox status. Accordingly, cell resistance appeared related to the efficiency of the biochemical equipment leading any cell line to sustain the activity of Ca2+ pumps while maintaining under control the oxidative stress associated with the increased metabolic rate. Our data depict membrane destabilization and the subsequent early derangement of ion balance and intracellular redox status as key events in targeting exposed cells to apoptotic death.

Calcium Dysregulation and Membrane Disruption as a Ubiquitous Neurotoxic Mechanism of Soluble Amyloid Oligomers*♦

Increasing evidence suggests that amyloid peptides associated with a variety of degenerative diseases induce neurotoxicity in their intermediate oligomeric state, rather than as monomers or fibrils. To test this hypothesis and investigate the possible involvement of Ca2+ signaling disruptions in amyloid-induced cytotoxicity, we made homogeneous preparations of disease-related amyloids (Abeta, prion, islet amyloid polypeptide, polyglutamine, and lysozyme) in various aggregation states and tested their actions on fluo-3-loaded SH-SY5Y cells. Application of oligomeric forms of all amyloids tested (0.6-6 microg ml-1) rapidly (approximately 5 s) elevated intracellular Ca2+, whereas equivalent amounts of monomers and fibrils did not. Ca2+ signals evoked by Abeta42 oligomers persisted after depletion of intracellular Ca2+ stores, and small signals remained in Ca2+-free medium, indicating contributions from both extracellular and intracellular Ca2+ sources. The increased membrane permeability to Ca2+ cannot be attributed to activation of endogenous Ca2+ channels, because responses were unaffected by the potent Ca2+-channel blocker cobalt (20 microm). Instead, observations that Abeta42 and other oligomers caused rapid cellular leakage of anionic fluorescent dyes point to a generalized increase in membrane permeability. The resulting unregulated flux of ions and molecules may provide a common mechanism for oligomer-mediated toxicity in many amyloidogenic diseases, with dysregulation of Ca2+ ions playing a crucial role because of their strong trans-membrane concentration gradient and involvement in cell dysfunction and death.

Amyloid formation from HypF-N under conditions in which the protein is initially in its native state

Aggregation of the N-terminal domain of the Escherichia coli HypF (HypF-N) was investigated in mild denaturing conditions, generated by addition of 6-12% (v/v) trifluoroethanol (TFE). Atomic force microscopy indicates that under these conditions HypF-N converts into the same type of protofibrillar aggregates previously shown to be highly toxic to cultured cells. These convert subsequently, after some weeks, into well-defined fibrillar structures. The rate of protofibril formation, monitored by thioflavin T (ThT) fluorescence, depends strongly on the concentration of TFE. Prior to aggregation the protein has far-UV circular dichroism (CD) and intrinsic fluorescence spectra identical with those observed for the native protein in the absence of co-solvent; the quenching of the intrinsic tryptophan fluorescence by acrylamide and the ANS binding properties are also identical in the two cases. These findings indicate that HypF-N is capable of forming amyloid protofibrils and fibrils under conditions in which the protein is initially in a predominantly native-like conformation. The rate constants for folding and unfolding of HypF-N, determined in 10% TFE using the stopped-flow technique, indicate that a partially folded state is in rapid equilibrium with the native state and populated to ca 1%. A kinetic analysis reveals that aggregation results from molecules accessing such a partially folded state. The approach described here shows that it is possible to probe the mechanism of aggregation of a specific protein under conditions in which the protein is initially native and hence relevant to a physiological environment. In addition, the results indicate that toxic protofibrils can be formed from globular proteins under conditions that are only marginally destabilising and in which the large majority of molecules have the native fold. This conclusion emphasises the importance for cells to constantly combat the propensity for even the most stable of these proteins to aggregate.

Investigating the effects of mutations on protein aggregation in the cell

The conversion of peptides and proteins into highly ordered and intractable aggregates is associated with a range of debilitating human diseases and represents a widespread problem in biotechnology. Protein engineering studies carried out in vitro have shown that mutations promote aggregation when they either destabilize the native state of a globular protein or accelerate the conversion of unfolded or partially folded conformations into oligomeric structures. We have extended such studies to investigate protein aggregation in vivo where a number of additional factors able to modify dramatically the aggregation behavior of proteins are present. We have expressed, in Escherichia coli cells, an E. coli protein domain, HypF-N. The results for a range of mutational variants indicate that although mutants with a conformational stability similar to that of the wild-type protein are soluble in the E. coli cytosol, variants with single point mutations predicted to destabilize the protein invariably aggregate after expression. We show, however, that aggregation of destabilized variants can be prevented by incorporating multiple mutations designed to reduce the intrinsic propensity of the polypeptide chain to aggregate; in the cases discussed here, this is achieved by an increase in the net charge of the protein. These results suggest that the principles being established to rationalize aggregation behavior in vitro have general validity for situations in vivo where aggregation has both biotechnological and medical relevance.

Protein misfolding and aggregation: new examples in medicine and biology of the dark side of the protein world

The data reported in the past 5 years have highlighted new aspects of protein misfolding and aggregation. Firstly, it appears that protein aggregation may be a generic property of polypeptide chains possibly linked to their common peptide backbone that does not depend on specific amino acid sequences. In addition, it has been shown that even the toxic effects of protein aggregates, mainly in their pre-fibrillar organization, result from common structural features rather than from specific sequences of side chains. These data lead to hypothesize that every polypeptide chain, in itself, possesses a previously unsuspected hidden dark side leading it to transform into a generic toxin to cells in the presence of suitable destabilizing conditions. This new view of protein biology underscores the key importance, in protein evolution, of the negative selection against molecules with significant tendency to aggregate as well as, in biological evolution, of the development of the complex molecular machineries aimed at hindering the appearance of misfolded proteins and their toxic early aggregates. These data also suggest that, in addition to the well-known amyloidoses, a number of degenerative diseases whose molecular basis are presently unknown might be determined by the intra- or extracellular deposition of aggregates of presently unsuspected proteins. From these considerations one could also envisage the possibility that protein aggregation may be exploited by nature to perform specific physiological functions in differing biological contexts. The present review focuses the most recent reports supporting these ideas and discusses their clinical and biological significance.

Ultrastructural organization of ex vivo amyloid fibrils formed by the apolipoprotein A-I Leu174Ser variant: an atomic force microscopy study

Atomic force microscopy was employed to study ex vivo amyloid material isolated from the transplanted hearts of two patients affected by systemic amyloidosis caused by the Leu174Ser apolipoprotein A-I variant. The purified material consists of fibrils and globular aggregates. For both patients the same morphological patterns are observed; in addition, fibril diameters obtained for the two patients turn out to be compatible, both in air (2.00+/-0.02 and 2.04+/-0.04 nm) and under liquid (10.7+/-0.4 and 11.3+/-0.5 nm). Fibrils display heterogeneous morphologies, occasionally showing a left-handed twist. Inspection of fibril ends, the study of fibril contour shape and the analysis of partially unfolded fibrils yield independent evidences suggesting that most twisted fibrils are composed of three protofilaments. The size of globular aggregates is the same for both patients (4.4+/-0.4 and 5.1+/-0.5 nm, measured under liquid) and is compatible with the protofilament expected diameter, suggesting that globules may represent protofilament precursors.

Prefibrillar amyloid protein aggregates share common features of cytotoxicity

The intracellular free Ca(2+) concentration and redox status of murine fibroblasts exposed to prefibrillar aggregates of the HypF N-terminal domain have been investigated in vitro and in vivo using a range of fluorescent probes. Aggregate entrance into the cytoplasm is followed by an early rise of reactive oxygen species and free Ca(2+) levels and eventually by cell death. Such changes correlate directly with the viability of the cells and are not observed when cell are cultured in the presence of reducing agents or in Ca(2+)-free media. In addition, moderate cell stress following exposure to the aggregates was found to be fully reversible. The results show that the cytotoxicity of prefibrillar aggregates of HypF-N, a protein not associated with clinical disease, has the same fundamental origin as that produced by similar types of aggregates of proteins linked with specific amyloidoses. These findings suggest that misfolded proteinaceous aggregates stimulate generic cellular responses as a result of the exposure of regions of the structure (such as hydrophobic residues and the polypeptide main chain) that are buried in the normally folded proteins. They also support the idea that a higher number of degenerative pathologies than previously known might be considered as protein deposition diseases.