Formation of morphologically similar globular aggregates from diverse aggregation-prone proteins in mammalian cells

General Principles Underpinning Amyloid Structure

Amyloid fibrils are a pathologically and functionally relevant state of protein folding, which is generally accessible to polypeptide chains and differs fundamentally from the globular state in terms of molecular symmetry, long-range conformational order, and supramolecular scale. Although amyloid structures are challenging to study, recent developments in techniques such as cryo-EM, solid-state NMR, and AFM have led to an explosion of information about the molecular and supramolecular organization of these assemblies. With these rapid advances, it is now possible to assess the prevalence and significance of proposed general structural features in the context of a diverse body of high-resolution models, and develop a unified view of the principles that control amyloid formation and give rise to their unique properties. Here, we show that, despite system-specific differences, there is a remarkable degree of commonality in both the structural motifs that amyloids adopt and the underlying principles responsible for them. We argue that the inherent geometric differences between amyloids and globular proteins shift the balance of stabilizing forces, predisposing amyloids to distinct molecular interaction motifs with a particular tendency for massive, lattice-like networks of mutually supporting interactions. This general property unites previously characterized structural features such as steric and polar zippers, and contributes to the long-range molecular order that gives amyloids many of their unique properties. The shared features of amyloid structures support the existence of shared structure-activity principles that explain their self-assembly, function, and pathogenesis, and instill hope in efforts to develop broad-spectrum modifiers of amyloid function and pathology.

Protein Misfolding Diseases and Therapeutic Approaches

Protein folding is the process by which a polypeptide chain acquires its functional, native 3D structure. Protein misfolding, on the other hand, is a process in which protein fails to fold into its native functional conformation. This misfolding of proteins may lead to precipitation of a number of serious diseases such as Cystic Fibrosis (CF), Alzheimer's Disease (AD), Parkinson's Disease (PD), and Amyotrophic Lateral Sclerosis (ALS) etc. Protein Quality-control (PQC) systems, consisting of molecular chaperones, proteases and regulatory factors, help in protein folding and prevent its aggregation. At the same time, PQC systems also do sorting and removal of improperly folded polypeptides. Among the major types of PQC systems involved in protein homeostasis are cytosolic, Endoplasmic Reticulum (ER) and mitochondrial ones. The cytosol PQC system includes a large number of component chaperones, such as Nascent-polypeptide-associated Complex (NAC), Hsp40, Hsp70, prefoldin and T Complex Protein-1 (TCP-1) Ring Complex (TRiC). Protein misfolding diseases caused due to defective cytosolic PQC system include diseases involving keratin/collagen proteins, cardiomyopathies, phenylketonuria, PD and ALS. The components of PQC system of Endoplasmic Reticulum (ER) include Binding immunoglobulin Protein (BiP), Calnexin (CNX), Calreticulin (CRT), Glucose-regulated Protein GRP94, the thiol-disulphide oxidoreductases, Protein Disulphide Isomerase (PDI) and ERp57. ER-linked misfolding diseases include CF and Familial Neurohypophyseal Diabetes Insipidus (FNDI). The components of mitochondrial PQC system include mitochondrial chaperones such as the Hsp70, the Hsp60/Hsp10 and a set of proteases having AAA+ domains similar to the proteasome that are situated in the matrix or the inner membrane. Protein misfolding diseases caused due to defective mitochondrial PQC system include medium-chain acyl-CoA dehydrogenase (MCAD)/Short-chain Acyl-CoA Dehydrogenase (SCAD) deficiency diseases, hereditary spastic paraplegia. Among therapeutic approaches towards the treatment of various protein misfolding diseases, chaperones have been suggested as potential therapeutic molecules for target based treatment. Chaperones have been advantageous because of their efficient entry and distribution inside the cells, including specific cellular compartments, in therapeutic concentrations. Based on the chemical nature of the chaperones used for therapeutic purposes, molecular, chemical and pharmacological classes of chaperones have been discussed.

Glutathionylation primes soluble glyceraldehyde-3-phosphate dehydrogenase for late collapse into insoluble aggregates

Protein aggregation is a complex physiological process, primarily determined by stress-related factors revealing the hidden aggregation propensity of proteins that otherwise are fully soluble. Here we report a mechanism by which glycolytic glyceraldehyde-3-phosphate dehydrogenase of Arabidopsis thaliana (AtGAPC1) is primed to form insoluble aggregates by the glutathionylation of its catalytic cysteine (Cys149). Following a lag phase, glutathionylated AtGAPC1 initiates a self-aggregation process resulting in the formation of branched chains of globular particles made of partially misfolded and totally inactive proteins. GSH molecules within AtGAPC1 active sites are suggested to provide the initial destabilizing signal. The following removal of glutathione by the formation of an intramolecular disulfide bond between Cys149 and Cys153 reinforces the aggregation process. Physiological reductases, thioredoxins and glutaredoxins, could not dissolve AtGAPC1 aggregates but could efficiently contrast their growth. Besides acting as a protective mechanism against overoxidation, S-glutathionylation of AtGAPC1 triggers an unexpected aggregation pathway with completely different and still unexplored physiological implications.

Tadpole-like Conformations of Huntingtin Exon 1 Are Characterized by Conformational Heterogeneity that Persists regardless of Polyglutamine Length

Soluble huntingtin exon 1 (Httex1) with expanded polyglutamine (polyQ) engenders neurotoxicity in Huntington's disease. To uncover the physical basis of this toxicity, we performed structural studies of soluble Httex1 for wild-type and mutant polyQ lengths. Nuclear magnetic resonance experiments show evidence for conformational rigidity across the polyQ region. In contrast, hydrogen-deuterium exchange shows absence of backbone amide protection, suggesting negligible persistence of hydrogen bonds. The seemingly conflicting results are explained by all-atom simulations, which show that Httex1 adopts tadpole-like structures with a globular head encompassing the N-terminal amphipathic and polyQ regions and the tail encompassing the C-terminal proline-rich region. The surface area of the globular domain increases monotonically with polyQ length. This stimulates sharp increases in gain-of-function interactions in cells for expanded polyQ, and one of these interactions is with the stress-granule protein Fus. Our results highlight plausible connections between Httex1 structure and routes to neurotoxicity.

Sedimentation Velocity Analysis with Fluorescence Detection of Mutant Huntingtin Exon 1 Aggregation in Drosophila melanogaster and Caenorhabditis elegans

At least nine neurodegenerative diseases that are caused by the aggregation induced by long tracts of glutamine sequences have been identified. One such polyglutamine-containing protein is huntingtin, which is the primary factor responsible for Huntington's disease. Sedimentation velocity with fluorescence detection is applied to perform a comparative study of the aggregation of the huntingtin exon 1 protein fragment upon transgenic expression in Drosophila melanogaster and Caenorhabditis elegans. This approach allows the detection of aggregation in complex mixtures under physiologically relevant conditions. Complementary methods used to support this biophysical approach included fluorescence microscopy and semidenaturing detergent agarose gel electrophoresis, as a point of comparison with earlier studies. New analysis tools developed for the analytical ultracentrifuge have made it possible to readily identify a wide range of aggregating species, including the monomer, a set of intermediate aggregates, and insoluble inclusion bodies. Differences in aggregation in the two animal model systems are noted, possibly because of differences in levels of expression of glutamine-rich sequences. An increased level of aggregation is shown to correlate with increased toxicity for both animal models. Co-expression of the human Hsp70 in D. melanogaster showed some mitigation of aggregation and toxicity, correlating best with inclusion body formation. The comparative study emphasizes the value of the analytical ultracentrifuge equipped with fluorescence detection as a useful and rigorous tool for in situ aggregation analysis to assess commonalities in aggregation across animal model systems.

Huntington's Disease: Mechanisms of Pathogenesis and Therapeutic Strategies

Huntington's disease is a late-onset neurodegenerative disease caused by a CAG trinucleotide repeat in the gene encoding the huntingtin protein. Despite its well-defined genetic origin, the molecular and cellular mechanisms underlying the disease are unclear and complex. Here, we review some of the currently known functions of the wild-type huntingtin protein and discuss the deleterious effects that arise from the expansion of the CAG repeats, which are translated into an abnormally long polyglutamine tract. Finally, we outline some of the therapeutic strategies that are currently being pursued to slow down the disease.

Expression of the type VI intermediate filament proteins CP49 and filensin in the mouse lens epithelium

PURPOSE

The differentiated lens fiber cell assembles a filamentous cytoskeletal structure referred to as the beaded filament (BF). The BF requires CP49 (bfsp2) and filensin (bfsp1) for assembly, both of which are highly divergent members of the large intermediate filament (IF) family of proteins. Thus far, these two proteins have been reported only in the differentiated lens fiber cell. For this reason, both proteins have been considered robust markers of fiber cell differentiation. We report here that both proteins are also expressed in the mouse lens epithelium, but only after 5 weeks of age.

METHODS

Localization of CP49 was achieved with immunocytochemical probing of wild-type, CP49 knockout, filensin knockout, and vimentin knockout mice, in sections and in the explanted lens epithelium, at the light microscope and electron microscope levels. The relationship between CP49 and other cytoskeletal elements was probed using fluorescent phalloidin, as well as with antibodies to vimentin, GFAP, and α-tubulin. The relationship between CP49 and the aggresome was probed with antibodies to γ-tubulin, ubiquitin, and HDAC6.

RESULTS

CP49 and filensin were expressed in the mouse lens epithelium, but only after 5 weeks of age. At the light microscope level, these two proteins colocalize to a large tubular structure, approximately 7 × 1 μm, which was typically present at one to two copies per cell. This structure is found in the anterior and anterolateral lens epithelium, including the zone where mitosis occurs. The structure becomes smaller and largely undetectable closer to the equator where the cell exits the cell cycle and commits to fiber cell differentiation. This structure bears some resemblance to the aggresome and is reactive with antibodies to HDAC6, a marker for the aggresome. However, the structure does not colocalize with antibodies to γ-tubulin or ubiquitin, also markers for the aggresome. The structure also colocalizes with actin but appears to largely exclude vimentin and α-tubulin. In the CP49 and filensin knockouts, this structure is absent, confirming the identity of CP49 and filensin in this structure, and suggesting a requirement for the physiologic coassembly of CP49 and filensin.

CONCLUSIONS

CP49 and filensin have been considered robust markers for mouse lens fiber cell differentiation. The data reported here, however, document both proteins in the mouse lens epithelium, but only after 5 weeks of age, when lens epithelial growth and mitotic activity have slowed. Because of this, CP49 and filensin must be considered markers of differentiation for both fiber cells and the lens epithelium in the mouse. In addition, to our knowledge, no other protein has been shown to emerge so late in the development of the mouse lens epithelium, suggesting that lens epithelial differentiation may continue well into post-natal life. If this structure is related to the aggresome, it is a rare, or perhaps unique example of a large, stable aggresome in wild-type tissue.

Studying polyglutamine aggregation in Caenorhabditis elegans using an analytical ultracentrifuge equipped with fluorescence detection

This work explores the heterogeneity of aggregation of polyglutamine fusion constructs in crude extracts of transgenic Caenorhabditis elegans animals. The work takes advantage of the recent technical advances in fluorescence detection for the analytical ultracentrifuge. Further, new sedimentation velocity methods, such as the multi-speed method for data capture and wide distribution analysis for data analysis, are applied to improve the resolution of the measures of heterogeneity over a wide range of sizes. The focus here is to test the ability to measure sedimentation of polyglutamine aggregates in complex mixtures as a prelude to future studies that will explore the effects of genetic manipulation and environment on aggregation and toxicity. Using sedimentation velocity methods, we can detect a wide range of aggregates, ranging from robust analysis of the monomer species through an intermediate and quite heterogeneous population of oligomeric species, and all the way up to detecting species that likely represent intact inclusion bodies based on comparison to an analysis of fluorescent puncta in living worms by confocal microscopy. Our results support the hypothesis that misfolding of expanded polyglutamine tracts into insoluble aggregates involves transitions through a number of stable intermediate structures, a model that accounts for how an aggregation pathway can lead to intermediates that can have varying toxic or protective attributes. An understanding of the details of intermediate and large-scale aggregation for polyglutamine sequences, as found in neurodegenerative diseases such as Huntington's Disease, will help to more precisely identify which aggregated species may be involved in toxicity and disease.

Cell Biology of Prions and Prionoids: A Status Report

The coalescence of proteins into highly ordered aggregates is a hallmark of protein misfolding disorders (PMDs), which, when affecting the central nervous system, lead to progressive neurodegeneration. Although the chemical identity and the topology of each culprit protein are unique, the principles governing aggregation and propagation are strikingly stereotypical. It is now clear that such protein aggregates can spread from cell to cell and eventually affect entire organ systems - similarly to prion diseases. However, because most aggregates are not found to transmit between individuals, they are not infectious sensu strictiori. Therefore, they are not identical to prions and we prefer to define them as 'prionoids'. Here we review recent advances in understanding the toxicity of protein aggregation affecting the brain.

Targeting Hsp90/Hsp70-based protein quality control for treatment of adult onset neurodegenerative diseases

Currently available therapies for adult onset neurodegenerative diseases provide symptomatic relief but do not modify disease progression. Here we explore a new neuroprotective approach based on drugs targeting chaperone-directed protein quality control. Critical target proteins that unfold and aggregate in these diseases, such as the polyglutamine androgen receptor in spinal and bulbar muscular atrophy, huntingtin in Huntington's disease, α-synuclein in Parkinson's disease, and tau in Alzheimer's disease, are client proteins of heat shock protein 90 (Hsp90), and their turnover is regulated by the protein quality control function of the Hsp90/Hsp70-based chaperone machinery. Hsp90 and Hsp70 have opposing effects on client protein stability in protein quality control; Hsp90 stabilizes the clients and inhibits their ubiquitination, whereas Hsp70 promotes ubiquitination dependent on CHIP (C terminus of Hsc70-interacting protein) and proteasomal degradation. We discuss how drugs that modulate proteostasis by inhibiting Hsp90 function or promoting Hsp70 function enhance the degradation of the critical aggregating proteins and ameliorate toxic symptoms in cell and animal disease models.

Fluorescence anisotropy uncovers changes in protein packing with inclusion growth in a cellular model of polyglutamine aggregation

The aggregation of polyglutamine-rich proteins is closely linked with numerous neurodegenerative disorders. In pathological and cellular models, the appearance of protein-rich inclusions in cells acts as a read out of protein aggregation. The precise organization of aggregated protein in these inclusions and their mode of growth are still poorly understood. Here, fluorescence anisotropy-based measurements have been used to probe protein packing across inclusions of varying brightness, formed by an monomeric enhanced green fluorescent protein (mEGFP)-tagged polyglutamine model peptide in cells. High-resolution, confocal-based steady-state anisotropy measurements report a large depolarization, consistent with extensive homo-Förster (fluorescence) resonance energy transfer (FRET) between the sequestered mEGFP-tagged protein molecules. An inverse correlation of fluorescence anisotropy with intensity is seen across inclusions, which becomes emphasized when the observed fluorescence anisotropy values of inclusions are corrected for the fluorescence contribution of the diffusible protein, present within and around smaller inclusions. Homo-FRET becomes enhanced as inclusion size increases. This enhancement is confirmed by two-photon excitation-based time-resolved fluorescence anisotropy decay measurements, which also suggest that the mEGFP-tagged protein molecules are arranged in multiple ways within inclusions. Bright inclusions display faster FRET rates with a larger number of mEGFP moieties participating in homo-FRET than faint inclusions do. These results are consistent with a model in which the protein is more closely packed in the brighter inclusions. In such a possible mechanism, the higher packing density of protein molecules in brighter inclusions would suggest that inclusion growth could involve an intermolecular compaction event within the inclusion, as more monomers and aggregates are recruited into the growing inclusion.

Monomeric, oligomeric and polymeric proteins in huntington disease and other diseases of polyglutamine expansion

Huntington disease and other diseases of polyglutamine expansion are each caused by a different protein bearing an excessively long polyglutamine sequence and are associated with neuronal death. Although these diseases affect largely different brain regions, they all share a number of characteristics, and, therefore, are likely to possess a common mechanism. In all of the diseases, the causative protein is proteolyzed, becomes abnormally folded and accumulates in oligomers and larger aggregates. The aggregated and possibly the monomeric expanded polyglutamine are likely to play a critical role in the pathogenesis and there is increasing evidence that the secondary structure of the protein influences its toxicity. We describe here, with special attention to huntingtin, the mechanisms of polyglutamine aggregation and the modulation of aggregation by the sequences flanking the polyglutamine. We give a comprehensive picture of the characteristics of monomeric and aggregated polyglutamine, including morphology, composition, seeding ability, secondary structure, and toxicity. The structural heterogeneity of aggregated polyglutamine may explain why polyglutamine-containing aggregates could paradoxically be either toxic or neuroprotective.

Prions Ex Vivo: What Cell Culture Models Tell Us about Infectious Proteins

Prions are unconventional infectious agents that are composed of misfolded aggregated prion protein. Prions replicate their conformation by template-assisted conversion of the endogenous prion protein PrP. Templated conversion of soluble proteins into protein aggregates is also a hallmark of other neurodegenerative diseases. Alzheimer's disease or Parkinson's disease are not considered infectious diseases, although aggregate pathology appears to progress in a stereotypical fashion reminiscent of the spreading behavior ofmammalian prions. While basic principles of prion formation have been studied extensively, it is still unclear what exactly drives PrP molecules into an infectious, self-templating conformation. In this review, we discuss crucial steps in the life cycle of prions that have been revealed in ex vivo models. Importantly, the persistent propagation of prions in mitotically active cells argues that cellular processes are in place that not only allow recruitment of cellular PrP into growing prion aggregates but also enable the multiplication of infectious seeds that are transmitted to daughter cells. Comparison of prions with other protein aggregates demonstrates that not all the characteristics of prions are equally shared by prion-like aggregates. Future experiments may reveal to which extent aggregation-prone proteins associated with other neurodegenerative diseases can copy the replication strategies of prions.

Different ataxin-3 amyloid aggregates induce intracellular Ca(2+) deregulation by different mechanisms in cerebellar granule cells

This work aims at elucidating the relation between morphological and physicochemical properties of different ataxin-3 (ATX3) aggregates and their cytotoxicity. We investigated a non-pathological ATX3 form (ATX3Q24), a pathological expanded form (ATX3Q55), and an ATX3 variant truncated at residue 291 lacking the polyQ expansion (ATX3/291Δ). Solubility, morphology and hydrophobic exposure of oligomeric aggregates were characterized. Then we monitored the changes in the intracellular Ca(2+) levels and the abnormal Ca(2+) signaling resulting from aggregate interaction with cultured rat cerebellar granule cells. ATX3Q55, ATX3/291Δ and, to a lesser extent, ATX3Q24 oligomers displayed similar morphological and physicochemical features and induced qualitatively comparable time-dependent intracellular Ca(2+) responses. However, only the pre-fibrillar aggregates of expanded ATX3 (the only variant which forms bundles of mature fibrils) triggered a characteristic Ca(2+) response at a later stage that correlated with a larger hydrophobic exposure relative to the two other variants. Cell interaction with early oligomers involved glutamatergic receptors, voltage-gated channels and monosialotetrahexosylganglioside (GM1)-rich membrane domains, whereas cell interaction with more aged ATX3Q55 pre-fibrillar aggregates resulted in membrane disassembly by a mechanism involving only GM1-rich areas. Exposure to ATX3Q55 and ATX3/291Δ aggregates resulted in cell apoptosis, while ATX3Q24 was substantially innocuous. Our findings provide insight into the mechanisms of ATX3 aggregation, aggregate cytotoxicity and calcium level modifications in exposed cerebellar cells.

Cyclophilin-mediated reactivation pathway of inactive adenosine kinase aggregates

Monomeric adenosine kinase (AdK), a pivotal salvage enzyme of the purine auxotrophic parasite, Leishmania donovani, tends to aggregate naturally or selectively in presence of ADP, leading to inactivation. A cyclophilin (LdCyP) from the parasite reactivated the enzyme by disaggregating it. We studied the aggregation pathway of AdK with or without ADP. Transmission electron microscopy revealed that ADP-induced aggregates, as opposed to annular or torus-shaped natural aggregates, were mostly amorphous with protofibril-like structures. Interestingly, only the natural aggregates bound thioflavin T with a KD of 3.33 μM, indicating cross β-sheet structure. Dynamic light scattering experiments indicated that monomers formed aggregates either upon prolonged storage or ADP exposure. ADP-aggregates were disaggregated by LdCyP with concomitant reactivation of the enzyme. The activity revived with decrease in the aggregate size. Displacement of ADP from the ADP-aggregated enzyme by LdCyP resulted in reactivation. CD-spectral studies suggested that, like the natural aggregates, ADP induced formation of β-sheet structure in the ADP-aggregates. However, unlike the natural aggregate, it could be reconverted to α-helical conformation upon addition of LdCyP. Based on the results, a regulatory mechanism through interplay of ADP and/or LdCyP interaction with the enzyme is envisaged and a pathway of AdK reactivation by LdCyP-chaperone is proposed.

Simultaneous measurement of amyloid fibril formation by dynamic light scattering and fluorescence reveals complex aggregation kinetics

An apparatus that combines dynamic light scattering and Thioflavin T fluorescence detection is used to simultaneously probe fibril formation in polyglutamine peptides, the aggregating subunit associated with Huntington's disease, in vitro. Huntington's disease is a neurodegenerative disorder in a class of human pathologies that includes Alzheimer's and Parkinson's disease. These pathologies are all related by the propensity of their associated protein or polypeptide to form insoluble, β-sheet rich, amyloid fibrils. Despite the wide range of amino acid sequence in the aggregation prone polypeptides associated with these diseases, the resulting amyloids display strikingly similar physical structure, an observation which suggests a physical basis for amyloid fibril formation. Thioflavin T fluorescence reports β-sheet fibril content while dynamic light scattering measures particle size distributions. The combined techniques allow elucidation of complex aggregation kinetics and are used to reveal multiple stages of amyloid fibril formation.

Huntingtin fragments and SOD1 mutants form soluble oligomers in the cell

Diffusion coefficients of huntingtin (Htt) fragments and SOD1 mutants expressed in cells were measured using fluorescence correlation spectroscopy. The diffusion mobilities of both non-pathological Htt fragments (25 polyQs) and pathological Htt fragments (103 polyQs) were much slower than expected for monomers suggesting that they oligomerize. The mobility of these fragments was unaffected by duration of expression or by over-expression of Hsp70 and Hsp40. However in cells with HttQ103 inclusions, diffusion measurements showed that the residual cytosolic HttQ103 was monomeric. These results suggest that both non-pathological and pathological Htt fragments form soluble oligomers in the cytosol with the properties of the oligomers determining whether they cause pathology. SOD1 with point mutations (A4V, G37R, and G85R) also had slower diffusional mobility than the wild-type protein whose mobility was consistent with that of a dimer. However, the decrease in mobility of the different SOD1 mutantsdid not correlate with their known pathology. Therefore, while soluble oligomers always seem to be present under conditions where cell pathology occurs, the presence of the oligomers, in itself, does not determine the extent of neuropathology.

Membrane lipid composition and its physicochemical properties define cell vulnerability to aberrant protein oligomers

Increasing evidence suggests that the interaction of misfolded protein oligomers with cell membranes is a primary event resulting in the cytotoxicity associated with many protein-misfolding diseases, including neurodegenerative disorders. We describe here the results of a study on the relative contributions to toxicity of the physicochemical properties of protein oligomers and the cell membrane with which they interact. We altered the amount of cholesterol and the ganglioside GM1 in membranes of SH-SY5Y cells. We then exposed the cells to two types of oligomers of the prokaryotic protein HypF-N with different ultrastructural and cytotoxicity properties, and to oligomers formed by the amyloid-β peptide associated with Alzheimer's disease. We identified that the degree of toxicity of the oligomeric species is the result of a complex interplay between the structural and physicochemical features of both the oligomers and the cell membrane.

Amyloid-like fibril formation by polyQ proteins: a critical balance between the polyQ length and the constraints imposed by the host protein

Nine neurodegenerative disorders, called polyglutamine (polyQ) diseases, are characterized by the formation of intranuclear amyloid-like aggregates by nine proteins containing a polyQ tract above a threshold length. These insoluble aggregates and/or some of their soluble precursors are thought to play a role in the pathogenesis. The mechanism by which polyQ expansions trigger the aggregation of the relevant proteins remains, however, unclear. In this work, polyQ tracts of different lengths were inserted into a solvent-exposed loop of the β-lactamase BlaP and the effects of these insertions on the properties of BlaP were investigated by a range of biophysical techniques. The insertion of up to 79 glutamines does not modify the structure of BlaP; it does, however, significantly destabilize the enzyme. The extent of destabilization is largely independent of the polyQ length, allowing us to study independently the effects intrinsic to the polyQ length and those related to the structural integrity of BlaP on the aggregating properties of the chimeras. Only chimeras with 55Q and 79Q readily form amyloid-like fibrils; therefore, similarly to the proteins associated with diseases, there is a threshold number of glutamines above which the chimeras aggregate into amyloid-like fibrils. Most importantly, the chimera containing 79Q forms amyloid-like fibrils at the same rate whether BlaP is folded or not, whereas the 55Q chimera aggregates into amyloid-like fibrils only if BlaP is unfolded. The threshold value for amyloid-like fibril formation depends, therefore, on the structural integrity of the β-lactamase moiety and thus on the steric and/or conformational constraints applied to the polyQ tract. These constraints have, however, no significant effect on the propensity of the 79Q tract to trigger fibril formation. These results suggest that the influence of the protein context on the aggregating properties of polyQ disease-associated proteins could be negligible when the latter contain particularly long polyQ tracts.

In vitro oligomerization and fibrillogenesis of amyloid-beta peptides

The amyloid beta Ab(1-40) and Ab(1-42) peptides are the main components of the fibrillar plaques characteristically found in the brains affected by Alzheimer's disease. Fibril formation has been thoroughly studied in vitro using synthetic amyloid peptides and has been described to be a nucleation dependent polymerization process. During this process, defined by a slow nucleation phase followed by a rapid exponential elongation reaction, a whole range of aggregated species (low and high molecular weight aggregates) precede fibril formation. Toxic species related to the onset and development of Alzheimer's disease are thought to be found among these prefibrillar aggregates. Two main procedures are used to experimentally monitor fibril formation kinetics: through the measurement of the light scattered by the different peptide aggregates and using the fluorescent dye thioflavin T, which fluorescence increases when specifically interacting with amyloid fibrils. Reproducibility may, however, be difficult to achieve when measuring and characterizing fibril formation kinetics. This fact is mainly due to the difficulty in experimentally handling amyloid peptides, which is directly related to the difficulty of having them in a monomeric form at the beginning of the polymerization process. This has to do mainly with the type of solvent used for the preparation of the peptide stock solutions (water, DMSO, TFE, HFIP) and the control of determinant physicochemical parameters such as pH. Moreover, kinetic progression turns out to be highly dependent on the type of peptide counter-ion used, which will basically determine the duration of the nucleation phase and the rate at which high molecular weight oligomers are formed. Centrifugation and filtration procedures used in the preparation of the peptide stock solutions will also greatly influence the duration of the fibril formation process. In this chapter, a survey of the alluded experimental procedures is provided and a general frame is proposed for the interpretation of the fibril formation kinetics, intended to integrate the results from the different experimental approaches. The significance of the different aggregated species in terms of cell toxicity will be discussed. Special emphasis will be given to the influence of pH on the structural and toxic characteristics of amyloid aggregates, an aspect that may be particularly relevant in some specific physiological conditions.

The role of immunity in Huntington's disease

Huntington's disease (HD) is a devastating and incurable neurodegenerative disorder characterized by progressive cognitive, psychiatric and motor impairments. Although the disease has been seen as a disorder purely of the brain, there is now emerging evidence that abnormalities outside the central nervous system are commonly seen in HD. Indeed, the mutant huntingtin (mHtt) coded for by the abnormal gene in HD is found in every cell type where its presence has been sought. In particular, there are a number of recent observations in HD patients that mHtt interacts with the immune system with accumulating evidence that changes in the immune system may critically contribute to the pathology of HD. However, the nature of this contribution remains unclear, to the extent that it is not even known whether the immune system has a beneficial or detrimental role in HD patients. In this review, we attempt to bring a novel understanding to the interaction of the immune system to HD pathology, thereby shedding light on its potential pathogenic role. As part of this discussion, we revisit the clinical data on the anti-inflammatory drug trials in HD and propose new experimental approaches to interrogate the role of immunity in this currently incurable disorder.

Variables and strategies in development of therapeutic post-transcriptional gene silencing agents

Post-transcriptional gene silencing (PTGS) agents such as ribozymes, RNAi and antisense have substantial potential for gene therapy of human retinal degenerations. These technologies are used to knockdown a specific target RNA and its cognate protein. The disease target mRNA may be a mutant mRNA causing an autosomal dominant retinal degeneration or a normal mRNA that is overexpressed in certain diseases. All PTGS technologies depend upon the initial critical annealing event of the PTGS ligand to the target RNA. This event requires that the PTGS agent is in a conformational state able to support hybridization and that the target have a large and accessible single-stranded platform to allow rapid annealing, although such platforms are rare. We address the biocomplexity that currently limits PTGS therapeutic development with particular emphasis on biophysical variables that influence cellular performance. We address the different strategies that can be used for development of PTGS agents intended for therapeutic translation. These issues apply generally to the development of PTGS agents for retinal, ocular, or systemic diseases. This review should assist the interested reader to rapidly appreciate critical variables in PTGS development and facilitate initial design and testing of such agents against new targets of clinical interest.

Protein quality control during erythropoiesis and hemoglobin synthesis

Erythrocytes must regulate hemoglobin synthesis to limit the toxicities of unstable free globin chain subunits. This regulation is particularly relevant in β-thalassemia, in which β-globin deficiency causes accumulation of free α-globin, which forms intracellular precipitates that destroy erythroid precursors. Experimental evidence accumulated over more than 40 years indicates that erythroid cells can neutralize moderate amounts of free α-globin through generalized protein quality control mechanisms, including molecular chaperones, the ubiquitin-proteasome system, and autophagy. In many ways, β-thalassemia resembles protein aggregation disorders of the nervous system, liver, and other tissues, which occur when levels of unstable proteins overwhelm cellular compensatory mechanisms. Information gained from studies of nonerythroid protein aggregation disorders may be exploited to further understand and perhaps treat β-thalassemia.

Formation and toxicity of soluble polyglutamine oligomers in living cells

BACKGROUND

Aggregation and cytotoxicity of mutant proteins containing an expanded number of polyglutamine (polyQ) repeats is a hallmark of several diseases, including Huntington's disease (HD). Within cells, mutant Huntingtin (mHtt) and other polyglutamine expansion mutant proteins exist as monomers, soluble oligomers, and insoluble inclusion bodies (IBs). Determining which of these forms constitute a toxic species has proven difficult. Recent studies support a role for IBs as a cellular coping mechanism to sequester levels of potentially toxic soluble monomeric and oligomeric species of mHtt.

METHODOLOGY/PRINCIPAL FINDINGS

When fused to a fluorescent reporter (GFP) and expressed in cells, the soluble monomeric and oligomeric polyglutamine species are visually indistinguishable. Here, we describe two complementary biophysical fluorescence microscopy techniques to directly detect soluble polyglutamine oligomers (using Htt exon 1 or Htt(ex1)) and monitor their fates in live cells. Photobleaching analyses revealed a significant reduction in the mobilities of mHtt(ex1) variants consistent with their incorporation into soluble microcomplexes. Similarly, when fused to split-GFP constructs, both wildtype and mHtt(ex1) formed oligomers, as evidenced by the formation of a fluorescent reporter. Only the mHtt(ex1) split-GFP oligomers assembled into IBs. Both FRAP and split-GFP approaches confirmed the ability of mHtt(ex1) to bind and incorporate wildtype Htt into soluble oligomers. We exploited the irreversible binding of split-GFP fragments to forcibly increase levels of soluble oligomeric mHtt(ex1). A corresponding increase in the rate of IBs formation and the number formed was observed. Importantly, higher levels of soluble mHtt(ex1) oligomers significantly correlated with increased mutant cytotoxicity, independent of the presence of IBs.

CONCLUSIONS/SIGNIFICANCE

Our study describes powerful and sensitive tools for investigating soluble oligomeric forms of expanded polyglutamine proteins, and their impact on cell viability. Moreover, these methods should be applicable for the detection of soluble oligomers of a wide variety of aggregation prone proteins.

Neurotoxic protein oligomerisation associated with polyglutamine diseases

Polyglutamine (polyQ) diseases are associated with a CAG/polyQ expansion mutation in unrelated proteins. Upon elongation of the glutamine tract, disease proteins aggregate within cells, mainly in the central nervous system (CNS) and this aggregation process is associated with neurotoxicity. However, it remains unclear to what extent and how this aggregation causes neuronal dysfunction in the CNS. Aiming at preventing neuronal dysfunction, it will be crucial to determine the links between aggregation and cellular dysfunction, understand the folding pathway of polyQ proteins and discover the relative neurotoxicity of polyQ protein species formed along the aggregation pathway. Here, we review what is known about conformations of polyQ peptides and proteins in their monomeric state from experimental and modelling data, how conformational changes of polyQ proteins relate to their oligomerisation and morphology of aggregates and which cellular function are impaired by oligomers, in vitro and in vivo. We also summarise the key modulatory cellular mechanisms and co-factors, which could affect the folding pathway and kinetics of polyQ aggregation. Although many studies have investigated the relationship between polyQ aggregation and toxicity, these have mainly focussed on investigating changes in the formation of the classical hallmark of polyQ diseases, i.e. microscopically visible inclusion bodies. However, recent studies in which oligomeric species have been considered start to shed light on the identity of neurotoxic oligomeric species. Initial evidence suggests that conformational changes induced by polyQ expansions and their surrounding sequence lead to the formation of particular oligomeric intermediates that may differentially affect neurotoxicity.

Tracking mutant huntingtin aggregation kinetics in cells reveals three major populations that include an invariant oligomer pool

Huntington disease is caused by expanded polyglutamine sequences in huntingtin, which procures its aggregation into intracellular inclusion bodies (IBs). Aggregate intermediates, such as soluble oligomers, are predicted to be toxic to cells, yet because of a lack of quantitative methods, the kinetics of aggregation in cells remains poorly understood. We used sedimentation velocity analysis to define and compare the heterogeneity and flux of purified huntingtin with huntingtin expressed in mammalian cells under non-denaturing conditions. Non-pathogenic huntingtin remained as hydrodynamically elongated monomers in vitro and in cells. Purified polyglutamine-expanded pathogenic huntingtin formed elongated monomers (2.4 S) that evolved into a heterogeneous aggregate population of increasing size over time (100-6,000 S). However, in cells, mutant huntingtin formed three major populations: monomers (2.3 S), oligomers (mode s(20,w) = 140 S) and IBs (mode s(20,w) = 320,000 S). Strikingly, the oligomers did not change in size heterogeneity or in their proportion of total huntingtin over 3 days despite continued monomer conversion to IBs, suggesting that oligomers are rate-limiting intermediates to IB formation. We also determined how a chaperone known to modulate huntingtin toxicity, Hsc70, influences in-cell huntingtin partitioning. Hsc70 decreased the pool of 140 S oligomers but increased the overall flux of monomers to IBs, suggesting that Hsc70 reduces toxicity by facilitating transfer of oligomers into IBs. Together, our data suggest that huntingtin aggregation is streamlined in cells and is consistent with the 140 S oligomers, which remain invariant over time, as a constant source of toxicity to cells irrespective of total load of insoluble aggregates.

Gene- and cell-based approaches for neurodegenerative disease

Neurodegenerative diseases comprise an important group ofchronic diseases that increase in incidence with rising age. In particular, the two most common neurodegenerative diseases are Alzheimer's disease and Parkinson's disease, both of which will be discussed below. A third, Huntington's disease, occurs infrequently, but has been studied intensely. Each of these diseases shares characteristics which are also generalizeable to other neurodegenerative diseases: accumulation ofproteinaceous substances that leads inexorably to selective neuronal death and decline in neural function. Treatments for these diseases have historically focused on symptomatic relief, but recent advances in molecular research have identified more specific targets. Additionally, stem cell therapy, immunotherapy and trophic-factor delivery provide avenues for neuronal protection that may alter the natural progression of these devastating illnesses. Upcoming clinical trials will evaluate treatment strategies and provide hope that translational research will decrease the onset of debilitating disability associated with neurodegenerative disease.

A generic mechanism of beta2-microglobulin amyloid assembly at neutral pH involving a specific proline switch

Although numerous measurements of amyloid assembly of different proteins under distinct conditions in vitro have been performed, the molecular mechanisms underlying the specific self-association of proteins into amyloid fibrils remain obscure. Elucidating the nature of the events that initiate amyloid formation remains a particularly difficult challenge because of the heterogeneity and transient nature of the species involved. Here, we have used site-directed mutagenesis to create five proline to glycine variants in the naturally amyloidogenic protein beta2-microglobulin (beta2m). One of these variants, P5G, allowed us to isolate and characterise an intermediate containing a non-native trans Pro32 backbone conformation, a feature that is known to be required for amyloid elongation at neutral pH. By analysing oligomerisation and amyloid formation using analytical size-exclusion chromatography, multi-angle static light-scattering, analytical ultracentrifugation, circular dichroism and thioflavin T fluorescence we reveal a pathway for beta2m amyloid assembly at pH 7.5 that does not require the addition of metal ions, detergents, co-solvents or other co-factors that have been used to facilitate amyloid formation at physiological pH and temperature. Assembly is shown to involve the transient formation of a non-native monomer containing a trans P32 backbone conformation. This is followed by the formation of dimeric species and higher molecular mass oligomers that accumulate before the development of amyloid fibrils. On the basis of these results, we propose a generic mechanism for beta2m fibrillogenesis at neutral pH that is consistent with the wide range of published studies of this protein. In this mechanism, amyloid formation is initiated by a specific cis to trans proline switch, the rate of which we show to be controlled by the amino acid sequence proximal to P32 and to the applied solution conditions.

Huntington's disease: the current state of research with peripheral tissues

Huntington's disease (HD) is a genetically dominant condition caused by expanded CAG repeats. These repeats code for a glutamine tract in the HD gene product huntingtin (htt), which is a protein expressed in almost all tissues. Although most HD symptoms reflect preferential neuronal death in specific brain regions, even before the HD gene was identified numerous reports had described additional abnormalities in the peripheral tissues of HD patients, including weight loss, altered glucose homeostasis, and sub-cellular abnormalities in fibroblasts, lymphocytes and erythrocytes. Several years have elapsed since the HD mutation was discovered, and analyses of peripheral tissues from HD patients have helped to understand the molecular pathogenesis of the disease and revealed that the molecular mechanisms through which mutated htt leads to cell dysfunction are widely shared between central nervous system (CNS) and peripheral tissues. These studies show that in peripheral tissues, mutated htt causes accumulation of intracellular protein aggregates, impairment of energetic metabolism, transcriptional deregulation and hyperactivation of programmed cell-death mechanisms. Here, we review the current knowledge of peripheral tissue alterations in HD patients and in animal models of HD and focus on how this information can be used to identify potential therapeutic possibilities and biomarkers for disease progression.

Atomistic simulations of the effects of polyglutamine chain length and solvent quality on conformational equilibria and spontaneous homodimerization

Aggregation of expanded polyglutamine tracts is associated with nine different neurodegenerative diseases, including Huntington's disease. Experiments and computer simulations have demonstrated that monomeric forms of polyglutamine molecules sample heterogeneous sets of collapsed structures in water. The current work focuses on a mechanistic characterization of polyglutamine homodimerization as a function of chain length and temperature. These studies were carried out using molecular simulations based on a recently developed continuum solvation model that was designed for studying conformational and binding equilibria of intrinsically disordered molecules such as polyglutamine systems. The main results are as follows: Polyglutamine molecules form disordered, collapsed globules in aqueous solution. These molecules spontaneously associate at conditions approaching those of typical in vitro experiments for chains of length N>/=15. The spontaneity of these homotypic associations increases with increasing chain length and decreases with increasing temperature. Similar and generic driving forces govern both collapse and spontaneous homodimerization of polyglutamine in aqueous milieus. Collapse and dimerization maximize self-interactions and reduce the interface between polyglutamine molecules and the surrounding solvent. Other than these generic considerations, there do not appear to be any specific structural requirements for either chain collapse or chain dimerization; that is, both collapse and dimerization are nonspecific in that disordered globules form disordered dimers. In fact, it is shown that the driving force for intermolecular associations is governed by spontaneous conformational fluctuations within monomeric polyglutamine. These results suggest that polyglutamine aggregation is unlikely to follow a homogeneous nucleation mechanism with the monomer as the critical nucleus. Instead, the results support the formation of disordered, non-beta-sheet-like soluble molten oligomers as early intermediates--a proposal that is congruent with recent experimental data.

Do amyloid oligomers act as traps for misfolded proteins? A hypothesis

Mounting evidence points to soluble peptide oligomers as the primary agents in various amyloid and prion diseases. Multiple mechanisms appear to contribute to the cytotoxic effects of these oligomers. Here, an additional, general mechanism is proposed - that soluble amyloid peptide oligomers serve as "all-purpose"beta strands that can interact with transiently unfolded or nascent proteins where interior beta-sheet edges are exposed. The proteins, trapped in misfolded states through this interaction, become substrates for ubiquitination, targeting them for proteasomal degradation. The increased load of ubiquitinated proteins could contribute to the impairment of the ubiquitin/proteasome system (UPS) seen in many amyloid-related diseases. This "misfolding trap" mechanism could be especially stressful in the endoplasmic reticulum, where the amyloid oligomers would compete with chaperones for nascent beta-sheet proteins. If the bound amyloid oligomer dissociates at some point after the misfolded protein is committed to the UPS pathway, the oligomer could then repeat the process, adding a catalytic aspect to the misfolding mechanism. Direct proof of this proposed mechanism requires detection of amyloid oligomer-beta-sheet protein complexes, and a co-immunoprecipitation experiment is proposed. This hypothesis supports therapies that increase amyloid oligomer degradation or sequestration, as well as therapies that upregulate chaperone activity, for combating amyloid-related diseases.

Sodium dodecyl sulfate-insoluble oligomers are involved in polyglutamine degeneration

In polyglutamine (polyQ) degeneration, disease protein that carries an expanded polyQ tract is neurotoxic. Expanded polyQ protein exists in different conformations that display distinct solubility properties. In this study, an inducible transgenic Drosophila model is established to define the pathogenic form of polyQ protein at an early stage of degeneration in vivo. We show that microscopic polyQ aggregates are neither pathogenic nor protective. Further, no toxic effect of sodium dodecyl sulfate (SDS) -soluble polyQ protein is observed in our model. By means of filtration, 2 forms of SDS-insoluble protein species are identified according to their size. Coexpression of an ATPase-defective form of the molecular chaperone Hsc70 (Hsc70-K71S) selectively reduces the abundance of the large SDS-insoluble polyQ species, but such modulation has no modifying effects on degeneration. Notably, we detect a distinct Hsc70-K71S-resistant, small, SDS-insoluble polyQ oligomeric species that is closely correlated with degeneration. Our data highlight the toxic role of SDS-insoluble oligomers in polyQ degeneration in vivo.

Specific N-terminal mutations in the human androgen receptor induce cytotoxicity

Polyglutamine (polyQ) stretch amplification in different proteins causes neurodegenerative disease. These proteins form intracellular aggregates thought to be cytotoxic but differ in pathology and tissue specificity. Here, we demonstrate that specific sequences outside the polyQ stretch of the human androgen receptor contribute to polyQ pathology. An exchange of two N-terminal serine phosphorylation residues to alanine in the wild type androgen receptor (ARQ22dm) resulted in cytoplasmic accumulation and increased early hormone-dependent aggregation of the receptor. In a Drosophila model, the ARQ22dm was cytotoxic, and developing larvae expressing this receptor showed behavioral abnormalities and severely impaired locomotion. In contrast, the same double mutation in an androgen receptor with an extended polyQ stretch was less toxic. The response of the receptors to inhibitors of polyglutamine toxicity is altered by the amino acid exchanges suggesting that careful consideration is needed in the choice of potential therapies of disorders involving toxic polyQ species.

Protein stress and stress proteins: implications in aging and disease

Environmental stress induces damage that activates an adaptive response in any organism. The cellular stress response is based on the induction of cytoprotective proteins,the so called stress or heat shock proteins. The stress response as well as stress proteins are ubiquitous,highly conserved mechanism, and genes, respectively, already present in prokaryotes. Chaperones protect the proteome against conformational damage, promoting the function of protein networks. Protein damage takes place during aging and in several degenerative diseases, and presents a threat to overload the cellular defense mechanisms. The preservation of a robust stress response and protein disposal is indispensable for health and longevity. This review summarizes the present knowledge of protein damage, turnover, and the stress response in aging and degenerative diseases.

4-Phenylbutyrate rescues trafficking incompetent mutant alpha-galactosidase A without restoring its functionality

Fabry disease is a lysosomal storage disorder caused by deficiency of alpha-galactosidase A. Most mutant enzyme is catalytically active but due to misfolding retained in the endoplasmic reticulum. We have tested 4-phenylbutyrate for its potential to rescue various trafficking incompetent mutant alpha-galactosidase A. Although we found that the trafficking blockade for endoplasmic reticulum-retained mutant alpha-Gal A was released, neither a mature enzyme was detectable in transgenic mice fibroblasts nor a reversal of lysosomal Gb3 storage in fibroblasts from Fabry patients could be observed. Because of lack of functionality of rescued mutant alpha-galactosidase A, 4-phenylbutyrate seems to be of limited use as a chemical chaperone for Fabry disease.

The CFTR-derived peptides as a model of sequence-specific protein aggregation

Protein aggregation is a hallmark of a growing group of pathologies known as conformational diseases. Although many native or mutated proteins are able to form aggregates, the exact amino acid sequences involved in the process of aggregation are known only in a few cases. Hence, there is a need for different model systems to expand our knowledge in this area. The so-called ag region was previously found to cause the aggregation of the C-terminal fragment of the cystic fibrosis transmembrane conductance regulator (CFTR). To investigate whether this specific amino acid sequence is able to induce protein aggregation irrespective of the amino acid context, we altered its position within the CFTR-derived C-terminal peptide and analyzed the localization of such modified peptides in transfected mammalian cells. Insertion of the ag region into a different amino acid background affected not only the overall level of intracellular protein aggregation, but also the morphology and subcellular localization of aggregates, suggesting that sequences other than the ag region can substantially influence the peptide’s behavior. Also, the introduction of a short dipeptide (His-Arg) motif, a crucial component of the ag region, into different locations within the C-terminus of CFTR lead to changes in the aggregation pattern that were less striking, although still statistically significant. Thus, our results indicate that even subtle alterations within the aggregating peptide can affect many different aspects of the aggregation process.

Differential morphology and composition of inclusions in the R6/2 mouse and PC12 cell models of Huntington’s disease

The histological hallmark feature of Huntington's disease (HD) and other polyglutamine repeat diseases is the presence of intracellular inclusions. Much work has been devoted to trying to determine the relationship between inclusion formation and neuronal injury. However, little attention has been paid to the variability and characteristics of inclusions themselves. Here, we characterize the morphological and biochemical composition of inclusions in both a transgenic mouse model (R6/2 line) and an inducible cell culture model of HD (iPC12Q74). We identified several morphologically distinct kinds of inclusions in different locations (nuclei, cytoplasm and cellular processes). Ubiquitin colocalized completely with all of these inclusions in both the iPC12Q72 and R6/2 models. In the inclusions in iPC12Q74 cells, the 20S and 11S proteasome subunits colocalized variably, and the 19S subunit did not colocalize at all. In inclusions in R6/2 mouse neurons, the 20S subunit colocalized completely, but neither the 11S nor the 19S subunits colocalized at all. While the role of inclusions in the pathogenesis of HD continues to be debated, we suggest that the content and structure of inclusions vary considerably, not only from cell to cell but even within individual cells. Their role in the pathogenesis of HD is likely to depend on their location as well as their composition.

Huntington's disease

Huntington's disease is an autosomal-dominant, progressive neurodegenerative disorder with a distinct phenotype, including chorea and dystonia, incoordination, cognitive decline, and behavioural difficulties. Typically, onset of symptoms is in middle-age after affected individuals have had children, but the disorder can manifest at any time between infancy and senescence. The mutant protein in Huntington's disease--huntingtin--results from an expanded CAG repeat leading to a polyglutamine strand of variable length at the N-terminus. Evidence suggests that this tail confers a toxic gain of function. The precise pathophysiological mechanisms of Huntington's disease are poorly understood, but research in transgenic animal models of the disorder is providing insight into causative factors and potential treatments.

Kinetics of amyloid formation and membrane interaction with amyloidogenic proteins

Interest in amyloidogenesis has exploded in recent years, as scientists recognize the role of amyloid protein aggregates in degenerative diseases such as Alzheimer's and Parkinson's disease. Assembly of proteins or peptides into mature amyloid fibrils is a multistep process initiated by conformational changes, during which intermediate aggregation states such as oligomers, protofibrils, and filaments are sampled. Although once it was assumed that the mature fibril was the biologically toxic species, more recently it has been widely speculated that soluble intermediates are the most damaging. Because of its relevance to mechanism of disease, the paths traversed during fibrillogenesis, and the kinetics of the process, are of considerable interest. In this review we discuss various kinetic models used to describe amyloidogenesis. Although significant advances have been made, construction of rigorous, detailed, and experimentally validated quantitative models remains a work in progress. We briefly review recent literature that illustrates the interplay between kinetics and amyloid-membrane interactions: how do different intermediates interact with lipid bilayers, and how does the lipid bilayer affect kinetics of amyloidogenesis?

The interplay between PolyQ and protein context delays aggregation by forming a reservoir of protofibrils

Polyglutamine (polyQ) diseases are inherited neurodegenerative disorders caused by the expansion of CAG codon repeats, which code for polyQ in the corresponding gene products. These diseases are associated with the presence of amyloid-like protein aggregates, induced by polyQ expansion. It has been suggested that the soluble aggregates rather than the mature fibrillar aggregates are the toxic species, and that the aggregation properties of polyQ can be strongly modulated by the surrounding protein context. To assess the importance of the protein carrier in polyQ aggregation, we have studied the misfolding pathway and the kinetics of aggregation of polyQ of lengths above (Q41) and below (Q22) the pathological threshold fused to the well-characterized protein carrier glutathione S-transferase (GST). This protein, chosen as a model system, is per se able to misfold and aggregate irreversibly, thus mimicking the behaviour of domains of naturally occurring polyQ proteins. We prove that, while it is generally accepted that the aggregation kinetics of polyQ depend on its length and are faster for longer polyQ tracts, the presence of GST alters the polyQ aggregation pathway and reverses this trend. Aggregation occurs through formation of a reservoir of soluble intermediates whose populations and kinetic stabilities increase with polyQ length. Our results provide a new model that explains the toxicity of expanded polyQ proteins, in which the interplay between polyQ regions and other aggregation-prone domains plays a key role in determining the aggregation pathway.

Soluble androgen receptor oligomers underlie pathology in a mouse model of spinobulbar muscular atrophy

In polyglutamine diseases such as X-linked spinobulbar muscular atrophy (SBMA), it is unknown whether the toxic form of the protein is an insoluble or soluble aggregate or a monomer. We have addressed this question by studying a full-length androgen receptor (AR) mouse model of SBMA. We used biochemistry and atomic force microscopy to immunopurify oligomers soluble after ultracentrifugation that are comprised of a single approximately 50-kDa N-terminal polyglutamine-containing AR fragment. AR oligomers appeared several weeks prior to symptom onset, were distinct and temporally dissociated from intranuclear inclusions, and disappeared rapidly after castration, which halts disease. This is the first demonstration of soluble AR oligomers in vivo and suggests that they underlie neurodegeneration in SBMA.

Failure of protein quality control in amyotrophic lateral sclerosis

The protein chaperoning and ubiquitin-proteasome systems perform many homeostatic functions within cells involving protein folding, transport and degradation. Of paramount importance is ridding cells of mutant or post-translationally modified proteins that otherwise tend to aggregate into insoluble complexes and form inclusions. Such inclusions are characteristic of many neurodegenerative diseases and implicate protein misfolding and aggregation as common aspects of pathogenesis. In the most common familial form of ALS, mutations in SOD1 promote misfolding of the protein and target it for degradation by proteasomes. Although proteasomes can degrade the mutant proteins efficiently, altered solubility and aggregation of mutant SOD1 are features of the disease and occur most prominently in the most vulnerable cells and tissues. Indeed, lumbar spinal cord of mutant SOD1 transgenic mice show early reduction in their capacity for protein chaperoning and proteasome-mediated hydrolysis of substrates, and motor neurons are particularly vulnerable to aggregation of mutant SOD1. A high threshold for upregulating key pathways in response to the stress of added substrate load may contribute to this vulnerability. The broad spectrum neuroprotective capability and efficacy of some chaperone-based therapies in preclinical models makes these pathways attractive as targets for therapy in ALS, as well as other neurodegenerative diseases. A better understanding of the mechanisms governing the regulation of protein chaperones and UPS components would facilitate development of treatments that upregulate these pathways in a coordinated manner in neural tissue without long term toxicity.

A century-old debate on protein aggregation and neurodegeneration enters the clinic

The correlation between neurodegenerative disease and protein aggregation in the brain has long been recognized, but a causal relationship has not been unequivocally established, in part because a discrete pathogenic aggregate has not been identified. The complexity of these diseases and the dynamic nature of protein aggregation mean that, despite progress towards understanding aggregation, its relationship to disease is difficult to determine in the laboratory. Nevertheless, drug candidates that inhibit aggregation are now being tested in the clinic. These have the potential to slow the progression of Alzheimer's disease, Parkinson's disease and related disorders and could, if administered presymptomatically, drastically reduce the incidence of these diseases. The clinical trials could also settle the century-old debate about causality.