Protofibrils, pores, fibrils, and neurodegeneration: separating the responsible protein aggregates from the innocent bystanders

Tyrosine-to-Cysteine Modification of Human α-Synuclein Enhances Protein Aggregation and Cellular Toxicity

The deposition of alpha-synuclein and other cellular proteins in Lewy bodies in midbrain dopamine neurons is a pathological hallmark of Parkinson's disease. Nitrative and oxidative stress can induce alpha-synuclein protein aggregation, possibly initiated by the formation of stable cross-linking dimers. To determine whether enhanced dimer formation can accelerate protein aggregation and increase cellular toxicity, we have substituted cysteine for tyrosine at positions 39, 125, 133, and 136 in human wild-type (WT) alpha-synuclein, and in A53T and A30P mutant alpha-synuclein. To reduce the likelihood of cross-linking, phenylalanine was substituted for tyrosine at the same sites. We have found that overexpression of Y39C or Y125C mutant proteins leads to increased intracellular inclusions and apoptosis in a rat dopaminergic cell line (N27 cells) and in human embryonic kidney 293 cells. Expression of Y133C, Y136C, and all four Tyr-to-Phe mutations were not more cytotoxic than WT control. Exposure to oxidative stress increased Y39C and Y125C alpha-synuclein aggregation and toxicity. Dimers and oligomers were found in Triton X-100-soluble fractions from adenovirus-mediated overexpression of Y39C and Y125C in N27 cells. In contrast, WT beta-synuclein and all four Tyr-to-Cys mutant beta-synucleins did not cause protein aggregation and cell death. We conclude that cysteine substitution at critical positions in the alpha-synuclein molecule can increase dimer formation and accelerate protein aggregation and cellular toxicity of alpha-synuclein.

From conversion to aggregation: protofibril formation of the prion protein

The ability to diagnose and treat prion diseases is limited by our current understanding of the conversion process of the protein from healthy to harmful isoform. Whereas the monomeric, benign species is well characterized, the misfolded conformations responsible for infectivity and neurodegeneration remain elusive. There is mounting evidence that fibrillization intermediates, or protofibrils, but not mature fibrils or plaques, are the pathogenic species in amyloid diseases. Here, we use molecular dynamics to simulate the conversion of the prion protein. Molecular dynamics simulation produces a scrapie prion protein-like conformation enriched in beta-structure that is in good agreement with available experimental data. The converted conformation was then used to model a protofibril by means of the docking of hydrophobic patches of the template structure to form hydrogen-bonded sheets spanning adjacent subunits. The resulting protofibril model provides a non-branching aggregate with a 3(1) axis of symmetry that is in good agreement with a wide variety of experimental data; importantly, it was derived from realistic simulation of the conversion process.

Islet amyloid: a complication of islet dysfunction or an aetiological factor in Type 2 diabetes?

The role of islet amyloidosis in the onset and progression of Type 2 diabetes remains obscure. Islet amyloid polypeptide is a 37 amino-acid, beta-cell peptide which is co-stored and co-released with insulin. Human islet amyloid polypeptide refolds to a beta-conformation and oligomerises to form insoluble fibrils; proline substitutions in rodent islet amyloid polypeptide prevent this molecular transition. Pro-islet amyloid polypeptide (67 amino acids in man) is processed in secretory granules. Refolding of islet amyloid polypeptide may be prevented by intragranular heterodimer formation with insulin (but not proinsulin). Diabetes-associated abnormal proinsulin processing could contribute to de-stabilisation of granular islet amyloid polypeptide. Increased pro-islet amyloid polypeptide secretion as a consequence of islet dysfunction could promote fibrillogenesis; the propeptide forms fibrils and binds to basement membrane glycosamino-glycans. Islet amyloid polypeptide gene polymorphisms are not universally associated with Type 2 diabetes. Transgenic mice expressing human islet amyloid polypeptide gene have increased islet amyloid polypeptide concentrations but develop islet amyloid only against a background of obesity and/or high fat diet. In transgenic mice, obese monkeys and cats, initially small perivascular deposits progressively increase to occupy 80% islet mass; the severity of amyloidosis in animal models is related to the onset of hyperglycaemia, suggesting that islet amyloid and the associated destruction of islet cells cause diabetes. In human diabetes, islet amyloid can affect less than 1% or up to 80% of islets indicating that islet amyloidosis largely results from diabetes-related pathologies and is not an aetiological factor for hyperglycaemia. However, the associated progressive beta-cell destruction leads to severe islet dysfunction and insulin requirement.

Roles of molecular chaperones in protein misfolding diseases

Human misfolding diseases result from the failure of proteins to reach their active state or from the accumulation of aberrantly folded proteins. The mechanisms by which molecular chaperones influence the development of these diseases is beginning to be understood. Mutations that compromise the activity of chaperones lead to several rare syndromes. In contrast, the more frequent amyloid-related neurodegenerative diseases are caused by a gain of toxic function of misfolded proteins. Toxicity in these disorders may result from an imbalance between normal chaperone capacity and production of dangerous protein species. Increased chaperone expression can suppress the neurotoxicity of these molecules, suggesting possible therapeutic strategies.

Monomeric Cu,Zn-superoxide dismutase is a common misfolding intermediate in the oxidation models of sporadic and familial amyotrophic lateral sclerosis

Proteinacious intracellular aggregates in motor neurons are a key feature of both sporadic and familial amyotrophic lateral sclerosis (ALS). These inclusion bodies are often immunoreactive for Cu,Zn-superoxide dismutase (SOD1) and are implicated in the pathology of ALS. On the basis of this and a similar clinical presentation of symptoms in the familial (fALS) and sporadic forms of ALS, we sought to investigate the possibility that there exists a common disease-related aggregation pathway for fALS-associated mutant SODs and wild type SOD1. We have previously shown that oxidation of fALS-associated mutant SODs produces aggregates that have the same morphological, structural, and tinctorial features as those found in SOD1 inclusion bodies in ALS. Here, we show that oxidative damage of wild type SOD at physiological concentrations ( approximately 40 microm) results in destabilization and aggregation in vitro. Oxidation of either mutant or wild type SOD1 causes the enzyme to dissociate to monomers prior to aggregation. Only small changes in secondary and tertiary structure are associated with monomer formation. These results indicate a common aggregation prone monomeric intermediate for wild type and fALS-associated mutant SODs and provides a link between sporadic and familial ALS.

Pathological and functional amyloid formation orchestrated by the secretory pathway

Amyloidogenesis has historically been associated with pathology in a class of neurodegenerative diseases known as amyloid diseases. Recent studies have shown that proteolysis by furin during secretion initiates both variant gelsolin amyloidogenesis, associated with the disease familial amyloidosis of Finnish type, and Pmel17 fiber formation, which is necessary for the functional biogenesis of melanosomes. Proteolysis combined with organelle-dependent environment changes orchestrate amyloidogenesis associated with both pathological processes and a functional pathway.

Protein folding and misfolding

The manner in which a newly synthesized chain of amino acids transforms itself into a perfectly folded protein depends both on the intrinsic properties of the amino-acid sequence and on multiple contributing influences from the crowded cellular milieu. Folding and unfolding are crucial ways of regulating biological activity and targeting proteins to different cellular locations. Aggregation of misfolded proteins that escape the cellular quality-control mechanisms is a common feature of a wide range of highly debilitating and increasingly prevalent diseases.

Copper-binding-site-null SOD1 causes ALS in transgenic mice: aggregates of non-native SOD1 delineate a common feature

Cu/Zn superoxide dismutase (SOD1), a crucial cellular antioxidant, can in certain settings mediate toxic chemistry through its Cu cofactor. Whether this latter property explains why mutations in SOD1 cause FALS has been debated. Here, we demonstrate motor neuron disease in transgenic mice expressing a SOD1 variant that mutates the four histidine residues that coordinately bind Cu. In-depth analyses of this new mouse model, previously characterized models and FALS human tissues revealed that the accumulation of detergent-insoluble forms of SOD1 is a common feature of the disease. These insoluble species include full-length SOD1 proteins, peptide fragments, stable oligomers and ubiquitinated entities. Moreover, chaperones Hsp25 and alphaB-crystallin specifically co-fractionated with insoluble SOD1. In cultured cells, all 11 of the FALS variants tested produced insoluble forms of mutant SOD1. Importantly, expression of recombinant peptide fragments of wild-type SOD1 in cultured cells also produced insoluble species, suggesting that SOD1 possesses elements with an intrinsic propensity to aggregate. Thus, modifications to the protein, such as FALS mutations, fragmentation and possibly covalent modification, may simply act to augment a natural, but potentially toxic, propensity to aggregate.

The Ubiquitin Proteasome System in Neurodegenerative Diseases

The ubiquitin-proteasome system targets numerous cellular proteins for degradation. In addition, modifications by ubiquitin-like proteins as well as proteins containing ubiquitin-interacting and -associated motifs modulate many others. This tightly controlled process involves multiple specific and general enzymes of the system as well as many modifying and ancillary proteins. Thus, it is not surprising that ubiquitin-mediated degradation/processing/modification regulates a broad array of basic cellular processes. Moreover, aberrations in the system have been implicated, either as a primary cause or secondary consequence, in the pathogenesis of both inherited and acquired neurodegenerative diseases. Recent findings indicate that the system is involved in the pathogenesis of Parkinson's, Alzheimer's, Huntington's, and Prion diseases as well as amyotrophic lateral sclerosis. This raises hopes for a better understanding of the pathogenetic mechanisms involved in these diseases and for the development of novel, mechanism-based therapeutic modalities.

Mixtures of wild-type and a pathogenic (E22G) form of Abeta40 in vitro accumulate protofibrils, including amyloid pores

Although APP mutations associated with inherited forms of Alzheimer's disease (AD) are relatively rare, detailed studies of these mutations may prove critical for gaining important insights into the mechanism(s) and etiology of AD. Here, we present a detailed biophysical characterization of the structural properties of protofibrils formed by the Arctic variant (E22G) of amyloid-beta protein (Abeta40(ARC)) as well as the effect of Abeta40(WT) on the distribution of the protofibrillar species formed by Abeta40(ARC) by characterizing biologically relevant mixtures of both proteins that may mimic the situation in the heterozygous patients. These studies revealed that the Arctic mutation accelerates both Abeta oligomerization and fibrillogenesis in vitro. In addition, Abeta40(ARC) was observed to affect both the morphology and the size distribution of Abeta protofibrils. Electron microscopy examination of the protofibrils formed by Abeta40(ARC) revealed several morphologies, including: (1) relatively compact spherical particles roughly 4-5 nm in diameter; (2) annular pore-like protofibrils; (3) large spherical particles 18-25 nm in diameter; and (4) short filaments with chain-like morphology. Conversion of Abeta40(ARC) protofibrils to fibrils occurred more rapidly than protofibrils formed in mixed solutions of Abeta40(WT)/Abeta40(ARC), suggesting that co-incubation of Abeta40(ARC) with Abeta40(WT) leads to kinetic stabilization of Abeta40(ARC) protofibrils. An increase in the ratio of Abeta(WT)/Abeta(MUT(Arctic)), therefore, may result in the accumulation of potential neurotoxic protofibrils and acceleration of disease progression in familial Alzheimer's disease mutation carriers.

Zeroing in on the pathogenic form of alpha-synuclein and its mechanism of neurotoxicity in Parkinson's disease

Parkinson's disease (PD) is linked to mutations in the protein alpha-synuclein, which can exist in vitro in several aggregation states, including a natively unfolded monomer, a beta-sheet rich oligomer, or protofibril, and a stable amyloid fibril. This work reviews the current literature that is relevant to two linked questions: which of these species is pathogenic, and what is the mechanism of neurotoxicity? The amyloid fibril, fibrillar aggregates, Lewy bodies, and the alpha-synuclein monomer, which is normally expressed at high levels, are all unlikely to be pathogenic, for reasons discussed here. We therefore favor a toxic protofibril scenario, and propose that the pathogenic species is transiently populated during the process of fibrillization. Toxicity may arise from pore-like protofibrils that cause membrane permeabilization. An approach to testing this hypothesis is discussed.

Neuropathology of prion diseases

In prion diseases, neuropathology has remained the most important tool to give a definite diagnosis, and neuropathological research has contributed significantly to our current pathogenetic understanding. Immunohistochemistry for the disease-associated prion protein (PrP(Sc)) is indispensable for the neuropathological confirmation of prion diseases. The amount and distribution of PrP(Sc) deposits do not always correlate with type and severity of local tissue damage. PrP(Sc) deposition occurs only where neuronal parenchyma is present; in scarred infarctions with prominent gliosis, PrP(Sc) does not accumulate. Early, severe and selective loss affects a subset of inhibitory GABAergic neurons both in human and experimental prion diseases. The central pathogenetic cascade includes oxidative stress to neurons and their apoptosis. New patterns of PrP(Sc) immunoreactivity include granular ganglionic and tiny adaxonal PrP(Sc) deposits in peripheral nervous tissue, and dendritic cells and macrophages in vessel walls, suggesting that mobile haematogenous cells may be involved in spread of prions.