Vesicle permeabilization by protofibrillar alpha-synuclein: implications for the pathogenesis and treatment of Parkinson's disease

C. elegans as a model organism to investigate molecular pathways involved with Parkinson's disease

Parkinson's disease (PD) is an age-related movement disorder resulting, in part, from selective loss of dopaminergic neurons. Both invertebrate and mammalian models have been developed to study the cellular mechanisms altered during disease progression; nevertheless there are limitations within each model. Mammalian models remain invaluable in studying PD, but are expensive and time consuming. Here, we review genetic and environmental factors associated with PD, and describe how the nematode roundworm, Caenorhabditis elegans, has been used as a model organism for studying various aspects of this neurodegenerative disease. Both genetic and chemical screens have been conducted in C. elegans to identify molecular pathways, proteins, and small molecules that can impact PD pathology. Lastly, we highlight future areas of investigation, in the context of emerging fields in biology, where the nematode can be exploited to provide mechanistic insights and potential strategies to accelerate the path toward possible therapeutic intervention for PD.

An Aß concatemer with altered aggregation propensities

We present an analysis of the conformational and aggregative properties of an Aß concatemer (Con-Alz) of interest for vaccine development against Alzheimer's disease. Con-Alz consists of 3 copies of the 43 residues of the Aß peptide separated by the P2 and P30 T-cell epitopes from the tetanus toxin. Even in the presence of high concentrations of denaturants or fluorinated alcohols, Con-Alz has a very high propensity to form aggregates which slowly coalesce over time with changes in secondary, tertiary and quaternary structure. Only micellar concentrations of SDS were able to inhibit aggregation. The increase in the ability to bind the fibril-binding dye ThT increases without lag time, which is characteristic of relatively amorphous aggregates. Confirming this, electron microscopy reveals that Con-Alz adopts a morphology resembling truncated protofibrils after prolonged incubation, but it is unable to assemble into classical amyloid fibrils. Despite its high propensity to aggregate, Con-Alz does not show any significant ability to permeabilize vesicles, which for fibrillating proteins is taken to be a key factor in aggregate cytotoxicity and is attributed to oligomers formed at an early stage in the fibrillation process. Physically linking multiple copies of the Aß-peptide may thus sterically restrict Con-Alz against forming cytotoxic oligomers, forcing it instead to adopt a less well-organized assembly of intermeshed polypeptide chains.

Oligomeric alpha-synuclein and its role in neuronal death

Alpha-synuclein is a natively unfolded protein associated with a number of neurodegenerative disorders that include Parkinson's disease. In the past, research has focused on the fibrillar form of the protein. Current research now indicates that oligomeric alpha-synuclein is the form of the protein most likely to causes neuronal death. Recent research has suggested that a unique oligomer associated with the copper binding capacity of the protein is the neurotoxic form of the protein. This review looks at the evidence for this possibility.

A stable lipid-induced aggregate of alpha-synuclein

The Parkinson's disease-related protein alpha-Synuclein (alphaS) is a 140 residue intrinsically disordered protein. Its membrane-binding properties are thought to be relevant for its physiological or pathologic activity. Here, the interaction of alphaS with POPG [1-Palmitoyl-2-Oleoyl-sn-Glycero-3-(Phosphorac-(1-glycerol))] small unilamellar vesicles (SUVs) is investigated by spin-label EPR using double electron-electron resonance (DEER). Intermolecular distances between four single mutants reveal that well-defined aggregates are formed. The data suggest a coexistence of two dimer structures with main interactions in the helix 2, encompassing residues 50-100. Previously, the horseshoe conformation was detected by intramolecular restraints obtained by DEER on alphaS double mutants (Drescher et al. J. Am. Chem. Soc. 2008, 130, 7796). The present study suggests that interdigitation of two monomers in the aggregate fills the void between the two helices of each of the monomers thus providing a rationale for the horseshoe structure. This aggregate is lipid induced and affects the structure of the POPG SUVs, which become leaky and diminish in size upon contact with alphaS suggesting a possible origin of conflicting results in the recent literature (Jao et al. Proc. Natl. Acad. Sci. U.S.A. 2008, 105 (50), 19666; Georgieva et al. J. Am. Chem. Soc. 2008, 130 (39), 12856; Bortolus et al. J. Am. Chem. Soc. 2008, 130, 6690).

Domain a' of protein disulfide isomerase plays key role in inhibiting alpha-synuclein fibril formation

alpha-Synuclein (alpha Syn) is the main component of Lewy bodies formed in midbrain dopaminergic neurons which is a pathological characteristic of Parkinson's disease. It has been recently showed to induce endoplasmic reticulum (ER) stress and impair ER functions. However, the mechanism of how ER responds to alpha Syn toxicity is poorly understood. In the present study, we found that protein disulfide isomerase (PDI), a stress protein abundant in ER, effectively inhibits alpha Syn fibril formation in vitro. In PDI molecule with a structure of abb'xa'c, domain a' was found to be essential and sufficient for PDI to inhibit alpha Syn fibril formation. PDI was further found to be more avid for binding with intermediate species formed during alpha Syn fibril formation, and the binding was more intensive in the later lag phase. Our results provide new insight into the role of PDI in protecting ER from the deleterious effects of misfolded protein accumulation in many neurodegenerative diseases.

Alpha-synuclein mediates alterations in membrane conductance: a potential role for alpha-synuclein oligomers in cell vulnerability

alpha-Synuclein has been linked to the pathogenesis of Parkinson's disease and other synucleinopathies through its propensity to form toxic oligomers. The exact mechanism for oligomeric synuclein-directed cell vulnerability has not been fully elucidated, but one hypothesis portends the formation of synuclein-containing pores within cell membranes leading to leak channel-mediated calcium influx and subsequent cell death. Here we demonstrate synuclein-induced formation of sodium dodecyl sulfate-stable oligomers, intracellular synuclein-positive aggregates, alterations in membrane conductance reminiscent of leak channels and subsequent cytotoxicity in a dopaminergic-like cell line. Furthermore we demonstrate that the synuclein-induced membrane conductance changes are blocked by direct extracellular application of an anti-synuclein antibody. The work presented here confirms that synuclein overexpression leads to membrane conductance changes and demonstrates for the first time through antibody-blocking studies that synuclein plays a direct role in the formation of leak channels.

Targeting the progression of Parkinson's disease

By the time a patient first presents with symptoms of Parkinson's disease at the clinic, a significant proportion (50-70%) of the cells in the substantia nigra (SN) has already been destroyed. This degeneration progresses until, within a few years, most of the cells have died. Except for rare cases of familial PD, the initial trigger for cell loss is unknown. However, we do have some clues as to why the damage, once initiated, progresses unabated. It would represent a major advance in therapy to arrest cell loss at the stage when the patient first presents at the clinic. Current therapies for Parkinson's disease focus on relieving the motor symptoms of the disease, these unfortunately lose their effectiveness as the neurodegeneration and symptoms progress. Many experimental approaches are currently being investigated attempting to alter the progression of the disease. These range from replacement of the lost neurons to neuroprotective therapies; each of these will be briefly discussed in this review. The main thrust of this review is to explore the interactions between dopamine, alpha synuclein and redox-active metals. There is abundant evidence suggesting that destruction of SN cells occurs as a result of a self-propagating series of reactions involving dopamine, alpha synuclein and redox-active metals. A potent reducing agent, the neurotransmitter dopamine has a central role in this scheme, acting through redox metallo-chemistry to catalyze the formation of toxic oligomers of alpha-synuclein and neurotoxic metabolites including 6-hydroxydopamine. It has been hypothesized that these feed the cycle of neurodegeneration by generating further oxidative stress. The goal of dissecting and understanding the observed pathological changes is to identify therapeutic targets to mitigate the progression of this debilitating disease.

The behavior of alpha-synuclein in neurons

Despite considerable evidence linking alpha-synuclein with membranes in vitro, it has proven difficult to demonstrate membrane association of the protein in vivo. alpha-Synuclein localizes to the nerve terminal, but biochemical experiments have not revealed a tight association with membranes. To address the dynamics of the protein in live cells, we have used photobleaching and found that alpha-synuclein exhibits high mobility, although distinctly less than an entirely soluble protein. Further, neural activity controls the distribution of alpha-synuclein, causing its dispersion from the synapse. In addition to the presumed role of alpha-synuclein dynamics in synaptic function, changes in its physiological behavior may underlie the pathological changes associated with Parkinson's disease.

Peripheral biomarkers of Parkinson's disease as early reporters of central neurodegeneration

Parkinson's disease (PD) is the most common age-related movement disorder, with a prevalence of approximately 2% among people over 65 years of age. The diagnosis of PD is currently based on the clinical manifestations of the disease; therefore, the availability of peripheral biomarkers would have a great impact. In this review, we discuss and compare several attempts made to find peripheral biomarkers of PD to achieve early diagnosis, differential diagnosis, therapy assessment and classification of disease subtypes. Several investigators focused on proteins that are involved in PD pathogenesis. However, the best choice for a sensible biomarker-discovery procedure makes use of global approaches such as metabolomics and proteomics. In addition, the tissue or compartment where biomarkers are located, plays a basic role. In this context, lymphocytes are of particular interest because they are circulating dopaminergic cells, and display several functional modifications in PD.

Cell-produced alpha-synuclein is secreted in a calcium-dependent manner by exosomes and impacts neuronal survival

alpha-Synuclein is central in Parkinson's disease pathogenesis. Although initially alpha-synuclein was considered a purely intracellular protein, recent data suggest that it can be detected in the plasma and CSF of humans and in the culture media of neuronal cells. To address a role of secreted alpha-synuclein in neuronal homeostasis, we have generated wild-type alpha-synuclein and beta-galactosidase inducible SH-SY5Y cells. Soluble oligomeric and monomeric species of alpha-synuclein are readily detected in the conditioned media (CM) of these cells at concentrations similar to those observed in human CSF. We have found that, in this model, alpha-synuclein is secreted by externalized vesicles in a calcium-dependent manner. Electron microscopy and liquid chromatography-mass spectrometry proteomic analysis demonstrate that these vesicles have the characteristic hallmarks of exosomes, secreted intraluminar vesicles of multivesicular bodies. Application of CM containing secreted alpha-synuclein causes cell death of recipient neuronal cells, which can be reversed after alpha-synuclein immunodepletion from the CM. High- and low-molecular-weight alpha-synuclein species, isolated from this CM, significantly decrease cell viability. Importantly, treatment of the CM with oligomer-interfering compounds before application rescues the recipient neuronal cells from the observed toxicity. Our results show for the first time that cell-produced alpha-synuclein is secreted via an exosomal, calcium-dependent mechanism and suggest that alpha-synuclein secretion serves to amplify and propagate Parkinson's disease-related pathology.

Curcumin reduces alpha-synuclein induced cytotoxicity in Parkinson's disease cell model

BACKGROUND

Overexpression and abnormal accumulation of aggregated alpha-synuclein (alphaS) have been linked to Parkinson's disease (PD) and other synucleinopathies. alphaS can misfold and adopt a variety of morphologies but recent studies implicate oligomeric forms as the most cytotoxic species. Both genetic mutations and chronic exposure to neurotoxins increase alphaS aggregation and intracellular reactive oxygen species (ROS), leading to mitochondrial dysfunction and oxidative damage in PD cell models.

RESULTS

Here we show that curcumin can alleviate alphaS-induced toxicity, reduce ROS levels and protect cells against apoptosis. We also show that both intracellular overexpression of alphaS and extracellular addition of oligomeric alphaS increase ROS which induces apoptosis, suggesting that aggregated alphaS may induce similar toxic effects whether it is generated intra- or extracellulary.

CONCLUSIONS

Since curcumin is a natural food pigment that can cross the blood brain barrier and has widespread medicinal uses, it has potential therapeutic value for treating PD and other neurodegenerative disorders.

Molecular mechanisms of neurodegeneration in Alzheimer's disease

Alzheimer's disease (AD) is characterized by cognitive impairment, progressive neurodegeneration and formation of amyloid-beta (Abeta)-containing plaques and neurofibrillary tangles composed of hyperphosphorylated tau. The neurodegenerative process in AD is initially characterized by synaptic damage accompanied by neuronal loss. In addition, recent evidence suggests that alterations in adult neurogenesis in the hippocampus might play a role. Synaptic loss is one of the strongest correlates to the cognitive impairment in patients with AD. Several lines of investigation support the notion that the synaptic pathology and defective neurogenesis in AD are related to progressive accumulation of Abeta oligomers rather than fibrils. Abnormal accumulation of Abeta resulting in the formation of toxic oligomers is the result of an imbalance between the levels of Abeta production, aggregation and clearance. Abeta oligomers might lead to synaptic damage by forming pore-like structures with channel activity; alterations in glutamate receptors; circuitry hyper-excitability; mitochondrial dysfunction; lysosomal failure and alterations in signaling pathways related to synaptic plasticity, neuronal cell and neurogenesis. A number of signaling proteins, including fyn kinase; glycogen synthase kinase-3beta (GSK3beta) and cyclin-dependent kinase-5 (CDK5), are involved in the neurodegenerative progression of AD. Therapies for AD might require the development of anti-aggregation compounds, pro-clearance pathways and blockers of hyperactive signaling pathways.

Tryptophan probes at the alpha-synuclein and membrane interface

Understanding how environmental factors affect the conformational dynamics of alpha-synuclein (alpha-syn) is of great importance because the accumulation and deposit of aggregated alpha-syn in the brain are intimately connected to Parkinson's disease etiology. Measurements of steady-state and time-resolved fluorescence of single tryptophan-containing alpha-syn variants have revealed distinct phospholipid vesicle and micelle interactions at residues 4, 39, 94, and 125. Our circular dichroism data confirm that Trp mutations do not affect alpha-syn membrane binding properties (apparent association constant K(a)app approximately 1 x 10(7) M(-1) for all synucleins) saturating at an estimated lipid-to-protein molar ratio of 380 or approximately 120 proteins covering approximately 7% of the surface area of an 80 nm diameter vesicle. Fluorophores at positions 4 and 94 are the most sensitive to the lipid bilayer with pronounced spectral blue-shifts (W4: Delta(lambda)max approximately 23 nm; W94: Delta(lambda)max approximately 10 nm) and quantum yield increases (W4, W94: approximately 3 fold), while W39 and W125 remain primarily water-exposed. Time-resolved fluorescence data show that all sites (except W125) have subpopulations that interact with the membrane.

Interplay between protein homeostasis networks in protein aggregation and proteotoxicity

The misfolding and aggregation of disease proteins is characteristic of numerous neurodegenerative diseases. Particular neuronal populations are more vulnerable to proteotoxicity while others are more apt to tolerate the misfolding and aggregation of disease proteins. Thus, the cellular environment must play a significant role in determining whether disease proteins are converted into toxic or benign forms. The endomembrane network of eukaryotes divides the cell into different subcellular compartments that possess distinct sets of molecular chaperones and protein interaction networks. Chaperones act as agonists and antagonists of disease protein aggregation to prevent the accumulation of toxic intermediates in the aggregation pathway. Interacting partners can also modulate the conformation and localization of disease proteins and thereby influence proteotoxicity. Thus, interplay between these protein homeostasis network components can modulate the self-association of disease proteins and determine whether they elicit a toxic or benign outcome.

Adsorption of alpha-synuclein on lipid bilayers: modulating the structure and stability of protein assemblies

The interaction of alpha-synuclein with phospholipid membranes has been examined using supported lipid bilayers and epi-fluorescence microscopy. The membranes contained phosphatidylcholine (PC) and phosphatidic acid (PA), which mix at physiological pH. Upon protein adsorption, the lipids undergo fluid-fluid phase separation into PC-rich and PA-rich regions. The protein preferentially adsorbs to the PA-rich regions. The adsorption and subsequent aggregation of alpha-synuclein was probed by tuning several parameters: the charge on the lipids, the charge on the protein, and the screening environment. Conditions which promoted the greatest extent of adsorption resulted in structurally heterogeneous aggregates, while comparatively homogeneous aggregates were observed under conditions whereby adsorption did not occur as readily. Our observation that different alterations to the system lead to different degrees of aggregation and different aggregate structures poses a challenge for drug discovery. Namely, therapies aimed at neutralizing alpha-synuclein must target a broad range of potentially toxic, membrane-bound assemblies.

Inhibition of FK506 binding proteins reduces alpha-synuclein aggregation and Parkinson's disease-like pathology

alpha-Synuclein (alpha-SYN) is a key player in the pathogenesis of Parkinson's disease (PD). In pathological conditions, the protein is present in a fibrillar, aggregated form inside cytoplasmic inclusions called Lewy bodies. Members of the FK506 binding protein (FKBP) family are peptidyl-prolyl isomerases that were shown recently to accelerate the aggregation of alpha-SYN in vitro. We now established a neuronal cell culture model for synucleinopathy based on oxidative stress-induced alpha-SYN aggregation and apoptosis. Using high-content analysis, we examined the role of FKBPs in aggregation and apoptotic cell death. FK506, a specific inhibitor of this family of proteins, inhibited alpha-SYN aggregation and neuronal cell death in this synucleinopathy model dose dependently. Knockdown of FKBP12 or FKBP52 reduced the number of alpha-SYN aggregates and protected against cell death, whereas overexpression of FKBP12 or FKBP52 accelerated both aggregation of alpha-SYN and cell death. Thus, FK506 likely targets FKBP members in the cell culture model. Furthermore, oral administration of FK506 after viral vector-mediated overexpression of alpha-SYN in adult mouse brain significantly reduced alpha-SYN aggregate formation and neuronal cell death. Our data explain previously described neuroregenerative and neuroprotective effects of immunophilin ligands and validate FKBPs as a novel drug target for the causative treatment of PD.

Differential phospholipid binding of alpha-synuclein variants implicated in Parkinson's disease revealed by solution NMR spectroscopy

Three familial variants of the presynaptic protein α-synuclein (αS), A30P, E46K, and A53T, correlate with rare inherited Parkinson’s disease (PD), while wild-type αS is implicated in sporadic PD. The classic manifestation of both familiar and sporadic PD is the formation of fibrillar structures of αS which accumulate as the main component in intraneuronal Lewy bodies. At presynaptic termini, the partitioning of αS between disordered cytosolic and membrane-bound states likely mediates its proposed role in regulation of reserve pools of synaptic vesicles. Previously, we reported on multiple distinct phospholipid binding modes of αS with slow binding kinetics. Here, we report the phospholipid binding properties of the disease variants, viewed by solution NMR in a residue-specific manner. Our results agree qualitatively with previous biophysical studies citing overall decreased lipid affinity for the A30P mutation, comparable affinity for A53T, and an increased level of binding of E46K, relative to wild-type αS. Additionally, our NMR results describe the distribution of lipid-bound states for αS: the population of the SL1 binding mode (residues 3−25 bound as a helix) is augmented by each of the disease variants, relative to wild-type αS. We propose that the SL1 binding mode, which anchors the N-terminus of αS in the lipoprotein complex while the hydrophobic NAC region remains dynamically disordered, is prone to intermolecular interactions which progress toward disease-associated oligomers and fibrils. The elevation of the SL1 binding mode, unchecked by a proportionate population of binding modes incorporating the full N-terminal domain, may well account for the increased toxicity of the A30P, E46K, and A53T disease variants of αS.

Dopamine quinones interact with alpha-synuclein to form unstructured adducts

alpha-Synuclein (alphasyn) fibril formation is considered a central event in the pathogenesis of Parkinson's disease (PD). In recent years, it has been proposed that prefibrillar annular oligomeric beta-sheet-rich species, called protofibrils, rather than fibrils themselves, may be the neurotoxic species. The oxidation products of dopamine (DAQ) can inhibit alphasyn fibril formation supporting the idea that DAQ might stabilize alphasyn protofibrils. In the present work, through different biochemical and biophysical techniques, we isolated and structurally characterized alphasyn/DAQ adducts. Contrary to protofibrils, we demonstrated that alphasyn/DAQ adducts retain an unfolded conformation. We then investigated the nature of the modifications induced on alphasyn by DAQ. Our results indicate that only a small fraction of alphasyn interacts with DAQ in a covalent way, so that non-covalent interaction appears to be the major modification induced by DAQ on alphasyn.

Distinct region-specific alpha-synuclein oligomers in A53T transgenic mice: implications for neurodegeneration

Aggregation of alpha-synuclein (alpha-syn), a process that generates oligomeric intermediates, is a common pathological feature of several neurodegenerative disorders. Despite the potential importance of the oligomeric alpha-syn intermediates in neuron function, their biochemical properties and pathobiological functions in vivo remain vastly unknown. Here we used two-dimensional analytical separation and an array of biochemical and cell-based assays to characterize alpha-syn oligomers that are present in the nervous system of A53T alpha-syn transgenic mice. The most prominent species identified were 53 A detergent-soluble oligomers, which preceded neurological symptom onset, and were found at equivalent amounts in regions containing alpha-syn inclusions as well as histologically unaffected regions. These oligomers were resistant to SDS, heat, and urea but were sensitive to proteinase-K digestion. Although the oligomers shared similar basic biochemical properties, those obtained from inclusion-bearing regions were prominently reactive to antibodies that recognize oxidized alpha-syn oligomers, significantly accelerated aggregation of alpha-syn in vitro, and caused primary cortical neuron degeneration. In contrast, oligomers obtained from non-inclusion-bearing regions were not toxic and delayed the in vitro formation of alpha-syn fibrils. These data indicate that specific conformations of alpha-syn oligomers are present in distinct brain regions of A53T alpha-syn transgenic mice. The contribution of these oligomers to the development of neuron dysfunction appears to be independent of their absolute quantities and basic biochemical properties but is dictated by the composition and conformation of the intermediates as well as unrecognized brain-region-specific intrinsic factors.

Strategies to increase the reproducibility of protein fibrillization in plate reader assays

There is great interest in developing reproducible high-throughput screens to identify small molecular inhibitors of protein fibrillization and aggregation for possible therapy against deposition diseases such as Alzheimer's and Parkinson's (PD). We have made a methodical analysis of factors increasing the reproducibility of the fibrillization of alpha-synuclein (alphaSN), a 140-amino-acid protein implicated in PD and notorious for its erratic fibrillization behavior. Salts and polyanionic polymers do not significantly improve the quality of the assay. However, an orbital agitation mode in the plate reader is a crucial first step toward reproducible alphaSN fibrillization. Higher reproducibility is achieved by the addition of glass beads, as evaluated by the percentage standard deviation of the nucleation and elongation rate constants and the end-stage fluorescence intensity of the fibril-binding dye thioflavin T (ThT). The highest reproducibility is obtained by either seeding the solution with preformed fibrils or by adding submicellar amounts of sodium dodecyl sulfate (SDS), where we obtain percentage standard deviations of 3-4% on the end ThT level. We conclude that there are multiple ways to achieve satisfactory levels of reproducibility, although the different conditions used to induce aggregation may lead to different fibrillization pathways.

Protein aggregation diseases: toxicity of soluble prefibrillar aggregates and their clinical significance

Amyloid diseases, the most clinically relevant protein misfolding pathologies due to the high prevalence of some of them in the population, are characterized by the presence, in specific tissues and organs, of fibrillar deposits of specific peptides or proteins. Increasing efforts are presently dedicated at investigating the structural features and the structure-toxicity relation of the soluble oligomeric precursors arising in the path of fibril formation. In fact, it is increasingly recognised that these unstable, dynamic assemblies are remarkably toxic to cells thus featuring these as the main factor responsible for cell impairment in amyloid diseases. This chapter will review shortly the data presently available on the structural and biochemical features of these assemblies, as well as on their biological and clinical significance.

APP transgenic modeling of Alzheimer's disease: mechanisms of neurodegeneration and aberrant neurogenesis

Neurodegenerative disorders of the aging population affect over 5 million people in the US and Europe alone. The common feature is the progressive accumulation of misfolded proteins with the formation of toxic oligomers. Alzheimer’s disease (AD) is characterized by cognitive impairment, progressive degeneration of neuronal populations in the neocortex and limbic system, and formation of amyloid plaques and neurofibrillary tangles. Amyloid-β (Aβ) is the product of proteolysis of amyloid precursor protein (APP) by β and γ-secretase enzymes. The neurodegenerative process in AD initiates with axonal and synaptic damage and is associated with progressive accumulation of toxic Aβ oligomers in the intracellular and extracellular space. In addition, neurodegeneration in AD is associated with alterations in neurogenesis. Aβ accumulation is the consequence of an altered balance between protein synthesis, aggregation rate, and clearance. Identification of genetic mutations in APP associated with familial forms of AD and gene polymorphisms associated with the more common sporadic variants of AD has led to the development of transgenic (tg) and knock out rodents as well as viral vector driven models of AD. While APP tg murine models with mutations in the N- and C-terminal flanking regions of Aβ are characterized by increased Aβ production with plaque formation, mutations in the mid-segment of Aβ result in increased formation of oligomers, and mutations toward the C-terminus (E22Q) segment results in amyloid angiopathy. Similar to AD, in APP tg models bearing familial mutations, formation of Aβ oligomers results in defective plasticity in the perforant pathway, selective neuronal degeneration, and alterations in neurogenesis. Promising results have been obtained utilizing APP tg models of AD to develop therapies including the use of β- and γ-secretase inhibitors, immunization, and stimulating neurogenesis.

Forebrain overexpression of alpha-synuclein leads to early postnatal hippocampal neuron loss and synaptic disruption

Transgenic (Tg) mouse models of Parkinson's disease (PD) generated to date have primarily been designed to overexpress human alpha-synuclein (alpha-syn) to recapitulate PD-like motor impairments as well as PD-like nigrostriatal degeneration and alpha-syn pathology. However, cognitive impairments and cortical alpha-syn pathology are also common in PD patients. To model these features of PD, we created forebrain-specific conditional Tg mice that overexpress human wild type (WT) or A53T mutant alpha-syn. Here we show that both WT and A53T mutant alpha-syn lead to massive degeneration of postmitotic neurons in the hippocampal dentate gyrus (DG) during postnatal development, with hippocampal synapse loss as evidenced by reduced levels of pre- and postsynaptic markers. However, when mutant and WT alpha-syn expression was repressed until the Tg mice were mature postnatally and then induced for several months, no hippocampal neuron loss was observed. These data imply that developing neurons are more vulnerable to degenerate than mature neurons as a consequence of forebrain WT and mutant alpha-syn overexpression.

Characterization of hydrophobic residue requirements for alpha-synuclein fibrillization

alpha-Synuclein is the major component of pathological inclusions characteristic of diseases like Parkinson's disease, dementia with Lewy bodies, and multiple-system atrophy. A role for alpha-synuclein in neurodegenerative diseases is further supported by point mutations and duplication and triplication of the alpha-synuclein gene (SNCA) that are causative of these disorders. The middle hydrophobic region of the alpha-synuclein protein, also termed the "non-Abeta component of Alzheimer's disease amyloid plaque (NAC)" domain, is required for alpha-synuclein to polymerize into amyloid filaments, which are the major components of alpha-synuclein pathological inclusions. In this study, we assessed the importance of specific stretches of hydrophobic residues in driving the intrinsic ability of alpha-synuclein to polymerize. Several small deletions, even one with as few as two amino acid residues (A76 and V77), dramatically impaired the ability of alpha-synuclein to polymerize into mature amyloidogenic fibrils, and instead, it preferentially formed oligomers. However, this inhibition of filament assembly was clearly dependent on the spatial context, since similar and larger hydrophobic deletions in other parts of the NAC domain reduced only the rate of fibril formation, without abrogating filament assembly. Further, mutation of residue E83 to an A rescued the ability of mutant Delta76-77 alpha-synuclein to polymerize. These findings support the notion that while both the location and hydrophobicity of protein segments are important elements that affect the propensity to form amyloid fibrils, the intrinsic ability of a polypeptide to fold structurally into amyloid is also critical.

Prion-like propagation of cytosolic protein aggregates: insights from cell culture models

Amyloid formation is a hallmark of several systemic and neurodegenerative diseases. Extracellular amyloid deposits or intracellular inclusions arise from the conformational transition of normally soluble proteins into highly ordered fibrillar aggregates. Amyloid fibrils are formed by nucleated polymerization, a process also shared by prions, proteinaceous infectious agents identified in mammals and fungi. Unlike so called non-infectious amyloids, the aggregation phenotype of prion proteins can be efficiently transmitted between cells and organisms. Recent discoveries in vivo now implicate that even disease-associated intracellular protein aggregates consisting of alpha-synuclein or Tau have the capacity to seed aggregation of homotypic native proteins and might propagate their amyloid states in a prion-like manner. Studies in tissue culture demonstrate that aggregation of diverse intracellular amyloidogenic proteins can be induced by exogenous fibrillar seeds. Still, a prerequisite for prion-like propagation is the fragmentation of proteinaceous aggregates into smaller seeds that can be transmitted to daughter cells. So far efficient propagation of the aggregation phenotype in the absence of exogenous seeds was only observed for a yeast prion domain expressed in tissue culture. Intrinsic properties of amyloidogenic protein aggregates and a suitable host environment likely determine if a protein polymer can propagate in a prion-like manner in the mammalian cytosol.

Suppression of Map Kinases Inhibits Microglial Activation and Attenuates Neuronal Cell Death Induced by α-Synuclein Protofibrils

α-Synuclein (α-Syn) accounts, as a major component of Lewy bodies (LB), for the filamentous deposits in many cases of neurodegenerative diseases. Yet, little is known about the molecular mechanisms of neuronal loss in these diseases. The correlation between α-Syn oligomerization/aggregation and pathologies raises the key question of which molecular form of α-Syn (i.e. monomeric α-Syn, protofibrils or mature fibrils) represents the damage-inducing culprit in the scenario of synucleinopathies. We show that human α-Syn protofibrils (PFs) are potent activators of parallel proinflammatory signalling pathways (p38 and ERK1/2 MAP kinases and NF-κB) in microglial cells in vitro. Furthermore, stereotactic injection of α-Syn PFs into the substantia nigra of adult rats leads to a profound activation of microglia and adjacent neuronal cell loss, which can be attenuated by the MAP kinase inhibitor semapimod. We propose that the neurodegenerative process of α-synucleinopathies involves microglial activation through α-Syn released or extruded from cells with pathogenic α-Syn metabolism. Compounds that inhibit the MAPK/NF-κB pathways might be a promising pharmacological strategy for the treatment of the inflammatory component of synucleinopathies including PD.

Thermodynamic selection of steric zipper patterns in the amyloid cross-beta spine

At the core of amyloid fibrils is the cross-beta spine, a long tape of beta-sheets formed by the constituent proteins. Recent high-resolution x-ray studies show that the unit of this filamentous structure is a beta-sheet bilayer with side chains within the bilayer forming a tightly interdigitating "steric zipper" interface. However, for a given peptide, different bilayer patterns are possible, and no quantitative explanation exists regarding which pattern is selected or under what condition there can be more than one pattern observed, exhibiting molecular polymorphism. We address the structural selection mechanism by performing molecular dynamics simulations to calculate the free energy of incorporating a peptide monomer into a beta-sheet bilayer. We test filaments formed by several types of peptides including GNNQQNY, NNQQ, VEALYL, KLVFFAE and STVIIE, and find that the patterns with the lowest binding free energy correspond to available atomistic structures with high accuracy. Molecular polymorphism, as exhibited by NNQQ, is likely because there are more than one most stable structures whose binding free energies differ by less than the thermal energy. Detailed analysis of individual energy terms reveals that these short peptides are not strained nor do they lose much conformational entropy upon incorporating into a beta-sheet bilayer. The selection of a bilayer pattern is determined mainly by the van der Waals and hydrophobic forces as a quantitative measure of shape complementarity among side chains between the beta-sheets. The requirement for self-complementary steric zipper formation supports that amyloid fibrils form more easily among similar or same sequences, and it also makes parallel beta-sheets generally preferred over anti-parallel ones. But the presence of charged side chains appears to kinetically drive anti-parallel beta-sheets to form at early stages of assembly, after which the bilayer formation is likely driven by energetics.

Mitochondrial dysfunction in Parkinson's disease

Mitochondria are highly dynamic organelles which fulfill a plethora of functions. In addition to their prominent role in energy metabolism, mitochondria are intimately involved in various key cellular processes, such as the regulation of calcium homeostasis, stress response and cell death pathways. Thus, it is not surprising that an impairment of mitochondrial function results in cellular damage and is linked to aging and neurodegeneration. Many lines of evidence suggest that mitochondrial dysfunction plays a central role in the pathogenesis of Parkinson's disease (PD), starting in the early 1980s with the observation that an inhibitor of complex I of the electron transport chain can induce parkinsonism. Remarkably, recent research indicated that several PD-associated genes interface with pathways regulating mitochondrial function, morphology, and dynamics. In fact, sporadic and familial PD seem to converge at the level of mitochondrial integrity.

Protective effect of 3,5,3'-triiodothyroacetic and 3,5,3',5'-tetraiodothyroacetic acids on serum albumin fibrillation

Inhibition or reversion of protein self-aggregation has been suggested as a possible preventive mechanism against amyloid diseases, and many efforts are underway to found out molecules capable to restrain the protein aggregation process. In this paper, the inhibitory effects of thyroid hormone analogues on heat-induced fibrillation process of serum albumin are reported. Among the analogues tested, 3,5,3',5'-tetraiodothyroacetic and 3,5,3'-triiodothyroacetic acid showed the most important inhibitory effects on amyloid formation. Thyroxine exhibits a lesser protective effect, while 3,5,3'-triiodothyronine showed no significant inhibition. The gaining of a negative charge together with a size reduction of the hormone molecule could play an essential role in the inhibition of fibrils formation. According to infrared spectroscopy results, the thyroid hormones analogues protective effects proceed via the stabilization of the protein native structure. The current work demonstrates the effectiveness of naturally occurring molecules in the inhibition of albumin fibril formation.

Immune-directed gene therapeutic development for Alzheimer's, prion, and Parkinson's diseases

The development of novel immune-based therapeutics for neurodegenerative diseases is an area of intense focus. Neurodegenerative diseases represent a particular challenge since in many cases the onset of symptoms occurs after considerable degeneration has ensued. Based on human genetic and histopathological evidence from patients with neurodegenerative diseases, animal models that recapitulate specific pathologic features have been developed. Utilizing these animal models in combination with viral vector-based gene therapeutics, specific epochs of disease can be targeted. One common feature of several neurodegenerative diseases is misfolded proteins. The mechanism by which these altered protein conformers lead to neurodegeneration is not completely understood but much effort has been put forward to either degrade aberrant protein or prevent the formation of misfolded conformers. In this review, we will summarize work that employs viral vector gene therapeutics to modulate the brain's response to misfolded proteins with a specific focus on neurodegeneration.

Multiple tight phospholipid-binding modes of alpha-synuclein revealed by solution NMR spectroscopy

'In dopaminergic neurons, alpha-synuclein (alphaS) partitions between a disordered cytosolic state and a lipid-bound state. Binding of alphaS to membrane phospholipids is implicated in its functional role in synaptic regulation, but also impacts fibril formation associated with Parkinson's disease. We describe here a solution NMR study in which alphaS is added to small unilamellar vesicles of a composition mimicking synaptic vesicles; the results provide evidence for multiple distinct phospholipid-binding modes of alphaS. Exchange between the free state and the lipid-bound alphaS state, and between different bound states is slow on the NMR timescale, being in the range of 1-10 s(-1). Partitioning of the binding modes is dependent on lipid/alphaS stoichiometry, and tight binding with slow-exchange kinetics is observed at stoichiometries as low as 2:1. In all lipid-bound states, a segment of residues starting at the N-terminus of alphaS adopts an alpha-helical conformation, while succeeding residues retain the characteristics of a random coil. The 40 C-terminal residues remain dynamically disordered, even at high-lipid concentrations, but can also bind to lipids to an extent that appears to be determined by the fraction of cis X-Pro peptide bonds in this region. While lipid-bound alphaS exhibits dynamic properties that preclude its direct observation by NMR, its exchange with the NMR-visible free form allows for its indirect characterization. Rapid amide-amide nuclear Overhauser enhancement buildup points to a large alpha-helical conformation, and a distinct increase in fluorescence anisotropy attributed to Tyr39 indicates an ordered environment for this "dark state." Titration of alphaS with increasing amounts of lipids suggests that the binding mode under high-lipid conditions remains qualitatively similar to that in the low-lipid case. The NMR data appear incompatible with the commonly assumed model where alphaS lies in an alpha-helical conformation on the membrane surface and instead suggest that considerable remodeling of the vesicles is induced by alphaS.

Unfoldomics of human diseases: linking protein intrinsic disorder with diseases

BACKGROUND

Intrinsically disordered proteins (IDPs) and intrinsically disordered regions (IDRs) lack stable tertiary and/or secondary structure yet fulfills key biological functions. The recent recognition of IDPs and IDRs is leading to an entire field aimed at their systematic structural characterization and at determination of their mechanisms of action. Bioinformatics studies showed that IDPs and IDRs are highly abundant in different proteomes and carry out mostly regulatory functions related to molecular recognition and signal transduction. These activities complement the functions of structured proteins. IDPs and IDRs were shown to participate in both one-to-many and many-to-one signaling. Alternative splicing and posttranslational modifications are frequently used to tune the IDP functionality. Several individual IDPs were shown to be associated with human diseases, such as cancer, cardiovascular disease, amyloidoses, diabetes, neurodegenerative diseases, and others. This raises questions regarding the involvement of IDPs and IDRs in various diseases.

RESULTS

IDPs and IDRs were shown to be highly abundant in proteins associated with various human maladies. As the number of IDPs related to various diseases was found to be very large, the concepts of the disease-related unfoldome and unfoldomics were introduced. Novel bioinformatics tools were proposed to populate and characterize the disease-associated unfoldome. Structural characterization of the members of the disease-related unfoldome requires specialized experimental approaches. IDPs possess a number of unique structural and functional features that determine their broad involvement into the pathogenesis of various diseases.

CONCLUSION

Proteins associated with various human diseases are enriched in intrinsic disorder. These disease-associated IDPs and IDRs are real, abundant, diversified, vital, and dynamic. These proteins and regions comprise the disease-related unfoldome, which covers a significant part of the human proteome. Profound association between intrinsic disorder and various human diseases is determined by a set of unique structural and functional characteristics of IDPs and IDRs. Unfoldomics of human diseases utilizes unrivaled bioinformatics and experimental techniques, paves the road for better understanding of human diseases, their pathogenesis and molecular mechanisms, and helps develop new strategies for the analysis of disease-related proteins.

Molecular mechanisms of alpha-synuclein neurodegeneration

alpha-Synuclein is an abundant highly charged protein that is normally predominantly localized around synaptic vesicles in presynaptic terminals. Although the function of this protein is still ill-defined, genetic studies have demonstrated that point mutations or genetic alteration (duplications or triplications) that increase the number of copies of the alpha-synuclein (SCNA) gene can cause Parkinson's disease or the related disorder dementia with Lewy bodies. alpha-Synuclein can aberrantly polymerize into fibrils with typical amyloid properties, and these fibrils are the major component of many types of pathological inclusions, including Lewy bodies, which are associated with neurodegenerative diseases, such as Parkinson's disease. Although there is substantial evidence supporting the toxic nature of alpha-synuclein inclusions, other modes of toxicity such as oligomers have been proposed. In this review, some of the evidence for the different mechanisms of alpha-synuclein toxicity is presented and discussed.

The influence of vesicle size and composition on alpha-synuclein structure and stability

Monomeric alpha-synuclein (alphaSN), which has no persistent structure in aqueous solution, is known to bind to anionic lipids with a resulting increase in alpha-helix structure. Here we show that at physiological pH and ionic strength, alphaSN incubated with different anionic lipid vesicles undergoes a marked increase in alpha-helical content at a temperature dictated either by the temperature of the lipid phase transition, or (in 1,2-DilauroylSN-Glycero-3-[Phospho-rac-(1-glycerol)] (DLPG), which is fluid down to 0 degrees C) by an intrinsic cold denaturation that occurs around 10-20 degrees C. This structure is subsequently lost in a thermal transition around 60 degrees C. Remarkably, this phenomenon is only observed for vesicles >100 nm in diameter and is sensitive to lipid chain length, longer chain lengths, and larger vesicles giving more cooperative unfolding transitions and a greater degree of structure. For both vesicle size and chain length, a higher degree of compressibility or permeability in the lipid thermal transition region is associated with a higher degree of alphaSN folding. Furthermore, the degree of structural change is strongly reduced by an increase in ionic strength or a decrease in the amount of anionic lipid. A simple binding-and-folding model that includes the lipid phase transition, exclusive binding of alphaSN to the liquid disordered phase, the thermodynamics of unfolding, and the electrostatics of binding of alphaSN to lipids is able to reproduce the two thermal transitions as well as the effect of ionic strength and anionic lipid. Thus the nature of alphaSN's binding to phospholipid membranes is intimately tied to the lipids' physico-chemical properties.

CD 4+ T cells in the pathobiology of neurodegenerative disorders

CD4+ T cells orchestrate innate and adaptive immunity. In the central nervous system they modulate immune responses including cell trafficking and glial neuroregulatory functions through an array of soluble molecules cell-cell interactions affecting tissue homeostasis. During disease their roles evolve to an auto-aggressive or, alternatively, protective phenotype. How such a balance is struck in the setting of neurodegenerative disorders may reflect a dichotomy between regulatory T cell, anti-inflammatory and neuroprotective activities versus effector T cell inflammation and neurodegeneration. Interestingly, such roles may show commonalities amongst neurodegenerative diseases. Herein we focus on strategies to modulate such CD4+ T cell responses for therapeutic gain.

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

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

Optical reporters for the conformation of alpha-synuclein reveal a specific interaction with mitochondria

The aggregation of abnormally folded proteins is a defining feature of neurodegenerative disease, but it has not previously been possible to assess the conformation of these proteins in a physiologically relevant context, before they form morphologically recognizable aggregates. We now describe FRET-based reporters for the conformation of alpha-synuclein, a protein central to the pathogenesis of Parkinson's disease (PD). Characterization in vitro shows that alpha-synuclein adopts a relatively "closed" conformation in solution that converts to "open" on membrane binding. In living cells, the closed conformation predominates. In neurons, however, cell bodies contain a much larger proportion of the open conformation than synaptic boutons. To account for these differences, we also used the reporters to characterize the interaction with native membranes. We find that the conformation of alpha-synuclein responds selectively to mitochondria, indicating a direct link between alpha-synuclein and an organelle strongly implicated in the pathogenesis of PD.

Molecular and neurochemical mechanisms in PD pathogenesis

Oxidation of dopamine to aminochrome seems to be a normal process leading to aminochrome polymerization to form neuromelanin, since normal individuals have this pigment in their dopaminergic neurons in the substantia nigra. The neurons lost in individuals with Parkinson's disease are dopaminergic neurons containing neuromelanin. This raises two questions. First, why are those cells containing neuromelanin lost in this disease? Second, what is the identity of the neurotoxin that induces this cell death? We propose that aminochrome is the agent responsible for the death of dopaminergic neurons containing neuromelanin in individuals with Parkinson's disease. The normal oxidative pathway of dopamine, in which aminochrome polymerizes to form neuromelanin, can be neurotoxic if DT-diaphorase is inhibited under certain conditions. Inhibition of DT-diaphorase allows two neurotoxic reactions to proceed: (i) the formation of aminochrome adducts with alpha-synuclein, which induce and stabilize the formation of neurotoxic protofibrils; and (ii) the one electron reduction of aminochrome to the neurotoxic leukoaminochrome o-semiquinone radical. Therefore, we propose that DT-diaphorase is an important neuroprotective enzyme in dopaminergic neurons containing neuromelanin.