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

Molecular chaperones antagonize proteotoxicity by differentially modulating protein aggregation pathways

The self-association of misfolded or damaged proteins into ordered amyloid-like aggregates characterizes numerous neurodegenerative disorders. Insoluble amyloid plaques are diagnostic of many disease states. Yet soluble, oligomeric intermediates in the aggregation pathway appear to represent the toxic culprit. Molecular chaperones regulate the fate of misfolded proteins and thereby influence their aggregation state. Chaperones conventionally antagonize aggregation of misfolded, disease proteins and assist in refolding or degradation pathways. Recent work suggests that chaperones may also suppress neurotoxicity by converting toxic, soluble oligomers into benign aggregates. Chaperones can therefore suppress or promote aggregation of disease proteins to ameliorate the proteotoxic accumulation of soluble, assembly intermediates.

Lipid bilayer disruption by oligomeric alpha-synuclein depends on bilayer charge and accessibility of the hydrophobic core

Soluble oligomeric aggregates of alpha-synuclein have been implicated to play a central role in the pathogenesis of Parkinson's disease. Disruption and permeabilization of lipid bilayers by alpha-synuclein oligomers is postulated as a toxic mechanism, but the molecular details controlling the oligomer-membrane interaction are still unknown. Here we show that membrane disruption strongly depends on the accessibility of the hydrophobic membrane core and that charge interactions play an important but complex role. We systematically studied the influence of the physical membrane properties and solution conditions on lipid bilayer disruption by oligomers using a dye release assay. Varying the lipid headgroup composition revealed that membrane disruption only occurs for negatively charged bilayers. Furthermore, the electrostatic repulsion between the negatively charged alpha-synuclein and the negative surface charge of the bilayer inhibits vesicle disruption at low ionic strength. The disruption of negatively charged vesicles further depends on lipid packing parameters. Bilayer composition changes that result in an increased lipid headgroup spacing make vesicles more prone to disruption, suggesting that the accessibility of the bilayer hydrocarbon core modulates oligomer-membrane interaction. These data shed important new insights into the driving forces governing the highly debated process of oligomer-membrane interactions.

Inducible over-expression of wild type alpha-synuclein in human neuronal cells leads to caspase-dependent non-apoptotic death

Alpha-synuclein (ASYN) is central in Parkinson's disease pathogenesis. Converging pieces of evidence suggest that the levels of ASYN expression play a critical role in both familial and sporadic Parkinson's disease. To elucidate the mechanism underlying wild type (WT) ASYN-mediated neurotoxicity, we have generated a novel Tet-Off SHSY-5Y cell line, conditionally expressing WT ASYN. Induction of human WT ASYN in retinoic acid-differentiated SHSY-5Y cells leads to accumulation of soluble ASYN oligomers, in the absence of inclusions, and to gradual cellular degeneration. Morphologically, the death observed is non-apoptotic. Caspases other than caspase 3, including caspase 9, are activated and caspase inhibition diminishes death by acting at a point upstream of cytochrome c release. Application of Scyllo-inositol, an oligomer-stabilizing compound, prevents neuronal death in this model. These findings are consistent with a model in which oligomeric ASYN triggers the initial activation of the apoptotic pathway, which is however blocked downstream of the mitochondrial checkpoint, thus leading to a death combining in a unique fashion both apoptotic and non-apoptotic features. This novel inducible cell model system may prove valuable in the deciphering of WT ASYN-induced pathogenic effects and in the assessment and screening of potential therapeutic strategies.

Calcium signaling and neurodegenerative diseases

Neurodegenerative disorders, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD) and spinocerebellar ataxias (SCAs), present an enormous medical, social, financial and scientific problem. Recent evidence indicates that neuronal calcium (Ca2+) signaling is abnormal in many of these disorders. Similar, but less severe, changes in neuronal Ca2+ signaling occur as a result of the normal aging process. The role of aberrant neuronal Ca2+ signaling in the pathogenesis of neurodegenerative disorders is discussed here. The potential utility of Ca2+ blockers for treatment of these disorders is also highlighted. It is reasoned that Ca2+ blockers will be most beneficial clinically when used in combination with other disease-specific therapeutic approaches.

Real-time analysis of amyloid fibril formation of α-synuclein using a fibrillation-state-specific fluorescent probe of JC-1

α-Synuclein is a pathological component of PD (Parkinson's disease) by participating in Lewy body formation. JC-1 (5,5′,6,6′-tetrachloro-1,1,3,3′-tetraethylbenzimidazolyl carbocyanine iodide) has been shown to interact with α-synuclein at the acidic C-terminal region with a Kd of 2.6 μM. JC-1 can discriminated between the fibrillation states of α-synuclein (monomeric, oligomeric intermediate and fibrillar forms) by emitting the enhanced binding fluorescence of different colours at 590, 560 and 538 nm respectively with the common excitation at 490 nm. The fibrillation-state-specific interaction of JC-1 allowed us to perform real-time analyses of the α-synuclein fibrillation in the presence of iron as a fibrillation inducer, rifampicin as a fibrillation inhibitor, baicalein as a defibrillation agent and dequalinium as a protofibril inducer. In addition, various α-synuclein fibrils with different morphologies prepared with specific ligands such as metal ions, glutathione, eosin and lipids were monitored with their characteristic JC-1-binding fluorescence spectra. FRET (fluorescence resonance energy transfer) between thioflavin-T and JC-1 was also employed to specifically identify the amyloid fibrils of α-synuclein. Taken together, we have introduced JC-1 as a powerful and versatile probe to explore the molecular mechanism of the fibrillation process of α-synuclein in vitro. It could be also useful in high-throughput drug screening. The specific α-synuclein interaction of JC-1 would therefore contribute to our complete understanding of the molecular aetiology of PD and eventual development of diagnostic/therapeutic strategies for various α-synucleinopathies.

Detecting morphologically distinct oligomeric forms of alpha-synuclein

Neuropathologic and genetics studies as well as transgenic animal models have provided strong evidence linking misfolding and aggregation of alpha-synuclein to the progression of Parkinson disease (PD) and other related disorders. A growing body of evidence implicates various oligomeric forms of alpha-synuclein as the toxic species responsible for neurodegeneration and neuronal cell death. Although numerous different oligomeric forms of alpha-synuclein have been identified in vitro, it is not known which forms are involved in PD or how, when, and where different forms contribute to the progression of PD. Reagents that can interact with specific aggregate forms of alpha-synuclein would be very useful not only as tools to study how different aggregate forms affect cell function, but also as potential diagnostic and therapeutic agents for PD. Here we show that a single chain antibody fragment (syn-10H scFv) isolated from a phage display antibody library binds to a larger, later stage oligomeric form of alpha-synuclein than a previously reported oligomeric specific scFv isolated in our laboratory. The scFv described here inhibits aggregation of alpha-synuclein in vitro, blocks extracellular alpha-synuclein-induced toxicity in both undifferentiated and differentiated human neuroblastoma cell lines (SH-SY5Y), and specifically recognizes naturally occurring aggregates in PD but not in healthy human brain tissue.

Amyloid aggregation on lipid bilayers and its impact on membrane permeability

Fibrillar protein aggregates (amyloids) are involved in several common pathologies, e.g., Alzheimer's disease and type II diabetes. Accumulating evidence suggests that toxicity in amyloid-related diseases originates from the deposition of protein aggregates on the cell membrane, which results in bilayer disruption and cell leakage. The molecular mechanism of damage to the membrane, however, is still obscure. To shed light on it we have performed coarse-grained molecular dynamics simulations of fibril-forming amphipathic peptides in the presence of lipid vesicles. The simulation results show that highly amyloidogenic peptides fibrillate on the surface of the vesicle, damaging the bilayer and promoting leakage. In contrast, the ordered aggregation of peptides with low amyloidogenicity is hindered by the vesicles. Remarkably, leakage from the vesicle is caused by growing aggregates, but not mature fibrils. The simulation results provide a basis for understanding the range of aggregation behavior that is observed in experiments with fibril-forming (poly)peptides.

Absence of alpha-synuclein affects dopamine metabolism and synaptic markers in the striatum of aging mice

Despite numerous evidences for neurotoxicity of overexpressed α-synuclein, a protective function was suggested for endogenous α-synuclein and other members of the synuclein family. This protective role is most important for and evident in presynaptic terminals, where synucleins are normally accumulated. However, mice lacking synucleins display no adverse phenotype. In particular, no significant changes in striatal dopamine metabolism and only subtle deficit of dopaminergic neurons in the substantia nigra were found in juvenile or adult mice. To assess whether aging and synuclein deficiency may have additive detrimental effect on the nigrostriatal system, we studied dopaminergic neurons of the substantia nigra and their striatal synapses in 24–26-month-old α-synuclein and γ-synuclein null mutant mice. Significant ∼36% reduction of the striatal dopamine was found in aging α-synuclein, but not γ-synuclein null mutant mice when compared to age-matching wild type mice. This was accompanied by the reduction of TH-positive fibers in the striatum and decrease of striatal levels of TH and DAT. However, no progressive loss of TH-positive neurons was revealed in the substantia nigra of synuclein-deficient aging animals. Our results are consistent with a hypothesis that α-synuclein is important for normal function and integrity of synapses, and suggest that in the aging nervous system dysfunction of this protein could become a predisposition factor for the development of nigrostriatal pathology.

Protein folding and misfolding on surfaces

Protein folding, misfolding and aggregation, as well as the way misfolded and aggregated proteins affects cell viability are emerging as key themes in molecular and structural biology and in molecular medicine. Recent advances in the knowledge of the biophysical basis of protein folding have led to propose the energy landscape theory which provides a consistent framework to better understand how a protein folds rapidly and efficiently to the compact, biologically active structure. The increased knowledge on protein folding has highlighted its strict relation to protein misfolding and aggregation, either process being in close competition with the other, both relying on the same physicochemical basis. The theory has also provided information to better understand the structural and environmental factors affecting protein folding resulting in protein misfolding and aggregation into ordered or disordered polymeric assemblies. Among these, particular importance is given to the effects of surfaces. The latter, in some cases make possible rapid and efficient protein folding but most often recruit proteins/peptides increasing their local concentration thus favouring misfolding and accelerating the rate of nucleation. It is also emerging that surfaces can modify the path of protein misfolding and aggregation generating oligomers and polymers structurally different from those arising in the bulk solution and endowed with different physical properties and cytotoxicities.

Pink1 suppresses α-synuclein-induced phenotypes in a Drosophila model of Parkinson’s disease

Parkinson’s disease (PD) is the most prevalent human neurodegenerative movement disorder and is characterized by a selective and progressive loss of the dopaminergic neurons. Mutations in the genes parkin and PTEN-induced putative kinase 1 (PINK1) result in autosomal recessive forms of PD. It has been suggested that parkin and Pink1 function in the same pathway in Drosophila , with Pink1 acting upstream of parkin. Previous work in our laboratory has shown the ability of parkin to rescue an α-synuclein-induced PD-like phenotype in Drosophila. To investigate the ability of Pink1 to protect against α-synuclein-induced toxicity, we have performed longevity, mobility, and histological studies to determine whether Drosophila Pink1 can rescue the α-synuclein phenotypes. We have found that overexpression of Pink1 results in the rescue of the α-synuclein-induced phenotype of premature loss of climbing ability, suppression of degeneration of the ommatidial array, and the suppression of α-synuclein-induced developmental defects in the Drosophila eye. These results mark the first demonstration of Pink1 counteracting PD phenotypes in a protein toxicity animal model, and they show that Pink1 is able to impart protection against potentially harmful proteins such as α-synuclein that would otherwise result in cellular stress.

alpha-Synuclein: a therapeutic target for Parkinson's disease?

Parkinson's disease is a progressive age-related neurodegenerative disease with invariant loss of substantia nigra dopamine neurons and striatal projections. This disorder is well known for the associated motoric symptoms including resting tremor and the inability to initiate movement. However, it is now apparent that Parkinson's disease is a multisystem disorder with patients exhibiting symptoms derived from peripheral nervous system and extra-nigral dysfunctions in addition to the prototypical nigrostriatal damage. Although the etiology for sporadic Parkinson's disease is unknown, information gleaned from both familial forms of the disease and animal models places misfolded alpha-synuclein at the forefront. The disease is currently without a cure and most therapies target the motoric symptoms relying on increasing dopamine tone. In this review, the role of alpha-synuclein in disease pathogenesis and as a potential therapeutic target focusing on toxic conformers of this protein is considered. The addition of protofibrillar/oligomer-directed neurotherapeutics to the existing armamentarium may extend the symptom-free stage of Parkinson's disease as well as alleviate pathogenesis.

The effect of membranes on the in vitro fibrillation of an amyloidogenic light-chain variable-domain SMA

Light chain (or AL) amyloidosis is the most common form of systemic amyloidosis, characterized by the pathological deposition of insoluble fibrils of immunoglobulin light-chain fragments in various organs and tissues, especially in the kidney and heart. Both the triggering factors and the mechanisms involved in the abnormal formation of the insoluble fibrillar aggregates from the soluble proteins are poorly understood. For example, although the fibrillar deposits are typically found associated with the extracellular matrix and basement membranes, it is not clear whether fibrils are initially formed intra- or extracellularly, nor it is understood what determines where the deposits will occur; i.e., site tropism. In the present investigation, we studied the interaction of a recombinant amyloidogenic light-chain variable domain, SMA, with lipid vesicles. The nature of the interaction was dependent on the lipid composition and the SMA to lipid ratio. The most pronounced effect was found from vesicles composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphate, which dramatically accelerated fibril growth. Interestingly, spectral probes, such as intrinsic fluorescence and far-UV CD spectroscopy did not show significant conformational changes in the presence of the vesicles. The presence of cholesterol or divalent cations, such as Ca(2+) and Mg(2+), lead to decreased membrane-induced SMA fibrillation. Thus, membranes may have significant effects on light-chain fibrillation and may contribute to the site selectivity observed in AL amyloidosis.

Cathepsin D is the main lysosomal enzyme involved in the degradation of alpha-synuclein and generation of its carboxy-terminally truncated species

Alpha-synuclein is likely to play a key role in the development of Parkinson's disease as well as other synucleinopathies. In animal models, overexpression of full-length or carboxy-terminally truncated alpha-synuclein has been shown to produce pathology. Although the proteosome and lysosome have been proposed to play a role in the degradation of alpha-synuclein, the enzyme(s) involved in alpha-synuclein clearance and generation of its carboxy-terminally truncated species have not been identified. In this study, the role of cathepsin D and calpain I in these processes was analyzed. In vitro experiments, using either recombinant or endogenous alpha-synuclein as substrates and purified cathepsin D or lysosomes, demonstrated that cathepsin D degraded alpha-synuclein very efficiently, and that limited proteolysis resulted in the generation of carboxy-terminally truncated species. Purified calpain I also cleaved alpha-synuclein, but carboxy-terminally truncated species were not the main cleavage products, and calpain I activity present in cellular lysates was not able to degrade the protein. Knockdown of cathepsin D in cells overexpressing wild-type alpha-synuclein increased total alpha-synuclein levels by 28% and lysosomal alpha-synuclein by 2-fold. In in vitro experiments, pepstatin A completely blocked the degradation of alpha-synuclein in purified lysosomes. Furthermore, lysosomes isolated from cathepsin D knockdown cells showed a marked reduction in alpha-synuclein degrading activity, indicating that cathepsin D is the main lysosomal enzyme involved in alpha-synuclein degradation. Our findings suggest that upregulation of cathepsin D could be an additional therapeutic strategy to lessen alpha-synuclein burden in synucleinopathies.

Approaches to prevent dopamine quinone-induced neurotoxicity

Dopamine (DA) and its metabolites containing two hydroxyl residues exert cytotoxicity in dopaminergic neuronal cells, primarily due to the generation of highly reactive DA and DOPA quinones. Quinone formation is closely linked to other representative hypotheses such as mitochondrial dysfunction, inflammation, oxidative stress, and dysfunction of the ubiquitin-proteasome system, in the pathogenesis of neurodegenerative diseases such as Parkinson's disease and methamphetamine-induced neurotoxicity. Therefore, pathogenic effects of the DA quinone have focused on dopaminergic neuron-specific oxidative stress. Recently, various studies have demonstrated that some intrinsic molecules and several drugs exert protective effects against DA quinone-induced damage of dopaminergic neurons. In this article, we review recent studies on some neuroprotective approaches against DA quinone-induced dysfunction and/or degeneration of dopaminergic neurons.

Mechanisms of hybrid oligomer formation in the pathogenesis of combined Alzheimer's and Parkinson's diseases

BACKGROUND

Misfolding and pathological aggregation of neuronal proteins has been proposed to play a critical role in the pathogenesis of neurodegenerative disorders. Alzheimer's disease (AD) and Parkinson's disease (PD) are frequent neurodegenerative diseases of the aging population. While progressive accumulation of amyloid beta protein (Abeta) oligomers has been identified as one of the central toxic events in AD, accumulation of alpha-synuclein (alpha-syn) resulting in the formation of oligomers and protofibrils has been linked to PD and Lewy body Disease (LBD). We have recently shown that Abeta promotes alpha-syn aggregation and toxic conversion in vivo, suggesting that abnormal interactions between misfolded proteins might contribute to disease pathogenesis. However the molecular characteristics and consequences of these interactions are not completely clear.

METHODOLOGY/PRINCIPAL FINDINGS

In order to understand the molecular mechanisms involved in potential Abeta/alpha-syn interactions, immunoblot, molecular modeling, and in vitro studies with alpha-syn and Abeta were performed. We showed in vivo in the brains of patients with AD/PD and in transgenic mice, Abeta and alpha-synuclein co-immunoprecipitate and form complexes. Molecular modeling and simulations showed that Abeta binds alpha-syn monomers, homodimers, and trimers, forming hybrid ring-like pentamers. Interactions occurred between the N-terminus of Abeta and the N-terminus and C-terminus of alpha-syn. Interacting alpha-syn and Abeta dimers that dock on the membrane incorporated additional alpha-syn molecules, leading to the formation of more stable pentamers and hexamers that adopt a ring-like structure. Consistent with the simulations, under in vitro cell-free conditions, Abeta interacted with alpha-syn, forming hybrid pore-like oligomers. Moreover, cells expressing alpha-syn and treated with Abeta displayed increased current amplitudes and calcium influx consistent with the formation of cation channels.

CONCLUSION/SIGNIFICANCE

These results support the contention that Abeta directly interacts with alpha-syn and stabilized the formation of hybrid nanopores that alter neuronal activity and might contribute to the mechanisms of neurodegeneration in AD and PD. The broader implications of such hybrid interactions might be important to the pathogenesis of other disorders of protein misfolding.

Self-assembling Peptides: From Bio-inspired Materials to Bone Regeneration

In recent years, the development of new biomaterials with specifications for tissue and organ functional requirements—such as proper biological, structural, and biomechanical properties as well as designed control for biodegradation and therapeutic drug-release capacity—is the main aim of many academic and industrial programs. Hence, the concept of molecular self-assembly is the driving force for the development of new biomaterials that support the growth and functional differentiation of cells and tissues in a controlled manner. The discovery, properties, and development of self-assembling peptides to be used as three-dimensional (3D) scaffolds based on their similarity (in structure and mechanical features) to extracellular matrices are described. Self-assembling peptides can be used for in vitro applications for cell 3D culture as well as in vivo for tissue regeneration such as bone and optical nerve repair, as well as for drug delivery of mediators to improve therapy, as in the case of myocardial infarction. Finally, the use of self-assembling materials in combination with a bioengineering platform is proposed to assist functional bone regeneration in cases of larger bone defects, including exposed fractures due to trauma and spinal disorders dealing with high loadings, as well as replacement of big bone structures due to tumors.

Calcium-activated membrane interaction of the islet amyloid polypeptide: implications in the pathogenesis of type II diabetes mellitus

The role played by Ca(2+) ions in the interaction of the human islet amyloid polypeptide (hIAPP) with model membranes has been investigated by differential scanning calorimetry (DSC) and circular dichroism (CD) experiments. In particular, the interaction of hIAPP and its rat isoform (rIAPP) with zwitterionic dipalmitoyl-phosphatidylcholine (DPPC), negatively charged dipalmitoyl-phosphatidylserine (DPPS) vesicles and with a 3:1 mixtures of them, has been studied in the presence of Ca(2+) ions. The experiments have evidenced that amorphous, soluble hIAPP assemblies interact with the hydrophobic core of DPPC bilayers. Conversely, the presence of Ca(2+) ions is necessary to activate a preferential interaction of hIAPP with the hydrophobic core of DPPS membranes. These findings support the hypothesis that an impaired cellular homeostasis of Ca(2+) ions may promote the insertion of hIAPP into the hydrophobic core of carrier vesicles which is thought to contribute to an eventual intracellular accumulation of beta-sheet rich hIAPP aggregates.

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

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

Statins reduce neuronal alpha-synuclein aggregation in in vitro models of Parkinson's disease

Aggregation of alpha-synuclein (alpha-syn) is believed to play a critical role in the pathogenesis of disorders such as dementia with Lewy bodies and Parkinson's disease. The function of alpha-syn remains unclear, although several lines of evidence suggest that alpha-syn is involved in synaptic vesicle trafficking probably via lipid binding. Moreover, interactions with cholesterol and lipids have been shown to be involved in alpha-syn aggregation. In this context, the main objective of this study was to determine if statins--cholesterol synthesis inhibitors--might interfere with alpha-syn accumulation in cellular models. For this purpose, we studied the effects of lovastatin, simvastatin, and pravastatin on the accumulation of alpha-syn in a stably transfected neuronal cell line and in primary human neurons. Statins reduced the levels of alpha-syn accumulation in the detergent insoluble fraction of the transfected cells. This was accompanied by a redistribution of alpha-syn in caveolar fractions, a reduction in oxidized alpha-syn, and enhanced neurite outgrowth. In contrast, supplementation of the media with cholesterol increased alpha-syn aggregation in detergent insoluble fractions of transfected cells and was accompanied by reduced neurite outgrowth. Taken together, these results suggest that regulation of cholesterol levels with cholesterol inhibitors might be a novel approach for the treatment of Parkinson's disease.

Tissue transglutaminase modulates alpha-synuclein oligomerization

We have studied the interaction of the enzyme tissue transglutaminase (tTG), catalyzing cross-link formation between protein-bound glutamine residues and primary amines, with Parkinson's disease-associated alpha-synuclein protein variants at physiologically relevant concentrations. We have, for the first time, determined binding affinities of tTG for wild-type and mutant alpha-synucleins using surface plasmon resonance approaches, revealing high-affinity nanomolar equilibrium dissociation constants. Nanomolar tTG concentrations were sufficient for complete inhibition of fibrillization by effective alpha-synuclein cross-linking, resulting predominantly in intramolecularly cross-linked monomers accompanied by an oligomeric fraction. Since oligomeric species have a pathophysiological relevance we further investigated the properties of the tTG/alpha-synuclein oligomers. Atomic force microscopy revealed morphologically similar structures for oligomers from all alpha-synuclein variants; the extent of oligomer formation was found to correlate with tTG concentration. Unlike normal alpha-synuclein oligomers the resultant structures were extremely stable and resistant to GdnHCl and SDS. In contrast to normal beta-sheet-containing oligomers, the tTG/alpha-synuclein oligomers appear to be unstructured and are unable to disrupt phospholipid vesicles. These data suggest that tTG binds equally effective to wild-type and disease mutant alpha-synuclein variants. We propose that tTG cross-linking imposes structural constraints on alpha-synuclein, preventing the assembly of structured oligomers required for disruption of membranes and for progression into fibrils. In general, cross-linking of amyloid forming proteins by tTG may prevent the progression into pathogenic species.

Golgi apparatus and neurodegenerative diseases

Neurodegenerative disorders are typically characterized by progressive and extensive neuronal loss in specific populations of neurons and brain areas which lead to the observed clinical manifestations. Despite the recent advances in molecular neuroscience, the subcellular bases such as Golgi apparatus (GA) for most neurodegenerative diseases are poorly understood. This review gives a brief overview of the contribution of the neuronal GA in the pathogeneses of neurodegeneration, summarizes what is known of the GA machinery in these diseases, and present the relationship between GA fragmentation and the aggregation and accumulation of misfolded or aberrant proteins including mutant SOD1, a-synuclein, tau, which is considered to be a key event in the pathogenic process, and perturbating in calcium homeostasis, regulation of hormones, lipid metabolism are also linkage to the function of the GA thought to underlie neurodegeneration. Although these precise diseases mechanisms remain to be clarified, more research is needed to better understand how GA function for it and to enable physicians to use this knowledge for the benefit of the patients.

Structure of alpha-helical membrane-bound human islet amyloid polypeptide and its implications for membrane-mediated misfolding

Human islet amyloid polypeptide (hIAPP) misfolding is thought to play an important role in the pathogenesis of type II diabetes mellitus. It has recently been shown that membranes can catalyze the misfolding of hIAPP via an alpha-helical intermediate of unknown structure. To better understand the mechanism of membrane-mediated misfolding, we used site-directed spin labeling and EPR spectroscopy to generate a three-dimensional structural model of this membrane-bound form. We find that hIAPP forms a single alpha-helix encompassing residues 9-22. The helix is flanked by N- and C-terminal regions that do not take up a clearly detectable secondary structure and are less ordered. Residues 21 and 22 are located in a transitional region between the alpha-helical structure and C terminus and exhibit significant mobility. The alpha-helical structure presented here has important implications for membrane-mediated aggregation. Anchoring hIAPP to the membrane not only increases the local concentration but also reduces the encounter between peptides to essentially a two-dimensional process. It is significant to note that the alpha-helical membrane-bound form leaves much of an important amyloidogenic region of hIAPP (residues 20-29) exposed for misfolding. Misfolding of this and other regions is likely further aided by the low dielectric environment near the membrane that is known to promote secondary structure formation. Based upon these considerations, a structural model for membrane-mediated aggregation is discussed.

Drug Targeting of α-Synuclein Oligomerization in Synucleinopathies

The heterogeneity of symptoms and disease progression observed in synucleinopathies, of which Parkinson's disease (PD) is the most common representative, poses large problems for the discovery of novel therapeutics. The molecular basis for pathology is currently unclear, both in familial and in sporadic cases. While the therapeutic effects of L-DOPA and dopamine receptor agonists constitute good options for symptomatic treatment in PD, the development of neuroprotective and/or neurorestorative treatments for PD and other synucleinopathies faces significant challenges due to the poor knowledge of the putative targets. Recent experimental evidence strongly suggests a central role for neurotoxic α-synuclein oligomeric species in neurodegeneration. The events leading to protein oligomerization, as well as the oligomeric species themselves, are likely amenable to modulation by small molecules, which are beginning to emerge in high throughput compound screens in a variety of model organisms. The therapeutic potential of small molecule modulators of oligomer formation demands further exploration and validation in cellular and animal disease models in order to accelerate human drug development.

Dopamine-modified alpha-synuclein blocks chaperone-mediated autophagy

Altered degradation of alpha-synuclein (alpha-syn) has been implicated in the pathogenesis of Parkinson disease (PD). We have shown that alpha-syn can be degraded via chaperone-mediated autophagy (CMA), a selective lysosomal mechanism for degradation of cytosolic proteins. Pathogenic mutants of alpha-syn block lysosomal translocation, impairing their own degradation along with that of other CMA substrates. While pathogenic alpha-syn mutations are rare, alpha-syn undergoes posttranslational modifications, which may underlie its accumulation in cytosolic aggregates in most forms of PD. Using mouse ventral medial neuron cultures, SH-SY5Y cells in culture, and isolated mouse lysosomes, we have found that most of these posttranslational modifications of alpha-syn impair degradation of this protein by CMA but do not affect degradation of other substrates. Dopamine-modified alpha-syn, however, is not only poorly degraded by CMA but also blocks degradation of other substrates by this pathway. As blockage of CMA increases cellular vulnerability to stressors, we propose that dopamine-induced autophagic inhibition could explain the selective degeneration of PD dopaminergic neurons.

Globular and pre-fibrillar prion aggregates are toxic to neuronal cells and perturb their electrophysiology

Prion diseases are characterised at autopsy by neuronal loss and accumulation of amorphous protein aggregates and/or amyloid fibrils in the brains of humans and animals. These protein deposits result from the conversion of the cellular, mainly alpha-helical prion protein (PrP(C)) to the beta-sheet-rich isoform (PrP(Sc)). Although the pathogenic mechanism of prion diseases is not fully understood, it appears that protein aggregation is itself neurotoxic and not the product of cell death. The precise nature of the neurotoxic species and mechanism of cell death are yet to be determined, although recent studies with other amyloidogenic proteins suggest that ordered pre-fibrillar or oligomeric forms may be responsible for cellular dysfunction. In this study we have refolded recombinant prion protein (rPrP) to two distinct forms rich in beta-sheet structure with an intact disulphide bond. Here we report on the structural properties of globular aggregates and pre-fibrils of rPrP and show that both states are toxic to neuronal cells in culture. We show that exogenous rPrP aggregates are internalised by neuronal cells and found in the cytoplasm. We also measured the changes in electrophysiological properties of cultured neuronal cells on exposure to exogenous prion aggregates and discuss the implications of these findings.

Dequalinium-induced cell death of yeast expressing alpha-synuclein-GFP fusion protein

Intracellular toxic effects of the dequalinium-induced protofibrils of alpha-synuclein have been investigated with the yeast system expressing alpha-synuclein-GFP fusion protein in single copy, which appears in the green halo around the plasma membrane. Intracellular responses of the green fluorescent protein were analyzed as the cells were treated with dequalinium (DQ) and lactacystin. Yeast cells expressing alpha-synuclein-GFP were susceptible to both compounds in alpha-synuclein-dependent manner. Upon DQ treatment, the green halo became smeared throughout the cytoplasm while lactacystin induced a few discrete green dots, reflecting intracellular formation of the protofibrils and the protein inclusions, respectively. The DQ-treated yeast cells were intensely stained with the nucleic acid stains of cell-permeable Hoechst 33342 and cell-impermeable propidium imidione, indicating that nucleus has been disrupted in addition to plasma membrane destabilization. Those DQ-treated yeast cells, however, still contained active mitochondria identified with MitoTracker Red. Therefore, the DQ-induced protofibrillar state of alpha-synuclein-GFP has been suggested to cause the nuclear damage either independently or in combination with the membrane destabilization without affecting mitochondria.

Effects of vitamin E supplementation on plasma membrane permeabilization and fluidization induced by chlorpromazine in the rat brain

Neurotransmitter receptors play a key role in most research on antipsychotic drugs, but little is known about the effects of these drugs on the plasma membrane in the central nervous system. Therefore, we investigated whether chlorpromazine (CPZ), a typical phenothiazine antipsychotic drug, affects the plasma membrane integrity in the rat brain, and if so, whether these membrane alterations can be prevented by dietary supplementation with vitamin E, which has been shown to be an antioxidant and also a membrane-stabilizer. Leakage of [(18)F]2-fluoro-2-deoxy-D-glucose ([(18)F]FDG)-6-phosphate from rat striatal slices and decrease in 1,6-diphenyl-1,3,5-hexatriene fluorescence anisotropy were used as indexes for plasma membrane permeabilization and fluidization, respectively. CPZ induced leakage of [(18)F]FDG-6-phosphate from striatal slices, and the leakage was delayed in the vitamin E-supplemented group compared to that in the normal diet group. The decrease in plasma membrane anisotropy induced by CPZ was significantly attenuated by vitamin E supplementation. Chronic treatment with alpha-phenyl-N-tert-butyl nitrone, a free radical scavenger, had no effect on CPZ-induced plasma membrane permeabilization, and the treatment with CPZ did not induce lipid peroxidation. CPZ can reduce plasma membrane integrity in the brain, and this reduction can be prevented by vitamin E via its membrane-stabilizing properties, not via its antioxidant activity.

Environmental factors differently affect human and rat IAPP: conformational preferences and membrane interactions of IAPP17-29 peptide derivatives

Interest in the 37-residue human islet amyloid polypeptide (hIAPP) is related to its ability to form amyloid deposits in patients affected by type II diabetes. Attempts to unravel the molecular features of this disease have indicated several regions of this polypeptide to be responsible for either the ability to form insoluble fibrils or the abnormal interaction with membranes. To extend these studies to peptides that enclose His18, whose ionization state is believed to play a key role in the aggregation of hIAPP, we report on the synthesis of two peptides, hIAPP17-29 and rIAPP17-29, encompassing the 17-29 sequences of human and rat IAPP, respectively, as well as on their conformational features in water and in several membrane-mimicking environments as revealed by circular dichroism (CD) and 2D-NMR studies. hIAPP17-29 adopts a beta-sheet structure in water and its solubility increases at low pH. Anionic sodium dodecyl sulfate (SDS) micelles promoted the formation of an alpha-helical structure in the peptide chain, which was poorly influenced by pH variations. rIAPP17-29 was soluble and unstructured in all the environments investigated, with a negligible effect of pH. The membrane interactions of hIAPP17-29 and rIAPP17-29 were assessed by recording differential scanning calorimetry (DSC) measurements aimed at elucidating the peptide-induced changes in the thermotropic behaviour of zwitterionic (DPPC) and negatively charged (DPPC/DPPS 3:1) model membranes (DPPC=1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPS=1,2-dipalmitoyl-sn-glycero-3-phosphoserine). Results of DSC experiments demonstrated the high potential of hIAPP17-29 to interact with DPPC membranes. hIAPP17-29 exhibited a negligible affinity for negatively charged DPPC/DPPS model membranes at neutral pH. On the other hand, rIAPP17-29 did not interact with neutral or negatively charged membranes. The role played by His18 in the modulation of the biophysical properties of this hIAPP region was assessed by synthesising and studying the R18HrIAPP17-29 peptide; the replacement of a single Arg with a His residue is not sufficient to induce either amyloidogenic propensity or membrane interaction in this region. The results show that the 17-29 domain of hIAPP has many properties of the full-length protein "in vitro" and this opens up new perspectives for both research and eventually therapy.

Genetic factors involved in the pathogenesis of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease characterized by a loss of nigrostriatal dopaminergic neurons. Recently, PD research has been stimulated by the identification of genes that are implicated in rare familial forms of PD. However, despite these discoveries, the primary cause of PD is still unclear. Various pathogenic mechanisms may be involved including mitochondrial dysfunction, proteasomal dysfunction/protein aggregation, oxidative damage, environmental factors and genetic disposition. Furthermore, dopamine has also been implicated in contributing to the pathogenesis of PD. This review will focus on the genes that have been identified to be associated with PD and how they may impair dopamine metabolism. Understanding the role of these PD-related genes in dopamine neurobiology may provide insight into the underpinning pathogenic mechanisms of PD.

Single particle characterization of iron-induced pore-forming alpha-synuclein oligomers

Aggregation of alpha-synuclein is a key event in several neurodegenerative diseases, including Parkinson disease. Recent findings suggest that oligomers represent the principal toxic aggregate species. Using confocal single-molecule fluorescence techniques, such as scanning for intensely fluorescent targets (SIFT) and atomic force microscopy, we monitored alpha-synuclein oligomer formation at the single particle level. Organic solvents were used to trigger aggregation, which resulted in small oligomers ("intermediate I"). Under these conditions, Fe(3+) at low micromolar concentrations dramatically increased aggregation and induced formation of larger oligomers ("intermediate II"). Both oligomer species were on-pathway to amyloid fibrils and could seed amyloid formation. Notably, only Fe(3+)-induced oligomers were SDS-resistant and could form ion-permeable pores in a planar lipid bilayer, which were inhibited by the oligomer-specific A11 antibody. Moreover, baicalein and N'-benzylidene-benzohydrazide derivatives inhibited oligomer formation. Baicalein also inhibited alpha-synuclein-dependent toxicity in neuronal cells. Our results may provide a potential disease mechanism regarding the role of ferric iron and of toxic oligomer species in Parkinson diseases. Moreover, scanning for intensely fluorescent targets allows high throughput screening for aggregation inhibitors and may provide new approaches for drug development and therapy.

Aggregation of alpha-synuclein by DOPAL, the monoamine oxidase metabolite of dopamine

Parkinson's disease (PD) is a neurodegenerative disease characterized by the selective loss of dopamine (DA) neurons and the presence of alpha-synuclein (AS) aggregates as Lewy bodies (LBs) in the remaining substantia nigra (SN) neurons. A continuing puzzle in studying PD pathogenesis is that although AS is expressed throughout the brain, LBs and selective dopaminergic cell loss lead to characteristic clinical signs of PD, suggesting that there is a link between AS aggregation and DA metabolism. One potential candidate for this link is the monoamine oxidase (MAO) metabolite of DA, 3,4-dihydroxyphenylacetaldehyde (DOPAL), as neither DA nor DA metabolites other than DOPAL are toxic to SN neurons at physiological concentrations. We tested DOPAL-induced AS aggregation in a cell-free system, in vitro in DA neuron cultures and in vivo with stereotactic injections into the SN of Sprague-Dawley rats by Western blots, fluorescent confocal microscopy and immunohistochemistry. We demonstrate that DOPAL in physiologically relevant concentrations, triggers AS aggregation in the cell-free system, and in cell cultures resulting in the formation of potentially toxic AS oligomers and aggregates. Furthermore, DOPAL injection into the SN of Sprague-Dawley rats resulted in DA neuron loss and the accumulation of high molecular weight oligomers of AS detected by Western blot. Our findings support the hypothesis that DA metabolism via DOPAL can cause both DA neuron loss and AS aggregation observed in PD.

The two faces of protein misfolding: gain- and loss-of-function in neurodegenerative diseases

The etiologies of neurodegenerative diseases may be diverse; however, a common pathological denominator is the formation of aberrant protein conformers and the occurrence of pathognomonic proteinaceous deposits. Different approaches coming from neuropathology, genetics, animal modeling and biophysics have established a crucial role of protein misfolding in the pathogenic process. However, there is an ongoing debate about the nature of the harmful proteinaceous species and how toxic conformers selectively damage neuronal populations. Increasing evidence indicates that soluble oligomers are associated with early pathological alterations, and strikingly, oligomeric assemblies of different disease-associated proteins may share common structural features. A major step towards the understanding of mechanisms implicated in neuronal degeneration is the identification of genes, which are responsible for familial variants of neurodegenerative diseases. Studies based on these disease-associated genes illuminated the two faces of protein misfolding in neurodegeneration: a gain of toxic function and a loss of physiological function, which can even occur in combination. Here, we summarize how these two faces of protein misfolding contribute to the pathomechanisms of Alzheimer's disease, frontotemporal lobar degeneration, Parkinson's disease and prion diseases.

Beer and bread to brains and beyond: can yeast cells teach us about neurodegenerative disease?

For millennia, humans have harnessed the astonishing power of yeast, producing such culinary masterpieces as bread, beer and wine. Therefore, in this new millennium, is it very farfetched to ask if we can also use yeast to unlock some of the modern day mysteries of human disease? Remarkably, these seemingly simple cells possess most of the same basic cellular machinery as the neurons in the brain. We and others have been using the baker's yeast, Saccharomyces cerevisiae, as a model system to study the mechanisms of devastating neurodegenerative diseases such as Parkinson's, Huntington's, Alzheimer's and amyotrophic lateral sclerosis. While very different in their pathophysiology, they are collectively referred to as protein-misfolding disorders because of the presence of misfolded and aggregated forms of various proteins in the brains of affected individuals. Using yeast genetics and the latest high-throughput screening technologies, we have identified some of the potential causes underpinning these disorders and discovered conserved genes that have proven effective in preventing neuron loss in animal models. Thus, these genes represent new potential drug targets. In this review, I highlight recent work investigating mechanisms of cellular toxicity in a yeast Parkinson's disease model and discuss how similar approaches are being applied to additional neurodegenerative diseases.

Alpha-synuclein selectively binds to anionic phospholipids embedded in liquid-disordered domains

Previous studies indicate that binding of alpha-synuclein to membranes is critical for its physiological function and the development of Parkinson's disease (PD). Here, we have investigated the association of fluorescence-labeled alpha-synuclein variants with different types of giant unilamellar vesicles using confocal microscopy. We found that alpha-synuclein binds with high affinity to anionic phospholipids, when they are embedded in a liquid-disordered as opposed to a liquid-ordered environment. This indicates that not only electrostatic forces but also lipid packing and hydrophobic interactions are critical for the association of alpha-synuclein with membranes in vitro. When compared to wild-type alpha-synuclein, the disease-causing alpha-synuclein variant A30P bound less efficiently to anionic phospholipids, while the variant E46K showed enhanced binding. This suggests that the natural association of alpha-synuclein with membranes is altered in the inherited forms of Parkinson's disease.

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

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