Reversible Inhibition of α-Synuclein Fibrillization by Dopaminochrome-mediated Conformational Alterations

Protective and toxic roles of dopamine in Parkinson's disease

The molecular mechanisms causing the loss of dopaminergic neurons containing neuromelanin in the substantia nigra and responsible for motor symptoms of Parkinson's disease are still unknown. The discovery of genes associated with Parkinson's disease (such as alpha synuclein (SNCA), E3 ubiquitin protein ligase (parkin), DJ-1 (PARK7), ubiquitin carboxyl-terminal hydrolase isozyme L1 (UCHL-1), serine/threonine-protein kinase (PINK-1), leucine-rich repeat kinase 2 (LRRK2), cation-transporting ATPase 13A1 (ATP13A), etc.) contributed enormously to basic research towards understanding the role of these proteins in the sporadic form of the disease. However, it is generally accepted by the scientific community that mitochondria dysfunction, alpha synuclein aggregation, dysfunction of protein degradation, oxidative stress and neuroinflammation are involved in neurodegeneration. Dopamine oxidation seems to be a complex pathway in which dopamine o-quinone, aminochrome and 5,6-indolequinone are formed. However, both dopamine o-quinone and 5,6-indolequinone are so unstable that is difficult to study and separate their roles in the degenerative process occurring in Parkinson's disease. Dopamine oxidation to dopamine o-quinone, aminochrome and 5,6-indolequinone seems to play an important role in the neurodegenerative processes of Parkinson's disease as aminochrome induces: (i) mitochondria dysfunction, (ii) formation and stabilization of neurotoxic protofibrils of alpha synuclein, (iii) protein degradation dysfunction of both proteasomal and lysosomal systems and (iv) oxidative stress. The neurotoxic effects of aminochrome in dopaminergic neurons can be inhibited by: (i) preventing dopamine oxidation of the transporter that takes up dopamine into monoaminergic vesicles with low pH and dopamine oxidative deamination catalyzed by monoamino oxidase (ii) dopamine o-quinone, aminochrome and 5,6-indolequinone polymerization to neuromelanin and (iii) two-electron reduction of aminochrome catalyzed by DT-diaphorase. Furthermore, dopamine conversion to NM seems to have a dual role, protective and toxic, depending mostly on the cellular context. Dopamine oxidation to dopamine o-quinone, aminochrome and 5,6-indolequinone plays an important role in neurodegeneration in Parkinson's disease since they induce mitochondria and protein degradation dysfunction; formation of neurotoxic alpha synuclein protofibrils and oxidative stress. However, the cells have a protective system against dopamine oxidation composed by dopamine uptake mediated by Vesicular monoaminergic transporter-2 (VMAT-2), neuromelanin formation, two-electron reduction and GSH-conjugation mediated by Glutathione S-transferase M2-2 (GSTM2).

Glutathione transferase mu 2 protects glioblastoma cells against aminochrome toxicity by preventing autophagy and lysosome dysfunction

U373MG cells constitutively express glutathione S-transferase mu 2 (GSTM2) and exhibit (3)H-dopamine uptake, which is inhibited by 2 µM of nomifensine and 15 µM of estradiol. We generated a stable cell line (U373MGsiGST6) expressing an siRNA against GSTM2 that resulted in low GSTM2 expression (26% of wild-type U373MG cells). A significant increase in cell death was observed when U373MGsiGST6 cells were incubated with 50 µM purified aminochrome (18-fold increase) compared with wild-type cells. The incubation of U373MGsiGST6 cells with 75 µM aminochrome resulted in the formation of autophagic vacuoles containing undigested cellular components, as determined using transmission electron microscopy. A significant increase in autophagosomes was determined by measuring endogenous LC3-II, a significant decrease in cell death was observed in the presence of bafilomycin A 1, and a significant increase in cell death was observed in the presence of trehalose. A significant increase in LAMP2 immunostaining was observed, a significant decrease in bright red fluorescence of lysosomes with acridine orange was observed, and bafilomycin A 1 pretreatment reduced the loss of lysosome acidity. A significant increase in cell death was observed in the presence of lysosomal protease inhibitors. Aggregation of TUBA/α-tubulin (tubulin, α) and SQSTM1 protein accumulation were also observed. Moreover, a significant increase in the number of lipids droplets was observed compared with U373MG cells with normal expression of GSTM2. These results support the notion that GSTM2 is a protective enzyme against aminochrome toxicity in astrocytes and that aminochrome cell death in U373MGsiGST6 cells involves autophagic-lysosomal dysfunction.

Are dopamine derivatives implicated in the pathogenesis of Parkinson's disease?

Parkinson's disease (PD) is the most common motor system disorder affecting 1-2% of people over the age of sixty-five. Although PD is generally a sporadic neurological disorder, the discovery of monogenic, hereditable forms of the disease, representing 5-10% of all cases, has been very important in helping to partially delineate the molecular pathways that lead to this pathology. These mechanisms include impairment of the intracellular protein-degradation pathways, protein aggregation, mitochondria dysfunction, oxidative stress and neuroinflammation. Some of these features are also supported by post-mortem analyses. One of the main pathological hallmarks of PD is the preferential degeneration of dopaminergic neurons, which supports a direct role of dopamine itself in promoting the disorder. This review presents a comprehensive overview of the existing literature that links the aforementioned pathways to the oxidative chemistry of dopamine, ultimately leading to the formation of free radicals and reactive quinone species. We emphasize, in particular, how the reaction of dopamine-derived quinones with several cellular targets could foster the processes involved in the pathogenesis of PD and contribute to the progression of the disorder.

Modulation of human α-synuclein aggregation by a combined effect of calcium and dopamine

Parkinson's disease is characterized by the deposition of aggregated α-syn and its familial mutants into Lewy bodies leading to death of dopaminergic neurons. α-syn is involved in Ca(II) and dopamine (DA) signaling and their adequate balance inside neuronal cytoplasm is essential for maintaining healthy dopaminergic neurons. We have probed the binding energetics of Ca(II) and DA to human α-syn and its familial mutants A30P, A53T and E46K using isothermal titration calorimetry and have investigated the conformational and aggregation aspects using circular dichroism and fluorescence spectroscopy. While binding of Ca(II) to α-syn and its familial mutants was observed to be endothermic in nature, interaction of DA with α-syn was not detectable. Ca(II) enhanced fibrillation of α-syn and its familial mutants while DA promoted the formation of oligomers. However, Ca(II) and DA together critically favored the formation of protofibrils that are more cytotoxic than the mature fibrils. Using fluorescently labeled cysteine mutant A90C, we have shown that different aggregating species of α-syn formed in the presence of Ca(II) and DA are internalized into the human neuroblastoma cells with different rates and are responsible for the differential cytotoxicity depending on their nature. The findings put together suggest that an interplay between the concentrations of Ca(II), DA and α-syn can critically regulate the formation of various aggregating species responsible for the survival of dopaminergic neurons. Modulating this balance leading to either complete suppression of α-syn aggregation or promoting the formation of mature fibrils could be used as a strategy for the development of drugs to cure Parkinson's disease.

The function of α-synuclein

Human genetics has indicated a causal role for the protein α-synuclein in the pathogenesis of familial Parkinson's disease (PD), and the aggregation of synuclein in essentially all patients with PD suggests a central role for this protein in the sporadic disorder. Indeed, the accumulation of misfolded α-synuclein now defines multiple forms of neural degeneration. Like many of the proteins that accumulate in other neurodegenerative disorders, however, the normal function of synuclein remains poorly understood. In this article, we review the role of synuclein at the nerve terminal and in membrane remodeling. We also consider the prion-like propagation of misfolded synuclein as a mechanism for the spread of degeneration through the neuraxis.

The interplay between lipids and dopamine on α-synuclein oligomerization and membrane binding

The deposition of α-syn (α-synuclein) as amyloid fibrils and the selective loss of DA (dopamine) containing neurons in the substantia nigra are two key features of PD (Parkinson's disease). α-syn is a natively unfolded protein and adopts an α-helical conformation upon binding to lipid membrane. Oligomeric species of α-syn have been proposed to be the pathogenic species associated with PD because they can bind lipid membranes and disrupt membrane integrity. DA is readily oxidized to generate reactive intermediates and ROS (reactive oxygen species) and in the presence of DA, α-syn form of SDS-resistant soluble oligomers. It is postulated that the formation of the α-syn:DA oligomers involves the cross-linking of DA-melanin with α-syn, via covalent linkage, hydrogen and hydrophobic interactions. We investigate the effect of lipids on DA-induced α-syn oligomerization and studied the ability of α-syn:DA oligomers to interact with lipids vesicles. Our results show that the interaction of α-syn with lipids inhibits the formation of DA-induced α-syn oligomers. Moreover, the α-syn:DA oligomer cannot interact with lipid vesicles or cause membrane permeability. Thus, the formation of α-syn:DA oligomers may alter the actions of α-syn which require membrane association, leading to disruption of its normal cellular function.

Binding of Dopamine to Alpha-Synuclein is Mediated by Specific Conformational States

Parkinson's disease is the second most common neurodegenerative disorder, in which both alpha-synuclein (α-syn) and dopamine (DA) have a critical role. α-Syn is known to be natively unstructured in equilibrium with subpopulations of more compact structures. It is these compact structures that are thought to be linked to amyloid formation. In the presence of DA, α-syn yields a diverse range of SDS-resistant, non-amyloid oligomers, however the precursor state conformation has not been established. Here, three DA molecules have been observed to bind per α-syn monomer by electrospray-ionization-ion mobility spectrometry-mass spectrometry (ESI-IMS-MS). Each of these DA molecules binds exclusively to the extended conformation of α-syn, and binding is not observed in the compact state of the protein. Measurements of collisional cross sectional areas show that the incremental uptake of DA pushes the protein towards a highly extended population, becoming fully populated upon the binding of three DA ligands. Tyrosine (Tyr) as a closely related structural analog, exhibited limited binding to the protein as compared with DA, with a maximum of two ligands being observed. Those Tyr ligands that do bind were observed as adducts to the extended conformation akin to DA. These findings suggest DA is able to modulate α-syn self-assembly by inducing the population of a highly extended state.

Reprint of: revisiting oxidative stress and mitochondrial dysfunction in the pathogenesis of Parkinson disease-resemblance to the effect of amphetamine drugs of abuse

Parkinson disease (PD) is a chronic and progressive neurological disease associated with a loss of dopaminergic neurons. In most cases the disease is sporadic but genetically inherited cases also exist. One of the major pathological features of PD is the presence of aggregates that localize in neuronal cytoplasm as Lewy bodies, mainly composed of α-synuclein (α-syn) and ubiquitin. The selective degeneration of dopaminergic neurons suggests that dopamine itself may contribute to the neurodegenerative process in PD. Furthermore, mitochondrial dysfunction and oxidative stress constitute key pathogenic events of this disorder. Thus, in this review we give an actual perspective to classical pathways involving these two mechanisms of neurodegeneration, including the role of dopamine in sporadic and familial PD, as well as in the case of abuse of amphetamine-type drugs. Mutations in genes related to familial PD causing autosomal dominant or recessive forms may also have crucial effects on mitochondrial morphology, function, and oxidative stress. Environmental factors, such as MPTP and rotenone, have been reported to induce selective degeneration of the nigrostriatal pathways leading to α-syn-positive inclusions, possibly by inhibiting mitochondrial complex I of the respiratory chain and subsequently increasing oxidative stress. Recently, increased risk for PD was found in amphetamine users. Amphetamine drugs have effects similar to those of other environmental factors for PD, because long-term exposure to these drugs leads to dopamine depletion. Moreover, amphetamine neurotoxicity involves α-syn aggregation, mitochondrial dysfunction, and oxidative stress. Therefore, dopamine and related oxidative stress, as well as mitochondrial dysfunction, seem to be common links between PD and amphetamine neurotoxicity.

Oxidative and nitrative alpha-synuclein modifications and proteostatic stress: implications for disease mechanisms and interventions in synucleinopathies

Alpha-synuclein (ASYN) is a major constituent of the typical protein aggregates observed in several neurodegenerative diseases that are collectively referred to as synucleinopathies. A causal involvement of ASYN in the initiation and progression of neurological diseases is suggested by observations indicating that single-point (e.g., A30P, A53T) or multiplication mutations of the gene encoding for ASYN cause early onset forms of Parkinson's disease (PD). The relative regional specificity of ASYN pathology is still a riddle that cannot be simply explained by its expression pattern. Also, transgenic over-expression of ASYN in mice does not recapitulate the typical dopaminergic neuronal death observed in PD. Thus, additional factors must contribute to ASYN-related toxicity. For instance, synucleinopathies are usually associated with inflammation and elevated levels of oxidative stress in affected brain areas. In turn, these conditions favor oxidative modifications of ASYN. Among these modifications, nitration of tyrosine residues, formation of covalent ASYN dimers, as well as methionine sulfoxidations are prominent examples that are observed in post-mortem PD brain sections. Oxidative modifications can affect ASYN aggregation, as well as its binding to biological membranes. This would affect neurotransmitter recycling, mitochondrial function and dynamics (fission/fusion), ASYN's degradation within a cell and, possibly, the transfer of modified ASYN to adjacent cells. Here, we propose a model on how covalent modifications of ASYN link energy stress, altered proteostasis, and oxidative stress, three major pathogenic processes involved in PD progression. Moreover, we hypothesize that ASYN may act physiologically as a catalytically regenerated scavenger of oxidants in healthy cells, thus performing an important protective role prior to the onset of disease or during aging.

One-electron reduction of 6-hydroxydopamine quinone is essential in 6-hydroxydopamine neurotoxicity

6-Hydroxydamine has widely been used as neurotoxin in preclinical studies related on the neurodegenerative process of dopaminergic neurons in Parkinson's disease based on its ability to be neurotoxic as a consequence of free radical formation during its auto-oxidation to topaminequinone. We report that 50-µM 6-hydroxydopamine is not neurotoxic in RCSN-3 cells derived from substantia nigra incubated during 24 h contrasting with a significant sixfold increase in cell death (16 ± 2 %; P < 0.001) was observed in RCSN-3NQ7 cells expressing a siRNA against DT-diaphorase that silence the enzyme expression. To observe a significant cell death in RCSN-3 cells induced by 6-hydroxydopamine (24 ± 1 %; P < 0.01), we have to increase the concentration to 250 μm while a 45 ± 2 % cell death (P < 0.001) was observed at this concentration in RCSN-3NQ7 cells. The cell death induced by 6-hydroxydopamine in RCSN-3NQ7 cells was accompanied with a (i) significant increase in oxygen consumption (P < 0.01), (ii) depletion of reduced glutathione and (iii) a significant decrease in ATP level (P < 0.05) in comparison with RCSN-3 cells. In conclusion, our results suggest that one-electron reduction of 6-hydroxydopamine quinone seems to be the main reaction responsible for 6-hydroxydopamine neurotoxic effects in dopaminergic neurons and DT-diaphorase seems to play an important neuroprotective role by preventing one-electron reduction of topaminequinone.

The role of oxidative stress in Parkinson's disease

Oxidative stress plays an important role in the degeneration of dopaminergic neurons in Parkinson's disease (PD). Disruptions in the physiologic maintenance of the redox potential in neurons interfere with several biological processes, ultimately leading to cell death. Evidence has been developed for oxidative and nitrative damage to key cellular components in the PD substantia nigra. A number of sources and mechanisms for the generation of reactive oxygen species (ROS) are recognized including the metabolism of dopamine itself, mitochondrial dysfunction, iron, neuroinflammatory cells, calcium, and aging. PD causing gene products including DJ-1, PINK1, parkin, alpha-synuclein and LRRK2 also impact in complex ways mitochondrial function leading to exacerbation of ROS generation and susceptibility to oxidative stress. Additionally, cellular homeostatic processes including the ubiquitin-proteasome system and mitophagy are impacted by oxidative stress. It is apparent that the interplay between these various mechanisms contributes to neurodegeneration in PD as a feed forward scenario where primary insults lead to oxidative stress, which damages key cellular pathogenetic proteins that in turn cause more ROS production. Animal models of PD have yielded some insights into the molecular pathways of neuronal degeneration and highlighted previously unknown mechanisms by which oxidative stress contributes to PD. However, therapeutic attempts to target the general state of oxidative stress in clinical trials have failed to demonstrate an impact on disease progression. Recent knowledge gained about the specific mechanisms related to PD gene products that modulate ROS production and the response of neurons to stress may provide targeted new approaches towards neuroprotection.

Revisiting oxidative stress and mitochondrial dysfunction in the pathogenesis of Parkinson disease--resemblance to the effect of amphetamine drugs of abuse

Parkinson disease (PD) is a chronic and progressive neurological disease associated with a loss of dopaminergic neurons. In most cases the disease is sporadic but genetically inherited cases also exist. One of the major pathological features of PD is the presence of aggregates that localize in neuronal cytoplasm as Lewy bodies, mainly composed of α-synuclein (α-syn) and ubiquitin. The selective degeneration of dopaminergic neurons suggests that dopamine itself may contribute to the neurodegenerative process in PD. Furthermore, mitochondrial dysfunction and oxidative stress constitute key pathogenic events of this disorder. Thus, in this review we give an actual perspective to classical pathways involving these two mechanisms of neurodegeneration, including the role of dopamine in sporadic and familial PD, as well as in the case of abuse of amphetamine-type drugs. Mutations in genes related to familial PD causing autosomal dominant or recessive forms may also have crucial effects on mitochondrial morphology, function, and oxidative stress. Environmental factors, such as MPTP and rotenone, have been reported to induce selective degeneration of the nigrostriatal pathways leading to α-syn-positive inclusions, possibly by inhibiting mitochondrial complex I of the respiratory chain and subsequently increasing oxidative stress. Recently, increased risk for PD was found in amphetamine users. Amphetamine drugs have effects similar to those of other environmental factors for PD, because long-term exposure to these drugs leads to dopamine depletion. Moreover, amphetamine neurotoxicity involves α-syn aggregation, mitochondrial dysfunction, and oxidative stress. Therefore, dopamine and related oxidative stress, as well as mitochondrial dysfunction, seem to be common links between PD and amphetamine neurotoxicity.

Vitamins K interact with N-terminus α-synuclein and modulate the protein fibrillization in vitro. Exploring the interaction between quinones and α-synuclein

In the last decades, a series of compounds, including quinones and polyphenols, has been described as having anti-fibrillogenic action on α-synuclein (α-syn) whose aggregation is associated to the pathogenesis of Parkinson's disease (PD). Most of these molecules act as promiscuous anti-amyloidogenic agents, interacting with the diverse amyloidogenic proteins (mostly unfolded) through non-specific hydrophobic interactions. Herein we investigated the effect of the vitamins K (phylloquinone, menaquinone and menadione), which are 1,4-naphthoquinone (1,4-NQ) derivatives, on α-syn aggregation, comparing them with other anti-fibrillogenic molecules such as quinones, polyphenols and lipophilic vitamins. Vitamins K delayed α-syn fibrillization in substoichiometric concentrations, leading to the formation of short, sheared fibrils and amorphous aggregates, which are less prone to produce leakage of synthetic vesicles. In seeding conditions, menadione and 1,4-NQ significantly inhibited fibrils elongation, which could be explained by their ability to destabilize preformed fibrils of α-syn. Bidimensional NMR experiments indicate that a specific site at the N-terminal α-syn (Gly31/Lys32) is involved in the interaction with vitamins K, which is corroborated by previous studies suggesting that Lys is a key residue in the interaction with quinones. Together, our data suggest that 1,4-NQ, recently showed up by our group as a potential scaffold for designing new monoamine oxidase inhibitors, is also capable to modulate α-syn fibrillization in vitro.

Mechanisms of small-molecule binding to intrinsically disordered proteins

IDPs (intrinsically disordered proteins) play crucial roles in many important cellular processes such as signalling or transcription and are attractive therapeutic targets for several diseases. The considerable structural flexibility of IDPs poses a challenge for rational drug discovery approaches. Consequently, structure-based drug design efforts to date have mostly focused on inhibiting interactions of IDPs with other proteins whose structure can be solved by conventional biophysical methods. Yet, in recent years, several examples of small molecules that bind to monomeric IDPs in their disordered states have been reported, suggesting that this approach may offer new opportunities for therapeutic interventions. Further developments of this strategy will greatly benefit from an improved understanding of molecular recognition mechanisms between small molecules and IDPs. The present article summarizes findings from experimental and computational studies of the mechanisms of interaction between small molecules and three IDPs in their disordered states: c-Myc, Aβ (amyloid β-peptide) and α-synuclein.

Dopamine oxidation and autophagy

The molecular mechanisms involved in the neurodegenerative process of Parkinson's disease remain unclear. Currently, there is a general agreement that mitochondrial dysfunction, α-synuclein aggregation, oxidative stress, neuroinflammation, and impaired protein degradation are involved in the neurodegeneration of dopaminergic neurons containing neuromelanin in Parkinson's disease. Aminochrome has been proposed to play an essential role in the degeneration of dopaminergic neurons containing neuromelanin by inducing mitochondrial dysfunction, oxidative stress, the formation of neurotoxic α-synuclein protofibrils, and impaired protein degradation. Here, we discuss the relationship between the oxidation of dopamine to aminochrome, the precursor of neuromelanin, autophagy dysfunction in dopaminergic neurons containing neuromelanin, and the role of dopamine oxidation to aminochrome in autophagy dysfunction in dopaminergic neurons. Aminochrome induces the following: (i) the formation of α-synuclein protofibrils that inactivate chaperone-mediated autophagy; (ii) the formation of adducts with α- and β-tubulin, which induce the aggregation of the microtubules required for the fusion of autophagy vacuoles and lysosomes.

γ-Synuclein: seeding of α-synuclein aggregation and transmission between cells

Protein misfolding and aggregation is a ubiquitous phenomenon associated with a wide range of diseases. The synuclein family comprises three small naturally unfolded proteins implicated in neurodegenerative diseases and some forms of cancer. α-Synuclein is a soluble protein that forms toxic inclusions associated with Parkinson's disease and several other synucleinopathies. However, the triggers inducing its conversion into noxious species are elusive. Here we show that another member of the family, γ-synuclein, can be easily oxidized and form annular oligomers that accumulate in cells in the form of deposits. Importantly, oxidized γ-synuclein can initiate α-synuclein aggregation. Two amino acid residues in γ-synuclein, methionine and tyrosine located in neighboring positions (Met(38) and Tyr(39)), are most easily oxidized. Their oxidation plays a key role in the ability of γ-synuclein to aggregate and seed the aggregation of α-synuclein. γ-Synuclein secreted from neuronal cells into conditioned medium in the form of exosomes can be transmitted to glial cells and cause the aggregation of intracellular proteins. Our data suggest that post-translationally modified γ-synuclein possesses prion-like properties and may induce a cascade of events leading to synucleinopathies.

Protective effects of nicotine against aminochrome-induced toxicity in substantia nigra derived cells: implications for Parkinson's disease

Parkinson's disease is a debilitating progressive neurodegenerative disorder that results from the loss of or damage to dopaminergic cells containing neuromelanin in the substantia nigra (SN). The underlying neurodegenerative mechanism(s), however, remain elusive. Aminochrome, the precursor of neuromelanin is an endogenous substance capable of inducing selective neurotoxicity to dopaminergic neurons in SN. Nicotine, on the other hand, may offer protective effects against dopaminergic cell damage induced by various neurotoxins including MPTP and salsolinol. In this study, we sought to determine whether nicotine may also protect against aminochrome-induced toxicity in SN derived RCSN-3 cells. Exposure of RCSN-3 cells to a combination of aminochrome (50 μM) and dicoumarol (50 μM) for 48 h induced approximately 70 % cell death. Pretreatment with nicotine, dose-dependently blocked this toxicity. The effects of nicotine in turn were dose-dependently blocked by mecamylamine, a non-selective nicotinic receptor antagonist. These results suggest involvement of nicotinic receptors in protective effects of nicotine against aminochrome-induced toxicity and provide further evidence for possible therapeutic effects of nicotine or nicotinic agonists in Parkinson's disease.

Overexpression of VMAT-2 and DT-diaphorase protects substantia nigra-derived cells against aminochrome neurotoxicity

We tested the hypothesis that both VMAT-2 and DT-diaphorase are an important cellular defense against aminochrome-dependent neurotoxicity during dopamine oxidation. A cell line with VMAT-2 and DT-diaphorase over-expressed was created. The transfection of RCSN-3 cells with a bicistronic plasmid coding for VMAT-2 fused with GFP-IRES-DT-diaphorase cDNA induced a significant increase in protein expression of VMAT-2 (7-fold; P<0.001) and DT-diaphorase (9-fold; P<0.001), accompanied by a 4- and 5.5-fold significant increase in transport and enzyme activity, respectively. Studies with synaptic vesicles from rat substantia nigra revealed that VMAT-2 uptake of ³H-aminochrome 6.3 ± 0.4nmol/min/mg was similar to dopamine uptake 6.2 ± 0.3nmol/min/mg that which were dependent on ATP. Interestingly, aminochrome uptake was inhibited by 2μM lobeline but not reserpine (1 and 10μM). Incubation of cells overexpressing VMAT-2 and DT-diaphorase with 20μM aminochrome resulted in (i) a significant decrease in cell death (6-fold, P<0.001); (ii) normal ultra structure determined by transmission electron microscopy contrasting with a significant increase of autophagosome and a dramatic remodeling of the mitochondrial inner membrane in wild type cells; (iii) normal level of ATP (256 ± 11μM) contrasting with a significant decrease in wild type cells (121±11μM, P<0.001); and (iv) a significant decrease in DNA laddering (21 ± 8pixels, P<0.001) cells in comparison with wild type cells treated with 20μM aminochrome (269 ± 9). These results support our hypothesis that VMAT-2 and DT-diaphorase are an important defense system against aminochrome formed during dopamine oxidation.

Inhibition of amyloid formation

Amyloid is aggregated protein in the form of insoluble fibrils. Amyloid deposition in human tissue-amyloidosis-is associated with a number of diseases including all common dementias and type II diabetes. Considerable progress has been made to understand the mechanisms leading to amyloid formation. It is, however, not yet clear by which mechanisms amyloid and protein aggregates formed on the path to amyloid are cytotoxic. Strategies to prevent protein aggregation and amyloid formation are nevertheless, in many cases, promising and even successful. This review covers research on intervention of amyloidosis and highlights several examples of how inhibition of protein aggregation and amyloid formation has been achieved in practice. For instance, rational design can provide drugs that stabilize a native folded state of a protein, protein engineering can provide new binding proteins that sequester monomeric peptides from aggregation, small molecules and peptides can be designed to block aggregation or direct it into non-cytotoxic paths, and monoclonal antibodies have been developed for therapies towards neurodegenerative diseases based on inhibition of amyloid formation and clearance.

α-Synuclein aggregation and modulating factors

Aggregated a-synuclein is the major component of inclusions in Parkinson's disease and other synucleinopathy brains indicating that a-syn aggregation is associated with the pathogenesis of neurodegenerative disorders. Although the mechanisms underlying a-syn aggregation and toxicity are not fully elucidated, it is clear that a-syn undergoes post-translational modifications and interacts with numerous proteins and other macromolecules, metals, hormones, neurotransmitters, drugs and poisons that can all modulate its aggregation propensity. The current and most recent findings regarding the factors modulating a-syn aggregation process are discussed in detail.

Molecular insights into Parkinson's disease

Mutations in SNCA, PINK1, parkin, and DJ-1 are associated with autosomal-dominant or autosomal-recessive forms of Parkinson's disease (PD), the second most common neurodegenerative disorder. Studies on the structural and functional properties of the corresponding gene products have provided significant insights into the molecular underpinnings of familial PD and the much more common sporadic forms of the disease. Here, we review recent advances in our understanding of four PD-related gene products: α-synuclein, parkin, PINK1, and DJ-1. In Part 1, we review new insights into the role of α-synuclein in PD. In Part 2, we summarize the latest developments in understanding the role of mitochondrial dysfunction in PD, emphasizing the role of the PINK1/parkin pathway in regulating mitochondrial dynamics and mitophagy. The role of DJ-1 is also discussed. In Part 3, we point out converging pathways and future directions.

α-Synuclein synaptic pathology and its implications in the development of novel therapeutic approaches to cure Parkinson's disease

Parkinson's disease (PD) is characterized by a progressive loss of dopamine (DA) neurons of the nigrostriatal system and by the presence of Lewy bodies (LB), proteinaceous inclusions mainly composed of filamentous α-synuclein aggregates. Alpha-synuclein is a natively unfolded protein which plays a central role in the control of dopaminergic neuronal functions and which is thought to be critically implicated in PD pathophysiology. Indeed, besides the fact that α-synuclein is the main protein component of LB, genetic studies showed that mutations and multiplications of the α-synuclein gene are responsible for the onset of familial forms of PD. A large body of evidence indicates that α-synuclein pathology at dopaminergic synapses may underlie the onset of neuronal cell dysfunction and degeneration in the PD brain. Thus, since the available therapeutic approaches to cure this disease are still limited, we hypothesized that the analysis of the α-synuclein synaptic proteome/lipidome may represent a tool to identify novel potential therapeutic targets to cure this disorder. We thus performed a critical review of studies describing α-synuclein pathophysiology at synaptic sites in experimental models of PD and in this paper we outline the most relevant findings regarding the specific modulatory effects exerted by α-synuclein in the control of synaptic functions in physiological and pathological conditions. The conclusions of these studies allow to single out novel potential therapeutic targets among the α-synuclein synaptic partners. These targets may be considered for the development of new pharmacological and gene-based strategies to cure PD.

A30P α-synuclein impairs dopaminergic fiber regeneration and interacts with L-DOPA replacement in MPTP-treated mice

Parkinson's disease (PD) is the most common neurodegenerative movement disorder and is characterized by the loss of dopaminergic neurons from the substantia nigra pars compacta (SNpc). α-synuclein (αsyn) has been linked to the pathophysiology of PD, because of its mutations causing familial PD and its accumulation in brains of patients with familial and sporadic PD. Dopamine (DA) replacement is the most effective therapy for ameliorating the motor symptoms of PD; however, it remains controversial whether DA-replacement boosts regeneration in the dopaminergic system or accelerates disease progression and enhances neuronal loss. Here, we studied the effect of chronic L-DOPA treatment on dopaminergic neurons in wild-type (WT) and A30P αsyn transgenic mice after MPTP treatment. Acute MPTP intoxication induced degeneration of dopaminergic neurons in both WT and A30P αsyn transgenic mice. A strong regeneration of dopaminergic fibers at 90 days after MPTP was observed in WT mice. In contrast, regeneration was less pronounced in A30P αsyn mice. Chronic L-DOPA treatment after MPTP intoxication did not only reduce the regeneration of nigrostriatal fibers but also led to an increased apoptotic gene-expression profile in the SNpc and to a decline of TH-positive neurons in A30P αsyn. Our findings reveal that the presence of A30P αsyn inhibits the regeneration of nigrostriatal dopaminergic fibers, and that L-DOPA treatment might interact with the pathogenesis in PD.

Multiple system atrophy: a clinical and neuropathological perspective

Multiple system atrophy (MSA) is a neurodegenerative disease involving motor abnormalities that include akinesia, rigidity and postural instability. While improved diagnostic criteria have aided the accurate diagnosis of MSA, our understanding of the neuropathological aspects underlying MSA was bolstered by the identification of α-synuclein (α-syn) as the primary constituent of the abnormal protein aggregates observed in the brains of MSA patients. The generation of transgenic animal models of MSA coupled with an increasing understanding of the biochemical structure and function of α-syn has highlighted a number of key pathological pathways thought to underlie the neurodegeneration observed in MSA. This review summarizes key findings in the field, discusses current areas of debate, and describes current experimental approaches towards disease-modifying therapies.

E46K human alpha-synuclein transgenic mice develop Lewy-like and tau pathology associated with age-dependent, detrimental motor impairment

Synucleinopathies are a group of neurodegenerative disorders associated with the formation of aberrant amyloid inclusions composed of the normally soluble presynaptic protein α-synuclein (α-syn). Parkinson disease is the most well known of these disorders because it bears α-syn pathological inclusions known as Lewy bodies (LBs). Mutations in the gene for α-syn, including the E46K missense mutation, are sufficient to cause Parkinson disease as well as other synucleinopathies like dementia with LBs. Herein, we describe transgenic mice expressing E46K human α-syn in CNS neurons that develop detrimental age-dependent motor impairments. These animals accumulate age-dependent intracytoplasmic neuronal α-syn inclusions that parallel disease and recapitulate the biochemical, histological, and morphological properties of LBs. Surprisingly, the morphology of α-syn inclusions in E46K human α-syn transgenic mice more closely resemble LBs than the previously described transgenic mice (line M83) that express neuronal A53T human α-syn. E46K human α-syn mice also develop abundant neuronal tau inclusions that resemble neurofibrillary tangles. Subsequent studies on the ability of E46K α-syn to induce tau inclusions in cellular models suggest that both direct and indirect mechanisms of protein aggregation are probably involved in the formation of the tau inclusions observed here in vivo. Re-evaluation of presymptomatic transgenic mice expressing A53T human α-syn reveals that the formation of α-syn inclusions in mice must be synchronized; however, inclusion formation is diffuse within affected areas of the neuroaxis such that there was no clustering of inclusions. Collectively, these findings provide insights in the mechanisms of formation of these aberrant proteinaceous inclusions and support the notion that α-syn aggregates are involved in the pathogenesis of human diseases.

Dopamine and paraquat enhance α-synuclein-induced alterations in membrane conductance

We have previously demonstrated that α-synuclein overexpression increases the membrane conductance of dopaminergic-like cells. Although α-synuclein is thought to play a central role in the pathogenesis of several neurodegenerative diseases including Parkinson's disease, multiple system atrophy, and diffuse Lewy body disease, the mechanism of action is not completely understood. In this study, we sought to determine whether multiple factors act together with α-synuclein to engender cell vulnerability through an augmentation of membrane conductance. In this article, we employed a cell model that mimics dopaminergic neurons coupled with α-synuclein overexpression and oxidative stressors. We demonstrate an enhancement of α-synuclein-induced toxicity in the presence of combined treatment with dopamine and paraquat, two molecules known to incite oxidative stress. In addition, we show that combined dopamine and paraquat treatment increases the expression of heme oxygenase-1, an antioxidant response protein. Finally, we demonstrate for the first time that combined treatment of dopaminergic cells with paraquat and dopamine enhances α-synuclein-induced leak channel properties resulting in increased membrane conductance. Importantly, these increases are most robust when both paraquat and dopamine are present suggesting the need for multiple oxidative insults to augment α-synuclein-induced disruption of membrane integrity.

Autophagy protects against aminochrome-induced cell death in substantia nigra-derived cell line

Aminochrome, the precursor of neuromelanin, has been proposed to be involved in the neurodegeneration neuromelanin-containing dopaminergic neurons in Parkinson's disease. We aimed to study the mechanism of aminochrome-dependent cell death in a cell line derived from rat substantia nigra. We found that aminochrome (50μM), in the presence of NAD(P)H-quinone oxidoreductase, EC 1.6.99.2 (DT)-diaphorase inhibitor dicoumarol (DIC) (100μM), induces significant cell death (62 ± 3%; p < 0.01), increase in caspase-3 activation (p < 0.001), release of cytochrome C, disruption of mitochondrial membrane potential (p < 0.01), damage of mitochondrial DNA, damage of mitochondria determined with transmission electron microscopy, a dramatic morphological change characterized as cell shrinkage, and significant increase in number of autophagic vacuoles. To determine the role of autophagy on aminochrome-induced cell death, we incubated the cells in the presence of vinblastine and rapamycin. Interestingly, 10μM vinblastine induces a 5.9-fold (p < 0.001) and twofold (p < 0.01) significant increase in cell death when the cells were incubated with 30μM aminochrome in the absence and presence of DIC, respectively, whereas 10μM rapamycin preincubated 24 h before addition of 50μM aminochrome in the absence and the presence of 100μM DIC induces a significant decrease (p < 0.001) in cell death. In conclusion, autophagy seems to be an important protective mechanism against two different aminochrome-induced cell deaths that initially showed apoptotic features. The cell death induced by aminochrome when DT-diaphorase is inhibited requires activation of mitochondrial pathway, whereas the cell death induced by aminochrome alone requires inhibition of autophagy-dependent degrading of damaged organelles and recycling through lysosomes.

Modulation of α-synuclein aggregation by dopamine in the presence of MPTP and its metabolite

The neurotransmitter dopamine has been shown to inhibit fibrillation of α-synuclein by promoting the formation of nonamyloidogenic oligomers. Fibrillation of α-synuclein is accelerated in the presence of pesticides and the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The aim of this study was to determine whether dopamine continues to have an adverse effect on the fibrillation of α-synuclein in the presence of MPTP and its metabolite 1-methyl-4-phenylpyridinum ion (MPP(+) ). We also attempted to answer the ambiguous question of whether conversion of MPTP to MPP(+) is required for the fibrillation of α-synuclein. For this, α-synuclein was incubated in the presence of MPTP and MPP(+) along with dopamine. The fibrillation of α-synuclein was monitored by Thioflavin T fluorescence and immunoblotting. The morphology of the aggregates formed was observed using scanning electron microscopy. The concentrations of the neurotoxin and its metabolite were estimated by reverse phase HPLC. We found definitive evidence that the conversion of MPTP to MPP(+) is not required for aggregation of α-synuclein. MPP(+) was found to accelerate the rate of α-synuclein aggregation even in the absence of components of mitochondrial complex I. In contrast to the effect of dopamine on the aggregation of α-synuclein alone, in the presence of MPTP or MPP(+) , the aggregates formed are Thioflavin T-positive and amyloidogenic. Thus, the effect of dopamine on the nature of aggregates formed in case of α-synuclein alone and in the presence of MPTP/MPP(+) is different.

The association between Parkinson's disease and melanoma

Parkinson's disease (PD) is a neurodegenerative disorder characterized by a loss of melanin-positive, dopaminergic neurons in the substantia nigra. Although there is convincing epidemiologic evidence of a negative association between PD and most cancers, a notable exception to this is that melanoma, a malignant tumor of melanin-producing cells in skin, occurs with higher-than-expected frequency among subjects with PD and that melanoma patients are more likely to have PD. A clear biological explanation for this epidemiological observation is lacking. Here, we present a comprehensive review of published literature exploring the association between PD and melanoma. On the basis of published findings, we conclude that (i) changes in pigmentation including melanin synthesis and/or melanin synthesis enzymes, such as tyrosinase and tyrosine hydroxylase, play important roles in altered vulnerability for both PD and melanoma; (ii) changes of PD-related genes such as Parkin, LRRK2 and α-synuclein may increase the risk of melanoma; (iii) changes in some low-penetrance genes such as cytochrome p450 debrisoquine hydroxylase locus, glutathione S-transferase M1 and vitamin D receptor could increase the risk for both PD and melanoma and (iv) impaired autophagy in both PD and melanoma could also explain the association between PD and melanoma. Future studies are required to address whether altered pigmentation, PD- or melanoma-related gene changes and/or changes in autophagy function induce oncogenesis or apoptosis. From a clinical point of view, early diagnosis of melanoma in PD patients is critical and can be enhanced by periodic dermatological surveillance, including skin biopsies.

The neurotransmitter serotonin interrupts α-synuclein amyloid maturation

Indolic derivatives can affect fibril growth of amyloid forming proteins. The neurotransmitter serotonin (5-HT) is of particular interest, as it is an endogenous molecule with a possible link to neuropsychiatric symptoms of Parkinson disease. A key pathomolecular mechanism of Parkinson disease is the misfolding and aggregation of the intrinsically unstructured protein α-synuclein. We performed a biophysical study to investigate an influence between these two molecules. In an isolated in vitro system, 5-HT interfered with α-synuclein amyloid fiber maturation, resulting in the formation of partially structured, SDS-resistant intermediate aggregates. The C-terminal region of α-synuclein was essential for this interaction, which was driven mainly by electrostatic forces. 5-HT did not bind directly to monomeric α-synuclein molecules and we propose a model where 5-HT interacts with early intermediates of α-synuclein amyloidogenesis, which disfavors their further conversion into amyloid fibrils.

VMAT2-Deficient Mice Display Nigral and Extranigral Pathology and Motor and Nonmotor Symptoms of Parkinson's Disease

Dopamine is transported into synaptic vesicles by the vesicular monoamine transporter (VMAT2; SLC18A2). Disruption of dopamine storage has been hypothesized to damage the dopamine neurons that are lost in Parkinson's disease. By disrupting vesicular storage of dopamine and other monoamines, we have created a progressive mouse model of PD that exhibits catecholamine neuron loss in the substantia nigra pars compacta and locus coeruleus and motor and nonmotor symptoms. With a 95% reduction in VMAT2 expression, VMAT2-deficient animals have decreased motor function, progressive deficits in olfactory discrimination, shorter latency to behavioral signs of sleep, delayed gastric emptying, anxiety-like behaviors at younger ages, and a progressive depressive-like phenotype. Pathologically, the VMAT2-deficient mice display progressive neurodegeneration in the substantia nigra (SNpc), locus coeruleus (LC), and dorsal raphe (DR) coupled with α-synuclein accumulation. Taken together, these studies demonstrate that reduced vesicular storage of monoamines and the resulting disruption of the cytosolic environment may play a role in the pathogenesis of parkinsonian symptoms and neurodegeneration. The multisystem nature of the VMAT2-deficient mice may be useful in developing therapeutic strategies that go beyond the dopamine system.

The anti-Parkinsonian drug selegiline delays the nucleation phase of α-synuclein aggregation leading to the formation of nontoxic species

Parkinson's disease (PD) is a movement disorder characterized by the loss of dopaminergic neurons in the substantia nigra and the formation of intraneuronal inclusions called Lewy bodies, which are composed mainly of α-synuclein (α-syn). Selegiline (Sel) is a noncompetitive monoamino oxidase B inhibitor that has neuroprotective effects and has been administered to PD patients as monotherapy or in combination with l-dopa. Besides its known effect of increasing the level of dopamine (DA) by monoamino oxidase B inhibition, Sel induces other effects that contribute to its action against PD. We evaluated the effects of Sel on the in vitro aggregation of A30P and wild-type α-syn. Sel delays fibril formation by extending the lag phase of aggregation. In the presence of Sel, electron microscopy reveals amorphous heterogeneous aggregates, including large annular species, which are innocuous to a primary culture enriched in dopaminergic neurons, while their age-matched counterparts are toxic. The inhibitory effect displayed by Sel is abolished when seeds (small fibril pieces) are added to the aggregation reaction, reinforcing the hypothesis that Sel interferes with early nuclei formation and, to a lesser extent, with fibril elongation. NMR experiments indicate that Sel does not interact with monomeric α-syn. Interestingly, when added in combination with DA (which favors the formation of toxic protofibrils), Sel overrides the inhibitory effect of DA and favors fibrillation. Additionally, Sel blocks the formation of smaller toxic aggregates by perturbing DA-dependent fibril disaggregation. These effects might be beneficial for PD patients, since the sequestration of protofibrils into fibrils or the inhibition of fibril dissociation could alleviate the toxic effects of protofibrils on dopaminergic neurons. In nondopaminergic neurons, Sel might slow the fibrillation, giving rise to the formation of large nontoxic aggregates.

New insights into the role of mitochondrial dysfunction and protein aggregation in Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative movement disorder that affects increasing number of elderly in the world population. The disease is caused by a selective degeneration of dopaminergic neurons in the substantia nigra pars compacta with the molecular mechanism underlying this neurodegeneration still not fully understood. However, various studies have shown that mitochondrial dysfunction and abnormal protein aggregation are two of the major contributors for PD. In fact this notion has been supported by recent studies on genes that are linked to familial PD (FPD). For instance, FPD linked gene products such as PINK1 and parkin have been shown to play critical roles in the quality control of mitochondria, whereas α-synuclein has been found to be the major protein aggregates accumulated in PD patients. These findings suggest that further understanding of how dysfunction of these pathways in PD will help develop new approaches for the treatment of this neurodegenerative disorder.

The Drosophila vesicular monoamine transporter reduces pesticide-induced loss of dopaminergic neurons

Dopamine is cytotoxic and may play a role in the development of Parkinson's disease. However, its interaction with environmental risk factors such as pesticides remains poorly understood. The vesicular monoamine transporter (VMAT) regulates intracellular dopamine content, and we have tested the neuroprotective effects of VMAT in vivo using the model organism Drosophila melanogaster. We find that Drosophila VMAT (dVMAT) mutants contain fewer dopaminergic neurons than wild type, consistent with a developmental effect, and that dopaminergic cell loss in the mutant is exacerbated by the pesticides rotenone and paraquat. Overexpression of DVMAT protein does not increase the survival of animals exposed to rotenone, but blocks the loss of dopaminergic neurons caused by this pesticide. These results are the first to demonstrate an interaction between a VMAT and pesticides in vivo, and provide an important model to investigate the mechanisms by which pesticides and cellular DA may interact to kill dopaminergic cells.

[Search for antiamyloidogenic compounds based on a nucleation-dependent polymerization model]

We have proposed that a nucleation-dependent polymerization model could explain the general mechanisms of amyloid fibril formation in vitro. Based on this model, we systematically demonstrated that several classes of organic compounds (e.g., wine-related polyphenols, non-steroidal anti-inflammatory drugs) not only inhibit the formation of Abeta amyloid fibrils from Abeta and their extension, but also destabilize Abeta amyloid fibrils dose-dependently in vitro. We found significant positive correlations of the effective concentrations (EC(50)) of these compounds ranging from 10 nM to 10 microM, for the formation and destabilization of Abeta amyloid fibrils. We next investigated the anti-amyloidogenic effects of five flavonoids on Abeta amyloid fibrils in vitro. Oxidized flavonoids generally inhibited fibril formation significantly more potently than fresh compounds. By surface plasmon resonance (SPR) analysis, distinct association and dissociation reactions of myricetin (Myr) to Abeta amyloid fibrils were observed, in contrast to the very weak binding to the Abeta monomer. A significant decrease in the rate of fibril extension was observed when>0.5 microM of Myr was injected into the SPR experimental system. These findings suggest that flavonoids, especially Myr exert an anti-amyloidogenic effect in vitro by preferentially and reversibly binding to the amyloid fibril structure of fibrils, rather than to Abeta monomers. This working model should prove useful not only for the rational development of preventive and therapeutics for Alzheimer's disease and other human amyloidosis, but also for understanding the basic mode of action of amyloid imaging compounds.

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.

The structure of dopamine induced alpha-synuclein oligomers

Inclusions of aggregated alpha-synuclein (alpha-syn) in dopaminergic neurons are a characteristic histological marker of Parkinson's disease (PD). In vitro, alpha-syn in the presence of dopamine (DA) at physiological pH forms SDS-resistant non-amyloidogenic oligomers. We used a combination of biophysical techniques, including sedimentation velocity analysis, small angle X-ray scattering (SAXS) and circular dichroism spectroscopy to study the characteristics of alpha-syn oligomers formed in the presence of DA. Our SAXS data show that the trimers formed by the action of DA on alpha-syn consist of overlapping worm-like monomers, with no end-to-end associations. This lack of structure contrasts with the well-established, extensive beta-sheet structure of the amyloid fibril form of the protein and its pre-fibrillar oligomers. We propose on the basis of these and earlier data that oxidation of the four methionine residues at the C- and N-terminal ends of alpha-syn molecules prevents their end-to-end association and stabilises oligomers formed by cross linking with DA-quinone/DA-melanin, which are formed as a result of the redox process, thus inhibiting formation of the beta-sheet structure found in other pre-fibrillar forms of alpha-syn.

Tyrosinase-expressing neuronal cell line as in vitro model of Parkinson's disease

Oxidized metabolites of dopamine known as dopamine quinone derivatives are thought to play a pivotal role in the degeneration of nigrostriatal dopaminergic neurons in Parkinson’s disease. Although such quinone derivatives are usually produced via the autoxidation of catecholamines, tyrosinase, which is a key enzyme in melanin biosynthesis via the production of DOPA and subsequent molecules, can potentially accelerate the induction of catecholamine quinone derivatives by its oxidase activity. We have developed neuronal cell lines in which the expression of human tyrosinase was inducible. Overexpression of tyrosinase resulted in increased intracellular dopamine content in association with the formation of melanin pigments in neuronal somata, which eventually causes apoptotic cell death. This cellular model will provide a useful tool for detailed analyses of the neurotoxicity of oxidized catechol metabolites.

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.

Modulation of alpha-synuclein aggregation by dopamine analogs

The action of dopamine on the aggregation of the unstructured alpha-synuclein (α-syn) protein may be linked to the pathogenesis of Parkinson's disease. Dopamine and its oxidation derivatives may inhibit α-syn aggregation by non-covalent binding. Exploiting this fact, we applied an integrated computational and experimental approach to find alternative ligands that might modulate the fibrillization of α-syn. Ligands structurally and electrostatically similar to dopamine were screened from an established library. Five analogs were selected for in vitro experimentation from the similarity ranked list of analogs. Molecular dynamics simulations showed they were, like dopamine, binding non-covalently to α-syn and, although much weaker than dopamine, they shared some of its binding properties. In vitro fibrillization assays were performed on these five dopamine analogs. Consistent with our predictions, analyses by atomic force and transmission electron microscopy revealed that all of the selected ligands affected the aggregation process, albeit to a varying and lesser extent than dopamine, used as the control ligand. The in silico/in vitro approach presented here emerges as a possible strategy for identifying ligands interfering with such a complex process as the fibrillization of an unstructured protein.

Entacapone and tolcapone, two catechol O-methyltransferase inhibitors, block fibril formation of alpha-synuclein and beta-amyloid and protect against amyloid-induced toxicity

Parkinson disease (PD) is the second most common neurodegenerative disorder after Alzheimer disease (AD). There is considerable consensus that the increased production and/or aggregation of alpha-synuclein (alpha-syn) plays a central role in the pathogenesis of PD and related synucleinopathies. Current therapeutic strategies for treating PD offer mainly transient symptomatic relief and aim at the restitution of dopamine levels to counterbalance the loss of dopaminergic neurons. Therefore, the identification and development of drug-like molecules that block alpha-synuclein aggregation and prevent the loss of dopaminergic neurons are desperately needed to treat or slow the progression of PD. Here, we show that entacapone and tolcapone are potent inhibitors of alpha-syn and beta-amyloid (Abeta) oligomerization and fibrillogenesis, and they also protect against extracellular toxicity induced by the aggregation of both proteins. Comparison of the anti-aggregation properties of entacapone and tolcapone with the effect of five other catechol-containing compounds, dopamine, pyrogallol, gallic acid, caffeic acid, and quercetin on the oligomerization and fibrillization of alpha-syn and Abeta, demonstrate that the catechol moiety is essential for the anti-amyloidogenic activity. Our findings present the first characterization of the anti-amyloidogenic properties of tolcapone and entacapone against both alpha-synuclein and Abeta42 and highlight the potential of this class of nitro-catechol compounds as anti-amyloidogenic agents. Their inhibitory properties, mode of action, and structural properties suggest that they constitute promising lead compounds for further optimization.

Aminochrome induces disruption of actin, alpha-, and beta-tubulin cytoskeleton networks in substantia-nigra-derived cell line

In previous studies, we observed that cells treated with aminochrome obtained by oxidizing dopamine with oxidizing agents dramatically changed cell morphology, thus posing the question if such morphological changes were dependent on aminochrome or the oxidizing agents used to produce aminochrome. Therefore, to answer this question, we have now purified aminochrome on a CM-Sepharose 50-100 column and, using NMR studies, we have confirmed that the resulting aminochrome was pure and that it retained its structure. Fluorescence microscopy with calcein-AM and transmission electron microscopy showed that RCSN-3 cells presented an elongated shape that did not change when the cells were incubated with 50 muM aminochrome or 100 muM dicoumarol, an inhibitor of DT-diaphorase. However, the cell were reduced in size and the elongated shape become spherical when the cells where incubated with 50 muM aminochrome in the presence of 100 muM dicoumarol. Under these conditions, actin, alpha-, and beta-tubulin cytoskeleton filament networks became condensed around the cell membrane. Actin aggregates were also observed in cells processes that connected the cells in culture. These results suggest that aminochrome one-electron metabolism induces the disruption of the normal morphology of actin, alpha-, and beta-tubulin in the cytoskeleton, and that DT-diaphorase prevents these effects.