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

The aggregation and fibrillation of alpha-synuclein

alpha-Synuclein is a small (14 kDa), abundant, intrinsically disordered presynaptic protein, whose aggregation is believed to be a critical step in Parkinson's disease (PD). The kinetics of alpha-synuclein fibrillation are consistent with a nucleation-dependent mechanism, in which the critical early stage of the structural transformation involves a partially folded intermediate. Although the basis for the toxic effects of aggregated alpha-synuclein are unknown, it has been proposed that transient oligomers are responsible, possibly by forming pores in membranes. In this Account, I discuss our investigations into the molecular basis for alpha-synuclein aggregation/fibrillation, including factors that either accelerate or inhibit fibrillation, effects of molecular crowding, oxidation, point mutations, and lipid membranes, as well as the variety of conformational and oligomeric states that alpha-synuclein can adopt. It is apparent that neuronal cells must have a very fine balance of factors that control the levels and potential aggregation of alpha-synuclein.

Alpha-synuclein structure, posttranslational modification and alternative splicing as aggregation enhancers

Alpha-synuclein aggregation is thought to be a key event in the pathogenesis of synucleinopathies. Although different alpha-synuclein alterations and modifications have been proposed to be responsible for early aggregation steps, the mechanisms underlying these events remain unclarified. Alpha-synuclein is a small protein localized to synaptic terminals and its intrinsic structure has been claimed to be an important factor for self-oligomerization and self-aggregation. Alpha-synuclein expression studies in cell cultures have demonstrated that posttranslational modifications, such as phosphorylation, oxidation, and sumoylation, are primarily involved in alpha-synuclein aggregation. Furthermore, in the last few years accumulating evidence has pointed to alternative splicing as a crucial mechanism in the development of neurodegenerative disorders. At least three different alpha-synuclein isoforms have been described as products of alternative splicing. Two of these isoforms (alpha-synuclein 112 and alpha-synuclein 126) are shorter proteins with probably altered functions and aggregation propensity. The present review attempts to summarize the data so far available on alpha-synuclein structure, posttranslational modifications, and alternative splicing as possible enhancers of aggregation.

Small molecule inhibitors of alpha-synuclein filament assembly

Alpha-synuclein is the major component of the filamentous inclusions that constitute defining characteristics of Parkinson's disease and other alpha-synucleinopathies. Here we have tested 79 compounds belonging to 12 different chemical classes for their ability to inhibit the assembly of alpha-synuclein into filaments in vitro. Several polyphenols, phenothiazines, porphyrins, polyene macrolides, and Congo red and its derivatives, BSB and FSB, inhibited alpha-synuclein filament assembly with IC(50) values in the low micromolar range. Many compounds that inhibited alpha-synuclein assembly were also found to inhibit the formation of Abeta and tau filaments. Biochemical analysis revealed the formation of soluble oligomeric alpha-synuclein in the presence of inhibitory compounds, suggesting that this may be the mechanism by which filament formation is inhibited. Unlike alpha-synuclein filaments and protofibrils, these soluble oligomeric species did not reduce the viability of SH-SY5Y cells. These findings suggest that the soluble oligomers formed in the presence of inhibitory compounds may not be toxic to nerve cells and that these compounds may therefore have therapeutic potential for alpha-synucleinopathies and other brain amyloidoses.

Interaction of alpha-synuclein and dopamine metabolites in the pathogenesis of Parkinson's disease: a case for the selective vulnerability of the substantia nigra

Parkinson's disease (PD) is the most common movement disorder. Major disease symptoms are due to the loss of dopaminergic (DA) neurons in substantia nigra (SN). The pathologic hallmark of PD is Lewy bodies (LBs) in the SN and the major protein in LBs is alpha-synuclein (AS). A plethora of evidence points towards the culpability of AS in the pathogenesis of PD including: (1) linkage of AS mutations to familial forms of PD, (2) triplication of the AS locus causing PD, and (3) overexpression of AS in transgenic mice and Drosophila leads to PD-like phenotypes. Studies of purified AS have revealed its ability to interact with diverse molecules including monoamines. Monoamine metabolism is associated with oxidative stress conditions that may contribute to DA-AS interactions promoting aggregation and neuronal damage. However, in order to explain the selective vulnerability of DA neurons there needs to be a link between DA metabolism and AS aggregation. Since only the DA neurons contain significant amounts of DA, this has been hypothesized to account for the selective vulnerability of SN neurons. However, DA itself may not be toxic at physiologic relevant doses, so it is probable that other DA metabolites may play a major role in AS aggregation. In this review, we discuss the role of the DA metabolite 3,4-dihydroxyphenylacetaldehyde to provide a plausible link between DA production and metabolism, AS aggregation and the pathogenesis of PD.

Intranigral Dopamine Toxicity and α-Synuclein Response in Rats

There is increasing evidence that, in addition to its function as the main neurotransmitter in the nigrostriatal pathway, dopamine (DA) may be neurotoxic in certain conditions. In this study, the toxicity of DA was assessed by direct injection into the substantia nigra of anaesthetised rats, and its effects were compared with those of 6-hydroxydopamine. Brains were removed 1, 2 and 3 weeks after the lesion for histological or neurochemical analysis. DA caused a significant loss of 35% of tyrosine hydroxylase-positive neurons in the pars compacta of substantia nigra and a 40% reduction of striatal DA content. Cells with signs compatible with both apoptosis and autophagy were observed. GADD153, a parameter of endoplasmic reticulum stress, was strongly induced by 6-hydroxydopamine but not by DA. DA increased the alpha-synuclein content 1 week after the lesion (but not at the later times analyzed) in tyrosine hydroxylase-positive and in non-dopaminergic fibers of pars reticulata. The alpha-synuclein increase may be a physiological temporal response to DA accumulation and/or to cell damage, but the simultaneous presence of alpha-synuclein and DA in the cell cytoplasm at concentration higher than normal is not exempt from risk. In fact, their incubation in a free cell system gives a stable dimerized form of alpha-synuclein that has been described as the critical rate-limiting step for its abnormal fibrillation.

A potential role for cyclized quinones derived from dopamine, DOPA, and 3,4-dihydroxyphenylacetic acid in proteasomal inhibition

We examined the ability of oxidation products of dopamine, DOPA, and 3,4-dihydroxyphenylacetic acid (DOPAC) to inhibit proteasomal activity. Dopamine, DOPA, and DOPAC underwent tyrosinase-catalyzed oxidation to generate aminochrome, dopachrome, and furanoquinone, respectively. In these studies, the oxidation of dopamine by tyrosinase generated product(s) that inhibited the proteasome, and proteasomal inhibition correlated with the presence of the UV-visible spectrum of aminochrome. The addition of superoxide dismutase and catalase did not prevent proteasomal inhibition. The addition of NADH and the quinone reductase NAD(P)H:quinone oxidoreductase 1 (NQO1) protected against aminochrome-induced proteasome inhibition. Although NQO1 protected against dopamine-induced proteasomal inhibition, the metabolism of aminochrome by NQO1 led to oxygen uptake because of the generation of a redox-labile cyclized hydroquinone, further demonstrating the lack of involvement of oxygen radicals in proteasomal inhibition. DOPA underwent tyrosinase-catalyzed oxidation to form dopachrome, and similar to aminochrome, proteasomal inhibition correlated with the presence of a dopachrome UV-visible spectrum. The inclusion of NQO1 did not protect against proteasomal inhibition induced by dopachrome. Oxidation of DOPAC by tyrosinase generated furanoquinone, which was a poor proteasome inhibitor. These studies demonstrate that oxidation products, including cyclized quinones derived from dopamine and related compounds, rather than oxygen radicals have the ability to inhibit the proteasome. They also suggest an important protective role for NQO1 in protecting against dopamine-induced proteasomal inhibition. The ability of endogenous intermediates formed during dopaminergic metabolism to cause proteasomal inhibition provides a potential basis for the selectivity of dopaminergic neuron damage in Parkinson's disease.

Pathological changes in dopaminergic nerve cells of the substantia nigra and olfactory bulb in mice transgenic for truncated human alpha-synuclein(1-120): implications for Lewy body disorders

Dysfunction of the 140 aa protein alpha-synuclein plays a central role in Lewy body disorders, including Parkinson's disease, as well as in multiple system atrophy. Here, we show that the expression of truncated human alpha-synuclein(1-120), driven by the rat tyrosine hydroxylase promoter on a mouse alpha-synuclein null background, leads to the formation of pathological inclusions in the substantia nigra and olfactory bulb and to a reduction in striatal dopamine levels. At the behavioral level, the transgenic mice showed a progressive reduction in spontaneous locomotion and an increased response to amphetamine. These findings suggest that the C-terminal of alpha-synuclein is an important regulator of aggregation in vivo and will help to understand the mechanisms underlying the pathogenesis of Lewy body disorders and multiple system atrophy.

6-Hydroxydopamine but Not 1-Methyl-4-phenylpyridinium Abolishes α-Synuclein Anti-apoptotic Phenotype by Inhibiting Its Proteasomal Degradation and by Promoting Its Aggregation

We established previously that alpha-synuclein displayed a protective anti-apoptotic phenotype in neurons, mainly by down-regulating p53-dependent caspase-3 activation (Alves da Costa, C., Ancolio, K., and Checler, F. (2000) J. Biol. Chem. 275, 24065-24069; Alves da Costa, C., Paitel, E., Vincent, B., and Checler, F. (2002) J. Biol. Chem. 277, 50980-50984). This function was abolished by Parkinson disease-linked pathogenic mutations and by the dopaminergic toxin, 6-hydroxydopamine (6OH-DOPA) (Alves da Costa, C., Paitel, E., Vincent, B., and Checler, F. (2002) J. Biol. Chem. 277, 50980-50984). However, the mechanisms by which 6OH-DOPA interfered with alpha-synuclein function remained unclear. Here we showed that 6OH-DOPA prevents alpha-synuclein-mediated anti-apoptotic function by altering its degradation. Thus, 6OH-DOPA treatment of TSM1 neurons and SH-SY5Y neuroblastoma cells enhances endogenous alpha-synuclein-like immunoreactivity and inhibits the catabolism of endogenous and recombinant alpha-synucleins by purified 20 S proteasome. Furthermore, we demonstrated that 6OH-DOPA directly inhibits endogenous proteasomal activity in TSM1 and SH-SY5Y cells and also blocks purified proteasome activity in vitro. This inhibitory effect can be prevented by the anti-oxidant phenyl-N-butylnitrone. We also established that 6OH-DOPA triggers the aggregation of recombinant alpha-synuclein in vitro. Therefore, we conclude that 6OH-DOPA abolishes alpha-synuclein anti-apoptotic phenotype by inhibiting its proteasomal degradation, thereby increasing its intracellular concentration and potential propensity to aggregation, the latter phenomenon being directly exacerbated by 6OH-DOPA itself. Interestingly, 1-methyl-4-phenylpyridinium (MPP(+)), another toxin inducer of Parkinson disease-like pathology, does not affect alpha-synuclein protective function and fails to trigger aggregation of recombinant alpha-synuclein. Furthermore, MPP(+) does not alter cellular proteasomal activity, and only high concentrations of the toxin affect purified 20 S proteasome by a mechanism that remains insensitive to phenyl-N-butylnitrone. The drastically distinct effects of 6OH-DOPA and MPP(+) on alpha-synuclein function are discussed with respect to Parkinson disease pathology and animal models mimicking this pathology.

Interaction with phospholipids modulates alpha-synuclein nitration and lipid-protein adduct formation

Intracellular aggregates of alpha-syn (alpha-synuclein) represent pathoanatomical hallmarks of neurodegenerative disorders (synucleinopathies). The molecular mechanisms underlying alpha-syn aggregation into filamentous inclusions may involve oxidation and nitration of the protein. Whereas the effects of oxidants and nitrating species on soluble alpha-syn have been studied in detail, the effect of these reactive species on alpha-syn associated with lipids is still unknown. In the present paper, we report that alpha-syn bound to small unilamellar liposomes composed of phosphatidylcholine/phosphatidic acid is resistant to oxidation and nitration when compared with soluble alpha-syn. Additionally, increasing concentrations of unsaturated fatty acids diminished the oxidation and nitration of alpha-syn upon exposure to fluxes of peroxynitrite (8-20 microM x min(-1)). To investigate the effect of oxidized lipids on alpha-syn, the protein was incubated with the bifunctional electrophile 4-HNE [4-hydroxy-2(E)-nonenal]. MS analysis showed the formation of three major products corresponding to the native protein and alpha-syn plus one or two 4-HNE molecules. Trypsin digestion of the modified protein followed by peptide 'finger-printing' revealed that 4-HNE modified the peptide E46GVVHGVATVAEK58. Further analysis of the peptides with liquid chromatography-tandem MS identified the modified residue as His50. The data indicate that the association of alpha-syn with biological membranes protects the protein from oxidation and nitration and thus diminishes the formation of protein molecules capable of forming aggregates. However, products of lipid peroxidation can also modify alpha-syn, generating novel protein adducts that could serve as biomarkers for documenting oxidative processes in human as well as animal and cellular models of alpha-syn aggregation and pathology.

Dopamine promotes alpha-synuclein aggregation into SDS-resistant soluble oligomers via a distinct folding pathway

Dopamine (DA) and alpha-synuclein (alpha-SN) are two key molecules associated with Parkinson's disease (PD). We have identified a novel action of DA in the initial phase of alpha-SN aggregation and demonstrate that DA induces alpha-SN to form soluble, SDS-resistant oligomers. The DA:alpha-SN oligomeric species are not amyloidogenic as they do not react with thioflavin T and lack the typical amyloid fibril structures as visualized with electron microscopy. Circular dichroism studies indicate that in the presence of lipid membranes DA interacts with alpha-SN, causing an alteration to the structure of the protein. Furthermore, DA inhibited the formation of iron-induced alpha-SN amyloidogenic aggregates, suggesting that DA acts as a dominant modulator of alpha-SN aggregation. These observations support the paradigm emerging for other neurodegenerative diseases that the toxic species is represented by a soluble oligomer and not the insoluble fibril.