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

Dopamine Affects the Stability, Hydration, and Packing of Protofibrils and Fibrils of the Wild Type and Variants of α-Synuclein†

Parkinson's disease (PD) is characterized by the presence of cytoplasmic inclusions composed of alpha-synuclein (alpha-syn) in dopaminergic neurons. This suggests a pivotal role of dopamine (DA) on PD development. Here, we show that DA modulates differently the stability of protofibrils (PF) and fibrils (F) composed of wild type or variants of alpha-syn (A30P and A53T) as probed by high hydrostatic pressure (HHP). While in the absence of DA, all alpha-syn PF exhibited identical stability, in its presence, the variant-composed PF acquired a greater stability (DAPFwt < DAPFA30P = DAPFA53T), implying that they would last longer, which could shed light onto why these mutations are so aggressive. When alpha-syn was incubated for long times (18 days) in the presence of DA, we observed the formation of F by electronic microscopy, suggesting that the PF trapped in the presence of DA in short times can evolve into F. The stability of F was also altered by DA. DAFwt was more labile than Fwt, indicating that the former would be more susceptible to breakage. PFA30P and DAPFA30P, when added to mesencephalic and cortical neurons in culture, decreased the number and length of neurites and increased the number of apoptotic cells. Surprisingly, these toxic effects of PFA30P and DAPFA30P were practically abolished with HHP treatment, which was able to break the PF into smaller aggregates, as seen by atomic force microscopy. These results suggest that strategies aimed at breaking and/or clearing these aggregates is promising in alleviating the symptoms of PD.

Plasma alpha-synuclein is decreased in subjects with Parkinson's disease

Alpha-synuclein (alphaSN) is implicated in Parkinson's disease (PD) and is the major component of Lewy bodies (LBs). Although alphaSN is mainly expressed in neuronal cells and exists as a cytoplasmic protein, it has been found in body fluids including cerebrospinal fluid and blood. This study explored plasma alphaSN as a diagnostic marker for PD. Western blot analysis was used to characterize plasma alphaSN compared to brain alphaSN. Plasma alphaSN of 16 kDa migrates with the same mobility as its brain counterpart and recombinant alphaSN on denatured polyacrylamide gels and reacted with three different antibodies against the C-terminal and NAC regions of the alphaSN protein. The alphaSN levels in plasma from PD subjects are significantly lower than that in age-matched controls (p=0.001), and the alphaSN levels in patients with early-onset PD are lower than that in both late-onset PD and controls. This initial study indicates that measurement of alphaSN in plasma can provide support for a clinical diagnosis of Parkinson's disease and warrants further study in a larger population.

Cytosolic catechols inhibit alpha-synuclein aggregation and facilitate the formation of intracellular soluble oligomeric intermediates

Aberrant aggregation of alpha-synuclein (alpha-syn) to form fibrils and insoluble aggregates has been implicated in the pathogenic processes of many neurodegenerative diseases. Despite the dramatic effects of dopamine in inhibiting the formation of alpha-syn fibrils by stabilization of oligomeric intermediates in cell-free systems, no studies have examined the effects of intracellular dopamine on alpha-syn aggregation. To study this process and its association with neurodegeneration, intracellular catechol levels were increased to various levels by expressing different forms of tyrosine hydroxylase, in cells induced to form alpha-syn aggregates. The increase in the steady-state dopamine levels inhibited the formation of alpha-syn aggregates and induced the formation of innocuous oligomeric intermediates. Analysis of transgenic mice expressing the disease-associated A53T mutant alpha-syn revealed the presence of oligomeric alpha-syn in nondegenerating dopaminergic neurons that do contain insoluble alpha-syn. These data indicate that intraneuronal dopamine levels can be a major modulator of alpha-syn aggregation and inclusion formation, with important implications on the selective degeneration of these neurons in Parkinson's disease.

Ventral tegmental area dopamine neurons are resistant to human mutant alpha-synuclein overexpression

Parkinson's disease (PD) is characterized by the formation of intracytoplasmic inclusions, which contain alpha-synuclein (alpha-syn) protein. While most profound neurodegeneration is seen in the dopamine (DA) synthesizing neurons located in the ventral midbrain, it is unclear why some DA cell groups are more susceptible than others. In the midbrain, the degeneration of the substantia nigra (SN) DA neurons is severe, whereas the involvement of the ventral tegmental area (VTA) neurons is relatively spared. In the present study, we overexpressed human A53T alpha-syn in the VTA neurons and found that A53T toxicity did not affect their survival. There was, however, a mild functional impairment seen as altered open field locomotor activity. Overexpression of A53T in the SN, on the other hand, led to profound cell loss. These results suggest that the selective susceptibility of nigral DA neurons is at least in part associated with factor(s) involved in handling of alpha-syn that is not shared by the VTA neurons. Secondly, these results highlight the fact that impaired but surviving neurons can have a substantial impact on DA-dependent behavior and should therefore be considered as a critical part of animal models where novel therapeutic interventions are tested.

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.

Relationship Among α-Synuclein Accumulation, Dopamine Synthesis, and Neurodegeneration in Parkinson Disease Substantia Nigra

The histologic hallmark of Parkinson disease (PD) is loss of pigmented neurons in the substantia nigra (SN) and locus ceruleus (LC) with accumulation of alpha-synuclein (alphaS). It has been reported that tyrosine hydroxylase (TH)-negative pigmented neurons are present in these nuclei of patients with PD. However, the relationship between TH immunoreactivity and alphaS accumulation remains uncertain. We immunohistochemically examined the SN and LC from patients with PD (n = 10) and control subjects (n = 7). A correlation study indicated a close relationship among decreased TH immunoreactivity, alphaS accumulation, and neuronal loss. In addition, 10% of pigmented neurons in the SN and 54.9% of those in the LC contained abnormal alphaS aggregates. Moreover, 82.3% of pigmented neurons bearing alphaS aggregates in the SN and 39.2% of those in the LC lacked TH immunoreactivity, suggesting that pigmented neurons in the SN have a greater tendency to lack TH activity than those in the LC. Recent studies have shown that this decrease of TH activity leads to a decrease of cytotoxic substances and that decreased dopamine synthesis leads to a reduction of cytotoxic alphaS oligomers. Therefore, the decrease of TH immunoreactivity in pigmented neurons demonstrated here can be considered to represent a cytoprotective mechanism in PD.

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.

Linker histone H1 binds to disease associated amyloid-like fibrils

Alzheimer's disease (AD) and Parkinson's disease (PD) are the two most prevalent neurodegenerative diseases of the central nervous system. These two diseases share a common feature in that a normally soluble peptide (amyloid-beta) or protein (alpha-synuclein) aggregates into an ordered fibrillar structure. As well as structural similarities observed between fibrillar aggregates related to these diseases, common pathological processes of increased oxidative injury, excitotoxicity and altered cell cycle are also evident. It was the aim of this study to identify novel interacting proteins to the amyloid-like motif and therefore identify common potential pathways between neurodegenerative diseases that share biophysical properties common to classical amyloid fibrils. Optimal ageing of recombinant proteins to form amyloid-like fibrils was determined by electron microscopy, Congo red birefringement and photo-induced cross-linking. Using pull-down assays the strongest detected interacting protein to the amyloid-like motifs of amyloid-beta, alpha-synuclein and lysozyme was identified as histone H1. The interaction with the amyloid-like motif was confirmed by techniques including surface plasmon resonance and immunohistochemistry. Histone H1 is known to be an integral part of chromatin within the nucleus, with a primary role of binding DNA that enters and exits from the nucleosome, and facilitating the shift in equilibrium of chromatin towards a more condensed form. However, phosphorylated histone H1 is predominantly present in the cytoplasm and as yet the functional significance of this translocation is unknown. This study also found that histone H1 is localised within the cytoplasm of neurons and astrocytes from areas affected by disease as well as amyloid plaques, supporting the hypothesis that histone H1 favoured binding to an ordered fibrillar motif. We conclude that the binding of histone H1 to a general amyloid-like motif indicates that histone H1 may play an important common role in diseases associated with amyloid-like fibrils.

Inhibitors of alpha-synuclein oligomerization and toxicity: a future therapeutic strategy for Parkinson's disease and related disorders

An abundance of genetic, histopathological, and biochemical evidence has implicated the neuronal protein, alpha-synuclein (alpha-syn) as a key player in the development of several neurodegenerative diseases, the so-called synucleinopathies, of which Parkinson's disease (PD) is the most prevalent. Development of disease appears to be linked to events that increase the intracellular concentration of alpha-syn or cause its chemical modification, either of which can accelerate the rate at which it forms aggregates. Examples of such events include increased copy number of genes, decreased rate of degradation via the proteasome or other proteases, or altered forms of alpha-syn, such as truncations, missense mutations, or chemical modifications by oxidative reactions. Aggregated forms of the protein, especially newly formed soluble aggregates, are toxic to cells, so that one therapeutic strategy would be to reduce the rate at which such oligomerization occurs. We have therefore designed several peptides and also identified small molecules that can inhibit alpha-syn oligomerization and toxicity in vitro. These compounds could serve as lead compounds for the design of new drugs for the treatment of PD and related disorders in the future.

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.

High density lipoproteins bind Abeta and apolipoprotein C-II amyloid fibrils

Disease-associated amyloid deposits contain both fibrillar and nonfibrillar components. The majority of these amyloid components originate or coexist in the bloodstream. To understand the nature of the interaction between the nonfibrillar and fibrillar components, we have developed a centrifugation method to isolate fibril binding proteins from human serum. Amyloid fibrils composed of either Abeta peptide or apolipoprotein C-II (apoC-II) cosedimented with specific serum proteins. Gel electrophoresis, mass spectrometry peptide fingerprinting, and Western analysis identified the major binding species as proteins found in HDL particles, including apoA-I, apoA-II, apoE, clusterin, and serum amyloid A. Sedimentation analysis showed that purified human HDL and recombinant apoA-I lipid particles bound directly to Abeta and apoC-II amyloid fibrils. These studies reveal a novel function of HDL that may contribute to the well-established protective effect of this lipoprotein class in heart disease.

Ubiquitin–proteasome system and Parkinson's disease

Increasing genetic, pathological, and experimental evidence suggest that neurodegeneration in both familial and sporadic forms of Parkinson's disease (PD) may be related to a defect in the capacity of the ubiquitin-proteasome system (UPS) to clear unwanted proteins, resulting in protein accumulation, aggregation, and cytotoxicity. This concept is supported by in vitro and in vivo laboratory experiments which show that inhibition of UPS function can cause neurodegeneration coupled with the formation of Lewy body-like inclusions. This hypothesis could account for the presence of protein aggregates and Lewy bodies in PD, the other biochemical features seen in the disorder, and the age-related vulnerability of the substantia nigra pars compacta. It also suggests novel targets for putative neuroprotective therapies for PD.

α-Synuclein aggregation in neurodegenerative diseases and its inhibition as a potential therapeutic strategy

There is strong evidence for the involvement of α-synuclein in the pathologies of several neurodegenerative disorders, including PD (Parkinson's disease). Development of disease appears to be linked to processes that increase the rate at which α-synuclein forms aggregates. These processes include increased protein concentration (via either increased rate of synthesis or decreased rate of degradation), and altered forms of α-synuclein (such as truncations, missense mutations, or chemical modifications by oxidative reactions). Aggregated forms of the protein are toxic to cells and one therapeutic strategy would be to reduce the rate at which aggregation occurs. To this end we have designed several peptides that reduce α-synuclein aggregation. A cell-permeable version of one such peptide was able to inhibit the DNA damage induced by Fe(II) in neuronal cells transfected with α-synuclein (A53T), a familial PD-associated mutation.

Neurochemical insights

The 20th biennial meeting of the International Society for Neurochemistry was recently held in Innsbruck, Austria. This meeting gave an overview of the latest findings in the field of molecular mechanisms and diagnosis of neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and prion disease. There was a focus on the molecular pathogenesis of protein misfolding in these disorders as well as on the association between oxidative metabolism and neurological diseases. RNA interference, metal chelators, and the use of metallopeptidases were discussed as possible therapeutic strategies.