Inhibition of alpha-synuclein fibrillization by dopamine analogs via reaction with the amino groups of alpha-synuclein. Implication for dopaminergic neurodegeneration

A review on nanotechnological perspective of "the amyloid cascade hypothesis" for neurodegenerative diseases

Neurodegenerative diseases (NDs) are characterized by progressive degeneration of neurons which deteriorates the brain functions. An early detection of the onset of NDs is utmost important, as it will provide the fast treatment strategies to prevent further progression of the disease. Conventionally, accurate diagnosis of the brain related disorders is difficult in their early phase. To solve this problem, nanotechnology based neurofunctional imaging and biomarker detection techniques have been developed which allows high specificity and sensitivity towards screening and diagnosis of NDs. Another challenge to treat the brain related disorders is to overcome the complex integrity of blood-brain-barrier (BBB) for the delivery of theranostic agents. Fortunately, utilization of nanomaterials has been pursued as promising strategy to address this challenge. Herein, we critically highlighted the recent improvements in the field of neurodiagnostic and therapeutic approaches involving innovative strategies for diagnosis, and inhibition of protein aggregates. We have provided particular emphasis on the use of nanotechnology which can push forward the blooming research growth in this field to win the battle against devastating NDs.

Inhibition of Protein Aggregation and Endoplasmic Reticulum Stress as a Targeted Therapy for α-Synucleinopathy

α-synuclein (α-syn) is an intrinsically disordered protein abundant in the central nervous system. Physiologically, the protein regulates vesicle trafficking and neurotransmitter release in the presynaptic terminals. Pathologies related to misfolding and aggregation of α-syn are referred to as α-synucleinopathies, and they constitute a frequent cause of neurodegeneration. The most common α-synucleinopathy, Parkinson's disease (PD), is caused by abnormal accumulation of α-syn in the dopaminergic neurons of the midbrain. This results in protein overload, activation of endoplasmic reticulum (ER) stress, and, ultimately, neural cell apoptosis and neurodegeneration. To date, the available treatment options for PD are only symptomatic and rely on dopamine replacement therapy or palliative surgery. As the prevalence of PD has skyrocketed in recent years, there is a pending issue for development of new disease-modifying strategies. These include anti-aggregative agents that target α-syn directly (gene therapy, small molecules and immunization), indirectly (modulators of ER stress, oxidative stress and clearance pathways) or combine both actions (natural compounds). Herein, we provide an overview on the characteristic features of the structure and pathogenic mechanisms of α-syn that could be targeted with novel molecular-based therapies.

Oxidative Stress and Neuroinflammation in Parkinson's Disease: The Role of Dopamine Oxidation Products

Parkinson's disease (PD) is a chronic neurodegenerative condition affecting more than 1% of people over 65 years old. It is characterized by the preferential degeneration of nigrostriatal dopaminergic neurons, which is responsible for the motor symptoms of PD patients. The pathogenesis of this multifactorial disorder is still elusive, hampering the discovery of therapeutic strategies able to suppress the disease's progression. While redox alterations, mitochondrial dysfunctions, and neuroinflammation are clearly involved in PD pathology, how these processes lead to the preferential degeneration of dopaminergic neurons is still an unanswered question. In this context, the presence of dopamine itself within this neuronal population could represent a crucial determinant. In the present review, an attempt is made to link the aforementioned pathways to the oxidation chemistry of dopamine, leading to the formation of free radical species, reactive quinones and toxic metabolites, and sustaining a pathological vicious cycle.

Development of Small Molecules Targeting α-Synuclein Aggregation: A Promising Strategy to Treat Parkinson’s Disease

Parkinson’s disease, the second most common neurodegenerative disorder worldwide, is characterized by the accumulation of protein deposits in the dopaminergic neurons. These deposits are primarily composed of aggregated forms of α-Synuclein (α-Syn). Despite the extensive research on this disease, only symptomatic treatments are currently available. However, in recent years, several compounds, mainly of an aromatic character, targeting α-Syn self-assembly and amyloid formation have been identified. These compounds, discovered by different approaches, are chemically diverse and exhibit a plethora of mechanisms of action. This work aims to provide a historical overview of the physiopathology and molecular aspects associated with Parkinson’s disease and the current trends in small compound development to target α-Syn aggregation. Although these molecules are still under development, they constitute an important step toward discovering effective anti-aggregational therapies for Parkinson’s disease.

The Strategies of Development of New Non-Toxic Inhibitors of Amyloid Formation

In recent years, due to the aging of the population and the development of diagnostic medicine, the number of identified diseases associated with the accumulation of amyloid proteins has increased. Some of these proteins are known to cause a number of degenerative diseases in humans, such as amyloid-beta (Aβ) in Alzheimer's disease (AD), α-synuclein in Parkinson's disease (PD), and insulin and its analogues in insulin-derived amyloidosis. In this regard, it is important to develop strategies for the search and development of effective inhibitors of amyloid formation. Many studies have been carried out aimed at elucidating the mechanisms of amyloid aggregation of proteins and peptides. This review focuses on three amyloidogenic peptides and proteins-Aβ, α-synuclein, and insulin-for which we will consider amyloid fibril formation mechanisms and analyze existing and prospective strategies for the development of effective and non-toxic inhibitors of amyloid formation. The development of non-toxic inhibitors of amyloid will allow them to be used more effectively for the treatment of diseases associated with amyloid.

Recent advances in dopamine D2 receptor ligands in the treatment of neuropsychiatric disorders

Dopamine is a biologically active amine synthesized in the central and peripheral nervous system. This biogenic monoamine acts by activating five types of dopamine receptors (D_1-5 Rs), which belong to the G protein-coupled receptor family. Antagonists and partial agonists of D₂ Rs are used to treat schizophrenia, Parkinson's disease, depression, and anxiety. The typical pharmacophore with high D₂ R affinity comprises four main areas, namely aromatic moiety, cyclic amine, central linker and aromatic/heteroaromatic lipophilic fragment. From the literature reviewed herein, we can conclude that 4-(2,3-dichlorophenyl), 4-(2-methoxyphenyl)-, 4-(benzo[b]thiophen-4-yl)-1-substituted piperazine, and 4-(6-fluorobenzo[d]isoxazol-3-yl)piperidine moieties are critical for high D₂ R affinity. Four to six atoms chains are optimal for D₂ R affinity with 4-butoxyl as the most pronounced one. The bicyclic aromatic/heteroaromatic systems are most frequently occurring as lipophilic appendages to retain high D₂ R affinity. In this review, we provide a thorough overview of the therapeutic potential of D₂ R modulators in the treatment of the aforementioned disorders. In addition, this review summarizes current knowledge about these diseases, with a focus on the dopaminergic pathway underlying these pathologies. Major attention is paid to the structure, function, and pharmacology of novel D₂ R ligands, which have been developed in the last decade (2010-2021), and belong to the 1,4-disubstituted aromatic cyclic amine group. Due to the abundance of data, allosteric D₂ R ligands and D₂ R modulators from patents are not discussed in this review.

An Unrecognized Fundamental Relationship between Neurotransmitters: Glutamate Protects against Catecholamine Oxidation

Neurotransmitter catecholamines (dopamine, epinephrine, and norepinephrine) are liable to undergo oxidation, which copper is deeply involved in. Catecholamine oxidation-derived neurotoxicity is recognized as a pivotal pathological mechanism in neurodegenerative diseases. Glutamate, as an excitatory neurotransmitter, is enriched in the brain at extremely high concentrations. However, the chemical biology relationship of these two classes of neurotransmitters remains largely unknown. In the present study, we assessed the influences of glutamate on the autoxidation of catecholamines, the copper- and copper-containing ceruloplasmin-mediated oxidation of catecholamines, the catecholamine-induced formation of quinoprotein, catecholamine/copper-induced hydroxyl radicals, and DNA damage in vitro. The results demonstrate that glutamate, at a physiologically achievable molar ratio of glutamate/catecholamines, has a pronounced inhibitory effect on catecholamine oxidation, catecholamine oxidation-evoked hydroxyl radicals, quinoprotein, and DNA damage. The protective mechanism of glutamate against catecholamine oxidation could be attributed to its restriction of the redox activity of copper via chelation. This previously unrecognized link between glutamate, catecholamines, and copper suggests that neurodegenerative disorders may occur and develop once the built-in equilibrium is disrupted and brings new insight into developing more effective prevention and treatment strategies for neurodegenerative diseases.

Supramolecular Regulation of Polydopamine Formation by Amyloid Fibers

Polydopamine (PD) and melanin species are chemically complex systems, the formation and properties of which are incompletely understood. Inspired by the role of functional amyloids in melanin biosynthesis, this paper examines the influences of the supramolecular structure of amyloids on oxidative polymerization of dopamine. Kinetic analyses on the formation of PD species in the presence of hen egg white lysozyme (HEWL) fibers or soluble HEWL revealed that both forms gave rise to the total quantity of PD species, but the rate of their formation could be accelerated only by the amyloid form. PD species formed with HEWL fibers showed a morphology of bundled fibers, whereas those with soluble HEWL had a mesh-like structure. Amyloid fibers of recombinant Pmel17 had properties similar to those of HEWL fibers in modulating PD formation. The results presented here suggest how nature designs functionality with an amyloid structure and can help understand and engineer chemistries of other functional amyloids.

Advances in the development of imaging probes and aggregation inhibitors for alpha-synuclein

Abnormal protein aggregation has been linked to many neurodegenerative diseases, including Parkinson’s disease (PD). The main pathological hallmark of PD is the formation of Lewy bodies (LBs) and Lewy neurites, both of which contain the presynaptic protein alpha-synuclein (α-syn). Under normal conditions, native α-syn exists in a soluble unfolded state but undergoes misfolding and aggregation into toxic aggregates under pathological conditions. Toxic α-syn species, especially oligomers, can cause oxidative stress, membrane penetration, synaptic and mitochondrial dysfunction, as well as other damage, leading to neuronal death and eventually neurodegeneration. Early diagnosis and treatments targeting PD pathogenesis are urgently needed. Given its critical role in PD, α-syn is an attractive target for the development of both diagnostic tools and effective therapeutics. This review summarizes the progress toward discovering imaging probes and aggregation inhibitors for α-syn. Relevant strategies and techniques in the discovery of α-syn-targeted drugs are also discussed.

Targeting α-synuclein for PD Therapeutics: A Pursuit on All Fronts

Parkinson's Disease (PD) is characterized both by the loss of dopaminergic neurons in the substantia nigra and the presence of cytoplasmic inclusions called Lewy Bodies. These Lewy Bodies contain the aggregated α-synuclein (α-syn) protein, which has been shown to be able to propagate from cell to cell and throughout different regions in the brain. Due to its central role in the pathology and the lack of a curative treatment for PD, an increasing number of studies have aimed at targeting this protein for therapeutics. Here, we reviewed and discussed the many different approaches that have been studied to inhibit α-syn accumulation via direct and indirect targeting. These analyses have led to the generation of multiple clinical trials that are either completed or currently active. These clinical trials and the current preclinical studies must still face obstacles ahead, but give hope of finding a therapy for PD with time.

Scutellarin inhibits the uninduced and metal-induced aggregation of α-Synuclein and disaggregates preformed fibrils: implications for Parkinson's disease

The aggregation of the protein alpha synuclein (α-Syn), a known contributor in Parkinson's disease (PD) pathogenesis is triggered by transition metal ions through occupational exposure and disrupted metal ion homeostasis. Naturally occurring small molecules such as polyphenols have emerged as promising inhibitors of α-Syn fibrillation and toxicity and could be potential therapeutic agents against PD. Here, using an array of biophysical tools combined with cellular assays, we demonstrate that the novel polyphenolic compound scutellarin efficiently inhibits the uninduced and metal-induced fibrillation of α-Syn by acting at the nucleation stage and stabilizes a partially folded intermediate of α-Syn to form SDS-resistant, higher-order oligomers (∼680 kDa) and also disaggregates preformed fibrils of α-Syn into similar type of higher-order oligomers. ANS binding assay, fluorescence lifetime measurements and cell-toxicity experiments reveal scutellarin-generated oligomers as compact, low hydrophobicity structures with modulated surface properties and significantly reduced cytotoxicity than the fibrillation intermediates of α-Syn control. Fluorescence spectroscopy and isothermal titration calorimetry establish the binding between scutellarin and α-Syn to be non-covalent in nature and of moderate affinity (Ka ∼ 105 M−1). Molecular docking approaches suggest binding of scutellarin to the residues present in the NAC region and C-terminus of monomeric α-Syn and the C-terminal residues of fibrillar α-Syn, demonstrating inhibition of fibrillation upon binding to these residues and possible stabilization of the autoinhibitory conformation of α-Syn. These findings reveal interesting insights into the mechanism of scutellarin action and establish it as an efficient modulator of uninduced as well as metal-induced α-Syn fibrillation and toxicity.

Vitamin K1 As A Potential Molecule For Reducing Single-Walled Carbon Nanotubes-Stimulated α-Synuclein Structural Changes And Cytotoxicity

Aims

Different kinds of vitamins can be used as promising candidates to mitigate the structural changes of proteins and associated cytotoxicity stimulated by NPs. Therefore, the structural changes of α-syn molecules and their associated cytotoxicity in the presence of SWCNTs either alone or co-incubated with vitamin K1 were studied by spectroscopic, bioinformatical, and cellular assays.

Methods

Intrinsic and ThT fluorescence, CD, and Congo red absorption spectroscopic approaches as well as TEM investigation, molecular docking, and molecular dynamics were used to explore the protective effect of vitamin K1 on the structural changes of α-syn induced by SWCNTs. The cytotoxicity of α-syn/SWCNTs co-incubated with vitamin K1 against SH-SY5Y cells was also carried out by MTT, LDH, and caspase-3 assays.

Results

Fluorescence spectroscopy showed that vitamin K1 has a significant effect in reducing SWCNT-induced fluorescence quenching and aggregation of α- syn. CD, Congo red adsorption, and TEM investigations determined that co-incubation of α- syn with vitamin K1 inhibited the propensity of α-syn into the structural changes and amorphous aggregation in the presence of SWCNT. Docking studies determined the occupation of preferred docked site of SWCNT by vitamin K1 on α- syn conformation. A molecular dynamics study also showed that vitamin K1 reduced the structural changes of α- syn induced by SWCNT. Cellular data exhibited that the cytotoxicity of α- syn co-incubated with vitamin K1 in the presence of SWCNTs is less than the outcomes obtained in the absence of the vitamin K1.

Conclusion

It may be concluded that vitamin K1 decreases the propensity of α- syn aggregation in the presence of SWCNTs and induction of cytotoxicity.

Coordination of Platinum to α-Synuclein Inhibits Filamentous Aggregation in Solution

Accumulation of filamentous aggregates of α-synuclein (AS) in Lewy bodies and neurites is characteristic of neurodegenerative diseases such as Parkinson's disease. Inhibition of AS fibrillation is helpful for understanding of AS aggregate structure and for developing chemical therapies. Herein, we report that the Pt^II -containing antitumor drug cisplatin suppresses filamentous aggregation of AS in solution. Pt^II thus contrasts strongly with reported transition-metal ions such as Mn^II , Fe^III , and Cu^II , which accelerate AS aggregation. Interaction between Pt^II and the side chains of methionine and histidine residues was essential for inhibition of AS fibrillation. Binding of Pt^II to AS did not change the protein's overall random coil structure, as indicated by solution-state two-dimensional NMR and circular dichroism spectroscopy; and a solution of the AS⋅Pt^II complex remained free of filamentous aggregates. Our results constitute interesting new information about the biological chemistry of metal ions in Parkinson's disease and might open new lines of research into the suppression of filamentous aggregation.

α-Synuclein-Confocal Nanoscanning (ASYN-CONA), a Bead-Based Assay for Detecting Early-Stage α-Synuclein Aggregation

α-Synuclein fibrils are considered a hallmark of Parkinson’s disease and other synucleinopathies. However, small oligomers that formed during the early stages of α-synuclein aggregation are thought to be the main toxic species causing disease. The formation of α-synuclein oligomers has proven difficult to follow, because of the heterogeneity and transient nature of the species formed. Here, a novel bead-based aggregation assay for monitoring the earliest stages of α-synuclein oligomerization, α-Synuclein–Confocal Nanoscanning (ASYN-CONA), is presented. The α-synuclein A91C single cysteine mutant is modified with a trifunctional chemical tag, which allows simultaneous fluorescent labeling with a green dye (tetramethylrhodamine, TMR) and attachment to microbeads. Beads with bound TMR-labeled α-synuclein are then incubated with a red dye (Cy5)-labeled variant of α-synuclein A91C, and EtOH (20%) to induce aggregation. Aggregation is detected by confocal scanning imaging, below the equatorial plane of the beads, which is known as the CONA technique. On-bead TMR-labeled α-synuclein and aggregated Cy5-labeled α-synuclein from the solution are quantitatively monitored in parallel by detection of fluorescent halos or “rings”. α-Synuclein on-bead oligomerization results in a linear increase of red bead ring fluorescence intensity over a period of 5 h. Total internal reflection fluorescence microscopy was performed on oligomers cleaved from the beads, and it revealed that (i) oligomers are sufficiently stable in solution to investigate their composition, consisting of 6 ± 1 monomer units, and (ii) oligomers containing a mean of 15 monomers bind Thioflavin-T. Various known inhibitors of α-synuclein aggregation were used to validate the ASYN-CONA assay for drug screening. Baicalein, curcumin, and rifampicin showed concentration-dependent inhibition of the α-synuclein aggregation and the IC₅₀ (the concentration of the compound at which the maxiumum intensity was reduced by one-half) were calculated.

Toward the discovery and development of effective modulators of α-synuclein amyloid aggregation

A host of human diseases, including Parkinson's disease and Dementia with Lewy bodies, are suspected to be directly linked to protein aggregation. Amyloid protein aggregates and oligomeric intermediates of α-synuclein are observed in synucleinopathies and considered to be mediators of cellular toxicity. Hence, α-synuclein has seen as one of the leading and most compelling targets and is receiving a great deal of attention from researchers. Nevertheless, there is no neuroprotective approach directed toward Parkinson's disease or other synucleinopathies so far. In this review, we summarize the available data concerning inhibitors of α-synuclein aggregation and their advancing towards clinical use. The compounds are grouped according to their chemical structures, providing respective insights into their mechanism of action, pharmacology, and pharmacokinetics. Overall, shared structure-activity elements are emerging, as well as specific binding modes related to the ability of the modulators to establish hydrophobic and hydrogen bonds interactions with the protein. Some molecules with encouraging in vivo data support the possibility of translation to the clinic.

The usual suspects, dopamine and alpha-synuclein, conspire to cause neurodegeneration

Parkinson's disease (PD) is primarily a movement disorder driven by the loss of dopamine-producing neurons in the substantia nigra (SN). Early identification of the oxidative properties of dopamine implicated it as a potential source of oxidative stress in PD, yet few studies have investigated dopamine neurotoxicity in vivo. The discovery of PD-causing mutations in α-synuclein and the presence of aggregated α-synuclein in the hallmark Lewy body pathology of PD revealed another important player. Despite extensive efforts, the precise role of α-synuclein aggregation in neurodegeneration remains unclear. We recently manipulated both dopamine levels and α-synuclein expression in aged mice and found that only the combination of these 2 factors caused progressive neurodegeneration of the SN and an associated motor deficit. Dopamine modified α-synuclein aggregation in the SN, resulting in greater abundance of α-synuclein oligomers and unique dopamine-induced oligomeric conformations. Furthermore, disruption of the dopamine-α-synuclein interaction rescued dopaminergic neurons from degeneration in transgenic Caenorhabditis elegans models. In this Perspective, we discuss these findings in the context of known α-synuclein and dopamine biology, review the evidence for α-synuclein oligomer toxicity and potential mechanisms, and discuss therapeutic implications. © 2019 International Parkinson and Movement Disorder Society.

East Indian sandalwood (Santalum album L.) oil confers neuroprotection and geroprotection in Caenorhabditis elegans via activating SKN-1/Nrf2 signaling pathway

East Indian Sandalwood Oil (EISO) has diverse beneficial effects and has been used for thousands of years in traditional folk-medicine for treatment of different human ailments. However, there has been no in-depth scientific investigation to decipher the neuroprotective and geroprotective mechanism of EISO and its principle components, α- and β-santalol. Hence the current study was undertaken to assess the protective effects of EISO, and α- and β-santalol against neurotoxic (6-OHDA/6-hydroxydopamine) and proteotoxic (α-synuclein) stresses in a Caenorhabditis elegans model. Initially, we found that EISO and its principle components exerted an excellent antioxidant and antiapoptotic activity as it was able to extend the lifespan, and inhibit the ROS generation, and germline cell apoptosis in 6-OHDA-intoxicated C. elegans. Further, we showed that supplementation of EISO, and α- and β-santalol reduced the 6-OHDA and α-synuclein-induced Parkinson's disease associated pathologies and improved the physiological functions. The genetic and reporter gene expression analysis revealed that an EISO, or α- and β-santalol-mediated protective effect does not appear to rely on DAF-2/DAF-16, but selectively regulates SKN-1 and its downstream targets involved in antioxidant defense and geroprotective processes. Together, our findings indicated that EISO and its principle components are worth exploring further as a candidate redox-based neuroprotectant for the prevention and management of age-related neurological disorders.

Mitochondrial dynamics in Parkinson's disease: a role for α-synuclein?

The distinctive pathological hallmarks of Parkinson's disease are the progressive death of dopaminergic neurons and the intracellular accumulation of Lewy bodies enriched in α-synuclein protein. Several lines of evidence from the study of sporadic, familial and pharmacologically induced forms of human Parkinson's disease also suggest that mitochondrial dysfunction plays an important role in disease progression. Although many functions have been proposed for α-synuclein, emerging data from human and animal models of Parkinson's disease highlight a role for α-synuclein in the control of neuronal mitochondrial dynamics. Here, we review the α-synuclein structural, biophysical and biochemical properties that influence relevant mitochondrial dynamic processes such as fusion-fission, transport and clearance. Drawing on current evidence, we propose that α-synuclein contributes to the mitochondrial defects that are associated with the pathology of this common and progressive neurodegenerative disease.

Summary: The authors review the α-synuclein structural, biophysical and biochemical properties that influence relevant mitochondrial physiological processes such as fusion-fission, transport and clearance, and propose that α-synuclein contributes to the mitochondrial defects that are associated with Parkinson's disease.

Anti-Parkinsonian effects of β-amyrin are regulated via LGG-1 involved autophagy pathway in Caenorhabditis elegans

BACKGROUND

Parkinson's disease (PD) is a neurodegenerative disease that is associated with aging and is characterized as a movement disorder. Currently, there is still no complete therapy for PD. In recent years, the identification and characterization of medicinal plants to cure or treat PD has gained increasing scientific interest.

PURPOSE

In this study, we investigated a pentacyclic triterpenoid compound, β-amyrin, which is found in many medicinal plants for its anti-Parkinsonian effects, using Caenorhabditis elegans (C. elegans) disease models and their underlying mechanisms.

METHODS

C. elegans treated or untreated with β-amyrin were investigated for oxidative stress resistance, neurodegeneration, and α-synuclein aggregation assays. The C. elegans ortholog of Atg8/LC3, LGG-1 that is involved in the autophagy pathway was also evaluated by quantitative RT-PCR and transgenic strain experiments.

RESULTS

β-Amyrin exerted excellent antioxidant activity and reduced intracellular oxygen species in C. elegans. Using the transgenic strain BZ555, β-amyrin showed a protective effect on dopaminergic neurons reducing cell damage induced by 6-hydroxydopamine (6-OHDA). In addition, β-amyrin significantly reduced the α-synuclein aggregation in the transgenic strain NL5901. Moreover, β-amyrin up-regulated LGG-1 mRNA expression and increased the number of localized LGG-1 puncta in the transgenic strain DA2123.

CONCLUSION

The results from this study suggest that the anti-Parkinsonian effects of β-amyrin might be regulated via LGG-1 involved autophagy pathway in C. elegans. Therefore, β-amyrin may be useful for therapeutic applications or supplements to treat or slow the progression of PD.

Effects of small-molecule amyloid modulators on a Drosophila model of Parkinson's disease

Alpha-synuclein (aS) amyloid formation is involved in Parkinson's disease (PD); therefore, small molecules that target aS and affect its aggregation are of interest as future drug candidates. We recently reported modified ring-fused 2-pyridones that modulate aS amyloid formation in vitro. Here, we describe the effects of such molecules on behavioral parameters of a Drosophila model of PD (i.e., flies expressing human aS), using a new approach (implemented in a commercially available FlyTracker system) to quantify fly mobility. FlyTracker allows for automated analysis of walking and climbing locomotor behavior, as it collects large sequences of data over time in an unbiased manner. We found that the molecules per se have no toxic or kinetic effects on normal flies. Feeding aS-expressing flies with the amyloid-promoting molecule FN075, remarkably, resulted in increased fly mobility at early time points; however, this effect switched to reduced mobility at later time points, and flies had shorter life spans than controls. In contrast, an amyloid inhibitor increased both fly kinetics and life span. In agreement with increased aS amyloid formation, the FN075-fed flies had less soluble aS, and in vitro aS-FN075 interactions stimulated aS amyloid formation. In addition to a new quantitative approach to probe mobility (available in FlyTracker), our results imply that aS regulates brain activity such that initial removal (here, by FN075-triggered assembly of aS) allows for increased fly mobility.

α-Synuclein aggregation modulation: an emerging approach for the treatment of Parkinson's disease

Parkinson's disease (PD) is a multifactorial progressive neurological disorder. Pathological hallmarks of PD are characterized by the presence of α-synuclein (αSyn) aggregates known as Lewy bodies. αSyn aggregation is one of the leading causes for the neuronal dysfunction and death in PD. It is also associated with neurotransmitter and calcium release. Current therapies of PD are limited to only symptomatic relief without addressing the underlying pathogenic factors of the disease process such as aggregation of αSyn. Consequently, the progression of the disease continues with the current therapies. Therefore, the modulation of αSyn aggregation is an emerging approach as a novel therapeutic target to treat PD. There are two major aspects that might be targeted therapeutically: first, protein is prone to aggregation, therefore, anti-aggregative or compounds that can break the pre-existing aggregates should be helpful. Second, there are number of molecular events that may be targeted to combat the disease.

A Redox-Active, Compact Molecule for Cross-Linking Amyloidogenic Peptides into Nontoxic, Off-Pathway Aggregates: In Vitro and In Vivo Efficacy and Molecular Mechanisms

Chemical reagents targeting and controlling amyloidogenic peptides have received much attention for helping identify their roles in the pathogenesis of protein-misfolding disorders. Herein, we report a novel strategy for redirecting amyloidogenic peptides into nontoxic, off-pathway aggregates, which utilizes redox properties of a small molecule (DMPD, N,N-dimethyl-p-phenylenediamine) to trigger covalent adduct formation with the peptide. In addition, for the first time, biochemical, biophysical, and molecular dynamics simulation studies have been performed to demonstrate a mechanistic understanding for such an interaction between a small molecule (DMPD) and amyloid-β (Aβ) and its subsequent anti-amyloidogenic activity, which, upon its transformation, generates ligand-peptide adducts via primary amine-dependent intramolecular cross-linking correlated with structural compaction. Furthermore, in vivo efficacy of DMPD toward amyloid pathology and cognitive impairment was evaluated employing 5xFAD mice of Alzheimer's disease (AD). Such a small molecule (DMPD) is indicated to noticeably reduce the overall cerebral amyloid load of soluble Aβ forms and amyloid deposits as well as significantly improve cognitive defects in the AD mouse model. Overall, our in vitro and in vivo studies of DMPD toward Aβ with the first molecular-level mechanistic investigations present the feasibility of developing new, innovative approaches that employ redox-active compounds without the structural complexity as next-generation chemical tools for amyloid management.

Oligomerization and Membrane-binding Properties of Covalent Adducts Formed by the Interaction of α-Synuclein with the Toxic Dopamine Metabolite 3,4-Dihydroxyphenylacetaldehyde (DOPAL)

Oxidative deamination of dopamine produces the highly toxic aldehyde 3,4-dihydroxyphenylacetaldehyde (DOPAL), enhanced production of which is found in post-mortem brains of Parkinson disease patients. When injected into the substantia nigra of rat brains, DOPAL causes the loss of dopaminergic neurons accompanied by the accumulation of potentially toxic oligomers of the presynaptic protein α-synuclein (aS), potentially explaining the synergistic toxicity described for dopamine metabolism and aS aggregation. In this work, we demonstrate that DOPAL interacts with aS via formation of Schiff-base and Michael-addition adducts with Lys residues, in addition to causing oxidation of Met residues to Met-sulfoxide. DOPAL modification leads to the formation of small aS oligomers that may be cross-linked by DOPAL. Both monomeric and oligomeric DOPAL adducts potently inhibit the formation of mature amyloid fibrils by unmodified aS. The binding of aS to either lipid vesicles or detergent micelles, which results in a gain of α-helix structure in its N-terminal lipid-binding domain, protects the protein against DOPAL adduct formation and, consequently, inhibits DOPAL-induced aS oligomerization. Functionally, aS-DOPAL monomer exhibits a reduced affinity for small unilamellar vesicles with lipid composition similar to synaptic vesicles, in addition to diminished membrane-induced α-helical content in comparison with the unmodified protein. These results suggest that DOPAL could compromise the functionality of aS, even in the absence of protein oligomerization, by affecting the interaction of aS with lipid membranes and hence its role in the regulation of synaptic vesicle traffic in neurons.

Molecular, Neurochemical, and Behavioral Hallmarks of Reserpine as a Model for Parkinson's Disease: New Perspectives to a Long-Standing Model

The administration of reserpine to rodents was one of the first models used to investigate the pathophysiology and screening for potential treatments of Parkinson's disease (PD). The reserpine model was critical to the understanding of the role of monoamine system in the regulation of motor and affective disorders, as well as the efficacy of current PD treatments, such as L-DOPA and dopamine agonists. Nevertheless, with the introduction of toxin-induced and genetic models of PD, reserpine became underused. The main rationale to this drawback was the supposed absence of reserpine construct validity with PD. Here, we highlight classical and recent experimental findings that support the face, pharmacological, and construct validity of reserpine PD model and reason against the current rationale for its underuse. We also aim to shed a new perspective upon the model by discussing the main challenges and potentials for the reserpine model of PD.

DT-Diaphorase Prevents Aminochrome-Induced Alpha-Synuclein Oligomer Formation and Neurotoxicity

It was reported that aminochrome induces the formation of alpha synuclein (SNCA) oligomers during dopamine oxidation. We found that DT-diaphorase (NQO1) prevents the formation of SNCA oligomers in the presence of aminochrome determined by Western blot, transmission electron microscopy, circular dichroism, and thioflavin T fluorescence, suggesting a protective role of NQO1 by preventing the formation of SNCA oligomers in dopaminergic neurons. In order to test NQO1 protective role in SNCA neurotoxicity in cellular model, we overexpressed SNCA in both RCSN-3 cells (wild-type) and RCSN-3Nq7 cells, which have constitutive expression of a siRNA against NQO1. The expression of SNCA in RCSN-3SNCA and RCSN-3Nq7SNCA cells increased 4.2- and 4.4-fold, respectively. The overexpression of SNCA in RCSN-3Nq7SNCA cells induces a significant increase in cell death of 2.8- and 3.2-fold when they were incubated with 50 and 70 µM aminochrome, respectively. The cell death was found to be of apoptotic character determined by annexin/propidium iodide technique with flow cytometry and DNA laddering. A Western blot demonstrated that SNCA in RCSN-3SNCA is only found in monomer form both in the presence of 20 µM aminochrome or cell culture medium contrasting with RCSN-3Nq7SNCA cells where the majority SNCA is found as oligomer. The antioligomer compound scyllo-inositol induced a significant decrease in aminochrome-induced cell death in RCSN-3Nq7SNCA cells in comparison to cells incubated in the absence of scyllo-inositol. Our results suggest that NQO1 seems to play an important role in the prevention of aminochrome-induced SNCA oligomer formation and SNCA oligomers neurotoxicity in dopaminergic neurons.

Multifunctional D2/D3 agonist D-520 with high in vivo efficacy: modulator of toxicity of alpha-synuclein aggregates

We have developed a series of dihydroxy compounds and related analogues based on our hybrid D2/D3 agonist molecular template to develop multifunctional drugs for symptomatic and neuroprotective treatment for Parkinson's disease (PD). The lead compound (-)-24b (D-520) exhibited high agonist potency at D2/D3 receptors and produced efficacious activity in the animal models for PD. The data from thioflavin T (ThT) assay and from transmission electron microscopy (TEM) analysis demonstrate that D-520 is able to modulate aggregation of alpha-synuclein (αSN). Additionally, coincubation of D-520 with αSN is able to reduce toxicity of preformed aggregates of αSN compared to control αSN alone. Finally, in a neuroprotection study with dopaminergic MN9D cells, D-520 clearly demonstrated the effect of neuroprotection from toxicity of 6-hydroxydopamine. Thus, compound D-520 possesses properties characteristic of multifunctionality conducive to symptomatic and neuroprotective treatment of PD.

Cu(II) and dopamine bind to α-synuclein and cause large conformational changes

α-Synuclein (AS) is an intrinsically disordered protein that can misfold and aggregate to form Lewy bodies in dopaminergic neurons, a classic hallmark of Parkinson's disease. The binding of Cu(II) and dopamine to AS was evaluated by nanopore analysis with α-hemolysin. In the absence of Cu(II), wild-type AS (1 μM) readily translocated through the pore with a blockade current of--85 pA, but mostly bumping events were observed in the presence of 25 μM Cu(II). A binding site in the N-terminus was confirmed, because Cu(II) had no effect on the event profile of a peptide consisting of the C-terminal 96-140 residues. In the presence of dopamine (25 μM), the translocation events at--85 pA shifted to--80 pA, which also represents translocation events, because the event time decreases with increasing voltage. Events at--80 pA were also observed for the mutant A30P AS in the presence of dopamine. Event profiles for an N-terminal 1-60-residue peptide and a C-terminal 96-140-residue peptide were both altered in the presence of 25 μM dopamine. In contrast, dopamine had little effect on the CD spectrum of AS, and a single binding site with a Ka of 3.5 × 10(3) m(-1) was estimated by isothermal titration calorimetry. Thus, dopamine can interact with both the N-terminus and the C-terminus. Two-dimensional NMR spectroscopy of AS in the presence of dopamine showed that there were significant changes in the spectra in all regions of the protein. According to these findings, a model is presented in which dopamine induces folding between the N-terminus and C-terminus of AS. Partially folding conformations such as this may represent important intermediates in the misfolding of AS that leads to fibrillization.

Pitfalls associated with the use of Thioflavin-T to monitor anti-fibrillogenic activity

Thioflavin-T (ThT) is a cationic benzothiazole dye that displays enhanced fluorescence upon binding to amyloid fibrils. This property makes ThT the current reagent of choice for the quantification of amyloid fibrils. Herein, we investigate the main pitfalls associated with the use of ThT-based assays to monitor the fibrillation of α-synuclein (α-syn), a protein linked to Parkinson's disease and other α-synucleinopathies. We demonstrated for the first time that ThT interacts with α-syn disordered monomer and accelerates the protein fibrillation in vitro. As a consequence, misleading conclusions may arise from the use of ThT-based real-time assays in the evaluation of anti-fibrillogenic compounds. Interestingly, NMR experiments indicated that C-terminal domain of α-syn is the main region perturbed by ThT interaction, similarly to that found for the pesticide paraquat, a well-documented accelerator of α-syn fibrillation. Moreover, we demonstrated that certain potent inhibitors of α-syn fibrillation, such as oxidized catecholamines and polyphenols, undergo spontaneous oxidation in aqueous solution, generating compounds that strongly quench ThT fluorescence. In light of these findings, we alert for possible artifacts associated to the measure of the anti-fibrillogenic activity based only on ThT fluorescence approach.

Structural modifications of neuroprotective anti-Parkinsonian (-)-N6-(2-(4-(biphenyl-4-yl)piperazin-1-yl)-ethyl)-N6-propyl-4,5,6,7-tetrahydrobenzo[d]thiazole-2,6-diamine (D-264): an effort toward the improvement of in vivo efficacy of the parent molecule

In our overall goal to develop multifunctional dopamine D2/D3 agonist drugs for the treatment of Parkinson's disease (PD), we previously synthesized potent D3 preferring agonist D-264 (1a), which exhibited neuroprotective properties in two animal models of PD. To enhance the in vivo efficacy of 1a, a structure-activity relationship study was carried out. Competitive binding and [(35)S]GTPγS functional assays identified compound (-)-9b as one of the lead molecules with preferential D3 agonist activity (EC50(GTPγS); D3 = 0.10 nM; D2/D3 (EC50): 159). Compounds (-)-9b and (-)-8b exhibited high in vivo activity in two PD animal models, reserpinized and 6-hydroxydopamine (OHDA)-induced unilateral lesioned rats. On the other hand, 1a failed to show any in vivo activity in these models unless the compound was dissolved in 5-10% beta-hydroxy propyl cyclodextrin solution. Lead compounds exhibited appreciable radical scavenging activity. In vitro experiments with dopaminergic MN9D cells indicated neuroprotection by both 1a and (-)-9b from toxicity of MPP+.

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.

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.

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.

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.

Dopamine promotes formation and secretion of non-fibrillar alpha-synuclein oligomers

Parkinson's disease (PD) is characterized by selective and progressive degeneration of dopamine (DA)-producing neurons in the substantia nigra pars compacta (SNpc) and by abnormal aggregation of α-synuclein. Previous studies have suggested that DA can interact with α-synuclein, thus modulating the aggregation process of this protein; this interaction may account for the selective vulnerability of DA neurons in patients with PD. However, the relationship between DA and α-synuclein, and the role in progressive degeneration of DA neurons remains elusive. We have shown that in the presence of DA, recombinant human α-synuclein produces non-fibrillar, SDS-resistant oligomers, while β-sheet-rich fibril formation is inhibited. Pharmacologic elevation of the cytoplasmic DA level increased the formation of SDS-resistant oligomers in DA-producing neuronal cells. DA promoted α-synuclein oligomerization in intracellular vesicles, but not in the cytosol. Furthermore, elevation of DA levels increased secretion of α-synuclein oligomers to the extracellular space, but the secretion of monomers was not changed. DA-induced secretion of α-synuclein oligomers may contribute to the progressive loss of the dopaminergic neuronal population and the pronounced neuroinflammation observed in the SNpc in patients with PD.

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.

Converging environmental and genetic pathways in the pathogenesis of Parkinson's disease

As a prototypic neurodegenerative disorder Parkinson's disease (PD) is characterized by the progressive loss of specific neuronal subpopulations leading to a late-onset movement disorder. Based on familial forms of PD, to date a significant number of genes were identified that allowed first insight into the molecular pathogenesis of this common movement disorder. These pathways include impaired protein degradation and subsequent aggregation within neuronal cells and impaired mitochondrial function followed by energy depletion due to oxidative stress leading to cell death. The respective disease models were supported by pathoanatomical and biochemical findings in brains of sporadic PD patients without apparent genetic contribution to pathogenesis. Indeed recent genetic and epidemiological studies hint to a complex interplay of genetic susceptibility factors and environmental risk factors to converge to processes of pathological protein accumulation and mitochondrial damage that trigger neurodegeneration in PD. Therefore large-scale geneticoepidemiological studies combining genetic whole genome approaches with a detailed ascertainment of environmental exposures are expected to provide important clues to decipher the complexity of neurodegeneration of this most frequent neurodegenerative movement disorder.

Synthesis and evaluation of arylquinones as BACE1 inhibitors, β-amyloid peptide aggregation inhibitors, and destabilizers of preformed β-amyloid fibrils

BACE1 activity, inhibition of Aβ aggregation, and disaggregation of preformed Aβ fibrils constitute the three major targets in the development of small-molecule lipophilic new drugs for the treatment of Alzheimer's disease (AD). Quinones are widely distributed among natural products and possess relevant and varied biological activities including antitumor and antibiotic, inhibition of HIV-1 reverse transcriptase, antidiabetic, or COX-inhibition, among others. We report herein the interaction of several arylquinones and their derivatives with the amyloidogenic pathway of the amyloid precursor protein processing. Our studies put forward that these compounds are promising candidates in the development of new drugs which are effective simultaneously towards the three major targets of AD.