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

Rethinking Parkinson's disease: could dopamine reduction therapy have clinical utility?

Following reports of low striatal dopamine content in Parkinson's disease, levodopa was shown to rapidly reverse hypokinesis, establishing the model of disease as one of dopamine deficiency. Dopaminergic therapy became standard of care, yet it failed to reverse the disease, suggesting the understanding of disease was incomplete. The literature suggests the potential for toxicity of dopamine and its metabolites, perhaps more relevant given the recent evidence for elevated cytosolic dopamine levels in the dopaminergic neurons of people with Parkinson's. To understand the relevance of these data, multiple investigations are reviewed that tested dopamine reduction therapy as an alternative to dopaminergic agents. The data from use of an inhibitor of dopamine synthesis in experimental models suggest that such an approach could reverse disease pathology, which suggests that cytosolic dopamine excess is a primary driver of disease. These data support clinical investigation of dopamine reduction therapy for Parkinson's disease. Doing so will determine whether these experimental models are predictive and this treatment strategy is worth pursuing further. If clinical data are positive, it could warrant reconsideration of our disease model and treatment strategies, including a shift from dopaminergic to dopamine reduction treatment of the disease.

Sequestosome-1 (SQSTM1/p62) as a target in dopamine catabolite-mediated cellular dyshomeostasis

Alterations in the dopamine catabolic pathway are known to contribute to the degeneration of nigrostriatal neurons in Parkinson's disease (PD). The progressive cellular buildup of the highly reactive intermediate 3,4-dihydroxyphenylacetaldehye (DOPAL) generates protein cross-linking, oligomerization of the PD-linked αSynuclein (αSyn) and imbalance in protein quality control. In this scenario, the autophagic cargo sequestome-1 (SQSTM1/p62) emerges as a target of DOPAL-dependent oligomerization and accumulation in cytosolic clusters. Although DOPAL-induced oxidative stress and activation of the Nrf2 pathway promote p62 expression, p62 oligomerization rather seems to be a consequence of direct DOPAL modification. DOPAL-induced p62 clusters are positive for ubiquitin and accumulate within lysosomal-related structures, likely affecting the autophagy-lysosomal functionality. Finally, p62 oligomerization and clustering is synergistically augmented by DOPAL-induced αSyn buildup. Hence, the substantial impact on p62 proteostasis caused by DOPAL appears of relevance for dopaminergic neurodegeneration, in which the progressive failure of degradative pathways and the deposition of proteins like αSyn, ubiquitin and p62 in inclusion bodies represent a major trait of PD pathology.

Toxic interactions between dopamine, α-synuclein, monoamine oxidase, and genes in mitochondria of Parkinson's disease

Parkinson's disease is characterized by its distinct pathological features; loss of dopamine neurons in the substantia nigra pars compacta and accumulation of Lewy bodies and Lewy neurites containing modified α-synuclein. Beneficial effects of L-DOPA and dopamine replacement therapy indicate dopamine deficit as one of the main pathogenic factors. Dopamine and its oxidation products are proposed to induce selective vulnerability in dopamine neurons. However, Parkinson's disease is now considered as a generalized disease with dysfunction of several neurotransmitter systems caused by multiple genetic and environmental factors. The pathogenic factors include oxidative stress, mitochondrial dysfunction, α-synuclein accumulation, programmed cell death, impaired proteolytic systems, neuroinflammation, and decline of neurotrophic factors. This paper presents interactions among dopamine, α-synuclein, monoamine oxidase, its inhibitors, and related genes in mitochondria. α-Synuclein inhibits dopamine synthesis and function. Vice versa, dopamine oxidation by monoamine oxidase produces toxic aldehydes, reactive oxygen species, and quinones, which modify α-synuclein, and promote its fibril production and accumulation in mitochondria. Excessive dopamine in experimental models modifies proteins in the mitochondrial electron transport chain and inhibits the function. α-Synuclein and familiar Parkinson's disease-related gene products modify the expression and activity of monoamine oxidase. Type A monoamine oxidase is associated with neuroprotection by an unspecific dose of inhibitors of type B monoamine oxidase, rasagiline and selegiline. Rasagiline and selegiline prevent α-synuclein fibrillization, modulate this toxic collaboration, and exert neuroprotection in experimental studies. Complex interactions between these pathogenic factors play a decisive role in neurodegeneration in PD and should be further defined to develop new therapies for Parkinson's disease.

Levothyroxine attenuates behavioral impairment and improves oxidative stress and histological alteration 3-nitropropionic acid induced experimental Huntington's disease in rats

Huntington's disease (HD) is a neurodegenerative disorder characterized by degeneration of the striatum; it results in oxidative stress and motor deficits. Thyroid hormones regulate oxidative metabolism. In the present study, we evaluated the effect of administration of levothyroxine (LT-4) on neurobehavioral, oxidative stress, and histological changes in a rat model of HD. Forty-eight Wistar male rats were divided into the following six groups (n = 8): Group 1 (control) received physiological saline intraperitoneally (ip). Groups 2 and 3 received L-T4,30 and L-T4100 (μg/kg, ip, respectively) daily for 7 days. Group 4 (HD) received 3-nitropropionic acid (3-NP) (25 mg/kg, ip) daily for 7 days. Groups 5 and 6 received L-T4,30 and L-T4100 (μg/kg, ip, respectively) 30 min after 3-NP (25 mg/kg, ip) injection for the same duration. On the 8th day, behavioral parameters were evaluated with the Rotarod, Narrow beam walk, and Limb withdrawal tests. Oxidative markers such as Malondialdehyde (MDA) and Glutathione (GSH) levels and Superoxide dismutase (SOD) activity, in striatum tissue were measured. Moreover, striatum tissues were analyzed by Hematoxylin-eosin staining for histological alterations. We found that 3-NP administration caused motor incoordination and induced oxidative stress increased but reduced free radical scavenging. Also, increased amounts of lipid peroxides caused striatal damage as shown by histopathological evaluation. Administration of L-T4 led to increased falling time in the Rotarod, but reduced the time taken in Narrow beam walking and Limb withdrawal test. Furthermore, L-T4 increased antioxidant activity, decreased lipid peroxidation and ameliorated 3-NP-induced degeneration in neurons.

Transcriptomic profiling of early synucleinopathy in rats induced with preformed fibrils

Examination of early phases of synucleinopathy when inclusions are present, but long before neurodegeneration occurs, is critical to both understanding disease progression and the development of disease modifying therapies. The rat alpha-synuclein (α-syn) preformed fibril (PFF) model induces synchronized synucleinopathy that recapitulates the pathological features of Parkinson's disease (PD) and can be used to study synucleinopathy progression. In this model, phosphorylated α-syn (pSyn) inclusion-containing neurons and reactive microglia (major histocompatibility complex-II immunoreactive) peak in the substantia nigra pars compacta (SNpc) months before appreciable neurodegeneration. However, it remains unclear which specific genes are driving these phenotypic changes. To identify transcriptional changes associated with early synucleinopathy, we used laser capture microdissection of the SNpc paired with RNA sequencing (RNASeq). Precision collection of the SNpc allowed for the assessment of differential transcript expression in the nigral dopamine neurons and proximal glia. Transcripts upregulated in early synucleinopathy were mainly associated with an immune response, whereas transcripts downregulated were associated with neurotransmission and the dopamine pathway. A subset of 29 transcripts associated with neurotransmission/vesicular release and the dopamine pathway were verified in a separate cohort of males and females to confirm reproducibility. Within this subset, fluorescent in situ hybridization (FISH) was used to localize decreases in the Syt1 and Slc6a3 transcripts to pSyn inclusion-containing neurons. Identification of transcriptional changes in early synucleinopathy provides insight into the molecular mechanisms driving neurodegeneration.

Small Animal Multisubject PET/CT Workflow

Positron emission tomography (PET) is a highly sensitive molecular imaging technique that uses radioactive tracers to map molecular and metabolic processes in living animals. PET can be performed as a stand-alone modality but is often combined with CT to provide for objective anatomical localization of PET signals in a multimodality approach. In order to outline the general approach to evaluating four mice simultaneously by dynamic PET imaging, the use of the aldehyde-targeted radiotracer [18F]NA3BF3 in mouse models of hepatotoxicity will be described. Indeed the production of aldehydes is upregulated in a wide range of disease and injury, making them a suitable biomarker for PET imaging of numerous pathologies.

Distribution, connection and function of ALDH1A1+/TH+ neurons in substantia nigra pars reticulata of mouse

The massive cell death of dopaminergic neurons (DANs) in substantia nigra pars compacta (SNC) is associated with motor diseases, such as Parkinson's disease. Moreover, as a subtype of DANs in SNC, ALDH1A1+ neurons show better resistance to PD related neurotoxin. DANs can also be found in the substantia nigra pars reticulata (SNR), however, whether they are ALDH1A1+ neurons are rarely reported, as well as their projection, function, and reaction in the PD pathology. We studied the distribution of ALDH1A1+ neurons and track their projection by injecting pAAV. We figured out that, in SNR, 87 % neurons are ALDH1A1+/TH+ in ALDH1A1+ cluster averagely, while ALDH1A1+/TH+: TH+ is 52 % averagely. There are two enrichment regions of ALDH1A1+/TH+ neurons at brgma -3.40 mm and brgma -3.70 mm in the SNR of the nTg mice. Nevertheless, in one type of PD-liked mice model, the proportion of ALDH1A1+/TH+: ALDH1A1+ neurons are 98 % averagely, while ALHD1A1+/TH+: TH+ is 57 %. Intriguingly, neuro-tracing discovered that there may be a previously unreported connection between SNR and anterior dorsal thalamus (ADT). The mouse received MPTP stereotactic injection to destroy TH+ neurons in SNR showed depression behavior, indicated the DANs death in SNR may contribute to depression behavior.

Prodromal Parkinson's disease and the catecholaldehyde hypothesis: Insight from olfactory bulb organotypic cultures

Parkinson's disease (PD) is a progressive, neurodegenerative disorder with an increasing incidence, unknown etiology, and is currently incurable. Advances in understanding the pathological mechanisms at a molecular level have been slow, with little attention focused on the early prodromal phase of the disease. Consequently, the development of early-acting disease-modifying therapies has been hindered. The olfactory bulb (OB), the brain region responsible for initial processing of olfactory information, is particularly affected early in PD at both functional and molecular levels but there is little information on how the cells in this region are affected by disease. Organotypic and primary OB cultures were developed and characterized. These platforms were then used to assess the effects of 3,4-dihydroxyphenylacetylaldehyde (DOPAL), a metabolite of dopamine present in increased levels in post-mortem PD tissue and which is thought to contribute to PD pathogenesis. Our findings showed that DOPAL exposure can recapitulate many aspects of PD pathology. Oxidative stress, depolarization of mitochondrial membranes, and neurodegeneration were all induced by DOPAL addition, as were measured transcriptomic changes consistent with those reported in PD clinical studies. These olfactory models of prodromal disease lend credence to the catecholaldehyde hypothesis of PD and provide insight into the mechanisms by which the OB may be involved in disease progression.

The SARS-CoV-2 spike glycoprotein interacts with MAO-B and impairs mitochondrial energetics

SARS-CoV-2 infection is associated with both acute and post-acute neurological symptoms. Emerging evidence suggests that SARS-CoV-2 can alter mitochondrial metabolism, suggesting that changes in brain metabolism may contribute to the development of acute and post-acute neurological complications. Monoamine oxidase B (MAO-B) is a flavoenzyme located on the outer mitochondrial membrane that catalyzes the oxidative deamination of monoamine neurotransmitters. Computational analyses have revealed high similarity between the SARS-CoV-2 spike glycoprotein receptor binding domain on the ACE2 receptor and MAO-B, leading to the hypothesis that SARS-CoV-2 spike glycoprotein may alter neurotransmitter metabolism by interacting with MAO-B. Our results empirically establish that the SARS-CoV-2 spike glycoprotein interacts with MAO-B, leading to increased MAO-B activity in SH-SY5Y neuron-like cells. Common to neurodegenerative disease pathophysiological mechanisms, we also demonstrate that the spike glycoprotein impairs mitochondrial bioenergetics, induces oxidative stress, and perturbs the degradation of depolarized aberrant mitochondria through mitophagy. Our findings also demonstrate that SH-SY5Y neuron-like cells expressing the SARS-CoV-2 spike protein were more susceptible to MPTP-induced necrosis, likely necroptosis. Together, these results reveal novel mechanisms that may contribute to SARS-CoV-2-induced neurodegeneration.

The heterogeneity of Parkinson's disease

The heterogeneity of Parkinson's disease (PD), i.e. the various clinical phenotypes, pathological findings, genetic predispositions and probably also the various implicated pathophysiological pathways pose a major challenge for future research projects and therapeutic trail design. We outline several pathophysiological concepts, pathways and mechanisms, including the presumed roles of α-synuclein misfolding and aggregation, Lewy bodies, oxidative stress, iron and melanin, deficient autophagy processes, insulin and incretin signaling, T-cell autoimmunity, the gut-brain axis and the evidence that microbial (viral) agents may induce molecular hallmarks of neurodegeneration. The hypothesis is discussed, whether PD might indeed be triggered by exogenous (infectious) agents in susceptible individuals upon entry via the olfactory bulb (brain first) or the gut (body-first), which would support the idea that disease mechanisms may change over time. The unresolved heterogeneity of PD may have contributed to the failure of past clinical trials, which attempted to slow the course of PD. We thus conclude that PD patients need personalized therapeutic approaches tailored to specific phenomenological and etiologic subtypes of disease.

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.

DOPAL initiates αSynuclein-dependent impaired proteostasis and degeneration of neuronal projections in Parkinson's disease

Dopamine dyshomeostasis has been acknowledged among the determinants of nigrostriatal neuron degeneration in Parkinson's disease (PD). Several studies in experimental models and postmortem PD patients underlined increasing levels of the dopamine metabolite 3,4-dihydroxyphenylacetaldehyde (DOPAL), which is highly reactive towards proteins. DOPAL has been shown to covalently modify the presynaptic protein αSynuclein (αSyn), whose misfolding and aggregation represent a major trait of PD pathology, triggering αSyn oligomerization in dopaminergic neurons. Here, we demonstrated that DOPAL elicits αSyn accumulation and hampers αSyn clearance in primary neurons. DOPAL-induced αSyn buildup lessens neuronal resilience, compromises synaptic integrity, and overwhelms protein quality control pathways in neurites. The progressive decline of neuronal homeostasis further leads to dopaminergic neuron loss and motor impairment, as showed in in vivo models. Finally, we developed a specific antibody which detected increased DOPAL-modified αSyn in human striatal tissues from idiopathic PD patients, corroborating the translational relevance of αSyn-DOPAL interplay in PD neurodegeneration.

Scavenging neurotoxic aldehydes using lysine carbon dots

Reactive aldehydes generated in cells and tissues are associated with adverse physiological effects. Dihydroxyphenylacetaldehyde (DOPAL), the biogenic aldehyde enzymatically produced from dopamine, is cytotoxic, generates reactive oxygen species, and triggers aggregation of proteins such as α-synuclein implicated in Parkinson's disease. Here, we demonstrate that carbon dots (C-dots) prepared from lysine as the carbonaceous precursor bind DOPAL molecules through interactions between the aldehyde units and amine residues on the C-dot surface. A set of biophysical and in vitro experiments attests to attenuation of the adverse biological activity of DOPAL. In particular, we show that the lysine-C-dots inhibit DOPAL-induced α-synuclein oligomerization and cytotoxicity. This work underlines the potential of lysine-C-dots as an effective therapeutic vehicle for aldehyde scavenging.

C/EBPβ/AEP is age-dependently activated in Parkinson's disease and mediates α-synuclein in the gut and brain

Parkinson's disease (PD) is the most common neurodegenerative motor disorder, and its pathologic hallmarks include extensive dopaminergic neuronal degeneration in the Substantia nigra associated with Lewy bodies, predominantly consisting of phosphorylated and truncated α-Synuclein (α-Syn). Asparagine endopeptidase (AEP) cleaves human α-Syn at N103 residue and promotes its aggregation, contributing to PD pathogenesis. However, how AEP mediates Lewy body pathologies during aging and elicits PD onset remains incompletely understood. Knockout of AEP or C/EBPβ from α-SNCA mice, and their chronic rotenone exposure models were used, and the mechanism of α-Syn from the gut that spread to the brain was observed. Here we report that C/EBPβ/AEP pathway, aggravated by oxidative stress, is age-dependently activated and cleaves α-Syn N103 and regulates Lewy body-like pathologies spreading from the gut into the brain in human α-SNCA transgenic mice. Deletion of C/EBPβ or AEP substantially diminished the oxidative stress, neuro-inflammation, and PD pathologies, attenuating motor dysfunctions in aged α-SNCA mice. Noticeably, PD pathologies initiate in the gut and progressively spread into the brain. Chronic gastric exposure to a low dose of rotenone initiates Lewy body-like pathologies in the gut that propagate into the brain in a C/EBPβ/AEP-dependent manner. Hence, our studies demonstrate that C/EBPβ/AEP pathway is critical for mediating Lewy body pathology progression in PD.

Review on the interactions between dopamine metabolites and α-Synuclein in causing Parkinson's disease

Parkinson's disease (PD) is characterized by an abnormal post-translational modifications (PTM) in amino acid sequence and aggregation of alpha-synuclein (α-Syn) protein. It is generally believed that dopamine (DA) metabolite in dopaminergic (DAergic) neurons promotes the aggregation of toxic α-Syn oligomers and protofibrils, whereas DA inhibits the formation of toxic fibers and even degrades the toxic fibers. Therefore, the study on interaction between DA metabolites and α-Syn oligomers is one of the current hot topics in neuroscience, because this effect may have direct relevance to the selective DAergic neuron loss in PD. Several mechanisms have been reported for DA metabolites induced α-Syn oligomers viz. i) The reactive oxygen species (ROS) released during the auto-oxidation or enzymatic oxidation of DA changes the structure of α-Syn by the oxidation of amino acid residue leading to misfolding, ii) The oxidized DA metabolites directly interact with α-Syn through covalent or non-covalent bonding leading to the formation of oligomers, iii) DA interacts with lipid or autophagy related proteins to decreases the degradation efficiency of α-Syn aggregates. However, there is no clear-cut mechanism proposed for the interaction between DA and α-Syn. However, it is believed that the lysine (Lys) side chain of α-Syn sequence is the initial trigger site for the oligomer formation. Herein, we review different chemical mechanism involved during the interaction of Lys side chain of α-Syn with DA metabolites such as dopamine-o-quinone (DAQ), dopamine-chrome (DAC), dopamine-aldehyde (DOPAL) and neuromelanin. This review also provides the promotive effect of divalent Cu²⁺ ions on DA metabolites induced α-Syn oligomers and its inhibition effect by antioxidant glutathione (GSH).

Synucleinopathies

The α-synucleinopathies include pure autonomic failure, multiple system atrophy, dementia with Lewy bodies, and Parkinson disease. The past two decades have witnessed significant advances in the diagnostic strategies and symptomatic treatment of motor and nonmotor symptoms of the synucleinopathies. This chapter provides an in-depth review of the pathophysiology, pathology, genetic, epidemiology, and clinical and laboratory autonomic features that distinguish the different synucleinopathies with an emphasis on autonomic failure as a common feature. The treatment of the different synucleinopathies is discussed along with the proposal for multidisciplinary, individualized care models that optimally address the various symptoms. There is an urgent need for clinical scientific studies addressing patients at risk of developing synucleinopathies and the investigation of disease mechanisms, biomarkers, potential disease-modifying therapies, and further advancement of symptomatic treatments for motor and nonmotor symptoms.

Quantitative Determination of Endogenous Tetrahydroisoquinolines, Potential Parkinson's Disease Biomarkers, in Mammals

Current diagnostic options for Parkinson's disease are very limited and primarily based on characteristic clinical symptoms. Thus, there are urgent needs for reliable biomarkers that enable us to diagnose the disease in the early stages, differentiate it from other atypical Parkinsonian syndromes, monitor its progression, increase knowledge of its pathogenesis, and improve the development of potent therapies. A promising group of potential biomarkers are endogenous tetrahydroisoquinoline metabolites, which are thought to contribute to the multifactorial etiology of Parkinson's disease. The aim of this critical review is to highlight trends and limitations of available traditional and modern analytical techniques for sample pretreatment (extraction and derivatization procedures) and quantitative determination of tetrahydroisoquinoline derivatives in various types of mammalian fluids and tissues (urine, plasma, cerebrospinal fluid, brain tissue, liver tissue). Particular attention is paid to the most sensitive and specific analytical techniques, involving immunochemistry and gas or liquid chromatography coupled with mass spectrometric, fluorescence, or electrochemical detection. The review also includes a discussion of other relevant agents proposed and tested in Parkinson's disease.

Gastric Enteric Glial Cells: A New Contributor to the Synucleinopathies in the MPTP-Induced Parkinsonism Mouse

Accumulating evidence has shown that Parkinson's disease (PD) is a systemic disease other than a mere central nervous system (CNS) disorder. One of the most important peripheral symptoms is gastrointestinal dysfunction. The enteric nervous system (ENS) is regarded as an essential gateway to the environment. The discovery of the prion-like behavior of α-synuclein makes it possible for the neurodegenerative process to start in the ENS and spread via the gut-brain axis to the CNS. We first confirmed that synucleinopathies existed in the stomachs of chronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)/probenecid (MPTP/p)-induced PD mice, as indicated by the significant increase in abnormal aggregated and nitrated α-synuclein in the TH-positive neurons and enteric glial cells (EGCs) of the gastric myenteric plexus. Next, we attempted to clarify the mechanisms in single MPTP-injected mice. The stomach naturally possesses high monoamine oxidase-B (MAO-B) activity and low superoxide dismutase (SOD) activity, making the stomach susceptible to MPTP-induced oxidative stress, as indicated by the significant increase in reactive oxygen species (ROS) in the stomach and elevated 4-hydroxynonenal (4-HNE) in the EGCs after MPTP exposure for 3 h. Additionally, stomach synucleinopathies appear before those of the nigrostriatal system, as determined by Western blotting 12 h after MPTP injection. Notably, nitrated α-synuclein was considerably increased in the EGCs after 3 h and 12 h of MPTP exposure. Taken together, our work demonstrated that the EGCs could be new contributors to synucleinopathies in the stomach. The early-initiated synucleinopathies might further influence neighboring neurons in the myenteric plexus and the CNS. Our results offer a new experimental clue for interpreting the etiology of PD.

Human Induced Pluripotent Stem Cell Phenotyping and Preclinical Modeling of Familial Parkinson's Disease

Parkinson's disease (PD) is primarily idiopathic and a highly heterogenous neurodegenerative disease with patients experiencing a wide array of motor and non-motor symptoms. A major challenge for understanding susceptibility to PD is to determine the genetic and environmental factors that influence the mechanisms underlying the variations in disease-associated traits. The pathological hallmark of PD is the degeneration of dopaminergic neurons in the substantia nigra pars compacta region of the brain and post-mortem Lewy pathology, which leads to the loss of projecting axons innervating the striatum and to impaired motor and cognitive functions. While the cause of PD is still largely unknown, genome-wide association studies provide evidence that numerous polymorphic variants in various genes contribute to sporadic PD, and 10 to 15% of all cases are linked to some form of hereditary mutations, either autosomal dominant or recessive. Among the most common mutations observed in PD patients are in the genes LRRK2, SNCA, GBA1, PINK1, PRKN, and PARK7/DJ-1. In this review, we cover these PD-related mutations, the use of induced pluripotent stem cells as a disease in a dish model, and genetic animal models to better understand the diversity in the pathogenesis and long-term outcomes seen in PD patients.

Role of Protein Damage Inflicted by Dopamine Metabolites in Parkinson's Disease: Evidence, Tools, and Outlook

Dopamine is an important neurotransmitter that plays a critical role in motivational salience and motor coordination. However, dysregulated dopamine metabolism can result in the formation of reactive electrophilic metabolites which generate covalent adducts with proteins. Such protein damage can impair native protein function and lead to neurotoxicity, ultimately contributing to Parkinson's disease etiology. In this Review, the role of dopamine-induced protein damage in Parkinson's disease is discussed, highlighting the novel chemical tools utilized to drive this effort forward. Continued innovation of methodologies which enable detection, quantification, and functional response elucidation of dopamine-derived protein adducts is critical for advancing this field. Work in this area improves foundational knowledge of the molecular mechanisms that contribute to dopamine-mediated Parkinson's disease progression, potentially assisting with future development of therapeutic interventions.

Modeling the Progression of Cardiac Catecholamine Deficiency in Lewy Body Diseases

Background Lewy body diseases (LBDs) feature deficiency of the sympathetic neurotransmitter norepinephrine in the left ventricular myocardium and sympathetic intra-neuronal deposition of the protein alpha-synuclein (αS). LBDs therefore are autonomic synucleinopathies. Computational modeling has revealed multiple functional abnormalities in residual myocardial sympathetic noradrenergic nerves in LBDs, including decreased norepinephrine synthesis, vesicular storage, and recycling. We report an extended model that enables predictions about the progression of LBDs and effects of genetic predispositions and treatments on that progression. Methods and Results The model combines cardiac sympathetic activation with autotoxicity mediated by the dopamine metabolite 3,4-dihydroxyphenylacetaldehyde. We tested the model by its ability to predict longitudinal empirical data based on cardiac sympathetic neuroimaging, effects of genetic variations related to particular intra-neuronal reactions, treatment by monoamine oxidase inhibition to decrease 3,4-dihydroxyphenylacetaldehyde production, and post-mortem myocardial tissue contents of catecholamines and αS. The new model generated a triphasic decline in myocardial norepinephrine content. This pattern was confirmed by empirical data from serial cardiac ¹⁸F-dopamine positron emission tomographic scanning in patients with LBDs. The model also correctly predicted empirical data about effects of genetic variants and monoamine oxidase inhibition and about myocardial levels of catecholamines and αS. Conclusions The present computational model predicts a triphasic decline in myocardial norepinephrine content as LBDs progress. According to the model, disease-modifying interventions begun at the transition from the first to the second phase delay the onset of symptomatic disease. Computational modeling coupled with biomarkers of preclinical autonomic synucleinopathy may enable early detection and more effective treatment of LBDs.

Aldehyde Dehydrogenase 2 as a Therapeutic Target in Oxidative Stress-Related Diseases: Post-Translational Modifications Deserve More Attention

Aldehyde dehydrogenase 2 (ALDH2) has both dehydrogenase and esterase activity; its dehydrogenase activity is closely related to the metabolism of aldehydes produced under oxidative stress (OS). In this review, we recapitulate the enzyme activity of ALDH2 in combination with its protein structure, summarize and show the main mechanisms of ALDH2 participating in metabolism of aldehydes in vivo as comprehensively as possible; we also integrate the key regulatory mechanisms of ALDH2 participating in a variety of physiological and pathological processes related to OS, including tissue and organ fibrosis, apoptosis, aging, and nerve injury-related diseases. On this basis, the regulatory effects and application prospects of activators, inhibitors, and protein post-translational modifications (PTMs, such as phosphorylation, acetylation, S-nitrosylation, nitration, ubiquitination, and glycosylation) on ALDH2 are discussed and prospected. Herein, we aimed to lay a foundation for further research into the mechanism of ALDH2 in oxidative stress-related disease and provide a basis for better use of the ALDH2 function in research and the clinic.

The rat rotenone model reproduces the abnormal pattern of central catecholamine metabolism found in Parkinson's disease

Recent reports indicate that Parkinson's disease (PD) involves specific functional abnormalities in residual neurons – decreased vesicular sequestration of cytoplasmic catecholamines via the vesicular monoamine transporter (VMAT) and decreased aldehyde dehydrogenase (ALDH) activity. This double hit builds up the autotoxic metabolite 3,4-dihydroxyphenylacetaldehyde (DOPAL), the focus of the catecholaldehyde hypothesis for the pathogenesis of PD. An animal model is needed that reproduces this abnormal catecholamine neurochemical pattern. Adult rats received subcutaneous vehicle or the mitochondrial complex 1 inhibitor rotenone (2 mg/kg/day via a minipump) for 10 days. Locomotor activity was recorded, and striatal tissue sampled for catechol contents and catechol ratios that indicate the above abnormalities. Compared to vehicle, rotenone reduced locomotor activity (P=0.002), decreased tissue dopamine concentrations (P=0.00001), reduced indices of vesicular sequestration (3,4-dihydroxyphenylacetic acid (DOPAC)/dopamine) and ALDH activity (DOPAC/DOPAL) (P=0.0025, P=0.036), and increased DOPAL levels (P=0.04). The rat rotenone model involves functional abnormalities in catecholaminergic neurons that replicate the pattern found in PD putamen. These include a vesicular storage defect, decreased ALDH activity and DOPAL build-up. The rat rotenone model provides a suitable in vivo platform for studying the catecholaldehyde hypothesis.

Summary: This study presents an animal model that reflects the neurochemical pattern found in Parkinson's patients, the basis of the new and evolving catecholaldehyde hypothesis for the disease.

The essential role of transcription factor Pitx3 in preventing mesodiencephalic dopaminergic neurodegeneration and maintaining neuronal subtype identities during aging

Pituitary homeobox 3 (Pitx3) is required for the terminal differentiation of nigrostriatal dopaminergic neurons during neuronal development. However, whether Pitx3 contributes to the normal physiological function and cell-type identity of adult neurons remains unknown. To explore the role of Pitx3 in maintaining mature neurons, we selectively deleted Pitx3 in the mesodiencephalic dopaminergic (mdDA) neurons of Pitx3fl/fl/DATCreERT2 bigenic mice using a tamoxifen inducible CreERT2/loxp gene-targeting system. Pitx3fl/fl/DATCreERT2 mice developed age-dependent progressive motor deficits, concomitant with a rapid reduction of striatal dopamine (DA) content and a profound loss of mdDA neurons in the substantia nigra pars compacta (SNc) but not in the adjacent ventral tegmental area (VTA), recapitulating the canonical neuropathological features of Parkinson's disease (PD). Mechanistic studies showed that Pitx3-deficiency significantly increased the number of cleaved caspase-3+ cells in SNc, which likely underwent neurodegeneration. Meanwhile, the vulnerability of SNc mdDA neurons was increased in Pitx3fl/fl/DATCreERT2 mice, as indicated by an early decline in glial cell line-derived neurotrophic factor (GDNF) and aldehyde dehydrogenase 1a1 (Aldh1a1) levels. Noticeably, somatic accumulation of α-synuclein (α-syn) was also significantly increased in the Pitx3-deficient neurons. Together, our data demonstrate that the loss of Pitx3 in fully differentiated mdDA neurons results in progressive neurodegeneration, indicating the importance of the Pitx3 gene in adult neuronal survival. Our findings also suggest that distinct Pitx3-dependent pathways exist in SNc and VTA mdDA neurons, correlating with the differential vulnerability of SNc and VTA mdDA neurons in the absence of Pitx3.

Parkinson Disease: Translating Insights from Molecular Mechanisms to Neuroprotection

Parkinson disease (PD) used to be considered a nongenetic condition. However, the identification of several autosomal dominant and recessive mutations linked to monogenic PD has changed this view. Clinically manifest PD is then thought to occur through a complex interplay between genetic mutations, many of which have incomplete penetrance, and environmental factors, both neuroprotective and increasing susceptibility, which variably interact to reach a threshold over which PD becomes clinically manifested. Functional studies of PD gene products have identified many cellular and molecular pathways, providing crucial insights into the nature and causes of PD. PD originates from multiple causes and a range of pathogenic processes at play, ultimately culminating in nigral dopaminergic loss and motor dysfunction. An in-depth understanding of these complex and possibly convergent pathways will pave the way for therapeutic approaches to alleviate the disease symptoms and neuroprotective strategies to prevent disease manifestations. This review is aimed at providing a comprehensive understanding of advances made in PD research based on leveraging genetic insights into the pathogenesis of PD. It further discusses novel perspectives to facilitate identification of critical molecular pathways that are central to neurodegeneration that hold the potential to develop neuroprotective and/or neurorestorative therapeutic strategies for PD. SIGNIFICANCE STATEMENT: A comprehensive review of PD pathophysiology is provided on the complex interplay of genetic and environmental factors and biologic processes that contribute to PD pathogenesis. This knowledge identifies new targets that could be leveraged into disease-modifying therapies to prevent or slow neurodegeneration in PD.

Diverse midbrain dopaminergic neuron subtypes and implications for complex clinical symptoms of Parkinson's disease

Parkinson’s disease (PD), the most common degenerative movement disorder, is clinically manifested with various motor and non-motor symptoms. Degeneration of midbrain substantia nigra pas compacta (SNc) dopaminergic neurons (DANs) is generally attributed to the motor syndrome. The underlying neuronal mechanisms of non-motor syndrome are largely unexplored. Besides SNc, midbrain ventral tegmental area (VTA) DANs also produce and release dopamine and modulate movement, reward, motivation, and memory. Degeneration of VTA DANs also occurs in postmortem brains of PD patients, implying an involvement of VTA DANs in PD-associated non-motor symptoms. However, it remains to be established that there is a distinct segregation of different SNc and VTA DAN subtypes in regulating different motor and non-motor functions, and that different DAN subpopulations are differentially affected by normal ageing or PD. Traditionally, the distinction among different DAN subtypes was mainly based on the location of cell bodies and axon terminals. With the recent advance of single cell RNA sequencing technology, DANs can be readily classified based on unique gene expression profiles. A combination of specific anatomic and molecular markers shows great promise to facilitate the identification of DAN subpopulations corresponding to different behavior modules under normal and disease conditions. In this review, we first summarize the recent progress in characterizing genetically, anatomically, and functionally diverse midbrain DAN subtypes. Then, we provide perspectives on how the preclinical research on the connectivity and functionality of DAN subpopulations improves our current understanding of cell-type and circuit specific mechanisms of the disease, which could be critically informative for designing new mechanistic treatments.

Toxic Feedback Loop Involving Iron, Reactive Oxygen Species, α-Synuclein and Neuromelanin in Parkinson's Disease and Intervention with Turmeric

Parkinson's disease (PD) is a movement disorder associated with severe loss of mainly dopaminergic neurons in the substantia nigra. Pathological hallmarks include Lewy bodies, and loss of neuromelanin, due to degeneration of neuromelanin-containing dopaminergic neurons. Despite being described over 200 years ago, the etiology of PD remains unknown. Here, we highlight the roles of reactive oxygen species (ROS), iron, alpha synuclein (α-syn) and neuromelanin in a toxic feedback loop culminating in neuronal death and spread of the disease. Dopaminergic neurons are particularly vulnerable due to decreased antioxidant concentration with aging, constant exposure to ROS and presence of neurotoxic compounds (e.g. ortho-quinones). ROS and iron increase each other's levels, creating a state of oxidative stress. α-Syn aggregation is influenced by ROS and iron but also increases ROS and iron via its induced mitochondrial dysfunction and ferric-reductase activity. Neuromelanin's binding affinity is affected by increased ROS and iron. Furthermore, during neuronal death, neuromelanin is degraded in the extracellular space, releasing its bound toxins. This cycle of events continues to neighboring neurons in the form of a toxic loop, causing PD pathology. The increase in ROS and iron may be an important target for therapies to disrupt this toxic loop, and therefore diets rich in certain 'nutraceuticals' may be beneficial. Turmeric is an attractive candidate, as it is known to have anti-oxidant and iron chelating properties. More studies are needed to test this theory and if validated, this would be a step towards development of lifestyle-based therapeutic modalities to complement existing PD treatments.

Enhanced Catecholamine Flux and Impaired Carbonyl Metabolism Disrupt Cardiac Mitochondrial Oxidative Phosphorylation in Diabetes Patients

Aims: Catecholamine metabolism via monoamine oxidase (MAO) contributes to cardiac injury in models of ischemia and diabetes, but the pathogenic mechanisms involved are unclear. MAO deaminates norepinephrine (NE) and dopamine to produce H₂O₂ and highly reactive "catecholaldehydes," which may be toxic to mitochondria due to the localization of MAO to the outer mitochondrial membrane. We performed a comprehensive analysis of catecholamine metabolism and its impact on mitochondrial energetics in atrial myocardium obtained from patients with and without type 2 diabetes. Results: Content and maximal activity of MAO-A and MAO-B were higher in the myocardium of patients with diabetes and they were associated with body mass index. Metabolomic analysis of atrial tissue from these patients showed decreased catecholamine levels in the myocardium, supporting an increased flux through MAOs. Catecholaldehyde-modified protein adducts were more abundant in myocardial tissue extracts from patients with diabetes and were confirmed to be MAO dependent. NE treatment suppressed mitochondrial ATP production in permeabilized myofibers from patients with diabetes in an MAO-dependent manner. Aldehyde dehydrogenase (ALDH) activity was substantially decreased in atrial myocardium from these patients, and metabolomics confirmed lower levels of ALDH-catalyzed catecholamine metabolites. Proteomic analysis of catechol-modified proteins in isolated cardiac mitochondria from these patients identified >300 mitochondrial proteins to be potential targets of these unique carbonyls. Innovation and Conclusion: These findings illustrate a unique form of carbonyl toxicity driven by MAO-mediated metabolism of catecholamines, and they reveal pathogenic factors underlying cardiometabolic disease. Importantly, they suggest that pharmacotherapies targeting aldehyde stress and catecholamine metabolism in heart may be beneficial in patients with diabetes and cardiac disease. Antioxid. Redox Signal. 35, 235-251.

Monoamine Oxidase-B Inhibition Facilitates α-Synuclein Secretion In Vitro and Delays Its Aggregation in rAAV-Based Rat Models of Parkinson's Disease

Cell-to-cell transmission of α-synuclein (α-syn) pathology is considered to underlie the spread of neurodegeneration in Parkinson's disease (PD). Previous studies have demonstrated that α-syn is secreted under physiological conditions in neuronal cell lines and primary neurons. However, the molecular mechanisms that regulate extracellular α-syn secretion remain unclear. In this study, we found that inhibition of monoamine oxidase-B (MAO-B) enzymatic activity facilitated α-syn secretion in human neuroblastoma SH-SY5Y cells. Both inhibition of MAO-B by selegiline or rasagiline and siRNA-mediated knock-down of MAO-B facilitated α-syn secretion. However, TVP-1022, the S-isomer of rasagiline that is 1000 times less active, failed to facilitate α-syn secretion. Additionally, the MAO-B inhibition-induced increase in α-syn secretion was unaffected by brefeldin A, which inhibits endoplasmic reticulum (ER)/Golgi transport, but was blocked by probenecid and glyburide, which inhibit ATP-binding cassette (ABC) transporter function. MAO-B inhibition preferentially facilitated the secretion of detergent-insoluble α-syn protein and decreased its intracellular accumulation under chloroquine-induced lysosomal dysfunction. Moreover, in a rat model (male Sprague Dawley rats) generated by injecting recombinant adeno-associated virus (rAAV)-A53T α-syn, subcutaneous administration of selegiline delayed the striatal formation of Ser129-phosphorylated α-syn aggregates, and mitigated loss of nigrostriatal dopaminergic neurons. Selegiline also delayed α-syn aggregation and dopaminergic neuronal loss in a cell-to-cell transmission rat model (male Sprague Dawley rats) generated by injecting rAAV-wild-type α-syn and externally inoculating α-syn fibrils into the striatum. These findings suggest that MAO-B inhibition modulates the intracellular clearance of detergent-insoluble α-syn via the ABC transporter-mediated non-classical secretion pathway, and temporarily suppresses the formation and transmission of α-syn aggregates.SIGNIFICANCE STATEMENT The identification of a neuroprotective agent that slows or stops the progression of motor impairments is required to treat Parkinson's disease (PD). The process of α-synuclein (α-syn) aggregation is thought to underlie neurodegeneration in PD. Here, we demonstrated that pharmacological inhibition or knock-down of monoamine oxidase-B (MAO-B) in SH-SY5Y cells facilitated α-syn secretion via a non-classical pathway involving an ATP-binding cassette (ABC) transporter. MAO-B inhibition preferentially facilitated secretion of detergent-insoluble α-syn protein and reduced its intracellular accumulation under chloroquine-induced lysosomal dysfunction. Additionally, MAO-B inhibition by selegiline protected A53T α-syn-induced nigrostriatal dopaminergic neuronal loss and suppressed the formation and cell-to-cell transmission of α-syn aggregates in rat models. We therefore propose a new function of MAO-B inhibition that modulates α-syn secretion and aggregation.

Too much for your own good: Excessive dopamine damages neurons and contributes to Parkinson's disease: An Editorial Highlight for "Enhanced tyrosine hydroxylase activity induces oxidative stress, causes accumulation of autotoxic catecholamine metabolites, and augments amphetamine effects in vivo"

Dopamine dyshomeostasis is a driving factor of nigrostriatal degeneration in Parkinson's disease (PD). Accumulation of cytosolic dopamine at striatal projections results in the buildup of autoxidation products, which generates protein adducts and exacerbate oxidative stress. Moreover, an excessive rate of dopamine degradation results in accumulation of 3,4-dihydroxyphenylacetaldehyde (DOPAL), a toxic metabolite which rapidly reacts with other proteins. These events lead to protein misfolding and cross-linking as well as mitochondrial and lysosomal dysfunction, the main pathological mechanisms underscoring dopaminergic neuron loss in PD. In this issue of Journal of Neurochemistry, Vecchio et al. generated and characterized a new in vivo model of chronic dopamine accumulation through the overexpression of a hyperactive form of tyrosine hydroxylase (TH-HI), the rate-limiting step enzyme in dopamine biosynthesis. At 3-5 months of age, TH-HI mice displayed increased striatal dopamine content, exacerbated dopamine catabolism, and augmented responses to amphetamine. This correlated with enhanced oxidative stress and DOPAL buildup, highlighting a catechol-induced neurotoxic vicious cycle that may anticipate a parkinsonian-like phenotype in aged mice. This novel TH-HI animal model represents an exciting new tool to unravel the molecular mechanisms underlying dopamine disequilibrium, catecholamine autotoxicity, and neurodegeneration in PD.

Enhanced tyrosine hydroxylase activity induces oxidative stress, causes accumulation of autotoxic catecholamine metabolites, and augments amphetamine effects in vivo

In Parkinson's disease, dopamine‐containing nigrostriatal neurons undergo profound degeneration. Tyrosine hydroxylase (TH) is the rate‐limiting enzyme in dopamine biosynthesis. TH increases in vitro formation of reactive oxygen species, and previous animal studies have reported links between cytosolic dopamine build‐up and oxidative stress. To examine effects of increased TH activity in catecholaminergic neurons in vivo, we generated TH‐over‐expressing mice (TH‐HI) using a BAC‐transgenic approach that results in over‐expression of TH with endogenous patterns of expression. The transgenic mice were characterized by western blot, qPCR, and immunohistochemistry. Tissue contents of dopamine, its metabolites, and markers of oxidative stress were evaluated. TH‐HI mice had a 3‐fold increase in total and phosphorylated TH levels and an increased rate of dopamine synthesis. Coincident with elevated dopamine turnover, TH‐HI mice showed increased striatal production of H₂O₂ and reduced glutathione levels. In addition, TH‐HI mice had elevated striatal levels of the neurotoxic dopamine metabolites 3,4‐dihydroxyphenylacetaldehyde and 5‐S‐cysteinyl‐dopamine and were more susceptible than wild‐type mice to the effects of amphetamine and methamphetamine. These results demonstrate that increased TH alone is sufficient to produce oxidative stress in vivo, build up autotoxic dopamine metabolites, and augment toxicity.

Structural Features and Toxicity of α-Synuclein Oligomers Grown in the Presence of DOPAC

The interplay between α-synuclein and dopamine derivatives is associated with oxidative stress-dependent neurodegeneration in Parkinson’s disease (PD). The formation in the dopaminergic neurons of intraneuronal inclusions containing aggregates of α-synuclein is a typical hallmark of PD. Even though the biochemical events underlying the aberrant aggregation of α-synuclein are not completely understood, strong evidence correlates this process with the levels of dopamine metabolites. In vitro, 3,4-dihydroxyphenylacetaldehyde (DOPAL) and the other two metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylethanol (DOPET), share the property to inhibit the growth of mature amyloid fibrils of α-synuclein. Although this effect occurs with the formation of differently toxic products, the molecular basis of this inhibition is still unclear. Here, we provide information on the effect of DOPAC on the aggregation properties of α-synuclein and its ability to interact with membranes. DOPAC inhibits α-synuclein aggregation, stabilizing monomer and inducing the formation of dimers and trimers. DOPAC-induced oligomers did not undergo conformational transition in the presence of membranes, and penetrated the cell, where they triggered autophagic processes. Cellular assays showed that DOPAC reduced cytotoxicity and ROS production induced by α-synuclein aggregates. Our findings show that the early radicals resulting from DOPAC autoxidation produced covalent modifications of the protein, which were not by themselves a primary cause of either fibrillation or membrane binding inhibition. These findings are discussed in the light of the potential mechanism of DOPAC protection against the toxicity of α-synuclein aggregates to better understand protein and catecholamine biology and to eventually suggest a scaffold that can help in the design of candidate molecules able to interfere in α-synuclein aggregation.

The Catecholaldehyde Hypothesis for the Pathogenesis of Catecholaminergic Neurodegeneration: What We Know and What We Do Not Know

3,4-Dihydroxyphenylacetaldehyde (DOPAL) is the focus of the catecholaldehyde hypothesis for the pathogenesis of Parkinson’s disease and other Lewy body diseases. The catecholaldehyde is produced via oxidative deamination catalyzed by monoamine oxidase (MAO) acting on cytoplasmic dopamine. DOPAL is autotoxic, in that it can harm the same cells in which it is produced. Normally, DOPAL is detoxified by aldehyde dehydrogenase (ALDH)-mediated conversion to 3,4-dihydroxyphenylacetic acid (DOPAC), which rapidly exits the neurons. Genetic, environmental, or drug-induced manipulations of ALDH that build up DOPAL promote catecholaminergic neurodegeneration. A concept derived from the catecholaldehyde hypothesis imputes deleterious interactions between DOPAL and the protein alpha-synuclein (αS), a major component of Lewy bodies. DOPAL potently oligomerizes αS, and αS oligomers impede vesicular and mitochondrial functions, shifting the fate of cytoplasmic dopamine toward the MAO-catalyzed formation of DOPAL—destabilizing vicious cycles. Direct and indirect effects of DOPAL and of DOPAL-induced misfolded proteins could “freeze” intraneuronal reactions, plasticity of which is required for neuronal homeostasis. The extent to which DOPAL toxicity is mediated by interactions with αS, and vice versa, is poorly understood. Because of numerous secondary effects such as augmented spontaneous oxidation of dopamine by MAO inhibition, there has been insufficient testing of the catecholaldehyde hypothesis in animal models. The clinical pathophysiological significance of genetics, emotional stress, environmental agents, and interactions with numerous proteins relevant to the catecholaldehyde hypothesis are matters for future research. The imposing complexity of intraneuronal catecholamine metabolism seems to require a computational modeling approach to elucidate clinical pathogenetic mechanisms and devise pathophysiology-based, individualized treatments.

[The role of aluminum and lead in the development of Alzheimer's and Parkinson's diseases]

The article summarizes the data available in the literature on the toxic effects of aluminum and lead on the human brain and assesses the relationship of these effects to the etiopathogenesis of the most common neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases. The accumulation of ions of these metals in the brain structures leads to chronic intoxication that is manifested by the morphological signs that are typical for Alzheimer's disease, such as deposits of β-amyloid and τ-protein mainly in the frontal and temporal regions of the cortex, and for Parkinson's disease, such as degeneration of dopamine neurons in the substantia nigra and their accumulation of α-synuclein. The most likely forms of participation of aluminum and lead ions in the mechanisms of neurodegeneration are the replacement of bivalent metal ions necessary for brain functioning, oxidative stress initiation, epigenetic modifications of histones, and increased expression of noncoding ribonucleic acids.

A model of dopamine and serotonin-kynurenine metabolism in cortisolemia: Implications for depression

A major factor contributing to the etiology of depression is a neurochemical imbalance of the dopaminergic and serotonergic systems, which is caused by persistently high levels of circulating stress hormones. Here, a computational model is proposed to investigate the interplay between dopaminergic and serotonergic-kynurenine metabolism under cortisolemia and its consequences for the onset of depression. The model was formulated as a set of nonlinear ordinary differential equations represented with power-law functions. Parameter values were obtained from experimental data reported in the literature, biological databases, and other general information, and subsequently fine-tuned through optimization. Model simulations predict that changes in the kynurenine pathway, caused by elevated levels of cortisol, can increase the risk of neurotoxicity and lead to increased levels of 3,4-dihydroxyphenylaceltahyde (DOPAL) and 5-hydroxyindoleacetaldehyde (5-HIAL). These aldehydes contribute to alpha-synuclein aggregation and may cause mitochondrial fragmentation. Further model analysis demonstrated that the inhibition of both serotonin transport and kynurenine-3-monooxygenase decreased the levels of DOPAL and 5-HIAL and the neurotoxic risk often associated with depression. The mathematical model was also able to predict a novel role of the dopamine and serotonin metabolites DOPAL and 5-HIAL in the ethiology of depression, which is facilitated through increased cortisol levels. Finally, the model analysis suggests treatment with a combination of inhibitors of serotonin transport and kynurenine-3-monooxygenase as a potentially effective pharmacological strategy to revert the slow-down in monoamine neurotransmission that is often triggered by inflammation.

ALDH1A1 Genetic Variations May Modulate Risk of Parkinson's Disease in Han Chinese Population

Background: Studies in animal models have suggested that aldehyde dehydrogenase 1 (encoded by ALDH1A1) protects against Parkinson’s disease (PD) by reducing toxic metabolites of dopamine. Herein we aimed to investigate whether ALDH1A1 was genetically associated with PD susceptibility in humans.

Methods: A Han Chinese population of 1,039 subjects was recruited to analyze six tag-single nucleotide polymorphisms (SNPs), followed by haplotype analyses and variants interaction analyses. Real-time PCR was used to analyze mRNA levels of ALDH1A1 in peripheral blood of 42 subjects.

Results: The tag-SNP rs7043217 of ALDH1A1 was significantly associated with PD susceptibility with the T serving as a risk allele (genotype frequency, P = 0.030; allele frequency, P = 0.013, OR = 1.258, 95% CI = 1.050–1.508). Multiple haplotypes were linked to abnormalities of PD risk, topped by a 4-SNP GGTA module in the order of rs4646547, rs1888202, rs7043217, and rs647880 (P = 9.610 × 10^–8, OR = 6.420, 95% CI = 2.944–13.998). Interaction analyses showed that a simultaneous presence of the CC genotype of rs7043217 and the TT genotype of ALDH2 variant rs4767944 conferred an elevated protection against PD (P = 4.68 × 10^–4, OR = 0.378, 95% CI = 0.219–0.652). The mRNA expression of ALDH1A1 showed a trend of reduction (P = 0.084) in PD patients compared to the controls.

Conclusion: Our results provide novel genetic insights into the role of ALDH1 in PD pathogenesis.

C/EBPβ/δ-secretase signaling mediates Parkinson's disease pathogenesis via regulating transcription and proteolytic cleavage of α-synuclein and MAOB

Parkinson's disease (PD) is characterized by dopaminergic neuronal loss and the presence of intra-neuronal Lewy body (LB) inclusions with aggregated α-synuclein (α-Syn) as the major component. MAOB, a crucial monoamine oxidase for dopamine metabolism, triggers oxidative stress in dopaminergic neurons and α-Syn aggregation. However, the key molecular mechanism that mediates PD pathogenesis remains elusive. Here we show that C/EBPβ acts as an age-dependent transcription factor for both α-Syn and MAOB, and initiates the PD pathologies by upregulating these two pivotal players, in addition to escalating δ-secretase activity to cleave α-Syn and promotes its neurotoxicity. Overexpression of C/EBPβ in human wild-type α-Syn transgenic mice facilitates PD pathologies and elicits motor disorders associated with augmentation of δ-secretase, α-Syn, and MAOB. In contrast, depletion of C/EBPβ from human α-Syn Tg mice abolishes rotenone-elicited PD pathologies and motor impairments via downregulating the expression of these key factors. Hence, our study supports that C/EBPβ/δ-secretase signaling mediates PD pathogenesis via regulating the expression and cleavage of α-Syn and MAOB.

A dopamine metabolite stabilizes neurotoxic amyloid-β oligomers

Aberrant soluble oligomers formed by the amyloid-β peptide (Aβ) are major pathogenic agents in the onset and progression of Alzheimer's disease. A variety of biomolecules can influence the formation of these oligomers in the brain, although their mechanisms of action are still largely unknown. Here, we studied the effects on Aβ aggregation of DOPAL, a reactive catecholaldehyde intermediate of dopamine metabolism. We found that DOPAL is able to stabilize Aβ oligomeric species, including dimers and trimers, that exert toxic effects on human neuroblastoma cells, in particular increasing cytosolic calcium levels and promoting the generation of reactive oxygen species. These results reveal an interplay between Aβ aggregation and key biochemical processes regulating cellular homeostasis in the brain.

Role and Mechanism of Vitamin A Metabolism in the Pathophysiology of Parkinson's Disease

Evidence shows that altered retinoic acid signaling may contribute to the pathogenesis and pathophysiology of Parkinson's disease (PD). Retinoic acid is the bioactive derivative of the lipophilic vitamin A. Vitamin A is involved in several important homeostatic processes, such as cell differentiation, antioxidant activity, inflammation and neuronal plasticity. The role of vitamin A and its derivatives in the pathogenesis and pathophysiology of neurodegenerative diseases, and their potential as therapeutics, has drawn attention for more than 10 years. However, the literature sits in disparate fields. Vitamin A could act at the crossroad of multiple environmental and genetic factors of PD. The purpose of this review is to outline what is known about the role of vitamin A metabolism in the pathogenesis and pathophysiology of PD. We examine key biological systems and mechanisms that are under the control of vitamin A and its derivatives, which are (or could be) exploited for therapeutic potential in PD: the survival of dopaminergic neurons, oxidative stress, neuroinflammation, circadian rhythms, homeostasis of the enteric nervous system, and hormonal systems. We focus on the pivotal role of ALDH1A1, an enzyme expressed by dopaminergic neurons for the detoxification of these neurons, which is under the control of retinoic acid. By providing an integrated summary, this review will guide future studies on the potential role of vitamin A in the management of symptoms, health and wellbeing for PD patients.

The Convergence of Alpha-Synuclein, Mitochondrial, and Lysosomal Pathways in Vulnerability of Midbrain Dopaminergic Neurons in Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disease, characterized by progressive bradykinesia, rigidity, resting tremor, and gait impairment, as well as a spectrum of non-motor symptoms including autonomic and cognitive dysfunction. The cardinal motor symptoms of PD stem from the loss of substantia nigra (SN) dopaminergic (DAergic) neurons, and it remains unclear why SN DAergic neurons are preferentially lost in PD. However, recent identification of several genetic PD forms suggests that mitochondrial and lysosomal dysfunctions play important roles in the degeneration of midbrain dopamine (DA) neurons. In this review, we discuss the interplay of cell-autonomous mechanisms linked to DAergic neuron vulnerability and alpha-synuclein homeostasis. Emerging studies highlight a deleterious feedback cycle, with oxidative stress, altered DA metabolism, dysfunctional lysosomes, and pathological alpha-synuclein species representing key events in the pathogenesis of PD. We also discuss the interactions of alpha-synuclein with toxic DA metabolites, as well as the biochemical links between intracellular iron, calcium, and alpha-synuclein accumulation. We suggest that targeting multiple pathways, rather than individual processes, will be important for developing disease-modifying therapies. In this context, we focus on current translational efforts specifically targeting lysosomal function, as well as oxidative stress via calcium and iron modulation. These efforts could have therapeutic benefits for the broader population of sporadic PD and related synucleinopathies.

In Vitro Protective Action of Monomeric and Fibrillar α-Synuclein on Neuronal Cells Exposed to the Dopaminergic Toxins Salsolinol and DOPAL

The aggregation of α-synuclein (aSyn) is believed to be mechanistically linked to the degeneration of dopamine (DA)-producing neurons in Parkinson's disease (PD). In this respect, one crucial question that yet remains unsolved is whether aSyn aggregation is associated with either a gain- or loss-of-function of the protein in neuronal cells. Herein, we investigated the effect of monomeric versus fibrillar aSyn on mesencephalic dopaminergic neurons in primary cultures challenged with the neurotoxic catechols: salsolinol (SALSO; 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline) and 3,4-dihydroxyphenylacetaldehyde (DOPAL). aSyn monomer protected cells against either SALSO- or DOPAL-induced toxicity via inhibition of caspase-3-mediated apoptosis. While fibrillar aSyn failed in attenuating SALSO neurotoxicity, it increased the viability of DOPAL-treated cells, which was apparently not associated with the inhibition of caspase-3 cleavage. The fact that DOPAL-derived aSyn adducts exhibit lower toxicity compared with DOPAL itself raises the question of whether the generation of these adducts could be part of or a collateral effect of aSyn-mediated protection in neurons exposed to DOPAL. Overall, our work provides important evidence on the impact of the fibrillation of aSyn on its protective role in neuronal cells exposed to the toxic catechols SALSO and DOPAL.

The "Sick-but-not-Dead" Phenomenon Applied to Catecholamine Deficiency in Neurodegenerative Diseases

The catecholamines dopamine and norepinephrine are key central neurotransmitters that participate in many neurobehavioral processes and disease states. Norepinephrine is also the main neurotransmitter mediating regulation of the circulation by the sympathetic nervous system. Several neurodegenerative disorders feature catecholamine deficiency. The most common is Parkinson's disease (PD), in which putamen dopamine content is drastically reduced. PD also entails severely decreased myocardial norepinephrine content, a feature that characterizes two other Lewy body diseases-pure autonomic failure and dementia with Lewy bodies. It is widely presumed that tissue catecholamine depletion in these conditions results directly from loss of catecholaminergic neurons; however, as highlighted in this review, there are also important functional abnormalities in extant residual catecholaminergic neurons. We refer to this as the "sick-but-not-dead" phenomenon. The malfunctions include diminished dopamine biosynthesis via tyrosine hydroxylase (TH) and L-aromatic-amino-acid decarboxylase (LAAAD), inefficient vesicular sequestration of cytoplasmic catecholamines, and attenuated neuronal reuptake via cell membrane catecholamine transporters. A unifying explanation for catecholaminergic neurodegeneration is autotoxicity exerted by 3,4-dihydroxyphenylacetaldehyde (DOPAL), an obligate intermediate in cytoplasmic dopamine metabolism. In PD, putamen DOPAL is built up with respect to dopamine, associated with a vesicular storage defect and decreased aldehyde dehydrogenase activity. Probably via spontaneous oxidation, DOPAL potently oligomerizes and forms quinone-protein adducts with ("quinonizes") α-synuclein (AS), a major constituent in Lewy bodies, and DOPAL-induced AS oligomers impede vesicular storage. DOPAL also quinonizes numerous intracellular proteins and inhibits enzymatic activities of TH and LAAAD. Treatments targeting DOPAL formation and oxidation therefore might rescue sick-but-not-dead catecholaminergic neurons in Lewy body diseases.

Calcium, Bioenergetics, and Parkinson's Disease

Degeneration of substantia nigra (SN) dopaminergic (DAergic) neurons is responsible for the core motor deficits of Parkinson’s disease (PD). These neurons are autonomous pacemakers that have large cytosolic Ca²⁺ oscillations that have been linked to basal mitochondrial oxidant stress and turnover. This review explores the origin of Ca²⁺ oscillations and their role in the control of mitochondrial respiration, bioenergetics, and mitochondrial oxidant stress.

[The role of inhibitors of COMT and MAO-B in the therapy of Parkinson's disease]

Inhibitors of COMT and MAO-B are well established in the pharmacotherapy of Parkinson's disease (PD). MAO-B inhibitors are used as monotherapy as well as in combination with levodopa, whereas COMT inhibitors exert their effects only in conjungtion with levodopa. Both classes of compounds prolong the response duration of levodopa and optimise its clinical benefit. As a result, the ON-times are prolonged significantly. In the past, MAO-B inhibitors were also adminstered for neuroprotection; however, despite convincing scientific reasoning in support of neuroprotective effects, these could not be substantiated in clinical studies performed so far.

Dopamine promotes the neurodegenerative potential of β-synuclein

A contribution of α-Synuclein (α-Syn) to etiology of Parkinson´s disease (PD) and Dementia with Lewy bodies (DLB) is currently undisputed, while the impact of the closely related β-Synuclein (β-Syn) on these disorders remains enigmatic. β-Syn has long been considered to be an attenuator of the neurotoxic effects of α-Syn, but in a rodent model of PD β-Syn induced robust neurodegeneration in dopaminergic neurons of the substantia nigra. Given that dopaminergic nigral neurons are selectively vulnerable to neurodegeneration in PD, we now investigated if dopamine can promote the neurodegenerative potential of β-Syn. We show that in cultured rodent and human neurons a dopaminergic neurotransmitter phenotype substantially enhanced β-Syn-induced neurodegeneration, irrespective if dopamine is synthesized within neurons or up-taken from extracellular space. Nuclear magnetic resonance interaction and thioflavin-T incorporation studies demonstrated that dopamine and its oxidized metabolites 3,4-dihydroxyphenylacetaldehyde (DOPAL) and dopaminochrome (DCH) directly interact with β-Syn, thereby enabling structural and functional modifications. Interaction of DCH with β-Syn inhibits its aggregation, which might result in increased levels of neurotoxic oligomeric β-Syn. Since protection of outer mitochondrial membrane integrity prevented the additive neurodegenerative effect of dopamine and β-Syn, such oligomers might act at a mitochondrial level similar to what is suggested for α-Syn. In conclusion, our results suggest that β-Syn can play a significant pathophysiological role in etiology of PD through its interaction with dopamine metabolites and thus should be re-considered as a disease-relevant factor, at least for those symptoms of PD that depend on degeneration of nigral dopaminergic neurons.

Delta-secretase cleavage of Tau mediates its pathology and propagation in Alzheimer's disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disease with age as a major risk factor. AD is the most common dementia with abnormal structures, including extracellular senile plaques and intraneuronal neurofibrillary tangles, as key neuropathologic hallmarks. The early feature of AD pathology is degeneration of the locus coeruleus (LC), which is the main source of norepinephrine (NE) supplying various cortical and subcortical areas that are affected in AD. The spread of Tau deposits is first initiated in the LC and is transported in a stepwise manner from the entorhinal cortex to the hippocampus and then to associative regions of the neocortex as the disease progresses. Most recently, we reported that the NE metabolite DOPEGAL activates delta-secretase (AEP, asparagine endopeptidase) and triggers pathological Tau aggregation in the LC, providing molecular insight into why LC neurons are selectively vulnerable to developing early Tau pathology and degenerating later in the disease and how δ-secretase mediates the spread of Tau pathology to the rest of the brain. This review summarizes our current understanding of the crucial role of δ-secretase in driving and spreading AD pathologies by cleaving multiple critical players, including APP and Tau, supporting that blockade of δ-secretase may provide an innovative disease-modifying therapeutic strategy for treating AD.

The identification of an enzyme that plays a critical role in the progression of Alzheimer’s disease (AD) could lead to novel therapeutic interventions. In the earliest stage of AD, the build-up of Tau protein aggregates causes degeneration of a site in the brainstem. These abnormal Tau accumulations then spread to other parts of the brain. Recent research suggests that an enzyme called delta-secretase cleaves Tau and other key molecules, making Tau more prone to forming aggregates and thus facilitating disease progression. Keqiang Ye and co-workers at Emory University School of Medicine in Atlanta, USA, reviewed current understanding of the role of delta-secretase in AD pathology. Studies show that delta-secretase expression levels are high in aged mice and AD brains. Inhibiting delta-secretase could therefore limit neurodegeneration and alleviate cognitive deficits in patients.

Norepinephrine metabolite DOPEGAL activates AEP and pathological Tau aggregation in locus coeruleus

Aberrant Tau inclusions in the locus coeruleus (LC) are the earliest detectable Alzheimer's disease-like (AD-like) neuropathology in the human brain. However, why LC neurons are selectively vulnerable to developing early Tau pathology and degenerating later in disease and whether the LC might seed the stereotypical spread of Tau pathology to the rest of the brain remain unclear. Here, we show that 3,4-dihydroxyphenylglycolaldehyde, which is produced exclusively in noradrenergic neurons by monoamine oxidase A metabolism of norepinephrine, activated asparagine endopeptidase that cleaved Tau at residue N368 into aggregation- and propagation-prone forms, thus leading to LC degeneration and the spread of Tau pathology. Activation of asparagine endopeptidase-cleaved Tau aggregation in vitro and in intact cells was triggered by 3,4-dihydroxyphenylglycolaldehyde, resulting in LC neurotoxicity and propagation of pathology to the forebrain. Thus, our findings reveal that norepinephrine metabolism and Tau cleavage represent the specific molecular mechanism underlying the selective vulnerability of LC neurons in AD.