Novel enhancement mechanism of tyrosine hydroxylase enzymatic activity by nitric oxide through S-nitrosylation

The role of age-associated alpha-synuclein aggregation in a conditional transgenic mouse model of Parkinson's disease: Implications for Lewy body formation

Parkinson's disease (PD) is a common neurodegenerative disorder that is affecting an increasing number of older adults. Although PD is mostly sporadic, genetic mutations have been found in cohorts of families with a history of familial PD (FPD). The first such mutation linked to FPD causes a point mutation (A53T) in α-synuclein (α-syn), a major component of Lewy bodies, which are a classical pathological hallmark of PD. These findings suggest that α-syn is an important contributor to the development of PD. In our previous study, we developed an adenoviral mouse model of PD and showed that the expression of wild-type (WT) α-syn or a mutant form with an increased propensity to aggregate, designated as WT-CL1 α-syn, could be used to study how α-syn aggregation contributes to PD. In this study, we established a transgenic mouse model that conditionally expresses WT or WT-CL1 α-syn in dopaminergic (DA) neurons and found that the expression of either WT or WT-CL1 α-syn was associated with an age-dependent degeneration of DA neurons and movement dysfunction. Using this model, we were able to monitor the process of α-syn aggregate formation and found a correlation between age and the number and sizes of α-syn aggregates formed. These results provide a potential mechanism by which age-dependent α-syn aggregation may lead to the formation of Lewy bodies in PD pathogenesis.

Protein Oxidative Modifications in Neurodegenerative Diseases: From Advances in Detection and Modelling to Their Use as Disease Biomarkers

Oxidation-reduction post-translational modifications (redox-PTMs) are chemical alterations to amino acids of proteins. Redox-PTMs participate in the regulation of protein conformation, localization and function, acting as signalling effectors that impact many essential biochemical processes in the cells. Crucially, the dysregulation of redox-PTMs of proteins has been implicated in the pathophysiology of numerous human diseases, including neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. This review aims to highlight the current gaps in knowledge in the field of redox-PTMs biology and to explore new methodological advances in proteomics and computational modelling that will pave the way for a better understanding of the role and therapeutic potential of redox-PTMs of proteins in neurodegenerative diseases. Here, we summarize the main types of redox-PTMs of proteins while providing examples of their occurrence in neurodegenerative diseases and an overview of the state-of-the-art methods used for their detection. We explore the potential of novel computational modelling approaches as essential tools to obtain insights into the precise role of redox-PTMs in regulating protein structure and function. We also discuss the complex crosstalk between various PTMs that occur in living cells. Finally, we argue that redox-PTMs of proteins could be used in the future as diagnosis and prognosis biomarkers for neurodegenerative diseases.

Neuronal Nitric Oxide Synthase and Post-Translational Modifications in the Development of Central Nervous System Diseases: Implications and Regulation

In the Central Nervous System (CNS), Nitric Oxide (NO) is mainly biosynthesized by neuronal Nitric Oxide Synthase (nNOS). The dysregulated activation of nNOS in neurons is critical in the development of different conditions affecting the CNS. The excessive production of NO by nNOS is responsible for a number of proteins' post-translational modifications (PTMs), which can lead to aberrant biochemical pathways, impairing CNS functions. In this review, we briefly revise the main implications of dysregulated nNOS in the progression of the most prevalent CNS neurodegenerative disorders, i.e., Alzheimer's disease (AD) and Parkinson's disease, as well as in the development of neuronal disorders. Moreover, a specific focus on compounds able to modulate nNOS activity as promising therapeutics to tackle different neuronal diseases is presented.

Bile duct ligation increased dopamine levels in the cerebral cortex of rats partly due to induction of tyrosine hydroxylase

BACKGROUND AND PURPOSE

Liver failure is associated with psychiatric alterations, partly resulting from the increased brain dopamine levels. We investigated the relationship between increased dopamine levels and mental abnormalities using bile duct ligation (BDL) rats and the mechanism by which liver failure increased dopamine levels in SH-SY5Y cells. Behavioural tests were carried out on day 13 and 27 following BDL, along with measurements of dopamine and metabolites, expressions of enzymes and transporters related to dopamine metabolism, and its transport into the cortex and the hippocampus. SH-SY5Y cells were used to investigate whether NH₄ Cl, bile acids and bilirubin affected expression of tyrosine hydroxylase or not. Tyrosine hydroxylase (TH) expression in SH-SY5Y cells co-incubated with bilirubin and signal pathway inhibitors was measured.

KEY RESULTS

Open-field test results demonstrated BDL rats showed anxiety-like behaviour, accompanied by increased dopamine levels and expression of TH protein in the cortex. Membrane bound long form (MB)-COMT, slightly but significantly decreased. SH-SY5Y cells indicated that increased bilirubin levels was a factor in inducing TH expression. Both inhibitor of NF-κB pathway BAY 11-7082 and silencing NF-κB p65 reversed bilirubin-induced upregulation of TH protein. NF-κB activator TNF-α increased expression of TH protein. Roles of bilirubin in increases of TH protein expressions and dopamine levels were measured using hyperbilirubinemia rats. Anxiety-like behaviour, was associated with increased dopamine levels and TH protein expressions in hyperbilirubinemia rats.

CONCLUSION AND IMPLICATIONS

BDL significantly increased dopamine levels in rat cortex partly due to bilirubin-mediated TH induction. Increased bilirubin induced TH expression via activating NF-κB signalling pathway.

Tyrosine hydroxylase activity is regulated through the modification of the 176th cysteine residue

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the biosynthesis of dopamine (DA), and the regulation of its activity is important for DA homeostasis. In this study, we focused on the modification of TH through a cysteine residue. We found that incubation with N-ethylmaleimide (NEM), a cysteine modification reagent, inactivated TH. The responsible cysteine was identified as Cys176 of human TH with recombinant mutant proteins. We further examined how NEM modification was affected by the states of TH. DA binding, a feedback inhibition mechanism of TH, delayed the modification and inactivation of TH by NEM. In contrast, the S40E mutant, which mimics the phosphorylation of Ser40 that suppresses DA binding and is thus considered as an active state of TH, did not affect modification and inactivation. These results suggest that the modification of Cys176 can inhibit even phosphorylated active TH. In addition, we found that DA oxides, which are generated by oxidative stress in dopaminergic neurons, reacted with TH through Cys176 and inhibited its activity, similar to NEM. These results suggest that the modification of Cys176 of TH could be involved in the mechanisms of neurotoxicity caused by DA oxides.

ASL expression in ALDH1A1+ neurons in the substantia nigra metabolically contributes to neurodegenerative phenotype

Argininosuccinate lyase (ASL) is essential for the NO-dependent regulation of tyrosine hydroxylase (TH) and thus for catecholamine production. Using a conditional mouse model with loss of ASL in catecholamine neurons, we demonstrate that ASL is expressed in dopaminergic neurons in the substantia nigra pars compacta, including the ALDH1A1⁺ subpopulation that is pivotal for the pathogenesis of Parkinson disease (PD). Neuronal loss of ASL results in catecholamine deficiency, in accumulation and formation of tyrosine aggregates, in elevation of α-synuclein, and phenotypically in motor and cognitive deficits. NO supplementation rescues the formation of aggregates as well as the motor deficiencies. Our data point to a potential metabolic link between accumulations of tyrosine and seeding of pathological aggregates in neurons as initiators for the pathological processes involved in neurodegeneration. Hence, interventions in tyrosine metabolism via regulation of NO levels may be therapeutic beneficial for the treatment of catecholamine-related neurodegenerative disorders.

TNFα increases tyrosine hydroxylase expression in human monocytes

Most, if not all, peripheral immune cells in humans and animals express tyrosine hydroxylase (TH), the rate limiting enzyme in catecholamine synthesis. Since TH is typically studied in the context of brain catecholamine signaling, little is known about changes in TH production and function in peripheral immune cells. This knowledge gap is due, in part, to the lack of an adequately sensitive assay to measure TH in immune cells expressing lower TH levels compared to other TH expressing cells. Here, we report the development of a highly sensitive and reproducible Bio-ELISA to quantify picogram levels of TH in multiple model systems. We have applied this assay to monocytes isolated from blood of persons with Parkinson's disease (PD) and to age-matched, healthy controls. Our study unexpectedly revealed that PD patients' monocytes express significantly higher levels of TH protein in peripheral monocytes relative to healthy controls. Tumor necrosis factor (TNFα), a pro-inflammatory cytokine, has also been shown to be increased in the brains and peripheral circulation in human PD, as well as in animal models of PD. Therefore, we investigated a possible connection between higher levels of TH protein and the known increase in circulating TNFα in PD. Monocytes isolated from healthy donors were treated with TNFα or with TNFα in the presence of an inhibitor. Tissue plasminogen activator (TPA) was used as a positive control. We observed that TNFα stimulation increased both the number of TH+ monocytes and the quantity of TH per monocyte, without increasing the total numbers of monocytes. These results revealed that TNFα could potentially modify monocytic TH production and serve a regulatory role in peripheral immune function. The development and application of a highly sensitive assay to quantify TH in both human and animal cells will provide a novel tool for further investigating possible PD immune regulatory pathways between brain and periphery.

Nitric oxide and dopamine metabolism converge via mitochondrial dysfunction in the mechanisms of neurodegeneration in Parkinson's disease

The molecular mechanisms underlying the degeneration and neuronal death associated with Parkinson's disease (PD) are not clearly understood. Several pathways and models have been explored in an overwhelming number of studies. Overall, from these studies, mitochondrial dysfunction and nitroxidative stress have emerged as major contributors to degeneration of dopaminergic neurons in PD. In addition, an excessive or inappropriate production of nitric oxide (^•NO) and an abnormal metabolism of dopamine have been independently implicated in both processes. However, the participation of ^•NO in reactions with dopamine relevant to neurotoxicity strongly suggests that dopamine or its metabolites may be potential targets for ^•NO, affecting the physiological chemistry of both, ^•NO and dopamine. In this short review, we provide a critical and integrative appraisal of the nitric oxide-dopamine pathway we have previously suggested and that might be operative in PD. This pathway emphasizes a connection between abnormal dopamine and ^•NO metabolism, which may potentially converge in an integrated mechanism with toxic cellular outcomes. In particular, it encompasses the synergistic interaction of ^•NO with 3,4-dihydroxyphenylacetic acid (DOPAC), a major dopamine metabolite, leading to dopaminergic cell death via mechanisms that involve mitochondrial dysfunction, gluthathione depletion and nitroxidative stress.

Restoring pars intermedia dopamine concentrations and tyrosine hydroxylase expression levels with pergolide: evidence from horses with pituitary pars intermedia dysfunction

Background

Pituitary pars intermedia dysfunction (PPID) develops slowly in aged horses as degeneration of hypothalamic dopaminergic neurons leads to proliferation of pars intermedia (PI) melanotropes through hyperplasia and adenoma formation. Dopamine (DA) concentrations and tyrosine hydroxylase (TH) immunoreactivity are markedly reduced in PI tissue of PPID-affected equids and treatment with the DA receptor agonist pergolide results in notable clinical improvement. Thus, we hypothesized that pergolide treatment of PPID-affected horses would result in greater DA and TH levels in PI tissue collected from PPID-affected horses versus untreated PPID-affected horses.

To test this hypothesis, pituitary glands were removed from 18 horses: four untreated PPID-affected horses, four aged and four young horses without signs of PPID, and six PPID-affected horses that had been treated with pergolide at 2 µg/kg orally once daily for 6 months. DA concentrations and TH expression levels in PI tissues were determined by high performance liquid chromatography with electrochemical detection and Western blot analyses, respectively.

Results

DA and TH levels were lowest in PI collected from untreated PPID-affected horses while levels in the pergolide treated horses were similar to those of aged horses without signs of PPID.

Conclusions

These findings provide evidence of restoration of DA and TH levels following treatment with pergolide. Equine PPID is a potential animal model of dopaminergic neurodegeneration, which could provide insight into human neurodegenerative diseases.

ASL Metabolically Regulates Tyrosine Hydroxylase in the Nucleus Locus Coeruleus

Patients with germline mutations in the urea-cycle enzyme argininosuccinate lyase (ASL) are at risk for developing neurobehavioral and cognitive deficits. We find that ASL is prominently expressed in the nucleus locus coeruleus (LC), the central source of norepinephrine. Using natural history data, we show that individuals with ASL deficiency are at risk for developing attention deficits. By generating LC-ASL-conditional knockout (cKO) mice, we further demonstrate altered response to stressful stimuli with increased seizure reactivity in LC-ASL-cKO mice. Depletion of ASL in LC neurons leads to reduced amount and activity of tyrosine hydroxylase (TH) and to decreased catecholamines synthesis, due to decreased nitric oxide (NO) signaling. NO donors normalize catecholamine levels in the LC, seizure sensitivity, and the stress response in LC-ASL-cKO mice. Our data emphasize ASL importance for the metabolic regulation of LC function with translational relevance for ASL deficiency (ASLD) patients as well as for LC-related pathologies.

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ASL is expressed in the locus coeruleus (LC) and regulates catecholamine synthesis

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LC-ASL deficiency in mice promotes abnormal stress response and seizure sensitivity

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LC-ASL deficiency decreases nitric-oxide levels and tyrosine hydroxylase activity

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NO donors normalize catecholamine production and rescue LC-ASL deficiency phenotype