Compensatory role of the Nrf2–ARE pathway against paraquat toxicity: Relevance of 26S proteasome activity

Interactions Between the Ubiquitin-Proteasome System, Nrf2, and the Cannabinoidome as Protective Strategies to Combat Neurodegeneration: Review on Experimental Evidence

Neurodegenerative disorders are chronic brain diseases that affect humans worldwide. Although many different factors are thought to be involved in the pathogenesis of these disorders, alterations in several key elements such as the ubiquitin-proteasome system (UPS), the nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway, and the endocannabinoid system (ECS or endocannabinoidome) have been implicated in their etiology. Impairment of these elements has been linked to the origin and progression of neurodegenerative disorders, while their potentiation is thought to promote neuronal survival and overall neuroprotection, as proved with several experimental models. These key neuroprotective pathways can interact and indirectly activate each other. In this review, we summarize the neuroprotective potential of the UPS, ECS, and Nrf2 signaling, both separately and combined, pinpointing their role as a potential therapeutic approach against several hallmarks of neurodegeneration.

Regulation of Heme Oxygenase and Its Cross-Talks with Apoptosis and Autophagy under Different Conditions in Drosophila

Heme oxygenase (HO) is one of the cytoprotective enzymes that can mitigate the effects of oxidative stress. Here, we found that the ho mRNA level oscillates in the brain of Drosophila melanogaster with two minima at the beginning of the day and night. This rhythm was partly masked by light as its pattern changed in constant darkness (DD). It followed a similar trend in the clock mutant per⁰¹ under light/dark regime (LD12:12); however, differences between time points were not statistically significant. In older flies (20 days old), the rhythm was vanished; however, 15 days of curcumin feeding restored this rhythm with an elevated ho mRNA level at all time points studied. In addition, flies exposed to paraquat had higher ho expression in the brain, but only at a specific time of the day which can be a protective response of the brain against stress. These findings suggest that the expression of ho in the fly’s brain is regulated by the circadian clock, light, age, exposure to stress, and the presence of exogenous antioxidants. We also found that HO cross-talks with apoptosis and autophagy under different conditions. Induction of neuronal ho was accompanied by increased transcription of apoptosis and autophagy-related genes. However, this trend changed after exposure to curcumin and paraquat. Our results suggest that HO is involved in the control of apoptotic and autophagic key processes protecting the brain against oxidative damage.

Paraquat Treatment Compromises the Clearance of β-Amyloid and Tau Proteins and Induces Primary Hippocampal Neuronal Cell Death through HSP70, P20S, and TFEB Disruption

The herbicide paraquat (PQ) induces hippocampal neuronal cell loss and cognitive dysfunction after one and repeated treatment. All the mechanisms involved in these effects are not well understood. Single and repeated PQ treatment increased Aβ and tau protein levels, through HSP70 and TFEB downregulation and proteasome 20S inhibition, producing cell death in primary hippocampal neurons associated with cognitive decline. Our results reveal the mechanisms through which PQ could induce the accumulation of abnormal proteins and neurodegeneration that could originate the cognitive decline produced by it and could help managing its degenerative effects.

Redox toxicology of environmental chemicals causing oxidative stress

Living organisms are surrounded with heavy metals such as methylmercury, manganese, cobalt, cadmium, arsenic, as well as pesticides such as deltamethrin and paraquat, or atmospheric pollutants such as quinone. Extensive studies have demonstrated a strong link between environmental pollutants and human health. Redox toxicity is proposed as one of the main mechanisms of chemical-induced pathology in humans. Acting as both a sensor of oxidative stress and a positive regulator of antioxidants, the nuclear factor erythroid 2-related factor 2 (NRF2) has attracted recent attention. However, the role NRF2 plays in environmental pollutant-induced toxicity has not been systematically addressed. Here, we characterize NRF2 function in response to various pollutants, such as metals, pesticides and atmospheric quinones. NRF2 related signaling pathways and epigenetic regulations are also reviewed.

The Nrf2-mediated defense mechanism associated with HFE genotype limits vulnerability to oxidative stress-induced toxicity

There is considerable interest in gene and environment interactions in neurodegenerative diseases. The HFE (homeostatic iron regulator) gene variant (H63D) is highly prevalent in the population and has been investigated as a disease modifier in multiple neurodegenerative diseases. We have developed a mouse model to interrogate the impact of this gene variant in a model of paraquat toxicity. Using primary astrocytes, we found that the H67D-Hfe(equivalent of the human H63D variant) astrocytes are less vulnerable than the WT-Hfe astrocytes to paraquat-induced cell death, mitochondrial damage, and cellular senescence. We hypothesized that the Hfe variant-associated protection is a result of the activation of the Nrf2 antioxidant defense system and found a significant increase in Nrf2 levels after paraquat exposure in the H67D-Hfe astrocytes than the WT-Hfe astrocytes. Moreover, decreasing Nrf2 by molecular or pharmaceutical manipulation resulted in increased vulnerability to paraquat in the H67D-Hfe astrocytes. To further elucidate the role of Hfe variant genotype in neuroprotection mediated by astrocytes, we added media from the paraquat-treated astrocytes to differentiated SH-SY5Y neuroblastoma cells and found a significantly larger reduction in the viability when treated with WT-Hfe astrocyte media than the H67D-Hfe astrocyte media possibly due to higher secretion of IL-6 observed in the WT-Hfe astrocytes. To further explore the mechanism of Nrf2 protection, we measured NQO1, the Nrf2-mediated antioxidant, in primary astrocytes and found a significantly higher NQO1 level in the H67D-Hfe astrocytes. To consider the translational potential of our findings, we utilized the PPMI (Parkinson's Progression Markers Initiative) clinical database and found that, consistent with the mouse study, H63D-HFE carriers had a significantly higher NQO1 level in the CSF than the WT-HFE carriers. Consistent with our previous reports on H63D-HFE in disease, these data further suggest that HFE genotype in the human population impacts the antioxidant defense system and can therefore alter pathogenesis.

Effect of Quamoclit angulata Extract Supplementation on Oxidative Stress and Inflammation on Hyperglycemia-Induced Renal Damage in Type 2 Diabetic Mice

Type 2 diabetes mellitus (T2DM) is caused by abnormalities of controlling blood glucose and insulin homeostasis. Especially, hyperglycemia causes hyper-inflammation through activation of NLRP3 inflammasome, which can lead to cell apoptosis, hypertrophy, and fibrosis. Quamoclit angulata (QA), one of the annual winders, has been shown ameliorative effects on diabetes. The current study investigated whether the QA extract (QAE) attenuated hyperglycemia-induced renal inflammation related to NLRP inflammasome and oxidative stress in high fat diet (HFD)-induced diabetic mice. After T2DM was induced, the mice were treated with QAE (5 or 10 mg/kg/day) by gavage for 12 weeks. The QAE supplementation reduced homeostasis model assessment insulin resistance (HOMA-IR), kidney malfunction, and glomerular hypertrophy in T2DM. Moreover, the QAE treatment significantly attenuated renal NLRP3 inflammasome dependent hyper-inflammation and consequential renal damage caused by oxidative stress, apoptosis, and fibrosis in T2DM. Furthermore, QAE normalized aberrant energy metabolism (downregulation of p-AMPK, sirtuin (SIRT)-1, and PPARγ-coactivator α (PGC-1 α)) in T2DM mice. Taken together, the results suggested that QAE as a natural product has ameliorative effects on renal damage by regulation of oxidative stress and inflammation in T2DM.

Primary hippocampal estrogenic dysfunction induces synaptic proteins alteration and neuronal cell death after single and repeated paraquat exposure

The extensively utilized herbicide Paraquat (PQ) was reported to generate cognitive disorders and hippocampal neuronal cell death after unique and extended exposure. Although, most of the mechanisms that mediate these actions remain unknown. We researched whether PQ induces synaptic protein disruption, Tau and amyloid beta protein formation, oxidative stress generation, and hippocampal neuronal cell loss through anti-estrogen action in primary hippocampal neurons, after day and two weeks PQ treatment, as a probable mechanism of such learning and memory impairment. Our results reveal that PQ did not alter estrogen receptors (ERα and ERβ) gene expression, yet it decreased ER activation, which led to synaptic proteins disruption and amyloid beta proteins generation and Tau proteins hyperphosphorylation. Estrogenic signaling disruption induced by PQ also downregulated the NRF2 pathway leading to oxidative stress generation. Finally, PQ exposure induced cell death mediated by ER dysfunction partially through oxidative stress and amyloid beta proteins generation and Tau proteins hyperphosphorylation. The results presented provide a therapeutic strategy to protect against PQ toxic effects, possibly giving an explanation for the learning and memory impairment generated following PQ exposure.

Ukgansan protects dopaminergic neurons from 6-hydroxydopamine neurotoxicity via activation of the nuclear factor (erythroid-derived 2)-like 2 factor signaling pathway

The sustenance of redox homeostasis in brain is the crucial factor to treat Parkinson's disease (PD). Nuclear factor (erythroid-derived 2)-like 2 factor (Nrf2)-mediated antioxidant response is well known for the main cellular endogenous defense mechanisms against oxidative stress. This study investigated for the first time the effects and possible mechanisms of action of Ukgansan on 6-hydroxydopamine (6-OHDA)-induced neurotoxicity in both in vitro and in vivo models of PD. We investigated the protective effect of Ukgansan against 6-OHDA with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. In addition, we demonstrated that Ukgansan significantly increased the expression of antioxidant response elements (ARE) and pro-survival protein as Bcl2 and suppressed the expression of pro-apoptotic factors, such as Bax, cytochrome c, and caspase-3 using immunoblotting. For the in vivo study, we used a mouse model of PD involving stereotaxic injection of 6-OHDA into the striatum (ST). Ukgansan alleviated motor dysfunctions induced by 6-OHDA followed by pole, open-field, and rotation tests. Dopaminergic neuronal loss and Nrf2 activation were evaluated by immunohistochemistry in the mouse ST and substantia nigra pars compacta (SNpc) regions. Ukgansan significantly protected dopaminergic neurons from 6-OHDA toxicity in mouse ST and SNpc by activating Nrf2. These results indicate that Ukgansan inhibited 6-OHDA-induced dopaminergic neuronal cell damage via activation of Nrf2 and its related factors in 6-OHDA-induced dopaminergic loss in vitro and in vivo. Thus, Ukgansan might delay the progression of PD via maintenance of redox homeostasis.

HFE Genotype Restricts the Response to Paraquat in a Mouse Model of Neurotoxicity

Parkinson's disease is marked clinically by motor dysfunction and pathologically by dopaminergic cell loss in the substantia nigra and iron accumulation in the substantia nigra. The driver underlying iron accumulation remains unknown and could be genetic or environmental. The HFE protein is critical for the regulation of cellular iron uptake. Mutations within this protein are associated with increased iron accumulation including in the brain. We have focused on the commonly occurring H63D variant of the HFE gene as a disease modifier in a number of neurodegenerative diseases. To investigate the role of H63D HFE genotype, we generated a mouse model in which the wild-type (WT) HFE gene is replaced by the H67D gene variant (mouse homolog of the human H63D gene variant). Using paraquat toxicity as the model for Parkinson's disease, we found that WT mice responded as expected with significantly greater motor function, loss of tyrosine hydroxylase staining and increase microglial staining in the substantia nigra, and an increase in R2 relaxation rate within the substantia nigra of the paraquat-treated mice compared to their saline-treated counterparts. In contrast, the H67D mice showed a remarkable resistance to paraquat treatment; specifically differing from the WT mice with no changes in motor function or changes in R2 relaxation rates following paraquat exposure. At baseline, there were differences between the H67D HFE mice and WT mice in gut microbiome profile and increased L-ferritin staining in the substantia nigra that could account for the resistance to paraquat. Of particular note, the H67D HFE mice regardless of whether or not they were treated with paraquat had significantly less tyrosine hydroxylase immunostaining than WT. Our results clearly demonstrate that the HFE genotype impacts the expression of tyrosine hydroxylase in the substantia nigra, the gut microbiome and the response to paraquat providing additional support that the HFE genotype is a disease modifier for Parkinson's disease. Moreover, the finding that the HFE mutant mice are resistant to paraquat may provide a model in which to study resistant mechanisms to neurotoxicants.

Exposure to far-infrared rays attenuates methamphetamine-induced recognition memory impairment via modulation of the muscarinic M1 receptor, Nrf2, and PKC

We demonstrated that activation of protein kinase Cδ (PKCδ) and inactivation of the glutathione peroxidase-1 (GPx-1)-dependent systems are critical for methamphetamine (MA)-induced recognition memory impairment. We also demonstrated that exposure to far-infrared rays (FIR) causes induction of the glutathione (GSH)-dependent system, including induction of the GPx-1 gene. Here, we investigated whether exposure to FIR rays affects MA-induced recognition memory impairment and whether it modulates PKC, cholinergic receptors, and the GSH-dependent system. Because the PKC activator bryostatin-1 mainly induces PKCα, PKCε, and PKCδ, we assessed expression of these proteins after MA treatment. MA treatment selectively increased PKCδ expression and its phosphorylation. Exposure to FIR rays significantly attenuated MA-induced increases in PKCδ phosphorylation. Importantly, bryostatin-1 potentiated MA-induced phosphorylation of PKCδ. MA treatment significantly decreased M1, M3, and M4 muscarinic acetylcholine receptors (mAChRs) and β2 nicotinic acetylcholine receptor expression. Of these, the decrease was most pronounced in M1 mAChR. Exposure to FIR significantly attenuated MA-induced decreases in the M1 mAChR and phospho-ERK_1/2, while it facilitated Nrf2-dependent GSH induction. Dicyclomine, an M1 mAChR antagonist, and l-buthionine-(S, R)-sulfoximine (BSO), an inhibitor of GSH synthesis, counteracted against the protective potentials mediated by FIR. More importantly, the memory-enhancing potential of FIR rays was significantly counteracted by bryostatin-1, dicyclomine, and BSO. Our results suggest that exposure to FIR rays attenuates MA-induced impairment in recognition memory via up-regulation of M1 mAChR, Nrf2-dependent GSH induction, and ERK_1/2 phosphorylation by inhibiting PKCδ phosphorylation by bryostatin-1.

Alpha-synuclein aggregation, Ubiquitin proteasome system impairment, and L-Dopa response in zinc-induced Parkinsonism: resemblance to sporadic Parkinson's disease

Alpha-synuclein (α-synuclein) aggregation and impairment of the Ubiquitin proteasome system (UPS) are implicated in Parkinson's disease (PD) pathogenesis. While zinc (Zn) induces dopaminergic neurodegeneration resulting in PD phenotype, its effect on protein aggregation and UPS has not yet been deciphered. The current study investigated the role of α-synuclein aggregation and UPS in Zn-induced Parkinsonism. Additionally, levodopa (L-Dopa) response was assessed in Zn-induced Parkinsonian model to establish its closeness with idiopathic PD. Male Wistar rats were treated with zinc sulfate (Zn; 20 mg/kg; i.p.) twice weekly for 12 weeks along with respective controls. In few subsets, animals were subsequently treated with L-Dopa for 21 consecutive days following Zn exposure. A significant increase in total and free Zn content was observed in the substantia nigra of the brain of exposed groups. Zn treatment caused neurobehavioral anomalies, striatal dopamine decline, and dopaminergic neuronal cell loss accompanied with a marked increase in α-synuclein expression/aggregation and Ubiquitin-conjugated protein levels in the exposed groups. Zn exposure substantially reduced UPS-associated trypsin-like, chymotrypsin-like, and caspase-like activities along with the expression of SUG1 and β-5 subunits of UPS in the nigrostriatal tissues of exposed groups. L-Dopa treatment rescued from Zn-induced neurobehavioral deficits and restored dopamine levels towards normalcy; however, Zn-induced dopaminergic neuronal loss, reduction in tyrosine hydroxylase expression, and increase in oxidative stress were unaffected. The results suggest that Zn caused UPS impairment, resulting in α-synuclein aggregation subsequently leading to dopaminergic neurodegeneration, and that Zn-induced Parkinsonism exhibited positive L-Dopa response similar to sporadic PD.

The Proteasome Inhibition Model of Parkinson's Disease

The pathological hallmarks of Parkinson's disease are the progressive loss of nigral dopaminergic neurons and the formation of intracellular inclusion bodies, termed Lewy bodies, in surviving neurons. Accumulation of proteins in large insoluble cytoplasmic aggregates has been proposed to result, partly, from a failure in the function of intracellular protein degradation pathways. Evidence in support for such a hypothesis emerged in the beginning of the years 2000 with studies demonstrating structural and functional deficits in the ubiquitin-proteasome pathway in post-mortem nigral tissue of patients with Parkinson's disease. These fundamental findings have inspired the development of a new generation of animal models based on the use of proteasome inhibitors to disturb protein homeostasis and trigger nigral dopaminergic neurodegeneration. In this review, we provide an updated overview of the current approaches in employing proteasome inhibitors to model Parkinson's disease, with particular emphasis on rodent studies. In addition, the mechanisms underlying proteasome inhibition-induced cell death and the validity criteria (construct, face and predictive validity) of the model will be critically discussed. Due to its distinct, but highly relevant mechanism of inducing neuronal death, the proteasome inhibition model represents a useful addition to the repertoire of toxin-based models of Parkinson's disease that might provide novel clues to unravel the complex pathogenesis of this disorder.

Sexual dimorphism in oxidant-induced adaptive homeostasis in multiple wild-type D. melanogaster strains

Sexual dimorphism includes the physical and reproductive differences between the sexes, including differences that are conserved across species, ranging from the common fruit fly, Drosophila melanogaster, to humans. Sex-dependent variations in adaptive homeostasis, and adaptive stress responses may offer insight into the underlying mechanisms for male and female survival differences and into differences in chronic disease incidence and severity in humans. Earlier work showed sex-specific differences in adaptive responses to oxidative stressors in hybrid laboratory strains of D. melanogaster. The present study explored whether this phenomenon is also observed in wild-type D. melanogaster strains Oregon-R (Or-R) and Canton-S (Ca-S), as well as the common mutant reference strain w[1118], in order to better understand whether such findings are descriptive of D. melanogaster in general. Flies of each strain were pretreated with non-damaging, adaptive concentrations of hydrogen peroxide (H₂O₂) or of different redox cycling agents (paraquat, DMNQ, or menadione). Adaptive homeostasis, and changes in the expression of the Proteasome and overall cellular proteasomal proteolytic capacity were assessed. Redox cycling agents exhibited a male-specific adaptive response, whereas H₂O₂ exposure provoked female-specific adaptation. These findings demonstrate that different oxidants can elicit sexually dimorphic adaptive homeostatic responses in multiple fly strains. These results (and those contained in a parallel study [1]) highlight the need to address sex as a biological variable in fundamental science, clinical research, and toxicology.

Reduction of Immunoreactivity Against the C-Terminal Region of the Intracellular α-Synuclein by Exogenous α-Synuclein Aggregates: Possibility of Conformational Changes

BACKGROUND

The formation of intracellular aggregates containing α-synuclein (α-syn) is a main pathological feature of Parkinson disease. The propagation of α-syn aggregation via cell-to-cell transmission has been implicated in the progression of Parkinson disease.

OBJECTIVE

Our aim is to clarify the molecular mechanisms underlying the formation of intracellular aggregation by extracellular α-syn.

METHODS

We investigated the effects of exogenous α-syn aggregates on intracellular α-syn immunoreactivity in α-syn-overexpressing SH-SY5Y cells using two antibodies to distinct epitopes of α-syn. To obtain α-syn aggregates, α-syn solution was aged with continuous agitation.

RESULTS

Immunoreactivity against the acidic C-terminal domain of the intracellular α-syn was reduced by exposure to agedα-syn, whereas that against the hydrophobic non-amyloid component region was not changed. The reduction in immunoreactivity was not suppressed by protease inhibitors but was mimicked by neutralization of the negative charges on the C-terminal of the intracellular α-syn induced by spermine or extracellular acidification.

CONCLUSIONS

These results suggest that the reduction in immunoreactivity is attributed not to proteolytic cleavage but to a conformational change at the C-terminus of the intracellular α-syn. The conformational change at the C-terminus of the intracellular α-syn might be involved in an initial step of fibril formation by exogenous α-syn aggregates.

Ameliorative Effect of Ecklonia cava Polyphenol Extract on Renal Inflammation Associated with Aberrant Energy Metabolism and Oxidative Stress in High Fat Diet-Induced Obese Mice

Immoderate fat accumulation causes both oxidative stress and inflammation, which can induce kidney damage in obesity. Previously, Ecklonia cava has shown anti-inflammatory and antioxidative effects. Our group aimed to investigate whether E. cava polyphenol extract (ECPE) improves renal damage in high fat diet (HFD)-induced obese mice through regulation of not only energy metabolism but also oxidative stress and inflammation. After obesity induction by HFD, the mice were treated with different dosages of ECPE (100 or 500 mg/kg/day) by gavage for 12 weeks. ECPE treatment lowered the protein levels related to lipid accumulation (SREBP1c, ACC & FAS), inflammation (NLRP3 inflammasome, NFκB, MCP-1, TNF-α & CRP), and oxidative stress (Nrf2, HO-1, MnSOD, NQO1, GPx, 4-HNE and protein carbonyls) in HFD induced obese mice. Moreover, ECPE supplementation significantly up-regulated renal SIRT1, PGC-1α, and AMPK, which are associated with renal energy metabolism. Consequently, the results provide novel insights into the anti-inflammatory roles of ECPE in obesity-induced renal inflammation.

Integrin α5β1 expression on dopaminergic neurons is involved in dopaminergic neurite outgrowth on striatal neurons

During development, dopaminergic neurons born in the substantia nigra extend their axons toward the striatum. However, the mechanisms by which the dopaminergic axons extend the striatum to innervate their targets remain unclear. We previously showed that paired-cultivation of mesencephalic cells containing dopaminergic neurons with striatal cells leads to the extension of dopaminergic neurites from the mesencephalic cell region to the striatal cell region. The present study shows that dopaminergic neurites extended along striatal neurons in the paired-cultures of mesencephalic cells with striatal cells. The extension of dopaminergic neurites was suppressed by the pharmacological inhibition of integrin α5β1. Using lentiviral vectors, short hairpin RNA (shRNA)-mediated knockdown of integrin α5 in dopaminergic neurons suppressed the neurite outgrowth to the striatal cell region. In contrast, the knockdown of integrin α5 in non-dopaminergic mesencephalic and striatal cells had no effect. Furthermore, overexpression of integrin α5 in dopaminergic neurons differentiated from embryonic stem cells enhanced their neurite outgrowth on striatal cells. These results indicate that integrin α5β1 expression on dopaminergic neurons plays an important role in the dopaminergic neurite outgrowth on striatal neurons.