NAD+ Supplementation Attenuates Methylmercury Dopaminergic and Mitochondrial Toxicity in Caenorhabditis Elegans

Study of ATF4/CHOP axis-mediated mitochondrial unfolded protein response in neuronal apoptosis induced by methylmercury

Methylmercury (MeHg) is a widely distributed environmental pollutant that can easily cross the blood-brain barrier and accumulate in the brain, thereby damaging the central nervous system. Studies have shown that MeHg-induced mitochondrial damage and apoptosis play a crucial role in its neurotoxic effects. Mitochondrial unfolded protein response (UPRmt) is indispensable to maintain mitochondrial protein homeostasis and ensure mitochondrial function, and the ATF4/CHOP axis is one of the signaling pathways to activate UPRmt. In this study, the role of the ATF4/CHOP axis-mediated UPRmt in the neurotoxicity of MeHg has been investigated by C57BL/6 mice and the HT22 cell line. We discovered that mice exposed to MeHg had abnormal neurobehavioral patterns. The pathological section showed a significant decrease in the number of neurons. MeHg also resulted in a reduction in mtDNA copy number and mitochondrial membrane potential (MMP). Additionally, the ATF4/CHOP axis and UPRmt were found to be significantly activated. Subsequently, we used siRNA to knock down ATF4 or CHOP and observed that the expression of UPRmt-related proteins and the apoptosis rate were significantly reduced. Our research showed that exposure to MeHg can over-activate the UPRmt through the ATF4/CHOP axis, leading to mitochondrial damage and ultimately inducing neuronal apoptosis.

Expression Profiling of Adipogenic and Anti-Adipogenic MicroRNA Sequences following Methylmercury Exposure in Caenorhabditis elegans

MicroRNA (miRNA) are important regulators of gene expression that respond not only to developmental and pathological cues, but also to environmental stimuli. Dyslipidemia is a hallmark of metabolic conditions and has been shown to significantly affect the expression of circulating miRNA sequences. Recently, our lab has shown that the environmental toxicant methylmercury (MeHg) causes dyslipidemia in the Caenorhabditis elegans model organism. While 10 and 20 μM MeHg increases the expression of adipogenic transcription factors and lipid-binding proteins in worms, there is limited information on how the toxicant affects the miRNA regulators of these genes. We hypothesized that MeHg would increase the expression of adipogenic miRNA sequences and/or decrease the expression of anti-adipogenic miRNA sequences. We further hypothesized that the target mRNA sequences for the miRNAs affected by MeHg would be consequently altered. We selected three potentially adipogenic (mir-34, mir-124, and mir-355) and three potentially anti-adipogenic (mir-240, mir-786, and let-7) miRNA sequences homologous to known human miRNA sequences altered in obesity, and quantified their levels 24 h and 48 h post MeHg treatment. At 24 h post exposure, MeHg significantly increased expression of both the adipogenic and anti-adipogenic miRNA sequences 1.5-3x above untreated control. By 48 h post exposure, only the adipogenic miRNA sequences were elevated, while the anti-adipogenic miRNA sequences were decreased by 50% compared to untreated control. These data suggest that there are developmental changes in miRNA expression over time following MeHg exposure. We next selected one target mRNA sequence for each miRNA sequence based on miRNA-mRNA relationships observed in humans. MeHg altered the gene expression of all the target genes assayed. Except for mir-34, all the tested miRNA-mRNA sequences showed a conserved relationship between nematode and humans. To determine whether the selected miRNA sequences were involved in lipid accumulation in response to MeHg, lipid storage was investigated in transgenic worm strains that lacked the specific miRNA strains. Of the six strains investigated, only the mir-124 and let-7 mutant worms had lipid storage levels that were statistically different from wild type, suggesting that these two sequences can be potential mediators of MeHg-induced lipid dysregulation.

A Reliable Method Based on Liquid Chromatography-Tandem Mass Spectrometry for the Simultaneous Quantification of Neurotransmitters in Caenorhabditis elegans

Neurotransmitters like dopamine (DA), serotonin (SRT), γ-aminobutyric acid (GABA) and acetylcholine (ACh) are messenger molecules that play a pivotal role in transmitting excitation between neurons across chemical synapses, thus enabling complex processes in the central nervous system (CNS). Balance in neurotransmitter homeostasis is essential, and altered neurotransmitter levels are associated with various neurological disorders, e.g., loss of dopaminergic neurons (Parkinson's disease) or altered ACh synthesis (Alzheimer's disease). Therefore, it is crucial to possess adequate tools to assess precise neurotransmitter levels, and to apply targeted therapies. An established in vivo model to study neurotoxicity is the model organism Caenorhabditis elegans (C. elegans), as its neurons have been well characterized and functionally are analogous to mammals. We have developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method including a sample preparation assuring neurotransmitter stability, which allows a simultaneous neurotransmitter quantification of DA, SRT, GABA and ACh in C. elegans, but can easily be applied to other matrices. LC-MS/MS combined with isotope-labeled standards is the tool of choice, due to its otherwise unattainable sensitivity and specificity. Using C. elegans together with our analytically validated and verified method provides a powerful tool to evaluate mechanisms of neurotoxicity, and furthermore to identify possible therapeutic approaches.

Progressive Motor and Non-Motor Symptoms in Park7 Knockout Zebrafish

DJ-1 is a redox sensitive protein with a wide range of functions related to oxidative stress protection. Mutations in the park7 gene, which codes for DJ-1 are associated with early onset familial Parkinson’s disease and increased astrocytic DJ-1 levels are found in pathologic tissues from idiopathic Parkinson’s disease. We have previously established a DJ-1 knockout zebrafish line that developed normally, but with aging the DJ-1 null fish had a lowered level of tyrosine hydroxylase, respiratory mitochondrial failure and a lower body mass. Here we have examined the DJ-1 knockout from the early adult stage and show that loss of DJ-1 results in a progressive, age-dependent increase in both motoric and non-motoric symptoms associated to Parkinson’s disease. These changes coincide with changes in mitochondrial and mitochondrial associated proteins. Recent studies have suggested that a decline in NAD+ can contribute to Parkinson’s disease and that supplementation of NAD+ precursors may delay disease progression. We found that the brain NAD+/NADH ratio decreased in aging zebrafish but did not correlate with DJ-1 induced altered behavior. Differences were first observed at the late adult stage in which NAD+ and NADPH levels were decreased in DJ-1 knockouts. Considering the experimental power of zebrafish and the development of Parkinson’s disease-related symptoms in the DJ-1 null fish, this model can serve as a useful tool both to understand the progression of the disease and the effect of suggested treatments.

Importance of NAD+ Anabolism in Metabolic, Cardiovascular and Neurodegenerative Disorders

The role of nicotinamide adenine dinucleotide (NAD+) in ageing has emerged as a critical factor in understanding links to a wide range of chronic diseases. Depletion of NAD+, a central redox cofactor and substrate of numerous metabolic enzymes, has been detected in many major age-related diseases. However, the mechanisms behind age-associated NAD+ decline remains poorly understood. Despite limited conclusive evidence, supplements aimed at increasing NAD+ levels are becoming increasingly popular. This review provides renewed insights regarding the clinical utility and benefits of NAD+ precursors, namely nicotinamide (NAM), nicotinic acid (NA), nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN), in attenuating NAD+ decline and phenotypic characterization of age-related disorders, including metabolic, cardiovascular and neurodegenerative diseases. While it is anticipated that NAD+ precursors can play beneficial protective roles in several conditions, they vary in their ability to promote NAD+ anabolism with differing adverse effects. Careful evaluation of the role of NAD⁺, whether friend or foe in ageing, should be considered.

The intertwining between lead and ethanol in the model organism Caenorhabditis elegans

Caenorhabditis elegans (C. elegans) is a model organism widely used to evaluate the mechanistic aspects of toxicants with the potential to predict responses comparable to those of mammals. We report here the consequences of developmental lead (Pb) exposure on behavioral responses to ethanol (EtOH) in C. elegans. In addition, we present data on morphological alterations in the dopamine (DA) synapse and DA-dependent behaviors aimed to dissect the neurobiological mechanisms that underlie the relationship between these neurotoxicants. Finally, the escalation to superior animals that parallels the observed effects in both experimental models with references to EtOH metabolism and oxidative stress is also discussed. Overall, the literature revised here underpins the usefulness of C. elegans to evidence behavioral responses to a combination of neurotoxicants in mechanistic-orientated studies.

MeHg exposure impairs both the catecholaminergic and cholinergic systems resulting in motor and non-motor behavioral changes in Drosophila melanogaster

Human exposure to the natural environmental contaminant methylmercury (MeHg) has been associated to adverse health effects. Importantly, the mechanisms by which this organomercurial exerts its neurotoxicity have yet to be fully clarified. Therefore, the aim of this study was to evaluate whether exposure to MeHg alters dopamine (DA) and octopamine (OA) levels, acetylcholinesterase (AChE) activity and impacts both motor and non-motor behaviours. We studied the effect of MeHg by feeding 1-2 d old flies (male and females) with 25 and 50 μM MeHg for 4 d and determined effects on survival, motor and non-motor behaviours, oxidative stress, AChE and tyrosine hydroxylase (TH) activities, as well as DA and OA levels. We found that Drosophila melanogaster (D. melanogaster) exposed to MeHg showed a reduction in survival rate, associated with the inhibition of AChE and TH activities in head of flies and decreased DA and OA levels. These changes were accompanied by behavioural alterations, such as locomotor deficit and increased grooming behaviour, in addition to an increase in oxidative stress markers both in head and in body of flies, and an increase in glutathione-S-transferase (GST) activity in head of flies. Collectively, our data support the hypothesis that MeHg neurotoxicity is associated with altered OA and DA levels, AChE inhibition, which may serve, at least in part, as the underpinnings of both motor and non-motor behavioural changes.

Methylmercury induced apoptosis of human neuroblastoma cells through the reactive oxygen species mediated caspase and poly ADP-ribose polymerase/apoptosis-inducing factor dependent pathways

Methylmercury (MeHg) is an environmental neurotoxic substance, which can easily cross the blood-brain barrier, causing irreversible damage to the human central nervous system. Reactive oxygen species (ROS) are involved in various ways of intracellular physiological or pathological processes including neuronal apoptosis. This study attempted to explore the role of ROS-mediated poly ADP-ribose polymerase (PARP)/apoptosis-inducing factor (AIF) apoptosis signaling pathway in the process of MeHg-induced cell death of human neuroblastoma cells (SH-SY5Y). Here, we found that SH-SY5Y cells underwent apoptosis in response to MeHg, which was accompanied by the increased levels of ROS and calcium ion, and the activation of caspase cascades and PARP. Inhibiting the production of ROS can reduce the apoptosis rate to a certain extent. PARP/AIF apoptotic pathway is independent of caspase dependent signaling pathway and regulates it. In conclusion, these results suggest that ROS mediated activation of caspase pathway and PARP/AIF signaling pathway are involved in MeHg induced apoptosis, and these two pathways interact with each other.

ESR1 Regulates the Obesity- and Metabolism-Differential Gene MMAA to Inhibit the Occurrence and Development of Hepatocellular Carcinoma

Obesity is often regarded as a factor that promotes tumorigenesis, but the role of obesity in promoting hepatocellular carcinoma (HCC) is still controversial. We compared the trend change of 14 obesity-related genes in the formation and development of HCC in normal, adjacent, and HCC tissues. Mendelian randomization (MR) analysis was used to verify the relationship between obesity and HCC occurrence. Metabolism of cobalamin-associated A (MMAA) was discovered as an obesity- and metabolism-differential gene, and its function in HCC was tested in vitro and in vivo. Finally, we explored how obese female patients with an originally high expression of female estrogen receptor (ESR1) directly upregulated MMAA to interfere with the progression of HCC. Fourteen obesity-related genes were downregulated in adjacent and tumoral tissues compared with normal liver tissues, which indicated that obesity may be inversely related to the occurrence of HCC and was consistent with the results of MR analysis. We also discovered that MMAA is a metabolic gene closely related to the occurrence and development of HCC by mining the TCGA database, and it functioned an anti-tumor-promoting role in HCC by damaging the mitochondrial function and preserving the redox balance. We further verified that obese females with a high expression of ESR1 can regulate MMAA to protect HCC from progression. This study elucidates that obesity might be a protective factor for female HCC patients, as they originally highly expressed ESR1, which could upregulate MMAA to suppress tumor growth and participate in metabolic reprogramming.

MicroRNA Expression Influences Methylmercury-Induced Lipid Accumulation and Mitochondrial Toxicity in Caenorhabditis elegans

Metabolic effects of methylmercury (MeHg) are gaining wider attention. We have previously shown that MeHg causes lipid dysregulation in Caenorhabditis elegans (C. elegans), leading to altered gene expression, increased triglyceride levels and lipid storage, and altered feeding behaviors. Transcriptional regulators, such as transcription factors and microRNAs (miRNAs), have been shown to regulate lipid storage, serum triglycerides, and adipogenic gene expression in human and rodent models of metabolic diseases. As we recently investigated adipogenic transcription factors induced by MeHg, we were, therefore, interested in whether MeHg may also regulate miRNA sequences to cause metabolic dysfunction. Lipid dysregulation, as measured by triglyceride levels, lipid storage sites, and feeding behaviors, was assessed in wild-type (N2) worms and in transgenic worms that either were sensitive to miRNA expression or were unable to process miRNAs. Worms that were sensitive to the miRNA expression were protected from MeHg-induced lipid dysregulation. In contrast, the mutant worms that were unable to process miRNAs had exacerbated MeHg-induced lipid dysregulation. Concurrent with differential lipid homeostasis, miRNA-expression mutants had altered MeHg-induced mitochondrial toxicity as compared to N2, with the miRNA-sensitive mutants showing mitochondrial protection and the miRNA-processing mutants showing increased mitotoxicity. Taken together, our data demonstrate that the expression of miRNAs is an important determinant in MeHg toxicity and MeHg-induced metabolic dysfunction in C. elegans.

Methylmercury-Induced Metabolic Alterations in Caenorhabditis elegans Are Diet-Dependent

Methylmercury (MeHg) is a well-known neurotoxicant; however, its role in metabolic diseases has been gaining wider attention. Chronic exposure to MeHg in human populations shows an association with diabetes mellitus and metabolic syndrome (MS). As the incidences of both obesity and MS are on the rise globally, it is important to understand the potential role of MeHg in the development of the disease. There is a dearth of information on dietary interactions between MeHg and lipids, which play an important role in developing MS. We have previously shown that MeHg increases food seeking behaviors, lipid levels, fat storage, and pro-adipogenic gene expression in C. elegans fed the standard OP50 Escherichia coli diet. However, we hypothesized that these metabolic changes could be prevented if the worms were fed a bacterial diet lower in lipid content. We tested whether C. elegans developed metabolic alterations in response to MeHg if they were fed two alternative E. coli strains (HT115 and HB101) that are known absorb significantly less lipids from their media. Additionally, to explore the effect of a high-lipid and high-cholesterol diet on MeHg-induced metabolic dysfunction, we supplemented the OP50 strain with twice the standard concentration of cholesterol in the nematode growth media. Wild-type worms fed either the HB101 or HT115 diet were more resistant to MeHg than the worms fed the OP50 diet, showing a significant right-hand shift in the dose–response survival curve. Worms fed the OP50 diet supplemented with cholesterol were more sensitive to MeHg, showing a significant left-hand shift in the dose–response survival curve. Changes in sensitivity to MeHg by differential diet were not due to altered MeHg intake in the worms as measured by inductively coupled mass spectrometry. Worms fed the low-fat diets showed protection from MeHg-induced metabolic changes, including decreased food consumption, lower triglyceride content, and lower fat storage than the worms fed either of the higher-fat diets. Oxidative stress is a common characteristic of both MeHg exposure and high-fat diets. Worms fed either OP50 or OP50 supplemented with cholesterol and treated with MeHg had significantly higher levels of reactive oxygen species, carbonylated proteins, and loss of glutathione than the worms fed the HT115 or HB101 low-lipid diets. Taken together, our data suggest a synergistic effect of MeHg and dietary lipid levels on MeHg toxicity and fat metabolism in C. elegans, which may affect the ability of MeHg to cause metabolic dysfunction.

Mercury Toxicity and Neurogenesis in the Mammalian Brain

The mammalian brain is formed from billions of cells that include a wide array of neuronal and glial subtypes. Neural progenitor cells give rise to the vast majority of these cells during embryonic, fetal, and early postnatal developmental periods. The process of embryonic neurogenesis includes proliferation, differentiation, migration, the programmed death of some newly formed cells, and the final integration of differentiated neurons into neural networks. Adult neurogenesis also occurs in the mammalian brain, but adult neurogenesis is beyond the scope of this review. Developing embryonic neurons are particularly susceptible to neurotoxicants and especially mercury toxicity. This review focused on observations concerning how mercury, and in particular, methylmercury, affects neurogenesis in the developing mammalian brain. We summarized information on models used to study developmental mercury toxicity, theories of pathogenesis, and treatments that could be used to reduce the toxic effects of mercury on developing neurons.

Methylmercury Induces Metabolic Alterations in Caenorhabditis elegans: Role for C/EBP Transcription Factor

Methylmercury (MeHg) is a well-known neurotoxicant; however, its role in metabolic diseases has been gaining wider attention. We have previously shown that MeHg causes metabolic alterations in Caenorhabditis elegans, leading to decreased nicotinamide adenine dinucleotide cofactor, mitochondrial dysfunction, and oxidative stress. We were, therefore, interested in whether MeHg also affects nutrient metabolism, particularly lipid homeostasis, which may contribute to the development of metabolic conditions such as obesity or metabolic syndrome (MS). RNA from wild-type worms exposed to MeHg was collected immediately after treatment and used for gene expression analysis by DNA microarray. MeHg differentially regulated 215 genes, 17 genes involved in lipid homeostasis, and 12 genes involved in carbohydrate homeostasis. Of particular interest was cebp-1, the worm ortholog to human C/EBP, a pro-adipogenic transcription factor implicated in MS. MeHg increased the expression of cebp-1 as well as pro-adipogenic transcription factors sbp-1 and nhr-49, triglyceride synthesis enzyme acl-6, and lipid transport proteins vit-2 and vit-6. Concurrent with the altered gene expression, MeHg increased triglyceride levels, lipid storage, and feeding behaviors. Worms expressing mutant cebp-1 were protected from MeHg-induced alterations in lipid content, feeding behaviors, and gene expression, highlighting the importance of this transcription factor in the worm's response to MeHg. Taken together, our data demonstrate that MeHg induces biochemical, metabolic, and behavioral changes in C. elegans that can lead to metabolic dysfunction.

Latent alterations in swimming behavior by developmental methylmercury exposure are modulated by the homolog of tyrosine hydroxylase in Caenorhabditis elegans

Methylmercury (MeHg) is a persistent environmental neurotoxicant that may cause adverse neurodevelopmental effects. Previous studies showed that developmental MeHg exposure caused damage to brain functions that were unmasked after a silent period of years or decades. However, the underlying mechanisms of the latent neurotoxicity associated with MeHg exposure from earlier developmental stages have yet to be fully understood. Herein, we established a Caenorhabditis elegans (C. elegans) model of developmental MeHg latent toxicity. Synchronized L1 stage worms were exposed to MeHg (0, 0.05, 0.5 and 5 μM) for 48 h. Swimming moving speeds at adulthood were analyzed in worms exposed to MeHg exposure at early larvae stages. Worms developmentally exposed to MeHg had a significant decline in swimming moving speed on day 10 adult stage, but not on day 1 or 5 adult stage, even though the mercury level in the worms exposed to 0.05 or 0.5 μM MeHg were below the quantification limit on day 10 adult. Day 10 adult worms treated with MeHg showed a significant decrease in bending angle and bending frequency during swimming. Furthermore, their reduced moving speeds tended to increase during the 300-s swimming experiment. Dopamine signaling is known to be involved in the regulation of worms' moving speed. Accordingly, the moving speed of worms with cat-2 (mammalian tyrosine hydroxylase homolog) mutation or dat-1 deletion were assayed on day 10 adult. The cat-2 mutant worms did not show a decline in moving speeds, body bends or bending angles during swimming on day 10 adult stage. Analyses of moving speeds of worms with dat-1 deletion showed that the moving speeds were further reduced after MeHg exposure. However, the effects of MeHg and dat-1 deletion were not synergistic, as the interaction between these parameters did not attain statistical significance. Altogether, our results suggest that developmental MeHg exposure reduced moving speed, and this latent toxicity was less pronounced in the context of deficient production of dopamine synthesis. Tyrosine hydroxylase plays an important role in regulating dopamine-mediated modulation of neurobehavioral functions. These findings uncovered a pivotal role of dopamine and its metabolism in the latent neurotoxic effects of MeHg.

Effects of NAD+ in Caenorhabditis elegans Models of Neuronal Damage

Nicotinamide adenine dinucleotide (NAD⁺) is an essential cofactor that mediates numerous biological processes in all living cells. Multiple NAD⁺ biosynthetic enzymes and NAD⁺-consuming enzymes are involved in neuroprotection and axon regeneration. The nematode Caenorhabditis elegans has served as a model to study the neuronal role of NAD⁺ because many molecular components regulating NAD⁺ are highly conserved. This review focuses on recent findings using C. elegans models of neuronal damage pertaining to the neuronal functions of NAD⁺ and its precursors, including a neuroprotective role against excitotoxicity and axon degeneration as well as an inhibitory role in axon regeneration. The regulation of NAD⁺ levels could be a promising therapeutic strategy to counter many neurodegenerative diseases, as well as neurotoxin-induced and traumatic neuronal damage.

Chronic exposure to methylmercury induces puncta formation in cephalic dopaminergic neurons in Caenorhabditis elegans

The neurotransmitter dopamine is a neuromodulator in the positive and negative regulation of brain circuits. Dopamine insufficiency or overload has been implicated in aberrant activities of neural circuits that play key roles in the pathogenesis of neurological and psychiatric diseases. Dopaminergic neurons are vulnerable to environmental insults. The neurotoxin methylmercury (MeHg) produces dopaminergic neuron damage in rodent as well as in Caenorhabditis elegans (C. elegans) models. Previous studies have demonstrated the utility of C. elegans as an alternative and complementary experimental model in dissecting out mechanism of MeHg-induced dopaminergic neurodegeneration. However, a sensitive pathological change that marks early events in neurodegeneration induced by environmental level of MeHg, is still lacking. By establishing a chronic exposure C. elegans model, for the first time, we have shown the propensity of MeHg (5 μM, 10 days) to induce bright puncta of dat-1::mCherry aggreagtes in the dendrites of cephalic (2 CEPs) dopaminergic neurons in a dose- and time-dependent manner, while these changes were not found in other dopaminergic neurons: anterior deirids (2 ADEs) and posterior deirids (2 PDEs), cholinergic neurons (2 AIYs) or glutamatergic neurons (2 PVDs). The bright puncta appear as an aggregation of mCherry proteins accumulating in dendrites. Further staining shows that the puncta were not inclusions in lysosome, or amyloid protein aggregates. In addition, features of the puncta including enlarged sphere shape (0.5-2 μm diameters), bright and accompanying with the shrinkage of the dendrite suggest that the puncta are likely composed of homologous mCherry molecules packaged at the dendritic site for exportation. Moreover, in the glutathione S-transferase 4 (gst-4) transcriptional reporter strain and RT-PCR assay, the expression levels of gst-4 and tubulins (tba-1 and tba-2) genes were not significantly modified under this chronic exposure paradigm, but gst-4 did show significant changes in an one day exposure paradigm. Collectively, these results suggest that CEP dopaminergic neurons are a sensitive target of MeHg, and the current exposure paradigm could be used as a model to investigate mechanism of dopaminergic neurotoxicity.

NAD+ therapy in age-related degenerative disorders: A benefit/risk analysis

Nicotinamide adenine dinucleotide (NAD+) is an essential pyridine nucleotide that is present in all living cells. NAD+ acts as an important cofactor and substrate for a multitude of biological processes including energy production, DNA repair, gene expression, calcium-dependent secondary messenger signalling and immunoregulatory roles. The de novo synthesis of NAD+ is primarily dependent on the kynurenine pathway (KP), although NAD+ can also be recycled from nicotinic acid (NA), nicotinamide (NAM) and nicotinamide riboside (NR). NAD+ levels have been reported to decline during ageing and age-related diseases. Recent studies have shown that raising intracellular NAD+ levels represents a promising therapeutic strategy for age-associated degenerative diseases in general and to extend lifespan in small animal models. A systematic review of the literature available on Medline, Embase and Pubmed was undertaken to evaluate the potential health and/or longevity benefits due to increasing NAD+ levels. A total of 1545 articles were identified and 147 articles (113 preclinical and 34 clinical) met criteria for inclusion. Most studies indicated that the NAD+ precursors NAM, NR, nicotinamide mononucleotide (NMN), and to a lesser extent NAD+ and NADH had a favourable outcome on several age-related disorders associated with the accumulation of chronic oxidative stress, inflammation and impaired mitochondrial function. While these compounds presented with a limited acute toxicity profile, evidence is still quite limited and long-term human clinical trials are still nascent in the current literature. Potential risks in raising NAD+ levels in various clinical disorders using NAD+ precursors include the accumulation of putative toxic metabolites, tumorigenesis and promotion of cellular senescence. Therefore, NAD+ metabolism represents a promising target and further studies are needed to recapitulate the preclinical benefits in human clinical trials.

The role of poly(ADP-ribose) polymerases in manganese exposed Caenorhabditis elegans

BACKGROUND AND AIM

When exceeding the homeostatic range, manganese (Mn) might cause neurotoxicity, characteristic of the pathophysiology of several neurological diseases. Although the underlying mechanism of its neurotoxicity remains unclear, Mn-induced oxidative stress contributes to disease etiology. DNA damage caused by oxidative stress may further trigger dysregulation of DNA-damage-induced poly(ADP-ribosyl)ation (PARylation), which is of central importance especially for neuronal homeostasis. Accordingly, this study was designed to assess in the genetically traceable in vivo model Caenorhabditis elegans the role of PARylation as well as the consequences of loss of pme-1 or pme-2 (orthologues of PARP1 and PARP2) in Mn-induced toxicity.

METHODS

A specific and sensitive isotope-dilution liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed to quantify PARylation in worms. Next to monitoring the PAR level, pme-1 and pme-2 gene expression as well as Mn-induced oxidative stress was studied in wildtype worms and the pme deletion mutants.

RESULTS AND CONCLUSION

While Mn failed to induce PARylation in wildtype worms, toxic doses of Mn led to PAR-induction in pme-1-deficient worms, due to an increased gene expression of pme-2 in the pme-1 deletion mutants. However, this effect could not be observed at sub-toxic Mn doses as well as upon longer incubation times. Regarding Mn-induced oxidative stress, the deletion mutants did not show hypersensitivity. Taken together, this study characterizes worms to model PAR inhibition and addresses the consequences for Mn-induced oxidative stress in genetically manipulated worms.

NAD+ in Brain Aging and Neurodegenerative Disorders

NAD⁺ is a pivotal metabolite involved in cellular bioenergetics, genomic stability, mitochondrial homeostasis, adaptive stress responses, and cell survival. Multiple NAD⁺-dependent enzymes are involved in synaptic plasticity and neuronal stress resistance. Here, we review emerging findings that reveal key roles for NAD⁺ and related metabolites in the adaptation of neurons to a wide range of physiological stressors and in counteracting processes in neurodegenerative diseases, such as those occurring in Alzheimer's, Parkinson's, and Huntington diseases, and amyotrophic lateral sclerosis. Advances in understanding the molecular and cellular mechanisms of NAD⁺-based neuronal resilience will lead to novel approaches for facilitating healthy brain aging and for the treatment of a range of neurological disorders.

Bacteria affect Caenorhabditis elegans responses to MeHg toxicity

The organic form of mercurial complex, methylmercury (MeHg), is a neurotoxin that bioaccumulates in the food web. Studies in model organisms, such as Caenorhabditis elegans (C. elegans), provide powerful insights on the role of genetic factors in MeHg-induced toxicity. C. elegans is a free living worm that is commonly cultured in nematode growth medium (NGM) agar plates seeded with bacteria. The bacteria have broad impact on C. elegans biology, including development, reproduction and lifespan, as well as metabolism of experimental chemicals. We hypothesized that MeHg toxicity in C. elegans could be modified by the bacterial food. Using a C. elegans wild-type (WT) strain and transgenic reporter strains, we found that bacterial food reduced the chronic toxicity of MeHg in C. elegans in a dose- and live-status-dependent manner. The MeHg-induced death rate in C. elegans was highest in fasted worms, followed by dehydrated dead bacteria, dead bacteria and live bacteria fed worms. Among the different bacterial foods, dehydrated dead bacteria fed worms were most sensitive to the toxicity of MeHg. The distinct bacteria food (dehydrated dead bacteria food) attenuated oxidative stress and development delay in C. elegans exposed to MeHg. The FOXO transcriptional factor DAF16 was not changed by MeHg but modified by the distinct bacteria food. Independent of MeHg treatment, daf-16 expression in fed worms migrated from the intestine to muscle. We conclude that, in chronic exposure studies in C. elegans, the effects of bacteria on toxicological outcomes should be considered.

Caenorhabditis elegans and its applicability to studies on restless legs syndrome

Restless legs syndrome (RLS) is a common neurological disorder in the United States. This disorder is characterized by an irresistible urge to move the legs, although the symptoms vary in a wide range. The pathobiology of RLS has been linked to iron (Fe) deficiency and dopaminergic (DAergic) dysfunction. Several genetic factors have been reported to increase the risk of RLS. Caenorhabditis elegans (C. elegans) is a well-established animal model with a fully sequenced genome, which is highly conserved with mammals. Given the detailed knowledge of its genomic architecture, ease of genetic manipulation and conserved biosynthetic and metabolic pathways, as well as its small size, ease of maintenance, speedy generation time and large brood size, C. elegans provides numerous advantages in studying RLS-associated gene-environment interactions. Here we will review current knowledge about RLS symptoms, pathology and treatments, and discuss the application of C. elegans in RLS study, including the worm homologous genes and methods that could be performed to advance the pathophysiology RLS.

Combined exposure to methylmercury and manganese during L1 larval stage causes motor dysfunction, cholinergic and monoaminergic up-regulation and oxidative stress in L4 Caenorhabditis elegans

Humans are exposed simultaneously to a variety of neurotoxic agents, including manganese (Mn) and methylmercury (MeHg). Therefore, the study of combined exposures to toxicants is timely. This work aimed to study changes in cholinergic system focusing on acetylcholinesterase (ace-2), monoaminergic system focusing on vesicular monoamine transporter (VMAT, cat-1) expression, to address changes in antioxidant enzymatic systems, namely, the expression of superoxide dismutase (sod-3 and sod-4) and catalase (ctl-3), as well as worm reproduction and locomotion. C. elegans in the L1 larval stage were exposed to Mn, MeHg or both. All analyses were done 24 h after the end of exposure, except for behavior and reproduction tests that were assessed in L4 larval stage worms. The values obtained for lethal dose 50% (LD50) were 17.78 mM for Mn and 30.63 μM for MeHg. It was observed that body bends, pharyngeal pumping and brood size decreased in worms exposed to metals when undergoing combined exposures. Relative mRNA content of ace-2, cat-1, sod-3, sod-4 and ctl-3 was increased at the highest concentration of the interaction (50 mM Mn + 50 μM MeHg). Cholinergic degeneration was observed in all groups co-exposed to both metals. Notably, combined exposure to metals was more toxic to the worms than when exposed to a single metal.

Post-translational modifications in MeHg-induced neurotoxicity

Mercury (Hg) exposure remains a major public health concern due to its widespread distribution in the environment. Organic mercurials, such as MeHg, have been extensively investigated especially because of their congenital effects. In this context, studies on the molecular mechanism of MeHg-induced neurotoxicity are pivotal to the understanding of its toxic effects and the development of preventive measures. Post-translational modifications (PTMs) of proteins, such as phosphorylation, ubiquitination, and acetylation are essential for the proper function of proteins and play important roles in the regulation of cellular homeostasis. The rapid and transient nature of many PTMs allows efficient signal transduction in response to stress. This review summarizes the current knowledge of PTMs in MeHg-induced neurotoxicity, including the most commonly PTMs, as well as PTMs induced by oxidative stress and PTMs of antioxidant proteins. Though PTMs represent an important molecular mechanism for maintaining cellular homeostasis and are involved in the neurotoxic effects of MeHg, we are far from understanding the complete picture on their role, and further research is warranted to increase our knowledge of PTMs in MeHg-induced neurotoxicity.

Extracellular Degradation Into Adenosine and the Activities of Adenosine Kinase and AMPK Mediate Extracellular NAD+-Produced Increases in the Adenylate Pool of BV2 Microglia Under Basal Conditions

Cumulating evidence has indicated NAD⁺ deficiency as a common central pathological factor of multiple diseases and aging. NAD⁺ supplement is highly protective in various disease and aging models, while two key questions have remained unanswered: (1) Does extracellular NAD⁺ also produce its effects through its degradation product adenosine? (2) Does extracellular NAD⁺ produce the protective effects by affecting cells under pathological insults only, or by affecting both normal cell and the cells under pathological insults? Since extracellular NAD⁺ can be degraded into adenosine, and endogenous adenosine levels are in the nanomolar range under physiological conditions, extracellular NAD⁺ may produce its effects through its degradation into adenosine. In this study we used BV2 microglia as a cellular model to test our hypothesis that NAD⁺ treatment can increase the intracellular adenylate pool under basal conditions through its extracellular degradation into adenosine. Our study has shown that extracellular NAD⁺ is degraded into adenosine extracellularly, which enters BV2 microglia through equilibrative nucleoside transporters under basal conditions. The intracellular adenosine is converted to AMP by adenosine kinase, which increases the intracellular ATP levels by both activating AMPK and increasing the intracellular adenylate pool. Collectively, our study has suggested a novel mechanism underlying the protective effects of NAD⁺ administration, which is mediated by extracellular NAD⁺ degradation into adenosine as well as the activities of adenosine kinase and AMPK. Our findings have also suggested that NAD⁺ administration in various disease and aging models may also produce its effects by affecting the microglia that are not under pathological insults.

Mutation of hop-1 and pink-1 attenuates vulnerability of neurotoxicity in C. elegans: the role of mitochondria-associated membrane proteins in Parkinsonism

Mitochondrial dysfunction is considered as a critical mechanism in the pathogenesis of Parkinson's disease (PD). Increasing evidence supports the notion of mitochondria-associated membranes (MAMs) in mitochondrial dysfunction; yet little is known about the role of MAMs-related proteins in the pathogenesis of PD. Herein we exposed the nematode Caenorhabditis elegans to 0.5-10.0 μM rotenone (RO) or 0.2-1.6 mM paraquat (PQ) for 3 days. Our results showed that both RO and PQ induced similar Parkinsonism including motor deficits and dopaminergic degeneration. RO/PQ caused mitochondrial damages characterized by the increase of vacuole areas and autophagy vesicles, but the decrease of mitochondrial cristae. RO/PQ-impacted mitochondrial function was also demonstrated by the decrease of ATP level and mitochondrial membrane potential. Additionally, the attachment or surrounding of endoplasmic reticulum to the damaged mitochondria indicates ultrastructural alterations in MAMs. Using fluorescently labeled transgenic nematodes, we further found that the expression of tomm-7 and genes of Complex I, II and III was reduced, whereas the expression of pink-1 was increased in the exposed animals. To determine MAMs in toxicity toward PD, we investigated the mutants of hop-1 and pink-1, encoding presenilin and PTEN-induced putative kinase 1 (PINK1) in mitochondria-associated membranes, respectively. Results demonstrated that the mutation of both hop-1 and pink-1 reduced the vulnerability of lethal, behavioral, and mitochondrial toxicity induced by RO/PQ. These findings suggest that presenilin and PINK1 play important roles in the RO/PQ-induced neurotoxicity through the mechanisms involved in mitochondria-associated membranes.

Oxidative stress, caspase-3 activation and cleavage of ROCK-1 play an essential role in MeHg-induced cell death in primary astroglial cells

Methylmercury is a toxic environmental contaminant that elicits significant toxicity in humans. The central nervous system is the primary target of toxicity, and is particularly vulnerable during development. Rho-associated protein kinase 1 (ROCK-1) is a major downstream effector of the small GTPase RhoA and a direct substrate of caspase-3. The activation of ROCK-1 is necessary for membrane blebbing during apoptosis. In this work, we examined whether MeHg could affect the RhoA/ROCK-1 signaling pathway in primary cultures of mouse astrocytes. Exposure of cells with 10 μM MeHg decreased cellular viability after 24 h of incubation. This reduction in viability was preceded by a significant increase in intracellular and mitochondrial reactive oxygen species levels, as well as a reduced NAD⁺/NADH ratio. MeHg also induced an increase in mitochondrial-dependent caspase-9 and caspase-3, while the levels of RhoA protein expression were reduced or unchanged. We further found that MeHg induced ROCK-1 cleavage/activation and promoted LIMK1 and MYPT1 phosphorylation, both of which are the best characterized ROCK-1 downstream targets. Inhibiting ROCK-1 and caspases activation attenuated the MeHg-induced cell death. Collectively, these findings are the first to show that astrocytes exposed to MeHg showed increased cleavage/activation of ROCK-1, which was independent of the small GTPase RhoA.

Methylmercury exposure causes a persistent inhibition of myogenin expression and C2C12 myoblast differentiation

Methylmercury (MeHg) is a ubiquitous environmental toxicant, best known for its selective targeting of the developing nervous system. MeHg exposure has been shown to cause motor deficits such as impaired gait and coordination, muscle weakness, and muscle atrophy, which have been associated with disruption of motor neurons. However, recent studies have suggested that muscle may also be a target of MeHg toxicity, both in the context of developmental myogenic events and of low-level chronic exposures affecting muscle wasting in aging. We therefore investigated the effects of MeHg on myotube formation, using the C2C12 mouse myoblast model. We found that MeHg inhibits both differentiation and fusion, in a concentration-dependent manner. Furthermore, MeHg specifically and persistently inhibits myogenin (MyoG), a transcription factor involved in myocyte differentiation, within the first six hours of exposure. MeHg-induced reduction in MyoG expression is contemporaneous with a reduction of a number of factors involved in mitochondrial biogenesis and mtDNA transcription and translation, which may implicate a role for mitochondria in mediating MeHg-induced change in the differentiation program. Unexpectedly, inhibition of myoblast differentiation with MeHg parallels inhibition of Notch receptor signaling. Our research establishes muscle cell differentiation as a target for MeHg toxicity, which may contribute to the underlying etiology of motor deficits with MeHg toxicity.