Disruption of dopamine metabolism by exposure to 6-PPD quinone in Caenorhabditis elegans

Polylactic acid microplastics cause transgenerational reproductive toxicity associated with activation of insulin and hedgehog ligands in C. elegans

As a member of biodegradable plastics, exposure risk of polylactic acid microplastic (PLA-MP) has received attention recently. Toxicity of PLA-MP at parental generation (P0-G) has been observed in some organisms; however, its possible transgenerational toxicity and underlying mechanisms remain unclear. In Caenorhabditis elegans, 10 and 100 μg/L PLA-MP resulted in transgenerational inhibition in reproductive capacity and transgenerational damage on gonad development. Meanwhile, transgenerational increase in germline apoptosis was detected after PLA-MP exposure at P0-G, which was associated with transgenerational dysregulation in expressions of genes governing apoptosis (ced-3, ced-4, egl-1, and ced-9) and DNA damage related genes (cep-1, mrt-2, hus-1, and clk-2). Among secreted ligand genes, PLA-MP exposure induced transgenerational increase in expression of ins-39 and wrt-3, and RNAi of ins-39 and wrt-3 inhibited germline apoptosis in PLA-MP exposed nematodes. Additionally, PLA-MP caused transgenerational increase in expression of met-2 and set-6 encoding histone methylation transferases, and germline apoptosis induced by PLA-MP could be suppressed by RNAi of met-2 and set-6. Dysregulated expressions of some apoptosis and DNA damage related genes caused by PLA-MP were reversed by RNAi of ins-39, wrt-3, met-2, and set-6. Moreover, in PLA-MP exposed animals, expression of ins-39 and wrt-3 could be further inhibited by RNAi of met-2 and set-6. Therefore, PLA-MP potentially induced reproductive toxicity across multiple generations, which was under the control of MET-2 and SET-6 activated ligands of INS-39 and WRT-3.

Neurotoxicity and accumulation of CPPD quinone at environmentally relevant concentrations in Caenorhabditis elegans

CPPD quinone (CPPDQ) is a member of PPDQs, which was widely distributed in different environments. Using Caenorhabditis elegans as an animal model, we here examined neurotoxicity and accumulation of CPPDQ and the underlying mechanism. After exposure to 0.01-10 μg/L CPPDQ, obvious body accumulation of CPDDQ was detected. Meanwhile, exposure to CPPDQ (0.01-10 μg/L) decreased head thrash, body bend, and forward turn, and increased backward turn. Nevertheless, only exposure to 10 μg/L CPPDQ induced neurodegeneration in GABAergic system. Exposure to CPPDQ (0.01-10 μg/L) further decreased expressions of daf-7 encoding TGF-β ligand, jnk-1 encoding JNK MAPK, and mpk-1 encoding ERK MAPK. Additionally, among examined G protein-coupled receptor (GPCR) genes, exposure to CPPDQ (0.01-10 μg/L) decreased dcar-1 expression and increased npr-8 expression. RNAi of daf-7, jnk-1, mpk-1, and dcar-1 resulted in susceptibility, and nhr-8 RNAi caused resistance to CPPDQ neurotoxicity and accumulation. Moreover, in CPPDQ exposed nematodes, RNAi of dcar-1 decreased jnk-1 and mpk-1 expressions, and RNAi of npr-8 increased mpk-1 expression. Therefore, exposure to CPPDQ potentially resulted in neurotoxicity by inhibiting TGF-β, JNK MAPK, and ERK MAPK signals. The inhibition in JNK MAPK and ERK MAPKs signals in CPPDQ exposed nematodes was further related to alteration in GPCRs of DCAR-1 and NHR-8 in nematodes.

Exposure to 6-PPD quinone causes damage on mitochondrial complex I/II associated with lifespan reduction in Caenorhabditis elegans

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ) is an emerging pollutant transformed from 6-PPD. However, the effect of 6-PPDQ exposure on mitochondrion and underlying mechanism remains largely unclear. Using Caenorhabditis elegans as animal model, exposed to 6-PPDQ at 0.1-10 μg/L was performed form L1 larvae to adult day-1. Exposure to 6-PPDQ (1 and 10 μg/L) could increase oxygen consumption rate and decease adenosine 5'-triphosphate (ATP) content, suggesting induction of mitochondrial dysfunction. Activities of NADH dehydrogenase (complex I) and succinate dehydrogenase (complex II) were inhibited, accompanied by a decrease in expressions of gas-1, nuo-1, and mev-1. RNAi of gas-1 and mev-1 enhanced mitochondrial dysfunction and reduced lifespan of 6-PPDQ exposed nematodes. GAS-1 and MEV-1 functioned in parallel to regulate 6-PPDQ toxicity to reduce the lifespan. Insulin peptides and the insulin signaling pathway acted downstream of GAS-1 and MEV-1 to control the 6-PPDQ toxicity on longevity. Moreover, RNAi of sod-2 and sod-3, targeted genes of daf-16, caused susceptibility to 6-PPDQ toxicity in reducing lifespan and in causing reactive oxygen species (ROS) production. Therefore, 6-PPDQ at environmentally relevant concentrations (ERCs) potentially caused mitochondrial dysfunction by affecting mitochondrial complexes I and II, which was associated with lifespan reduction by affecting insulin signaling in organisms.

Transgenerational Response of Germline Nuclear Hormone Receptor Genes to Nanoplastics at Predicted Environmental Doses in Caenorhabditis elegans

Transgenerational nanoplastic toxicity could be detected in Caenorhabditis elegans after exposure at the parental generation (P0-G); however, the underlying mechanisms remain largely unclear. We aimed to examine the role of germline nuclear hormone receptors (NHRs) in controlling the transgenerational toxicity of polystyrene nanoparticles (PS-NPs) based on gene expression screening and functional analysis. Among germline NHR genes, daf-12, nhr-14, and nhr-47 expressions were increased and nhr-12 expression was decreased by PS-NPs (1 and 10 μg/L). Transgenerational alterations in expressions of these four NHR genes were also induced by PS-NPs (1 and 10 μg/L). RNAi of daf-12, nhr-14, and nhr-47 caused resistance, whereas RNAi of nhr-12 conferred susceptibility to transgenerational PS-NP toxicity. After PS-NP exposure, expressions of ins-3, daf-28, and ins-39 encoding insulin ligands, efn-3 encoding Ephrin ligand, and lin-44 encoding Wnt ligand, as well as expressions of their receptor genes (daf-2, vab-1, and/or mig-1), were dysregulated by the RNAi of daf-12, nhr-14, nhr-47, and nhr-12. Therefore, alteration in certain germline NHRs could mediate the induction of transgenerational nanoplastic toxicity by affecting secreted ligands and their receptors in the offspring of exposed organisms.

Exposure to 6-PPD quinone causes ferroptosis activation associated with induction of reproductive toxicity in Caenorhabditis elegans

Exposure to N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ) caused toxicity on Caenorhabditis elegans, including reproductive toxicity. However, the underlying mechanisms for this induced reproductive toxicity by 6-PPDQ remain largely unclear. We examined possible association of ferroptosis activation with reproductive toxicity of 6-PPDQ. In 1-100 μg/L 6-PPDQ exposed nematodes, Fe2+ content was increased, which was accompanied with enhanced lipid peroxidation, increased malonydialdehyde (MDA) content, and decreased L-glutathione (GSH) content. Exposure to 1-100 μg/L 6-PPDQ decreased expressions of ftn-1 encoding ferritin, ads-1 encoding AGPS, and gpx-6 encoding GPX4 and increased expression of bli-3 encoding dual oxidase. After 6-PPDQ exposure, RNAi of ftn-1 decreased ads-1 and gpx-6 expressions and increased bli-3 expression. RNAi of ftn-1, ads-1, and gpx-6 strengthened alterations in ferroptosis related indicators, and RNAi of bli-3 suppressed changes of ferroptosis related indicators in 6-PPDQ exposed nematodes. Meanwhile, RNAi of ftn-1, ads-1, and gpx-6 induced susceptibility, and RNAi of bli-3 caused resistance to 6-PPDQ reproductive toxicity. Moreover, expressions of DNA damage checkpoint genes (clk-2, mrt-2, and hus-1) could be increased by RNAi of ftn-1, ads-1, and gpx-6 in 6-PPDQ exposed nematodes. Therefore, our results demonstrated activation of ferroptosis in nematodes exposed to 6-PPDQ at environmentally relevant concentrations, and this ferroptosis activation was related to reproductive toxicity of 6-PPDQ.

Comparison of intestinal toxicity in enhancing intestinal permeability and in causing ROS production of six PPD quinones in Caenorhabditis elegans

As the derivatives of p-phenylenediamines (PPDs), PPD quinones (PPDQs) have received increasing attention due to their possible exposure risk. We compared the intestinal toxicity of six PPDQs (6-PPDQ, 77PDQ, CPPDQ, DPPDQ, DTPDQ and IPPDQ) in Caenorhabditis elegans. In the range of 0.01-10 μg/L, only 77PDQ (10 μg/L) moderately induced the lethality. All the examined PPDQs at 0.01-10 μg/L did not affect intestinal morphology. Different from this, exposure to 6-PPDQ (1-10 μg/L), 77PDQ (0.1-10 μg/L), CPPDQ (1-10 μg/L), DPPDQ (1-10 μg/L), DTPDQ (1-10 μg/L), and IPPDQ (10 μg/L) enhanced intestinal permeability to different degrees. Meanwhile, exposure to 6-PPDQ (0.1-10 μg/L), 77PDQ (0.01-10 μg/L), CPPDQ (0.1-10 μg/L), DPPDQ (0.1-10 μg/L), DTPDQ (1-10 μg/L), and IPPDQ (1-10 μg/L) resulted in intestinal reactive oxygen species (ROS) production and activation of both SOD-3::GFP and GST-4::GFP. In 6-PPDQ, 77PDQ, CPPDQ, DPPDQ, DTPDQ, and/or IPPDQ exposed nematodes, the ROS production was strengthened by RNAi of genes (acs-22, erm-1, hmp-2, and pkc-3) governing functional state of intestinal barrier. Additionally, expressions of acs-22, erm-1, hmp-2, and pkc-3 were negatively correlated with intestinal ROS production in nematodes exposed to 6-PPDQ, 77PDQ, CPPDQ, DPPDQ, DTPDQ, and/or IPPDQ. Therefore, exposure to different PPDQs differentially induced the intestinal toxicity on nematodes. Our data highlighted potential exposure risk of PPDQs at low concentrations to organisms by inducing intestinal toxicity.

Environmental profiles, hazard identification, and toxicological hallmarks of emerging tire rubber-related contaminants 6PPD and 6PPD-quinone

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is commonly used in rubber compounds as antioxidants to protect against degradation from heat, oxygen, and ozone exposure. This practice extends the lifespan of rubber products, including tires, by preventing cracking, aging, and deterioration. However, the environmental consequences of waste generated during rubber product use, particularly the formation of 6PPD-quinone (6PPD-Q) through the reaction of 6PPD with ozone, have raised significant concerns due to their detrimental effects on ecosystems. Extensive research has revealed the widespread occurrence of 6PPD and its derivate 6PPD-Q in various environmental compartments, including air, water, and soil. The emerging substance of 6PPD-Q has been shown to pose acute mortality and long-term hazards to aquatic and terrestrial organisms at concentrations below environmentally relevant levels. Studies have demonstrated toxic effects of 6PPD-Q on a range of organisms, including zebrafish, nematodes, and mammals. These effects include neurobehavioral changes, reproductive dysfunction, and digestive damage through various exposure pathways. Mechanistic insights suggest that mitochondrial stress, DNA adduct formation, and disruption of lipid metabolism contribute to the toxicity induced by 6PPD-Q. Recent findings of 6PPD-Q in human samples, such as blood, urine, and cerebrospinal fluid, underscore the importance of further research on the public health and toxicological implications of these compounds. The distribution, fate, biological effects, and underlying mechanisms of 6PPD-Q in the environment highlight the urgent need for additional research to understand and address the environmental and health impacts of these compounds.

Long-term exposure to 6-PPD quinone at environmentally relevant concentrations causes neurotoxicity by affecting dopaminergic, serotonergic, glutamatergic, and GABAergic neuronal systems in Caenorhabditis elegans

6-PPD quinone (6-PPDQ), an emerging environmental pollutant, is converted based on 6-PPD via ozonation. However, a systematic evaluation on possible neurotoxicity of long-term and low-dose 6-PPDQ exposure and the underlying mechanism remain unknown. In the present work, 0.1-10 μg/L 6-PPDQ was added to treat Caenorhabditis elegans for 4.5 days, with locomotion behavior, neuronal development, sensory perception behavior, neurotransmitter content, and levels of neurotransmission-related genes being the endpoints. 6-PPDQ exposure at 0.1-10 μg/L significantly reduced locomotion behavior, and that at 1-10 μg/L decreased sensory perception behavior in nematodes. Moreover, 6-PPDQ exposure at 10 μg/L notably induced damage to the development of dopaminergic, glutamatergic, serotonergic, and GABAergic neurons. Importantly, nematodes with chronic 6-PPDQ exposure at 10 μg/L were confirmed to suffer obviously decreased dopamine, serotonin, glutamate, dopamine, and GABA contents and altered neurotransmission-related gene expression. Meanwhile, the potential binding sites of 6-PPDQ and neurotransmitter synthesis-related proteins were further shown by molecular docking method. Lastly, Pearson's correlation analysis showed that locomotion behavior and sensory perception behavior were positively correlated with the dopaminergic, serotonergic, glutamatergic, and GABAergic neurotransmission. Consequently, 6-PPDQ exposure disturbed neurotransmitter transmission, while such changed molecular foundation for neurotransmitter transmission was related to 6-PPDQ toxicity induction. The present work sheds new lights on the mechanisms of 6-PPDQ and its possible neurotoxicity to organisms at environmentally relevant concentrations.

Evaluation of 6-PPD quinone toxicity on lung of male BALB/c mice by quantitative proteomics

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ), a transformation product of tyre-derived 6-PPD, has been frequently detected in different environments. After 6-PPDQ exposure, we here aimed to examine dynamic lung bioaccumulation, lung injury, and the underlying molecular basis in male BALB/c mice. After single injection at concentration of 4 mg/kg, 6-PPDQ remained in lung up to day 28, and higher level of 6-PPDQ bioaccumulation in lung was observed after repeated injection. Severe inflammation was observed in lung after both single and repeated 6-PPDQ injection as indicated by changes of inflammatory cytokines (TNF-α, IL-6 and IL-10). Sirius red staining and hydroxyproline content analysis indicated that repeated rather than single 6-PPDQ injection induced fibrosis in lung. Repeated 6-PPDQ injection also severely impaired lung function in mice by influencing chord compliance (Cchord) and enhanced pause (Penh). Proteomes analysis was further carried out to identify molecular targets of 6-PPDQ after repeated injection, which was confirmed by transcriptional expression analysis and immunohistochemistry staining. Alterations in Ripk1, Fadd, Il-6st, and Il-16 expressions were identified to be associated with inflammation induction of lung after repeated 6-PPDQ injection. Alteration in Smad2 expression was identified to be associated with fibrosis formation in lung of 6-PPDQ exposed mice. Therefore, long-term and repeated 6-PPDQ exposure potentially resulted in inflammation and fibrosis in lung by affecting certain molecular signals in mammals. Our results suggested several aspects of lung injury caused by 6-PPDQ and provide the underlying molecular basis. These observations implied the possible risks of long-term 6-PPDQ exposure to human health.

Polyethylene nanoparticles at environmentally relevant concentrations enhances neurotoxicity and accumulation of 6-PPD quinone in Caenorhabditis elegans

The exposure risk of 6-PPD quinone (6-PPDQ) has aroused increasing concern. In the natural environment, 6-PPDQ could interact with other pollutants, posing more severe environmental problems and toxicity to organisms. We here examined the effect of polyethylene nanoplastic (PE-NP) on 6-PPDQ neurotoxicity and the underling mechanisms in Caenorhabditis elegans. In nematodes, PE-NP (1 and 10 μg/L) decreased locomotion behavior, but did not affect development of D-type neurons. Exposure to PE-NP (1 and 10 μg/L) strengthened neurotoxicity of 6-PPDQ (10 μg/L) on the aspect of locomotion and neurodegeneration induction of D-type motor neurons. Exposure to PE-NPs (10 μg/L) caused increase in expressions of mec-4, asp-3, and asp-4 governing neurodegeneration in 10 μg/L 6-PPDQ exposed nematodes. Moreover, exposure to PE-NP (10 μg/L) increased expression of some neuronal genes (daf-7, dbl-1, jnk-1, and mpk-1) in 6-PPDQ exposed nematodes, and RNAi of these genes resulted in susceptibility to neurotoxicity of PE-NP and 6-PPDQ. 6-PPDQ could be adsorbed by PE-NPs, and resuspension of PE-NP and 6-PPDQ after adsorption equilibrium exhibited similar neurotoxicity to co-exposure of PE-NP and 6-PPDQ. In addition, exposure to PE-NP (1 and 10 μg/L) increased 6-PPDQ accumulation in body of nematodes and increased defecation cycle length in 6-PPDQ exposed nematodes. Therefore, 6-PPDQ could be adsorbed on nanoplastics (such as PE-NPs) and enhance both neurotoxicity and accumulation of 6-PPDQ in organisms.

Comparison of reproductive toxicity between pristine and aged polylactic acid microplastics in Caenorhabditis elegans

Caenorhabditis elegans was employed as model to compare reproductive toxicity between pristine and aged polylactic acid microplastics (PLA-MPs). Aged PLA-MPs induced by UV irradiation showed degradation reflected by decrease in size and alteration in morphological surface. Aged PLA-MPs also exhibited some certain changes of chemical properties compared to pristine PLA-MP. Compared with pristine PLA-MPs, more severe toxicity on reproductive capacity and gonad development was detected in 1-100 μg/L aged PLA-MPs. Meanwhile, aged PLA-MPs caused more severe enhancement in germline apoptosis and alterations in expressions of ced-9, ced-4, ced-3, and egl-1 governing cell apoptosis. In addition, aged PLA-MPs resulted in more severe increase in expression of DNA damage related genes (cep-1, mrt-2, hus-1, and clk-2) compared to pristine PLA-MPs, and the alterations in expression of ced-9, ced-4, ced-3, and egl-1 in pristine and aged PLA-MPs could be reversed by RNAi of cep-1, mrt-2, hus-1, and clk-2. Besides this, enhanced germline apoptosis in pristine and aged PLA-MPs exposed animals was also suppressed by RNAi of cep-1, mrt-2, hus-1, and clk-2. Therefore, our results suggested the more severe exposure risk of aged PLA-MPs than pristine PLA-MPs in causing reproductive toxicity, which was associated with the changed physicochemical properties and DNA damage induced germline apoptosis.

Polystyrene nanoparticles strengthen high glucose toxicity associated with alteration in insulin signaling pathway in C. elegans

Using Caenorhabditis elegans as animal model, we investigated the effect of exposure to polystyrene nanoparticles (PS-NPs) in the range of μg/L on high glucose toxicity induction. With lifespan and locomotion behavior as endpoints, we observed that PS-NP (10 and 100 μg/L) enhanced toxicity in 50 mM glucose treated animals. In insulin signaling pathway, expressions of genes encoding insulin receptor (daf-2), kinases (age-1 and akt-1/2), and insulin peptides (ins-9, ins-6, and daf-28) were increased, and expressions of daf-16 and its target of sod-3 were decreased in high glucose treated nematodes followed by PS-NP exposure. Toxicity enhancement in high glucose treated nematodes by PS-NP exposure was inhibited by RNAi of daf-2, age-1, akt-2, akt-1, and 3 insulin peptides genes, but increased by RNAi of daf-16 and sod-3. The resistance of animals with RNAi of daf-2 to toxicity in high glucose treated nematodes followed by PS-NP exposure could be suppressed by RNAi of daf-16. Moreover, in high glucose treated animals followed by PS-NP exposure, daf-2 expression was inhibited by RNAi of ins-6, ins-9, and daf-28. Our data demonstrated the risk of PS-NP exposure in enhancing the high glucose toxicity. More importantly, alteration in expression of genes in insulin signaling pathway was associated with the toxicity enhancement in high glucose treated nematodes followed by PS-NP exposure.