Environmental concentrations of tire rubber-derived 6PPD-quinone alter CNS function in zebrafish larvae

Sublethal 6PPD-quinone exposure impairs swimming performance and aerobic metabolism in juvenile lake trout (Salvelinus namaycush)

6PPD-quinone, an environmental oxidation product of the rubber tire antioxidant 6PPD, has recently gained recognition as a chemical of concern. Frequently detected in road runoff and surface waters, studies have reported this compound to cause acute lethality in several salmonid species at extremely low concentrations, including lake trout (Salvelinus namaycush; 24-h LC50 = 0.51 μg/L). Following exposure, species experiencing acute lethality show characteristic symptoms such as gasping, spiraling, increased ventilation, loss of equilibrium, erratic movements, and tumbling. However, there is a deficit of research targeted at understanding sublethal toxicities of 6PPD-quinone exposure, particularly concerning swimming capability and metabolic function. To evaluate these effects, juvenile lake trout were exposed for 20 h to a measured concentration of 0.46 μg/L 6PPD-quinone in a swim tunnel respirometer to assess temporal changes in standard metabolic rate (SMR) compared to controls. Following exposure, fish underwent a swim trial to determine critical swimming speed (Ucrit), oxygen consumption rate (MO2), active metabolic rate (AMR), aerobic scope (AS) and energetic cost of transport (CoT), followed by analysis of muscle triglyceride and glycogen concentrations. Results showed that 6PPD-quinone exposure impaired swimming performance, evident by a decrease in Ucrit. Additionally, exposure resulted in decreased AMR, although alterations in SMR were not observed. Decreased concentrations of muscle triglycerides of swam fish were also observed. These findings suggest that environmentally relevant concentrations of 6PPD-quinone disrupt aerobic metabolic capacity in juvenile lake trout, producing adverse effects that diminish endurance and maximum swim speeds, which may affect survival of fish populations.

Environmental fate, toxicity, and mitigation of 6PPD and 6PPD-Quinone: Current understanding and future directions

N'-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), a widely used antioxidant in the rubber industry, has garnered global attention due to the high toxicity and ecological-health risks posed by its environmental oxidation product, 6PPD-quinone (6PPD-Q). With the continuous release of tire wear particles (TWPs), 6PPD-Q is ubiquitously distributed in atmospheric, aquatic, and terrestrial environments, as well as within organisms, where it bioaccumulates through food chains. Notably, 6PPD-Q has been detected in human urine, serum, and cerebrospinal fluid, and its association with abnormal α-synuclein aggregation in the brains of Parkinson's patients further underscores its neurotoxic risks. This review systematically examines the environmental occurrence and migration patterns of 6PPD and 6PPD-Q, their multisystem toxicity, highly sensitive detection technologies, and pollution control strategies, while highlighting critical gaps in current research, such as chronic exposure mechanisms, combined pollution effects, and environmental safety thresholds. By synthesizing existing knowledge, this review provides a scientific foundation for elucidating the ecological and health risks of 6PPD-Q, offering critical insights to advance environmental regulatory policies, promote green transformation in the rubber industry, and safeguard global ecological security. Future research should prioritize long-term toxicity studies, refined detection techniques, and sustainable regulatory frameworks to mitigate the ecological and health risks posed by these emerging contaminants.

p-Phenylenediamines and their derived quinones: A review of their environmental fate, human exposure, and biological toxicity

p-Phenylenediamines (PPDs) are widely used as antioxidants in numerous rubber products to prevent or delay oxidation and corrosion. However, their derived quinones (PPD-Qs), generated through reactions with ozone, are ubiquitous in the environment and raise significant health and toxicity concerns. This review summarizes the current state of knowledge on environmental distribution and fate, human exposure, and biological toxicity of PPDs and PPD-Qs, and makes recommendations for future research directions. Although PPDs and PPD-Qs have been monitored in a variety of environmental matrices, studies on soil, sediment, and organisms remain limited. This shortcoming hinders our understanding of their distribution patterns and migration mechanisms in these specific environments. These contaminants can enter the human body through various exposure routes, but toxicological studies have not yielded sufficient results to derive risk thresholds for the assessment of human health. Most studies examining biological and toxicological effects have focused on acute exposure scenarios, which do not accurately reflect the long-term interactions that occur in natural settings. The toxic effects of PPDs and PPD-Qs on zebrafish, nematodes, and mammals include neurobehavioral changes, reproductive dysfunction, and digestive damage, which are linked to mitochondrial stress, DNA adduct formation, and disrupted lipid metabolism, respectively. However, the underlying toxicological mechanisms remain poorly understood. Future research should prioritize the investigation of the impacts of PPDs and PPD-Qs on various organizational levels within biota to provide a scientific basis for developing effective risk management measures.

Molecular insight on conformational alterations and functional changes of acetylcholinesterase induced by an emerging environmental pollutant 6PPD-quinone

The emerging pollutant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-quinone) has attracted broad attention because of its widespread presence and harmful impacts, including hepatotoxicity and neurotoxicity. Acetylcholinesterase (AChE) is commonly used as a classical biomarker for assessing toxicity in the nervous system. Here, the interaction mechanism between AChE and 6PPD-quinone was investigated using a combination of multispectral and computational approaches, including enzyme activity assay, fluorescence thermodynamic titration, circular dichroism (CD) spectroscopy, molecular dynamics (MD) simulation, computational alanine scanning (CAS), and free energy landscape (FEL) analysis, among others. The result indicates that 6PPD-quinone spontaneously binds into the active site of AChE, thereby competitively inhibiting enzyme's activity. The interaction is primarily facilitated by hydrogen bonds and van der Waals forces, exhibiting a binding constant (Kb) of 1.044 × 104 M-1 at 298 K. The introduction of 6PPD-quinone causes a reduction in the α-helix content of AChE, making the structure less stable and more relaxed. Furthermore, the FEL analysis of AChE revealed that, with the presence of 6PPD-quinone, the number of global minima of AChE increased from 2 to 2-3. Additionally, Molecular docking outcomes exhibit that 6PPD-quinone interacted with tyrosine (TYR) 337, TYR124, tryptophan (TRP) 86, serine (SER) 203, glycine (GLY) 120 and other residues of AChE. CAS analysis shows binding free energy changes (ΔΔGbinding) of TRP86, TYR337 were 5.17 and 2.57 kcal mol-1, respectively, highlighting their key roles in the binding process of 6PPD-quinone with AChE. The interactions of 6PPD-quinone with the TRP86 and TYR337 may be the reason for the decrease in AChE activity.

Inhalation exposure to tire rubber particle-sourced pollutant 6PPD-quinone involving basolateral amygdala impairment in male ICR mice

INTRODUCTION

The extensive use of 6PPD in tire rubber has led to its increase in atmospheric tire wear particles. 6PPD reacts with ozone to form 6PPD-quinone (6PPD-Q), a respiratory toxicant enriched in tire and road wear particles.

OBJECTIVES

The aim of this study is to decipher the potential sensitive target organs and toxic actions by inhalation exposure to atmospheric 6PPD-Q.

METHODS

This study employed a mouse inhalation exposure model to simulate environmental levels of 6PPD-Q at three concentrations of 0, 0.14, and 14 mg/m3. Using a 28-day exposure period followed by an equivalent recovery phase, we systematically evaluated the toxicological targets and effects of subacute exposure.

RESULTS

The findings revealed that, compared to pulmonary, cardiovascular, and metabolic organ damage, 6PPD-Q-induced neurotoxicity was more persistent and irreversible, particularly characterized by prolonged anxiety-like behaviors. Histopathological analyses of the basolateral amygdala, using Nissl staining and markers of neuronal aging, indicated substantial neuronal degeneration linked to elevated oxidative stress, identifying this region as a critical target of 6PPD-Q neurotoxicity. Transcriptomic analysis uncovered that the expression of Egr1, a transcription factor crucial for neuronal plasticity, was markedly dysregulated. Findings of significant downregulation at the gene level and an upward trend in protein expression suggest thatEgr1expression is influenced by translational efficiency, epigenetic modifications, and post-translational regulatory mechanisms. Egr1dysregulation disrupted downstream networks involving solute carrier proteins and calcium-binding proteins, contributing to aberrant neurobehavioral outcomes. Notably, the elevation ofEgr1protein levels in the basolateral amygdala but not in the cerebral cortex highlights the region-specific nature of 6PPD-Q's neurotoxic effects.

CONCLUSION

This study provides the first insights into the neurotoxicity and irreversibility of inhaled 6PPD-Q exposure, paving the way for future research into the long-term neurological consequences and regulatory mechanisms of 6PPD-Q.

Environmental factors ultraviolet a and ozone exacerbate the repeated inhalation toxicity of 6PPD in mice via accelerating the aging reaction

The burden of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its oxidized products on human health can no longer be ignored due to the detection types and concentrations in the environment continue to increase. Environmental ozone (O3) and ultraviolet A (UVA) may induce ozonation and photoaging of 6PPD to produce toxic products. However, the impact of specific environmental conditions on the aging and toxic effects of 6PPD is unclear. This study investigated the aging effects of O3 and UVA on 6PPD, and compared the repeated inhalation toxicity of differently aging 6PPDs in C57BL/6 male mice. The result showed that UVA and O3 accelerated 6PPD aging, and the aging products varied depending on the O3 and irradiation conditions. After 10 weeks of inhalation intoxication at human comparable level, mice exhibited significant neurobehavior alterations, respiratory dysfunction, and DNA damage in the blood, showing significant heterogeneity among groups. Notably, 6PPD treated with perozonation and UVA aging may be the most toxic. The study suggests inhalation health risks of transportation derived tire pollutants under the influence of ground-level ozone and ultraviolet light need more attention, and provides new insights into risk assessment and pollution control of 6PPD and other pollutants from the perspective of environmental factors.

Enantioselective disruption of circadian rhythm behavior in goldfish (Carassius auratus) induced by chiral fungicide triadimefon at environmentally-relevant concentration

The pollution of triadimefon (TDF) fungicides significantly hinders the "One Health" frame achievement. However, the enantioselective effects of chiral TDF on the circadian rhythm of fish remained unclear. Herein, TDF enantiomers (R(-)-TDF and S(+)-TDF) and racemic Rac-TDF were selected to investigate their enantioselective effects and mechanisms on circadian rhythm of goldfish (Carassius auratus) at an environmentally-relevant concentration (100 µg L⁻¹). S(+)-TDF reduced the diurnal-nocturnal differences in schooling behavior more strongly than R(-)-TDF, proving the enantioselectively weakened circadian rhythm of goldfish by TDF. S(+)-TDF more preferentially bioaccumulated in goldfish than R(-)-TDF, mainly contributed to the enantioselectively disrupted circadian rhythm. On one hand, TDF enantiomers in brains differentially inhibited neuronal activity, leading to cholinergic system dysfunction. On the other hand, TDF enantiomers in intestines differentially disrupted intestinal barriers, thus potentially dysregulating the "brain-gut" axis. Importantly, the commercial probiotics alleviated the behavioral disorder, indirectly confirming that the dysbiosis of intestinal bacteria contributed to the TDF-induced circadian rhythm disruption. These findings provide novel insights into the enantioselective disruption of fish circadian rhythm behaviors by chiral fungicides at enantiomer levels, and offer novel strategies for early assessing the ecological risks of chiral agrochemicals in aquatic ecosystems.

Acute cardiorespiratory effects of 6PPD-quinone on juvenile rainbow trout (Oncorhynchus mykiss) and arctic char (Salvelinus alpinus)

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-quinone) is an environmental transformation product of the widely used rubber tire antioxidant, 6PPD. Found in stormwater runoff, 6PPD-quinone has been reported to cause acute lethality at ≤1 μg/L in salmonids like coho salmon, rainbow trout, and brook trout. Conversely, other species such as Arctic char and brown trout are insensitive, even when exposed to significantly greater concentrations (3.8-50 μg/L). Sensitive species exhibit symptoms such as gasping, spiraling, increased ventilation, and loss of equilibrium, suggesting a possible impact on cardiorespiratory physiology. This study investigated sublethal 6PPD-quinone toxicities, focusing on cardiovascular and metabolic effects in two salmonids of varying sensitivity: a sensitive species, rainbow trout (Oncorhynchus mykiss) and a tolerant species, Arctic char (Salvelinus alpinus). Fish were exposed to measured concentrations of 0.59 or 7.15 μg/L 6PPD-quinone, respectively, in respirometry chambers for 48 h to assess temporal changes in resting oxygen consumption compared to unexposed controls. Following exposure, cardiac ultrasound and electrocardiography characterized cardiac function in vivo, while blood gas analysis examined blood composition changes. In both species, changes in resting oxygen consumption were observed. In rainbow trout only, a decrease in end systolic volume and an increase in passive ventricular filling, cardiac output, and PR interval length were observed, indicating cardiac stimulation. Cardiorespiratory symptoms observed following rainbow trout exposure might partly be driven by a significant increase in methemoglobin, resulting in an impaired ability to oxygenate tissues. This study is the first to examine the effects of 6PPD-quinone exposure on the cardiorespiratory system of salmonid fishes and provides information invaluable to a better understanding of the mechanism of 6PPD-quinone toxicity.

Tire rubber-derived contaminants 6PPD and 6PPD-quinone reduce attachment and outgrowth of trophoblast spheroids onto endometrial epithelial cells

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), a synthetic additive widely used in the rubber industry, and its oxidized product 6PPD-quinone (6PPDQ), have garnered widespread attention as an emerging hazardous chemicals owing to their potential detrimental effects on aquatic ecosystem and human health. The effects of 6PPD and 6PPDq on the female reproductive tract, especially embryo implantation, remain unknown and were investigated in this study. We used the spheroid attachment and outgrowth models of BeWo trophoblastic spheroids and Ishikawa cells as surrogates for the human blastocyst and endometrial epithelium, respectively. Treatment with the chemicals for up to 48 h decreased the viability of the cells in a dose- and cell line-dependent manner (20-100 μM 6PPD and 10-100 μM 6PPDQ for both the cell lines). At a noncytotoxic concentration, exposure of Ishikawa cells to 1 and 10 μM 6PPD reduced the attachment of BeWo spheroids and further inhibited their invasion and outgrowth on the endometrial epithelial monolayer. A similar result was observed in 1 μM 6PPDQ-exposed groups. Gene expression profiling of 6PPD- and 6PPDQ-exposed endometrial epithelial cells revealed that both 6PPD and 6PPDQ differentially regulated a panel of transcript markers toward overall downregulation of receptivity and invasion. The study provides the first proof of the adverse effects of 6PPD and 6PPDQ on human endometrial receptivity and trophoblast invasion during the window of implantation, warranting the need for further in vivo and clinical studies.

Transgenerational intestinal toxicity of 6-PPD quinone in causing ROS production, enhancement in intestinal permeability and suppression in innate immunity in C. elegans

Toxicity of 6-PPD quinone (6-PPDQ) on organisms at various aspects has been frequently observed at parental generation (P0-G). In contrast, we know little about its possible transgenerational toxicity and underlying mechanisms. In Caenorhabditis elegans, exposure to 6-PPDQ (0.1-10 μg/L) at P0-G induced transgenerational reactive oxygen species (ROS) production in intestine. Accompanied with this, transgenerational increase in intestinal permeability and decrease in expressions of genes governing intestinal function were observed. Exposure to 6-PPDQ (1 and 10 μg/L) at P0-G caused transgenerational suppression in expressions of antimicrobial genes (lys-7 and spp-1) and LYS-7::RFP. Meanwhile, intestinal ROS production could be enhanced by RNAi of acs-22, hmp-2, pkc-3, lys-7, and spp-1. Moreover, acs-22, hmp-2, and pkc-3 RNAi could inhibit innate immune response induced by 6-PPDQ. Additionally, lys-7 and spp-1 RNAi could strengthen intestinal permeability in 6-PPDQ exposed nematodes. Therefore, 6-PPDQ caused transgenerational intestinal toxicity, which was associated with both enhanced intestinal permeability and suppressed innate immunity.

A transcriptomics-based analysis of mechanisms involved in the neurobehavioral effects of 6PPD-quinone on early life stages of zebrafish

As an emerging pollutant frequently detected in aquatic ecosystems, the toxicity of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-quinone) on fish has been confirmed, but insight into the mechanisms underlying those adverse effects is still limited. Thus, we exposed zebrafish embryos to 6PPD-quinone at 0, 0.25, 2.5, and 25 μg/L until 120 h post-fertilization (hpf), and investigated the variations in their development, behavior, monoamine neurotransmitter levels, and transcriptional profile. Exposure to 6PPD-quinone notably elevated the heart rate of zebrafish at 48 hpf (at 2.5 and 25 μg/L) and 72 hpf (at 0.25, 2.5, and 25 μg/L). In the dark-light transition test, the locomotor activity of zebrafish larvae exposed to 6PPD-quinone significantly increased, especially in the dark periods. Exposure to 6PPD-quinone also altered the dopamine level and its turnover in zebrafish, which exhibited significant correlations to their locomotor activity. RNA sequencing identified 394 differentially expressed genes (DEGs), most of which have the molecular function of binding and catalytic activity. Five DEGs were predicted as the key driver genes in the protein-protein interaction networks associated with circadian rhythm (i.e., npas2), protein processing in endoplasmic reticulum (i.e., hsp90b1 and pdia4), and estrogen signaling pathway (i.e., hsp90aa1.1 and hsp90aa1.2). Our findings provide more insights into mechanisms underlying the toxicity of 6PPD-quinone to teleosts and highlight the necessity to assess its potential risks to aquatic ecosystems.

P-phenylenediamine antioxidants and their quinone derivatives: A review of their environmental occurrence, accessibility, potential toxicity, and human exposure

Substituted p-phenylenediamines (PPDs), a class of antioxidants, have been widely used to extend the lifespan of rubber products, such as tires and pipes. During use, PPDs will generate their quinone derivatives (PPD-Qs). In recent years, PPDs and PPD-Qs have been detected in the global environment. Among them, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q), the oxidation product of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), has been identified as highly toxic to coho salmon, with the lethal concentration of 50 % (LC50) being 95 ng/L, highlighting it as an emerging pollutant of great concern. This review summarizes the physicochemical properties, global environmental distribution, bioaccessibility, potential toxicity, human exposure risk, and green measures of PPDs and PPD-Qs. These chemicals exhibit lipophilicity, bioaccumulation potential, and poor aqueous stability. They have been found in water, air, dust, soil, and sediment worldwide, indicating their significance as emerging pollutants. Notably, current studies have identified electronic waste (e-waste), such as discarded wires and cables, as a non-negligible source of PPDs and PPD-Qs, in addition to tire wear. PPDs and PPD-Qs exhibit strong bioaccumulation in aquatic organisms and mammals, with a tendency for biomagnification within the food web, posing health threats to humans. Available toxicity data indicate that PPDs and PPD-Qs have negative effects on aquatic organisms, mammals, and invertebrates. Acute exposure leads to death and acute damage, and long-term exposure can cause a series of adverse effects, including growth and development toxicity, reproductive toxicity, neurotoxicity, intestinal toxicity, and multi-organ damage. This paper discusses current research gaps and offers recommendations to understand better the occurrence, behavior, toxicity, and environmental exposure risks of PPDs and PPD-Qs.

Protective role of ghrelin against 6PPD-quinone-induced neurotoxicity in zebrafish larvae (Danio rerio) via the GHSR pathway

The toxicity mechanisms of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q), an antioxidant derivative of 6PPD via ozone reaction commonly used in rubber and tire industries, were investigated in zebrafish larvae with concentrations ranging from 0 to 50 μg/L. Despite normal hatchability, 6PPD-Q exposure led to reduced body length and swimming distance in 120 hours post-fertilization (hpf) larvae. At the highest concentration (50 μg/L), 6PPD-Q significantly impaired dopaminergic neuron development and neurotransmitter levels, including dopamine, 5-hydroxytryptamine, and glutamate. Transcriptome profiling unveiled perturbations in growth and developmental gene expression, such as upregulation of runx2a, runx2b, and ghrl (ghrelin and obestatin prepropeptide), and downregulation of stat1b, auto1, and cidea. Notably, anamorelin, a growth hormone secretagogue receptor (GHSR) agonist, recovered the behavioral deficits induced by 6PPD-Q, implying a neuroprotective role of ghrelin possibly mediated via the ghrelin/GHSR pathway. Collectively, our findings indicate that ghrelin upregulation may counteract 6PPD-Q toxicity in zebrafish larvae, shedding light on potential therapeutic avenues for mitigating the adverse effects of this antioxidant byproduct.

The spatio-temporal accumulation of 6 PPD-Q in greenbelt soils and its effects on soil microbial communities

6 PPD-Q (6 PPD-Quinone) is an ozone-induced byproduct derived from the degradation of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6 PPD), commonly found in road dust resulting from tire wear. However, the extent of 6 PPD-Q pollution in urban soil remains unclear. This study investigates the spatial and temporal accumulation patterns of 6 PPD-Q in greenbelt soils in Ningbo, and explores the correlation between 6 PPD-Q accumulation and soil microbial community composition and functions. Our findings indicate that 6 PPD-Q is present (ranging from 0.85 to 12.58 μg/kg) in soil samples collected from both sides of urban traffic arteries. Soil fungi exhibit higher sensitivity to 6 PPD-Q accumulation compared to bacteria, and associated fungi (Basidiomycota) may be potential biomarkers for environmental 6 PPD-Q contamination. Co-occurrence network analysis reveals that the bacterial microbial network in summer exhibits greater stability and resilience in response to 6 PPD-Q inputs than in winter. However, 6 PPD-Q accumulation disrupts the network structure of fungal communities to some extent, leading to reduced diversity in fungal microbial communities. Long-term accumulation of 6 PPD-Q weakens the nitrogen and phosphorus cycling potential within urban soil, while the enhancement of carbon cycling may further promote 6 PPD-Q degradation in urban soil. Taken together, this study provides new insights into the ecological risks of 6 PPD-Q in urban soils.

Neurotoxicity from long-term exposure to 6-PPDQ: Recent advances

The recent acceleration of industrialization and urbanization has brought significant attention to N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ), an emerging environmental pollutant from tire wear, due to its long-term effects on the environment and organisms. Recent studies suggest that 6-PPDQ can disrupt neurotransmitter synthesis and release, impact receptor function, and alter signaling pathways, potentially causing oxidative stress, inflammation, and apoptosis. This review investigates the potential neurotoxic effects of prolonged 6-PPDQ exposure, the mechanisms underlying its cytotoxicity, and the associated health risks. We emphasize the need for future research, including precise exposure assessments, identification of individual differences, and development of risk assessments and intervention strategies. This article provides a comprehensive overview of 6-PPDQ's behavior, impact, and neurotoxicity in the environment, highlighting key areas and challenges for future research.

Behind conventional (micro)plastics: An ecotoxicological characterization of aqueous suspensions from End-of-Life Tire particles

Million tons of tires become waste every year, and the so-called End-of-Life Tires (ELTs) are ground into powder (ELT-dp; size < 0.8 mm) and granules (ELT-dg; 0.8 < size < 2.5 mm) for recycling. The aim of this study was to evaluate the sub-lethal effects of three different concentrations (0.1, 1, and 10 mg/L) of aqueous suspensions from ELT-dp and ELT-dg on Danio rerio (zebrafish) larvae exposed from 0 to 120 h post-fertilization (hpf). Chronic effects were assessed through biomarkers, real-time PCR, and proteomics. We observed a significant increase in swimming behavior and heart rate only in specimens exposed to ELT-dp suspensions at 1 and 10 mg/L, respectively. Conversely, the activities of detoxifying enzymes ethoxyresorufin-O-deethylase (EROD) and glutathione-S-transferase (GST) showed significant modulation only in specimens exposed to ELT-dg groups. Although no effects were observed through real-time PCR, proteomics highlighted alterations induced by the three ELT-dp concentrations in over 100 proteins involved in metabolic pathways of aromatic and nitrogen compounds. The results obtained suggest that the toxic mechanism of action (MoA) of ELT suspensions is mainly associated with the induction of effects by released chemicals in water, with a higher toxicity of ELT-dp compared to ELT-dg.

Short- and Long-Term Neurobehavioral Effects of Developmental Exposure to Valproic Acid in Zebrafish

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impairments in social interaction and communication, anxiety, hyperactivity, and interest restricted to specific subjects. In addition to the genetic factors, multiple environmental factors have been related to the development of ASD. Animal models can serve as crucial tools for understanding the complexity of ASD. In this study, a chemical model of ASD has been developed in zebrafish by exposing embryos to valproic acid (VPA) from 4 to 48 h post-fertilization, rearing them to the adult stage in fish water. For the first time, an integrative approach combining behavioral analysis and neurotransmitters profile has been used for determining the effects of early-life exposure to VPA both in the larval and adult stages. Larvae from VPA-treated embryos showed hyperactivity and decreased visual and vibrational escape responses, as well as an altered neurotransmitters profile, with increased glutamate and decreased acetylcholine and norepinephrine levels. Adults from VPA-treated embryos exhibited impaired social behavior characterized by larger shoal sizes and a decreased interest for their conspecifics. A neurotransmitter analysis revealed a significant decrease in dopamine and GABA levels in the brain. These results support the potential predictive validity of this model for ASD research.

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.

Navigating the environmental dynamics, toxicity to aquatic organisms and human associated risks of an emerging tire wear contaminant 6PPD quinone

N-1,3-Dimethylbutyl-N'-phenyl-p-quinone diamine (6PPDQ) is a derivative of 6PPD, a synthetic antioxidant used in tire manufacturing to control the degradation caused by oxidation and heat aging. Its discovery in 2020 has raised important environmental concern, particularly regarding its association with acute mortality in coho salmon, prompting surge in research on its occurrence, fate, and transport in aquatic ecosystems. Despite this attention, there remain notable gaps in grasping the knowledge, demanding an in depth overview. Thus, this review consolidates recent studies to offer a thorough investigation of 6PPDQ's environmental dynamics, pathways into aquatic ecosystems, toxicity to aquatic organisms, and human health implications. Various aquatic species exhibit differential susceptibility to 6PPDQ toxicity, manifesting in acute mortalities, disruption of metabolic pathways, oxidative stress, behavioral responses, and developmental abnormalities. Whereas, understanding the species-specific responses, molecular mechanisms, and broader ecological implications requires further investigation across disciplines such as ecotoxicology, molecular biology, and environmental chemistry. Integration of findings emphasizes the complexity of 6PPDQ toxicity and its potential risks to human health. However, urgent priorities should be given to the measures like long-term monitoring studies to evaluate the chronic effects on aquatic ecosystems and the establishment of standardized toxicity testing protocols to ensure the result comparability and reproducibility. This review serves as a vital resource for researchers, policymakers, and environmental professionals seeking appraisals into the impacts of 6PPDQ contamination on aquatic ecosystems and human health.

Polluted water from a storage dam (Villa Victoria, méxico) induces oxidative damage, AChE activity, embryotoxicity, and behavioral changes in Cyprinus carpio larvae

The Villa Victoria dam is one of the most important storage reservoirs in Mexico since it distributes water to more than 20 million inhabitants in the Metropolitan Zone of Mexico City. In this dam, the common carp (Cyprinus carpio) is an important food resource for the inhabitants, so the aim of this work was to evaluate the oxidative damage (lipoperoxidation, oxidized proteins, antioxidant enzymes activity and gene expression), AChE, embryotoxicity and behavioral changes in C. carpio embryos and larvae exposed to water from Villa Victoria dam for 24, 48, 72 and 96 h. The embryotoxicity was evaluated trough the General Morphology Score (GMS) and the teratogenic index. Behavioral changes in basal locomotor activity and thigmotaxis were evaluated in a DanioVision, Noldus ™. An increase in lipid and protein oxidation as well as modification of CAT, SOD and GPx enzymatic activity was observed during the exposure times. The GMS indicated a low development in the embryos, the teratogenic index was less than 1, however teratogenic effects as yolk edema, fin malformation, head malformation and scoliosis were observed. In parallel, an increase in AChE activity and gene expression was observed reflecting changes in distance traveled of the basal locomotor activity and thigmotaxis at the sampling points. In conclusion, pollutants in water from Villa Victoria dam caused oxidative damage, changes in SOD, CAT, GPx and AChE activity as well as embryotoxicity and modifications in the behavior of C. carpio larvae. This study demonstrates the need to implement restoration programs for this reservoir since, contamination in the Villa Victoria dam could eventually endanger aquatic life and human health.

A Review of N-(1,3-Dimethylbutyl)-N'-phenyl-p-Phenylenediamine (6PPD) and Its Derivative 6PPD-Quinone in the Environment

As an antioxidant and antiozonant, N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is predominantly used in the rubber industry to prevent degradation. However, 6PPD can be ozonated to generate a highly toxic transformation product called N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-quinone), which is toxic to aquatic and terrestrial organisms. Thus, 6PPD and 6PPD-quinone, two emerging contaminants, have attracted extensive attention recently. This review discussed the levels and distribution of 6PPD and 6PPD-quinone in the environment and investigated their toxic effects on a series of organisms. 6PPD and 6PPD-quinone have been widely found in air, water, and dust, while data on soil, sediment, and biota are scarce. 6PPD-quinone can cause teratogenic, developmental, reproductive, neuronal, and genetic toxicity for organisms, at environmentally relevant concentrations. Future research should pay more attention to the bioaccumulation, biomagnification, transformation, and toxic mechanisms of 6PPD and 6PPD-quinone.

Effects of 6PPD-Quinone on Human Liver Cell Lines as Revealed with Cell Viability Assay and Metabolomics Analysis

N-(1,3-Dimethyl butyl)-N'-phenyl-phenylenediamine-quinone (6PPD-Q) is a derivative of the widely used rubber tire antioxidant 6PPD, which was first found to be acutely toxic to coho salmon. Subsequent studies showed that 6PPD-Q had species-specific acute toxicity in fishes and potential hepatotoxicity in mice. In addition, 6PPD-Q has been reported in human urine, demonstrating the potential widespread exposure of humans to this chemical. However, whether 6PPD-Q poses a higher risk to humans than its parent compound, 6PPD, and could cause adverse effects in humans is still unclear. In this study, we utilized two human liver cell models (the human proto-hepatocyte model L02 and the human hepatocellular carcinoma cell line HepG2) to investigate the potentially differential effects of these two chemicals. Cell viability curve analysis showed that 6PPD-Q had lower IC50 values than 6PPD for both liver cell lines, suggesting higher toxicity of 6PPD-Q to human liver cells than 6PPD. In addition, L02 cells are more sensitive to 6PPD-Q exposure, which might be derived from its weaker metabolic transformation of 6PPD-Q, since significantly lower levels of phase I and phase II metabolites were detected in 6PPD-Q-exposed L02 cell culture medium. Furthermore, pathway analysis showed that 6PPD-Q exposure induced changes in phenylalanine, tyrosine, and tryptophan biosynthesis and tyrosine metabolism pathways in L02 cells, which might be the mechanism underlying its liver cell toxicity. Gene expression analysis revealed that exposure to 6PPD-Q induced excessive ROS production in L02 cells. Our results further supported the higher risk of 6PPD-Q than 6PPD and provided insights for understanding the effects of 6PPD-Q on human health.

Analyzing the Effects of Age, Time of Day, and Experiment on the Basal Locomotor Activity and Light-Off Visual Motor Response Assays in Zebrafish Larvae

The recent availability of commercial platforms for behavioral analyses in zebrafish larvae based on video-tracking technologies has exponentially increased the number of studies analyzing different behaviors in this model organism to assess neurotoxicity. Among the most commonly used assays in zebrafish larvae are basal locomotor activity (BLA) and visual motor responses (VMRs). However, the effect of different intrinsic and extrinsic factors that can significantly alter the outcome of these assays is still not well understood. In this work, we have analyzed the influence of age (5-8 days post-fertilization), time of day (8:00, 10:00, 12:00, 14:00; 16:00, 18:00, and 20:00 h), and experiment (three experiments performed at different days) on BLA and VMR results (4004 analyses for each behavior) in 143 larvae. The results from both behaviors were adjusted to a random-effects linear regression model using generalized least squares (GLSs), including in the model the effect of the three variables, the second-way interactions between them, and the three-way interaction. The results presented in this manuscript show a specific effect of all three intrinsic factors and their interactions on both behaviors, supporting the view that the most stable time period for performing these behavioral assays is from 10:00 am to 04:00 pm, with some differences depending on the age of the larva and the behavioral test.

Mixture toxicity of 6PPD-quinone and polystyrene nanoplastics in zebrafish

Plastic pollution, including micro- and nanoplastics, is a growing concern. Tyre-wear particles (TWPs) are the second largest source of microplastics in the ocean following abrasion of synthetic fibres. In addition to the particles themselves, TWPs contain many harmful chemicals, including 6PPD. This chemical reacts with atmospheric ozone and forms the toxic compound 6PPD-quinone (6PPDq), which poses a danger to aquatic life. There is a knowledge gap in understanding risks associated with the combined toxicity of nanoplastics (NPs) and 6PPDq. The present study aimed to investigate the toxicity of NPs and 6PPDq on adult zebrafish using phenotypic (behaviour, histology) and transcriptomic endpoints. Zebrafish were exposed to four treatments: control (contaminant-free), 50 μg/L 6PPDq, 3 mg/L polystyrene (PS)-NPs, and a combination of 50 μg/L 6PPDq and 3 mg/L PS-NPs. We did not observe locomotory dysregulation in zebrafish exposed to NPs. However, we found significant hyperlocomotion in zebrafish exposed to 6PPDq and this effect was even more substantial after co-exposure with PS-NPs. This study explores the molecular mechanisms behind these effects, identifying genes associated with neurotransmitters and fatty acid metabolism that were dysregulated by the co-exposure. Transcriptomic analysis further showed that both 6PPDq and PS-NPs impacted cellular processes associated with sterol biosynthesis, cholesterol metabolism, and muscle tissue development. The effects on these mechanisms were stronger in co-exposed zebrafish, indicating a heightened risk to cellular integrity and mitochondrial dysfunction. These results highlight the significance of mixture toxicity when studying the effects of NPs and associated chemicals like 6PPDq.

Bacteriophage EPP-1, a potential antibiotic alternative for controlling edwardsiellosis caused by Edwardsiella piscicida while mitigating drug-resistant gene dissemination

Edwardsiella piscicida causes significant economic losses to the aquaculture industry worldwide. Phage-based biocontrol methods are experiencing a renaissance because of the spread of drug-resistant genes and bacteria resulting from the heavy use of antibiotics. Here, we showed that the novel Edwardsiella phage EPP-1 could achieve comparable efficacy to florfenicol using a zebrafish model of Edwardsiella piscicida infection and could reduce the content of the floR resistance gene in zebrafish excreta. Specifically, phage EPP-1 inhibited bacterial growth in vitro and significantly improved the zebrafish survival rate in vivo (P = 0.0035), achieving an efficacy comparable to that of florfenicol (P = 0.2304). Notably, integrating the results of 16S rRNA sequencing, metagenomic sequencing, and qPCR, although the effects of phage EPP-1 converged with those of florfenicol in terms of the community composition and potential function of the zebrafish gut microbiota, it reduced the floR gene content in zebrafish excreta and aquaculture water. Overall, our study highlights the feasibility and safety of phage therapy for edwardsiellosis control, which has profound implications for the development of antibiotic alternatives to address the antibiotic crisis.

6PPD-quinone exposure induces neuronal mitochondrial dysfunction to exacerbate Lewy neurites formation induced by α-synuclein preformed fibrils seeding

The emerging toxicant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q) is of wide concern due to its ubiquitous occurrence and high toxicity. Despite regular human exposure, limited evidence exists about its presence in the body and potential health risks. Herein, we analyzed cerebrospinal fluid (CSF) samples from Parkinson's disease (PD) patients and controls. The CSF levels of 6PPD-Q were twice as high in PD patients compared to controls. Immunostaining assays performed with primary dopaminergic neurons confirm that 6PPD-Q at environmentally relevant concentrations can exacerbate the formation of Lewy neurites induced by α-synuclein preformed fibrils (α-syn PFF). Assessment of cellular respiration reveals a considerable decrease in neuronal spare respiratory and ATP-linked respiration, potentially due to changes in mitochondrial membrane potential. Moreover, 6PPD-Q-induced mitochondrial impairment correlates with an upsurge in mitochondrial reactive oxygen species (mROS), and Mito-TEMPO-driven scavenging of mROS can lessen the amount of pathologic phospho-serine 129 α-synuclein. Untargeted metabolomics provides supporting evidence for the connection between 6PPD-Q exposure and changes in neuronal metabolite profiles. In-depth targeted metabolomics further unveils an overall reduction in glycolysis metabolite pool and fluctuations in the quantity of TCA cycle intermediates. Given its potentially harmful attributes, the presence of 6PPD-Q in human brain could potentially be a risk factor for PD.

A mini review on 6PPD quinone: A new threat to aquaculture and fisheries

Numerous toxic substances are directly and indirectly discharged by humans into water bodies, causing distress to the organisms living on it. 6PPD, an amino antioxidant from tires reacts with ozone to form 6PPD-Q, which has garnered global attention due to its lethal nature to various organisms. This review aims to provide an understanding of the sources, transformation, and fate of 6PPD-Q in water and the current knowledge on its effects on aquatic organisms. Furthermore, we discuss research gaps pertaining to the mechanisms by which 6PPD-Q acts within fish bodies. Previous studies have demonstrated the ubiquitous presence of 6PPD-Q in the environment, including air, water, and soil. Moreover, this compound has shown high lethality to certain fish species while not affecting others. Toxicological studies have revealed its impact on the nervous system, intestinal barrier function, cardiac function, equilibrium loss, and oxidative stress in various fish species. Additionally, exposure to 6PPD-Q has led to organ injury, lipid accumulation, and cytokine production in C. elegans and mice. Despite studies elucidating the lethal dose and effects of 6PPD-Q in fish species, the underlying mechanisms behind these symptoms remain unclear. Future studies should prioritize investigating the mechanisms underlying the lethality of 6PPD-Q in fish species to gain a better understanding of its potential effects on different organisms.

(Micro)Plastics Are Toxic Pollutants

Plastics, including microplastics, have generally been regarded as harmful to organisms because of their physical characteristics. There has recently been a call to understand and regard them as persistent, bioaccumulative, and toxic. This review elaborates on the reasons that microplastics in particular should be considered as "toxic pollutants". This view is supported by research demonstrating that they contain toxic chemicals within their structure and also adsorb additional chemicals, including polychlorinated biphenyls (PCBs), pesticides, metals, and polycyclic aromatic hydrocarbons (PAHs), from the environment. Furthermore, these chemicals can be released into tissues of animals that consume microplastics and can be responsible for the harmful effects observed on biological processes such as development, physiology, gene expression, and behavior. Leachates, weathering, and biofilm play important roles in the interactions between microplastics and biota. Global policy efforts by the United Nations Environmental Assembly via the international legally binding treaty to address global plastic pollution should consider the designation of harmful plastics (e.g., microplastics) with associated hazardous chemicals as toxic pollutants.