Role of Toxicokinetic and Toxicodynamic Parameters in Explaining the Sensitivity of Zebrafish Larvae to Four Metals

Predicting the Metal Mixture Toxicity with a Toxicokinetic-Toxicodynamic Model Considering the Time-Dependent Adverse Outcome Pathways

Chemicals mainly exist in ecosystems as mixtures, and understanding and predicting their effects are major challenges in ecotoxicology. While the adverse outcome pathway (AOP) and toxicokinetic-toxicodynamic (TK-TD) models show promise as mechanistic approaches in chemical risk assessment, there is still a lack of methodology to incorporate the AOP into a TK-TD model. Here, we describe a novel approach that integrates the AOP and TK-TD models to predict mixture toxicity using metal mixtures (specifically Cd-Cu) as a case study. We preliminarily constructed an AOP of the metal mixture through temporal transcriptome analysis together with confirmatory bioassays. The AOP revealed that prolonged exposure time activated more key events and adverse outcomes, indicating different modes of action over time. We selected a potential key event as a proxy for damage and used it as a measurable parameter to replace the theoretical parameter (scaled damage) in the TK-TD model. This refined model, which connects molecular responses to organism outcomes, effectively predicts Cd-Cu mixture toxicity over time and can be extended to other metal mixtures and even multicomponent mixtures. Overall, our results contribute to a better understanding of metal mixture toxicity and provide insights for integrating the AOP and TK-TD models to improve risk assessment for chemical mixtures.

Non-negligible vector effect of micro(nano)plastics on tris(1,3-dichloro-2-propyl) phosphate in zebrafish quantified by toxicokinetic model

Micro(nano)plastics (MNPs) inevitably interact with coexisting contaminants and can act as vectors to affect their fate in organisms. However, the quantitative contribution of MNPs in the in vivo bioaccumulation and distribution of their coexisting contaminants remains unclear. Here, by selecting tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) as the typical coexisting contaminant, we quantified the contribution of MNPs to bioaccumulation and distribution of TDCIPP with toxicokinetic models. Results indicated that MNPs differentially facilitated TDCIPP bioaccumulation and distribution, and NPs slowed down TDCIPP depuration more significantly than MPs. Model analysis further revealed increasing contributions of MNPs to whole-fish TDCIPP bioaccumulation over time, with NPs (33-42%) contributing more than MPs (12-32%) at 48 h exposure. NPs contributed more than MPs to TDCIPP distribution in the liver (13-19% for MPs; 36-52% for NPs) and carcass (24-45% for MPs; 57-71% for NPs). The size-dependent vector effect might be attributed to the fact that MNPs promote contaminant transfer by damaging biofilm structure and increasing tissue membrane permeability, with NPs exerting stronger effects. This work demonstrated the effectiveness of using modeling tools to understand the relative importance of MNPs as contaminant vectors in the TK process and highlighted the higher contaminant transfer potential of NPs under combined exposure scenarios.

Physiological trade-off of marine fish under Zn deficient and excess conditions

Fish can regulate their Zn body bioaccumulation, but the mechanisms and physiological responses at the organ level are still largely unknown. In the present study, we exposed the marine seabreams under different Zn levels (deficient, optimum and excess levels) over a period of 4 weeks and examined how fish maintained its regulation of bioaccumulation with associated physiological effects at the fish intestinal organ. Our results indicated that fish intestinal organs constantly controlled the Zip family to "rob" more Zn under Zn-deficiency (with a dietary level of 7.9 mg/kg), whereas restricted the Zn efflux to preserve the intestinal function. Under Zn-excess conditions (193.3 mg/kg), the fish intestine maintained a limited Zn homeostasis (37.8-44.6 μg/mg) by initially inhibiting the influx through the Zip family receptor, but later accelerating both influx and efflux of Zn. Based on the WGCNA method, Zn deficient dietary exposure first resulted in defense response with subsequent switching to antioxidant defense. Instead, excess Zn first triggered the immunological response, but then led to physiological toxicity (abnormal in lipid metabolism). Although Zn had multiple biological functions, it was preferentially involved in lipid metabolism under different dietary Zn doses. This study provided direct evidence for Zn regulation at the organ level and detoxification mechanisms against potential environmental toxicity in fish.

Near-Infrared-II In Vivo Visualization and Quantitative Tracking of Micro/Nanoplastics in Fish

As emerging contaminants, micro/nanoplastics (MNPs) are widely present in aquatic environments and are often ingested by aquatic organisms. However, the in vivo trafficking and fate of MNPs remain largely unknown. Here, we developed near-infrared (NIR) aggregated-induced emission (AIE) fluorophore-labeled microplastics (2 μm) and nanoplastics (100 nm) as models of MNPs. This model was based on the NIR-AIE technique with strong emission at the second near-infrared (NIR-IIII) window, which overcomes the interference of autofluorescence and observation artifacts in the detection of commercial fluorescent-labeled particles. Due to its deep tissue penetration and noninvasiveness, the dynamic process of accumulation and transport of MNPs in individuals can be tracked with NIR imaging. We then directly visualized and quantified the uptake and depuration processes of MPs and NPs in zebrafish. The results showed that the MPs and NPs were mainly accumulated in the fish gut, and the distribution was heterogeneous. MPs tended to accumulate more in the fore and mid areas of the gut compared with NPs. Besides, both MPs and NPs could accumulate in large quantities locally in the gut and might cause intestinal obstruction. MNPs accumulated slowly during the initial exposure followed by rapid and sustained accumulation in gut. Based on these kinetic accumulation and depuration, we developed a refined toxicokinetic (TK) model to describe the dynamic changes in the uptake and depuration of MNPs. Overall, this study proposed a MNP model based on the NIR-AIE technique, which provided a reliable tracer technology for the visualization, tracking and quantification of MNPs in vivo.

A refined toxicokinetic model for quantifying the interaction between Cd and Cu in zebrafish larvae

The interaction between metals is ubiquitous, but there is still a lack of quantitative models considering the interaction between metals, which leads to the deviations in predicting the joint toxicity of metals. The present study estimated the uptake rate constants (kin) and elimination rate constants (kout) and elucidated how the presence of one metal (Cu or Cd) affects the absorption and excretion of another metal (Cd or Cu) in zebrafish larvae. The results showed that Cd and Cu inhibited each other in the process of absorption and excretion by comparing separately kin and kout of Cd or Cu with the other metal Cu or Cd mixed concentrations increased, thereby affecting the Cd and Cu bioaccumulation in the zebrafish larvae. Then the interactions between Cd and Cu in the uptake and elimination processes were quantified to obtain a refined toxicokinetic model. Verification with independent experiment data showed that the refined toxicokinetic model could significantly improve the prediction of the Cd or Cu bioaccumulation in the zebrafish larvae. This study contributes to understand the toxicokinetic process of the Cd-Cu mixture in the zebrafish larvae, and the developed model could be used to predict the toxicity of the metal mixtures.

Mitochondrial dysfunction in metabolic disorders induced by per- and polyfluoroalkyl substance mixtures in zebrafish larvae

Several per- and polyfluoroalkyl substances (PFAS) have been linked to metabolic disorders in organisms. However, few studies have considered their combined effects, which would be more representative of PFAS occurring in the environment. In this study, zebrafish embryos were exposed to a mixture of 18 PFAS at three environmentally relevant concentrations for 5 days to assess their bioconcentration and metabolic consequences. The burdens of ∑PFAS in zebrafish larvae were 0.12, 1.58, and 9.63 mg/kg in the 0.5, 5, and 50 μg/L treatment groups, respectively. Exposure to the PFAS mixture accelerated hatching and larval heart rates, increased energy expenditure, and reduced ATP levels and glucose contents due to decreased feed intake and glucose uptake. Metabolomic analysis revealed that exposure to the PFAS mixture enhanced glycolysis but inhibited phospholipid synthesis, and significantly increased the expression of lipid metabolism related genes (srebf1, acox, and pparα), which indicated enhanced β-oxidation. The significant changes in mitochondrial membrane potential, mitochondrial content, and the transcription of genes involved in the mitochondrial respiratory chain (mfn2, ndufs1, atp5fa1, and mt-nd1) and mitochondrial DNA replication and transcription (18rs-rrn, and polg1) suggested that exposure to the PFAS mixture could cause mitochondrial dysfunction and further disrupt glucose and lipid metabolic pathways, ultimately causing metabolic disorders in zebrafish larvae. These findings demonstrate the importance of assessing the metabolic effects of PFAS mixtures on early development in wildlife and humans.

Toxicokinetics and Particle Number-Based Trophic Transfer of a Metallic Nanoparticle Mixture in a Terrestrial Food Chain

Herein, we investigated to which extent metallic nanoparticles (MNPs) affect the trophic transfer of other coexisting MNPs from lettuce to terrestrial snails and the associated tissue-specific distribution using toxicokinetic (TK) modeling and single-particle inductively coupled plasma mass spectrometry. During a period of 22 days, snails were fed with lettuce leaves that were root exposed to AgNO3 (0.05 mg/L), AgNPs (0.75 mg/L), TiO2NPs (200 mg/L), and a mixture of AgNPs and TiO2NPs (equivalent doses as for single NPs). The uptake rate constants (ku) were 0.08 and 0.11 kg leaves/kg snail/d for Ag and 1.63 and 1.79 kg leaves/kg snail/d for Ti in snails fed with NPs single- and mixture-exposed lettuce, respectively. The elimination rate constants (ke) of Ag in snails exposed to single AgNPs and mixed AgNPs were comparable to the corresponding ku, while the ke for Ti were lower than the corresponding ku. As a result, single TiO2NP treatments as well as exposure to mixtures containing TiO2NPs induced significant biomagnification from lettuce to snails with kinetic trophic transfer factors (TTFk) of 7.99 and 6.46. The TTFk of Ag in the single AgNPs treatment (1.15 kg leaves/kg snail) was significantly greater than the TTFk in the mixture treatment (0.85 kg leaves/kg snail), while the fraction of Ag remaining in the body of snails after AgNPs exposure (36%) was lower than the Ag fraction remaining after mixture exposure (50%). These results indicated that the presence of TiO2NPs inhibited the trophic transfer of AgNPs from lettuce to snails but enhanced the retention of AgNPs in snails. Biomagnification of AgNPs from lettuce to snails was observed in an AgNPs single treatment using AgNPs number as the dose metric, which was reflected by the particle number-based TTFs of AgNPs in snails (1.67, i.e., higher than 1). The size distribution of AgNPs was shifted across the lettuce-snail food chain. By making use of particle-specific measurements and fitting TK processes, this research provides important implications for potential risks associated with the trophic transfer of MNP mixtures.

Occurrence characteristics of uranium mineral-related substances in various environmental media in China: A critical review

The high demand and extensive exploitation of uranium resources resulted in the ubiquity and high detection levels of uranium mineral-related substances in various environment media in China. The potential adverse effects of uranium mineral-related substances on environment and human health have received extensive attention. Therefore, we reviewed the occurrence and spatial distribution of uranium mineral-related substances in various basins and environmental media in China to obtain an overall understanding. We collected information from over 70 papers reporting the occurrence and distribution of uranium mineral-related substances in multiple environments and 183 articles on the genesis of uranium deposits in China from 2001 to 2021. Then the occurrence of uranium mineral-related substances and corresponding correlation in different basins, environmental media and depth ranges were compared in detail. And this review assessed the uranium mineral-related pollution in China based on various environmental quality standards of China, EPA and WHO, and proposed the priority uranium mineral-related heavy metals and radioactive substances based on cluster analysis. This review showed that there were obvious differences in the occurrence characteristics of various uranium mineral-related substances in different environmental media, especially in the surrounding environment of sandstone type and hard rock type uranium deposits. These results will guide us to tackle the challenge of uranium mineral-related pollution in China. The correlation analysis of uranium mineral-related pollutants in different environmental media and the identification of priority pollutants will also provide instructions for us to control uranium mineral-related pollution. Finally, we put forward a series of urgent and practical suggestions on risk management and control of uranium mining according to the current situation of uranium mining environment in China, which is of guiding significance for the realization of "green uranium mining".

Elucidating the Differences in Metal Toxicity by Quantitative Adverse Outcome Pathways

Numerous studies have reported that the toxicity differences among metals are widespread; however, little is known about the mechanism of differences in metal toxicity to aquatic organisms due to the lack of quantitative understanding of their adverse outcome pathway. Here, we investigated the effects of Cd and Cu on bioaccumulation, gene expression, physiological responses, and apical effects in zebrafish larvae. RNA sequencing was conducted to provide supplementary mechanistic information for the effects of Cd and Cu exposure. On this basis, we proposed a quantitative adverse outcome pathway (qAOP) suitable for metal risk assessment of aquatic organisms. Our work provides a mechanistic explanation for the differences in metal toxicity where the strong bioaccumulation of Cu enables the newly accumulated Cu to reach the threshold that causes different adverse effects faster than Cd in zebrafish larvae, resulting in a higher toxicity of Cu than that of Cd. Furthermore, we proposed a parameter C_IT/BCF (the ratio of internal threshold concentration and bioaccumulation factor) that helps to understand the toxicity differences by combining the information of bioaccumulation and internal threshold of adverse effects. This work demonstrated that qAOP is an effective quantitative tool for understanding the toxicity mechanism and highlight the importance of toxicokinetics and toxicodynamics at different biological levels in determining the metal toxicity.

A review of interactions of microplastics and typical pollutants from toxicokinetics and toxicodynamics perspective

The widespread microplastics (MPs) pollution has become a concerning environmental issue. The interactions between MPs and typical pollutants may change the bioaccumulation, and toxicity of pollutants, leading to high uncertainty in risk assessment. Still, significant gaps remain in the knowledge available to integrate these interactions in the perspectives of toxicokinetics (TK) and toxicodynamics (TD), which is also an essential part of quantitative toxicological research. This review systematically summarizes the interaction between MPs and typical pollutants in TK and TD processes. MPs can be acted as the vector or sink of pollutants to increase or decrease their bioaccumulation, and also may not affect their bioaccumulation due to no interaction. The adverse outcome pathway (AOP) framework enables novel approaches for determining the interaction between MPs and pollutants in the TD process. MPs can directly or indirectly enhance, reduce and not affect the toxicity of pollutants. A series of factors influencing the interaction in TK and TD processes are summarized, including MPs characteristics and exposure scenarios. TK-TD approach can quantitatively understand the interaction between MPs and pollutants based on the mechanism. Given the current knowledge gap in TK and TD processes, future perspectives on combined exposure research are proposed.

Nutrient potentiate the responses of plankton community structure and metabolites to cadmium: A microcosm study

Metal pollution is a worldwide concern and may pose risks to aquatic organisms, communities, and ecosystems. The toxic effects of metals at the organism level are relatively clear. However, their impacts at the community level are still poorly understood, especially with concurred eutrophication in surface water. In the present study, the effects of Cd on the plankton community structure and function under varying nutrient conditions were evaluated using a microcosm study. The employed concentrations of Cd and nutrient were based on the values currently measured in the freshwater ecosystem. For the plankton structure, our results showed that the Chl a concentration, the abundances of total phytoplankton, Cyanophyta, and Chlorophyta, and the abundance of Copepoda decreased by Cd consistently. The Cyanophyta Oscillatoria tenuis and Copepoda nauplius were the most sensitive species to Cd in the phytoplankton and zooplankton community, respectively. For the community effects, we found the inhibitory effects of Cd on the photosystem II (PSII) activity of phytoplankton community because of the consistent decrease in the chlorophyll fluorescence parameters (F_v/F_m, Y(Ⅱ), and ETR). Furthermore, the reductions of DOC and pH by Cd were only found in the high nutrient condition, which indicated that the toxic effects of Cd on the community structure and community metabolites were aggravated by the increased nutrient. This study emphasizes the importance of considering nutrient conditions when assessing the metal ecotoxicological effects at environmentally relevant concentrations.

Exposure to trace levels of metals and fluoroquinolones increases inflammation and tumorigenesis risk of zebrafish embryos

Exposure to trace-level heavy metals and antibiotics may elicit metabolic disorder, alter protein expression, and then induce pathological changes in zebrafish embryos, despite negligible physiological and developmental toxicity. This study investigated the single and combined developmental toxicity of fluoroquinolones (enrofloxacin [ENR] and ciprofloxacin [CIP]) (≤0.5 μM) and heavy metals (Cu and Cd) (≤0.5 μM) to zebrafish embryos, and molecular responses of zebrafish larvae upon exposure to the single pollutant (0.2 μM) or a binary metal-fluoroquinolone mixture (0.2 μM). In all single and mixture exposure groups, no developmental toxicity was observed, but oxidative stress, inflammation, and lipid depletion were found in zebrafish embryos, which was more severe in the mixture exposure groups than in the single exposure groups, probably due to increased metal bioaccumulation in the presence of ENR or CIP. Metabolomics analysis revealed the up-regulation of amino acids and down-regulation of fatty acids, corresponding to an active response to oxidative stress and the occurrence of inflammation. The up-regulation of antioxidase and immune proteins revealed by proteomics analysis further confirmed the occurrence of oxidative stress and inflammation. Furthermore, the KEGG pathway enrichment analysis showed a significant disturbance of pathways related to immunity and tumor, indicating the potential risk of tumorigenesis in zebrafish larvae. The findings provide molecular-level insights into the adverse effects of heavy metals and antibiotics (especially in chemical mixtures) on zebrafish embryos, and highlight the potential ecotoxicological risks of trace-level heavy metals and antibiotics in the environment.