The agrochemical fungicide triadimefon induces abnormalities in Xenopus laevis embryos

PCNB exposure during early embryogenic development induces developmental delay and teratogenicity by altering the gene expression in Xenopus laevis

Pentachloronitrobenzene (PCNB) is an organochlorine fungicide commonly used to treat seeds against seedling infections and controlling snow mold on golf courses. PCNB has been demonstrated to be toxic to living organisms, including fish and several terrestrial organisms. However, only phenotypical deformities have been studied, and the effects of PCNB on early embryogenesis, where primary organogenesis occurs, have not been completely studied. In the current study, the developmental toxicity and teratogenicity of PCNB is evaluated by using frog embryo teratogenesis assay Xenopus (FETAX). Our results confirmed the teratogenic potential of PCNB revealing the teratogenic index of 1.29 during early embryogenesis. Morphological studies revealed tiny head, bent axis, reduced inter ocular distance, hyperpigmentation, and reduced total body lengths. Whole mount in situ hybridization and reverse transcriptase polymerase chain reaction were used to identify PCNB teratogenic effects at the gene level. The gene expression analyses revealed that PCNB was embryotoxic to the liver and heart of developing embryos. Additionally, to determine the most sensitive developmental stages to PCNB, embryos were exposed to the compound at various developmental stages, demonstrating that the most sensitive developmental stage to PCNB is primary organogenesis. Taken together, we infer that PCNB's teratogenic potential affects not just the phenotype of developing embryos but also the associated genes and involving the oxidative stress as a possible mechanism of toxicity, posing a hazard to normal embryonic growth. However, the mechanisms of teratogenesis require additional extensive investigation to be defined completely.

Predictive assays for craniofacial malformations: evaluation in Xenopus laevis embryos exposed to triadimefon

Craniofacial defects are one of the most frequent abnormalities at birth, but their experimental evaluation in animal models requires complex procedures. The aim of the present work is the comparison of different methodologies to identify dose- and stage-related craniofacial malformations in Xenopus laevis assay (R-FETAX, where the full cartilage evaluation, including flat mount technique, is the gold standard for skeletal defect detection). Different methods (external morphological evaluation of fresh samples, deglutition test, whole mount cartilage evaluation and Meckel-palatoquadrate angle measurements) were applied. Triadimefon (FON) was selected as the causative molecule as it is known to induce craniofacial defects in different animal models, including the amphibian X. laevis.FON exposure (0-31.25 μM) was scheduled to cover the whole 6-day test (from gastrula to free swimming tadpole stage) or each crucial developmental phases: gastrula, neurula, early morphogenesis, late morphogenesis, tadpole. Dose-dependent effects (fusions among craniofacial cartilages) were evident for groups exposed during the morphogenetic periods (neurula, early morphogenesis, late morphogenesis); gastrula was insensitive to the tested concentrations, tadpole group showed malformations only at 31.25 μM. The overall NOAEL was set at 3.9 μM. Results were evaluated applying benchmark dose (BMD) approach. The comparison of relative potencies from different methods showed deglutition as the only assay comparable with the gold standard (cartilage full evaluation).In conclusion, we suggest deglutition test as a reliable method for a rapid screening of craniofacial abnormalities in the alternative model X. laevis. This is a rapid, inexpensive and vital test allowing to preserve samples for the application of further morphological or molecular investigations.

An adverse outcome pathway on the disruption of retinoic acid metabolism leading to developmental craniofacial defects

Adverse outcome pathway (AOP) is a conceptual framework that links a molecular initiating event (MIE) via intermediate key events (KEs) with adverse effects (adverse outcomes, AO) relevant for risk assessment, through defined KE relationships (KERs). The aim of the present work is to describe a linear AOP, supported by experimental data, for skeletal craniofacial defects as the AO. This AO was selected in view of its relative high incidence in humans and the suspected relation to chemical exposure. We focused on inhibition of CYP26, a retinoic acid (RA) metabolizing enzyme, as MIE, based on robust previously published data. Conazoles were selected as representative stressors. Intermediate KEs are RA disbalance, aberrant HOX gene expression, disrupted specification, migration, and differentiation of neural crest cells, and branchial arch dysmorphology. We described the biological basis of the postulated events and conducted weight of evidence (WoE) assessments. The biological plausibility and the overall empirical evidence were assessed as high and moderate, respectively, the latter taking into consideration the moderate evidence for concordance of dose-response and temporal relationships. Finally, the essentiality assessment of the KEs, considered as high, supported the robustness of the presented AOP. This AOP, which appears of relevance to humans, thus contributes to mechanistic underpinning of selected test methods, thereby supporting their application in integrated new approach test methodologies and strategies and application in a regulatory context.

Effect of nano-encapsulation of β-carotene on Xenopus laevis embryos development (FETAX)

Vitamin A plays a vital role during embryo development as most precursor of embryonic retinoic acid, a key morphogen during embryogenesis. Carotenoids, including β-carotene, are important vegetal source of Vitamin A and in contrast to teratogenic potential of animal-derived retinoids, β-carotene is usually considered freed from embryotoxic effects and supplements in pregnancy with β-carotene are suggested. The aim of the present work is to evaluate the effect of bulk and nano-encapsulated β-carotene on embryo development, by using the animal model Frog Embryo Teratogenesis Assay: Xenopus- FETAX. Xenopus laevis embryos were exposed from late gastrula till pharyngula (the phylotypic stage for vertebrates) to the concentrations of BULK β-carotene 150-3000 ng/mL and NANO β-carotene 0.75-30 ng/mL. At pharyngula stage, some embryos were processed for whole mount neural crest cell immunostaining, while others embryos were allowed to develop till tadpole for morphological and histological evaluation of neural crest cells-derived structures. In this model, the nano-encapsulated β-carotene induced specific malformations at craniofacial and eye level, while the bulk formulation only induced developmental delays. Finally, the applied alternative animal model resulted a rapid and sensitive screening method able to re-evaluate the teratogenic profile of nano-encapsulated micronutrients.

Comparison of triadimefon and its metabolite on acute toxicity and chronic effects during the early development of Rana nigromaculata tadpoles

Pesticides are one of major causes for amphibian population declines and the behavior of pesticide metabolite products to amphibians has become a rising concern. In this study, the acute toxicity and the chronic effects of triadimefon and triadimenol (the metabolite of triadimefon) on Rana. nigromaculata were investigated. In the acute assay, significant differences were observed in antioxidant enzyme activities and malondialdehyde levels between the triadimefon and triadimenol. The 96 h-acute toxicity of triadimefon (25.97 mg/L) and triadimenol (34.55 mg/L) to tadpoles was low. In 28d-chronic exposure, we studied the relative expression of tadpoles genes related to thyroid hormone-dependent metamorphic development, histological examination of liver and some biological index, including wet weight, snout-to-vent length (SVL) and development stages. The results revealed that the effects of triadimefon and triadimenol on tadpole development are driven by a disruption of the hormonal pathways involved in metamorphosis. Interestingly, triadimefon was more harmful on R. nigromaculata than triadimenol at high dose, whereas the reverse result was observed at low doses. According to the relative expression of thyroid hormone-dependent genes, we also found that the two compounds may have different mechanisms of toxic action on R. nigromaculata. Our study developed a pragmatic approach for use in the risk assessment of pesticide and its metabolite，and increased the information and understanding of the impacts of fungicides and other potential endocrine disrupting environmental contaminants on amphibians.

Synergistic potential of fenvalerate and triadimefon on endocrine disruption and oxidative stress during rare minnow embryo development

Pyrethroids have been reported to interact synergistically when co-exposed with azoles fungicides in different organisms. In the present study, we investigated the mixture toxicity of fenvalerate (FEN) and triadimefon (TDF) toward embryos of Gobiocypris rarus after 96 h exposure. Results demonstrated that TDF enhanced the acute toxicity of FEN. Exposure to binary mixtures of FEN and TDF resulted in synergistic responses of endocrine disruption by inducing the transcripts of several genes including vtg, erα, erβ1, erβ2, cyp19a, cyp1a, cyp4, cyp11a, gnrh3, gnrhr1a, star, and dmrt1. Furthermore, FEN and TDF mixture increased the VTG level and aromatase activity in rare minnow embryos. FEN and TDF co-exposure also regulated the mRNA of vezf, hsp70, p53, gadd45α, induced the synthesis of ROS and activity of GST, suggesting the synergistic potential of oxidative stress induced by FEN and TDF co-exposure. The results indicated that binary mixtures of FEN and TDF could simultaneously induce endocrine disruption and oxidative stress in a synergistic manner during rare minnow embryo development.

Single and mixture toxicity of strobilurin and SDHI fungicides to Xenopus tropicalis embryos

The decline in amphibian populations is a critical threat to global biodiversity, and pesticide pollution is considered as one of the major factors. Although effects of single pesticides on amphibians have been documented, toxicological interactions prevailing in mixtures of pesticides have not been well elucidated. Strobilurin and succinate dehydrogenase inhibitor (SDHI) fungicides are new types of commonly used pesticides. In this study, effects of three strobilurins (pyraclostrobin, trifloxystrobin and azoxystrobin), two SDHIs (isopyrazam and bixafen), and their mixtures on X. tropicalis embryos were fully investigated. Results showed that exposure to individual fungicides induced lethal and teratogenetic effects; and malformed embryos displayed similar phenotypes including microcephaly, hypopigmentation, somite segmentation and narrow fin. Exposure to two strobilurins or two SDHIs at equitoxic concentrations caused additive or synergetic effects at environmentally relevant concentrations. TU for mixtures of isopyrazam and bixafen was 0.53 and 0.30 for lethal and teratogenic toxicity, respectively. Finally, binary mixtures of strobilurins and SDHIs also exhibited additive or synergetic effects on amphibian embryos. Overall, these results reveal that the mixtures of multiple fungicides caused a higher incidence of lethality and teratogenicity of amphibian embryos, compared to a single fungicide at the corresponding doses. Our findings provide important data about the ecotoxicology of agricultural fungicides on non-target organisms, which is useful for guiding management practices for pesticides.

A glyphosate micro-emulsion formulation displays teratogenicity in Xenopus laevis

Glyphosate is the active ingredient in broad-spectrum herbicide formulations used in agriculture, domestic area and aquatic weed control worldwide. Its market is growing steadily concurrently with the cultivation of glyphosate-tolerant transgenic crops and emergence of weeds less sensitive to glyphosate. Ephemeral and lentic waters near to agricultural lands, representing favorite habitats for amphibian reproduction and early life-stage development, may thus be contaminated by glyphosate based herbicides (GBHs) residues. Previous studies on larval anuran species highlighted increased mortality and growth effects after exposure to different GBHs in comparison to glyphosate itself, mainly because of the surfactants such as polyethoxylated tallow amine present in the formulations. Nevertheless, these conclusions are not completely fulfilled when the early development, characterized by primary organogenesis events, is considered. In this study, we compare the embryotoxicity of Roundup® Power 2.0, a new GBH formulation currently authorized in Italy, with that of technical grade glyphosate using the Frog Embryo Teratogenesis Assay-Xenopus (FETAX). Our results evidenced that glyphosate was not embryolethal and only at the highest concentration (50 mg a.e./L) caused edemas. Conversely, Roundup® Power 2.0 exhibited a 96 h LC50 of 24.78 mg a.e./L and a 96 h EC50 of 7.8 mg a.e./L. A Teratogenic Index of 3.4 was derived, pointing out the high teratogenic potential of the Roundup® Power 2.0. Specific concentration-dependent abnormal phenotypes, such as craniofacial alterations, microphthalmia, narrow eyes and forebrain regionalization defects were evidenced by gross malformation screening and histopathological analysis. These phenotypes are coherent with those evidenced in Xenopus laevis embryos injected with glyphosate, allowing us to hypothesize that the teratogenicity observed for Roundup® Power 2.0 may be related to the improved efficacy in delivering glyphosate to cells, guaranteed by the specific surfactant formulation. In conclusion, the differences in GBH formulations should be carefully considered by the authorities, since sub-lethal and/or long-term effects (e.g. teratogenicity) can be significantly modulated by the active ingredient salt type and concentration of the adjuvants. Finally, the mechanistic toxicity of glyphosate and GBHs are worthy of further research.

Tissue distribution and metabolism of triadimefon and triadimenol enantiomers in Chinese lizards (Eremias argus)

Triadimefon (TF, S-(+)-TF, R-(-)-TF) and its metabolite triadimenol (TN, TN-A1, A2 and TN-B1, B2) are two systemic fungicides and both of them are chiral pharmaceuticals which are widely used in agricultural industry. Many researches focused on the toxicity effects of triadimefon on mammals, while the ecotoxicological data of tiradimefon on reptiles is limited. In order to understand the toxicity mechanism of triadimefon in reptiles, the current study administrated S-(+)-TF or R-(-)-TF traidimefon (50mg/kg^bw) to Chinese lizards (Eremias argus) respectively, the absorption, distribution of triadimefon and the formation of triadimenol were analysed at different sampling times. The metabolic pathways were demonstrated through relative gene expression using quantitative real-time PCR reaction. During the experiment time, triadimefon was quickly peaked to the maximum concentration within 12h in liver, brain, kidney, and plasma, eliminated slowly. The biotransformation in kidney was the lowest and fat possessed the worst degradation ability among others. The metabolite, triadimenol was detected in blood in 2h and reached to a plateau at about 12h in most organs (fat excepted), while the process of metabolism is stereoselective. The mainly metabolite in R-(-)-TF treated group was TN-B1, and TN-A2 in S-(+)-TF group which showed the selective metabolism to other species caused by environmental conditions, differences in the animal models and concentration of TF. The related gene expression of cyp1a1, cyp3a1 and hsd11β mRNA level in lizards showed different metabolic pathways in the liver and brain. Both P450s enzymes and 11β-hydroxysteroid dehydrogenase participated in metabolic reaction in liver, while no 11β-hydroxysteroid dehydrogenase pathway observed in brain. This diversity in liver and brain may cause different degradation rate and ecotoxicological effect in different organs.

Waterborne exposure to triadimefon causes thyroid endocrine disruption and developmental delay in Xenopus laevis tadpoles

Triadimefon (TDF) is a triazole-derivative fungicide that is detectable in the environment and target agricultural products, prompting concern over its risk to wildlife and human health. In our study, Nieuwkoop & Faber stage 51 Xenopus laevis tadpoles were exposed to different nominal concentrations TDF (0, 0.112, and 1.12mg/L) for 21 days while the tadpoles were undergoing pre-morphological development. Developmental condition, bioaccumulation and thyroid hormone levels, and mRNA expression of genes involved in the hypothalamic-pituitary-thyroid (HPT) axis were examined. Exposure to TDF caused a reduction in developmental rates on pre-metamorphosis of X. laevis. TDF exposure significantly decreased thyroid hormone (T4 and T3) concentrations, indicating thyroid endocrine disruption. The downregulation of thyroglobulin and upregulation of genes related to thyroid hormone metabolism (ugt1ab) might be responsible for the decreased thyroid hormone concentrations. Treatment with TDF also significantly increased mRNA expression of genes involved in thyroid-stimulating hormone as a compensatory mechanism response to decreased thyroid hormone concentrations. Gene expression and in silico ligand docking studies were combined to study the interaction between TDF and thyroid hormone receptor. Results showed that TDF could consequently affect the HPT axis signaling pathway. In addition, bioconcentration of TDF was observed in tadpoles, indicating the bioactivity of this compound. Taken together, the results suggest that TDF alters the HPT axis-related genes and changes thyroid hormone contents in X. laevis tadpoles, thus causing thyroid endocrine disruption and consequently delaying thyroid hormones-dependent metamorphic development.

Strong lethality and teratogenicity of strobilurins on Xenopus tropicalis embryos: Basing on ten agricultural fungicides

Agricultural chemical inputs have been considered as a risk factor for the global declines in amphibian populations, yet the application of agricultural fungicides has increased dramatically in recent years. Currently little is known about the potential toxicity of fungicides on the embryos of amphibians. We studied the effects of ten commonly used fungicides (four strobilurins, two SDHIs, two triazoles, fludioxonil and folpet) on Xenopus tropicalis embryos. Lethal and teratogenic effects were respectively examined after 48 h exposure. The median lethal concentrations (LC50s) and the median teratogenic concentrations (TC50s) were determined in line with actual exposure concentrations. These fungicides except two triazoles showed obvious lethal effects on embryos; however LC50s of four strobilurins were the lowest and in the range of 6.81-196.59 μg/L. Strobilurins, SDHIs and fludioxonil induced severe malformations in embryos. Among the ten fungicides, the lowest TC50s were observed for four strobilurins in the range of 0.61-84.13 μg/L. The teratogenicity shared similar dose-effect relationship and consistent phenotypes mainly including microcephaly, hypopigmentation, somite segmentation and narrow fins. The findings indicate that the developmental toxicity of currently-used fungicides involved with ecologic risks on amphibians. Especially strobilurins are highly toxic to amphibian embryos at μg/L level, which is close to environmentally relevant concentrations.