Toxaphene detoxification and acclimation in Daphnia magna: do cytochrome P-450 enzymes play a role?

Mutation of the Cytochrome P450 CYP360A8 Gene Increases Sensitivity to Paraquat in Daphnia magna

The freshwater crustacean Daphnia magna has traditionally been a model for ecotoxicological studies owing to its sensitivity to many xenobiotics. Because it is used in many toxicity assessments, its detoxification mechanism for xenobiotics is important and requires further study. However, studies related to detoxification genes are limited to transcriptomic profiling, and there are no D. magna mutants for use in the understanding of xenobiotic metabolism in vivo. We report the generation of a D. magna CYP360A8 mutant-the gene is a cytochrome P450 (CYP) clan 3 gene. Based on RNA sequencing of adult D. magna, we found that CYP360A8 has the highest expression level among all CYP genes. At ovarian maturation, its expression level is up-regulated 6-fold compared to the juvenile stages and is maintained thereafter. Using the CRISPR/CRISPR-associated 9 (Cas9) system, we disrupted CYP360A8 by coinjecting CYP360A8-targeting guide RNA and Cas9 proteins into D. magna eggs and established one monoallelic CYP360A8 mutant line. This CYP360A8 mutant had a higher sensitivity to the herbicide paraquat compared to the wild type. We confirmed the up-regulation of CYP360A8 by paraquat. The results demonstrate the role of CYP360A8 in paraquat detoxification. The present study establishes a CYP mutant of D. magna, and this strategy can be a basic platform to document a range of CYP gene-xenobiotic relationships in this species. Environ Toxicol Chem 2021;40:1279-1288. © 2020 SETAC.

Multi-scale impact of chronic exposure to environmental concentrations of chlordecone in freshwater cnidarian, Hydra circumcincta

Chlordecone (CLD) is an organochlorine pesticide widely used in the past to control pest insects in banana plantations in the French West Indies. Due to its persistence in the environment, CLD has contaminated the soils where it has been spread, as well as the waters, and is still present in them. The objective of our study was to evaluate the effects of chronic exposure to environmentally relevant CLD concentrations in an animal model, the freshwater hydra (Hydra circumcincta). In a multi-marker approach, we have studied the expression of some target stress genes, the morphology, and the asexual reproduction rates. Our data showed that exposure to low concentrations of chlordecone leads to (i) a modulation of the expression of target genes involved in oxidative stress, detoxification, and neurobiological processes, and (ii) morphological damages and asexual reproduction impairment. We have observed non-monotonic dose-response curves, which agree with endocrine-disrupting chemical effects. Thus, "U-shaped" dose-response curves were observed for SOD, GRed, Hym355, and potentially GST gene expressions; inverted "U-shaped" curves for GPx and CYP1A gene expressions and reproductive rates; and a biphasic dose-response curve for morphological damages. Therefore, in the range of environmental concentrations tested, very low concentrations of CLD can produce equally or more important deleterious effects than higher ones. Finally, to our knowledge, this study is the first one to fill the lack of knowledge concerning the effects of CLD in Hydra circumcincta and confirms that this diploblastic organism is a pertinent freshwater model in the risk assessment.

Species-specific transcriptomic responses in Daphnia magna exposed to a bio-plastic production intermediate

Hydroxymethylfurfural (HMF) is a plant-based chemical building block that could potentially substitute petroleum-based equivalents, yet ecotoxicological data of this compound is currently limited. In this study, the effects of HMF on the reproduction and survival of Daphnia magna were assessed through validated ecotoxicological tests. The mechanism of toxicity was determined by analysis of transcriptomic responses induced by exposure to different concentrations of HMF using RNA sequencing. HMF exerted toxicity to D. magna with an EC₅₀ for effects on reproduction of 17.2 mg/l. HMF exposure affected molecular pathways including sugar and polysaccharide metabolism, lipid metabolism, general stress metabolism and red blood cell metabolism, although most molecular pathways affected by HMF exposure were dose specific. Hemoglobin genes, however, responded in a sensitive and dose-related manner. No induction of genes involved in the xenobiotic metabolism or oxidative stress metabolism pathway could be observed, which contrasted earlier observations on transcriptional responses of the terrestrial model Folsomia candida exposed to the same compound in a similar dose. We found 4189 orthologue genes between D. magna and F. candida, yet only twenty-one genes of those orthologues were co-regulated in both species. The contrasting transcriptional responses to the same compound exposed at a similar dose between D. magna and F. candida indicates limited overlap in stress responses among soil and aquatic invertebrates. The dose-related expression of hemoglobin provides further support for using hemoglobin expression as a biomarker for general stress responses in daphnids.

Toxaphene affects the levels of mRNA transcripts that encode antioxidant enzymes in Hydra

We evaluated toxaphene-induced acute toxicity in Hydra magnipapillata. The median lethal concentrations of the animals (LC(50)) were determined to be 34.5 mg/L, 25.0 mg/L and 12.0 mg/L after exposure to toxaphene for 24 h, 48 h and 72 h, respectively. Morphological responses of hydra polyps to a range of toxaphene concentrations suggested that toxaphene negatively affects the nervous system of H. magnipapillata. We used real-time quantitative PCR of RNA extracted from polyps exposed to two concentrations of toxaphene (0.3 mg/L and 3 mg/L) for 24 h to evaluate the differential regulation of levels of transcripts that encode six antioxidant enzymes (CAT, G6PD, GPx, GR, GST and SOD), two proteins involved in detoxification and molecular stress responses (CYP1A and UB), and two proteins involved in neurotransmission and nerve cell differentiation (AChE and Hym-355). Of the genes involved in antioxidant responses, the most striking changes were observed for transcripts that encode GPx, G6PD, SOD, CAT and GST, with no evident change in levels of transcripts encoding GR. Levels of UB and CYP1A transcripts increased in a dose-dependent manner following exposure to toxaphene. Given that toxaphene-induced neurotoxicity was not reflected in the level of AChE transcripts and only slight accumulation of Hym-355 transcript was observed only at the higher of the two doses of toxaphene tested, there remains a need to identify transcriptional biomarkers for toxaphene-mediated neurotoxicity in H. magnipapillata. Transcripts that respond to toxaphene exposure could be valuable biomarkers for stress levels in H. magnipapillata and may be useful for monitoring the pollution of aquatic environments.

A new method for ranking mode-specific sensitivity of freshwater arthropods to insecticides and its relationship to biological traits

The problem of how to deal with species sensitivity differences to toxic substances has been addressed successfully with the species sensitivity distribution (SSD), yet this has not increased understanding about the underlying mechanisms of sensitivity. Other researchers have identified the mode of action of chemicals and also biological traits of species as determinants for sensitivity, yet no systematic approach combines these factors. To achieve this, first existing data on organophosphate, carbamate, and pyrethroid toxicity and mode of action and also species trait information were mined. Second, we linked taxon sensitivity to their traits at the family level to generate empirical and mechanistic hypotheses about sensitivity-trait relationships. In this way, a mode-specific sensitivity (MSS) ranking method was developed, and tested at the taxonomic level of family and genus. The application of several quality criteria indicated overall confidence in rankings, but confidence in exact taxon rank was less certain, due to data insufficiency for certain groups. The MSS rankings were found to be applicable for trait-based approaches and were successfully linked to existing trait data to identify traits with predictive potential. Although this empirical analysis cannot test causality relationships between traits and sensitivity, testable hypotheses were generated, for further experimental investigation. Single traits as well as combinations of traits can be used to predict laboratory sensitivity to the substances tested, although associations were not as strong as in previous studies. We conclude that existing trait data are not suitable for every trait-based research question and that important traits remain to be identified and quantified in relation to the processes of toxicity, i.e., the toxicokinetics and toxicodynamics.