The onset of active gill respiration in post-embryonic zebrafish (Danio rerio) larvae triggers an increased sensitivity to neurotoxic compounds

Neurotoxic pesticides change respiratory parameters in early gill-breathing, but not in skin-breathing life-stages of zebrafish (Danio rerio)

Neurotoxic compounds can interfere with active gill ventilation in fish, which might lead to premature death in adult fish, but not in skin-breathing embryos of zebrafish, since these exclusively rely on passive diffusion across the skin. Regarding lethality, this respiratory failure syndrome (RFS) has been discussed as one of the main reasons for the higher sensitivity of adult fish in the acute fish toxicity test (AFT), if compared to embryos in the fish embryo toxicity test (FET). To further elucidate the relationship between the onset of gill respiration and death by a neurotoxic mode of action, a comparative study into oxygen consumption (MO2), breathing frequency (fv) and amplitude (fampl) was performed with 4 d old skin-breathing and 12 d old early gill-breathing zebrafish. Neurotoxic model substances with an LC50 FET/AFT ratio of > 10 were used: chlorpyrifos, permethrin, aldicarb, ziram, and fluoxetine. Exposure to hypoxia served as a positive control, whereas aniline was tested as an example of a narcotic substance interfering non-specifically with gill membranes. In 12 d old larvae, all substances caused an increase in MO2, fv and partly fampl, whereas effects were minor in 4 d old embryos. An increase of fv in 4 d old embryos following exposure to chlorpyrifos, aldicarb and hypoxia could not be correlated with an increased MO2 and might be attributed either to (1) to the successfully postponed decrease of arterial partial pressure of oxygen (PO2) through support of skin respiration by increased fv, (2) to an unspecific stimulation of the sphincter muscles at the base of the gill filaments, or (3) to the establishment of oxygen sensing for later stages. In gill-breathing 12 d old zebrafish, a concentration-dependent increase of fv was detected for aniline and chlorpyrifos, whereas for aldicarb, fluoxetine and permethrin, a decline of fv at higher substance concentrations was measured, most likely due to the onset of paralysis and/or fatigue of the gill filament sphincter muscles. Since alterations of fv serve to postpone the decrease in arterial PO2 and MO2 increased with decreasing fv, the respiratory failure syndrome could clearly be demonstrated in 12 d old zebrafish larvae. Passive respiration across the skin in zebrafish embryos could thus be confirmed as a probable reason for the lower sensitivity of early life-stages to neurotoxicants. Integration of respiratory markers into existing testing protocols with non-protected developmental stages such as embryos might help to not underestimate the toxicity of early life-stages of fish.

Physiology-informed toxicokinetic model for the zebrafish embryo test developed for bisphenols

Zebrafish embryos (ZFE) is a widely used model organism, employed in various research fields including toxicology to assess e.g., developmental toxicity and endocrine disruption. Variation in effects between chemicals are difficult to compare using nominal dose as toxicokinetic properties may vary. Toxicokinetic (TK) modeling is a means to estimate internal exposure concentration or dose at target and to enable extrapolation between experimental conditions and species, thereby improving hazard assessment of potential pollutants. In this study we advance currently existing TK models for ZFE with physiological ZFE parameters and novel experimental bisphenol data, a class of chemicals with suspected endocrine activity. We developed a five-compartment model consisting of water, plastic, chorion, yolk sack and embryo in which surface area and volume changes as well as the processes of biotransformation and blood circulation influence mass fluxes. For model training and validation, we measured internal concentrations in ZFE exposed individually to BPA, bisphenol AF (BPAF) and Z (BPZ). Bayesian inference was applied for parameter calibration based on the training data set of BPZ. The calibrated TK model predicted internal ZFE concentrations of the majority of external test data within a 5-fold error and half of the data within a 2-fold error for bisphenols A, AF, F, and tetrabromo bisphenol A (TBBPA). We used the developed model to rank the hazard of seven bisphenols based on predicted internal concentrations and measured in vitro estrogenicity. This ranking indicated a higher hazard for BPAF, BPZ, bisphenol B and C (BPB, BPC) than for BPA.

Integrating 4-D light-sheet fluorescence microscopy and genetic zebrafish system to investigate ambient pollutants-mediated toxicity

Ambient air pollutants, including PM2.5 (aerodynamic diameter d ~2.5 μm), PM10 (d ~10 μm), and ultrafine particles (UFP: d < 0.1 μm) impart both short- and long-term toxicity to various organs, including cardiopulmonary, central nervous, and gastrointestinal systems. While rodents have been the principal animal model to elucidate air pollution-mediated organ dysfunction, zebrafish (Danio rerio) is genetically tractable for its short husbandry and life cycle to study ambient pollutants. Its electrocardiogram (ECG) resembles that of humans, and the fluorescent reporter-labeled tissues in the zebrafish system allow for screening a host of ambient pollutants that impair cardiovascular development, organ regeneration, and gut-vascular barriers. In parallel, the high spatiotemporal resolution of light-sheet fluorescence microscopy (LSFM) enables investigators to take advantage of the transparent zebrafish embryos and genetically labeled fluorescent reporters for imaging the dynamic cardiac structure and function at a single-cell resolution. In this context, our review highlights the integrated strengths of the genetic zebrafish system and LSFM for high-resolution and high-throughput investigation of ambient pollutants-mediated cardiac and intestinal toxicity.

Influence of exposure scenario on the sensitivity to caffeine

The chorion acts as a protective barrier, restricting some chemical absorption into the embryo and the surrounding fluids. In this sense, larvae may only have direct contact with some chemicals after dechorionation. This study aimed to evaluate the effects of caffeine (CAF) (0, 13, 20, 44, 67, and 100 mg.L-1) under different exposure scenarios (embryos with chorion or embryos/larvae already hatched) and rank the stage sensitivity. Thus, three scenarios were investigated: from 2 to 120 hours post fertilization (hpf) (5 days of exposure- 5dE), from 72 to 120 hpf (2dE), and from 96 to 120 hpf (1dE). Heart rate (48 hpf) and energy reserves (120 hpf) were measured in the 5dE scenario, and behavior and acetylcholinesterase (AChE) activity were evaluated at 120 hpf in all scenarios (5dE, 2dE, and 1dE). At 120 hpf, some of the fish was transferred to clean medium for a 10 days depuration period (10dPE). Behavior and AChE activity were assessed after this period. In the 5dE scenario, CAF increased heartbeat (13, 20, and 30 mg.L-1) and reduced carbohydrates (67, and 100 mg.L-1), while inhibiting AChE activity (100 mg.L-1) in the 5dE, 2dE, and 1dE scenarios. CAF reduced the total distance moved in the 5dE (67, and 100 mg.L-1), 2dE (20, 30, 44, 67, and 100 mg.L-1), and 1dE fish (67, and 100 mg.L-1) and increased erratic movements. Based on the lowest observed effect concentration (LOEC) for total distance moved (20 mg.L-1) and higher inhibition of AChE activity (100 mg.L-1) (65%), 2dE fish appear to be more sensitive to CAF. After 10dPE, a recovery in behavior was detected in all scenarios (5dE, 2dE, and 1dE). AChE activity remained inhibited in the 2dE scenario while increasing in the 1dE scenario. This study demonstrated that the presence of the chorion is an important factor for the analysis of CAF toxicity. After the loss of the chorion, organisms show greater sensitivity to CAF and can be used to evaluate the toxicity of various substances, including nanomaterials or chemicals with low capacity to cross the chorion. Therefore, the use of hatched embryos in toxicity tests is suggested, as they allow a shorter and less expensive exposure scenario that provides similar outcome as the conventional scenario.

Impact of Drug Administration Routes on the In Vivo Efficacy of the Natural Product Sorangicin a Using a Staphylococcus aureus Infection Model in Zebrafish Embryos

Staphylococcus aureus causes a wide range of infections, and it is one of the leading pathogens responsible for deaths associated with antimicrobial resistance, the rapid spread of which among S. aureus urges the discovery of new antibiotics. The evaluation of in vivo efficacy of novel drug candidates is usually performed using animal models. Recently, zebrafish (Danio rerio) embryos have become increasingly attractive in early drug discovery. Herein, we established a zebrafish embryo model of S. aureus infection for evaluation of in vivo efficacy of novel potential antimicrobials. A local infection was induced by microinjecting mCherry-expressing S. aureus Newman followed by treatment with reference antibiotics via microinjection into different injection sites as well as via waterborne exposure to study the impact of the administration route on efficacy. We successfully used the developed model to evaluate the in vivo activity of the natural product sorangicin A, for which common mouse models were not successful due to fast degradation in plasma. In conclusion, we present a novel screening platform for assessing in vivo activity at the antibiotic discovery stage. Furthermore, this work provides consideration for the choice of an appropriate administration route based on the physicochemical properties of tested drugs.

Microbial Diversity and Enzyme Activity as Indicators of Permethrin-Exposed Soil Health

Owing to their wide range of applications in the control of ticks and insects in horticulture, forestry, agriculture and food production, pyrethroids pose a significant threat to the environment, including a risk to human health. Hence, it is extremely important to gain a sound understanding of the response of plants and changes in the soil microbiome induced by permethrin. The purpose of this study has been to show the diversity of microorganisms, activity of soil enzymes and growth of Zea mays following the application of permethrin. This article presents the results of the identification of microorganisms with the NGS sequencing method, and of isolated colonies of microorganisms on selective microbiological substrates. Furthermore, the activity of several soil enzymes, such as dehydrogenases (Deh), urease (Ure), catalase (Cat), acid phosphatase (Pac), alkaline phosphatase (Pal), β-glucosidase (Glu) and arylsulfatase (Aryl), as well as the growth of Zea mays and its greenness indicators (SPAD), after 60 days of growth following the application of permethrin, were presented. The research results indicate that permethrin does not have a negative effect on the growth of plants. The metagenomic studies showed that the application of permethrin increases the abundance of Proteobacteria, but decreases the counts of Actinobacteria and Ascomycota. The application of permethrin raised to the highest degree the abundance of bacteria of the genera Cellulomonas, Kaistobacter, Pseudomonas, Rhodanobacter and fungi of the genera Penicillium, Humicola, Iodophanus, Meyerozyma. It has been determined that permethrin stimulates the multiplication of organotrophic bacteria and actinomycetes, decreases the counts of fungi and depresses the activity of all soil enzymes in unseeded soil. Zea mays is able to mitigate the effect of permethrin and can therefore be used as an effective phytoremediation plant.

The sensitivity of the zebrafish embryo coiling assay for the detection of neurotoxicity by compounds with diverse modes of action

In the aim to determine neurotoxicity, new methods are being validated, including tests and test batteries comprising in vitro and in vivo approaches. Alternative test models such as the zebrafish (Danio rerio) embryo have received increasing attention, with minor modifications of the fish embryo toxicity test (FET; OECD TG 236) as a tool to assess behavioral endpoints related to neurotoxicity during early developmental stages. The spontaneous tail movement assay, also known as coiling assay, assesses the development of random movement into complex behavioral patterns and has proven sensitive to acetylcholine esterase inhibitors at sublethal concentrations. The present study explored the sensitivity of the assay to neurotoxicants with other modes of action (MoAs). Here, five compounds with diverse MoAs were tested at sublethal concentrations: acrylamide, carbaryl, hexachlorophene, ibuprofen, and rotenone. While carbaryl, hexachlorophene, and rotenone consistently induced severe behavioral alterations by ~ 30 h post fertilization (hpf), acrylamide and ibuprofen expressed time- and/or concentration-dependent effects. At 37-38 hpf, additional observations revealed behavioral changes during dark phases with a strict concentration-dependency. The study documented the applicability of the coiling assay to MoA-dependent behavioral alterations at sublethal concentrations, underlining its potential as a component of a neurotoxicity test battery.

A novel automated method for the simultaneous detection of breathing frequency and amplitude in zebrafish (Danio rerio) embryos and larvae

Stress responses of fish to disruption of oxygen homeostasis include adjusted oxygen consumption rate (MO₂) as well as the hyperventilation consisting of changes in breathing frequency (f_v) and amplitude (f_ampl). However, studying the HVR in very small organisms such as zebrafish (Danio rerio) embryos and larvae is challenging, and breathing movements (i.e., f_v) are usually manually counted, which is time- and human resource-intense, error-prone and does not provide information on the amplitude of breathing movements of the response, the breathing amplitude (f_ampl). Hence, in the present study, a new automated method was developed to simultaneously measure f_v and f_ampl in small zebrafish embryos and larvae with the computer software DanioScope™. To compare HVR strategies at different life-stages of zebrafish and the physiologically linked MO₂, hatched 4 d old embryos and early gill-breathing 12 d old larvae were treated with the HVR-inducing neurotoxic compound lindane (γ-hexachlorocyclohexane; γ-HCH) as a model substance. Comparison of manually counted f_v with f_v data measured by DanioScope™ at both life-stages showed high to moderate agreement between the two methods with respect to f_v in control fish and in fish treated with lower lindane concentrations (3 - 18% deviation at 25 µg/L γ-HCH). With increasing lindane concentrations (100 and 400 µg/L γ-HCH), however, manual counts showed an average underestimation of f_v by up to 30%, mainly due to very fast, rapidly successive, and indistinct movements of the fish, which cannot be properly detected by manual counts. Automated measurement thus proved significantly more sensitive, although several pre- and post-processing steps are needed. The improved automated detection of f_v and the first reliable estimation of f_ampl in small fish embryos and larvae, as well as the inclusion of MO₂, may provide new insights into different respiratory strategies and may, thus, represent a tool to lower the detection limit for reactions of different life-stages of fish to environmental stressors. In the present study, this became evident, as early gill-breathing 12 d old zebrafish larvae showed symptoms of respiratory failure (i.e., increase in f_v, f_ampl and MO₂, followed by subsequent lethargy) after exposure to lindane, whereas skin-breathing in 4 d old embryos proved mainly insensitive to the paralytic effects of lindane.