Integrative assessment of low-dose gamma radiation effects on Daphnia magna reproduction: Toxicity pathway assembly and AOP development

History of exposure to copper influences transgenerational gene expression responses in Daphnia magna

The establishment of transgenerational effects following chemical exposure is a powerful phenomenon, capable of modulating ecosystem health beyond exposure periods. This study assessed the transgenerational effects occurring due to copper exposure in the invertebrate D. magna at the transcriptional level, while evaluating the role of exposure history on such responses. Thus, daphnids acclimated for several generations in a copper vs. clean medium were then exposed for one generation (F0) to this metal, and monitored for the following non-exposed generations (F1, F2 and F3). Organisms differing in exposure histories showed remarkably different transcriptional profiles at the F0, with naïve organisms being more profoundly affected. These trends were confirmed for F3 treatments, which presented different transcriptional patterns for genes involved in detoxification, oxidative stress, DNA damage repair, circadian clock functioning and epigenetic regulation. Furthermore, regardless of exposure history, a great number of histone modifier genes were always found transcriptionally altered, thus suggesting the involvement of histone modifications in the response of Daphnia to metal exposure. Lastly, remarkably distinct transgenerational transcriptional responses were found between naïve and non-naïve organisms, thereby highlighting the influence of exposure history on gene expression and confirming the capacity of metals to determine transgenerational transcriptional effects across non-exposed generations.

Combined effects of nanoplastics and elevated temperature in the freshwater water flea Daphnia magna

Global warming and nanoplastics (NPs) are critical global issues. Among NPs, one of the most hazardous types of plastics, polystyrene (PS), poses ecotoxicological threats to several freshwater organisms. The degree of toxicity of PS-NPs is strongly influenced by various environmental factors. This study illustrates the combined effects of temperature and PS-NPs on the water flea Daphnia magna. The sensitivity of D. magna to PS-NPs was tested under control (23 °C) and elevated temperatures (28 °C). As a result, increased temperatures influenced the uptake and accumulation of PS-NPs. Co-exposure to both higher temperatures and PS-NPs resulted in a drastic decrease in reproductive performance. The level of oxidative stress was found to have increased in a temperature-dependent manner. Oxidative stress was stimulated by both stressors, leading to increased levels of reactive oxygen species and antioxidant enzyme activity supported by upregulation of antioxidant enzyme-related genes under combined PS-NPs exposure and elevated temperature. In the imbalanced status of intracellular redox, activation of the p38 mitogen-activated protein kinase signaling pathway was induced by exposure to PS-NPs at high temperatures, which supported the decline of the reproductive capacity of D. magna. Therefore, our results suggest that PS-NPs exposure along with an increase in temperature significantly affects physiological processes triggered by damage from oxidative stress, leading to severely inhibited reproduction of D. magna.

Thermal stress-stimulated ZnO toxicity inhibits reproduction of freshwater crustacean Daphnia magna

Elevated temperatures due to climate change pose a variety of environmental risks to the freshwater ecosystem. At the same time, zinc oxide (ZnO) has become widely used and has entered the freshwater environment. As thermal stress may potentially impact the physicochemical properties of ZnO, its toxicity to freshwater organisms in the face of global warming is poorly understood. The potential effects on reproductive performances, including oogenesis, are of particular concern. In this study, we investigate the reproductive performances and related mRNA abundance of the zooplankton Daphnia magna under conditions of ZnO exposure and heat stress. The results revealed that ZnO and elevated temperature delayed maturity and juvenile production of D. magna. Histological observations indicated that oogenesis was inhibited, and the number and size of oocytes were reduced in the condition of ZnO exposure under heat stress. Eventual offspring in the same treatment exhibited decreased numbers, size, and quality. Congenital juvenile anomalies were increased, such as deformed eye, and impaired antenna and tail spine. Moreover, both ZnO and elevated temperature treatments inhibited expression levels of reproduction-related genes (vtg, EcR and VMO1) and induced the dmrt93b gene involved in the production of male offspring. Furthermore, we found that D. magna tried to cope with ZnO and thermal stress by upregulating hsp90, HIF-1α and HIF-1β. ZnO and heat stress inhibited the reproductive capacity of D. magna, produced deleterious effects on reproduction-associated physiological pathways, and damaged reproductive outcomes.

Effects of petrogenic pollutants on North Atlantic and Arctic Calanus copepods: From molecular mechanisms to population impacts

Oil and gas industries in the Northern Atlantic Ocean have gradually moved closer to the Arctic areas, a process expected to be further facilitated by sea ice withdrawal caused by global warming. Copepods of the genus Calanus hold a key position in these cold-water food webs, providing an important energetic link between primary production and higher trophic levels. Due to their ecological importance, there is a concern about how accidental oil spills and produced water discharges may impact cold-water copepods. In this review, we summarize the current knowledge of the toxicity of petroleum on North Atlantic and Arctic Calanus copepods. We also review how recent development of high-quality transcriptomes from RNA-sequencing of copepods have identified genes regulating key biological processes, like molting, diapause and reproduction in Calanus copepods, to suggest linkages between exposure, molecular mechanisms and effects on higher levels of biological organization. We found that the available ecotoxicity threshold data for these copepods provide valuable information about their sensitivity to acute petrogenic exposures; however, there is still insufficient knowledge regarding underlying mechanisms of toxicity and the potential for long-term implications of relevance for copepod ecology and phenology. Copepod transcriptomics has expanded our understanding of how key biological processes are regulated in cold-water copepods. These advances can improve our understanding of how pollutants affect biological processes, and thus provide the basis for new knowledge frameworks spanning the effect continuum from molecular initiating events to adverse effects of regulatory relevance. Such efforts, guided by concepts such as adverse outcome pathways (AOPs), enable standardized and transparent characterization and evaluation of knowledge and identifies research gaps and priorities. This review suggests enhancing mechanistic understanding of exposure-effect relationships to better understand and link biomarker responses to adverse effects to improve risk assessments assessing ecological effects of pollutant mixtures, like crude oil, in Arctic areas.

Dose rate dependent reduction in chromatin accessibility at transcriptional start sites long time after exposure to gamma radiation

Ionizing radiation (IR) impact cellular and molecular processes that require chromatin remodelling relevant for cellular integrity. However, the cellular implications of ionizing radiation (IR) delivered per time unit (dose rate) are still debated. This study investigates whether the dose rate is relevant for inflicting changes to the epigenome, represented by chromatin accessibility, or whether it is the total dose that is decisive. CBA/CaOlaHsd mice were whole-body exposed to either chronic low dose rate (2.5 mGy/h for 54 d) or the higher dose rates (10 mGy/h for 14 d and 100 mGy/h for 30 h) of gamma radiation (⁶⁰Co, total dose: 3 Gy). Chromatin accessibility was analysed in liver tissue samples using Assay for Transposase-Accessible Chromatin with high-throughput sequencing (ATAC-Seq), both one day after and over three months post-radiation (>100 d). The results show that the dose rate contributes to radiation-induced epigenomic changes in the liver at both sampling timepoints. Interestingly, chronic low dose rate exposure to a high total dose (3 Gy) did not inflict long-term changes to the epigenome. In contrast to the acute high dose rate given to the same total dose, reduced accessibility at transcriptional start sites (TSS) was identified in genes relevant for the DNA damage response and transcriptional activity. Our findings link dose rate to essential biological mechanisms that could be relevant for understanding long-term changes after ionizing radiation exposure. However, future studies are needed to comprehend the biological consequence of these findings.

Temperature related toxicity features of acute acetamiprid and thiacloprid exposure in Daphnia magna and implications on reproductive performance

This study investigated the potential for elevated temperature to alter the toxicity of acetamiprid (ACE) and thiacloprid (Thia) in the ecotoxicity model Daphnia magna. The modulation of CYP450 monooxygenases (ECOD), ABC transporter activity (MXR) and incident cellular reactive oxygen species (ROS) overproduction was screened in premature daphnids following acute (48 h) exposure to sublethal concentrations of ACE and Thia (0.1-, 1.0 μM) at standard 21 °C and elevated 26 °C temperatures. Delayed outcomes of acute exposures were further evaluated based on the reproduction performance of daphnids monitored over 14 days of recovery. Exposures to ACE and Thia at 21^o C elicited moderate induction of ECOD activity, pronounced inhibition of MXR activity and severe ROS overproduction in daphnids. In the high thermal regime, treatments resulted in significantly lower induction of ECOD activity and inhibition of MXR activity, suggesting a suppressed metabolism of neonicotinoids and less impaired membrane transport activity in daphnids. Elevated temperature on its own, caused a three-fold rise in ROS levels in control daphnids, while ROS overproduction upon neonicotinoid exposure was less accentuated. Acute exposures to ACE and Thia caused significant decreases also in the reproduction of daphnids, indicating delayed outcomes even at environmentally relevant concentrations. Both the cellular alterations in exposed daphnids and decreases in their reproductive output post exposures evidenced closely similar toxicity patterns and potentials for the two neonicotinoids. While elevated temperature elicited only a shift in baseline cellular alterations evoked by neonicotinoids, it significantly worsened the reproductive performance of daphnids following neonicotinoid exposures.

Epigenetic analytical approaches in ecotoxicological aquatic research

Environmental epigenetics has become a key research focus in global climate change studies and environmental pollutant investigations impacting aquatic ecosystems. Specifically, triggered by environmental stress conditions, intergenerational DNA methylation changes contribute to biological adaptive responses and survival of organisms to increase their tolerance towards these conditions. To critically review epigenetic analytical approaches in ecotoxicological aquatic research, we evaluated 78 publications reported over the past five years (2016-2021) that applied these methods to investigate the responses of aquatic organisms to environmental changes and pollution. The results show that DNA methylation appears to be the most robust epigenetic regulatory mark studied in aquatic animals. As such, multiple DNA methylation analysis methods have been developed in aquatic organisms, including enzyme restriction digestion-based and methyl-specific immunoprecipitation methods, and bisulfite (in)dependent sequencing strategies. In contrast, only a handful of aquatic studies, i.e. about 15%, have been focusing on histone variants and post-translational modifications due to the lack of species-specific affinity based immunological reagents, such as specific antibodies for chromatin immunoprecipitation applications. Similarly, ncRNA regulation remains as the least popular method used in the field of environmental epigenetics. Insights into the opportunities and challenges of the DNA methylation and histone variant analysis methods as well as decreasing costs of next generation sequencing approaches suggest that large-scale epigenetic environmental studies in model and non-model organisms will soon become available in the near future. Moreover, antibody-dependent and independent methods, such as mass spectrometry-based methods, can be used as an alternative epigenetic approach to characterize global changes of chromatin histone modifications in future aquatic research. Finally, a systematic guide for DNA methylation and histone variant methods is offered for ecotoxicological aquatic researchers to select the most relevant epigenetic analytical approach in their research.

Disturbances in growth, oxidative stress, energy reserves and the expressions of related genes in Daphnia magna after exposure to ZnO under thermal stress

The toxicological effects of metal contamination are influenced by the ambient temperature. Therefore, global warming affects the toxicity of metal contamination in aquatic ecosystems. ZnO is widely used as a catalyst in many industries, and causes contamination in aquatic ecosystems. Here, we investigated the effects of ZnO concentration under elevated temperature by observing growth, oxidative stress, energy reserves and related gene expression in exposed Daphnia magna. Body length and growth rate increased in neonates exposed to ZnO for 2 days but decreased at 9 and 21 days under elevated temperature. ZnO concentration and elevated temperature induced oxidative stress in mature D. magna by reducing superoxide dismutase (SOD) activity and increasing malondialdehyde (MDA) levels. In contrast, juveniles were unaffected. Carbohydrate, protein and caloric contents were reduced throughout development in D. magna treated with ZnO and elevated temperature in all exposure periods (2, 9 and 21 days). However, lipid content also decreased in mature D. magna treated with ZnO cultured under elevated temperature, while that of juveniles showed an increase in lipid content. Therefore, energy was perhaps allocated to physiological processes for detoxification and homeostasis. Moreover, expression patterns of genes related to physiological processes changed under elevated temperature and ZnO exposure. Taken together, our results highlight that the combination of temperature and ZnO concentration induced toxicity in D. magna. This conclusion was confirmed by the Integrated Biological Response (IBR) index. This study shows that changes in biological levels of organization could be used to monitor environmental change using D. magna as a bioindicator.

Biological effects of ionizing radiation on aquatic biota - A critical review

Ionizing radiation from radionuclides impacts marine aquatic biota and the scope of investigation must be wider than just invertebrates. We intend to detail and illustrate numerous biological effects that occur in both aquatic vertebrates and invertebrates, at various dose rates from all three kinds of ionizing radiation. The characteristics of radiation sources and dosages that would most effectively generate the intended effects in the irradiated organism were assessed once the biological differentiation between vertebrates and invertebrates was determined through multiple lines of evidence. We contend that invertebrates are still more radiosensitive than vertebrates, due to their small genome size, rapid reproduction rates and lifestyle, which help them to compensate for the effects of radiation induced declines in fecundity, life span and individual health. We also identified various research gaps in this field and suggest future directions to be investigated to remedy the lack of data available in this area.

Multiomics Point of Departure (moPOD) Modeling Supports an Adverse Outcome Pathway Network for Ionizing Radiation

While adverse biological effects of acute high-dose ionizing radiation have been extensively investigated, knowledge on chronic low-dose effects is scarce. The aims of the present study were to identify hazards of low-dose ionizing radiation to Daphnia magna using multiomics dose-response modeling and to demonstrate the use of omics data to support an adverse outcome pathway (AOP) network development for ionizing radiation. Neonatal D. magna were exposed to γ radiation for 8 days. Transcriptomic analysis was performed after 4 and 8 days of exposure, whereas metabolomics and confirmative bioassays to support the omics analyses were conducted after 8 days of exposure. Benchmark doses (BMDs, 10% benchmark response) as points of departure (PODs) were estimated for both dose-responsive genes/metabolites and the enriched KEGG pathways. Relevant pathways derived using the BMD modeling and additional functional end points measured by the bioassays were overlaid with a previously published AOP network. The results showed that several molecular pathways were highly relevant to the known modes of action of γ radiation, including oxidative stress, DNA damage, mitochondrial dysfunction, protein degradation, and apoptosis. The functional assays showed increased oxidative stress and decreased mitochondrial membrane potential and ATP pool. Ranking of PODs at the pathway and functional levels showed that oxidative damage related functions had relatively low PODs, followed by DNA damage, energy metabolism, and apoptosis. These were supportive of causal events in the proposed AOP network. This approach yielded promising results and can potentially provide additional empirical evidence to support further AOP development for ionizing radiation.

Daphnia as a model organism to probe biological responses to nanomaterials-from individual to population effects via adverse outcome pathways

The importance of the cladoceran Daphnia as a model organism for ecotoxicity testing has been well-established since the 1980s. Daphnia have been increasingly used in standardised testing of chemicals as they are well characterised and show sensitivity to pollutants, making them an essential indicator species for environmental stress. The mapping of the genomes of D. pulex in 2012 and D. magna in 2017 further consolidated their utility for ecotoxicity testing, including demonstrating the responsiveness of the Daphnia genome to environmental stressors. The short lifecycle and parthenogenetic reproduction make Daphnia useful for assessment of developmental toxicity and adaption to stress. The emergence of nanomaterials (NMs) and their safety assessment has introduced some challenges to the use of standard toxicity tests which were developed for soluble chemicals. NMs have enormous reactive surface areas resulting in dynamic interactions with dissolved organic carbon, proteins and other biomolecules in their surroundings leading to a myriad of physical, chemical, biological, and macromolecular transformations of the NMs and thus changes in their bioavailability to, and impacts on, daphnids. However, NM safety assessments are also driving innovations in our approaches to toxicity testing, for both chemicals and other emerging contaminants such as microplastics (MPs). These advances include establishing more realistic environmental exposures via medium composition tuning including pre-conditioning by the organisms to provide relevant biomolecules as background, development of microfluidics approaches to mimic environmental flow conditions typical in streams, utilisation of field daphnids cultured in the lab to assess adaption and impacts of pre-exposure to pollution gradients, and of course development of mechanistic insights to connect the first encounter with NMs or MPs to an adverse outcome, via the key events in an adverse outcome pathway. Insights into these developments are presented below to inspire further advances and utilisation of these important organisms as part of an overall environmental risk assessment of NMs and MPs impacts, including in mixture exposure scenarios.

Adverse outcome pathways (AOPs) for radiation-induced reproductive effects in environmental species: state of science and identification of a consensus AOP network

BACKGROUND

Reproductive effects of ionizing radiation in organisms have been observed under laboratory and field conditions. Such assessments often rely on associations between exposure and effects, and thus lacking a detailed mechanistic understanding of causality between effects occurring at different levels of biological organization. The Adverse Outcome Pathway (AOP), a conceptual knowledge framework to capture, organize, evaluate and visualize the scientific knowledge of relevant toxicological effects, has the potential to evaluate the causal relationships between molecular, cellular, individual, and population effects. This paper presents the first development of a set of consensus AOPs for reproductive effects of ionizing radiation in wildlife. This work was performed by a group of experts formed during a workshop organized jointly by the Multidisciplinary European Low Dose Initiative (MELODI) and the European Radioecology Alliance (ALLIANCE) associations to present the AOP approach and tools. The work presents a series of taxon-specific case studies that were used to identify relevant empirical evidence, identify common AOP components and propose a set of consensus AOPs that could be organized into an AOP network with broader taxonomic applicability.

CONCLUSION

Expert consultation led to the identification of key biological events and description of causal linkages between ionizing radiation, reproductive impairment and reduction in population fitness. The study characterized the knowledge domain of taxon-specific AOPs, identified knowledge gaps pertinent to reproductive-relevant AOP development and reflected on how AOPs could assist applications in radiation (radioecological) research, environmental health assessment, and radiological protection. Future advancement and consolidation of the AOPs is planned to include structured weight of evidence considerations, formalized review and critical assessment of the empirical evidence prior to formal submission and review by the OECD sponsored AOP development program.

A high-level overview of the Organisation for Economic Co-operation and Development Adverse Outcome Pathway Programme

Background The Organisation for Economic Co-operation and Development (OECD), through its Chemical Safety Programme, is delegated to ensure the safety of humans and wildlife from harmful toxicants. To support these needs, initiatives to increase the efficiency of hazard identification and risk management are under way. Amongst these, the adverse outcome pathway (AOP) approach integrates information on biological knowledge and test methodologies (both established and new) to support regulatory decision making. AOPs collate biological knowledge from different sources, assess lines of evidence through considerations of causality and undergo rigorous peer-review before being subsequently endorsed by the OECD. It is envisioned that the OECD AOP Development Programme will transform the toxicity testing paradigm by leveraging the strengths of mechanistic and modelling based approaches and enhance the utility of high throughput screening assays. Since its launch, in 2012, the AOP Development Programme has matured with a greater number of AOPs endorsed since inception, and the attraction of new scientific disciplines (e.g. the radiation field). Recently, a Radiation and Chemical (Rad/Chem) AOP Joint Topical Group has been formed by the OECD Nuclear Energy Agency High-Level Group on Low-Dose Research (HLG-LDR) under the auspices of the Committee on Radiological Protection and Public Health (CRPPH). The topical group will work to evolve the development and use of the AOP framework in radiation research and regulation. As part of these efforts, the group will bring awareness and understanding on the programme, as it has matured from the chemical perspective. In this context, this paper provides the radiation community with a high-level overview of the OECD AOP Development Programme, including examples of application using knowledge gleaned from the field of chemical toxicology, and their work towards regulatory implementation. Conclusion: Although the drivers for developing AOPs in chemical sector differ from that of the radiation field, the principles and transparency of the approach can benefit both scientific disciplines. By providing perspectives and an understanding of the evolution of the OECD AOP Development Programme including case examples and work towards quantitative AOP development, it may motivate the expansion and implementation of AOPs in the radiation field.

An adverse outcome pathway-based approach to assess aurantio-obtusin-induced hepatotoxicity

Cassiae semen (CS), a traditional Chinese medicine, has various bioactivities in preclinical and clinical practice. Aurantio-obtusin (AO) is a major anthraquinone (AQ) ingredient derived from CS, and has drawn public concerns over its potential hepatotoxicity. We previously found that AO induces hepatic necroinflammation by activating NOD-like receptor protein 3 inflammasome signaling. However, the mechanisms contributing to AO-motivated hepatotoxicity remain unclear. Herein, we evaluated hepatotoxic effects of AO on three liver cell lines by molecular and biochemical analyses. We found that AO caused cell viability inhibition and biochemistry dysfunction in the liver cells. Furthermore, AO elevated reactive oxygen species (ROS), followed by mitochondrial dysfunction (decreases in mitochondrial membrane potential and adenosine triphosphate) and apoptosis (increased Caspase-3, Cleaved caspase-3, Cytochrome c and Bax expression, and decreased Bcl-2 expression). We also found that AO increased the lipid peroxidation (LPO) and enhanced ferroptosis by activating cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA)-cAMP response element-binding (CREB) pathway (increases in PKA, p-CREB, acyl-CoA synthetase long chain family member 4). Based on these results, we used an AOP framework to explore the mechanisms underlying AO's hepatotoxicity. It starts from molecular initiating event (ROS), and follows two critical toxicity pathways (i.e., mitochondrial dysfunction-mediated apoptosis and LPO-enhanced ferroptosis) over a series of key events (KEs) to the adverse outcome of hepatotoxicity. The results of an assessment confidence in the adverse outcome pathway (AOP) framework supported the evidence concordance in dose-response, temporal and incidence relationships between KEs in AO-induced hepatotoxicity. This study's findings offer a novel toxicity pathway network for AO-caused hepatotoxicity.

Altered life history traits and transcripts of molting- and reproduction-related genes by cadmium in Daphnia magna

Cadmium (Cd) is a non-essential element and can be toxic to aquatic organisms at low concentrations. Despite its well-known toxicity to Daphnia magna, the effects of Cd on physiological parameters (heart rate and thoracic limb activity) and molting- and reproduction-related genes are relatively understudied. In this study, D. magna were exposed to 0 (control), 25, 50 and 75 μg L^-1 of Cd for 7 d and 21 d to determine the toxicity of Cd. The results showed that the Cd body burden in D. magna was significantly increased with elevated Cd concentrations, up to 13.4 μg Cd/g dry weight (dw) after exposure to 75 μg L^-1 for 21 d. After 21 d of exposure, the body length and body weight of D. magna were significantly decreased in all Cd treatments compared to the control. The heart rate and thoracic limb activity were reduced by 4.3-11.7 and 5.0-10.3%, respectively. The levels of malondialdehyde (MDA) were increased by ~24-37% and the activity of catalase (CAT) was inhibited by ~50% compared to the control. The reproductive parameters (i.e., size of the first brood, the total number of offspring per female and the number of offspring per brood) were remarkably reduced, causing adverse effects on the population dynamics. In addition, the transcripts of genes (cyp314, cyp18a1, ecra, usp, hr3, cut, cht and cht3) related to the molting of D. magna were altered, whereas the transcripts of genes (vtg1, vtg2 and vmo1) related to reproduction were down-regulated. This study helps better understand the effects of Cd at different biological levels.

Application of the adverse outcome pathway concept for investigating developmental neurotoxicity potential of Chinese herbal medicines by using human neural progenitor cells in vitro

Adverse outcome pathways (AOPs) are organized sequences of key events (KEs) that are triggered by a xenobiotic-induced molecular initiating event (MIE) and summit in an adverse outcome (AO) relevant to human or ecological health. The AOP framework causally connects toxicological mechanistic information with apical endpoints for application in regulatory sciences. AOPs are very useful to link endophenotypic, cellular endpoints in vitro to adverse health effects in vivo. In the field of in vitro developmental neurotoxicity (DNT), such cellular endpoints can be assessed using the human "Neurosphere Assay," which depicts different endophenotypes for a broad variety of neurodevelopmental KEs. Combining this model with large-scale transcriptomics, we evaluated DNT hazards of two selected Chinese herbal medicines (CHMs) Lei Gong Teng (LGT) and Tian Ma (TM), and provided further insight into their modes-of-action (MoA). LGT disrupted hNPC migration eliciting an exceptional migration endophenotype. Time-lapse microscopy and intervention studies indicated that LGT disturbs laminin-dependent cell adhesion. TM impaired oligodendrocyte differentiation in human but not rat NPCs and activated a gene expression network related to oxidative stress. The LGT results supported a previously published AOP on radial glia cell adhesion due to interference with integrin-laminin binding, while the results of TM exposure were incorporated into a novel putative, stressor-based AOP. This study demonstrates that the combination of phenotypic and transcriptomic analyses is a powerful tool to elucidate compounds' MoA and incorporate the results into novel or existing AOPs for a better perception of the DNT hazard in a regulatory context.

Reactive Oxygen Species in the Adverse Outcome Pathway Framework: Toward Creation of Harmonized Consensus Key Events

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are formed as a result of natural cellular processes, intracellular signaling, or as adverse responses associated with diseases or exposure to oxidizing chemical and non-chemical stressors. The action of ROS and RNS, collectively referred to as reactive oxygen and nitrogen species (RONS), has recently become highly relevant in a number of adverse outcome pathways (AOPs) that capture, organize, evaluate and portray causal relationships pertinent to adversity or disease progression. RONS can potentially act as a key event (KE) in the cascade of responses leading to an adverse outcome (AO) within such AOPs, but are also known to modulate responses of events along the AOP continuum without being an AOP event itself. A substantial discussion has therefore been undertaken in a series of workshops named "Mystery or ROS" to elucidate the role of RONS in disease and adverse effects associated with exposure to stressors such as nanoparticles, chemical, and ionizing and non-ionizing radiation. This review introduces the background for RONS production, reflects on the direct and indirect effects of RONS, addresses the diversity of terminology used in different fields of research, and provides guidance for developing a harmonized approach for defining a common event terminology within the AOP developer community.

Emerging investigator series: metal nanoparticles in freshwater: transformation, bioavailability and effects on invertebrates

MNPs may undergo different environmental transformations in aquatic systems, consequently changing their mobility, bioavailability and toxicity to freshwater invertebrates.

Adverse Outcome Pathway: A Path towards better Data Consolidation and Global Co-ordination of Radiation Research

Background The purpose of toxicology is to protect human health and the environment. To support this, the Organisation for Economic Co-operation and Development (OECD), operating via its Extended Advisory Group for Molecular Screening and Toxicogenomics (EAGMST), has been developing the Adverse Outcome Pathway (AOP) approach to consolidate evidence for chemical toxicity spanning multiple levels of biological organization. The knowledge transcribed into AOPs provides a structured framework to transparently organize data, examine the weight of evidence of the AOP, and identify causal relationships between exposure to stressors and adverse effects of regulatory perspective. The AOP framework has undergone substantial maturation in the field of hazard characterization of chemicals over the last decade, and has also recently gained attention from the radiation community as a means to advance the mechanistic understanding of human and ecological health effects from exposure to ionizing radiation at low dose and low dose-rates. To fully exploit the value of such approaches for facilitating risk assessment and management in the field of radiation protection, solicitation of experiences and active cooperation between chemical and radiation communities are needed. As a result, the Radiation and Chemical (Rad/Chem) AOP joint topical group was formed on June 1, 2021 as part of the initiative from the High Level Group on Low Dose Research (HLG-LDR). HLG-LDR is overseen by the OECD Nuclear Energy Agency (NEA) Committee on Radiation Protection and Public Health (CRPPH). The main aims of the joint AOP topical group are to advance the use of AOPs in radiation research and foster broader implementation of AOPs into hazard and risk assessment. With global representation, it serves as a forum to discuss, identify and develop joint initiatives that support research and take on regulatory challenges. Conclusion: The Rad/Chem AOP joint topical group will specifically engage, promote, and implement the use of the AOP framework to: a) organize and evaluate mechanistic knowledge relevant to the protection of human and ecosystem health from radiation; b) identify data gaps and research needs pertinent to expanding knowledge of low dose and low dose-rate radiation effects; and c) demonstrate utility to support risk assessment by developing radiation-relevant case studies. It is envisioned that the Rad/Chem AOP joint topical group will actively liaise with the OECD EAGMST AOP developmental program to collectively advance areas of common interest and, specifically, provide recommendations for harmonization of the AOP framework to accommodate non-chemical stressors, such as radiation.

Deciphering Differential Life Stage Radioinduced Reproductive Decline in Caenorhabditis elegans through Lipid Analysis

Wildlife is chronically exposed to various sources of ionizing radiations, both environmental or anthropic, due to nuclear energy use, which can induce several defects in organisms. In invertebrates, reproduction, which directly impacts population dynamics, has been found to be the most radiosensitive endpoint. Understanding the underlying molecular pathways inducing this reproduction decrease can help in predicting the effects at larger scales (i.e., population). In this study, we used a life stage dependent approach in order to better understand the molecular determinants of reproduction decrease in the roundworm C. elegans. Worms were chronically exposed to 50 mGy·h⁻¹ external gamma ionizing radiations throughout different developmental periods (namely embryogenesis, gametogenesis, and full development). Then, in addition to reproduction parameters, we performed a wide analysis of lipids (different class and fatty acid via FAMES), which are both important signaling molecules for reproduction and molecular targets of oxidative stress. Our results showed that reproductive defects are life stage dependent, that lipids are differently misregulated according to the considered exposure (e.g., upon embryogenesis and full development) and do not fully explain radiation induced reproductive defects. Finally, our results enable us to propose a conceptual model of lipid signaling after radiation stress in which both the soma and the germline participate.

Uranium accumulation and toxicokinetics in the crustacean Daphnia magna provide perspective to toxicodynamic responses

The importance of incorporating kinetic approaches in order to gain information on underlying physiological processes explaining species sensitivity to environmental stressors has been highlighted in recent years. Uranium is present in the aquatic environment worldwide due to naturally occurring and anthropogenic sources, posing a potential risk to freshwater taxa in contaminated areas. Although literature shows that organisms vary widely with respect to susceptibility to U, information on toxicokinetics that may explain the variation in toxicodynamic responses is scarce. In the present work, Daphnia magna were exposed to a range of environmentally relevant U concentrations (0 - 200 µg L^-1) followed by a 48 h depuration phase to obtain information on toxicokinetic parameters and toxic responses. Results showed time-dependent and concentration-dependent uptake of U in daphnia (k_u = 1.2 - 3.8 L g^-1 day^-1) with bioconcentration factors (BCFs) ranging from 1,641 - 5,204 (L kg^-1), a high depuration rate constant (k_e = 0.75 day^-1), the majority of U tightly bound to the exoskeleton (~ 50 - 60%) and maternal transfer of U (1 - 7%). Effects on growth, survivorship and major ion homeostasis strongly correlated with exposure (external or internal) and toxicokinetic parameters (uptake rates, k_u, BCF), indicating that uptake and internalization drives U toxicity responses in D. magna. Interference from U with ion uptake pathways and homeostasis was highlighted by the alteration in whole-body ion concentrations, their ionic ratios (e.g., Ca:Mg and Na:K) and the increased expression in some ion regulating genes. Together, this work adds to the limited data examining U kinetics in freshwater taxa and, in addition, provides perspective on factors influencing stress, toxicity and adaptive response to environmental contaminants such as uranium.

High-throughput analyses and Bayesian network modeling highlight novel epigenetic Adverse Outcome Pathway networks of DNA methyltransferase inhibitor mediated transgenerational effects

A number of epigenetic modulating chemicals are known to affect multiple generations of a population from a single ancestral exposure, thus posing transgenerational hazards. The present study aimed to establish a high-throughput (HT) analytical workflow for cost-efficient concentration-response analysis of epigenetic and phenotypic effects, and to support the development of novel Adverse Outcome Pathway (AOP) networks for DNA methyltransferase (DNMT) inhibitor-mediated transgenerational effects on aquatic organisms. The model DNMT inhibitor 5-azacytidine (5AC) and the model freshwater crustacean Daphnia magna were used to generate new experimental data and served as prototypes to construct AOPs for aquatic organisms. Targeted HT bioassays (DNMT ELISA, MS-HRM and qPCR) in combination with multigenerational ecotoxicity tests revealed concentration-dependent transgenerational (F0-F3) effects of 5AC on total DNMT activity, DNA promoter methylation, gene body methylation, gene transcription and reproduction. Top sensitive toxicity pathways related to 5AC exposure, such as apoptosis and DNA damage responses were identified in both F0 and F3 using Gaussian Bayesian network modeling. Two novel epigenetic AOP networks on DNMT inhibitor mediated one-generational and transgenerational effects were developed for aquatic organisms and assessed for the weight of evidence. The new HT analytical workflow and AOPs can facilitate future ecological hazard assessment of epigenetic modulating chemicals.

Epigenetic, transcriptional and phenotypic responses in Daphnia magna exposed to low-level ionizing radiation

Ionizing radiation is known to induce oxidative stress and DNA damage as well as epigenetic effects in aquatic organisms. Epigenetic changes can be part of the adaptive responses to protect organisms from radiation-induced damage, or act as drivers of toxicity pathways leading to adverse effects. To investigate the potential roles of epigenetic mechanisms in low-dose ionizing radiation-induced stress responses, an ecologically relevant crustacean, adult Daphnia magna were chronically exposed to low and medium level external ⁶⁰Co gamma radiation ranging from 0.4, 1, 4, 10, and 40 mGy/h for seven days. Biological effects at the molecular (global DNA methylation, histone modification, gene expression), cellular (reactive oxygen species formation), tissue/organ (ovary, gut and epidermal histology) and organismal (fecundity) levels were investigated using a suite of effect assessment tools. The results showed an increase in global DNA methylation associated with loci-specific alterations of histone H3K9 methylation and acetylation, and downregulation of genes involved in DNA methylation, one-carbon metabolism, antioxidant defense, DNA repair, apoptosis, calcium signaling and endocrine regulation of development and reproduction. Temporal changes of reactive oxygen species (ROS) formation were also observed with an apparent transition from ROS suppression to induction from 2 to 7 days after gamma exposure. The cumulative fecundity, however, was not significantly changed by the gamma exposure. On the basis of the new experimental evidence and existing knowledge, a hypothetical model was proposed to provide in-depth mechanistic understanding of the roles of epigenetic mechanisms in low dose ionizing radiation induced stress responses in D. magna.

De Novo Development of a Quantitative Adverse Outcome Pathway (qAOP) Network for Ultraviolet B (UVB) Radiation Using Targeted Laboratory Tests and Automated Data Mining

Ultraviolet B (UVB) radiation is a natural nonchemical stressor posing potential hazards to organisms such as planktonic crustaceans. The present study was conducted to revisit the lethal effects of UVB on crustaceans, generate new experimental evidence to fill in knowledge gaps, and develop novel quantitative adverse outcome pathways (qAOPs) for UVB. A combination of laboratory and computational approaches was deployed to achieve the goals. For targeted laboratory tests, Daphnia magna was used as a prototype and exposed to a gradient of artificial UVB. Targeted bioassays were used to quantify the effects of UVB at multiple levels of biological organization. A toxicity pathway network was assembled based on the new experimental evidence and previously published data extracted using a novel computational tool, the NIVA Risk Assessment Database (NIVA RAdb). A network of AOPs was developed, and weight of evidence was assessed based on a combination of the current and existing data. In addition, quantitative key event relationships in the AOPs were developed by fitting the D. magna data to predefined models. A complete workflow for assembly and evaluation of qAOPs has been presented, which may serve as a good example for future de novo qAOP development for chemical and nonchemical stressors.