Casting a wide net: use of diverse model organisms to advance toxicology

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Hahn, M. E., & Sadler, K. C. Casting a wide net: use of diverse model organisms to advance toxicology. Disease Models & Mechanisms, 13, (2020): dmm.043844, doi: 10.1242/dmm.043844.Toxicology – the study of how chemicals interact with biological systems – has clear relevance to human health and disease. Persistent exposure to natural and synthetic chemicals is an unavoidable part of living on our planet; yet, we understand very little about the effects of exposure to the vast majority of chemicals. While epidemiological studies can provide strong statistical inference linking chemical exposure to disease, research in model systems is essential to elucidate the mechanisms of action and to predict outcomes. Most research in toxicology utilizes a handful of mammalian models that represent a few distinct branches of the evolutionary tree. This narrow focus constrains the understanding of chemical-induced disease processes and systems that have evolved in response to exposures. We advocate for casting a wider net in environmental toxicology research to utilize diverse model systems, including zebrafish, and perform more mechanistic studies of cellular responses to chemical exposures to shift the perception of toxicology as an applied science to that of a basic science. This more-inclusive perspective will enrich the field and should remain central to research on chemical-induced disease.K.C.S. acknowledges support from the National Institutes of Health (NIH)(5R01AA018886). M.E.H. acknowledges support from the National Institute ofEnvironmental Health Sciences (NIEHS) through the Boston University SuperfundResearch Program (P42ES007381) and the Woods Hole Center for Oceans andHuman Health (NIEHS grant P01ES028938 and National Science Foundation grantOCE-1840381)

Evolutionary concepts can benefit both fundamental research and applied research in toxicology (A comment on brady et al. 2017).

Author Posting. © John Wiley & Sons, 2019. This article is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Hahn, Mark E. Evolutionary concepts can benefit both fundamental research and applied research in toxicology (A comment on brady et al. 2017). Evolutionary Applications, 12(2), (2019):350-352, doi:10.1111/eva.12695.National Institute of Environmental Health Sciences (NIEHS) through the Boston University Superfund Research Program (P42ES007381) and the Woods Hole Center for Oceans and Human Health (NIH grant P01ES021923 and NSF Grant OCE-1314642)

Mechanistic research in aquatic toxicology : perspectives and future directions

Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Aquatic Toxicology 105 (2011): 67-71, doi:10.1016/j.aquatox.2011.06.001.On the thirtieth anniversary of the journal, I provide a perspective on some of the questions and opportunities for new understanding that will interest aquatic toxicologists during the next thirty years. I focus on mechanisms of toxicity involving transcription factors, signalling pathways, and gene networks involved in toxic and adaptive responses in aquatic animals. Prominent questions address the value of a toxicity pathways approach in aquatic systems, issues involving extrapolation among species, identification of susceptibility genes and useful biomarkers of adverse effect, new emerging contaminants, the importance of epigenetic mechanisms, effects of multiple stressors, evolutionary toxicology, and the relative roles of technical and conceptual limitations to our understanding of chemical effects on aquatic systems.I gratefully acknowledge the U.S. National Institutes of Health for support in preparation of this chapter and long-term funding that has allowed my group to conduct research on the comparative biology of AHRs, mechanisms of evolved resistance to PCBs, and (more recently) mechanisms of response to oxidative stress and the role of microRNAs in developmental toxicology [grants R01ES006272, P42ES007381 (Superfund Basic Research Program at Boston University), R01ES016366, and R21ES017304]. I also acknowledge valuable support from the WHOI Sea Grant program with funding from the National Oceanic and Atmospheric Administration for support of research on the role of AHRs in susceptibility of birds and marine mammals to dioxin-like compounds

Delayed effects of developmental exposure to low levels of the aryl hydrocarbon receptor agonist 3,3′,4,4′,5-pentachlorobiphenyl (PCB126) on adult zebrafish behavior

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in NeuroToxicology 52 (2016): 134-143, doi:10.1016/j.neuro.2015.11.012.Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants. The most toxic PCBs are the non-ortho-substituted ("dioxin-like") congeners that act through the aryl hydrocarbon receptor (AHR) pathway. In humans, perinatal exposure to dioxin-like PCBs is associated with neurodevelopmental toxicity in children. Yet, the full potential for later-life neurobehavioral effects that result from early-life low level exposure to dioxin-like PCBs is not well understood. The objective of this study was to determine the effects of developmental exposure to low levels of dioxin-like PCBs on early- and later-life behavioral phenotypes using zebrafish as a model system. We exposed zebrafish embryos to either vehicle (DMSO) or low concentrations of PCB126 (0.3, 0.6, 1.2 nM) for 20 hours (4-24 hours post fertilization), and then reared them to adulthood in clean water. Locomotor activity was tested at two larval stages (7 and 14 days post fertilization). Adult fish were tested for anxiety-related behavior using the novel tank and shoaling assays. Adult behavioral assays were repeated several times on the same group of fish and effects on intra- and inter-trial habituation were determined. While there was no effect of PCB126 on larval locomotor activity in response to changes in light conditions, developmental exposure to PCB126 resulted in impaired short- and long-term habituation to a novel environment in adult zebrafish. Cyp1a induction was measured as an indicator for AHR activation. Despite high induction at early stages, cyp1a expression was not induced in the brains of developmentally exposed adult fish that showed altered behavior, suggesting that AHR was not activated at this stage. Our results demonstrate the effectiveness of the zebrafish model in detecting subtle and delayed behavioral effects resulting from developmental exposure to an environmental contaminant.This work was supported by the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution (with funding provided by the Townsend Postdoctoral Scholarship Fund, and the John H. Steele Endowment in support of Postdoctoral Research) and by the Woods Hole Center for Oceans and Human Health (NIH grant P01ES021923 and National Science Foundation Grant OCE-1314642 to MEH and NA.)

The Ah receptor: adaptive metabolism, ligand diversity, and the xenokine model

Author Posting. © American Chemical Society, 2020. This is an open access article published under an ACS AuthorChoice License. The definitive version was published in Chemical Research in Toxicology, 33(4), (2020): 860-879, doi:10.1021/acs.chemrestox.9b00476.The Ah receptor (AHR) has been studied for almost five decades. Yet, we still have many important questions about its role in normal physiology and development. Moreover, we still do not fully understand how this protein mediates the adverse effects of a variety of environmental pollutants, such as the polycyclic aromatic hydrocarbons (PAHs), the chlorinated dibenzo-p-dioxins (“dioxins”), and many polyhalogenated biphenyls. To provide a platform for future research, we provide the historical underpinnings of our current state of knowledge about AHR signal transduction, identify a few areas of needed research, and then develop concepts such as adaptive metabolism, ligand structural diversity, and the importance of proligands in receptor activation. We finish with a discussion of the cognate physiological role of the AHR, our perspective on why this receptor is so highly conserved, and how we might think about its cognate ligands in the future.This review is dedicated in memory of the career of Alan Poland, one of the truly great minds in pharmacology and toxicology. This work was supported by the National Institutes of Health Grants R35-ES028377, T32-ES007015, P30-CA014520, P42-ES007381, and U01-ES1026127, The UW SciMed GRS Program, and The Morgridge Foundation. The authors would like to thank Catherine Stanley of UW Media Solutions for her artwork

Developmental neurotoxicity of the harmful algal bloom toxin domoic acid: Cellular and molecular mechanisms underlying altered behavior in the zebrafish model

This paper is not subject to U.S. copyright. The definitive version was published in Environmental Health Perspectives 128(11), (2020): 117002, doi:10.1289/EHP6652.Background: Harmful algal blooms (HABs) produce potent neurotoxins that threaten human health, but current regulations may not be protective of sensitive populations. Early life exposure to low levels of the HAB toxin domoic acid (DomA) produces long-lasting behavioral deficits in rodent and primate models; however, the mechanisms involved are unknown. The zebrafish is a powerful in vivo vertebrate model system for exploring cellular processes during development and thus may help to elucidate mechanisms of DomA developmental neurotoxicity. Objectives: We used the zebrafish model to investigate how low doses of DomA affect the developing nervous system, including windows of susceptibility to DomA exposure, structural and molecular changes in the nervous system, and the link to behavioral alterations. Methods: To identify potential windows of susceptibility, DomA (0.09–0.18 ng) was delivered to zebrafish through caudal vein microinjection during distinct periods in early neurodevelopment. Following exposure, structural and molecular targets were identified using live imaging of transgenic fish and RNA sequencing. To assess the functional consequences of exposures, we quantified startle behavior in response to acoustic/vibrational stimuli. Results: Larvae exposed to DomA at 2 d postfertilization (dpf), but not at 1 or 4 dpf, showed consistent deficits in startle behavior at 7 dpf, including lower responsiveness and altered kinematics. Similarly, myelination in the spinal cord was disorganized after exposure at 2 dpf but not 1 or 4 dpf. Time-lapse imaging revealed disruption of the initial stages of myelination. DomA exposure at 2 dpf down-regulated genes required for maintaining myelin structure and the axonal cytoskeleton. Discussion: These results in zebrafish reveal a developmental window of susceptibility to DomA-induced behavioral deficits and identify altered gene expression and disrupted myelin structure as possible mechanisms. The results establish a zebrafish model for investigating the mechanisms of developmental DomA toxicity, including effects with potential relevance to exposed sensitive human populations. https://doi.org/10.1289/EHP6652This research was supported by the Oceans Venture Fund, the Von Damm Fellowship, the Ocean Ridge Initiative Fellowship, and Woods Hole Sea grant (NA14OAR4170074) (all to J.M.P.), and by the Woods Hole Center for Oceans and Human Health (NIH: P01ES021923 and P01ES028938; NSF: OCE-1314642 and OCE-1840381; Center PI: John Stegeman, Project PI: M.E.H.)

Role of DNA methylation of AHR1 and AHR2 promoters in differential sensitivity to PCBs in Atlantic Killifish, Fundulus heteroclitus

Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Aquatic Toxicology 101 (2011): 288-294, doi:10.1016/j.aquatox.2010.10.010.Atlantic killifish (Fundulus heteroclitus) inhabiting the PCB-contaminated Superfund site in New Bedford Harbor (MA, USA) have evolved genetic resistance to the toxic effects of these compounds. They also lack induction of cytochrome P4501A (CYP1A) and other aryl hydrocarbon receptor (AHR)-dependent responses after exposure to AHR agonists, suggesting an overall down-regulation of the AHR signaling pathway. In this study, we hypothesized that the genetic resistance is due to altered AHR expression resulting from hypermethylation of DNA in the promoter region of AHR genes in fish inhabiting New Bedford Harbor. To test this hypothesis, we cloned and sequenced AHR1 and AHR2 promoter regions and employed bisulfite conversion-polymerase chain reaction (BS-PCR) followed by clonal analysis to compare the methylation status of CpG islands of AHR1 and AHR2 in livers of adult killifish collected from New Bedford Harbor and a reference site (Scorton Creek, MA). No significant differences in methylation profiles were observed in either AHR1 or AHR2 promoter regions between NBH and SC fish. However, hypermethylation of the AHR1 promoter correlated with low expression of transcripts in the liver in both populations. In comparison to AHR1, hepatic mRNA expression of AHR2 is high and its promoter is hypomethylated. Taken together, our results suggest that genetic resistance to contaminants in NBH fish is not due to altered methylation of AHR promoter regions, but that promoter methylation may control tissue-specific expression of AHR genes in killifish.This work is funded in part by the Superfund Basic Research Program at Boston University to MEH (NIH Grant P42ES007381) and the postdoctoral scholar program at WHOI, with funding provided by the Dr. George D. Grice Postdoctoral Scholarship Fund to NA

When evolution is the solution to pollution : key principles, and lessons from rapid repeated adaptation of killifish (Fundulus heteroclitus) populations

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Evolutionary Applications 10 (2017): 762–783, doi:10.1111/eva.12470.For most species, evolutionary adaptation is not expected to be sufficiently rapid to buffer the effects of human-mediated environmental changes, including environmental pollution. Here we review how key features of populations, the characteristics of environmental pollution, and the genetic architecture underlying adaptive traits, may interact to shape the likelihood of evolutionary rescue from pollution. Large populations of Atlantic killifish (Fundulus heteroclitus) persist in some of the most contaminated estuaries of the United States, and killifish studies have provided some of the first insights into the types of genomic changes that enable rapid evolutionary rescue from complexly degraded environments. We describe how selection by industrial pollutants and other stressors has acted on multiple populations of killifish and posit that extreme nucleotide diversity uniquely positions this species for successful evolutionary adaptation. Mechanistic studies have identified some of the genetic underpinnings of adaptation to a well-studied class of toxic pollutants; however, multiple genetic regions under selection in wild populations seem to reflect more complex responses to diverse native stressors and/or compensatory responses to primary adaptation. The discovery of these pollution-adapted killifish populations suggests that the evolutionary influence of anthropogenic stressors as selective agents occurs widely. Yet adaptation to chemical pollution in terrestrial and aquatic vertebrate wildlife may rarely be a successful “solution to pollution” because potentially adaptive phenotypes may be complex and incur fitness costs, and therefore be unlikely to evolve quickly enough, especially in species with small population sizes.National Science Foundation Grant Numbers: DEB-1265282, OCE-1314567, DEB-1120263; National Institutes of Environmental Health Sciences Grant Numbers: R01ES021934-01, P42ES007381; Postdoctoral Research Program at the US Environmental Protection (US EPA); Office of Research and Development; Oak Ridge Institute for Science and Education (ORISE) Grant Number: DW92429801; US Department of Energ

Aryl hydrocarbon receptor-mediated activity of gas-phase ambient air derived from passive sampling and an \u3cem\u3ein vitro\u3c/em\u3e bioassay

The gaseous fraction of hydrophobic organic contaminants (HOCs) in ambient air appears to be responsible for a significant portion of aryl hydrocarbon receptor (AhR)‐mediated activity, but the majority of compounds contributing to this activity remain unidentified. This study investigated the use of polyethylene passive samplers (PEs) to isolate gaseous HOCs from ambient air for use in in vitro bioassays and to improve our understanding of the toxicological relevance of the gaseous fraction of ambient air in urban and residential environments. Concentrations of polycyclic aromatic hydrocarbons (PAHs) and organic flame retardants (OFRs) were measured in PE extracts. Extracts were also analyzed using an in vitro bioassay to measure AhR‐mediated activity. Bioassay‐derived benzo[a]pyrene (BaP) equivalents (BaP‐Eqbio), a measure of potency of HOC mixtures, were greatest in the downtown Cleveland area and lowest at rural/residential sites further from the city center. BaP‐Eqbio was weakly correlated with concentrations of 2‐ring alkyl/substituted PAHs and one organophosphate flame retardant, ethylhexyl diphenyl phosphate (EHDPP). Potency predicted based on literature‐derived induction equivalency factors (IEFs) explained only 2‐23% of the AhR‐mediated potency observed in bioassay experiments. This study suggests that health risks of gaseous ambient air pollution predicted using data from targeted chemical analysis may underestimate risks of exposure, most likely due to augmentation of potency by unmonitored chemicals in the mixture, and the lack of relevant IEFs for many targeted analytes

Flood Inundation Mapping in the Logone Floodplain from Multi Temporal Landsat ETM+Imagery

Yearly flooding in the Logone floodplain makes an impact on agricultural, pastoral, and fishery systems in the Lake Chad Basin. Since the flooding extent and depth are highly variable, flood inundation mapping helps us make better use of water resources and prevent flood hazards in the Logone floodplain. The flood maps are generated from 33 multi temporal Landsat Enhanced Thematic Mapper Plus (ETM+) during three years 2006 to 2008. Flooded area is classified using a short-wave infrared band whereas open water is classified by Iterative Self-organizing Data Analysis (ISODATA) clustering. The maximum flooding extent in the study area increases up to approximately 5.8K km2 in late October 2008. The study also provides strong correlation of the flooding extents with water height variations in both the floodplain and the river based on a second polynomial regression model. The water heights are from ENIVSAT altimetry in the floodplain and gauge measurements in the river. Coefficients of determination between flooding extents and water height variations are greater than 0.91 with 4 to 36 days in phase lag. Floodwater drains back to the river and to the northeast during the recession period in December and January. The study supports understanding of the Logone floodplain dynamics in detail of spatial pattern and size of the flooding extent and assists the flood monitoring and prediction systems in the catchment