The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Frogs and Its Role in Frog Development and Reproduction

Hepatic enzyme induction and its potential effect on thyroid hormone metabolism in the metamorphosing tadpole of Xenopus laevis (African clawed frog)

Hepatic enzyme induction, an inherent defense system against xenobiotics, is known to simultaneously affect endocrine system functions in mammals under specific conditions, particularly thyroid hormone (TH) regulation. While this phenomenon has been studied extensively, the pathway leading to this indirect thyroid effect in mammals has unclear applicability to amphibians, despite the importance of amphibian species in assessing thyroid-disruptive chemicals. Here, we investigated the effects of three well-known mammalian enzyme inducers-β-naphthoflavone (BNF), pregnenolone carbonitrile (PCN), and sodium phenobarbital (NaPB)-on the gene expression of phase-I and phase-II metabolizing enzymes in Xenopus laevis tadpoles. Waterborne exposure to BNF and PCN significantly induced the expression of both phase-I (cytochrome P450, CYP) and phase-II enzymes (UDP-glucuronosyltransferase, UGT and sulfotransferase, SULT), but in different patterns, while NaPB exposure induced CYP2B expression without affecting phase-II enzymes in tadpoles, in contrast to mammals. Furthermore, an ex vivo hepatic enzyme activity assay confirmed that BNF treatment significantly increased phase-II metabolic activity (glucuronidation and sulfation) toward TH. These results suggest the potential for certain mammalian enzyme inducers to influence TH clearance in X. laevis tadpoles. Our findings provide insights into the profiles of xenosensing activity and enzyme induction in amphibians, which can facilitate a better understanding of the mechanisms of indirect effects on the thyroid system via hepatic enzyme induction in nonmammalian species.

Response characteristic and potential molecular mechanism of tail resorption in Bufo gargarizans after exposure to lead and copper, alone or combined

Tail resorption during amphibian metamorphosis is one of the most dramatic processes that is obligatorily dependent on thyroid hormone (TH). Heavy metals could result in thyroid gland damages and disturb TH homeostasis. Lead (Pb) and copper (Cu) often co-exist in natural aquatic ecosystems. However, there is still little information on how tail resorption responds to alone or combined exposure to Pb and Cu. Our study investigated the effects of Pb and Cu alone or combined exposure on the morphological parameters of the tail, histological changes of thyroid gland and tail, and gene expression programs involved in cell death of the tail in Bufo gargarizans tadpoles at the climax of metamorphosis. Results demonstrated that Pb, Cu and Pb-Cu mixture exposure resulted in a significantly longer tail compared with control. Damages to notochord, muscle, skin and spinal cord of the tail were found in Pb and Cu exposure groups. The colloid area, the height of follicular cells and number of phagocytic vesicles of thyroid gland in Pb-Cu mixture exposure groups were significantly reduced. In addition, the expression levels of TH, apoptosis, autophagy, degradation of cellular components and oxidative stress-related genes in the tail were significantly altered following Pb and Cu exposure. The present work revealed the relationship between environmental pollutants and tail resorption, providing scientific basis for amphibian protection.

Limited contribution of photoenzymatic DNA repair in mitigating carry-over effects from larval UVB exposure: Implications for frog recruitment

Solar ultraviolet-B (UVB) radiation has increased due to stratospheric ozone depletion, climate and ecosystem changes and is a driver of amphibian population declines. Photoenzymatic repair (PER) is a critical mechanism for limiting UVB lethality in amphibian larvae. However, the link between PER and the UVB-induced effects remains understudied through long-term investigations in vivo. Here, we assessed how larval PER determines the lethal and sublethal effects induced by environmentally relevant acute UVB exposure until the juvenile phase in the Neotropical frog Odontophrynus americanus. We conducted laboratory-based controlled experiments in which tadpoles were or were not exposed to UVB and subsequently were exposed to light (for PER activation) or dark treatments. Results showed that the rates of mortality and apoptosis observed in post-UVB dark treatment are effectively limited in post-UVB light treatment, indicating PER (and not dark repair, i.e. nucleotide excision repair) is critical to limit the immediate genotoxic impact of UVB-induced pyrimidine dimers. Nonetheless, even tadpoles that survived UVB exposure using PER showed sublethal complications that extended to the juvenile phase. Tadpole responses included alterations in morphology, chromosomal instability, increased skin susceptibility to fungal proliferation, as well as increased generation of reactive oxygen species. The short-term effects were carried over to later stages of life because metamorphosis time increased and juveniles were smaller. No body abnormalities were visualized in tadpoles, metamorphs, and juveniles, suggesting that O. americanus is UVB-resistant concerning these responses. This study reveals that even frog species equipped with an effective PER are not immune to carry-over effects from early UVB exposure, which are of great ecological relevance as late metamorphosis and smaller juveniles may impact individual performance and adult recruitment to breeding. Future ecological risk assessments and conservation and management efforts for amphibian species should exercise caution when linking PER effectiveness to UVB resistance.

Inefficacy of dietary test substance administration in Amphibian Metamorphosis Assay (AMA) studies

Traditionally, the Amphibian Metamorphosis Assay (AMA; OECD TG 231) is performed by exposing Xenopus laevis tadpoles to test substances dissolved in laboratory water. Recently, the use of dietary administration has been proposed to combat poorly soluble test substances in ecotoxicologically-based regulatory endocrine disruption (ED) studies, specifically the AMA warranting an investigation into the efficacy of dietary administration. An efficacy study comprised of two phases: 1) evaluation of the physical influence of the loading process via solvent and 10, 1, and 0.1 mg/l test substance or surrogate (sunflower oil, SFO) on the Sera® Micron Nature (SMN) diet, and 2) performance of a modified AMA in which Nieuwkoop and Faber (NF) stage 51 X. laevis larvae were exposed to dechlorinated tap water using one concentration of the SFO in the diet for 21 days, was performed. In phase 1, the addition of acetone or acetone with bis(2-propylheptyl) phthalate (DPHP) or SFO to SMN with subsequent solvent purge altered the diet reducing the density of the liquified diet and dietary pellet size following centrifugation indicative of alteration of the physical properties of the diet. Treatments used in the modified AMA were acetone alone and 0.1 mg/l SFO dissolved in acetone. These treatments were evaluated against an SMN benchmark using standard AMA endpoints. Both the acetone-treated SMN and 0.1 mg/l SFO-treated diets significantly reduced survival rates, 67 and 70% relative to the SMN benchmark (100%), decreased developmental stage distribution and snout-vent length-normalized hind limb length relative to the SMN benchmark, and slightly increased the prevalence and severity of thyroid follicular cell hypertrophy. Although the acetone-treated diets may have impacted the hypothalamo-pituitary-thyroid axis, clinical signs of gastrointestinal impaction and tail flexure were also observed in the acetone-treated diets, but not the SMN diet alone. Ultimately, test substance exposure via the diet in an AMA study can produce results that may confound data interpretation, which suggests that the traditional aqueous exposure route is generally more appropriate.

Assessment of toxicity, genotoxicity and oxidative stress in Fejervarya limnocharis exposed to tributyltin

Tributyltin (TBT) is widely used in various commercial applications due to its biocidal properties. Toxicological and genotoxicological data on TBT exposure to amphibians is insufficient. Our study aimed to determine the acute toxicity and genotoxic potential of TBT in Fejervarya limnocharis tadpoles. Furthermore, oxidative stress was also investigated in TBT-treated tadpoles. Tadpoles of Gosner stage (26-30) were screened and subjected to increasing concentrations of TBT (0, 3, 7, 11, 15, 19, 23 µg/L) for determining the LC50 values for 24 h, 48 h, 72 h, and 96 h. LC50 values of TBT for 24 h, 48 h, 72 h, and 96 h were found to be 19.45, 15.07, 13.12, and 11.84 μg/L respectively. Based on the 96 h LC50 value (11.84 µg/L), tadpoles were exposed to different sub-lethal concentrations of TBT for the evaluation of its genotoxic potential and effects on oxidative balance. The role of TBT on survivability, growth, and time to metamorphosis was also assessed. TBT exposure significantly altered the life history traits measured, increased mortality, and delayed the time taken to metamorphosis. Results indicated significant induction of micronucleus (MN, p < 0.001) and other erythrocytic nuclear aberrations (ENA, p < 0.01) in the TBT-treated groups. Significant alterations in comet parameters and oxidative balance were also observed in the treated groups. The present study findings might add to the cause of the gradual population decline seen in the amphibians. This study also demonstrates the alteration of the life-history traits, oxidative balance, and DNA damage upon TBT exposure which can have long-term consequences for the anuran amphibian F. limnocharis.

Effects of mifepristone, a model compound with anti-progestogenic activity, on the development of African clawed frog (Xenopus laevis)

The objective of this study was to assess the effects of a model substance with anti-progestogenic activity on development of African clawed frog (Xenopus laevis) from tadpole to juvenile stage. Mifepristone, a synthetic progesterone receptor-blocking steroid hormone used in medicine as an abortifacient, was chosen as a model compound with anti-progestogenic activity. In the experiment, African clawed frog tadpoles were exposed to mifepristone at three concentrations (2, 21, and 215 ng L-1). A control group was exposed to dimethyl sulfoxide (DMSO; 0.001 %). The experiment started when tadpoles reached stages 47-48 according to Nieuwkoop and Faber (NF; 1994) and continued until stage NF 66, when metamorphosis was complete. Exposure to mifepristone had no significant effect on the rate of tadpole development, occurrence of morphological anomalies, weight, body length, or sex ratio. Mortality was within an acceptable range of 0-3.6 % throughout the test and did not differ among the groups. Histopathological examination of the gonads and thyroid gland revealed no significant changes. Therefore, we can conclude that mifepristone had no negative effect on development of the African clawed frog up to juvenile stage. Nevertheless, at the highest tested mifepristone concentration (215 ng L-1), gene expression analysis revealed up-regulation of mRNA expression of nuclear progesterone receptor (npr), membrane progesterone receptor (mpr), estrogen receptor beta (esrβ), and luteinizing hormone (lh) in the brain-pituitary complex of exposed frogs at stage NF 66. Higher mRNA expression of npr was also found in frogs exposed to 22 ng L-1 mifepristone compared to the solvent control. These findings confirmed the anti-progestogenic activity of mifepristone in frogs because the up-regulation of progesterone receptors occurs if progesterone availability in the body is reduced. All the observed changes in combination may have negative consequences for reproduction and reproductive behavior later in life.

Exploring the repairing mechanisms of reduced graphene oxide (rGo) on the dysregulation of Xenopus Laevis larva hypothalamus-pituitary-thyroid (HPT) axis caused by chiral triazole fungicide metconazole

Replacing chair fungicide racemate marketed product by its enantiomer with high activity and low environmental risk for application is a more environmentally friendly methods to control crop diseases. Moreover, carbon-based nanomaterials, with the desirable chemical and mechanical properties, exhibits latent reduce fungicide toxicity capability, while the mechanism is still poorly understood. Therefore, the present study characterized the toxicity of rac-metconazole (Mez; (1RS,5RS;1RS,5SR)-5-(4-chlorobenzyl)-2,2-dimethyl-1-(1H)) and its two cis-enantiomers as well as the repairing effect of reduced graphene oxide (rGo) on Xenopus Laevis larva by examining growth appearance indexes, Mez bioaccumulation, and hypothalamus-pituitary-thyroid (HPT) axis related hormone contents and gene expression after 14 and 28 days exposure. Compared with two cis-Mez, rac-Mez was preferentially bioaccumulated in tadpoles, and rac-Mez treatment showed a higher toxicity effect on tadpole including growth stage and body weight inhibition by dysregulating tadpole thyroid stimulating hormone (TSH) and thyroid hormone (TH) contents and related gene expression. Enantioselectivity was observed in two cis-Mez treatments. Compared with R,S-Mez, S,R-Mez treatment showed more severe damage on tadpole HPT axis related physiological and biochemical processes. rGo could effectively decrease the toxicity of Mez, especially shown the capacity of repairing the hormone dysregulation caused by R,S-Mez treatment. Moreover, the addition of rGo can decrease the bioaccumulation of Mez in tadpoles. Therefore, R,S-Mez is less toxic to Xenopus Laevis larva growth, and its toxicity could be effectively repaired by the addition of rGO.

A biologically based computational model for the hypothalamic-pituitary-thyroid (HPT) axis in Xenopus laevis larvae

A biologically based computational model was developed to describe the hypothalamic-pituitary-thyroid (HPT) axis in developing Xenopus laevis larvae. The goal of this effort was to develop a tool that can be used to better understand mechanisms of thyroid hormone-mediated metamorphosis in X. laevis and predict organismal outcomes when those mechanisms are perturbed by chemical toxicants. In this report, we describe efforts to simulate the normal biology of control organisms. The structure of the model borrows from established models of HPT axis function in mammals. Additional features specific to X. laevis account for the effects of organism growth, growth of the thyroid gland, and developmental changes in regulation of thyroid stimulating hormone (TSH) by circulating thyroid hormones (THs). Calibration was achieved by simulating observed changes in stored and circulating levels of THs during a critical developmental window (Nieuwkoop and Faber stages 54-57) that encompasses widely used in vivo chemical testing protocols. The resulting model predicts that multiple homeostatic processes, operating in concert, can act to preserve circulating levels of THs despite profound impairments in TH synthesis. Represented in the model are several biochemical processes for which there are high-throughput in vitro chemical screening assays. By linking the HPT axis model to a toxicokinetic model of chemical uptake and distribution, it may be possible to use this in vitro effects information to predict chemical effects in X. laevis larvae resulting from defined chemical exposures.

Effects of the herbicide ametryn on development and thyroidogenesis of bullfrog tadpoles (Aquarana catesbeiana) under different temperatures

Thyroid hormones (TH) are essential for the metamorphosis of amphibians and their production can be influenced by environmental stressors, such as temperature fluctuations, and exposure to aquatic pollutants, such as herbicides. In the present study we evaluated the influence of different temperatures (25 and 32 °C) on the effects of the herbicide ametryn (AMT, 0 - control, 10, 50 and 200 ng.L^-1) for 16 days on thyroidogenesis of bullfrog tadpoles. Higher temperature and AMT exposure caused a delay in the development of tadpoles, despite no differences were noted in weight gain and total length of the animals. Levels of triiodothyronine (T3) and thyroxine (T4) were not altered neither by AMT nor by temperature, but the highest temperature caused a decrease in total area and number of follicles in the thyroid gland. Transcript levels of thyroid hormone receptors alpha and beta (TRα and TRβ) and iodothyronine deiodinase 3 (DIO3) were lower at 32 °C, which is consistent with developmental delay at the higher temperature. Tadpoles exposed to 200 ng.L^-1 of AMT at 25 °C also presented delayed development, which was consistent with lower TRα and DIO3 transcript levels. Lower levels of estradiol were noted in tadpoles exposed to AMT at the higher temperature, being also possibly related to a developmental delay. This study demonstrates that higher temperature and AMT exposure impair thyroidgenesis in bullfrog tadpoles, disrupting metamorphosis.

Influence of systemic copper toxicity on early development and metamorphosis in Xenopus laevis

The primary objective of the present study was to examine the influence of early systemic toxicity resulting from copper (Cu) exposure on metamorphic processes in Xenopus laevis. A 28-day exposure study with copper, initiated at developmental stage 10, was performed using test concentrations of 3.0, 9.0, 27.2, 82.5, and 250 μg Cu/L. The primary endpoints included mortality, developmental stage, embryo-larval malformation, behavioral effects, hindlimb length (HLL), growth (snout-vent length [SVL] and wet body weight), and histopathology. The 28-day LC50 value with 95% confidence intervals was 61.2 (51.4-72.9) μg Cu/L with 250 μg Cu/L resulting in complete lethality. Developmental arrest in the 82.5 and delay in the 27.2 μg Cu/L treatments was observed as early as study day 10 continuing throughout the remainder of exposure. SVL-normalized HLL, body weight, and SVL in the 27.2 and 82.5 μg Cu/L treatments were significantly decreased relative to control. At 82.5 μg Cu/L, and thyroid gland size was markedly reduced when compared with controls consistent with the stage of developmental and growth arrest. Concentration-dependent findings in the intestine, liver, gills, eyes, and pharyngeal mucosa were consistent with non-endocrine systemic toxicity. These were prevalent in the 9.0 and 27.2 μg Cu/L treatment groups but were minimally evident or absent in the 82.5 μg/L group, which was attributed to developmental arrest. In conclusion, developmental delay in larvae exposed to 27.2 and 82.5 μg Cu/L was the result of systemic toxicity occurring in early development prior hypothalomo-pituitary-thyroid axis (HPT)-driven metamorphosis and was not indicative of endocrine disruption.

Importance of diet in amphibian metamorphosis-based studies designed to assess the risk of thyroid active substances

The present study evaluated the hypothesis that dietary quality used in historical studies may impact the effects of chemical stressors on premetamorphic development and metamorphosis due to suboptimal nutritional quality. A modified Amphibian Metamorphosis Assay (AMA) was performed in which Nieuwkoop and Faber (NF) Stage 47 tadpoles of Xenopus laevis were exposed for 32 days to iodide (I^- )-deficient FETAX solution supplemented with <0.025, 0.17, 0.52, 1.58, and 4.80 μg I^- /L (measured concentrations 0.061, 0.220, 0.614, 1.65, and 4.73 μg I^- /L) and fed a pureed Frog Brittle (FB) diet. An AMA guideline benchmark group (four replicates) exposed to dechlorinated tap water and fed standard Sera Micron Nature® (SMN) diet was evaluated concurrently. Developmental delay, observed as changes in stage distribution or median developmental stage, occurred in FB treatments with 0.061, 0.220, and 0.614 μg/L I^- , respectively. Developmental rates and hind limb length of the 1.65 and 4.73 μg/L I^- groups were similar to each other, but both treatments fell short of the developmental rate achieved by the SMN benchmark. Iodide supplementation also had no impact on nonthyroidal growth endpoints, which were markedly reduced in FB-fed frogs compared with their SMN-fed counterparts. All larvae that received the FB diet had mildly to severely hypoplastic/atrophic thyroids, a condition for which iodine supplementation had little if any ameliorative effect. Collectively, these results suggested that nutritional deficiencies in the FB diet negatively affected both growth and metamorphic development, the latter of which was only compensated to a limited extent by iodine supplementation.

Adverse Outcome Pathways for Chronic Copper Toxicity to Fish and Amphibians

In the present review, we synthesize information on the mechanisms of chronic copper (Cu) toxicity using an adverse outcome pathway framework and identify three primary pathways for chronic Cu toxicity: disruption of sodium homeostasis, effects on bioenergetics, and oxidative stress. Unlike acute Cu toxicity, disruption of sodium homeostasis is not a driving mechanism of chronic toxicity, but compensatory responses in this pathway contribute to effects on organism bioenergetics. Effects on bioenergetics clearly contribute to chronic Cu toxicity with impacts at multiple lower levels of biological organization. However, quantitatively translating these impacts into effects on apical endpoints such as growth, amphibian metamorphosis, and reproduction remains elusive and requires further study. Copper-induced oxidative stress occurs in most tissues of aquatic vertebrates and is clearly a significant driver of chronic Cu toxicity. Although antioxidant responses and capacities differ among tissues, there is no clear indication that specific tissues are more sensitive than others to oxidative stress. Oxidative stress leads to increased apoptosis and cellular damage in multiple tissues, including some that contribute to bioenergetic effects. This also includes oxidative damage to tissues involved in neuroendocrine axes and this damage likely alters the normal function of these tissues. Importantly, Cu-induced changes in hormone concentrations and gene expression in endocrine-mediated pathways such as reproductive steroidogenesis and amphibian metamorphosis are likely the result of oxidative stress-induced tissue damage and not endocrine disruption. Overall, we conclude that oxidative stress is likely the primary driver of chronic Cu toxicity in aquatic vertebrates, with bioenergetic effects and compensatory response to disruption of sodium homeostasis contributing to some degree to observed effects on apical endpoints. Environ Toxicol Chem 2022;41:2911-2927. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

South American National Contributions to Knowledge of the Effects of Endocrine Disrupting Chemicals in Wild Animals: Current and Future Directions

Human pressure due to industrial and agricultural development has resulted in a biodiversity crisis. Environmental pollution is one of its drivers, including contamination of wildlife by chemicals emitted into the air, soil, and water. Chemicals released into the environment, even at low concentrations, may pose a negative effect on organisms. These chemicals might modify the synthesis, metabolism, and mode of action of hormones. This can lead to failures in reproduction, growth, and development of organisms potentially impacting their fitness. In this review, we focused on assessing the current knowledge on concentrations and possible effects of endocrine disruptor chemicals (metals, persistent organic pollutants, and others) in studies performed in South America, with findings at reproductive and thyroid levels. Our literature search revealed that most studies have focused on measuring the concentrations of compounds that act as endocrine disruptors in animals at the systemic level. However, few studies have evaluated the effects at a reproductive level, while information at thyroid disorders is scarce. Most studies have been conducted in fish by researchers from Brazil, Argentina, Chile, and Colombia. Comparison of results across studies is difficult due to the lack of standardization of units in the reported data. Future studies should prioritize research on emergent contaminants, evaluate effects on native species and the use of current available methods such as the OMICs. Additionally, there is a primary focus on organisms related to aquatic environments, and those inhabiting terrestrial environments are scarce or nonexistent. Finally, we highlight a lack of funding at a national level in the reviewed topic that may influence the observed low scientific productivity in several countries, which is often negatively associated with their percentage of protected areas.

Influence of temperature on growth, development and thyroid metabolism of American bullfrog tadpoles (Lithobates catesbeianus) exposed to the herbicide tebuthiuron

The influence of temperature (25 and 32 °C) on the negative effects of the herbicide tebuthiuron (TBU, 0, 10, 50 and 200 ng.L-1, 16 days) on thyroid function and metamorphosis of Lithobates catesbeianus tadpoles was evaluated. Metamorphosis was accelerated by TBU exposure at 25 ºC, but delayed at 32 ºC with considerable losses of body mass. T3 and T4 levels were not altered. The highest TBU concentrarion at 25 ºC increased TR β and DIO3 transcript levels, which is consistent with development acceleration in tadpoles. At 32 ºC TR β transcript levels were lower than the values recorded at 25 ºC, and those tadpoles exposed to the highest TBU concentration presented increased diameter of thyroid follicles compared to controls at same temperature. This study evidences that TBU at environmentally realistic concentrations is able to disrupt thyroidogenesis in bullfrog tadpoles, impairing their development. These effects are influenced by temperature.

Effects of nitrite exposure on metamorphosis and skeletal development of Bufo gargarizans

Nitrite, as a part of nitrogen cycle, is one of the most common toxic compounds in aquatic ecosystems. Since skeletal development is an essential process during amphibian metamorphosis, exposure of larval amphibians to nitrite might disrupt skeletal development. To evaluate whether nitrite affects skeletal development of amphibian larvae, Bufo gargarizans larvae at Gs26 were exposed to 10, 100, 500 and 1000 μg/L nitrite-nitrogen (NO₂-N) in the present study. The metamorphosis rate, body weight, body length, forelimb length and hindlimb length of B. gargarizans exposed to NO₂-N were decreased. The microscopic structures of thyroid gland were altered under NO₂-N exposure at Gs42. The skeletal lengths of the humerus, femur and fibulare of tadpole at Gs42 were significantly reduced under 100, 500 and 1000 μg/L NO₂-N treatment groups, and the lengths of humerus, tibia-fibula and tibiale of tadpole at Gs46 were significantly reduced under 1000 μg/L NO₂-N treatment groups. In addition, the expression levels of thyroid hormone (TH) and endochondral ossification-related genes of tadpoles at Gs42 and Gs46 were tested by qRT-PCR. Overall, NO₂-N exposure could affect the expressions of these genes and then may influence the activity and function of thyroid gland, further disturbing the amphibian metamorphosis and skeletal development of amphibian larvae.

Effects of Separate and Combined Exposure of Cadmium and Lead on the Endochondral Ossification in Bufo gargarizans

Cadmium (Cd) and lead (Pb) are ubiquitous in aquatic environments and most studies have examined the potential effects of Cd or Pb alone on aquatic organisms. In the present study, chronic effects of Cd and Pb, alone and in combination, on Bufo gargarizans were investigated by exposing embryos to these contaminants throughout metamorphosis. Significant reductions in body mass and snout-to-vent length were observed in B. gargarizans at Gosner stage 42 (Gs 42) and Gs 46 exposed to a Cd/Pb mixture. Single and combined exposure with Cd and Pb induced histological alterations of the thyroid gland characterized by reduced colloid area and thickness of epithelial cells. There was a significant decrease in the maximum jump distance of froglets exposed to Cd alone and the Cd/Pb mixture, and the jumping capacity showed a positive correlation with hind limb length and tibia/fibula. Moreover, single metals and their mixture induced reduction of endochondral bone formation in B. gargarizans. Transcriptomic and real-time quantitative polymerase chain reaction results showed that genes involved in skeletal ossification (TRα, TRβ, Dio2, Dio3, MMP9, MMP13, Runx1, Runx2, and Runx3) were transcriptionally dysregulated by Cd and Pb exposure alone or in combination. Our results suggested that despite the low concentration tested, the Cd/Pb mixture induced more severe impacts on B. gargarizans. In addition, the Cd/Pb mixture might reduce chances of survival for B. gargarizans froglets by decreasing size at metamorphosis, impaired skeletal ossification, and reduction in jumping ability, which might result from dysregulation of genes involved in thyroid hormone action and endochondral ossification. The findings obtained could add a new dimension to understanding of the mechanisms underpinning skeletal ossification response to heavy metals in amphibians. Environ Toxicol Chem 2022;41:1228-1245. © 2022 SETAC.

Effects of sodium perchlorate and 6-propylthiouracil on metamorphosis and thyroid gland histopathology in the European common frog (Rana temporaria)

Several chemicals have been identified as thyroid hormone axis disrupting chemicals (THADCs) able to interfere with the thyroid hormone system during fetal life and early life stages, thereby impairing neurodevelopment in mammals and inducing development and growth disorders in fish and amphibians. However, identification of THADCs is particularly challenging, and thyroid modalities are currently only assessed in vivo by mammalian and amphibian tests. The aquatic African clawed frog (Xenopus laevis/tropicalis) is the model species of the amphibian test guidelines developed by the OECD and the United States Environmental Protection Agency, but as most European amphibians are semi-aquatic, concern has been raised whether the sensitivity of native European species is comparable to Xenopus. A shortened version of the OEDC test guideline 241 (Larval Amphibian Growth and Development Assay, LAGDA) was used to investigate the effects of two model THADCs on the metamorphosis and thyroid histopathology in the European common frog (Rana temporaria). R. temporaria eggs were collected on the field and exposed till metamorphic climax to sodium perchlorate (11.9-426.5 μg/L perchlorate concentrations) and 6-propylthiouracil (PTU: 1.23-47.7 mg/L). PTU severely delayed metamorphosis and affected several thyroid gland histopathological endpoints at slightly lower concentrations compared to Xenopus. As opposed to what was described in similar Xenopus studies, we observed no effect of perchlorate on the investigated endpoints. Interspecies differences may be linked to mechanisms of action.

Exposure to the synthetic phenolic antioxidant 4,4'-thiobis(6-t-butyl-m-cresol) disrupts early development in the frog Silurana tropicalis

Many chemicals in commonly used household and industrial products are being released into the environment, yet their toxicity is poorly understood. The synthetic phenolic antioxidant, 4,4'-thiobis(6-t-butyl-m-cresol) (CAS 96-69-5; TBBC) is present in many common products made of rubber and plastic. Yet, this phenolic antioxidant has not been tested for potential toxicity and developmental disruption in amphibians, a sensitive and susceptible class. We investigated whether acute and chronic exposure to TBBC would interfere with thyroid hormone-dependent developmental processes in the frog Silurana tropicalis and thus affect its early life-stage development. We exposed S. tropicalis embryos at the Nieuwkoop-Faber (NF) 9-10 stage to TBBC at nominal concentrations (0, 25, 50, 75, 100, 200 and 400 μg/L) to determine the 96h lethal concentrations and sublethal effects. We conducted a chronic exposure starting at stage NF47-48 to three sublethal TBBC nominal concentrations (0, 0.002, 0.1 and 5 μg/L) for 48-52 days to evaluate effects on growth and metamorphosis. The 96h lethal and effective (malformations) TBBC concentrations (LC₅₀ and EC₅₀) were 70.5 and 76.5 μg/L, respectively. Acute exposure to all TBBC concentrations affected S. tropicalis growth and was lethal at 200 and 400 μg/L. Chronic exposure to sublethal TBBC concentrations reduced body size by 8% at 5 μg/L and body mass by 17% at 0.002 μg/L when metamorphosis was completed. This study demonstrates that TBBC is toxic, induces malformations and inhibits tadpole growth after acute and chronic exposures. These findings call for further investigations on the mode of actions of TBBC and related antioxidants for developmental disruption in amphibians.

Time to synchronize our clocks: Connecting developmental mechanisms and evolutionary consequences of heterochrony

Heterochrony, defined as a change in the timing of developmental events altering the course of evolution, was first recognized by Ernst Haeckel in 1866. Haeckel's original definition was meant to explain the observed parallels between ontogeny and phylogeny, but the interpretation of his work became a source of controversy over time. Heterochrony took its modern meaning following the now classical work in the 1970-80s by Steven J. Gould, Pere Alberch, and co-workers. Predicted and described heterochronic scenarios emphasize the many ways in which developmental changes can influence evolution. However, while important examples of heterochrony detected with comparative morphological methods have multiplied, the more mechanistic understanding of this phenomenon lagged conspicuously behind. Considering the rapid progress in imaging and molecular tools available now for developmental biologists, this review aims to stress the need to take heterochrony research to the next level. It is time to synchronize the different levels of heterochrony research into a single analysis flow: from studies on organismal-level morphology to cells to molecules and genes, using complementary techniques. To illustrate how to achieve a more comprehensive understanding of phyletic morphological diversification associated with heterochrony, we discuss several recent case studies at various phylogenetic scales that combine morphological, cellular, and molecular analyses. Such a synergistic approach offers to more fully integrate phylogenetic and ontogenetic dimensions of the fascinating evolutionary phenomenon of heterochrony.

Increasing Hormonal Control of Skeletal Development: An Evolutionary Trend in Amphibians

The biphasic life history of amphibians includes metamorphosis, a complex developmental event that involves drastic changes in the morphology, physiology and biochemistry accompanying the transition from the larval to adult stage of development. Thyroid hormones (THs) are widely known to orchestrate this remodeling and, in particular, to mediate the development of the bony skeleton, which is a model system in evolutionary morphological studies of amphibians. Detailed experimental studies of the role of THs in the craniogenesis of diverse urodelan amphibians revealed that (i) these hormones affect both the timing and sequence of bone formation, (ii) TH involvement increases in parallel with the increase in divergence between larval and adult skull morphology, and (iii) among urodelans, TH-involvement in skull development changes from a minimum in basal salamanders (Hynobiidae) to the most pronounced in derived ones (Salamandridae and Plethodontidae). Given the increasing regulatory function of THs in urodelan evolution, we hypothesized a stronger involvement of THs in the control of skeletogenesis in anurans with their most complex and dramatic metamorphosis among all amphibians. Our experimental study of skeletal development in the hypo- and hyperthyroid yellow-bellied toad (Bombina variegata: Bombinatoridae) supports the greater involvement of THs in the mediation of all stages of anuran cranial and postcranial bones formation. Similar to urodelans, B. variegata displays enhancing TH involvement in the development of cranial bones that arise during larval ontogeny: while the hormonal impact on early larval ossifications is minimal, the skull bones forming during metamorphosis are strictly TH-inducible. However, in contrast to urodelans, all cranial bones, including the earliest to form, are TH-dependent in B. variegata; moreover, the development of all elements of the axial and limb skeleton is affected by THs. The more accentuated hormonal control of skeletogenesis in B. variegata demonstrates the advanced regulatory and inductive function of THs in the orchestration of anuran metamorphosis. Based on these findings, we discuss (i) changes in THs function in amphibian evolution and (ii) the role of THs in the evolution of life histories in amphibians.

Gynogenetic diploids, tetraploids, or octoploids, and a path to polyploidy in Anuran Amphibians

Unreduced gametes have been implicated in the evolution of polyploid species of plants and animals and are normally produced by female anuran amphibians. Such eggs may initiate the evolution of polyploid species that have independently arisen in several anuran families. Polyploid females could also produce unreduced eggs that might lead to species with higher ploidy levels or their eggs may develop gynogenetically to reduce the ploidy level. Diploid Hyla chrysoscelis (2n=24) and tetraploid H. versicolor (4n=48) are sibling cryptic species of North American Grey Treefrogs. Artificial crosses using H. versicolor females and genetically distant diploid males were performed to produce haploid H. versicolor and to assess the production of unreduced eggs in this tetraploid species. Gynogenetic diploid (haploid H. versicolor), allotriploid, gynogenetic tetraploid, allopentaploid, autohexaploid, and gynogenetic octoploid tadpoles were confirmed using chromosome counts from tadpole tail tip squashes. Transformation and survival of the different ploidies varied. Gynogenetic diploids transformed but expressed aspects of the haploid syndrome and died before or shortly after transformation.

Targeted Pathway-based In Vivo Testing Using Thyroperoxidase Inhibition to Evaluate Plasma Thyroxine as a Surrogate Metric of Metamorphic Success in Model Amphibian Xenopus laevis

Chemical safety evaluation is in the midst of a transition from traditional whole-animal toxicity testing to molecular pathway-based in vitro assays and in silico modeling. However, to facilitate the shift in reliance on apical effects for risk assessment to predictive surrogate metrics having characterized linkages to chemical mechanisms of action, targeted in vivo testing is necessary to establish these predictive relationships. In this study, we demonstrate a means to predict thyroid-related metamorphic success in the model amphibian Xenopus laevis using relevant biochemical measurements during early prometamorphosis. The adverse outcome pathway for thyroperoxidase inhibition leading to altered amphibian metamorphosis was used to inform a pathway-based in vivo study design that generated response-response relationships. These causal relationships were used to develop Bayesian probabilistic network models that mathematically determine conditional dependencies between biochemical nodes and support the predictive capability of the biochemical profiles. Plasma thyroxine concentrations were the most predictive of metamorphic success with improved predictivity when thyroid gland sodium-iodide symporter gene expression levels (a compensatory response) were used in conjunction with plasma thyroxine as an additional regressor. Although thyroid-mediated amphibian metamorphosis has been studied for decades, this is the first time a predictive relationship has been characterized between plasma thyroxine and metamorphic success. Linking these types of biochemical surrogate metrics to apical outcomes is vital to facilitate the transition to the new paradigm of chemical safety assessments.

Effects of Environmental Contaminants at Great Bay National Wildlife Refuge on Anuran Development, Gonadal Histology, and Reproductive Steroidogenesis: A Comparison of In Situ and Laboratory Exposures

Previous monitoring at Great Bay National Wildlife Refuge (NWR), Newington, New Hampshire documented high prevalence of amphibian malformations at sites contaminated with potential endocrine active compounds. In the present study, a combination of in situ and laboratory experiments were used to determine whether contaminants present in the sites affect amphibian growth and reproductive development. Wood frog (Rana sylvatica) tadpoles were exposed in situ at four sites (Ferry Way, Beaver Pond, Lower Peverly, and Stubbs Pond) at Great Bay NWR and northern leopard frog (Rana pipiens) tadpoles were exposed in the lab to sediments collected from three sites (Beaver Pond, Ferry Way, Stubbs Pond) at Great Bay NWR as well as a positive (estradiol) and negative control. High mortality was observed at Stubbs Pond and extended larval period at Beaver Pond in the in situ exposure. Only three malformations were noted in the lab experiment, whereas there was a 63% prevalence of rounded femurs in Beaver Pond metamorphs in the in situ exposure. Only 2.4% (5 of 207) of R. sylvatica metamorphs exhibited abnormal reproductive development, whereas intersex metamorphs occurred in treatments and controls in the lab experiment at rates as high as 26%. Reproductive development was more advanced and estradiol to androgen ratios reduced in male metamorphs from Beaver Pond in both the in situ and lab exposures. DDT, PCBs, and PAHs were detected in sediments at Great Bay NWR at concentrations that exceed regulatory or guidance values, with concentrations of PAHs being highest at Lower Peverly Pond and DDT highest at Stubbs Pond. The effects on anuran development may be attributable to the primary contaminants-DDT and PCBs-acting on the thyroid and gonadal axes.

Developmental assays using invasive cane toads, Rhinella marina, reveal safety concerns of a common formulation of the rice herbicide, butachlor

Identifying the adverse impacts of pesticide exposure is essential to guide regulations that are protective of wildlife and human health. Within rice ecosystems, amphibians are valuable indicators because pesticide applications coincide with sensitive reproductive and developmental life stages. We conducted two experiments using wild cane toads (Rhinella marina) to test 1) whether environmentally relevant exposure to a commercial formulation of butachlor, an acetanilide herbicide used extensively in rice, affects amphibian development and 2) whether cane toad tadpoles are capable of acclimatizing to sub-lethal exposure. First, we exposed wild cane toads to 0.002, 0.02, or 0.2 mg/L of butachlor (Machete EC), during distinct development stages (as eggs and hatchlings, as tadpoles, or continuously) for 12 days. Next, we exposed a subset of animals from the first experiment to a second, lethal concentration and examined survivorship. We found that cane toads exposed to butachlor developed slower and weighed less than controls, and that development of the thyroid gland was affected: exposed individuals had smaller thyroid glands and thyrocyte cells, and more individual follicles. Analyses of the transcriptome revealed that butachlor exposure resulted in downregulation of transcripts related to metabolic processes, anatomic structure development, immune system function, and response to stress. Last, we observed evidence of acclimatization, where animals exposed to butachlor early in life performed better than naïve animals during a second exposure. Our findings indicate that the commercial formulation of butachlor, Machete EC, causes thyroid endocrine disruption in vertebrates, and suggest that exposure in lowland irrigated rice fields presents a concern for wildlife and human health. Furthermore, we establish that developmental assays with cane toads can be used to screen for adverse effects of pesticides in rice fields.

Does the spatial sorting of dispersal traits affect the phenotype of the non-dispersing stages of the invasive frog Xenopus laevis through coupling?

In amphibians, spatial sorting progressively enhances the dispersal capacities of dispersing stages in expanding populations but may enhance or limit the performance of the earlier non-dispersing stages. Phenotypic traits of non-dispersing tadpoles and metamorphs can be coupled, through carry-over effects and trade-offs, or decoupled to dispersal traits in adults. We used the globally invasive amphibian, Xenopus laevis, to examine whether spatial sorting of adult phenotypes affects the phenotype of larval stages to metamorphosis in the core and at the periphery of an invasive population in France. We combined common garden laboratory and outdoor experiments to test the effect of parental pond location (core or periphery) on morphology, development and survival to metamorphosis and found no differences between tadpoles. After metamorphosis, the only difference observed in either of the experiments was the larger body size of metamorphs from the periphery, and then only when reared in the laboratory. Differences in metamorph size may indicate that a shift of dispersal traits occur after metamorphosis in X. laevis. Thus, our findings illustrate that decoupled evolution through spatial sorting can lead to changes of X. laevis adult phenotypes that would enhance dispersal without affecting the phenotype of tadpoles before metamorphosis.

Effects of sodium perchlorate and exogenous L-thyroxine on growth, development and leptin signaling pathway of Bufo gargarizans tadpoles during metamorphosis

Sodium perchlorate (NaClO₄) and exogenous L-thyroxine (T4), two kinds of endocrine-disrupting chemicals (EDCs), mainly affect the circulating thyroid hormones, which regulate the initiation and rate of metamorphosis in amphibian. The aim of this study is to evaluate the potential role of EDCs in regulating the development of tadpoles and leptin signaling pathway of liver during the metamorphosis of Bufo gargarizans. There was completely opposite result of average development stage of tadpoles and morphological parameters between the NaClO₄ and T4 exposure groups. Histological analysis revealed that NaClO₄ and T4 exposure both caused liver injury, such as the decreased size of hepatocytes, atrophy of nucleus, increased melanomacrophage centres and disappearance of hepatocyte membranes. In addition, the results of RT-qPCR revealed that NaClO₄ treatment significantly inhibited the transcript levels of genes related to thyroid hormone (D2, TRα and TRβ) and leptin signaling pathway (LepR, JAK1, JAK2, and TYK2), while there was an increase of mRNA expression of these genes in the liver of tadpoles administrated with T4 compared with control. This work lays an important foundation for assessing the risk of EDCs in relation to amphibian development during metamorphosis.

Cross-Taxa Distinctions in Mechanisms of Developmental Effects for Aquatic Species Exposed to Trifluralin

Standard ecological risk assessment practices often rely on larval and juvenile fish toxicity data as representative of the amphibian aquatic phase. Empirical evidence suggests that endpoints measured in fish early life stage tests are often sufficient to protect larval amphibians. However, the process of amphibian metamorphosis relies on endocrine cues that affect development and morphological restructuring and are not represented by these test endpoints. The present study compares developmental endpoints for zebrafish (Danio rerio) and the African clawed frog (Xenopus laevis), 2 standard test species, exposed to the herbicide trifluralin throughout the larval period. Danio rerio were more sensitive and demonstrated a reduction in growth measurements with increasing trifluralin exposure. Size of X. laevis at metamorphosis was not correlated with exposure concentration; however, time to metamorphosis was delayed relative to trifluralin concentration. Gene expression patterns indicate discrepancies in response by D. rerio and X. laevis, and dose-dependent metabolic activity suggests that trifluralin exposure perturbed biological pathways differently within the 2 species. Although many metabolites were correlated with exposure concentration in D. rerio, nontargeted hepatic metabolomics identified a subset of metabolites that exhibited a nonmonotonic response to trifluralin exposure in X. laevis. Linking taxonomic distinctions in cellular-level response with ecologically relevant endpoints will refine assumptions used in interspecies extrapolation of standard test effects and improve assessment of sublethal impacts on amphibian populations. Environ Toxicol Chem 2020;39:1797-1812. Published 2020. This article is a US government work and is in the public domain in the USA.

Effect assessment of reclaimed water and carbamazepine exposure on the thyroid axis of X. laevis: Apical and histological effects

There is increasing environmental concern about the constant presence of pharmaceuticals and personal care products (PPCPs) in surface water, generally attributed to water discharge from wastewater treatment plants (WWTPs) that are unable to completely remove these compounds. The slight, but continuous, presence of these contaminants in reclaimed water (RW) poses a risk of chronic and sublethal toxicity, and the thyroid axis can likely be a target of many of these PPCPs. In this work, we addressed the effects of RW on the Xenopus laevis thyroid system. The Amphibian Metamorphosis Assay (AMA test) was used with modifications by exposing X. laevis tadpoles to RW samples, and to RW spiked with carbamazepine (CBZ) at 100 and 1000 higher than the average levels environmentally relevant (RW 100× and RW 1000×, respectively). Carbamazepine was selected because it is considered a marker of anthropogenic pollution and could have a potential effect on the thyroid axis. The morphological endpoints and histological alterations to the thyroid gland were evaluated. The results suggested the stimulation of the thyroid gland from exposures to the RW samples, supported by tadpoles' accelerated development and by the histological alterations observed in the thyroid gland. Developmental acceleration was also seen in the tadpoles exposed to the RW-100× and -1000× samples at comparable levels to those seen in exposures to RW samples alone. Hence CBZ did not seem to increase the effects of RW on the thyroid axis. Overall, our results suggested endocrine effects of these RW samples regardless of the CBZ concentration.

Chronic effects of lead on metamorphosis, development of thyroid gland, and skeletal ossification in Bufo gargarizans

We examined the Pb²⁺ exposure on tadpoles of Bufo gargarizans from Gosner stage 26-42. Mortality, growth and development, time to metamorphosis, size, and skeletal ossification at metamorphic climax of Bufo gargarizans were examined. Also, histological characteristics of thyroid glands in tadpoles at Gosner stage 33, 38, and 42 as well as transcript levels of thyroid hormone-related genes in the hind-limb, tail, and liver of tadpoles at metamorphic climax were examined. Pb²⁺ exposure induced mortality in a concentration-dependent manner in Bufo gargarizans larvae. The significant increase in growth and development, percent metamorphosis, size at metamorphic climax, and skeletal ossification were observed at 50 μg Pb²⁺ L^-1; however, exposure to 1000 μg Pb²⁺ L^-1 resulted in the opposite effects in tadpoles. In addition, histological alterations of thyroid gland, such as follicular cell hyperplasia and colloid depletion could be found in 50-1000 μg Pb²⁺ L^-1 treatments. Furthermore, Pb²⁺ exposure at 1000 μg L^-1 resulted in significantly decreased transcript levels of Dio2, TRα and TRβ, and increased transcript levels of Dio3. In contrast, 50 μg Pb²⁺ L^-1 significantly upregulated the mRNA levels of Dio2, TRα, and TRβ, but it reduced the Dio3 expression. These results suggested that Pb²⁺ might disrupt TH homeostasis in tadpoles by histological alterations of thyroid gland and disturb the transcript levels of Dio2, Dio3, TRα, and TRβ, leading to altered growth and development, as well as percent metamorphosis and skeletal ossification. Further studies are needed to elucidate the underlying mechanisms of low-dose stimulation and high-dose inhibition effects.

Development of central respiratory control in anurans: The role of neurochemicals in the emergence of air-breathing and the hypoxic response

Physiological and environmental factors impacting respiratory homeostasis vary throughout the course of an animal's lifespan from embryo to adult and can shape respiratory development. The developmental emergence of complex neural networks for aerial breathing dates back to ancestral vertebrates, and represents the most important process for respiratory development in extant taxa ranging from fish to mammals. While substantial progress has been made towards elucidating the anatomical and physiological underpinnings of functional respiratory control networks for air-breathing, much less is known about the mechanisms establishing these networks during early neurodevelopment. This is especially true of the complex neurochemical ensembles key to the development of air-breathing. One approach to this issue has been to utilize comparative models such as anuran amphibians, which offer a unique perspective into early neurodevelopment. Here, we review the developmental emergence of respiratory behaviours in anuran amphibians with emphasis on contributions of neurochemicals to this process and highlight opportunities for future research.

Evaluating Iodide Recycling Inhibition as a Novel Molecular Initiating Event for Thyroid Axis Disruption in Amphibians

The enzyme iodotyrosine deiodinase (dehalogenase, IYD) catalyzes iodide recycling and promotes iodide retention in thyroid follicular cells. Loss of function or chemical inhibition of IYD reduces available iodide for thyroid hormone synthesis, which leads to hormone insufficiency in tissues and subsequent negative developmental consequences. IYD activity is especially critical under conditions of lower dietary iodine and in low iodine environments. Our objective was to evaluate the toxicological relevance of IYD inhibition in a model amphibian (Xenopus laevis) used extensively for thyroid disruption research. First, we characterized IYD ontogeny through quantification of IYD mRNA expression. Under normal development, IYD was expressed in thyroid glands, kidneys, liver, and intestines, but minimally in the tail. Then, we evaluated how IYD inhibition affected developing larval X. laevis with an in vivo exposure to a known IYD inhibitor (3-nitro-l-tyrosine, MNT) under iodine-controlled conditions; MNT concentrations were 7.4-200 mg/L, with an additional 'rescue' treatment of 200 mg/L MNT supplemented with iodide. Chemical inhibition of IYD resulted in markedly delayed development, with larvae in the highest MNT concentrations arrested prior to metamorphic climax. This effect was linked to reduced glandular and circulating thyroid hormones, increased thyroidal sodium-iodide symporter gene expression, and follicular cell hypertrophy and hyperplasia. Iodide supplementation negated these effects, effectively rescuing exposed larvae. These results establish toxicological relevance of IYD inhibition in amphibians. Given the highly conserved nature of the IYD protein sequence and scarcity of environmental iodine, IYD should be further investigated as a target for thyroid axis disruption in freshwater organisms.

Endocrine regulation of regeneration: Linking global signals to local processes

Regeneration in amphibians and reptiles has been explored since the early 18th century, giving us a working in vivo model to study epimorphic regeneration in vertebrates. Studies aiming to uncover primary mechanisms of regeneration have predominantly focused on genetic pathways regulating specific stages of the regeneration process: wound healing, blastema formation and growth, and pattern formation. However, studies across organisms show that environmental conditions and physiological state of the animal can affect the rate or quality of regeneration, and endocrine signals are likely the mediators of these effects. Endocrine signals working/acting directly on receptors expressed in the structure or via neuroendocrine pathways can affect regeneration by modulating immune response to injury, allocation of energetic resources, or by enhancing or inhibiting proliferation and differentiation pathways in regenerating tissue. This review discusses the cumulative knowledge known about endocrine regulation of regeneration and important future research directions of interest to both ecological and biomedical research.

Is normalized hindlimb length measurement in assessment of thyroid disruption in the amphibian metamorphosis assay relevant?

The amphibian metamorphosis assay represents an OECD Level 3 and EDSP Tier 1 ecotoxicity test assessing thyroid activity of chemicals in African clawed frog (Xenopus laevis). To evaluate the effectiveness of snout-vent length (SVL) normalization of hindlimb length (HLL), correlation between the HLL and SVL or body weight was evaluated in the control groups of 10 individual studies from three laboratories. Two studies required separate analysis of the Nieuwkoop-Faber (NF) stage ≤60 and >60 animals creating a total of 12 data sets. On study day 7, significant positive correlation between HLL and SVL or body weight was observed in eight and seven of the 10 data sets, respectively (r = 0.608-0.843 and 0.583-0.876). On study day 21, significant positive correlation between HLL and SVL or body weight was found in three and four of the 12 data sets, respectively (r = 0.452, 0.480 and 0.553 and r = 0.621, 0.546, 0.564 and 0.378). Significant positive correlation between HLL and SVL was found in three of five studies, including ≤NF stage 60 data (r = 0.564, 0.546 and 0.621). In one of eight studies, including >NF stage 60 data, the positive correlation between HLL and body weight was determined (r = 0.378). Negative or no correlation between HLL and SVL or body weight was found in the other late stage data sets. Therefore, use of SVL-normalized HLL to assess thyroid-mediated effects in X. laevis tadpoles is not warranted. HL stage relative to body stage should be considered.

Octylphenol influence growth and development of Rana chensinensis tadpoles via disrupting thyroid function

Octylphenol (OP), a class of endocrine disrupting chemicals (EDCs), could produce adverse effects on developmental process of animals. Thyroid hormone is one of the important hormones involved in animal development. To determine whether OP affect the growth and development of amphibian larvae via interfering the thyroid function, Rana chensinensis larvae at Gosner stage 29 were exposed to 10^-8, 10^-7 and 10^-6 mol/L OP in the present study. Results demonstrated that OP could decrease the body length and mass and retard the development of tadpoles. The histologic evaluation showed microscopic structures of thyroid gland were changed in 10^-7 and 10^-6 mol/L OP treated groups on day 40. The expression levels of Dio2, Dio3, TRα and TRβ mRNA in the liver, brain, skin and tail of tadpoles were detected by qRT-PCR, when treated with OP for 20, 30, 40 and 50 day, respectively. The results of qRT-PCR showed OP could affect the expressions of Dio2, Dio3, TRα and TRβ mRNA in the four tissues, and then influence the activity and function of THs, further affecting the growth and development of the tadpoles.

Endocrine disruption, oxidative stress and lipometabolic disturbance of Bufo gargarizans embryos exposed to hexavalent chromium

The aim of the current study was to determine the potential developmental and metabolic abnormalities caused by Cr (VI) exposure on Bufo gargarizans (B. gargarizans) embryos. B. gargarizans embryos were treated with different concentrations of Cr (VI) (13, 52, 104, 208, and 416 μg Cr⁶⁺ L^-1) for 6 days. Morphological abnormalities, total length, weight and developmental stage were monitored. Malformations of embryos were also examined using scanning electron microscopy (SEM). In addition, the transcript levels of several genes associated with lipid metabolism, oxidative stress, and thyroid hormones signaling pathways were also determined. Our results showed a time-dependent inhibitory effect of Cr (VI) on the growth and development of B. gargarizans embryos. On day 4, total length, weight, and developmental stage were significantly lower at 416 μg Cr⁶⁺ L^-1 relative to control embryos. On day 6, significant reductions in total length, weight, and developmental stage were observed at 104, 208, and 416 μg Cr⁶⁺ L^-1. Malformed embryos were found in all Cr (VI) treatments, which were characterized by axial flexures, yolk sac edema and rupture, surface tissue hyperplasia, stunted growth, wavy fin and fin flexure. RT-qPCR results showed that exposure to Cr (VI) down-regulated TRβ and Dio2 mRNA expression and up-regulated Dio3 mRNA level at 416 μg Cr⁶⁺ L^-1. The transcript levels of SOD and GPx were upregulated at 52, 208, and 416 μg Cr⁶⁺ L^-1, while the transcript level of HSP90 was downregulated at 52, 208, and 416 μg Cr⁶⁺ L^-1. Also, mRNA expression of lipid synthesis-related genes (FAE and ACC) were significantly downregulated in embryos treated with 208 and 416 μg Cr⁶⁺ L^-1, but mRNA expression of fatty acid β-oxidation-related genes (ACOX, CPT, and SCP) was significantly upregulated at 416 μg Cr⁶⁺ L^-1. Therefore, our results suggested that Cr (VI) could disrupt thyroid endocrine pathways and lipid synthesis, leading to the inhibition of growth and development in B. gargarizans embryos. Furthermore, the decreased ability of scavenging ROS induced by Cr (VI) might be responsible for the teratogenic effects of Cr (VI).

Simulated developmental and reproductive impacts on amphibian populations and implications for assessing long-term effects

Fish endpoints measured in early life stage toxicity tests are often used as representative of larval amphibian sensitivity in Ecological Risk Assessment (ERA). This application potentially overlooks the impact of developmental delays on amphibian metamorphosis, and thereby reduced survival, in amphibian populations constrained by habitat availability. Likewise, the effects of reduced productivity or altered sexual development as a result of chemical exposure are not presented in terms of lower population fecundity in these surrogate tests. Translating endpoints measured in toxicity tests to those that are more representative of amphibian ecology and population dynamics provides a means of identifying how developmental effects result in long-term impacts. Here we compare effects of developmental delay on metamorphosis success in six anuran species and simulate population-level impacts of subsequent reductions in larval survival as well as potential reductions in fecundity as a result of developmental impacts. We use deterministic matrix models to compare realistic combinations of amphibian demographic rates and relative impacts of reduced growth on larval survival and subsequently on population growth. Developmental delays are less detrimental in species with longer and less synchronous larval periods. All six species were most sensitive to changes in first-year survival, and damping ratios were generally a good indicator of resilience to perturbations in both larval survival and fecundity. Further identification of species and population-level vulnerabilities can improve the evaluation of sublethal effects in relevant context for ERA.

Competition and pesticide exposure affect development of invasive (Rhinella marina) and native (Fejervarya vittigera) rice paddy amphibian larvae

Increased pesticide use in rice agricultural ecosystems may alter competitive interactions between invasive and native amphibian species. We conducted an experiment with two rice paddy amphibians found in Luzon, Philippines, the invasive cane toad (Rhinella marina) and the endemic Luzon wart frog (Fejervarya vittigera), to determine whether exposure to a common herbicide, butachlor, drives competitive interactions in favor of the invasive amphibian. Our results revealed that competition had a strong effect on the development of both species, but in opposing directions; Luzon wart frog tadpoles were smaller and developed slower than when raised alone, whereas cane toad tadpoles were larger and developed faster. Contrary to our predictions, development and survival of endemic wart frog tadpoles was not affected by butachlor, whereas invasive cane toad tadpoles were affected across several endpoints including gene expression, body size, and survival. We also observed an interaction between pesticide exposure and competition for the cane toad, where survival declined but body size and expression of thyroid sensitive genes increased. Taken together, our findings indicate that the success of the cane toad larvae in rice fields may be best explained by increased rates of development and larger body sizes of tadpoles in response to competition with native Luzon wart frog tadpoles rather than lower sensitivity to a common pesticide. Our results for the cane toad also provide evidence that butachlor can disrupt thyroid hormone mediated development in amphibians, and further demonstrate that important species interactions such as competition can be affected by pesticide exposure in aquatic ecosystems.

Responses of growth, malformation, and thyroid hormone-dependent genes expression in Bufo gargarizans embryos following chronic exposure to Pb2

The aim of this study was to examine the adverse effects of lead (Pb) exposure on Bufo gargarizans embryos. The 96 h-LC₅₀ of Pb²⁺ for B. gargarizans embryos was determined to be 26.6 mg L^-1 after an acute test. In the chronic test, B. gargarizans embryos at Gosner stage 3 were exposed to 10~2000 μg Pb²⁺ L^-1 during embryogenesis. Total length, weight, developmental stage, and malformation were monitored. In addition, the transcript levels of type II and type III iodothyronine deiodinase (Dio2 and Dio3) and thyroid hormone receptors (TRα and TRβ) were determined to assess the thyroid-disrupting effects of Pb²⁺. Slightly increased growth and development of B. gargarizans embryos were observed at low concentrations of Pb²⁺ (10, 50, and 100 μg L^-1), while retarded growth and development were found at high concentrations of Pb²⁺ (1000 and 2000 μg L^-1). In addition, Pb²⁺ exposure induced morphological abnormalities, which were characterized by edema at tail, wavy fin, abdominal edema, stunted growth, hyperplasia, and axial flexures in B. gargarizans embryos. Furthermore, our results showed that exposure to 2000 μg Pb²⁺ L^-1 decreased the transcript levels of Dio2, TRα, and TRβ, but it increased Dio3 mRNA level. In contrast, exposure to 50 μg Pb²⁺ L^-1 increased TRα mRNA level and decreased Dio3 mRNA level. These results suggested that Pb²⁺ might have thyroid-disrupting effects, leading to the disruption of growth and development in B. gargarizans embryos.

Developmental toxicity and thyroid hormone-disrupting effects of 2,4-dichloro-6-nitrophenol in Chinese rare minnow (Gobiocypris rarus)

In the present study, to evaluate embryonic toxicity and the thyroid-disrupting effects of 2,4-dichloro-6-nitrophenol (DCNP), embryos and adults of Chinese rare minnow (Gobiocypris rarus) were exposed to 2, 20, and 200μg/L DCNP. In the embryo-larval assay, increased percentages of mortality and occurrence of malformations, decreased percentage of hatching, and decreased body length and body weight were observed after DCNP treatment. Moreover, the whole-body T3 levels were significantly increased at 20 and 200μg/L treatments, whereas the T4 levels were markedly decreased significantly (p<0.05) for all DCNP concentrations. In the adult fish assay, plasma T3 levels were significantly increased whereas plasma T4 levels were significantly reduced in the fish treated with 20 and 200μg/L (p<0.05). In addition, DCNP exposure significantly changed the transcription levels of thyroid system related genes, including dio1, dio2, me, nis, tr, and ttr. The increased responsiveness of thyroid hormone and mRNA expression levels of thyroid system related genes suggested that DCNP could disrupt the thyroid hormone synthesis and transport pathways. Therefore, our findings provide new insights of DCNP as a thyroid hormone-disrupting chemical.

Chronic effects of triclosan on embryonic development of Chinese toad, Bufo gargarizans

Triclosan (TCS) is commonly used worldwide in a range of personal care and sanitizing products. The aim of this study was to evaluate potential effects of TCS exposure on embryonic development of Bufo gargarizans, an endemic frog species in China. Standard Gosner stage 3 B. gargarizans embryos were exposed to 10 ~ 150 μg/L TCS during embryogenesis. Survival, total length, weight, developmental stage, duration of different embryo stages, malformation, and type II and III deiodinase (D2 and D3) expression were measured. Inhibitory effects on embryo developmental stage, total length and weight were found at 30 ~ 150 μg/L TCS. Moreover, the duration of embryonic development was increased at gastrula, neural, circulation, and operculum development stage in TCS-treated groups. In addition, TCS exposure induced morphological malformations in B. gargarizans embryos, which are characterized by hyperplasia, abdominal edema, and axial flexures. Furthermore, our results showed that the expression of D2 in embryos was probably down-regulated at 60 and 150 μg/L TCS, but its spatial expression patterns was not affected by TCS. In summary, our study suggested that TCS exposure not only resulted in delayed growth and development but also caused teratogenic effects in B. gargarizans embryos, and the developmental effects of TCS at high concentrations may be associated with disruption of THs homeostasis. Although further studies are necessary, the present findings could provide a basis for understanding on harmful effects and the potential mechanisms of TCS in amphibian embryos.

The synthetic gestagen levonorgestrel directly affects gene expression in thyroid and pituitary glands of Xenopus laevis tadpoles

The synthetic gestagen levonorgestrel (LNG) was previously shown to perturb thyroid hormone-dependent metamorphosis in Xenopus laevis. However, so far the mechanisms underlying the anti-metamorphic effects of LNG remained unknown. Therefore, a series of in vivo and ex vivo experiments was performed to identify potential target sites of LNG action along the pituitary-thyroid axis of X. laevis tadpoles. Prometamorphic tadpoles were treated in vivo with LNG (0.01-10nM) for 72h and brain-pituitary and thyroid tissue was analyzed for marker gene expression. While no treatment-related changes were observed in brain-pituitary tissue, LNG treatment readily affected thyroidal gene expression in tadpoles including decreased slc5a5 and iyd mRNA expression and a strong induction of dio2 and dio3 expression. When using an ex vivo organ explant culture approach, direct effects of LNG on both pituitary and thyroid gland gene expression were detecTable Specifically, treatment of pituitary explants with 10nM LNG strongly stimulated dio2 expression and concurrently suppressed tshb expression. In thyroid glands, ex vivo LNG treatment induced dio2 and dio3 mRNA expression in a thyrotropin-independent manner. When thyroid explants were cultured in thyrotropin-containing media, LNG caused similar gene expression changes as seen after 72h in vivo treatment including a very strong repression of thyrotropin-induced slc5a5 expression. Concerning the anti-thyroidal activity of LNG as seen under in vivo conditions, our ex vivo data provide clear evidence that LNG directly affects expression of genes important for thyroidal iodide handling as well as genes involved in negative feedback regulation of pituitary tshb expression.

γ-Aminobutyric acid ameliorates fluoride-induced hypothyroidism in male Kunming mice

AIM

This study evaluated the protective effects of γ-aminobutyric acid (GABA), a non-protein amino acid and anti-oxidant, against fluoride-induced hypothyroidism in mice.

MAIN METHODS

Light microscope sample preparation technique and TEM sample preparation technique were used to assay thyroid microstructure and ultrastructure; enzyme immunoassay method was used to assay hormone and protein levels; immunohistochemical staining method was used to assay apoptosis of thyroid follicular epithelium cells.

KEY FINDINGS

Subacute injection of sodium fluoride (NaF) decreased blood T4, T3 and thyroid hormone-binding globulin (TBG) levels to 33.98 μg/l, 3 2.8 ng/ml and 11.67 ng/ml, respectively. In addition, fluoride intoxication induced structural abnormalities in thyroid follicles. Our results showed that treatment of fluoride-exposed mice with GABA appreciably decreased metabolic toxicity induced by fluoride and restored the microstructural and ultrastructural organisation of the thyroid gland towards normalcy. Compared with the negative control group, GABA treatment groups showed significantly upregulated T4, T3 and TBG levels (42.34 μg/l, 6.54 ng/ml and 18.78 ng/ml, respectively; P<0.05), properly increased TSH level and apoptosis inhibition in thyroid follicular epithelial cells.

SIGNIFICANCE

To the best of our knowledge, this is the first study to establish the therapeutic efficacy of GABA as a natural antioxidant in inducing thyroprotection against fluoride-induced toxicity.

Cord Blood Cells for Developmental Toxicology and Environmental Health

The Tox21 program initiated a shift in toxicology toward in vitro testing with a focus on the biological mechanisms responsible for toxicological response. We discuss the applications of these initiatives to developmental toxicology. Specifically, we briefly review current approaches that are widely used in developmental toxicology to demonstrate the gap in relevance to human populations. An important aspect of human relevance is the wide variability of cellular responses to toxicants. We discuss how this gap can be addressed by using cells isolated from umbilical cord blood, an entirely non-invasive source of fetal/newborn cells. Extension of toxicological testing to collections of human fetal/newborn cells would be useful for better understanding the effect of toxicants on fetal development in human populations. By presenting this perspective, we aim to initiate a discussion about the use of cord blood donor-specific cells to capture the variability of cellular toxicological responses during this vulnerable stage of human development.

Sodium perchlorate disrupts development and affects metamorphosis- and growth-related gene expression in tadpoles of the wood frog (Lithobates sylvaticus)

Numerous endocrine disrupting chemicals can affect the growth and development of amphibians. We investigated the effects of a targeted disruption of the endocrine axes modulating development and somatic growth. Wood frog (Lithobates sylvaticus) tadpoles were exposed for 2weeks (from developmental Gosner stage (Gs) 25 to Gs30) to sodium perchlorate (SP, thyroid inhibitor, 14mg/L), estradiol (E2, known to alter growth and development, 200nM) and a reduced feeding regime (RF, to affect growth and development in a chemically-independent manner). All treatments experienced developmental delay, and animals exposed to SP or subjected to RF respectively reached metamorphic climax (Gs42) approximately 11(±3) and 17(±3) days later than controls. At Gs42, only SP-treated animals showed increased weight and snout-vent length (P<0.05) relative to controls. Tadpoles treated with SP had 10-times higher levels of liver igf1 mRNA after 4days of exposure (Gs28) compared to controls. Tadpoles in the RF treatment expressed 6-times lower levels of liver igf1 mRNA and 2-times higher liver igf1r mRNA (P<0.05) at Gs30. Tadpoles treated with E2 exhibited similar developmental and growth patterns as controls, but had increased liver igf1 mRNA levels at Gs28, and tail igf1r at Gs42. Effects on tail trβ mRNA levels were detected in SP-treated tadpoles at Gs42, 40days post-exposure, suggesting that the chemical inhibition of thyroid hormone production early in development can have long-lasting effects. The growth effects observed in the SP-exposed animals suggest a relationship between TH-dependent development and somatic growth in L. sylvaticus tadpoles.

Effects of glyphosate-based herbicides on survival, development, growth and sex ratios of wood frog (Lithobates sylvaticus) tadpoles. II: agriculturally relevant exposures to Roundup WeatherMax® and Vision® under laboratory conditions

Glyphosate-based herbicides are currently the most commonly used herbicides in the world. They have been shown to affect survival, growth, development and sexual differentiation of tadpoles under chronic laboratory exposures but this has not been investigated under more environmentally realistic conditions. The purpose of this study is (1) to determine if an agriculturally relevant exposure to Roundup WeatherMax®, a relatively new and understudied formulation, influences the development of wood frog tadpoles (Lithobates sylvaticus) through effects on the mRNA levels of genes involved in the control of metamorphosis; (2) to compare results to the well-studied Vision® formulation (containing the isopropylamine salt of glyphosate [IPA] and polyethoxylated tallowamine [POEA] surfactant) and to determine which ingredient(s) in the formulations are responsible for potential effects on development; and (3) to compare results to recent field studies that used a similar experimental design. In the present laboratory study, wood frog tadpoles were exposed to an agriculturally relevant application (i.e., two pulses) of Roundup WeatherMax® and Vision® herbicides as well as the active ingredient (IPA) and the POEA surfactant of Vision®. Survival, development, growth, sex ratios and mRNA levels of genes involved in tadpole metamorphosis were measured. Results show that Roundup WeatherMax® (2.89 mg acid equivalent (a.e.)/L) caused 100% mortality after the first pulse. Tadpoles treated with a lower concentration of Roundup WeatherMax® (0.21 mg a.e./L) as well as Vision® (2.89 mg a.e./L), IPA and POEA had an increased condition factor (based on length and weight measures in the tadpoles) relative to controls at Gosner stage (Gs) 36/38. At Gs42, tadpoles treated with IPA and POEA had a decreased condition factor. Also at Gs42, the effect on condition factor was dependent on the sex of tadpoles and significant treatment effects were only detected in males. In most cases, treatment reduced the normal mRNA increase of key genes controlling development in tadpoles between Gs37 and Gs42, such as genes encoding thyroid hormone receptor beta in brain, glucocorticoid receptor in tail and deiodinase enzyme in brain and tail. We conclude that glyphosate-based herbicides have the potential to alter mRNA profiles during metamorphosis. However, studies in natural systems have yet to replicate these negative effects, which highlight the need for more ecologically relevant studies for risk assessment.

Effects of the glyphosate-based herbicide Roundup WeatherMax® on metamorphosis of wood frogs (Lithobates sylvaticus) in natural wetlands

Amphibian tadpoles develop in aquatic environments where they are susceptible to the effects of pesticides and other environmental contaminants. Glyphosate-based herbicides are currently the most commonly used herbicide in the world and have been shown to affect survival and development of tadpoles under laboratory and mesocosm conditions. In the present study, whole wetland manipulations were used to determine if exposure to an agriculturally relevant application of Roundup WeatherMax(®), a herbicide formulation containing the potassium salt of glyphosate and an undisclosed surfactant, influences the development of wood frog tadpoles (Lithobates sylvaticus) under natural conditions. Wetlands were divided in half with an impermeable curtain so that each wetland contained a treatment and control side. Tadpoles were exposed to two pulses of this herbicide at an environmentally realistic concentration (ERC, 0.21 mg acid equivalent (a.e.)/L) and the predicted maximum environmental concentration (PMEC, 2.89 mg a.e./L), after which abundance, growth, development, and mRNA levels of genes involved in tadpole metamorphosis were measured. Results present little evidence that exposure to this herbicide affects abundance, growth and development of wood frog tadpoles. As part of the Long-term Experimental Wetlands Area (LEWA) project, this research demonstrates that typical agricultural use of Roundup WeatherMax(®) poses minimal risk to larval amphibian development. However, our gene expression data (mRNA levels) suggests that glyphosate-based herbicides have the potential to alter hormonal pathways during tadpole development.