Comparison of transcriptome responses of the liver, tail fin, and olfactory epithelium of Rana [Lithobates] catesbeiana tadpoles disrupted by thyroid hormones and estrogen

Cross-taxa extrapolation: Is there a role for thyroid hormone conjugating liver enzymes during amphibian metamorphosis?

Chemical safety assessment includes evaluating the potential to disrupt the endocrine system in humans and wildlife. The thyroid hormone system shows high complexity which is conserved across vertebrates, allowing biological read-across between regulatory important taxa, namely mammals and amphibians. Potential thyroid disruption in aquatic vertebrates is typically investigated by activity assays (Amphibian Metamorphosis Assay (AMA), Xenopus Eleutheroembryo Thyroid Assay). Since neither assay is designed to provide detailed mechanistic information, mode of action analyses often rely on mammalian data, assuming overall cross-vertebrate conservation. This manuscript elaborates on the imperative that, despite overall conservation, the T-modality in metamorphosing amphibians needs to be understood in detail to justify biological read-across between mammals and amphibians. To this end, we revisit the AMA regarding amphibian developmental physiology, and the T-modality regarding mechanistic cross-vertebrate conservation. The importance of a mechanistic understanding for read-across is showcased based on the AMA's apparent insensitivity to at least one category of prototypical liver enzyme inducers. From a regulatory perspective, deeper mechanistic understanding is needed, not only to strengthen the scientific basis for designing testing strategies and interpreting study results, but also to allow the identification of data gaps and thus development of New Approach Methodologies (NAMs) to minimize vertebrate testing.

Single and combined chronic toxicity of cadmium and titanium dioxide nanoparticles in Aquarana catesbeiana (Anura: Ranidae) tadpoles

The rising production and improper disposal of titanium dioxide nanoparticles (TiO2 NPs) into aquatic systems present considerable environmental challenges, especially when these particles interact with other contaminants such as cadmium (Cd). Thus, the current study aimed to evaluate the potential toxic effects on the gills, chondrocranium, body growth, and mortality of Aquarana catesbeiana tadpoles. The tadpoles were exposed to environmentally relevant concentrations of TiO2 NPs (10 µg L-1), and CdCl2 (10 µg L-1), both individually and in combination, for 30 days (chronic exposure), along with a control group. Our results indicate that the co-exposure to TiO2 NPs and Cd induced a higher mortality rate. In the gills, TiO2 NPs led to epithelial simplification, while Cd exposure resulted in stratified epithelium formation. Additionally, there were notable changes in the index of degenerative alterations for the co-exposed group and the overall organ index for the groups exposed to Cd and the mixture. The viscerocranium showed significant malformations in the ceratobranchials and reticular processes, indicating the mixture's toxicological potential during the skeletal system's development. Morphometric analysis also revealed reduced body length and abnormal body ratios in tadpoles from the co-exposure group. In conclusion, TiO2 NPs and Cd, both alone and in combination, exhibit toxicological effects in A. catesbeiana tadpoles, indicating a potential ecological risk associated with releasing these contaminants into aquatic environments.

Transcriptomic profiling of Rana [Lithobates] catesbeiana back skin during natural and thyroid hormone-induced metamorphosis under different temperature regimes with particular emphasis on innate immune system components

As amphibians undergo thyroid hormone (TH)-dependent metamorphosis from an aquatic tadpole to the terrestrial frog, their innate immune system must adapt to the new environment. Skin is a primary line of defense, yet this organ undergoes extensive remodelling during metamorphosis and how it responds to TH is poorly understood. Temperature modulation, which regulates metamorphic timing, is a unique way to uncover early TH-induced transcriptomic events. Metamorphosis of premetamorphic tadpoles is induced by exogenous TH administration at 24 °C but is paused at 5 °C. However, at 5 °C a "molecular memory" of TH exposure is retained that results in an accelerated metamorphosis upon shifting to 24 °C. We used RNA-sequencing to identify changes in Rana (Lithobates) catesbeiana back skin gene expression during natural and TH-induced metamorphosis. During natural metamorphosis, significant differential expression (DE) was observed in >6500 transcripts including classic TH-responsive transcripts (thrb and thibz), heat shock proteins, and innate immune system components: keratins, mucins, and antimicrobial peptides (AMPs). Premetamorphic tadpoles maintained at 5 °C showed 83 DE transcripts within 48 h after TH administration, including thibz which has previously been identified as a molecular memory component in other tissues. Over 3600 DE transcripts were detected in TH-treated tadpoles at 24 °C or when tadpoles held at 5 °C were shifted to 24 °C. Gene ontology (GO) terms related to transcription, RNA metabolic processes, and translation were enriched in both datasets and immune related GO terms were observed in the temperature-modulated experiment. Our findings have implications on survival as climate change affects amphibia worldwide.

Utilization of temperature-mediated activation of thyroid hormone-induced molecular memory to evaluate early signaling events in the olfactory epithelium of Rana [Lithobates] catesbeiana tadpoles

The amphibian olfactory system is highly distinct between aquatic tadpole and terrestrial frog life stages and therefore must remodel extensively during thyroid hormone (TH)-dependent metamorphosis. Developmentally appropriate functioning of the olfactory epithelium is critical for survival. Previous studies in other Rana [Lithobates] catesbeiana premetamorphic tadpole tissues showed that initiation of TH-induced metamorphosis can be uncoupled from execution of TH-dependent programs by holding tadpoles in the cold rather than at warmer permissive temperatures. TH-exposed tadpoles at the nonpermissive (5 °C) temperature do not undergo metamorphosis but retain a "molecular memory" of TH exposure that is activated upon shift to a permissive warm temperature. Herein, premetamorphic tadpoles were held at permissive (24 °C) or nonpermissive (5 °C) temperatures and injected with 10 pmoles/g body weight 3,5,3'-triiodothyronine (T3) or solvent control. Olfactory epithelium was collected at 48 h post-injection. RNA-sequencing (RNA-Seq) and reverse transcriptase quantitative real-time polymerase chain reaction (RT-qPCR) analyses generated differentially expressed transcript profiles of 4328 and 54 contigs for permissive and nonpermissive temperatures, respectively. Translation, rRNA, spliceosome, and proteolytic processes gene ontologies were enriched by T3 treatment at 24 °C while negative regulation of cell proliferation was enriched by T3 at 5 °C. Of note, as found in other tissues, TH-induced basic leucine zipper-containing protein-encoding transcript, thibz, was significantly induced by T3 at both temperatures, suggesting a role in the establishment of molecular memory in the olfactory epithelium. The current study provides critical insights by deconstructing early TH-induced induction of postembryonic processes that may be targets for disruption by environmental contaminants.

Defining components of early thyroid hormone signalling through temperature-mediated activation of molecular memory in cultured Rana [lithobates] catesbeiana tadpole back skin

Thyroid hormones (THs) are essential signalling molecules for the postembryonic development of all vertebrates. THs are necessary for the metamorphosis from tadpole to froglet and exogenous TH administration precociously induces metamorphosis. In American bullfrog (Rana [Lithobates] catesbeiana) tadpoles, the TH-induced metamorphosis observed at a warm temperature (24 °C) is arrested at a cold temperature (4 °C) even in the presence of exogenous THs. However, when TH-exposed tadpoles are shifted from cold to warm temperatures (4 → 24 °C), they undergo TH-dependent metamorphosis at an accelerated rate even when the initial TH signal is no longer present. Thus, they possess a "molecular memory" of TH exposure that establishes the TH-induced response program at the cold temperature and prompts accelerated metamorphosis after a shift to a warmer temperature. The components of the molecular memory that allow the uncoupling of initiation from the execution of the metamorphic program are not understood. To investigate this, we used cultured tadpole back skin (C-Skin) in a repeated measures experiment under 24 °C only, 4 °C only, and 4 → 24 °C temperature shifted regimes and reverse transcription quantitative polymerase chain reaction (RT-qPCR) and RNA-sequencing (RNA-seq) analyses. RNA-seq identified 570, 44, and 890 transcripts, respectively, that were significantly changed by TH treatment. These included transcripts encoding transcription factors and proteins involved in mRNA structure and stability. Notably, transcripts associated with molecular memory do not overlap with those identified previously in cultured tail fin (C-fin) except for TH-induced basic leucine zipper-containing protein (thibz) suggesting that thibz may have a central role in molecular memory that works with tissue-specific factors to establish TH-induced gene expression programs.