Molecular characterization and developmental expression patterns of thyroid hormone receptors (TRs) and their responsiveness to TR agonist and antagonist in Rana nigromaculata

Molecular characterization and expression analysis of thyroid hormone receptors in protogynous rice field eel, Monopterus albus

Thyroid hormones (THs) play important roles in growth, development, morphogenesis, reproduction, and so on. They are mainly meditated by binding to thyroid hormone receptors (TRs) in vertebrates. As important members of the nuclear receptor superfamily, TRs and their ligands are involved in many biological processes. To investigate the potential roles of TRs in the gonadal differentiation and sex change, we cloned and characterized the TRs genes in protogynous rice field eel (Monopterus albus). In this study, three types of TRs were obtained, which were TRαA, TRαB and TRβ, encoding preproproteins of 336-, 409- and 415-amino acids, respectively. Multiple alignments of the three putative TRs protein sequences showed they had a higher similarity. Tissue expression analysis showed that TRαA mainly expressed in the gonad, while TRαB and TRβ in the brain. During female-to-male sex reversal, the expression levels of all the three TRs showed a similar trend of increase followed by a decrease in the gonad. Intraperitoneal injection of triiodothyronine (T3) stimulated the expression of TRαA and TRαB, while it had no significant change on the expression of TRβ in the ovary. Gonadotropin-releasing hormone analogue (GnRHa) injection also significantly upregulated the expression levels of TRαA and TRαB after 6 h, while it had no significant effect on TRβ. These results demonstrated that TRs were involved in the gonadal differentiation and sex reversal, and TRα may play more important roles than TRβ in reproduction by the regulation of GnRHa in rice field eel.

Effects of continuous and pulse lead exposure on the swimming behavior of tadpoles revealed by brain-gut axis analysis

Lead (Pb) is present in aquatic environments with a continuous or pulse form due to the regular or irregular discharge of wastewater. These two modes of exposure result in different toxicological effects on aquatic animals. To compare the effects of Pb exposure mode on the swimming behavior of amphibian larvae, this study proposed a combination method to examine the brain-gut axis (gut bacteria, histopathology, metabolomics, and ethology) in order to evaluate the ecotoxic differences in Pelophylax nigromaculatus tadpoles (Gs 21-28) when exposed to continuous (CE100) versus pulse exposure (PE100) of environmental concentrations of Pb (100 μg/L). The results showed that: 1) CE100 significantly decreased the movement distance and swimming activity of the tadpoles compared to PE100 and the control, while there were no significant differences between the control group and PE100. 2) At the phyla level, compared to PE100, CE100 treatment significantly decreased the abundance of Actinobacteria, Firmicutes, Proteobacteria, and Bacteroidetes and increased the abundance of Fusobacteria in the gut. At the genus level, compared to PE100, CE100 significantly increased the abundance of U114 and decreased the abundance of Anaerorhabdus, Exiguobacterium and Microbacterium. 3) Compared to PE100, CE100 changed the metabolites of the brain-gut axis pathway, such as quinolinic acid, L-valine, L-dopa, L-histidine, urocanic acid, L-threonine, γ-aminobutyric acid (GABA), L-glutamate (Glu), acetylcholine (Ach), L-tyrosine (Tyr), L-tryptophan (Trp), and levodopa (DOPA). 4) CE100 and PE100 played a repressive role in the histidine metabolism and tyrosine metabolism pathways and played a promoting role in the purine metabolism and pyrimidine metabolism pathways. This study provides a method for evaluating the toxic effects of heavy metal exposure via two different exposure modes (pulse versus continuous) which tadpoles may encounter in the natural environment from a combined study examining the brain-gut axis.

Multiple bioanalytical methods reveal a thyroid-disrupting mechanism related to the membrane receptor integrin αvβ3

Di(2-ethylhexyl) phthalate (DEHP), a manufactured chemical, is suitable for large-scale production and has extensive applications. Although restricted for use, DEHP is still ubiquitous in the environment and shows potential to disrupt the structure or function of the thyroid system. However, its toxic mechanism is complex and not clearly understood. In this study, a battery of methods was employed to investigate DEHP-induced thyroid-disrupting effects and their mechanism of action, focusing on a newly discovered membrane receptor-mediated mechanism. The results showed that DEHP promoted rat pituitary tumor (GH3) cell proliferation and c-fos gene expression at environment level concentrations (2 and 5 μmol/L) in a manner similar to that of the natural thyroid hormone 3,3',5-triiodo-L-thyronine (T3). The macromolecule DEHP-BSA cannot pass through the cell membrane to interact with nuclear receptors but upregulated the c-fos gene expression when administered at concentrations comparable to DEHP concentrations; molecular docking demonstrated that DEHP has affinity for the membrane receptor integrin α_vβ₃; DEHP at 2 μmol/L upregulated the β₃ gene expression in GH3 cells; after the addition of integrin α_vβ₃-inhibiting RGD peptide, DEHP-induced c-fos gene upregulation decreased. All of these findings support the supposition that DEHP-induced thyroid-disrupting effects might be mediated by the membrane receptor integrin α_vβ₃. Moreover, DEHP activated the downstream extracellular regulated protein kinase (ERK1/2) pathway, upregulating the gene expression of raf-1, MEK-1 and MAPK1 and increasing the protein levels of p-ERK; interestingly, ERK1/2 activation and c-fos upregulation induced by DEHP were attenuated by PD98059 (an ERK1/2 inhibitor). Taken together, the data suggest that the membrane receptor integrin α_vβ₃ and the downstream ERK1/2 pathway might be involved in DEHP-induced thyroid-disrupting effects. This study provides new insight into the thyroid-disrupting effect and the underlying mechanism and will advance the effort to construct adverse outcome pathways of DEHP and other thyroid hormone disrupting chemicals.

Bisphenol A and thyroid hormones: Bibliometric analysis of scientific publications

Bisphenol A (BPA) is a well-known endocrine-disrupting chemical which can cause potential health risks and interfere with thyroid hormones through multiple avenues. This study aimed to evaluate the hotspots and emerging trends on BPA and thyroid hormones by using a bibliometric method.Publications related on BPA and thyroid hormones were downloaded from Science Citation Index-Expanded database. Annual outputs, high yield journals, countries, institutions, authors and their cited times were summarized. In addition, keywords co-occurrence, burst references and citation networks were bibliometric analyzed.From 2000 to 2019, 418 articles were published. Both of the Environment International and Environmental Health Perspectives, United States, Chinese Academy of Sciences and Antonia M. Calafat were the most recorded journals, countries, institutions and authors, respectively. The main research area was Toxicology. In addition of the retrieve term "bisphenol-a" and "thyroid-hormone", "in-vitro", "exposure" and "endocrine disruptors", were the hotspot keywords and "triclosan", "oxidative stress" and "united-states" were the most recent trends keywords. "Thyroid hormone action is disrupted by Bisphenol A as an antagonist" published on The Journal of Clinical Endocrinology & Metabolism by Kenji Moriyama in 2002 got both the highest burst score and citation score. Six groups were clustered and the mechanism of BPA's effect on thyroid hormones, and the exposure of BPA and potential risks in children and pregnant women were the two main large fields.The number of publications in the field of BPA and thyroid hormones has increased tremendously since 2000. The research hotspot ranged from mechanism researches in animal models to epidemiological studies. "Thyroid hormone action is disrupted by bisphenol A as an antagonist" of Kenji Moriyama provided important building blocks in the field. The impact of BPA on thyroid hormones, especially pregnant women and children, was the latest research frontiers and might be the future direction of this filed in the following years.

Transcriptome analyses reveal molecular mechanisms that regulate endochondral ossification in amphibian Bufo gargarizans during metamorphosis

BACKGROUND

A developmental transition from aquatic to terrestrial existence is one of the most important events in the evolution of terrestrial vertebrates. Amphibian metamorphosis is a classic model to study this transition. The development of the vertebrate skeleton can reflect its evolutionary history. Endochondral ossification serves a vital role in skeletal development. Thus, we sought to unravel molecular mechanisms that regulate endochondral ossification during Bufo gargarizans metamorphosis.

METHODS

The alizarin red-alcian blue double staining method was used to visualize the skeletal development of B. gargarizans during metamorphosis. RNA sequencing (RNA-seq) was used to explore the transcriptome of B. gargarizans in four key developmental stages during metamorphosis. Real-time quantitative PCR (RT-qPCR) was used to validate the expression patterns of endochondral ossification related genes.

RESULTS

Endochondral ossification increased gradually in skeletal system of B. gargarizans during metamorphosis. A total of 137,264 unigenes were assembled and 44,035 unigenes were annotated. 10,352 differentially expressed genes (DEGs) were further extracted among four key developmental stages. In addition, 28 endochondral ossification related genes were found by searching for DEG libraries in B. gargarizans. Of the 28 genes, 10 genes were validated using RT-qPCR.

CONCLUSIONS

The exquisite coordination of the 28 genes is essential for regulation of endochondral ossification during B. gargarizans metamorphosis.

GENERAL SIGNIFICANCE

The present study will not only provide an invaluable genomic resource and background for further research of endochondral ossification in amphibians but will also aid in enhancing our understanding of the evolution of terrestrial vertebrates.

Bisphenol A alternatives bisphenol S and bisphenol F interfere with thyroid hormone signaling pathway in vitro and in vivo

The wide use of the alternatives to bisphenol A (BPA) has raised concerns about their potential toxicities. Considering the disrupting activity of BPA on thyroid hormone (TH) signaling, we investigated whether bisphenol S (BPS) and bisphenol F (BPF), two leading alternatives, could interfere with TH signaling pathway using a series of assays in vitro and in vivo. In the fluorescence competitive binding assay, we found BPS and BPF, like BPA, bound to TH receptors (TRα and TRβ), with the binding potencies an order of magnitude lower than BPA (BPA > BPF > BPS). Molecular docking data also show their binding potencies to TRs. In the coactivator recruitment assay, BPS and BPF recruited coactivator to TRβ but not TRα, with weaker potencies than BPA. Correspondingly, agonistic actions of the three bisphenols in the absence or presence of T3 were observed in the TR-mediated reporter gene transcription assay. Also, all the three bisphenols induced TH-dependent GH3 cell proliferation, whereas BPA and BPF inhibited T3 induction in the presence of T3. As for in vivo assay, the three bisphenols like T3 induced TH-response gene transcription in Pelophylax nigromaculatus tadpoles, but in the presence of T3 altered T3-induced gene transcription in a biphasic concentration-response manner. These results for the first time demonstrate that BPS and BPF, like BPA, have potential to interfere with TH signaling pathway, i.e., they generally activate TH signaling in the absence of T3, but in the presence of TH, display agonistic or/and antagonistic actions under certain condition. Our study highlights the potential risks of BPS and BPF as BPA alternatives.

Protective effects of Cervus nippon Temminck velvet antler polypeptides against MPP+‑induced cytotoxicity in SH‑SY5Y neuroblastoma cells

The aim of the present study was to examine the protective effects and mechanism of sika deer (Cervus nippon Temminck) velvet antler polypeptides (VAPs) against MPP⁺ exposure in the SH-SY5Y human neuroblastoma cell line. MPP⁺ cytotoxicity and the protective effects of VAPs on the SH-SY5Y cells were determined using an MTT assay. Cell apoptosis and mitochondrial membrane potential were detected using Hoechst 33342 and Rhodamine123 staining, respectively. Endoplasmic reticulum (ER) stress-related reactive oxygen species (ROS) production in the SH-SY5Y cells was detected using 2′,7′-dichlorodihydrofluorescein diacetate fluorescent probes. The expression levels of proteins, including caspase-12, glucose regulated protein 78 (GRP78), CCAAT/enhancer binding protein homologous protein (CHOP) and phosphorylated c-Jun N-terminal kinase (p-JNK) were detected using western blot analysis. The results showed that the half inhibitory concentration of MPP⁺ at 72 h was 120.9 µmol/l, and that 62.5, 125, and 250 µg/ml concentrations of VAPs protected the SH-SY5Y cells under MPP⁺ exposure. When exposed to 120.9 µmol/l MPP⁺, changes in cell nucleus morphology, mitochondrial membrane potential and intracellular ROS were observed. VAPs at concentrations of 62.5, 125, 250 µg/ml reduced this damage. Western blot analysis showed that protein expression levels of caspase-12, GRP78 and p-JNK were upregulated in the SH-SY5Y cells exposed to 120.9 µmol/l MPP⁺ for 72 h. In addition, 62.5, 125, and 250 µg/ml VAPs downregulated the expression levels of caspase-12 and p-JNK in a concentration- dependent manner, particularly the p-JNK pathway. The effects of VAPs on GRP78 and CHOP were weak. In conclusion, MPP⁺-induced SH-SY5Y cell death may be linked to ER stress. VAPs prevented MPP⁺-induced SH-SY5Y cell death by affecting the p-JNK pathway and caspase-12-mediated apoptosis. These findings assist in understanding the mechanism underlying the protective effect of VAPs on neurons.

Transcriptome profiles of metamorphosis in the ornamented pygmy frog Microhyla fissipes clarify the functions of thyroid hormone receptors in metamorphosis

Anuran metamorphosis is an excellent system in which to study postembryonic development. Studies on Xenopus (Mesobatrachia) show that thyroid hormone receptors (TRs) regulate metamorphosis in a ligand-dependent manner by coordinating the action of hundreds of genes. However, whether this mechanism is conserved among amphibians is still unknown. To understand the molecular mechanism of this universal phenomenon, we report the transcriptional profiles of the three key developmental stages in Microhyla fissipes (Neobatrachia): premetamorphosis (PM), metamorphic climax (MC) and completion of metamorphosis (CM). In total, 2,293 differentially expressed genes were identified from comparisons of transcriptomes, and these genes showed stage-specific expression patterns. Unexpectedly, we found that TRα was highly expressed in Xenopus laevis and Bufo gargarizans at premetamorphosis but showed low expression in M. fissipes. In contrast, TRβ was highly expressed during metamorphosis in M. fissipes and X. laevis. This result may imply that TRβ is essential for initiating metamorphosis, at least in M. fissipes. Thus, our work not only identifies genes that are likely to be involved in Neobatrachia metamorphosis but also clarifies the roles of unliganded TRα in regulating tadpole growth and timing of metamorphosis, which may be conserved in anurans, and the role of liganded TRβ in launching metamorphosis.

Effects of copper on growth, metamorphosis and endocrine disruption of Bufo gargarizans larvae

Chinese toad (Bufo gargarizans) tadpoles were exposed to copper (1, 6.4, 32 and 64μgL(-1) copper) from the beginning of larval period through completion of metamorphosis. We examined the effects of chronic copper exposure on mortality, growth, time to metamorphosis, tail resorption time, body size at the metamorphic climax (Gs 42) and completion of metamorphosis (Gs 46) and thyroid gland histology. In addition, type 2 and 3 iodothyronine deiodinase (Dio2 and Dio3), thyroid hormone receptors (TRα and TRβ) mRNA levels were also measured to assess disruption of TH synthesis. Our result showed that 6.4-64μgL(-1) copper concentration increased the mortality and inhibited the growth of B. gargarizans tadpoles. In addition, significant reduction in size at Gs 42 and a time delay to Gs 42 were observed at 6.4-64μgL(-1) copper treatments. Moreover, histological examinations have clearly revealed that 64μgL(-1) copper caused follicular cell hyperplasia in thyroid gland. According to real-time PCR results, exposure to 32 and 64μgL(-1) copper significantly up-regulated mRNA expression of Dio3, but down-regulated mRNA expression of TRα and TRβ mRNA level. We concluded that copper delayed amphibian metamorphosis through changing mRNA expression of Dio3, TRα and TRβ, which suggests that copper might have the endocrine-disrupting effect.

Low concentrations of dihydrotestosterone induce female-to-male sex reversal in the frog Pelophylax nigromaculatus

Previous studies have demonstrated that some amphibian species can be sex-reversed by high concentrations of androgens. Little attention has focused on the effects of androgenic endocrine-disrupting chemicals (EDCs) on amphibians. The present study aimed to investigate the effects of lower concentrations of the androgenic EDC 5α-dihydrotestosterone (DHT) on gonadal differentiation and development in Pelophylax nigromaculatus, a true frog distributed widely in East Asia. Tadpoles at Gosner stage 24/25 were exposed to nominal concentrations of 40 ng/L, 400 ng/L, and 4000 ng/L DHT to complete metamorphosis. In all DHT treatment groups, males and ambiguous sexes were identified based on gonadal morphology, whereas no females were found; thus, all treatment groups exhibited male-skewed ratios compared with the control group. Gonadal histological examination revealed that ambiguous sexes displayed overall testicular structure with certain ovarian characteristics, demonstrating that DHT-induced sex-ambiguous gonads were incomplete ovary-to-testis reversals (IOTTRs). The expression levels of some ovary-biased genes in the IOTTRs were significantly higher than in the control testes but lower than in the control ovaries. These results show that low concentrations of DHT induced complete or incomplete female-to-male sex reversal in P. nigromaculatus, and incomplete sex reversal retained certain ovarian characteristics not only at gonadal morphological and histological levels but also at the molecular level. They present study highlights potential risks of DHT and other androgenic EDCs for P. nigromaculatus.

A screening assay for thyroid hormone signaling disruption based on thyroid hormone-response gene expression analysis in the frog Pelophylax nigromaculatus

Amphibian metamorphosis provides a wonderful model to study the thyroid hormone (TH) signaling disrupting activity of environmental chemicals, with Xenopus laevis as the most commonly used species. This study aimed to establish a rapid and sensitive screening assay based on TH-response gene expression analysis using Pelophylax nigromaculatus, a native frog species distributed widely in East Asia, especially in China. To achieve this, five candidate TH-response genes that were sensitive to T3 induction were chosen as molecular markers, and T3 induction was determined as 0.2 nmol/L T3 exposure for 48 hr. The developed assay can detect the agonistic activity of T3 with a lowest observed effective concentration of 0.001 nmol/L and EC50 at around 0.118-1.229 nmol/L, exhibiting comparable or higher sensitivity than previously reported assays. We further validated the efficiency of the developed assay by detecting the TH signaling disrupting activity of tetrabromobisphenol A (TBBPA), a known TH signaling disruptor. In accordance with previous reports, we found a weak TH agonistic activity for TBBPA in the absence of T3, whereas a TH antagonistic activity was found for TBBPA at higher concentrations in the presence of T3, showing that the P. nigromaculatus assay is effective for detecting TH signaling disrupting activity. Importantly, we observed non-monotonic dose-dependent disrupting activity of TBBPA in the presence of T3, which is difficult to detect with in vitro reporter gene assays. Overall, the developed P. nigromaculatus assay can be used to screen TH signaling disrupting activity of environmental chemicals with high sensitivity.

Low concentrations of 17β-trenbolone induce female-to-male reversal and mortality in the frog Pelophylax nigromaculatus

Trenbolone, as a growth promoter in animal agriculture, has become an environmental androgen in surface water. Here, we aimed to reveal the effects of 17β-trenbolone on survival, growth, and gonadal differentiation in the frog Pelophylax nigromaculatus, which is widespread in East Asia and undergoing population decline. P. nigromaculatus tadpoles were exposed to 17β-trenbolone (0.1, 1, 10 μg/L) from Gosner stage 24/25 to complete metamorphosis. We found that 17β-trenbolone resulted in significantly high mortality in a concentration-dependent manner, with a decrease in body weight in the high concentration group compared with the solvent control. Based on gross gonadal morphology, no females were observed, instead of about 15% ambiguous sexes and 85% males, in all 17β-trenbolone treatment groups. Like normal testes, the gonads with sex-ambiguous morphology exhibited testicular histology, showing that the sex-ambiguous gonads were incomplete ovary-to-testis reversals (IOTTRs) with certain ovarian morphological features. In the IOTTRs, the transcriptional levels of ovary-biased genes decreased drastically relative to normal ovaries, and even declined to the levels in normal testes. These observations confirmed that all test concentrations of 17β-trenbolone resulted in 100% sex reversal, although some sex-reversed testes retained some ovarian characteristics at the morphological level. To our knowledge, this is the first report strongly demonstrating that trenbolone can cause female-to-male reversal in amphibians. Given that the lowest concentration tested is environmentally relevant, our study highlights the risks of trenbolone and other environmental androgens for P. nigromaculatus and other amphibians, in particular the species with high sensitivity of gonadal differentiation to androgenic chemicals.