Waterborne exposure to microcystin-leucine arginine induces endocrine disruption and gonadal dysplasia of Pelophylax nigromaculatus tadpoles via the hypothalamic-pituitary-gonadal-liver axis

Microcystin-LR Induces Lipid Metabolism Disorder in Pelophylax nigromaculatus Tadpoles via the Gut-Liver Axis

Disruption of lipid homeostasis in aquatic animals poses serious health risks, including tissue damage and systemic metabolic dysfunction. The precise mechanisms by which microcystin-LR, a potent cyanotoxin, disrupts lipid metabolism in amphibian tadpoles remain unclear. In this study, tadpoles (Pelophylax nigromaculatus) were exposed to MC-LR and fecal microbiota transplantation (FMT) experiments were performed to investigate whether or how MC-LR at environmental concentrations interfered with tadpole lipid metabolism from the perspective of the gut microbiota-gut-liver axis. Following exposure, the liver exhibited significant inflammation, hypertrophy, and fibrosis, accompanied by elevated serum lipid levels. Furthermore, the expression levels of the farnesoid X receptor (FXR), a nuclear receptor, were significantly downregulated. Molecular docking and molecular dynamics simulations indicated a strong and stable binding between FXR and MC-LR. Moreover, MC-LR suppressed liver FXR expression or activity, triggering: (1) upregulation of sterol regulatory element-binding protein 1 (SREBP1)-mediated triglyceride (TG) synthesis, (2) inhibition of free fatty acid (FFA) β-oxidation, and (3) activation of SREBP2-dependent bile acid biosynthesis. Moreover, MC-LR altered the composition of gut microbiota and specific bile acid levels (e.g., taurocholic acid and glycochenodeoxycholic acid) in the gut, thereby interfering with hepatic lipid metabolism, as evidenced by FMT-induced hepatic lipid accumulation in recipient tadpoles. These findings identify FXR as a potentially key molecular target for MC-LR and suggest that changes in bile acid levels of intestinal microbiota metabolism also may be an important pathway driving hepatic lipid dysregulation in amphibians exposed to environmental concentrations of MC-LR.

The accumulation of Microcystin-LR in the gonads of Pelophylax nigromaculatus during the reproductive periods induces reproductive endocrine disorders in their offspring

Microcystin-LR's reproductive (reproductive and non-reproductive periods) and transgenerational toxicity in amphibians remains poorly understood. Adult Pelophylax nigromaculatus in reproductive and non-reproductive periods were exposed to MC-LR to investigate whether there are differences in the effects of MC-LR on reproductive endocrinology between reproductive and non-reproductive periods of amphibians. Furthermore, cross-mating experiments between MC-LR-exposed and non-exposed frogs in reproductive periods were conducted to explore transgenerational effects. Compared to P. nigromaculatus without MC-LR exposure, exposure to MC-LR resulted in an increase in testosterone synthesis levels and a decrease in estradiol synthesis levels during the reproductive period, but a decrease in testosterone synthesis levels and an increase in estradiol synthesis levels during the non-reproductive period. High lipid contents in the gonads during the reproductive period substantially enriched MC-LR, increasing DNA damage and methylation levels. This may be the reason for the observed opposite trend in sex hormone synthesis levels compared to the non-reproductive period. Additionally, the hypothalamic-pituitary-gonadal-liver axis in F1 tadpoles was disrupted, leading to gonadal dysgenesis, particularly in the ovaries. The observed transgenerational reproductive toxicity may be attributed to decreased gamete quality, transgenerational transfer of MC-LR, and increased DNA methylation level. This study provides novel insights into the differential reproductive endocrine disruption effects of MC-LR during different periods and highlights its transgenerational reproductive toxicity for the first time, underscoring the need for further research on MC-LR's impact on amphibian population dynamics.

Perfluorobutanesulfonate Induces Hypothalamic-Pituitary-Gonadal Axis Disruption and Gonadal Dysplasia of Lithobates catesbeianus Tadpoles

It is uncertain whether exposure to environmental concentrations of perfluorobutanesulfonate (PFBS) disrupts the reproductive endocrine system in amphibian tadpoles. In this study, tadpoles (Lithobates catesbeianus) in G26 stage were treated with different levels of PFBS (0, 1, 3, and 10 μg/L) for 60 days to investigate whether and how PFBS affects the reproductive endocrine system and gonadal development in tadpoles. Tadpole testes exhibited structural damage to germ cells and significantly fewer spermatogonia following PFBS exposure, but the sex ratio remained unaffected. Further, PFBS exposure downregulated transcripts of genes associated with ovarian (figla and nobox) and testicular (sox9 and dmrt1) development in tadpoles. Encoding gonadotropin hormone genes were transcriptionally upregulated in the pituitary, and serum gonadotropins (FSH and LH) were elevated. Genes related to testosterone synthesis were transcriptionally upregulated, and serum testosterone concentrations were raised. The transcription of the cyp19a1 gene, which is involved in the synthesis of estradiol (E2), was downregulated, leading to decreased levels of serum E2. Furthermore, the transcript level of the vitellogenin gene was downregulated in the liver. Thus, PFBS exposure appears to disrupt the hypothalamic-pituitary-gonadal-liver axis in tadpoles, subsequently impacting gonadal development. The findings of this study indicate that environmental concentrations of PFBS threaten the reproductive endocrine system in amphibians for the first time. This provides important insights for further investigation into the risk that PFBS poses to the stability of the amphibian population.

Alteration of intestinal microbiota-intestinal barrier interaction interferes with intestinal health after microcystin-LR exposure in Lithobates catesbeianus tadpoles

There remains uncertainty regarding the influence of microcystin-leucine arginine (MC-LR) on amphibian intestinal health, specifically how MC-LR interferes with intestinal microbiota following exposure to environmental concentrations. In this study, Lithobates catesbeianus tadpoles were exposed to varying MC-LR concentrations (0, 0.5, and 2 µg/L) over a 30-day period. The aim was to investigate how altered interactions between tadpole intestinal microbiota and the intestinal barrier influence intestinal health following MC-LR exposure. Following exposure to the MC-LR at low ambient concentrations, tadpole intestinal tissue was damaged. It had increased permeability, reduced pathogen inhibition capacity, and impaired digestive function. Additionally, there was a significant increase in lipopolysaccharide content and upregulation of downstream response genes, including TLR4, MyD88, and NF-κB, within the intestinal tissue. Therefore, eosinophils' count and pro-inflammatory cytokines' expression increased. In addition, MC-LR exposure induced oxidative stress and mitochondrial structural damage by increasing the levels of reactive oxygen species in intestinal tissue. CytoC and Bax transcription, as well as caspase 9 and caspase 3 activities, increased significantly. Significant downregulation of Bcl-2 transcription promoted apoptosis in tadpole intestinal cells. MC-LR exposure disrupted intestinal microbiota and metabolism in tadpoles. Correlation analysis revealed a strong association between intestinal microbiota and oxidative stress, inflammation, immunity, and tissue damage in the intestine. Conclusively, this study provides the first demonstration that MC-LR significantly affects amphibian intestinal microbiota, highlighting tadpoles' susceptibility to environmental risks posed by MC-LR.

Long-term nanoplastics exposure contributes to impaired steroidogenesis by disrupting the hypothalamic-testis axis: Evidence from integrated transcriptome and metabolome analysis

Cumulative evidence suggested that nanoplastics (NPs) cause male toxicity, but the mechanisms of which are still misty. Steroidogenesis is a key biological event that responsible for maintaining reproductive health. However, whether dysregulated steroidogenesis is involved in NPs-induced impaired male reproductive function and the underlying mechanism remains unclear. In our study, Balb/c mice were continuously exposed to pristine-NPs or NH2-NPs for 12 weeks, spanning the puberty and adult stage. Upon the long-term NPs treatment, the hypothalamus and testis were subjected to transcriptome and metabolome analysis. And the results demonstrated that both primitive-NPs and NH2-NPs resulted in impaired spermatogenesis and steroidogenesis, as evidenced by a significant reduction in sperm quality, testosterone, FSH, and LH. The expression of genes involved in hypothalamic-pituitary-testis (HPT) axis, such as Kiss-1 and Cyp17a1 that encoded the key steroid hormone synthetase, was also diminished. Furthermore, the phosphatidylcholine and pantothenic acid that mainly enriched in glycerophospholipid metabolism were significantly reduced in the testis. Comprehensive analysis of the transcriptome and metabolome indicated that down-regulated Cyp17a1 was associated with decreased metabolites phosphatidylcholine and pantothenic acid. Overall, we speculate that the disturbed HPT axis induced by long-term NPs contributes to disordered glycerophospholipid metabolism and subsequently impaired steroidogenesis. Our findings deepen the understanding of the action of the mechanism responsible for NPs-induced male reproductive toxicology.

Effects of Harmful Algal Blooms on Amphibians and Reptiles are Under-Reported and Under-Represented

Harmful algal blooms (HABs) are a persistent and increasing problem globally, yet we still have limited knowledge about how they affect wildlife. Although semi-aquatic and aquatic amphibians and reptiles have experienced large declines and occupy environments where HABs are increasingly problematic, their vulnerability to HABs remains unclear. To inform monitoring, management, and future research, we conducted a literature review, synthesized the studies, and report on the mortality events describing effects of cyanotoxins from HABs on freshwater herpetofauna. Our review identified 37 unique studies and 71 endpoints (no-observed-effect and lowest-observed-effect concentrations) involving 11 amphibian and 3 reptile species worldwide. Responses varied widely among studies, species, and exposure concentrations used in experiments. Concentrations causing lethal and sublethal effects in laboratory experiments were generally 1 to 100 µg/L, which contains the mean value of reported HAB events but is 70 times less than the maximum cyanotoxin concentrations reported in the environment. However, one species of amphibian was tolerant to concentrations of 10,000 µg/L, demonstrating potentially immense differences in sensitivities. Most studies focused on microcystin-LR (MC-LR), which can increase systemic inflammation and harm the digestive system, reproductive organs, liver, kidneys, and development. The few studies on other cyanotoxins illustrated that effects resembled those of MC-LR at similar concentrations, but more research is needed to describe effects of other cyanotoxins and mixtures of cyanotoxins that commonly occur in the environment. All experimental studies were on larval and adult amphibians; there were no such studies on reptiles. Experimental work with reptiles and adult amphibians is needed to clarify thresholds of tolerance. Only nine mortality events were reported, mostly for reptiles. Given that amphibians likely decay faster than reptiles, which have tissues that resist decomposition, mass amphibian mortality events from HABs have likely been under-reported. We propose that future efforts should be focused on seven major areas, to enhance our understanding of effects and monitoring of HABs on herpetofauna that fill important roles in freshwater and terrestrial environments. Environ Toxicol Chem 2024;43:1936-1949. Published 2024. This article is a U.S. Government work and is in the public domain in the USA. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Toxicity comparison and risk assessment of two chlorinated organophosphate flame retardants (TCEP and TCPP) on Polypedates megacephalus tadpoles

Tris(2-chloroethyl) phosphate (TCEP) and tris(1‑chloro-2-propyl) phosphate (TCPP) are widely used as chlorinated organophosphate flame retardants (OPFRs) due to their fire-resistance capabilities. However, their extensive use has led to their permeation and pollution in aquatic environments. Using amphibians, which are non-model organisms, to test the toxic effects of OPFRs is relatively uncommon. This study examined the acute and chronic toxicity differences between TCEP and TCPP on Polypedates megacephalus tadpoles and evaluated the potential ecological risks to tadpoles in different aquatic environments using the risk quotient (RQ). In acute toxicity assay, the tadpole survival rates decreased with increased exposure time and concentrations, with TCEP exhibiting higher LC50 values than TCPP, at 305.5 mg/L and 70 mg/L, respectively. In the chronic assay, prolonged exposure to 300 μg/L of both substances resulted in similar adverse effects on tadpole growth, metamorphosis, and hepatic antioxidant function. Based on RQ values, most aquatic environments did not pose an ecological risk to tadpoles. However, the analysis showed that wastewater presented higher risks than rivers and drinking water, and TCPP posed a higher potential risk than TCEP in all examined aquatic environments. These findings provide empirical evidence to comprehend the toxicological effects of OPFRs on aquatic organisms and to assess the safety of aquatic environments.

Pleurotus ostreatus polysaccharide-mediated modulation of skin damage caused by microcystin-LR in tadpoles

In this study, we aimed to evaluate the effect of dietary supplementation with edible mushroom (Pleurotus ostreatus)-derived polysaccharides on microcystin leucine-arginine (MC-LR)-induced skin damage in Pelophylax nigromaculatus tadpoles. Tadpoles were exposed to 1 μg/L daily MC-LR, with or without 5.0 g/kg of dietary P. ostreatus polysaccharides, for 30 days. P. ostreatus polysaccharide supplementation significantly increased the dermal collagen fibrils, increased tight junction protein gene expression, decreased the amount of MC-LR accumulation in skin tissues, attenuated oxidative stress, downregulated apoptosis-associated gene transcription, decreased eosinophil numbers, and downregulated transcription of inflammation-related genes (e.g. TLR4, NF-κB, and TNF-α). The composition of the skin commensal microbiota of MC-LR-exposed tadpoles supplemented with P. ostreatus polysaccharides was similar to that of the no-treatment control group. Lipopolysaccharide (LPS) content was positively correlated with the abundance of Gram-negative bacteria, including Chryseobacterium and Thauera. Therefore, P. ostreatus polysaccharides may alleviate MC-LR-induced skin barrier damage in tadpoles in two ways: 1) attenuation of oxidative stress-mediated apoptosis mediated by increased glutathione (GSH) content and total superoxide dismutase activity; and 2) alteration of the skin commensal microbiota composition to attenuate the LPS/Toll-like receptor 4 inflammatory pathway response. Furthermore, P. ostreatus polysaccharides may increase skin GSH synthesis by promoting glycine production via the gut microbiota and may restore the MC-LR-damaged skin resistance to pathogenic bacteria by increasing antimicrobial peptide transcripts and lysozyme activity. This study highlights for the first time the potential application of P. ostreatus polysaccharides, an ecologically active substance, in mitigating the skin damage induced by MC-LR exposure, and may provide new insights for its further development in aquaculture.