Microcystin-leucine arginine exposure induced intestinal lipid accumulation and MC-LR efflux disorder in Lithobates catesbeianus tadpoles

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.

Integration of Fatty Acid-Targeted Metabolome and Transcriptomics Reveals the Mechanism of Chronic Environmental Microcystin-LR-Induced Hepatic Steatosis

Microcystin-LR (MC-LR) is a toxin that causes hepatic steatosis. Our previous study found that exposure to 60 μg/L MC-LR for 9 months resulted in liver lipid accumulation, but the underlying mechanisms remain elusive. Herein, for the first time, fatty acid-targeted metabolome and RNA-seq were combined to probe the effect and mechanism of chronic (12-month) MC-LR treatment on mice lipid metabolism at environmental-related levels (1, 60, and 120 μg/L). It was found that MC-LR dose-dependently raised serum and liver lipid levels. The total cholesterol (TC) levels in the liver were significantly increased following treatment with 1 μg/L MC-LR (equivalent to 0.004 μ/L in human). Treatment with 60 and 120 μg/L MC-LR significantly elevated TC and triglyceride (TG) levels in both serum and liver. Serum fatty acid-targeted metabolome analysis demonstrated that exposure to 1, 60, and 120 μg/L MC-LR caused significant alterations in the fatty acid profile. Chronic 1, 60, and 120 μg/L MC-LR treatment significantly increased serum polyunsaturated fatty acids (PUFAs), including conjugated linoleic acid and eicosapentaenoic acid, which positively correlated with serum or liver TG levels. Chronic exposure to 120 μg/L MC-LR led to a significant decrease in the accumulation of saturated fatty acids, including citramalic acid, pentadecanoic acid, and docosanoic acid, which were negatively correlated with serum or liver lipid levels. These findings suggested that 1 μg/L MC-LR exposure caused mild lipid metabolism disruption, while 60 and 120 μg/L MC-LR treatment resulted in pronounced hepatic steatosis in mice. Transcriptome analysis revealed that chronic environmental MC-LR treatment regulated the expression of genes involved in the phosphatidylinositol 3-kinase (PI3K) complex and fatty acid metabolism. Western blotting and RT-qPCR confirmed that chronic environmental MC-LR exposure activated the PI3K/AKT/mTOR signaling pathway, the downstream of fads3 gene that participates in fatty acid desaturation was upregulated, fatty acid degradation-related genes, including acsl1, acsl4, and ehhadh were inhibited, and lipid transport-related genes, including slc27a4 and apol7a, were promoted. Thus, chronic environmental MC-LR exposure boosts hepatic steatosis. Our work indicated that the limit concentration of 1 μg/L MC-LR in human drinking water for safety needs to be discussed. The study provides the first evidence of the fatty acid profile and gene changes and gains new insights into the mechanisms of chronic environmental MC-LR treatment-induced hepatic steatosis.

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.

Chronic exposure to low-dose MC-LR induces ileal inflammation in mice through the PI3K/AKT/mTOR pathway

The global phenomenon of cyanobacterial bloom pollution is spreading globally due to climate change and eutrophication. It is well established that harmful cyanobacteria produce a wide range of toxins including microcystin-LR (MC-LR), a cyclic heptapeptide toxin known to damage various organs. The intestinal tract is the main site of MC-LR absorption and one of the targets susceptible to toxicity. Currently, studies on the enterotoxic effects of MC-LR predominantly focused on the colorectum, with limited investigations addressing the impact of microcystins on the small intestine. Therefore, the aim of our study was to examine the impact of chronic 9-month exposure of mice to low-dose 120 μg/L MC-LR in drinking water on ileal inflammation and potential mechanisms underlying these effects. Our findings showed that in mice chronically administered with low-dose MC-LR disorganized intestinal epithelial cells, lymphocytic infiltration and disturbed crypt arrangement were detected. The results of qPCR and Western blot demonstrated that, in comparison to control, the mRNA expression levels of pro-inflammatory factors IL-6, IL-17, IL-18, and IFN-γ were markedly elevated in the ileal tissue of mice treated with MC-LR, associated with significant increases in protein expression levels of p-PI3K, p-AKT, and p-mTOR. Taken together, evidence indicates that MC-LR induces ileal inflammation and histopathological damage involved activation of the PI3K/AKT/mTOR signaling pathway.

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.

Effects of L-Leu-L-Leu peptide on growth, proliferation, and apoptosis in broiler intestinal epithelial cells

Small peptides are nutrients and bioactive molecules that have dual regulatory effects on nutrition and physiology. They are of great significance for maintaining the intestinal health and production performance of broilers. We here cultured the primary small intestinal epithelial cells (IEC) of chicken in a medium containing L-Leu (Leu) and L-Leu-L-Leu (Leu-Leu) for 24 h. The untreated cells were considered as the control group. The growth, proliferation, and apoptosis of IEC were examined. By combining RNA-seq and label-free sequencing technology, candidate genes, proteins, and pathways related to the growth, proliferation, and apoptosis of IEC were screened. Immunofluorescence detection revealed that the purity of the isolated primary IEC was >90%. The Leu-Leu group significantly promoted IEC growth and proliferation and significantly inhibited IEC apoptosis, and the effect was better than those of the Leu and control groups. Using transcriptome sequencing, four candidate genes, CCL20, IL8L1, IL8, and IL6, were screened in the Leu group, and one candidate gene, IL8, was screened in the Leu-Leu group. Two candidate genes, IL6 and RGN, were screened in the Leu-Leu group compared with the Leu group. Nonquantitative proteomic marker sequencing results revealed that through the screening of candidate proteins and pathways, found one growth-related candidate protein PGM3 and three proliferation-related candidate proteins RPS17, RPS11, and RPL23, and two apoptosis-related candidate proteins GPX4 and PDPK1 were found in the Leu-Leu group compared with Leu group. In short, Leu-Leu could promote IEC growth and proliferation and inhibit IEC apoptosis. On combining transcriptome and proteome sequencing technologies, multiple immune- and energy-related regulatory signal pathways were found to be related to IEC growth, proliferation, and apoptosis. Three candidate genes of IL8, IL6, and RGN were identified, and six candidate proteins of PGM3, RPS17, RPS11, RPL23, GPX4, and PDPK1 were involved in IEC growth, proliferation, and apoptosis. The results provide valuable data for preliminarily elucidating small peptide-mediated IEC regulation pathways, improving the small peptide nutrition theoretical system, and establishing small peptide nutrition regulation technology.

Microcystin-LR and polystyrene microplastics jointly lead to hepatic histopathological damage and antioxidant dysfunction in male zebrafish

The co-occurrence of cyanobacterial blooms and nano-microplastic pollution in the water is becoming an emerging risk. To assess the combined hepatotoxicity of microcystin-LR (MC-LR) and polystyrene microplastics (PSMPs) on zebrafish (Danio rerio), male adult zebrafish were exposed to single MC-LR (0, 1, 5, 25 μg/L) and a mixture of MC-LR and PSMPs (100 μg/L). After 60 d exposure, the results indicated that PSMPs significantly increased the MC-LR bioaccumulation in the livers in contrast to the single 25 μg/L MC-LR treatment group. Moreover, the severity of hepatic pathological lesions was aggravated in the MC-LR + PSMPs treatment groups, which were mainly characterized by cellular vacuolar degeneration, swollen hepatocytes, and pyknotic nucleus. The ultrastructural changes also proved that PSMPs combined with MC-LR could enhance the swollen mitochondria and dilated endoplasmic reticulum. The biochemical results, including increased malondialdehyde (MDA) and decreased glutathione (GSH), indicated that PSMPs intensified the MC-LR-induced oxidative damage in the combined treatment groups. Concurrently, alterations of sod1 and keap1a mRNA levels also confirmed that PSMPs together with MC-LR jointly lead to enhanced oxidative injury. Our findings demonstrated that PSMPs enhanced the MC-LR bioavailability by acting as a vector and exacerbating the hepatic injuries and antioxidant dysfunction in zebrafish.

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.

Lactobacillus fermentum Alleviates the Colorectal Inflammation Induced by Low-Dose Sub-Chronic Microcystin-LR Exposure

Microcystin-LR (MC-LR) contamination is a worldwide environmental problem that poses a grave threat to the water ecosystem and public health. Exposure to MC-LR has been associated with the development of intestinal injury, but there are no effective treatments for MC-LR-induced intestinal disease. Probiotics are "live microorganisms that are beneficial to the health of the host when administered in sufficient quantities". It has been demonstrated that probiotics can prevent or treat a variety of human diseases; however, their ability to mitigate MC-LR-induced intestinal harm has not yet been investigated. The objective of this study was to determine whether probiotics can mitigate MC-LR-induced intestinal toxicity and its underlying mechanisms. We first evaluated the pathological changes in colorectal tissues using an animal model with sub-chronic exposure to low-dose MC-LR, HE staining to assess colorectal histopathologic changes, qPCR to detect the expression levels of inflammatory factors in colorectal tissues, and WB to detect the alterations on CSF1R signaling pathway proteins in colorectal tissues. Microbial sequencing analysis and screening of fecal microorganisms differential to MC-LR treatment in mice. To investigate the role of microorganisms in MC-LR-induced colorectal injury, an in vitro model of MC-LR co-treatment with microorganisms was developed. Our findings demonstrated that MC-LR treatment induced an inflammatory response in mouse colorectal tissues, promoted the expression of inflammatory factors, activated the CSF1R signaling pathway, and significantly decreased the abundance of Lactobacillus. In a model of co-treatment with MC-LR and Lactobacillus fermentum (L. fermentum), it was discovered that L. fermentum substantially reduced the incidence of the colorectal inflammatory response induced by MC-LR and inhibited the protein expression of the CSF1R signaling pathway. This is the first study to suggest that L. fermentum inhibits the CSF1R signaling pathway to reduce the incidence of MC-LR-induced colorectal inflammation. This research may provide an excellent experimental foundation for the development of strategies for the prevention and treatment of intestinal diseases in MC-LR.

Microcystin-leucine arginine causes brain injury and functional disorder in Lithobates catesbeianus tadpoles by oxidative stress and inflammation

Microcystin-leucine arginine (MC-LR) is a toxin commonly found in eutrophic waters worldwide, but its potential effects on amphibian brain toxicity and exposure mechanisms are unclear. In this study, Lithobates catesbeianus tadpoles were exposed to MC-LR for 30 days at realistic ambient concentrations (0, 0.5, and 2 µg/L) to reveal its effects on brain health. The MC-LR bioaccumulation in the brain increased in dependence on the concentration of MC-LR exposure. Exposure to 0.5 and 2 µg/L MC-LR resulted in a significant down-regulation of the expression of structural components of the blood-brain barrier (CLDN1), while the expression of genes associated with inflammation (NLRP3, TNF, IL-1β, and CXCL12) was significantly up-regulated with increased number of eosinophils. In the hippocampal and hypothalamic regions, the number of vacuolated neuropils increased with increasing MC-LR exposure concentration, while the expression of genes associated with neuronal development (LGALS1, CACNA2D2, and NLGN4X) and neurotransmitter transmission (SLC6A13 and AChE) was significantly down-regulated. Moreover, the levels of neurotransmitters (5-HT, glutamate, GABA, and ACh) were significantly reduced. These results provide strong evidence that MC-LR exposure at realistic ambient concentrations of 0.5 and 2 µg/L can break the blood-brain barrier and raise the accumulation of MC-LR in the brain tissue, causing structural damage and functional disorder to brain neurons. Further, based on transcriptomic and biochemical analysis, it was revealed that MC-LR exposure induces DNA damage through oxidative stress and may be an important pathway causing brain structural damage and functional disorder. Overall, this study demonstrates the significant effects of MC-LR on the brain tissue of amphibians, highlighting the sensitivity of amphibians to MC-LR.

Microcystins Exposure Associated with Blood Lipid Profiles and Dyslipidemia: A Cross-Sectional Study in Hunan Province, China

Increasing evidence from experimental research suggests that exposure to microcystins (MCs) may induce lipid metabolism disorder. However, population-based epidemiological studies of the association between MCs exposure and the risk of dyslipidemia are lacking. Therefore, we conducted a population-based cross-sectional study involving 720 participants in Hunan Province, China, and evaluated the effects of MCs on blood lipids. After adjusting the lipid related metals, we used binary logistic regression and multiple linear regression models to examine the associations among serum MCs concentration, the risk of dyslipidemia and blood lipids (triglyceride (TG), total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C) and low-density lipoprotein cholesterol (LDL-C)). Moreover, the additive model was used to explore the interaction effects on dyslipidemia between MCs and metals. Compared to the lowest quartile of MCs exposure, the risk of dyslipidemia [odds ratios (OR) = 2.27, 95% confidence interval (CI): 1.46, 3.53] and hyperTG (OR = 3.01, 95% CI: 1.79, 5.05) in the highest quartile was significantly increased, and showed dose-response relationships. MCs were positively associated with TG level (percent change, 9.43%; 95% CI: 3.53%, 15.67%) and negatively associated with HDL-C level (percent change, -3.53%; 95% CI: -5.70%, -2.10%). In addition, an additive antagonistic effect of MCs and Zn on dyslipidemia was also reported [relative excess risk due to interaction (RERI) = -1.81 (95% CI: -3.56, -0.05)], and the attributable proportion of the reduced risk of dyslipidemia due to the antagonism of these two exposures was 83% (95% CI: -1.66, -0.005). Our study first indicated that MCs exposure is an independent risk factor for dyslipidemia in a dose-response manner.

Comprehensive proteomic and phosphoproteomic reveal that Microcystin-LR contributed to the malignant progression of gastric cancer by estrogenic potency

The widespread cyanotoxins in drinking water pose a threat to public health induced by Microcystins (MCs). MC-LR, a predominant toxic form of MCs, has been found to play critical roles in cancer progression. The role of MC-LR in hepatocarcinogenesis has attracted extensive attention. However, as a critical digestive organ, the precise mechanism of MC-LR-induced gastric cancer is still unclear. We found that 100 nM MC-LR promoted the proliferation, migration, invasion, and anti-apoptosis of SGC-7901 cells. Quantitative proteome and phosphoproteome analysis identified differential expression patterns and aberrant pathways of SGC-7901 cells exposed to MC-LR. The results indicated that 48,109 unique peptides from 6320 proteins and 1375 phosphoproteins with 3473 phosphorylation sites were detected after 24 h treatment of MC-LR. Proteome and phosphoproteome conjoint analysis indicated estrogen signaling pathway might play an essential step in MC-LR-treated molecular events. The mechanism underlying these changes may involve MC-LR excessively activating the estrogen signaling pathway by reducing Hsp90 phosphorylation, which results in nucleus translocation of activated ERα and Krt16 overexpression in gastric cells. In general, our results indicate multiple crucial signals triggered by MC-LR, among which MC-LR may promote the development of gastric cancer by exerting estrogenic potency.

Revealing microcystin-LR ecotoxicity to earthworm (Eisenia fetida) at the intestinal cell level

Potential adverse effects of microcystin-LR (MC-LR) on soil invertebrates have not been studied. Here we investigated the mechanism of MC-LR toxicity to earthworm (Eisenia fetida) intestine at the individual level and at the cellular level. The results showed an inverse relationship between the bodyweight and survival rate of earthworms over exposure time- and MC-LR doses in soil. Dose-dependent intestinal lesions and disturbances of enzymatic activities (e.g., cellulase, Na⁺/K⁺-ATPase, and AChE) were observed, which resulted in intestinal dysfunction. Excessive reactive oxygen species generation led to DNA damage and lipid peroxidation of intestinal cells. The oxidative damage to DNA prolonged cell cycle arrest at the G2/M-phase transition in mitosis, thus stimulating and accelerating apoptosis in earthworm intestine. MC-LR target earthworm intestine tissue. MC-LR at low concentrations can damage earthworm intestine regardless of exposure routes (oral or contact). High toxicity of MC-LR to earthworms delineates its ecological risks to terrestrial ecosystems.

Microcystin-leucine arginine exhibits adverse effects on human aortic vascular smooth muscle cells in vitro

Microcystin-leucine arginine (MC-LR) is a kind of toxin produced by cyanobacteria, which can do harm to human and livestock health. MC-LR can easily enter tissues and organs through the blood circulation and accumulate in certain target organs. Vessels are prone to contact with MC-LR during growth and development. Previous study had demonstrated that MC-LR had potential vascular toxicity. However, it is not clear whether MC-LR has adverse effects on vascular smooth muscle cells. In this study, we evaluated the cytotoxicity of MC-LR exposure (0.01, 0.05, 0.1, 0.5, and 1 μM) on human aortic vascular smooth muscle cells (HAVSMCs) in vitro. The data showed that MC-LR exposure inhibited the HAVSMC proliferation and migration, induced HAVSMC apoptosis, cytoskeleton destruction, S-phase arrest, mitochondrial transmembrane potential (MMP) loss, and reactive oxygen species (ROS) production. In addition, MC-LR exposure resulted in the imbalance between oxidants and antioxidants, increased the caspase-3 and caspase-9 activities, and down-regulated the gene expressions (integrin β1, Rho, ROCK, MLC). Taken together, MC-LR could induce the generation of ROS in HAVSMCs, leading to apoptosis by the mitochondrial signaling pathway. MC-LR could also induce cytoskeletal disruption by integrin-mediated FAK/ROCK signaling pathway, leading to cell cycle arrest and the inhibition of HAVSMCs proliferation and migration. The current findings facilitate an understanding of the mechanism of MC-LR toxicity involved in angiocardiopathy.

Gill Junction Injury and Microbial Disorders Induced by Microcystin-Leucine Arginine in Lithobates catesbeianus Tadpoles

Microcystin-LR (MC-LR) is widely present in waters around the world, but its potential toxic effects and mechanisms on amphibian gills remain unknown. In the present study, tadpoles (Lithobates catesbeianus) were exposed to environmentally realistic concentrations of 0.5, 2 μg/L MC-LR, and 0 μg/L MC-LR (Control) for 30 days with the objective to unveil the impairment of gill health. The lysozyme was downregulated, while pattern recognition receptors and complement and adaptive immune processes were upregulated and the ability of gill supernatant to inhibit pathogenic bacteria decreased in the 0.5 and 2 μg/L MC-LR groups. The transcriptions of epithelial barrier components (e.g., CLDN1) were significantly decreased in MC-LR-exposed gills, while the gill content of lipopolysaccharide (LPS) endotoxins and the transcriptions of downstream responsive genes (e.g., TLR4 and NF-κB) were concurrently increased. In addition, the number of eosinophils and the expression of pro-inflammatory cytokines (e.g., IL-1β and TNF-α) were increased. These results imply that exposure of tadpoles to low environmentally concentrations of MC-LR leads to inflammation, increased permeability, and a reduced ability to inhibit pathogenic bacteria. The epithelial cells of inner gill filaments increased and transcriptions of hypoxic stress genes (e.g., HIF-1α, FLT1, and SERPINE1) were upregulated within the exposed group. As a consequence, exposure to MC-LR may lead to hypoxic stress. MC-LR exposure also drove gill microbiota to a dysbiosis. The relative abundance of Elizabethkingia was positively correlated with content of LPS and transcriptions of NF-κB and TNF-α. Overall, this study presents the first evidence about the pronounced impacts of MC-LR exposure on gills of amphibians, highlighting the susceptibility of early developing tadpoles to the environmental risks of MC-LR.

Subchronic Toxicity of Microcystin-LR on Young Frogs (Xenopus laevis) and Their Gut Microbiota

Although toxic effects of microcystins (MCs) in mammals and fish have been extensively studied, the effects of MCs on the immune system and gut microbiota of amphibians have not received sufficient attention. As MCs cause general damage to the vertebrate liver and immune system and trigger an inflammatory response, and the gut microbiota is closely related to host metabolism and immunity, we speculated that MCs can cause changes in the immune system and gut microbiota of amphibians. To verify this, we examined the intestinal and liver injury of Xenopus laevis exposed to different microcystin-leucine-arginine (MC-LR) concentrations and the effects on the gut microbiota through high-throughput sequencing of 16S rDNA of the gut microbiota combined with histopathological analysis, enzyme activity determination, and qRT-PCR. Our results showed that MC-LR caused focal infiltration of inflammatory cells and increased the number of T cells and local congestion and vacuolization in X. laevis liver, but reduced the number, density, height, and regularity of villi. These liver and intestinal injuries became more obvious with an increase in MC-LR concentration. MC-LR significantly decreased the activities of malondialdehyde and alkaline phosphatase and the expression of TGF-β in the liver. Moreover, MC-LR significantly altered the gut microbiota of X. laevis. The relative abundance of Firmicutes and Bacteroidetes in high-concentration MC-LR groups was significantly reduced compared to that in low-concentration MC-LR groups, whereas Fusobacteria was significantly enriched. The metabolic gene composition of the gut microbiota in low-concentration MC-LR (≤5 μg/L) groups was significantly different from that in high-concentration MC-LR (≥20 μg/L) groups. These results deepen our understanding of the toxicity of MCs to aquatic organisms and assessment of the ecological risk of MCs in amphibians.