The proteomic study on cellular responses of the testes of zebrafish (Danio rerio ) exposed to microcystin-RR

Toxicity of three microcystin variants on the histology, physiological and metabolism of hepatopancreas and intestinal microbiota of Litopenaeus vannamei

Microcystins (MCs) are prevalent harmful contaminants within shrimp aquaculture systems, exhibiting a diverse array of variants. Gut microbiota can engage in mutual interactions with the host through the gut-liver axis. In this study, the shrimp Litopenaeus vannamei were subjected to three different variants of MCs (LR, YR, RR) at a concentration of 1 μg/L each, and elucidated the alterations in both intestinal microbiota and hepatopancreas physiological homeostasis. The results showed that all three variants of MCs prompted histological alterations in the hepatopancreas, induced elevated levels of oxidative stress biomarkers (H2O2, T-SOD, and CAT), disturbed the transcription levels of immune-related genes (Crus, ALF, and Lys), along with an increase in apoptotic genes (Casp-3 and P53). Furthermore, the metabolic profiles of the hepatopancreas were perturbed, particularly in amino acid metabolism such as "lysine degradation" and "β-alanine metabolism"; the mTOR and FoxO signaling were also influenced, encompassing alterations in the transcription levels of related genes. Additionally, the alterations were observed in the intestinal microbiota's diversity and composition, particularly potential beneficial bacteria (Alloprevotella, Bacteroides, Collinsella, Faecalibacterium, and Prevotellaceae UCG-001), which exhibited a positive correlation with the metabolite berberine. These findings reveal that the three MCs variants can impact the health of the shrimp by interfering with the homeostasis of intestinal microbial and hepatopancreas physiology.

Comparative toxicology of algal cell extracts and pure cyanotoxins: insights into toxic effects and mechanisms of harmful cyanobacteria Raphidiopsis raciborskii

Ongoing research on cyanotoxins, driven by the socioeconomic impact of harmful algal blooms, emphasizes the critical necessity of elucidating the toxicological profiles of algal cell extracts and pure toxins. This study comprehensively compares Raphidiopsis raciborskii dissolved extract (RDE) and cylindrospermopsin (CYN) based on Daphnia magna assays. Both RDE and CYN target vital organs and disrupt reproduction, development, and digestion, thereby causing acute and chronic toxicity. Disturbances in locomotion, reduced behavioral activity, and weakened swimming capability in D. magna have also been reported for both RDE and CYN, indicating the insufficiency of conventional toxicity evaluation parameters for distinguishing between the toxic effects of algal extracts and pure cyanotoxins. Additionally, chemical profiling revealed the presence of highly active tryptophan-, humic acid-, and fulvic acid-like fluorescence compounds in the RDE, along with the active constituents of CYN, within a 15-day period, demonstrating the chemical complexity and dynamics of the RDE. Transcriptomics was used to further elucidate the distinct molecular mechanisms of RDE and CYN. They act diversely in terms of cytotoxicity, involving oxidative stress and response, protein content, and energy metabolism, and demonstrate distinct modes of action in neurofunctions. In essence, this study underscores the distinct toxicity mechanisms of RDE and CYN and emphasizes the necessity for context- and objective-specific toxicity assessments, advocating nuanced approaches to evaluate the ecological and health implications of cyanotoxins, thereby contributing to the precision of environmental risk assessments.

Toxic effects of three variants of microcystins on the intestinal histology, physiological and metabolic response of Litopenaeus vannamei

Microcystins (MCs) are harmful substances to the health of cultured shrimp, and there are many variants of MCs. Intestinal is the immune and metabolic center of the shrimp, and is also the target organ for MCs toxicity. In this study, the shrimp Litopenaeus vannamei juvenile were separately exposed to 1 μg/L of three MCs variants (LR, YR, RR) for 72 h respectively, and the changes of intestinal morphology, physiological response and metabolic function were analyzed. The results showed the three MCs variants stress caused intestinal mucosal damage and disordered the homeostasis of antimicrobial genes (ALF and Lys) expression. The mRNA expression levels of antioxidant genes (Nrf2 and GPx) and apoptosis factors (CytC and Casp-3) were increased, but that of detoxification gene (CYP450) was decreased. Furthermore, the intestinal metabolic pattern was also influenced by stresses, among which MC-LR induced more differential metabolites than that of MC-YR and MC-RR. The function of purine metabolism was highly influenced by the stress of three MCs variants, followed by amino acid metabolism, but there were differences in the types of amino acids. The metabolites citric acid, L-glutamine, L-tryptophan, spermine, UMP, and indole contributed to the metabolic pathway network. Nineteen stress-related metabolites were identified as candidates for subsequent screening of potential biomarkers. These results revealed the toxic effects of three MCs variants on the intestinal physiological and metabolic homeostasis of the shrimp.

Paternal exposure to microcystin-LR triggers developmental neurotoxicity in zebrafish offspring via an epigenetic mechanism involving MAPK pathway

Microcystin-LR (MCLR) induced impairment to male reproductive system and revealed the effects of transgenerational toxicity on offspring. But very little is known about the inheritance of these effects to offspring and the mechanisms involved. Here, we used methylated DNA immunoprecipitation sequencing (MeDIP-Seq) and microarray to characterize whole-genome DNA methylation and mRNA expression patterns in zebrafish testis after 6-week exposure to 5 and 20 μg/L MCLR. Accompanied with these analyses it revealed that MAPK pathway and ER pathway significantly enriched in zebrafish testes. Apoptosis and testicular damage were also observed in testis. Next, we test the transmission of effects to compare control-father and MCLR exposure-father progenies. DNA methylation analyses (via reduced representation bisulfite sequencing) reveal that the enrichment of differentially methylated regions on neurodevelopment after paternal MCLR exposure. Meanwhile, several genes associated with neurodevelopment were markedly downregulated in zebrafish larvae, and swimming speed was also reduced in the larvae. Interestingly, paternal MCLR exposure also triggered activation the phosphorylation of mitogen-activated protein kinase (MAPK) pathway which is also associated with neurodevelopmental disorders. These results demonstrated the significant effect that paternal MCLR exposure may have on gene-specific DNA methylation patterns in testis. Inherited epigenetic alterations through the germline may be the mechanism leading to developmental neurotoxicity in the offspring.

Advances in the toxicology research of microcystins based on Omics approaches

Microcystins (MCs) are the most widely distributed cyanotoxins, which can be ingested by animals and human body in multiple ways, resulting in a threat to human health and the biodiversity of wildlife. Therefore, the study on toxic effects and mechanisms of MCs is one of the focuses of attention. Recently, the Omics techniques, i.e. genomics, transcriptomics, proteomics and metabolomics, have significantly contributed to the comprehensive understanding and revealing of the molecular mechanisms about the toxicity of MCs. This paper mainly reviews current literature using the Omics approaches to explore the toxicity mechanism of MCs in liver, gonad, spleen, brain, intestine and lung of multiple species. It was found that MCs can exert strong toxic effects on various metabolic activities and cell signal transduction in cell cycle, apoptosis, destruction of cell cytoskeleton and redox disorder, at protein, transcription and metabolism level. Meanwhile, it was also revealed that the alteration of non-coding RNAs (miRNA, circRNA and lncRNA, etc.) and gut microbiota plays an essential regulatory role in the toxic effects of MCs, especially in hepatotoxicity and reproductive toxicity. In addition, we summarized current research gaps and pointed out the future directions for research. The detailed information in this paper shows that the application and development of Omics techniques have significantly promoted the research on MCs toxicity, and it is also a valuable resource for exploring the toxic mechanism of MCs.

MCLR induces dysregulation of calcium homeostasis and endoplasmic reticulum stress resulting in apoptosis in Sertoli cells

Microcystins-LR (MCLR) is a potent reproductive system toxin. We have previously shown that MCLR induced endoplasmic reticulum (ER) stress and apoptosis in testis. ER is the main calcium storage site in cells, and its calcium homeostasis plays an important role in the regulation of apoptosis. Hence, in the present study, we have investigated the role of calcium (Ca²⁺) in inducing apoptosis and how it affect the mitochondria and endoplasmic reticulum in TM4 cells. Our study found that MCLR induced an increase in Ca²⁺ concentration in TM4 cells. Compared to the controls, MCLR induced phosphorylation of calmodulin-dependent protein kinase II (CaMKII) which was involved in MAPKs activation, resulting in the induction of mitochondrial apoptosis pathways. Ca²⁺ chelator Bapta-AM partially reversed MCLR-induced apoptosis, confirming the possible involvement of calcium homeostasis disruption after MCLR exposure. Meanwhile, MCLR activated unfolded protein response and activated the ER apoptotic pathway by activating caspase-12. In addition, exposure to MCLR causes mitochondrial defects and increased apoptosis by up-regulating caspase 3 and cytosol cytochrome c expression. Collectively, these results demonstrated that MCLR disturbed calcium homeostasis, which caused ER-mitochondria dysfunction, ultimately promoted cell apoptosis in Sertoli cells.

Epigenetic modification of H3K4 and oxidative stress are involved in MC-LR-induced apoptosis in testicular cells of SD rats

Microcystin-leucine arginine (MC-LR) is a cyclic heptapeptide, produced by aquatic cyanobacteria such as microcystis, with strong reproductive toxicity which poses greater threat to the reproductive abilities of humans and animals. By exploring the role of trimethylation of histone H3 at lysine 4 (H3K4me3) and the role of oxidative stress in MC-LR-induced apoptosis in testicular Sertoli cells in Sprague-Dawley (SD) rats, this study indicated that MC-LR increased the expression levels of apoptosis-related genes by raising the levels of H3K4me3. 5'-Deoxy-5'-methylthioadenosine (MTA), the inhibitor of H3K4me3, reduced apoptosis, indicating for the first time that epigenetic modification is closely related to the testicular reproductive toxicity induced by MC-LR. MC-LR also induced oxidative stress by stimulating the generation of reactive oxygen species (ROS), and subsequently triggering mitochondria-mediated apoptotic pathway by decreasing mitochondrial membrane potential and increasing the levels of Bax, Bcl-2, Caspase-3, and so on. MC-LR-induced apoptosis of testicular cells could be decreased after pretreatment with oxidative stress inhibitor N-acetyl-cysteine (NAC). Furthermore, the pathological damage to mitochondria and testes were observed in SD rats. These results show that MC-LR can induce apoptosis by raising the levels of H3K4me3, and pretreatment with MTA can ameliorate the MC-LR-induced apoptosis of cocultured cells by lowering the levels of H3K4me3. Furthermore, NAC has a protective effect on MC-LR-induced apoptosis of testicular cells in SD rats by inhibiting the oxidative stress.

Osmoregulatory disturbance in Neotropical fish exposed to the crude extracts of the cyanobacterium, Radiocystis fernandoi

Blooms of cyanobacteria, a common event in eutrophic environments, result in the release of potentially toxic substances into the water. The cyanobacterium Radiocystis fernandoi produces microcystin (MC) and other peptides that may disturb homeostasis. This study evaluated the effect of intraperitoneal injections containing the crude extract (CE) of R. fernandoi strain R28 on the gills and kidneys of neotropical fish, Piaractus mesopotamicus, 3, 6 and 24 h post-injection. CE contained MC-RR, MC-YR and minor other oligopeptides. Plasma ions and the activities of the enzymes PP1 and PP2A, Na⁺/K⁺-ATPase (NKA), H⁺-ATPase (HA) and carbonic anhydrase (CA) were determined and morphological changes in both the gills and kidneys were characterized. Compared to controls, the concentration of Na⁺ within the plasma of P. mesopotamicus decreased after treatment with CE 3 h post treatment and increased after 24 h; the concentration of K⁺ decreased after 6 h. The activity of the endogenous PP1 and PP2A was unchanged in the gills and was inhibited in the kidneys 6 h after i.p. injection. In the gills, NKA activity increased after 3 h and decreased 6 h post i.p. exposure. Further, NKA activity did not differ from the controls 24-h post injection. In the kidneys, NKA, HA and CA activities were unaffected by treatment. The mitochondria-rich cell (MRC) density in the gills decreased after 3 h in the filament and 3 and 6 h in the lamellae and was restored to the control levels 24 h post-exposure. Filament epithelial hyperplasia and hypertrophy, lamellar atrophy and rupture of the lamellar epithelium were the most common effects of treatment in the gills. No histopathological changes occurred in the kidneys. This study demonstrates that a single dose of toxic CE from R. fernandoi can cause a transitory ion imbalance in P. mesopotamicus which is related to the changes in MRC levels and NKA activity. Ionic balance was recovered 24 h post i.p. injection, however, morphological changes that occurred in the gills took a longer amount of time to return to normal. To conclude, the effects of components contained within the CE of R. fernandoi may be harmful to P. mesopotamicus. In particular, the recovery of ionic regulation depends on MRC responses and histopathological changes produced by CE may affect gas exchange and other gill functions.

Microcystin-LR disturbs testicular development of giant freshwater prawn Macrobrachium rosenbergii

Microcystins (MCs) are produced by cyanobacterial blooms and known for their hepatotoxicity. They could cause serious damage to the reproduction of higher vertebrate mice and fish. However, few studies have focused on the reproductive toxicity of MCs to invertebrates. Giant freshwater prawn Macrobrachium rosenbergii are highly cultivated in China. During their breeding process, M. rosenbergii are often infested by cyanobacteria blooms. In the present study, to investigate the toxic effect of MCs on the testicular development of M. rosenbergii. Male M. rosenbergii were exposed to environmental relevant concentration of MC-LR for 1, 2 and 3 weeks. Results showed that MC-LR entered M. rosenbergii testis, down-regulated hemolymph testosterone (T) levels, and damaged testicular germ cells, mitochondria and cell junctions, and inhibited testicular development. Moreover, MC-LR could significantly induce the expression of gonadal development related genes in testis and eyestalk). The present results indicate that MC-LR can disrupt the testicular development of M. rosenbergii by affecting T levels and gonadal development related genes in the testis and eyestalk.

Subcellular localization of microcystin in the liver and the gonads of medaka fish acutely exposed to microcystin-LR

Among the diverse toxic components produced by cyanobacteria, microcystins (MCs) are one of the most toxic and notorious cyanotoxin groups. Besides their potent hepatotoxicity, MCs have been revealed to induce potential reproductive toxicity in various animal studies. However, little is still known regarding the distribution of MCs in the reproductive organ, which could directly affect reproductive cells. In order to respond to this question, an acute study was conducted in adult medaka fish (model animal) gavaged with 10 μg.g^-1 body weight of pure MC-LR. The histological and immunohistochemical examinations reveal an intense distribution of MC-LR within hepatocytes along with a severe liver lesion in the toxin-treated female and male fish. Besides being accumulated in the hepatocytes, MC-LR was also found in the connective tissue of the ovary and the testis, as well as in oocytes and degenerative spermatocyte-like structures but not spermatocytes. Both liver and gonad play important roles in the reproductive process of oviparous vertebrates. This observation constitutes the first observation of the presence of MC-LR in reproductive cells (female, oocytes) of a vertebrate model with in vivo study. Our results, which provide intracellular localization of MC-LR in the gonad, advance our understanding of the potential reproductive toxicity of MC-LR in fish.

Microcystin-RR: Occurrence, content in water and food and toxicological studies. A review

Microcystins (MCs) are hepatotoxins, produced by various species of cyanobacteria, whose occurrence is increasing worldwide owing to climate change and anthropogenic activities. More than 100 variants have been reported, and among them MC-LR is the most extensively studied, but there are other MC congeners that deserve to be investigated. The need for data to characterize the toxicological profile of MC variants other than MC-LR has been identified in order to improve risk assessment in humans and wildlife. Accordingly, the aim of this study was to evaluate the information available in the scientific literature dealing with MC-RR, as this congener is the second most common cyanotoxin in the environment. The review focuses on aspects such as occurrence in water and food, and toxicity studies both in vitro and in vivo. It reveals that, although MC-RR is a real hazard with a high exposure potential in some countries, little is known yet about its specific toxicological properties that differ from those of MC-LR, and important aspects such as genotoxicity and chronic effects have not yet been sufficiently addressed.

N-acetylcysteine protects against microcystin-LR-induced endoplasmic reticulum stress and germ cell apoptosis in zebrafish testes

Previous studies have shown that microcystin-LR (MCLR) is a reproductive toxicant that induces germ cell apoptosis in the testes, but the underlying mechanisms have not been well understood. In this study, we investigated that MCLR induces germ cell apoptosis is through activation of endoplasmic reticulum (ER) stress and N-acetylcysteine (NAC), an antioxidant could protect against germ cell apoptosis by inhibiting the ER stress. Healthy male zebrafish were intraperitoneally injected with NAC (500 nM), beginning at 2 h before different doses of MCLR (0, 50, 100, 200 μg/kg). As expected, acute MCLR exposure resulted in oxidative stress and germ cell apoptosis in zebrafish testes. Further analysis showed that NAC significantly alleviated MCLR-induced testicular germ cell apoptosis and inhibited the caspase-dependent apoptotic proteins. Meanwhile H&E staining showed that NAC could rescue testicular damage induced by MCLR. Moreover, MCLR induced activation of ER stress which consequently triggered apoptosis in zebrafish testes. Interestingly, NAC was effective in improving the total antioxidant capacity (T-AOC) level and activity of antioxidant enzymes in NAC pretreated groups. NAC significantly attenuated MCLR-induced upregulation of GRP78 in testes. In addition, NAC significantly attenuated MCLR-triggered testicular eIF2s1 and MAPK8 activation, indicating that NAC counteracts MCLR-induced unfolded protein response (UPR) in testes. Taken together, the results observed in this study suggested that ER stress plays a critical role in germ cell apoptosis exposed to MCLR and NAC could protect against apoptosis via inhibiting ER stress in zebrafish testes.

Microcystin-LR induces changes in the GABA neurotransmitter system of zebrafish

It has been reported that exposure to microcystins altered adult zebrafish swimming performance parameters, but the possible mechanisms of action remain unknown. Neuronal activity depends on the balance between the number of excitatory and inhibitory processes which are associated with neurotransmitters. In the present study, zebrafish embryos (5 d post-fertilization) were exposed to 0, 0.3, 3 and 30μg/L (microcystin-LR) MCLR for 90day until reaching sexual maturity. To investigate the effects of MCLR on the neurotransmitter system, mRNA levels involved in amino acid g-aminobutyric acid (GABA) and glutamate metabolic pathways were tested using quantitative real-time PCR. Significant increase of GABAA receptor, alpha 1 (gabra1), glutamate decarboxylase (gad1b), glutaminase (glsa) and reduction of mRNA expression of GABA transporter (gat1) at transcriptional level were observed in the brain. Meanwhile, western blotting showed that the protein levels of gabra1, gad1b were induced by MCLR, whereas the expression of gat1 was decreased. In addition, MCLR induced severe damage to cerebrum ultrastructure, showing edematous and collapsed myelinated nerve fibers, distention of endoplasmic reticulum and swelling mitochondria. Our results suggested that MCLR showed neurotoxicity in zebrafish which might attribute to the disorder of GABA neurotransmitter pathway.

Cyanobacterial Toxins of the Laurentian Great Lakes, Their Toxicological Effects, and Numerical Limits in Drinking Water

Cyanobacteria are ubiquitous phototrophic bacteria that inhabit diverse environments across the planet. Seasonally, they dominate many eutrophic lakes impacted by excess nitrogen (N) and phosphorus (P) forming dense accumulations of biomass known as cyanobacterial harmful algal blooms or cyanoHABs. Their dominance in eutrophic lakes is attributed to a variety of unique adaptations including N and P concentrating mechanisms, N₂ fixation, colony formation that inhibits predation, vertical movement via gas vesicles, and the production of toxic or otherwise bioactive molecules. While some of these molecules have been explored for their medicinal benefits, others are potent toxins harmful to humans, animals, and other wildlife known as cyanotoxins. In humans these cyanotoxins affect various tissues, including the liver, central and peripheral nervous system, kidneys, and reproductive organs among others. They induce acute effects at low doses in the parts-per-billion range and some are tumor promoters linked to chronic diseases such as liver and colorectal cancer. The occurrence of cyanoHABs and cyanotoxins in lakes presents challenges for maintaining safe recreational aquatic environments and the production of potable drinking water. CyanoHABs are a growing problem in the North American (Laurentian) Great Lakes basin. This review summarizes information on the occurrence of cyanoHABs in the Great Lakes, toxicological effects of cyanotoxins, and appropriate numerical limits on cyanotoxins in finished drinking water.

Chemical proteomic analysis of the potential toxicological mechanisms of microcystin-RR in zebrafish (Danio rerio) liver

Microcystins (MCs) are common toxins produced by freshwater cyanobacteria, and they represent a potential health risk to aquatic organisms and animals, including humans. Specific inhibition of protein phosphatases 1 and 2A is considered the typical mechanism of MCs toxicity, but the exact mechanism has not been fully elucidated. To further our understanding of the toxicological mechanisms induced by MCs, this study is the first to use a chemical proteomic approach to screen proteins that exhibit special interactions with MC-arginine-arginine (MC-RR) from zebrafish (Danio rerio) liver. Seventeen proteins were identified via affinity blocking test. Integration of the results of previous studies and this study revealed that these proteins play a crucial role in various toxic phenomena of liver induced by MCs, such as the disruption of cytoskeleton assembly, oxidative stress, and metabolic disorder. Moreover, in addition to inhibition of protein phosphate activity, the overall toxicity of MCs was simultaneously modulated by the distribution of MCs in cells and their interactions with other target proteins. These results provide new insight into the mechanisms of hepatotoxicity induced by MCs. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 1206-1216, 2016.

Life-cycle exposure to microcystin-LR interferes with the reproductive endocrine system of male zebrafish

Recently, MC-LR reproductive toxicity drew great attention. Limited information was available on endocrine-disrupting effects of MC-LR on the reproduction system in fish. In the present study, zebrafish hatchlings (5 d post-fertilization) were exposed to 0, 0.3, 3 and 30μg/L MC-LR for 90 d until they reached sexual maturity. Male zebrafish were selected, and changes in growth and developmental parameters, testicular histological structure as well as the levels of gonadal steroid hormones were studied along with the related-gene transcriptional responses in the hypothalamic-pituitary-gonadal axis (HPG-axis). The results, for the first time, show a life cycle exposure to MC-LR causes growth inhibition, testicular damage and delayed sperm maturation. A significant decrease in T/E2 ratio indicated that MC-LR disrupted sex steroid hormones balance. The changes in transcriptional responses of HPG-axis related genes revealed that MC-LR promoted the conversion of T to E2 in circulating blood. It was also noted that vtg1 mRNA expression in the liver was up-regulated, which implied that MC-LR could induce estrogenic-like effects at environmentally relevant concentrations and long-term exposure. Our findings indicated that a life cycle exposure to MC-LR causes endocrine disruption with organic and functional damage of the testis, which might compromise the quality of life for the survivors and pose a potent threat on fish reproduction and thus population dynamics in MCs-contaminated aquatic environments.

A proteomic study on liver impairment in rat pups induced by maternal microcystin-LR exposure

There is mounting evidence indicating that microcystins (MCs) are heptapeptide toxins. Recent studies have also shown that MCLR can transfer from mother to offspring, but it is unclear whether maternal MCLR can influence the liver of offspring or not. In this study, pregnant SD rats were injected intraperitoneally with a saline solution (control) or 10 μg/kg MCLR per day from gestational day 8 (GD8) to postnatal day 15 (PD15) for a total of 4 weeks. 2-DE and MALDI-TOF-TOF mass spectrometry were used to screen for MCLR target proteins in the livers of rat pups. Our results demonstrated that MCLR could accumulate in the livers of neonatal rats. Proteomics studies also showed that MCLR significantly influenced many proteins, including those involved in the cytoskeleton, metabolism and particularly oxidative stress. In addition, MCLR induced cellular structural damage and resulted in the production of intracellular reactive oxygen species (ROS) and lipid peroxidation. Moreover, protein phosphatase (PP) activity was inhibited and some serum biochemistry parameters were altered. These results suggest an early molecular mechanism behind the hepatotoxicity induced by maternal MC exposure and highlight the importance of monitoring MC concentrations in new-born mammals.

Sex-dependent effects of microcystin-LR on hypothalamic-pituitary-gonad axis and gametogenesis of adult zebrafish

While microcystins (MCs) have been reported to exert reproductive toxicity on fish with a sex-dependent effect, the underlying mechanism has been rarely investigated. In the present study, zebrafish were exposed to 1, 5 and 20 μg/L MC-LR for 30 d. The gonad-somatic index declined in all treated males. 17β-estradiol (E2), testosterone (T), 11-keto testosterone (11-KT) and follicle-stimulating hormone (FSH) levels increased in serum from all treated females, while T, FSH and luteinizing hormone (LH) levels changed in all treated males. Histomorphological observation showed that MC-LR exposure evidently retarded oogenesis and spermatogenesis. Transcriptional changes of 22 genes of the hypothalamic-pituitary-gonad (HPG) axis exhibited sex-specific responses, and the relationship between gene transcriptions and gametogenesis was evaluated by principle component analysis (PCA). Major contributors to PC1 (gnrh2, gnrhr3, ar, lhr, hmgra, hmgrb and cyp19a) were positively correlated with the number of post-vitellogenic oocytes, while PC1 (gnrh2, lhβ, erβ, fshr, cyp11a and 17βhsd) were positively correlated with the number of spermatozoa. The protein levels of 17βHSD and CYP19a were affected in both females and males. In conclusion, this study first investigated the sex-dependent effects of microcystins on fish reproduction and revealed some important molecular biomarkers related to gametogenesis in zebrafish suffered from MC-LR.

A review of reproductive toxicity of microcystins

Animal studies provide strong evidence of positive associations between microcystins (MCs) exposure and reproductive toxicity, representing a threat to human reproductive health and the biodiversity of wild life. This paper reviews current knowledge of the reproductive toxicity of MCs, with regard to mammals, fishes, amphibians, and birds, mostly in males. Toxicity of MCs is primarily governed by the inhibition of protein phosphatases 1 and 2A (PP1 and PP2A) and disturbance of cellular phosphorylation balance. MCs exposure is related to excessive production of reactive oxygen species (ROS) and oxidative stress, leading to cytoskeleton disruption, mitochondria dysfunction, endoplasmic reticulum (ER) stress, and DNA damage. MCs induce cell apoptosis mediated by the mitochondrial and ROS and ER pathways. Through PP1/2A inhibition and oxidative stress, MCs lead to differential expression/activity of transcriptional factors and proteins involved in the pathways of cellular differentiation, proliferation, and tumor promotion. MC-induced DNA damage is also involved in carcinogenicity. Apart from a direct effect on testes and ovaries, MCs indirectly affect sex hormones by damaging the hypothalamic-pituitary-gonad (HPG) axis and liver. Parental exposure to MCs may result in hepatotoxicity and neurotoxicity of offspring. We also summarize the current research gaps which should be addressed by further studies.

The role of glutathione detoxification pathway in MCLR-induced hepatotoxicity in SD rats

In the present study, we investigated the role of glutathione (GSH) and its related enzymes in Sprague Dawley (SD) rats subjected to microcystin-leucine-arginine (MCLR)-induced hepatotoxicity. SD rats were intraperitoneally (i.p.) injected with MCLR after pretreating with or without buthionine-(S,R)-sulfoximine (BSO), an inhibitor of GSH synthesis. The depletion of GSH with BSO enhanced MCLR-induced oxidative stress, resulting in more severe liver damage and higher MCLR accumulation. Similarly, the contents of malondialdehyde (MDA), total GSH (T-GSH), oxidized GSH (GSSG) and GSH were significantly enhanced in BSO pretreated rats following MCLR treatment. The study showed that the transcription of GSH-related enzymes such as glutathione-S-transferase (GST), γ-glutamylcysteine synthetase (γ-GCS), glutathione reductase (GR) varied in different ways (expect for glutathione peroxidase (GPx), whose gene expression was induced in all treated groups) with or without BSO pretreatment before MCLR exposure, suggesting an adaptative response of GSH-related enzymes at transcription level to combat enhancement of oxidative stress induced by MCLR when pretreated with BSO. These data suggested the tissues with low GSH concentration are highly vulnerable to MCLR toxicity and GSH was critical for the detoxification in MCLR-induced hepatotoxicity in vivo.

Stressor-induced proteome alterations in zebrafish: a meta-analysis of response patterns

Proteomics approaches are being increasingly applied in ecotoxicology on the premise that the identification of specific protein expression changes in response to a particular chemical would allow elucidation of the underlying molecular pathways leading to an adverse effect. This in turn is expected to promote the development of focused testing strategies for specific groups of toxicants. Although both gel-based and gel-free global characterization techniques provide limited proteome coverage, the conclusions regarding the cellular processes affected are still being drawn based on the few changes detected. To investigate how specific the detected responses are, we analyzed a set of studies that characterized proteome alterations induced by various physiological, chemical and biological stressors in zebrafish, a popular model organism. Our analysis highlights several proteins and protein groups, including heat shock and oxidative stress defense proteins, energy metabolism enzymes and cytoskeletal proteins, to be most frequently identified as responding to diverse stressors. In contrast, other potentially more specifically responding protein groups are detected much less frequently. Thus, zebrafish proteome responses to stress reported by different studies appear to depend mostly on the level of stress rather than on the specific stressor itself. This suggests that the most broadly used current proteomics technologies do not provide sufficient proteome coverage to allow in-depth investigation of specific mechanisms of toxicant action. We suggest that the results of any differential proteomics experiment performed with zebrafish should be interpreted keeping in mind the list of the most frequent responders that we have identified. Similar reservations should apply to any other species where proteome responses are analyzed by global proteomics methods. Careful consideration of the reliability and significance of observed changes is necessary in order not to over-interpret the experimental results and to prevent the proliferation of false positive linkages between the chemical and the cellular functions it perturbs. We further discuss the implications of the identified "top lists" of frequently responding proteins and protein families, and suggest further directions for proteomics research in ecotoxicology. Apart from improving the proteome coverage, further research should focus on defining the significance of the observed stress response patterns for organism phenotypes and on searching for common upstream regulators that can be targeted by specific assays.

Microcystin-LR impairs zebrafish reproduction by affecting oogenesis and endocrine system

Previous studies have shown that microcystins (MCs) are able to exert negative effects on the reproductive system of fish. However, few data are actually available on the effects of MC-LR on the reproductive system of female fish. In the present study, female zebrafish were exposed to 2, 10, and 50 μg L(-1) of MC-LR for 21 d, and its effects on oogenesis, sex hormones, transcription of genes on the hypothalamic-pituitary-gonad (HPG) axis, and reproduction were investigated for the first time. It was observed that egg production significantly declined at ⩾ 10 μg L(-1) MC-LR. MC-LR exposure to zebrafish increased the concentrations of 17β-estradiol (E2) and vitellogenin (VTG) at 10 μg L(-1) level, whereas concentrations of E2, VTG and testosterone declined at 50 μg L(-1) MC-LR. The transcriptions of steroidogenic pathway gene (cyp19a, cyp19b, 17βhsd, cyp17 and hmgra) changed as well after the exposure and corresponded well with the alterations of hormone levels. A number of intra- and extra-ovarian factors, such as gnrh3, gnrhr1, fshβ, fshr, lhr, bmp15, mrpβ, ptgs2 and vtg1 which regulate oogenesis, were significantly changed with a different dose-related effect. Moreover, MC-LR exposure to female zebrafish resulted in decreased fertilization and hatching rates, and may suggest the possibility of trans-generational effects of MC-LR exposure. The results demonstrate that MC-LR could modulate endocrine function and oogenesis, eventually leading to disruption of reproductive performance in female zebrafish. These data suggest there is a risk for aquatic population living in MC polluted areas.

An overview of the toxic effect of potential human carcinogen Microcystin-LR on testis

The worldwide occurrence of cyanobacterial blooms due to water eutrophication evokes extreme concerns. These blooms produce cyanotoxins which are hazardous to living organisms. So far among these toxins, Microcystin-LR (MC-LR) is the most toxic and the most frequently encountered toxin produced by the cyanobacteria in the contaminated aquatic environment. Microcystin-LR is a potential carcinogen for animals and humans, and the International Agency for Research on Cancer has classified Microcystin-LR as a possible human carcinogen. After liver, testis has been considered as one of the most important target organs of Microcystin-LR toxicity. Microcystin-LR crosses the blood–testis barrier and interferes with DNA damage repair pathway and also increases expression of the proto-oncogenes, genes involved in the response to DNA damage, cell cycle arrest, and apoptosis in testis. Toxicity of MC-LR disrupts the motility and morphology of sperm and also affects the hormone levels of male reproductive system. MC-LR treated mice exhibit oxidative stress in testis through the alteration of antioxidant enzyme activity and also affect the histopathology of male reproductive system. In the present review, an attempt has been made to comprehensively address the impact of MC-LR toxicity on testis.

Effects of microcystin-LR on bacterial and fungal functional genes profile in rat gut

The short-term exposure to microcystin-LR (MC-LR, one of the most common and toxic variants generated by toxigenic cyanobacteria) induced gut dysfunction such as generation of reactive oxygen species, cell erosion and deficient intestinal absorption of nutrients. However, till now, little is known about its impact on gut microbial community, which has been considered as necessary metabolic assistant and stresses resistant entities for the host. This study was designed to reveal the shift of microbial functional genes in the gut of rat orally gavaged with MC-LR. GeoChip detected a high diversity of bacterial and fungal genes involved in basic metabolic processes and stress resistance. The results showed that the composition of functional genes was significantly changed in rat gut after one week of exposure to MC-LR, and we found some relatively enriched genes that are involved in carbon degradation including chitin, starch and limonene metabolism, and these genes were mainly derived from fungal and bacterial pathogens. In addition, we found large amounts of significantly enriched genes relevant to degradation of the specific carbon compounds, aromatics. The dysbiosis of bacterial and fungal flora gave an implication of pathogens invasion. The enriched gene functions could be linked to acute gastroenteritis induced by MC-LR.

Damage and recovery of the ovary in female zebrafish i.p.-injected with MC-LR

Up to now, in vivo studies on toxic effects of microcystins (MCs) on the reproductive system are limited and the underlying molecular mechanisms of MCs-induced reproductive toxicity remain to be elucidated. In an acute toxic experiment, female zebrafish (Danio rerio) were injected intraperitoneally (i.p.) at doses of 50 and 200 μg MC-LR/kg body weight (BW) respectively, and histopathological lesions and antioxidant enzymatic activities and gene expression in the ovary were studied at 1, 3, 12, 24, 48 and 168 h post injection (hpi). Pathological lesions of zebrafish ovary progressed in severity and extent with the increasing exposure time and dose within 12 hpi. Concurrently, the increases in malondialdehyde (MDA) contents as well as the enzymatic activities and transcriptional levels of antioxidant enzymes catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GPx) showed the occurrence of oxidative stress, indicating that MC-LR induced adverse effects on the structure and functional activity of zebrafish ovary. Oxidative stress plays a significant role in the reproductive toxicity of MC-LR. The significant decrease of glutathione (GSH) content in zebrafish ovary suggested the importance of MC-LR detoxification by glutathione S-transferases (GST) via GSH. The final recovery of histostructure and antioxidative indices indicated that ovarian efficient antioxidant defense system might be an important mechanism of zebrafish to counteract MC-LR. Although the negative effects of MC-LR can be overcome by ovarian antioxidant system in this study, the potential reproductive risks of MC-LR should not be neglected because of its wide occurrence.

Microcystin-LR exposure to adult zebrafish (Danio rerio) leads to growth inhibition and immune dysfunction in F1 offspring, a parental transmission effect of toxicity

Microcystins (MCs) are algal toxins produced intracellularly within the cyanobacteria cells. MCs exposure exerts great harm to the reproductive system of fish and deteriorates the quality of eggs and sperms, and has further adverse effects on early developmental stages of fish. Whether the MC toxicity can be parentally transmitted to offspring, even though the embryos and larvae are free of MC exposure? In the present study, adult zebrafish were continuously exposed to MC-LR (with dose of 1, 5 and 20 μg/L) for 30 days. After MC-LR exposure, fertilized eggs were collected and the following F1 generation was reared in water containing no MC-LR until 60 days post fertilization (dpf). In F1 offspring, both body weight and body length were evidently dropped. Some growth and immune related genes were detected using the real-time PCR. The transcriptional levels of these genes significantly decreased in F1 offspring of zebrafish whose parents were treated with 5 and 20 μg/L MC-LR. The activities of some antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) significantly dropped in 5 and 20 μg/L MC-LR groups, and the malondialdehyde (MDA) levels markedly increased in all the three treatment groups. Furthermore, distinct pathological changes in liver were observed in F1 zebrafish. Our findings show that the MC-LR exposure to parental zebrafish results in liver damage and evidently influences the growth and immune function in F1 offspring. We consider this damage as a parental transmission effect of microcystin toxicity. Further mechanism studies are necessary to elucidate this transmission effect.

NF-κB plays a key role in microcystin-RR-induced HeLa cell proliferation and apoptosis

Microcystins (MCs) are well-known cyanobacterial toxins produced in eutrophic waters and can act as potential carcinogens and have caused serious risk to human health. However, pleiotropic even paradoxical actions of cells exposure to MCs have been reported, and the mechanisms of MC-induced tumorigenesis and apoptosis are still unknown. In this study, we performed the first comprehensive in vitro investigation on carcinogenesis associated with nuclear factor kappa B (NF-κB) and its downstream genes in HeLa cells (Human cervix adenocarcinoma cell line from epithelial cells) exposure to MC-RR. HeLa cells were treated with 0, 20, 40, 60, and 80 µg/mL MC-RR for 4, 8, 12, and 24 h. HeLa cells presented dualistic responses to different doses of MCs. CCK8 assay showed that MC-RR exposure evidently enhanced cell viability of HeLa cells at lower MCs doses. Cell cycle and apoptosis analysis revealed that lower MCs doses promoted G1/S transition and cell proliferation while higher doses of MCs induced apoptosis, with a dose-dependent manner. Electrophoretic mobility shift assay (EMSA) revealed that MC-RR could increase/decrease NF-κB activity at lower/higher MC-RR doses, respectively. Furthermore, the expression of NF-κB downstream target genes including c-FLIP, cyclinD1, c-myc, and c-IAP2 showed the same variation trend as NF-κB activity both at mRNA and protein levels, which were induced by lower doses of MC-RR and suppressed by higher doses. Our data verified for the first time that NF-κB pathway may mediate MC-induced cell proliferation and apoptosis and provided a better understanding of the molecular mechanism for potential carcinogenicity of MC-RR.

Zebrafish as a model for systems biology

Zebrafish offer a unique vertebrate model for research areas such as drug development, disease modeling and other biological exploration. There is significant conservation of genetics and other cellular networks among zebrafish and other vertebrate models, including humans. Here we discuss the recent work and efforts made in different fields of biology to explore the potential of zebrafish. Along with this, we also reviewed the concept of systems biology. A biological system is made up of a large number of components that interact in a huge variety of combinations. To understand completely the behavior of a system, it is important to know its components and interactions, and this can be achieved through a systems biology approach. At the end of the paper we present a concept of integrating zebrafish into the systems biology approach.

Organic extract contaminants from drinking water activate Nrf2-mediated antioxidant response in a human cell line

Traditional risk assessment methods face challenges in estimating risks from drinking waters that contain low-levels of large numbers of contaminants. Here, we evaluate the toxicity of organic contaminant (OC) extracts from drinking water by examining activation of nuclear factor E2-related factor 2 (Nrf2)-mediated antioxidant response. In HepG2 cells, the Nrf2-mediated antioxidant response-measured as Nrf2 protein accumulation, expression of antioxidant response element (ARE)-regulated genes and ARE-luciferase reporter gene assays were activated by OC extracts from drinking water sources that detected 25 compounds in 9 classification groups. Individual OCs induced oxidative stress at concentrations much higher than their environmental levels; however, mixtures of contaminants induced oxidative stress response at only 8 times the environmental levels. Additionally, a synthetic OC mixture prepared based on the contamination profiling of drinking water induced ARE activity to the same extent as the real-world mixture, reinforcing our conclusion that these mixture exposures produce responses relevant for human exposure situations. Our study tested the possibility of assessing toxicity of OCs of drinking water using a specific ARE-pathway measurement. This approach should be broadly useful in assisting risk assessment of mixed environmental exposure.

The interactive effects of cytoskeleton disruption and mitochondria dysfunction lead to reproductive toxicity induced by microcystin-LR

The worldwide occurrence of cyanobacterial blooms evokes profound concerns. The presence of microcystins (MCs) in waters and aquatic food increases the risk to human health. Some recent studies have suggested that the gonad is the second most important target organ of MCs, however, the potential toxicity mechanisms are still unclear. For a better understanding of reproductive toxicity of MCs on animals, we conducted the present experimental investigation. Male rats were intraperitoneally injected with MC-LR for 50 d with the doses of 1 and 10 µg/kg body weight per day. After prolonged exposure to MC-LR, the testes index significantly decreased in 10 µg/kg group. Light microscope observation indicated that the space between the seminiferous tubules was increased. Ultrastructural observation showed some histopathological characteristics, including cytoplasmic shrinkage, cell membrane blebbing, swollen mitochondria and deformed nucleus. Using Q-PCR methods, the transcriptional levels of some cytoskeletal and mitochondrial genes were determined. MC-LR exposure affected the homeostasis of the expression of cytoskeletal genes, causing possible dysfunction of cytoskeleton assembly. In MC-LR treatments, all the 8 mitochondrial genes related with oxidative phosphorylation (OXPHOS) significantly increased. The reactive oxygen species (ROS) level significantly increased in 10 µg/kg group. The mitochondria swelling and DNA damage were also determined in 10 µg/kg group. Hormone levels of testis significantly changed. The present study verified that both cytoskeleton disruption possibly due to cytoskeletal reorganization or depolymerization and mitochondria dysfunction interact with each other through inducing of reactive oxygen species and oxidative phosphorylation, and jointly result in testis impairment after exposure to MC-LR.

Induction of apoptosis in testis of the marine teleost mummichog Fundulus heteroclitus after in vivo exposure to the antifouling biocide 4,5-dichloro-2-n-octyl-3(2H)-isothiazolone (Sea-Nine 211)

4,5-dichloro-2-n-octyl-3(2H)-isothiazolone (Sea-Nine 211) has been widely used as an effective antifouling biocide. However, little is known about its reproductive toxicity in fish. Here we investigated testicular toxicity in a marine teleost, the mummichog Fundulus heteroclitus, after exposure to Sea-Nine 211 for 28 d. Although Sea-Nine 211 exposure did not affect germ cell proliferation in testis, terminal deoxynucleotidyl transferase-mediated deoxy-UTP nick-end labeling revealed that the number of apoptotic spermatocytes was increased in the 1.0- and 3.0-μg L(-1) groups, and significant differences emerged between the 1.0-μg L(-1) group and control groups. Immunohistochemistry showed that the numbers of cysts expressing caspases 2, 3, 6, and 8 (apoptosis-associated proteins) were significantly increased in the 1.0-μg L(-1) group, whereas the signal intensity of an anti-apoptotic protein Bcl-xL was reduced in a dose-dependent manner. Moreover, the number of cysts positive for neuronal nitric oxide synthase was twofold higher in the 1.0-μg L(-1) group than in the control groups. These results suggest that long-term exposure to Sea-Nine 211 induces apoptosis in the testicular germ cells of mummichogs via a caspase-dependent pathway and that oxidative stress via nitric oxide synthesized by neuronal nitric oxide synthase is involved in this induction.