Analysis of microRNA expression in embryonic developmental toxicity induced by MC-RR

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.

MicroRNAs and Xenobiotic Toxicity: An Overview

The advent of new technologies has paved the rise of various chemicals that are being employed in industrial as well as consumer products. This leads to the accumulation of these xenobiotic compounds in the environment where they pose a serious threat to both target and non-target species. miRNAs are one of the key epigenetic mechanisms that have been associated with toxicity by modulating the gene expression post-transcriptionally. Here, we provide a comprehensive view on miRNA biogenesis, their mechanism of action and, their possible role in xenobiotic toxicity. Further, we review the recent in vitro and in vivo studies involved in xenobiotic exposure induced miRNA alterations and the mRNA-miRNA interactions. Finally, we address the challenges associated with the miRNAs in toxicological studies.

Zebrafish as an integrative vertebrate model to identify miRNA mechanisms regulating toxicity

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Zebrafish are an established vertebrate model for toxicity studies.

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Zebrafish have a fully sequenced genome and the capability to create genetic models.

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Zebrafish have over 80 % homology for genes related to human disease.

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Functions of miRNAs in the zebrafish genome are being characterized.

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Zebrafish are ideal for mechanistic studies on how miRNAs regulate toxicity.

Zebrafish (Danio rerio) are an integrative vertebrate model ideal for toxicity studies. The zebrafish genome is sequenced with detailed characterization of all life stages. With their genetic similarity to humans, zebrafish models are established to study biological processes including development and disease mechanisms for translation to human health. The zebrafish genome, similar to other eukaryotic organisms, contains microRNAs (miRNAs) which function along with other epigenetic mechanisms to regulate gene expression. Studies have now established that exposure to toxins and xenobiotics can change miRNA expression profiles resulting in various physiological and behavioral alterations. In this review, we cover the intersection of miRNA alterations from toxin or xenobiotic exposure with a focus on studies using the zebrafish model system to identify miRNA mechanisms regulating toxicity. Studies to date have addressed exposures to toxins, particulate matter and nanoparticles, various environmental contaminants including pesticides, ethanol, and pharmaceuticals. Current limitations of the completed studies and future directions for this research area are discussed.

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.

Analysis of MicroRNA Expression Profiling Involved in MC-LR-Induced Cytotoxicity by High-Throughput Sequencing

In recent years, microRNAs (miRNAs) in toxicology have attracted great attention. However, the underlying mechanism of miRNAs in the cytotoxicity of microcystin-LR (MC-LR) is lacking. The objective of this study is to analyze miRNA profiling in HepG2 cells after 24 h of MC-LR-exposure to affirm whether and how miRNAs were involved in the cytotoxicity of MC-LR. The results showed that totally 21 and 37 miRNAs were found to be significantly altered in the MC-LR treated cells at concentrations of 10 and 50 μM, respectively, when compared to the control cells. In these two groups, 37,566 and 39,174 target genes were predicted, respectively. The further analysis showed that MC-LR-exposure promoted the expressions of has-miR-149-3p, has-miR-449c-5p, and has-miR-454-3p while suppressed the expressions of has-miR-4286, has-miR-500a-3p, has-miR-500a-5p, and has-miR-500b-5p in MC-LR-treated groups when compared to the control group. Moreover, the result of qPCR confirmed the above result, suggesting that these miRNAs may be involved in MC-LR-hepatotoxicity and they may play an important role in the hepatitis and liver cancer caused by MC-LR. The target genes for differentially expressed miRNAs in MC-LR treatment groups were significantly enriched to totally 23 classes of GO, in which three were significantly enriched in both 10 and 50 μM MC-LR groups. Moreover, the results of KEGG pathway analysis showed that MC-LR-exposure altered some important signaling pathways such as MAPK, biosynthesis of secondary metabolites, and pyrimidine and purine metabolism, which were possibly negatively regulated by the corresponding miRNAs and might play important role in MC-LR-mediated cytotoxicity in HepG2 cells.

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.

miR-541 Contributes to Microcystin-LR-Induced Reproductive Toxicity through Regulating the Expression of p15 in Mice

Microcystin-leucine arginine (MC-LR) is a harmful cyanotoxin produced by cyanobacteria. MC-LR can exert endocrine-disrupting activities in many organisms. We have previously demonstrated that MC-LR exerts both acute and chronic reproductive toxicity in male mice, resulting in a decline in sperm quality and damage to testicular structure. Moreover, we also observed extensive alterations in a panel of microRNAs in spermatogonial cells after exposure to MC-LR. In this study, we have confirmed that miR-541 was significantly increased both in GC-1 cells (in vitro) and in mouse testes (in vivo) after exposure to MC-LR. Our data support that p15 was the target gene of miR-541. Increase in miR-541 led to a reduction of p15 and murine double minute2 (MDM2), promoting the activation of p53 signaling and MC-LR-mediated cell apoptosis. Moreover, cells responded to MC-LR with reduced viability and increased apoptosis. Consistently, inhibiting miR-541 could upregulate the expression of p15 and MDM2, resulting in the downregulation of phospho-p53. Downregulation of miR-541 promoted cell viability by reducing MC-LR-induced cell apoptosis. In conclusion, we demonstrate here a crucial role for miR-541 in MC-LR-induced toxic effects on the reproductive system, in an attempt to provide a rational strategy for the diagnosis and treatment of MC-LR-induced impairment in the reproductive system.

Neuronal developmental gene and miRNA signatures induced by histone deacetylase inhibitors in human embryonic stem cells

Human embryonic stem cells (hESCs) may be applied to develop human-relevant sensitive in vitro test systems for monitoring developmental toxicants. The aim of this study was to identify potential developmental toxicity mechanisms of the histone deacetylase inhibitors (HDAC) valproic acid (VPA), suberoylanilide hydroxamic acid (SAHA) and trichostatin A (TSA) relevant to the in vivo condition using a hESC model in combination with specific differentiation protocols and genome-wide gene expression and microRNA profiling. Analysis of the gene expression data showed that VPA repressed neural tube and dorsal forebrain (OTX2, ISL1, EMX2 and SOX10)-related transcripts. In addition, VPA upregulates axonogenesis and ventral forebrain-associated genes, such as SLIT1, SEMA3A, DLX2/4 and GAD2. HDACi-induced expression of miR-378 and knockdown of miR-378 increases the expression of OTX2 and EMX2, which supports our hypothesis that HDACi targets forebrain markers through miR-378. In conclusion, multilineage differentiation in vitro test system is very sensitive for monitoring molecular activities relevant to in vivo neuronal developmental toxicity. Moreover, miR-378 seems to repress the expression of the OTX2 and EMX2 and therefore could be a regulator of the development of neural tube and dorsal forebrain neurons.

Molecular fingerprints of environmental carcinogens in human cancer

Identification of specific molecular changes (fingerprints) is important to identify cancer etiology. Exploitable biomarkers are related to DNA, epigenetics, and proteins. DNA adducts are the turning point between environmental exposures and biological damage. DNA mutational fingerprints are induced by carcinogens in tumor suppressor and oncogenes. In an epigenetic domain, methylation changes occurs in specific genes for arsenic, benzene, chromium, and cigarette smoke. Alteration of specific microRNA has been reported for environmental carcinogens. Benzo(a)pyrene, cadmium, coal, and wood dust hits specific heat-shock proteins and metalloproteases. The multiple analysis of these biomarkers provides information on the carcinogenic mechanisms activated by exposure to environmental carcinogens.

Regulation of microcystin-LR-induced toxicity in mouse spermatogonia by miR-96

Microcystin (MC)-LR is a cyclic heptapeptide that acts as a potent reproductive system toxin, especially by decreasing sperm quality through affecting spermatogonia. However, the molecular mechanisms of MC-induced spermatogonial cytotoxicity still remain unclear. The present study was designed to investigate changes in microRNA (miRNA) profiles and their potential functions in spermatogonia (GC-1 cell line) following treatment with MC-LR. With microarray analysis, 101 miRNAs were identified to be significantly altered in GC-1 cells treated with MC-LR. Among the 25 miRNAs associated with spermatogenesis, miR-96 was down-regulated most dramatically and thus selected for further functional analysis. Deleted-in azoospermia-associated protein 2 (DAZAP2) was predicted to have a binding sequence for miR-96 within its 3'-untranslated region. Fluorescent reporter assay confirmed that DAZAP2 was the target gene of miR-96. The expression of DAZAP2 decreased significantly when miR-96 was up-regulated. Consistently, down-regulation of miR-96 significantly increased the level of DAZAP2. Up-regulation of miR-96 promoted cell viability in GC-1 cells as a result of exposure to MC-LR. Our study suggested a crucial role for miR-96 in the regulation of cytotoxic effects of MC-LR in spermatogonia, which provides new perspectives in the diagnosis and treatment strategies for MC-induced male infertility.

The effects of environmental chemical carcinogens on the microRNA machinery

The first evidence that microRNA expression is early altered by exposure to environmental chemical carcinogens in still healthy organisms was obtained for cigarette smoke. To date, the cumulative experimental data indicate that similar effects are caused by a variety of environmental carcinogens, including polycyclic aromatic hydrocarbons, nitropyrenes, endocrine disruptors, airborne mixtures, carcinogens in food and water, and carcinogenic drugs. Accordingly, the alteration of miRNA expression is a general mechanism that plays an important pathogenic role in linking exposure to environmental toxic agents with their pathological consequences, mainly including cancer development. This review summarizes the existing experimental evidence concerning the effects of chemical carcinogens on the microRNA machinery. For each carcinogen, the specific microRNA alteration signature, as detected in experimental studies, is reported. These data are useful for applying microRNA alterations as early biomarkers of biological effects in healthy organisms exposed to environmental carcinogens. However, microRNA alteration results in carcinogenesis only if accompanied by other molecular damages. As an example, microRNAs altered by chemical carcinogens often inhibits the expression of mutated oncogenes. The long-term exposure to chemical carcinogens causes irreversible suppression of microRNA expression thus allowing the transduction into proteins of mutated oncogenes. This review also analyzes the existing knowledge regarding the mechanisms by which environmental carcinogens alter microRNA expression. The underlying molecular mechanism involves p53-microRNA interconnection, microRNA adduct formation, and alterations of Dicer function. On the whole, reported findings provide evidence that microRNA analysis is a molecular toxicology tool that can elucidate the pathogenic mechanisms activated by environmental carcinogens.

Developmental exposure to valproic acid alters the expression of microRNAs involved in neurodevelopment in zebrafish

Congenital malformations are a prevalent cause of infant mortality in the United States and their induction has been linked to a variety of factors, including exposure to teratogens. However, the molecular mechanisms of teratogenicity are not fully understood. MicroRNAs are an important group of small, non-coding RNAs that regulate mRNA expression. MicroRNA roles in early embryonic development are well established, and their disruption during development can cause abnormalities. We hypothesized that developmental exposure to teratogens such as valproic acid alters microRNA expression profiles in developing embryos. Valproic acid is an anticonvulsant and mood-stabilizing drug used to treat epilepsy, bipolar disorder and migraines. To examine the effects of valproic acid on microRNA expression during development, we used zebrafish embryos as a model vertebrate developmental system. Zebrafish embryos were continuously exposed to valproic acid (1mM) or vehicle control (ethanol) starting from 4h post-fertilization (hpf) and sampled at 48 and 96hpf to determine the miRNA expression profiles prior to and after the onset of developmental defects. At 96hpf, 95% of the larvae showed skeletal deformities, abnormal swimming behavior, and pericardial effusion. Microarray expression profiling was done using Agilent zebrafish miRNA microarrays. Microarray results revealed changes in miRNA expression at both time points. Thirteen miRNAs were differentially expressed at 48hpf and 22 miRNAs were altered at 96hpf. Among them, six miRNAs (miR-16a, 18c, 122, 132, 457b, and 724) were common to both time points. Bioinformatic target prediction and examination of published literature revealed that these miRNAs target several genes involved in the normal functioning of the central nervous system. These results suggest that the teratogenic effects of valproic acid could involve altered miRNA expression.

In situ studies on growth, oxidative stress responses, and gene expression of juvenile bighead carp (Hypophthalmichthys nobilis) to eutrophic lake water dominated by cyanobacterial blooms

Cyanobacterial blooms have received increasing attention as a public biohazard for human and animal health. To assess the effect of cyanobacteria-dominant lake water on juvenile fish, we measured the responses of specific growth rate, condition factor, body weight and body length, oxidative stress, and related gene expression of juvenile bighead carp Hypophthalmichthys nobilis exposed to in situ eutrophic lake (Chl a was around 7.0μgL(-1)). Results showed in situ cyanobacteria-dominant lake water had no effect on the growth performance, but significantly elevated the contents of malondialdehyde, the expression of heat shock protein 70, and the activity of superoxide dismutase, indicating that oxidative stress occurred. Meanwhile in situ lake water significantly decreased the expression of catalase and glutathione S-transferase genes. We conclude that in situ cyanobacteria-dominated lake water was harmful to juvenile bighead carp based on the oxidative stress and changes in the related gene expression levels.

Expression profiling in vivo demonstrates rapid changes in liver microRNA levels of whitefish (Coregonus lavaretus) following microcystin-LR exposure

At present, little is known about the role of miRNAs in liver response of fish to the cyanobacterial hepatotoxin microcystin-LR (MC-LR) treatment, despite the fact that the exposure is thought to underlie multiple acute and chronic effects. To address this question, we used the Real-Time PCR method to examine the differential expression of 6 miRNAs putatively playing roles in signal transduction (let-7c, miR-9b), apoptosis and cell cycle (miR-16a, miR-21a, miR-34a) and fatty acid metabolism (miR-122) in whitefish (Coregonus lavaretus) liver, during the first 48h after intraperitoneal injection of MC-LR (100 μg/kg body weight). In addition, we analyzed expression levels of 8 mRNAs and p53 protein, known to be involved in the cell response on the exposure to environmental stressors. Following the challenge we observed a rapid and transient increase in the mean (n=5) levels of individual miRNA expression (from 2.7-fold for miR-122 to 6.8-fold for let-7c), compared to the respective levels in control fish, which mostly peaked at 24h of the experiment. This increase was correlated with a reduction in the expression of mRNAs of genes coding for ferritin H (frih) and HNK Ras -like protein (p-ras) and an overexpression of mRNAs of genes coding for bcl2-associated X protein (bax), cyclin dependent kinase inhibitor 1a (cdkn1a), dicer (dcr), histone 2A (h2a) and p53. Expression of the remaining caspase 6 (cas6) mRNA did not change over 48 h of the treatment. Moreover, exposure to MC-LR did not alter whitefish p53 protein levels. Bearing in mind a variety of likely silencing targets for, and the onset of, the aberrant miRNA expression it may be concluded that they are involved in molecular pathways, such as liver cell metabolism, cell cycle regulation and apoptosis, and may contribute to the early phase of MC-LR induced hepatotoxicity.

Acute Exposure to Microcystin-Producing Cyanobacterium Microcystis aeruginosa Alters Adult Zebrafish (Danio rerio) Swimming Performance Parameters

Microcystins (MCs) are toxins produced by cyanobacteria (blue-green algae), primarily Microcystis aeruginosa, forming water blooms worldwide. When an organism is exposed to environmental perturbations, alterations in normal behavioral patterns occur. Behavioral repertoire represents the consequence of a diversity of physiological and biochemical alterations. In this study, we assessed behavioral patterns and whole-body cortisol levels of adult zebrafish (Danio rerio) exposed to cell culture of the microcystin-producing cyanobacterium M. aeruginosa (MC-LR, strain RST9501). MC-LR exposure (100 μg/L) decreased by 63% the distance traveled and increased threefold the immobility time when compared to the control group. Interestingly, no significant alterations in the number of line crossings were found at the same MC-LR concentration and time of exposure. When animals were exposed to 50 and 100 μg/L, MC-LR promoted a significant increase (around 93%) in the time spent in the bottom portion of the tank, suggesting an anxiogenic effect. The results also showed that none of the MC-LR concentrations tested promoted significant alterations in absolute turn angle, path efficiency, social behavior, or whole-body cortisol level. These findings indicate that behavior is susceptible to MC-LR exposure and provide evidence for a better understanding of the ecological consequences of toxic algal blooms.