Mercury-induced toxicity: Mechanisms, molecular pathways, and gene regulation.

Mercury is a well-known neurotoxicant for humans and wildlife. The epidemic of mercury poisoning in Japan has clearly demonstrated that chronic exposure to methylmercury (MeHg) results in serious neurological damage to the cerebral and cerebellar cortex, leading to the dysfunction of the central nervous system (CNS), especially in infants exposed to MeHg in utero. The occurrences of poisoning have caused a wide public concern regarding the health risk emanating from MeHg exposure; particularly those eating large amounts of fish may experience the low-level and long-term exposure. There is growing evidence that MeHg at environmentally relevant concentrations can affect the health of biota in the ecosystem. Although extensive in vivo and in vitro studies have demonstrated that the disruption of redox homeostasis and microtube assembly is mainly responsible for mercurial toxicity leading to adverse health outcomes, it is still unclear whether we could quantitively determine the occurrence of interaction between mercurial and thiols and/or selenols groups of proteins linked directly to outcomes, especially at very low levels of exposure. Furthermore, intracellular calcium homeostasis, cytoskeleton, mitochondrial function, oxidative stress, neurotransmitter release, and DNA methylation may be the targets of mercury compounds; however, the primary targets associated with the adverse outcomes remain to be elucidated. Considering these knowledge gaps, in this article, we conducted a comprehensive review of mercurial toxicity, focusing mainly on the mechanism, and genes/proteins expression. We speculated that comprehensive analyses of transcriptomics, proteomics, and metabolomics could enhance interpretation of "omics" profiles, which may reveal specific biomarkers obviously correlated with specific pathways that mediate selective neurotoxicity.

Methylmercury induces ectopic expression of complement components and apoptotic cell death in the retina of the zebrafish embryo.

Methylmercury (MeHg) is a well-known neurotoxin of humans and wildlife. Visual impairments, including blindness, are frequently present in human patients with MeHg poisoning and in affected animals. It is widely assumed that MeHg-induced damage to the visual cortex is the sole or primary cause of vision loss. MeHg has been shown to accumulate in the outer segments of photoreceptor cells, and to alter the thickness of the inner nuclear layer of the fish retina. However, it is unclear whether the bioaccumulated MeHg has direct deleterious effects on the retina. Herein we report that the genes encoding complement components 5 (c5), c7a, c7b, and c9 were ectopically expressed in the inner nuclear layer of the retinas of zebrafish embryos exposed to MeHg (6-50 µg/L). The numbers of apoptotic cell deaths scored in the retinas of MeHg-treated embryos significantly increased in a concentration-dependent manner. In comparison with cadmium and arsenic, ectopic expression of c5, c7a, c7b, and c9, and the observed apoptotic cell death in the retina were specific to MeHg exposure. Our data provide evidence supporting the hypothesis that MeHg has deleterious impacts on the retinal cells, especially the inner nuclear layer. We propose that MeHg-induced retinal cell death may trigger the activation of the complement system.

Toxicogenomic signatures associated with methylmercury induced developmental toxicity in the zebrafish embryos.

Methylmercury (MeHg) is a toxicant with adverse effects on embryogenesis from fish to man. The developmental outcomes of MeHg are well understood, but molecular understanding of toxicity is rather limited. We performed here a genome-wide transcriptional analyses of 6, 30, and 50 µg/L MeHg exposed zebrafish embryos from 4 to 72 h post-fertilization (hpf) using RNA-sequencing and microarray, and conducted a systematical comparison of MeHg-induced transcriptomic responses reported in this and our previous studies. We observed MeHg significantly to disrupt expression of 1050, 1931, and 2996 genes, respectively including gene ontologies in terms of visual and sensory perception, phototransduction, ferroptosis, and GABAergic synapse. Significantly altered genes were associated with ontology categorized into metabolism, such as fatty acid, amino acid, and glutathione metabolism across all experiments. Expression of genes involved in Wnt, Shh, and Notch signaling pathways previously demonstrated to be crucial for development was changed at varying levels dependent on exposure concentrations and durations. Our findings show MeHg significantly to affect expression of genes associated with tissue and/or organs developmental processing including eye, lateral line, fins, and brain, especially in embryos exposed to 6 µg/L, which did not cause obviously toxic effects on zebrafish embryos. We obtain 21 genes being significantly altered by MeHg in a concentration and stage independent manner, and might be served as signatures for developmental toxicity and/or teratogenic effects.

Tal2 is required for generation of GABAergic neurons in the zebrafish midbrain.

BACKGROUND: In the zebrafish midbrain, GABAergic neurons develop from precursors located in the nucleus of the medial longitudinal fasciculus (nMLF). However, the precise mechanisms that underline generation of the nMLF GABAergic neuron are poorly understood. RESULTS: GABAergic neurons in the nMLF co-express transcription factors tal2, gata2a, gata3, and nkx1.2lb. The Nodal-related gene and shh signaling are required for differentiation of nMLF GABAergic neuron precursors. Tal2 is important for nMLF GABAergic neurogenesis. Disruption of Tal2, embryos completely lack the GABA-synthesizing enzyme glutamic acid decarboxylase 67 gene (gad67) expressing cells in the nMLF, and the whole nkx1.2lb expressing cells in the midbrain. Although almost all tal2-expressing cells in the diencephalon and/or nMLF are gata2a- and gata3-positive, simultaneous knockdown of gata2a and gata3 does not affect either tal2 or gad67 expression. CONCLUSIONS: In the zebrafish midbrain, expression of tal2, gata2a, and/or gata3 is independent of each other. The function of gata2a and gata3 is dispensable for generation of GABAergic neuron in the nMLF. This suggests that the functional connections of the regulatory genes leading to generation of nMLF GABAergic neurons have diverged between mouse and zebrafish.

Methylmercury-induced hair cell loss requires hydrogen peroxide production and leukocytes in zebrafish embryos.

Hearing impairment and deafness is frequently observed as one of the neurological signs in patients with Minamata disease caused by methylmercury (MeHg) poisoning. Loss of hair cells in humans and animals is a consequence of MeHg poisoning. However, it is still not clear how MeHg causes hearing deficits. We employed the hair cells of the lateral line system of zebrafish embryos as a model to explore this question. We exposed transgenic zebrafish embryos to MeHg (30-360 µg/L) at the different stages, and scored the numbers of hair cells. We find that MeHg-induced reduction of hair cells is in a concentration dependent manner. By employing antisense morpholino against to pu.1, we confirm that loss of hair cells involves the action of leukocytes. Moreover, hair cell loss is attenuated by co-treating MeHg-exposed embryos with pharmacological inhibitors of NADPH oxidases named diphenyleneiodonium (DPI) and VAS2870. In situ gene expression analysis showed that genes encoding the SQSTM1-Keap1-Nrf2 systems involved in combating oxidative stress and immune responses are highly expressed in the lateral line organs of embryos exposed to MeHg. This suggests that induction of hydrogen peroxide (H2O2) is the primary effect of MeHg on the hair cells. Genes induced by MeHg are also involved in regeneration of the hair cells. These features are likely related to the capacity of the zebrafish to regenerate the lost hair cells.

Antitumor and off-target effects of cholesterol-conjugated let-7a mimics in an orthotopic hepatocellular carcinoma xenograft nude mouse model.

To explore the antitumor and potential off-target effects of systemically delivered cholesterol-conjugated let-7a mimics (Chol-let-7a) and control mimics (Chol-miRCtrl) on hepatocellular carcinoma in vivo. Methods: The antitumor effects of two intravenous dosing regimens of Chol-let-7a on heptocellular carcinoma growth were compared using an orthotopic xenograft mouse model. Off-targets were analyzed with histopathological and ultrapathological features of heparenal tissue and cells in the Chol-let-7a-, Chol-miRCtrl-, and saline-treated (blank) xenograft mice and normal control mice. Then, let-7a abundance in orthotopic tumors, corresponding paracancerous hepatic tissue, and normal liver tissue from healthy nude mice was examined by reverse transcription-polymerase chain reaction. The distribution of Chol-let-7a and Chol-miRCtrl in vivo was examined by whole-animal imaging and frozen-sections observation. The experiments were approved by the Institutional Research Board of Peking Union Medical College Hospital. Results: Continuous treatment with Chol-let-7a resulted in tumors that were 35.86% and 40.02% the size of those in the Chol-miRCtrl and blank xenograft group (P < 0.01 and P < 0.01, respectively), while intermittent dosing with Chol-let-7a resulted in tumors that were 65.42% and 56.66% the size of those in the Chol-miRCtrl and the blank control group, respectively (P < 0.05 and P < 0.05). In addition, some histopathological and ultrapathological features were only observed after treatment with the two cholesterol-conjugated molecules, however mild with intermittent dosing Chol-let-7a treatment, such as diffuse sinusoidal dilation and edema, primarily around the centrolobular vein in heptic tissues; mild hypercellularity with dilated capillary lumens in the renal tissue; and some organelle abnormalities found in heptic and renal cells. Furthermore, whole-animal imaging showed that Chol-let-7a and Chol-miRCtrl were predominantly distributed in the liver, kidney, and bladder regions after injection, and that the concentration of Chol-let-7a and Chol-miRCtrl in the kidney and the bladder decreased much slowly in the xenograft animals, especially in the Chol-miRCtrl group. Finally, RT-PCR analysis showed that let-7a levels were significantly increased in Chol-let-7a-treated xenografts compared with Chol-miRCtrl group (P=0.003) and blank xenograft group (P=0.001); however, the level was only equivalent to 50.6% and 40.7% of that in paracancerous hepatic tissue and hepatic tissue in normal mice, respectively. Conclusions: Chol-let-7a, administered either continuously or intermittently, showed effective antitumor efficacy. Chol-let-7a had some off-target effects, such as mild acute hepatitis-like inflammation and non-specific drug-induced kidney injury. The intermittent dosing regimen resulted in less damage than the continuous regimen, while maintaining relatively satisfactory antitumor efficacy, which could be useful for the investigation and possible clinical use of miRNA treatment regimens in the future.

Toxicity of mercury: Molecular evidence.

Minamata disease in Japan and the large-scale poisoning by methylmercury (MeHg) in Iraq caused wide public concerns about the risk emanating from mercury for human health. Nowadays, it is widely known that all forms of mercury induce toxic effects in mammals, and increasing evidence supports the concern that environmentally relevant levels of MeHg could impact normal biological functions in wildlife. The information of mechanism involved in mercurial toxicity is growing but knowledge gaps still exist between the adverse effects and mechanisms of action, especially at the molecular level. A body of data obtained from experimental studies on mechanisms of mercurial toxicity in vivo and in vitro points to that disruption of the antioxidant system may play an important role in the mercurial toxic effects. Moreover, the accumulating evidence indicates that signaling transduction, protein or/and enzyme activity, and gene regulation are involving in mediating toxic and adaptive response to mercury exposure. We conducted here a comprehensive review of mercurial toxic effects on wildlife and human, in particular synthesized key findings of molecular pathways involved in mercurial toxicity from the cells to human. We discuss the molecular evidence related mercurial toxicity to the adverse effects, with particular emphasis on the gene regulation. The further studies relying on Omic analysis connected to adverse effects and modes of action of mercury will aid in the evaluation and validation of causative relationship between health outcomes and gene expression.

Transcriptome analysis of response mechanism to ammonia stress in Asian clam (Corbicula fluminea).

Corbicula fluminea is highly sensitive to ammonia, and its response mechanism to ammonia stress is unclear. In this study, C. fluminea was exposed to different levels of ammonia (control group, 10 mg/L, and 25 mg/L) for 24 h and 48 h. A comparative analysis of transcriptome sequencing (RNA-seq) of C. fluminea digestive gland showed that the expression of 6742 genes (11.54%) was significantly affected by ammonia stress. The TLR, NF-κB, FOXO, and apoptotic signaling pathways were involved in the regulation. The differential expression of 14 genes was confirmed by real-time PCR. In summary, the response mechanism of C. fluminea digestive gland under ammonia stress may be different from that of oxidative stress in marine vertebrates. Also, the NMDAR-mediated pathway may not be the main mechanism in the response to ammonia stress in C. fluminea. The present study is a preliminary study for further investigation into ammonia toxicity in shellfish.

Exposure to Ambient Air Particles Increases the Risk of Mental Disorder: Findings from a Natural Experiment in Beijing.

Epidemiology studies indicated that air pollution has been associated with adverse neurological effects in human. Moreover, the secretion of glucocorticoid (GC) affects the mood regulation, and the negative feedback of hippocampal glucocorticoid receptors (GR) inhibits the GC secretion. Meanwhile, the over secretion of GC can interfere the immune system and induce neurotoxicity. In the present study, the human test showed that the secretion of the cortisol in plasma was elevated after exposure in heavy air pollution. In the mouse model, we found that breathing the highly polluted air resulted in the negative responses of the mood-related behavioral tests and morphology of hippocampus, as well as the over secretion of GC in plasma, down regulation of GR, and up-regulation of cytokine and chemokine in the hippocampus. When considering the interrelated trends between the hippocampal GR, inflammatory factors, and plasmatic GC, we speculated that PM2.5 exposure could lead to the increased secretion of GC in plasma by decreasing the expression of GR in hippocampus, which activated the inflammation response, and finally induced neurotoxicity, suggesting that PM2.5 exposure negatively affects mood regulation. When combined with the results of the human test, it indicated that exposure to ambient air particles increased the risk of mental disorder.