Sublethal levels of cadmium down-regulate the gene expression of DNA mismatch recognition protein MutS homolog 6 (MSH6) in zebrafish (Danio rerio) embryos

Comparative Transcriptome Analysis of the Hepatopancreas from Macrobrachium rosenbergii Exposed to the Heavy Metal Copper

The contamination of aquatic ecosystems by the heavy metal copper (Cu) is an important environmental issue and poses significant risks to the physiological functions of aquatic organisms. Macrobrachium rosenbergii is one of the most important freshwater-cultured prawns in the world. The hepatopancreas of crustaceans is a key organ for immune defense, heavy metal accumulation, and detoxification, playing a pivotal role in toxicological research. However, research on the molecular response of the hepatopancreas in M. rosenbergii to Cu exposure is still lacking. In this study, the transcriptomic response in the hepatopancreas of M. rosenbergii was studied after Cu exposure for 3 and 48 h. Compared with the control group, 11,164 (7288 up-regulated and 3876 down-regulated genes) and 10,937 (6630 up-regulated and 4307 down-regulated genes) differentially expressed genes (DEGs) were identified after 3 and 48 h exposure, respectively. Most of these DEGs were up-regulated, implying that gene expressions were largely induced by Cu. Functional enrichment analysis of these DEGs revealed that immunity, copper homeostasis, detoxification, DNA damage repair, and apoptosis were differentially regulated by Cu. Seven genes involved in immunity, detoxification, and metabolism were selected for validation by qRT-PCR, and the results confirmed the reliability of RNA-Seq. All these findings suggest that M. rosenbergii attempts to resist the toxicity of Cu by up-regulating the expression of genes related to immunity, metabolism, and detoxification. However, with the excessive accumulation of reactive oxygen species (ROS), the antioxidant enzyme system was destroyed. As a result, DNA damage repair and the cellular stress response were inhibited, thereby exacerbating cell damage. In order to maintain the normal function of the hepatopancreas, M. rosenbergii removes damaged cells by activating the apoptosis mechanism. Our study not only facilitates an understanding of the molecular response mechanisms of M. rosenbergii underlying Cu toxicity effects but also helps us to identify potential biomarkers associated with the stress response in other crustaceans.

Susceptibility of DNA damage recognition activities linked to nucleotide excision and mismatch repair in zebrafish (Danio rerio) early and mid-early embryos to 2.5 to 4.5 °C heat stress

Fish at early life stages are sensitive to temperature change because of their narrower temperature tolerance ranges. Initiated by damage detection, DNA mismatch repair (MMR) and nucleotide excision repair (NER) maintain genome integrity respectively by eliminating mismatched nucleotides and helix-distorting DNA lesions. As discharge of heated effluent from power plants may elevate water temperatures to only 2 to 6 °C higher than ambient, this study explored if temperatures within this range affected MMR and NER-linked damage detection activities in fish embryos using zebrafish (Danio rerio) embryo as a model organism. Exposure of early embryos at 10 h post fertilization (hpf) to a warmer temperature at + 4.5 °C for 30 min enhanced damage recognition activities targeting UV-induced cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts (6-4PPs) that distorted helical structures. Conversely, photolesions sensing activities were inhibited in 24 hpf mid-early embryos under the same stress conditions. A much higher temperature at + 8.5 °C imposed similar effects on UV damage detection. A mild heat stress at + 2.5 °C for 30 min, however, repressed both CPD and 6-4PP binding activities in 10 and 24 hpf embryos. Inhibition of damage recognition under mild heat stress impeded the overall NER capacity evidenced by a transcription-based repair assay. Warmer water temperatures at + 2.5 and + 4.5 °C also inhibited G-T mismatch binding activities in 10 and 24 hpf embryos, but G-T recognition was more sensitive to + 4.5 °C stress. Inhibition of G-T binding partially correlated with a downregulation of Sp1 transcription factor activity. Our results showed the potential of water temperature elevation within 2 to 4.5 °C to disturb DNA damage repair in fish at embryonic stages.

MicroRNAs as the critical regulators of autophagy-mediated cisplatin response in tumor cells

Chemotherapy is one of the most common therapeutic methods in advanced and metastatic tumors. Cisplatin (CDDP) is considered as one of the main first-line chemotherapy drugs in solid tumors. However, there is a high rate of CDDP resistance in cancer patients. Multi-drug resistance (MDR) as one of the main therapeutic challenges in cancer patients is associated with various cellular processes such as drug efflux, DNA repair, and autophagy. Autophagy is a cellular mechanism that protects the tumor cells toward the chemotherapeutic drugs. Therefore, autophagy regulatory factors can increase or decrease the chemotherapy response in tumor cells. MicroRNAs (miRNAs) have a pivotal role in regulation of autophagy in normal and tumor cells. Therefore, in the present review, we discussed the role of miRNAs in CDDP response through the regulation of autophagy. It has been reported that miRNAs mainly increased the CDDP sensitivity in tumor cells by inhibition of autophagy. PI3K/AKT signaling pathway and autophagy-related genes (ATGs) were the main targets of miRNAs in the regulation of autophagy-mediated CDDP response in tumor cells. This review can be an effective step to introduce the miRNAs as efficient therapeutic options to increase autophagy-mediated CDDP sensitivity in tumor cells.

Mutation and selection processes regulating short tandem repeats give rise to genetic and phenotypic diversity across species

Short tandem repeats (STRs) are units of 1-6 bp that repeat in a tandem fashion in DNA. Along with single nucleotide polymorphisms and large structural variations, they are among the major genomic variants underlying genetic, and likely phenotypic, divergence. STRs experience mutation rates that are orders of magnitude higher than other well-studied genotypic variants. Frequent copy number changes result in a wide range of alleles, and provide unique opportunities for modulating complex phenotypes through variation in repeat length. While classical studies have identified key roles of individual STR loci, the advent of improved sequencing technology, high-quality genome assemblies for diverse species, and bioinformatics methods for genome-wide STR analysis now enable more systematic study of STR variation across wide evolutionary ranges. In this review, we explore mutation and selection processes that affect STR copy number evolution, and how these processes give rise to varying STR patterns both within and across species. Finally, we review recent examples of functional and adaptive changes linked to STRs.

Resistance of mitochondrial DNA to cadmium and Aflatoxin B1 damage-induced germline mutation accumulation in C. elegans

Mitochondrial DNA (mtDNA) is prone to mutation in aging and over evolutionary time, yet the processes that regulate the accumulation of de novo mtDNA mutations and modulate mtDNA heteroplasmy are not fully elucidated. Mitochondria lack certain DNA repair processes, which could contribute to polymerase error-induced mutations and increase susceptibility to chemical-induced mtDNA mutagenesis. We conducted error-corrected, ultra-sensitive Duplex Sequencing to investigate the effects of two known nuclear genome mutagens, cadmium and Aflatoxin B1, on germline mtDNA mutagenesis in Caenorhabditis elegans. Detection of thousands of mtDNA mutations revealed pervasive heteroplasmy in C. elegans and that mtDNA mutagenesis is dominated by C:G → A:T mutations generally attributed to oxidative damage. However, there was no effect of either exposure on mtDNA mutation frequency, spectrum, or trinucleotide context signature despite a significant increase in nuclear mutation rate after aflatoxin B1 exposure. Mitophagy-deficient mutants pink-1 and dct-1 accumulated significantly higher levels of mtDNA damage compared to wild-type C. elegans after exposures. However, there were only small differences in mtDNA mutation frequency, spectrum, or trinucleotide context signature compared to wild-type after 3050 generations, across all treatments. These findings suggest mitochondria harbor additional previously uncharacterized mechanisms that regulate mtDNA mutational processes across generations.

Exosomal CTCF Confers Cisplatin Resistance in Osteosarcoma by Promoting Autophagy via the IGF2-AS/miR-579-3p/MSH6 Axis

Cancer-derived exosomes participate in carcinogenesis and progression of cancers, including metastasis and drug-resistance. Of note, CTCF has been suggested to induce drug resistance in various cancers. Herein, we aim to investigate the role of cisplatin- (CDDP-) resistant osteosarcoma- (OS-) derived exosomal CTCF in OS cell resistance to CDDP and its mechanistic basis. Differentially expressed transcription factors, long noncoding RNAs (lncRNAs), miRNAs, and genes in OS were retrieved using bioinformatics approaches. Exosomes were extracted from CDDP-resistant OS cells and then cocultured with parental OS cells, followed by lentiviral transduction to manipulate the expression of CTCF, IGF2-AS, miR-579-3p, and MSH6. We assessed the in vitro and in vivo effects on malignant phenotypes, autophagy, CDDP sensitivity, and tumor formation of OS cells. It was established that CTCF and IGF2-AS were highly expressed in CDDP-resistant OS cells, and the CDDP-resistant OS cell-derived exosomal CTCF enhanced IGF2-AS transcription. CDDP-resistant OS-derived exosomes transmitted CTCF to OS cells and increased CDDP resistance in OS cells by activating an autophagy-dependent pathway. Mechanistically, CTCF activated IGF2-AS transcription and IGF2-AS competitively bound to miR-579-3p to upregulate MSH6 expression. Additionally, the promoting function of exosomal CTCF-mediated IGF2-AS/miR-579-3p/MSH6 in OS cell resistance to CDDP was confirmed in vivo. Taken together, CDDP-resistant OS-derived exosomal CTCF enhanced resistance of OS cells to CDDP via activating the autophagy-dependent pathway, providing a potential therapeutic consideration for OS treatment.

Protective role of zinc in Spodoptera exigua larvae under 135-generational cadmium exposure

The aim of this study was to investigate whether zinc supplementation modulates cadmium toxicity in the beet armyworm Spodoptera exigua selected for 135 generations towards cadmium tolerance. To achieve this, larvae originating from three laboratory populations of S. exigua (control strain - C; cadmium-intoxicated for 135 generations strain - Cd, and control strain intoxicated with Cd for 1 generation - CCd) were additionally exposed to zinc in three concentrations (Zn1, 400 μg Zn·g^-1 dry mass of food; Zn2; 200 μg Zn·g^-1 dry mass of food; Zn3, 100 μg Zn·g^-1 dry mass of food). As the markers of toxicity, a life history traits (the duration of L4 and L5 stages), cellular (DNA damage indices) and biochemical parameters (ADP/ATP ratio and ATP and HSP70 concentrations) were chosen. The duration of larval stages of Zn supplemented larvae was prolonged, while cellular and biochemical indicators, in general, appeared to be lower in comparison to the insects from respective reference groups in each laboratory populations. Moreover, the range of the differences depended on zinc concentration in food. We can suspect that zinc supplementation contributed to the protection of S. exigua individuals against negative effects of cadmium intoxication, probably at the cost of growth rate. Significant differences in the response pattern between insects from different laboratory populations indicate that the influence of additional stress factors is dependent on the overall condition of animals and their previous adaptation to other stressors.

Heat stress upregulates G-T mismatch binding activities in zebrafish (Danio rerio) embryos preexposed and nonexposed to a sublethal level of cadmium (Cd)

G-T mispair frequently appears in eukaryotic DNA due to the spontaneous deamination of 5-methylcytosine paired with guanine and is therefore an important target for DNA mismatch repair (MMR). Our earlier studies showed the downregulation of G-T binding activities in cadmium (Cd)-exposed (Danio rerio) embryos. Since elevation of water temperature was reported to increase Cd toxicity in zebrafish, this study explored whether heat stress affected zebrafish mismatch binding capacity in the absence or presence of Cd. Heat stress (37 °C for 30 min) induced heat shock protein 70 mRNA expression in embryos at 10 and 24 h post fertilization (hpf). Heat stress weakly upregulated normal G-T sensing machinery and inhibited G-T recognition activity in embryos preexposed to 3 μM Cd for 9 h. Either heat shock or a 23-h Cd treatment alone caused a 1.7-fold stimulation of G-T binding capacity in 24 hpf embryos and heat stress of Cd-preexposed embryos further enhanced G-T binding activity to 2.5 fold of control. Normal and Cd-downregulated loop binding activities in 10 and 24 hpf embryos were almost unreactive to heat shock. Heat stress-upregulated G-T sensing in nonexposed, but not in Cd-preexposed, 24 hpf embryos correlated with stronger gene activities encoding MMR-linked mismatch detecting factors MutS homolog 2 and 6 plus a higher DNA binding activity of the transcription factor Sp1 that regulates msh2/msh6 expression. Our results suggested the importance of heat shock response in facilitating the correction of G-T mismatch in developing zebrafish even under Cd exposure.

High-throughput gene expression in soil invertebrate embryos - Mechanisms of Cd toxicity in Enchytraeus crypticus

Gene expression can vary with the organisms' life stage. It is known that embryos can be more sensitive to toxicant exposure, as previously demonstrated for Enchytraeus crypticus (Oligochaeta) exposed to cadmium (Cd), known to cause embryotoxicity and hatching delay. It was shown that Ca enters embryos via the L-type Ca channels in the cocoon membrane, this being affected in Cd exposed embryos (Cd-Ca competition is well-known). In the present study, the embryotoxic mechanisms of Cd were studied via high-throughput gene expression for E. crypticus. Cocoons (1-2 days old), instead of the adult organism, were exposed in Cd spiked LUFA 2.2 soil during 1 day. Results showed that Cd affected Ca homeostasis which is implicated in several other molecular processes. Several of the major modulators of Cd toxicity (e.g., impaired gene expression, cell cycle arrest, DNA and mitochondrial damage) were identified in the embryos showing its relevancy as a model in ecotoxicogenomics. The draft Adverse Outcome Pathway was improved. Previously was hypothesized that gene regulation mechanisms were activated to synthesize more Ca channel proteins - this was confirmed here. Further, novel evidences were that, besides the extracellular competition, Cd competes intracellularly which causes a reduction in Ca efflux, and potentiates Cd embryotoxicity.

Linking arsenite- and cadmium-generated oxidative stress to microsatellite instability in vitro and in vivo

Mismatch repair (MMR) corrects replicative errors and minimizes DNA damage that occurs frequently in microsatellites. MMR deficiency is manifested as microsatellite instability (MSI), which contributes to hypermutability and cancer pathogenesis. Genomic instability, including MSI and chromosomal instability, appears to be responsible for the carcinogenesis of arsenic and cadmium, common contaminants in our environment. However, few studies have addressed arsenic- or cadmium-induced MSI, especially its potential link with arsenic- or cadmium-generated oxidative stress, due to the lack of quantifiable MSI assays and cost-effective animal models. Here, using a dual-fluorescent reporter, we demonstrate that sub-lethal doses of cadmium or arsenite, but not arsenate, increased the MSI frequency in human colorectal cancer cells. Arsenite- and cadmium-induced MSI occurred concomitantly with increased levels of reactive species and oxidative DNA damage, and with decreased levels of MMR proteins. However, N-acetyl-l-cysteine (NAC) suppressed arsenite- and cadmium-induced MSI and oxidative stress while restoring the levels of MMR proteins in the cells. Similarly, MSI was induced separately by arsenite and cadmium, and suppressed by NAC, in zebrafish in a fluorescinated PCR-based assay with newly-developed microsatellite markers and inter-segmental comparisons. Of five selected antioxidants examined, differential effects were exerted on the MSI induction and cytotoxicity of both arsenite and cadmium. Compared to MMR-proficient cells, MMR-deficient cells were more resistant to arsenic-mediated and cadmium-mediated cytotoxicity. Our findings demonstrate a novel linkage between arsenite-generated and cadmium-generated oxidative stress and MSI induction. Our findings also caution that antioxidants must be individually validated before being used for preventing arsenite- and cadmium-induced MSI that is associated with cancer development.

Oxidative stress intensity-related effects of cadmium (Cd) and paraquat (PQ) on UV-damaged-DNA binding and excision repair activities in zebrafish (Danio rerio) embryos

Our earlier studies showed the inhibitory effects of cadmium (Cd) and paraquat (PQ) on the gene expression of DNA mismatch recognition proteins in zebrafish (Danio rerio) embryos. This study explored the effects of Cd and PQ on nucleotide excision repair (NER) capacity in zebrafish embryos. Exposure of embryos at 1 h post fertilization (hpf) to 3-5 μM Cd or 30-100 μM PQ for 9 h induced a 2-3-fold increase of oxidative stress, while a 6.5-fold increase of oxidative stress was induced by 200 μM PQ. Real-time RT-PCR detected a down-regulated xeroderma pigmentosum C (XPC) and an up-regulated UV-DDB2 gene expression in mildly-stressed embryos, whereas 8-oxoguanine DNA glycosylase (OGG1) gene expression increased with PQ exposure levels. NER of UV-damaged DNA was enhanced in weakly oxidant-stressed embryos as shown by a transcription-based DNA repair assay, yet repair activities of both UV and cisplatin-damaged DNA were inhibited in embryos exposed to 200 μM PQ. Band shift assay showed a suppression of cyclobutane pyrimidine dimer (CPD) binding activity in all stressed embryos. In contrast, (6-4) photoproduct (6-4PP) recognition activity was weakly stimulated except in embryos exposed to 200 μM PQ, revealing a link of NER capacity to 6-4PP binding. Our results showed that Cd and PQ imposed similar inducing effects on UV-DDB2 gene expression, NER of UV-damaged DNA and 6-4PP binding activity in zebrafish embryo under low levels of oxidative stress and NER capacity could be inhibited if the intensity of oxidative stress increased to a critical level.

Molecular and phenotypic responses of Japanese medaka (Oryzias latipes) early life stages to environmental concentrations of cadmium in sediment

Japanese medaka embryos were exposed to environmental concentrations of cadmium (Cd) to investigate adverse and adaptive responses in fish early life stages. Embryos were exposed during their whole development by static sediment-contact to environmental Cd concentrations (2 and 20 μg/g dry weight). Cd bioaccumulation, developmental defects, biochemical and biomolecular (qRT-PCR) responses were analyzed in embryos and hatchlings. A dose-dependent increase of Cd bioaccumulation and developmental defects was observed at hatching. Cd had clear impacts on heartbeat and cardiac morphogenesis and also induced to spinal deformities. The profile and the level of gene transcription were differentially modulated according to the Cd concentration, the duration of exposure and/or the developmental stage of fish. Pro-apoptotic bax and DNA repair rad51 transcripts were significantly repressed in embryos exposed to the highest Cd concentration. Repression of these genes was correlated to the increase of heart rate in 6-day-old embryos. NADH-dehydrogenase nd5 gene transcription was inhibited in larvae at the lowest concentration suggesting mitochondrial respiratory chain impairment, in association with Cd-induced teratogenicity. Finally, wnt1 gene was overexpressed indicating putative deregulation of Wnt signaling pathway, and suggested to be implied in the occurrence of some spinal and cardiac deformities. Results of this study permitted to propose some promising markers at the transcriptional and phenotypical level, responding to environmental concentrations of Cd. The present work also highlights the usefulness of the modified version of the medaka embryo-larval assay with sediment-contact exposure (MELAc) to investigate the toxicity and the modes of action of sediment-bound pollutants.

In vivo DNA mismatch repair measurement in zebrafish embryos and its use in screening of environmental carcinogens

Impairment of DNA mismatch repair (MMR) function leads to the development and progression of certain cancers. Many environmental contaminants can target DNA MMR system. Currently, measurement of MMR activity is limited to in vitro or in vivo methods at the cell line level, and reports on measurement of MMR activity at the live organism level are lacking. Here, we report an efficient method to measure DNA MMR activity in zebrafish embryos. A G-T mismatch was introduced into enhanced green fluorescent protein (EGFP) gene. Repair of the G-T mismatch to G-C in the heteroduplex plasmid generates a functional EGFP expression. The heteroduplex plasmid and a similarly constructed homoduplex plasmid were injected in parallel into the same batch of embryos at 1-cell stage and EGFP expression in EGFP positive embryos was quantified at 24 h after injection. MMR efficiency was calculated as the total fluorescence intensity of embryos injected with the heteroduplex construct divided by that of embryos injected with the homoduplex construct. Our results showed 73% reduction of MMR activity in embryos derived from MMR-deficient mlh1 mutant fish (positive control) when compared with embryos from MMR-competent wild type AB line fish, indicating feasibility of in vivo MMR activity measurement in zebrafish embryos. We further applied this novel assay for measurement of MMR efficiency in embryos exposed to environmental chemicals such as cadmium chloride (CdCl2), benzo[a]pyrene (BaP), and perfluorooctanesulphonic acid (PFOS) from 6 hpf to 24 hpf. We observed significant reductions of MMR efficiency in embryos exposed to 0.1 μM CdCl2 (52%) and 0.5 μM BaP (34%), but no effect in embryos exposed to PFOS. Our study for the first time provides a model system for in vivo measurement of DNA MMR activity at the organism level, which has important implications in risk assessment of various environmental carcinogens.

Zebrafish genome instability after exposure to model genotoxicants

Sublethal exposure to environmental genotoxicants may impact genome integrity in affected organisms. It is therefore necessary to develop tools to measure the extent and longevity of genotoxicant-induced DNA damage, and choose appropriate model organisms for biomonitoring. To this end, markers of DNA damage were measured in zebrafish larvae and adults following exposure to model genotoxicants (benzo[a]pyrene and ethyl methanesulfonate). Specifically, we assessed primary DNA damage and the existence of potentially persistent genomic alterations through application of the comet assay, quantitative random amplified polymorphic DNA (qRAPD) and amplified fragment length polymorphism (AFLP) assays. Furthermore, expression of genes involved in DNA repair, oxidative stress response and xenobiotic metabolism was evaluated as well. Additionally, the AFLP method was applied to adult specimens 1 year after larval exposure to the genotoxicants to evaluate the longevity of the observed DNA alterations. Large numbers of DNA alterations were detected in larval DNA using the comet assay, qRAPD and AFLP, demonstrating that zebrafish larvae are a sensitive model for revealing genotoxic effects. Furthermore, some of these genomic alterations persisted into adulthood, indicating the formation of stable genomic modifications. qRAPD and AFLP methods proved to be highly sensitive to genotoxic effects, even in cases when the comet assay indicated a lack of significant damage. These results thus support the use of zebrafish larvae as a sensitive model for monitoring the impact of genotoxic insult and give evidence of the longevity of genomic modifications induced by genotoxic agents.

Cadmium effects on the retina of adult Danio rerio

The aim of this work is to describe the effects of cadmium pollution on the vision of adult zebrafish, Danio rerio. Retinal morpho-cytological alterations were investigated by light and electron microscopy, while the functionality of cadmium-exposed retinae was assessed by re-illumination behavioral tests with white or colored light. Our results demonstrate that cadmium toxicity causes significant degeneration and loss of organization at both macro and microscopic levels. These alterations impair functional responses particularly through an increase in light sensitivity. Metallothioneins were not seen to be up-regulated, while the recovery of visual acuity is due to a regenerative process by Müller cells.

Cell cycle and mismatch repair genes as potential biomarkers in Arabidopsis thaliana seedlings exposed to silver nanoparticles

The expression of cell cycle genes and DNA mismatch repair (MMR) genes were analyzed in Arabidopsis thaliana seedlings exposed to 0, 0.2, 0.5 and 1 mg/L of silver nanoparticles for 24, 48 and 72 h using real-time PCR. Significant up-regulation of AtPCNA1 was observed after 24 h exposure to 0.2 and 0.5 mg/L of silver nanoparticles. AtPCNA2 gene was up-regulated after 24, 48 and 72 h exposure to 0.5 and 1 mg/L of silver nanoparticles. AtMLH1 gene was up-regulated after 48 h exposure to 0.5 and 1 mg/L of silver nanoparticles and down-regulated after 72 h. Down-regulation of AtMSH2, AtMSH3, AtMSH6 and AtMSH7 mRNA was observed after exposure to all concentrations of silver nanoparticles for different time periods. Exposure to silver ions showed no significant change in the expression levels of AtPCNA and MMR genes. The results show that AtPCNA and MMR genes could be used as potential molecular biomarkers.

Assessment of Jatropha curcas L. biodiesel seed cake toxicity using the zebrafish (Danio rerio) embryo toxicity (ZFET) test

Consequent to the growing demand for alternative sources of energy, the seeds from Jatropha curcas remain to be the favorite for biodiesel production. However, a significant volume of the residual organic mass (seed cake) is produced during the extraction process, which raises concerns on safe waste disposal. In the present study, we assessed the toxicity of J. curcas seed cake using the zebrafish (Danio rerio) embryotoxicity test. Within 1-h post-fertilization (hpf), the fertilized eggs were exposed to five mass concentrations of J. curcas seed cake and were followed through 24, 48, and 72 hpf. Toxicity was evaluated based on lethal endpoints induced on zebrafish embryos namely egg coagulation, non-formation of somites, and non-detachment of tail. The lowest concentration tested, 1 g/L, was not able to elicit toxicity on embryos whereas 100 % mortality (based also on lethal endpoints) was recorded at the highest concentration at 2.15 g/L. The computed LC50 for the J. curcas seed cake was 1.61 g/L. No further increase in mortality was observed in the succeeding time points (48 and 72 hpf) indicating that J. curcas seed cake exerted acute toxicity on zebrafish embryos. Sublethal endpoints (yolk sac and pericardial edema) were noted at 72 hpf in zebrafish embryos exposed to higher concentrations. The observed lethal endpoints induced on zebrafish embryos were discussed in relation to the active principles, notably, phorbol esters that have remained in the seed cake even after extraction.

Zebrafish as a model system to study DNA damage and repair

Zebrafish (Danio rerio) have become a popular vertebrate model to study embryological development, because of unique advantages not found in other model systems. Zebrafish share many gene functions with other vertebrates including humans, making zebrafish a useful system for studying cancer etiology. However, systematic studies of DNA damage and repair pathways using adult or embryonic zebrafish have not been extensively reported. The zebrafish genome contains nearly all the genes involved in different DNA repair pathways in eukaryotes, including direct reversal (DR), mismatch repair (MMR) nucleotide excision repair (NER), base excision repair (BER), homologous recombination (HR), non-homologous end joining (NHEJ) and translesion synthesis (TLS). It also includes the genes of the p53-mediated damage recognition pathway. Therefore, zebrafish provide an ideal model for gaining fundamental insights into mechanisms of DNA damage and repair, especially during embryological development. This review introduces recent work on different DNA damage and repair studies in zebrafish, with special emphasis on the role of BER in zebrafish early embryological development. AP endonuclease 1 (Apex1), a critical protein in the BER pathway, not only regulates BER but also controls cyclic AMP response binding protein (Creb1), which itself regulates ∼25% of eukaryotic coding sequences. In addition, Apex1 indirectly regulates levels of p53. As these findings also occur in murine B cells, they illustrate the usefulness of the zebrafish system in elucidating fundamental mechanisms.

Cadmium(Cd)-induced oxidative stress down-regulates the gene expression of DNA mismatch recognition proteins MutS homolog 2 (MSH2) and MSH6 in zebrafish (Danio rerio) embryos

DNA mismatch repair (MMR) of simple base mismatches and small insertion-deletion loops in eukaryotes is initiated by the binding of the MutS homolog 2 (MSH2)-MSH6 heterodimer to mismatched DNA. Cadmium (Cd) is a genotoxic heavy metal that has been recognized as a human carcinogen. Oxidant stress and inhibition of DNA repair have been proposed as major factors underlying Cd genotoxicity. Our previous studies indicated the ability of Cd to disturb the gene expression of MSH6 in zebrafish (Danio rerio) embryos. This study was undertaken to explore if Cd-induced oxidative stress down-regulated MSH gene activities. Following the exposure of zebrafish embryos at 1 h post fertilization (hpf) to sublethal concentrations of Cd at 3-5 μM for 4 or 9 h, a parallel down-regulation of MSH2, MSH6 and Cu/Zn superoxide dismutase (Cu/Zn-SOD) gene expression was detected by real-time RT-PCR and the expression levels were 40-50% of control after a 9-h exposure. Cd exposure also induced oxidative stress, yet no inhibition of catalase gene activity was observed. Whole mount in situ hybridization revealed a wide distribution of msh6 mRNA in the head regions of 10 hpf embryos and pretreatment of embryos with antioxidants butylhydroxytoluene (BHT), d-mannitol or N-acetylcysteine (NAC) at 1-10 μM restored Cd-suppressed msh6 expression. QPCR confirmed the protective effects of antioxidants on Cd-suppressed msh2/msh6 mRNA production. Down-regulated MSH gene activities reaching about 50% of control were also induced in embryos exposed to paraquat, a reactive oxygen species (ROS)-generating herbicide, or hydrogen peroxide at 200 μM. Hence, Cd at sublethal levels down-regulates msh2/msh6 expression primarily via ROS as signaling molecules. The transcriptional activation of human msh6 is known to be fully dependent on the specificity factor 1 (Sp1). Cd failed to inhibit the DNA binding activity of zebrafish Sp1 unless at lethal concentrations based on band shift assay, therefore excluding the involvement of Sp1 inactivation in Cd-induced MSH gene inhibition in zebrafish embryos.

DNA strand breaks detected in embryos of the adult snails, Potamopyrgus antipodarum, and in neonates exposed to genotoxic chemicals

We tested the freshwater mudsnail Potamopyrgus antipodarum, which is a species that has already been used for endocrine-disrupting compounds (EDCs) to determine whether early life stages of aquatic organisms are sensitive to genotoxic chemicals. For this purpose, we first developed the alkaline comet assay on adults, embryos, and neonates. The comet assay protocol was validated on both embryonic cells exposed in vitro to hydrogen peroxide and adult snails in the reproducing stage exposed to methyl methane sulfonate. During the latter experiment, DNA strand breaks were investigated on both embryonic cells and on adult gill cells. The second part of this study investigated the stability of DNA strand breaks in adult reproducing snails and neonates exposed to cadmium (Cd) and bisphenol A for 8 days. Hydrogen peroxide-induced DNA strand breaks in vitro in isolated embryonic cells. Exposure of adult reproducing snails to methyl methane sulfonate for 24h induced DNA strand breaks in embryos. Bisphenol A induced a significant increase in the DNA strand-break level in whole embryonic cells and whole neonate cells. Cd was genotoxic for both embryos and neonates during the exposure time and also after 7 days of depuration, suggesting that Cd could inhibit DNA repair enzymes. These preliminary results on this original model have encouraged us to consider the impact of genotoxic environmental contaminants on the F1 generation.