Identification of a putative calcium-binding protein as a dioxin-responsive gene in zebrafish and rainbow trout

Toxicity effects of captan on different life stages of zebrafish (Danio rerio)

The objective of this study was to evaluate the toxicity and developmental effects of captan on different life stages (embryo and adult) of zebrafish (Danio rerio). The results showed that the 96-h lethal concentration 50 (LC₅₀) values of embryo and adult zebrafish (exposed to captan) were 0.81(0.75-0.87) mg/L and 0.65(0.62-0.68) mg/L, respectively. The results of developmental effect experiment showed that captan can significantly decrease the heartbeats and inhibit the hatching rate and growth of zebrafish embryos. Moreover, captan exposure can induce a series of deformities, including pericardial edema, yolk sac edema, spine curvature, and tail bending, in zebrafish embryos during the developmental period. Among these, the most significant were tail bending and spine curvature.

RNA-sequencing analysis of TCDD-induced responses in zebrafish liver reveals high relatedness to in vivo mammalian models and conserved biological pathways

TCDD is one of the most persistent environmental toxicants in biological systems and its effect through aryl hydrocarbon receptor (AhR) has been well characterized. However, the information on TCDD-induced toxicity in other molecular pathways is rather limited. To fully understand molecular toxicity of TCDD in an in vivo animal model, adult zebrafish were exposed to TCDD at 10 nM for 96 h and the livers were sampled for RNA-sequencing based transcriptomic profiling. A total of 1,058 differently expressed genes were identified based on fold-change>2 and TPM (transcripts per million) >10. Among the top 20 up-regulated genes, 10 novel responsive genes were identified and verified by RT-qPCR analysis on independent samples. Transcriptomic analysis indicated several deregulated pathways associated with cell cycle, endocrine disruptors, signal transduction and immune systems. Comparative analyses of TCDD-induced transcriptomic changes between fish and mammalian models revealed that proteomic pathway is consistently up-regulated while calcium signaling pathway and several immune-related pathways are generally down-regulated. Finally, our study also suggested that zebrafish model showed greater similarity to in vivo mammalian models than in vitro models. Our study indicated that the zebrafish is a valuable in vivo model in toxicogenomic analyses for understanding molecular toxicity of environmental toxicants relevant to human health. The expression profiles associated with TCDD could be useful for monitoring environmental dioxin and dioxin-like contamination.

Identification of a new cartilage-specific S100-like protein up-regulated during endo/perichondral mineralization in gilthead seabream

Calcium ions and calcium-binding proteins play a major role in many cellular processes, in particular skeletogenesis and bone formation. We report here the discovery of a novel S100 protein in fish and the analysis of its gene expression patterns. A 648-bp full-length cDNA encoding an 86-amino acid S100-like calcium-binding protein was identified through the subtractive hybridization of a gilthead seabream (Sparus aurata) cDNA library constructed to identify genes associated with in vitro mineralization. Deduced protein lacks an identifiable signal peptide and exhibits two EF-hand motifs characteristic of S100 proteins. Phylogenetic and bioinformatic analyses of S100 sequences suggested that gilthead seabream protein represents a novel and fish-specific member of the S100 protein family. Expression of S100-like gene was up-regulated during the in vitro mineralization of bone-derived cell lines and during seabream development, from larvae throughout adulthood, reflecting skeletogenesis. Restriction of S100-like gene expression to chondrocytes of cartilaginous tissues undergoing endo/perichondral mineralization in juvenile fish further confirmed the mineralogenic role of the protein in fish and emphasized the potential of S100-like as a marker of mineralizing cartilage in developing fish.

Identification of candidate genes and physiological pathways involved in gonad deformation in whitefish (Coregonus spp.) from Lake Thun, Switzerland

In 2000, fishermen reported the appearance of deformed reproductive organs in whitefish (Coregonus spp.) from Lake Thun, Switzerland. Despite intensive investigations, the causes of these abnormalities remain unknown. Using gene expression profiling, we sought to identify candidate genes and physiological processes possibly associated with the observed gonadal deformations, in order to gain insights into potential causes. Using in situ-synthesized oligonucleotide arrays, we compared the expression levels at 21,492 unique transcript probes in liver and head kidney tissue of male whitefish with deformed and normally developed gonads, respectively. The fish had been collected on spawning sites of two genetically distinct whitefish forms of Lake Thun. We contrasted the gene expression profiles of 56 individuals, i.e., 14 individuals of each phenotype and of each population. Gene-by-gene analysis revealed weak expression differences between normal and deformed fish, and only one gene, ictacalcin, was found to be up-regulated in head kidney tissue of deformed fish from both whitefish forms, However, this difference could not be confirmed with quantitative real-time qPCR. Enrichment analysis on the level of physiological processes revealed (i) the involvement of immune response genes in both tissues, particularly those linked to complement activation in the liver, (ii) proteolysis in the liver and (iii) GTPase activity and Ras protein signal transduction in the head kidney. In comparison with current literature, this gene expression pattern signals a chronic autoimmune disease in the testes. Based on the recent observations that gonad deformations are induced through feeding of zooplankton from Lake Thun we hypothesize that a xenobiotic accumulated in whitefish via the plankton triggering autoimmunity as the likely cause of gonad deformations. We propose several experimental strategies to verify or reject this hypothesis.

Comparative transcriptomics implicates mechanisms of evolved pollution tolerance in a killifish population

Wild populations of the killifish Fundulus heteroclitus resident in heavily contaminated North American Atlantic coast estuaries have recently and independently evolved dramatic, heritable, and adaptive pollution tolerance. We compared physiological and transcriptome responses to embryonic polychlorinated biphenyl (PCB) exposures between one tolerant population and a nearby sensitive population to gain insight into genomic, physiological and biochemical mechanisms of evolved tolerance in killifish, which are currently unknown. The PCB exposure concentrations at which developmental toxicity emerged, the range of developmental abnormalities exhibited, and global as well as specific gene expression patterns were profoundly different between populations. In the sensitive population, PCB exposures produced dramatic, dose-dependent toxic effects, concurrent with the alterations in the expression of many genes. For example, PCB-mediated cardiovascular system failure was associated with the altered expression of cardiomyocyte genes, consistent with sarcomere mis-assembly. In contrast, genome-wide expression was comparatively refractory to PCB induction in the tolerant population. Tolerance was associated with the global blockade of the aryl hydrocarbon receptor (AHR) signalling pathway, the key mediator of PCB toxicity, in contrast to the strong dose-dependent up-regulation of AHR pathway elements observed in the sensitive population. Altered regulation of signalling pathways that cross-talk with AHR was implicated as one candidate mechanism for the adaptive AHR signalling repression and the pollution tolerance that it affords. In addition to revealing mechanisms of PCB toxicity and tolerance, this study demonstrates the value of comparative transcriptomics to explore molecular mechanisms of stress response and evolved adaptive differences among wild populations.

Molecular Mechanisms of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin Cardiovascular Embryotoxicity

2,3,7,8 Tetrachlorodibenzo-p-dioxin (TCDD) and related planar halogenated aromatic hydrocarbons are widespread environmental contaminants and potent developmental toxicants. Hallmarks of embryonic exposure include edema, hemorrhage, and mortality. Recent studies in zebrafish and chicken have revealed direct impairment of cardiac muscle growth that may underlie these overt symptoms. TCDD toxicity is mediated by the aryl hydrocarbon receptor, but downstream targets remain unclear. Oxidative stress and growth factor modulation have been implicated in TCDD cardiovascular toxicity. Gene expression profiling is elucidating additional pathways by which TCDD might act. We review our understanding of the mechanism of TCDD embryotoxicity at morphological and molecular levels.

CYP1A/regucalcin gene expression and edema formation in zebrafish embryos exposed to 2,3,7,8-Tetrachlorodibenzo-p-dioxin

In this study, zebrafish eggs were exposed to a relatively low concentration (50 pg/mL) of 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) for 72 h and then transferred to vehicle/TCDD-free water for the remainder of the experiments. Mortality, heart rates, edema severity, CYP1A, and regucalcin gene expressions were investigated to study TCDD-induced toxicity in zebrafish during the early life stage. Results indicated that the 50 pg/mL TCDD caused severe and visible developmental toxicity. Further research of the long term and low concentration of TCDD exposure is required.

Structural and functional diversification in the teleost S100 family of calcium-binding proteins

BACKGROUND

Among the EF-Hand calcium-binding proteins the subgroup of S100 proteins constitute a large family with numerous and diverse functions in calcium-mediated signaling. The evolutionary origin of this family is still uncertain and most studies have examined mammalian family members.

RESULTS

We have performed an extensive search in several teleost genomes to establish the s100 gene family in fish. We report that the teleost S100 repertoire comprises fourteen different subfamilies which show remarkable similarity across six divergent teleost species. Individual species feature distinctive subsets of thirteen to fourteen genes that result from local gene duplications and gene losses. Eight of the fourteen S100 subfamilies are unique for teleosts, while six are shared with mammalian species and three of those even with cartilaginous fish. Several S100 family members are found in jawless fish already, but none of them are clear orthologs of cartilaginous or bony fish s100 genes. All teleost s100 genes show the expected structural features and are subject to strong negative selection. Many aspects of the genomic arrangement and location of mammalian s100 genes are retained in the teleost s100 gene family, including a completely conserved intron/exon border between the two EF hands. Zebrafish s100 genes exhibit highly specific and characteristic expression patterns, showing both redundancy and divergence in their cellular expression. In larval tissue expression is often restricted to specific cell types like keratinocytes, hair cells, ionocytes and olfactory receptor neurons as demonstrated by in situ hybridization.

CONCLUSION

The origin of the S100 family predates at least the segregation of jawed from jawless fish and some extant family members predate the divergence of bony from cartilaginous fish. Despite a complex pattern of gene gains and losses the total repertoire size is remarkably constant between species. On the expression level the teleost S100 proteins can serve as precise markers for several different cell types. At least some of their functions may be related to those of their counterparts in mammals. Accordingly, our findings provide an excellent basis for future studies of the functions and interaction partners of s100 genes and finally their role in diseases, using the zebrafish as a model organism.

The role of aryl hydrocarbon receptor in the pathogenesis of cardiovascular diseases

Numerous experimental and epidemiological studies have demonstrated that polycyclic aromatic hydrocarbons (PAHs), which are major constituents of cigarette tobacco tar, are strongly involved in the pathogenesis of the cardiovascular diseases (CVDs). Knowing that PAH-induced toxicities are mediated by the activation of a cytosolic receptor, aryl hydrocarbon receptor (AhR), which regulates the expression of a group of xenobiotic metabolizing enzymes (XMEs) such as CYP1A1, CYP1A2, CYP1B1, NQO1, and GSTA1, suggests a direct link between AhR-regulated XMEs and CVDs. Therefore, identifying the localization and expression of the AhR and its regulated XMEs in the cardiovascular system (CVS) is of major importance in understanding their physiological and pathological roles. Generally, it was believed that the levels of AhR-regulated XMEs are lower in the CVS than in the liver; however, it has been shown that similar or even higher levels of expression are demonstrated in the CVS in a tissue- and species-specific manner. Moreover, most, if not all, AhR-regulated XMEs are differentially expressed in most of the CVS, particularly in the endothelium cells, aorta, coronary arteries, and ventricles. Although the exact mechanisms of PAH-mediated cardiotoxicity are not fully understood, several mechanisms are proposed. Generally, induction of CYP1A1, CYP1A2, and CYP1B1 is considered cardiotoxic through generating reactive oxygen species (ROS), DNA adducts, and endogenous arachidonic acid metabolites. However the cardioprotective properties of NQO1 and GSTA1 are mainly attributed to the antioxidant effect by decreasing ROS and increasing the levels of endogenous antioxidants. This review provides a clear understanding of the role of AhR and its regulated XMEs in the pathogenesis of CVDs, in which imbalance in the expression of cardioprotective and cardiotoxic XMEs is the main determinant of PAH-mediated cardiotoxicity.

Understanding dioxin developmental toxicity using the zebrafish model

Zebrafish (Danio rerio) have advantages over mammals as an animal model for investigating developmental toxicity. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (dioxin, TCDD), a persistent global contaminant, is the most comprehensively studied developmental toxicant in zebrafish. The hallmark responses of TCDD developmental toxicity manifested in zebrafish larvae include edema, anemia, hemorrhage, and ischemia associated with arrested growth and development. Heart and vasculature development and function are severely impaired, and jaw malformations occur secondary to inhibited chondrogenesis. The swim bladder fails to inflate, and the switch from embryonic to adult erythropoiesis is blocked. This profile of developmental toxicity responses, commonly referred to as "blue sac syndrome" because the edematous yolk sac appears blue, is observed in the larval form of all freshwater fish species exposed to TCDD at the embryonic stage of development. Components of the aryl hydrocarbon receptor/aryl hydrocarbon receptor nuclear translocator (AHR/ARNT) signaling pathway in zebrafish have been identified and functionally characterized. Their role in mediating TCDD toxicity has been determined using morpholinos to specifically knockdown the translation of zfAHR1, zfAHR2, zfARNT1, and zfARNT2 mRNAs, respectively, and a line of zfARNT2 null mutant zebrafish has provided further insight. These studies have shown that zfAHR2 and zfARNT1 mediate TCDD developmental toxicity. In addition, the growing use of molecular and genomic tools for research on zebrafish have led to advances in our understanding of the mechanism of TCDD developmental toxicity at the molecular level, including the recent finding that toxicity is not mediated by increased cytochrome P4501A (zfCYP1A) expression.