Molecular cloning and developmental expression of zinc finger transcription factor MTF-1 gene in zebrafish, Danio rerio

Examining the subchronic (28-day) effects of aqueous Cd-BaP co-exposure on detoxification capacity and cardiac function in adult zebrafish (Danio rerio)

The present study aimed to examine the effects of environmentally relevant concentrations of cadmium (Cd) and Benzo[a]Pyrene (BaP) in the adult zebrafish (Danio rerio). To this end, fish were exposed to either 1 or 10 μg/L Cd or 0.1 or 1 μg/L BaP in isolation, or a co-exposure containing a mixture of the two toxicants. Our results showed extensive modulation of the expression of key antioxidant genes (GPx, SOD1, catalase), detoxifying genes (MT1, MT2, CYP1A1) and a stress biomarker (HSP70) differing between control, single toxicant groups and co-exposure groups. We additionally carried out histopathological analysis of the gills, liver, and hearts of exposed animals, noting no differences in tissue necrosis or apoptosis. Finally, we carried out ultrasonographic analysis of cardiac function, noting a significant decrease of E-wave peak velocity and end diastolic volume in exposed fish. This in turn was accompanied by a decrease in stroke volume and ejection fraction, but not cardiac output in co-exposed fish. The present study is the first to demonstrate that a subchronic aqueous exposure to a Cd-BaP mixture can extensively modulate detoxification capacity and cardiac function in adult zebrafish in a tissue-specific manner.

Genome-wide screening in pluripotent cells identifies Mtf1 as a suppressor of mutant huntingtin toxicity

Huntington's disease (HD) is a neurodegenerative disorder caused by CAG-repeat expansions in the huntingtin (HTT) gene. The resulting mutant HTT (mHTT) protein induces toxicity and cell death via multiple mechanisms and no effective therapy is available. Here, we employ a genome-wide screening in pluripotent mouse embryonic stem cells (ESCs) to identify suppressors of mHTT toxicity. Among the identified suppressors, linked to HD-associated processes, we focus on Metal response element binding transcription factor 1 (Mtf1). Forced expression of Mtf1 counteracts cell death and oxidative stress caused by mHTT in mouse ESCs and in human neuronal precursor cells. In zebrafish, Mtf1 reduces malformations and apoptosis induced by mHTT. In R6/2 mice, Mtf1 ablates motor defects and reduces mHTT aggregates and oxidative stress. Our screening strategy enables a quick in vitro identification of promising suppressor genes and their validation in vivo, and it can be applied to other monogenic diseases.

Analysis of the structure and activity of the promoter regions of the metallothionein genes of the freshwater pearl mussel Hyriopsis schlegelii

To investigate the regulation of metallothionein genes (HsMTs) of Hyriopsis schlegelii, 1,121-bp and 1,270-bp regions of the HsMT1 and HsMT2 promoters were cloned and analyzed, respectively. The two promoters shared partially conserved features and possessed distinct characteristics such as the number or position of metal response elements (MREs). Further analysis of the HsMT1 and HsMT2 promoters was performed by the reporter assay using the luciferase gene. Both promoters were activated by various metals, and presented different levels of metal ions inducibility in human hepatoblastoma cells. Deletion mutant assays demonstrated that both the longest promoter regions achieved the maximum inducibility, and the metal inducibility was dependent on the presence of the MRE in HsMT1 and the distal MRE in HsMT2. In addition, we cloned a putative metal responsive transcription factor (hereby designated as HsMTF-like) and studied its effect on HsMTs expression in human hepatoblastoma cells. An in vivo assay demonstrated that HsMTF-like activates basal HsMTs transcription level, and the MRE in the HsMTs promoter mediates this activation process. Moreover, this basal transcription level can be further boosted by zinc treatment. In conclusion, the regulation mechanism for MT activation in H. schlegelii should be evolutionarily conserved.

Solute carriers (SLCs) identified and characterized from kidney transcriptome of golden mahseer (Tor putitora) (Fam: Cyprinidae)

The solute carriers (SLC) are trans-membrane proteins, those regulate the transport of various substances (sugars, amino acids, nucleotides, inorganic cations/anions, metals, drugs etc.) across the cell membrane. There are more than 338 solute carriers (slc) reported in fishes that play crucial role in cellular influx and efflux. The study of solute carrier families may reveal many answers regarding the function of transporter genes in the species and their effect in the existing environment. Therefore, we performed RNA sequencing of kidney tissue of the golden mahseer (Tor putitora) using Illumina platform to identify the solute carrier families and characterized 24 putative functional genes under 15 solute carrier families. Out of 24 putative functional genes, 11 genes were differentially expressed in different tissues (head kidney, trunk kidney, spleen, liver, gill, muscle, intestine and brain) using qRT-PCR assay. The slc5a1, slc5a12, slc12a3, slc13a3, slc22a13 and slc26a6 were highly expressed in kidney. The slc15a2, slc25a47, slc33a1 and slc38a2 were highly expressed in brain and slc30a5 was over-expressed in gill. The unrooted phylogenetic trees of slc2, slc5, slc13 and slc33 were constructed using amino acid sequences of Homo sapiens, Salmo salar, Danio rerio, Cyprinus carpio and Tor putitora. It appears that all the putative solute carrier families are very much conserved in human and fish species including the present fish, golden mahseer. This study provides the first hand database of solute carrier families particularly transporter encoding proteins in the species.

Toxicity Evaluation of New Engineered Nanomaterials in Zebrafish

The effect of the nanoparticles on the marine organisms, depends on their size, chemical composition, surface structure, solubility and shape. In order to take advantage from their activity, preserving the surrounding environment from a possible pollution, we are trying to trap the nanoparticles into new nanomaterials. The nanomaterials tested were synthesized proposing a ground-breaking approach by an upside-down vision of the Au/TiO2 nano-system to avoid the release of nanoparticles. The system was synthesized by wrapping Au nanoparticles with a thin layer of TiO2. The non-toxicity of the nano-system was established by testing the effect of the material on zebrafish larvae. Danio rerio o zebrafish was considered an excellent model for the environmental biomonitoring of aquatic environments and the Zebrafish Embryo Toxicity Test (ZFET) is considered an alternative method of animal test. For this reason zebrafish larvae were exposed to different concentrations of nanoparticles of TiO2 and Au and new nanomaterials. As biomarkers of exposure, we evaluated the expression of metallothioneins by immunohistochemistry analysis and western blotting analysis also. The results obtained by toxicity test showed that neither mortality as well as sublethal effects were induced by the different nanomaterials and nanoparticles tested. Only zebrafish larvae exposed to free Au nanoparticles showed a different response to anti-MT antibody. In fact, the immunolocalization analysis highlighted an increase of the metallothioneins synthesis.

Metal-PAH mixtures in the aquatic environment: a review of co-toxic mechanisms leading to more-than-additive outcomes

Mixtures of metals and polycyclic aromatic hydrocarbons (PAHs) occur ubiquitously in aquatic environments, yet relatively little is known regarding their combined toxicities. Emerging reports investigating the additive mortality in metal-PAH mixtures have indicated that more-than-additive effects are equally as common as strictly-additive effects, raising concern for ecological risk assessment typically based on the summation of individual toxicities. Moreover, the current separation of focus between in vivo and in vitro studies, and fine- and coarse-scale endpoints, creates uncertainty regarding the mechanisms of co-toxicity involved in more-than-additive effects on whole organisms. Drawing from literature on metal and PAH toxicity in bacteria, protozoa, invertebrates, fish, and mammalian models, this review outlines several key mechanistic interactions likely to promote more-than-additive toxicity in metal-PAH mixtures. Namely, the deleterious effects of PAHs on membrane integrity and permeability to metals, the potential for metal-PAH complexation, the inhibitory nature of metals to the detoxification of PAHs via the cytochrome P450 pathway, the inhibitory nature of PAHs towards the detoxification of metals via metallothionein, and the potentiated production of reactive oxygenated species (ROS) in certain metal (e.g. Cu) and PAH (e.g., phenanthrenequinone) mixtures. Moreover, the mutual inhibition of detoxification suggests the possibility of positive feedback among these mechanisms. The individual toxicities and interactive aspects of contaminant transport, detoxification, and the production of ROS are herein discussed.

Inhibition of endogenous MTF-1 signaling in zebrafish embryos identifies novel roles for MTF-1 in development

The metal responsive element-binding transcription factor-1 (MTF-1) responds to changes in cellular zinc levels caused by zinc exposure or disruption of endogenous zinc homeostasis by heavy metals or oxygen-related stress. Here we report the functional characterization of a complete zebrafish MTF-1 in comparison with the previously identified isoform lacking the highly conserved cysteine-rich motif (Cys-X-Cys-Cys-X-Cys) found in all other vertebrate MTF-1 orthologs. In an effort to develop novel molecular tools, a constitutively nuclear dominant-negative MTF-1 (dnMTF-1) was generated as tool for inhibiting endogenous MTF-1 signaling. The in vivo efficacy of the dnMTF-1 was determined by microinjecting in vitro transcribed dnMTF-1 mRNA into zebrafish embryos (1-2 cell stage) followed by transcriptomic profiling using an Agilent 4x44K array on 28- and 36-hpf embryos. A total of 594 and 560 probes were identified as differentially expressed at 28hpf and 36hpf, respectively, with interesting overlaps between timepoints. The main categories of genes affected by the inhibition of MTF-1 signaling were: nuclear receptors and genes involved in stress signaling, neurogenesis, muscle development and contraction, eye development, and metal homeostasis, including novel observations in iron and heme homeostasis. Finally, we investigate both the transcriptional activator and transcriptional repressor role of MTF-1 in potential novel target genes identified by transcriptomic profiling during early zebrafish development.

Zinc'ing sensibly: controlling zinc homeostasis at the transcriptional level

Zinc-responsive transcription factors are found in all kingdoms of life and include the transcriptional activators ZntR, SczA, Zap1, bZip19, bZip23, and MTF-1, and transcriptional repressors Zur, AdcR, Loz1, and SmtB. These factors have two defining features; their activity is regulated by zinc and they all play a central role in zinc homeostasis by controlling the expression of genes that directly affect zinc levels or its availability. This review summarizes what is known about the mechanisms by which each of these factors sense changes in intracellular zinc levels and how they control zinc homeostasis through target gene regulation. Other factors that influence zinc ion sensing are also discussed.

Molecular cloning, characterization and expression analysis of melanotransferrin from the sea cucumber Apostichopus japonicus

Melanotransferrin (MTf), a member of the transferrin families, plays an important role in immune response. But the research about MTf in sea cucumber is limited till now. In this study, the Melanotransferrin (Aj-MTf) gene was firstly cloned and characterized from the sea cucumber Apostichoupus japonicus by reverse transcriptase polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends. The full-length cDNA of Aj-MTf is 2,840 bp in length and contains a 2,184 bp open reading frame that encodes a polypeptide of 727 amino acids. An iron-responsive element-like structure is located at the 5'-UTR of Aj-MTf cDNA. Sequence analysis shows that the Aj-MTf contains two conserved domains, and the binding-iron (III) sites, including eight amino acid residues (D81,Y109,Y215,H283,D425,Y454,Y565 and H634) and three N-linked glycosylation sites (N121V122S123,N173A174S175 and N673S674T675). Quantitative real-time polymerase chain reaction (qRT-PCR) analyses suggested that the Aj-MTf expressions in the coelomic fluid, body cavity wall and respiratory trees were significantly changed from 4 to 24 h post lipopolysaccharide (LPS) injection. The mRNA levels of Aj-MTf in coelomic fluid was significantly up-regulated at 12 and 24 h in treatment group, and Aj-MTf shared a similar expression pattern with C-type lectin in coelomic fluid, while both genes appears to gradually increase after 4 h of LPS injection. These results indicate that the Aj-MTf plays a pivotal role in immune responses to the LPS challenge in sea cucumber, and provide new information that it is complementary to the sea cucumber immune genes and initiate new researches concerning the genetic basis of the holothurian immune response.

Identification and functional characterization of MRE-binding transcription factor (MTF) in Crassostrea gigas and its conserved role in metal-induced response

The full-length cDNA that encodes the MRE-binding transcription factor (MTF) was cloned from the Pacific oyster (Crassostrea gigas) using reverse transcription polymerase chain reaction and the rapid amplification of cDNA ends. The cgMTF cDNA sequence is 2892 bp long, with a 2508 bp open reading frame that encodes an 835-amino acid polypeptide. Multiple alignment revealed that cgMTF has four putative zinc finger-like regions in cgMTF with three C2C2-type zinc fingers and one C2H2-type zinc finger. After 12 h of exposure to Cd(2+), the cgMTF mRNA level was increased in a dose-dependent manner, which then subsided with time. cgMTF stimulates the cgMT promoter reporter in the HEK293 cell line in a dose-dependent manner. When either of the metal-responsive elements (MRE1 or MRE2) of the cgMT promoter was mutated, the cgMT promoter reporter activity was significantly reduced. After the two MREs were mutated simultaneously, the promoter activity was completely abolished. In conclusion, we identified an MTF in C. gigas and revealed the presence of an evolutionarily conserved molecular mechanism for coping with environmental metal stress.

The taste of heavy metals: gene regulation by MTF-1

The metal-responsive transcription factor-1 (MTF-1, also termed MRE-binding transcription factor-1 or metal regulatory transcription factor-1) is a pluripotent transcriptional regulator involved in cellular adaptation to various stress conditions, primarily exposure to heavy metals but also to hypoxia or oxidative stress. MTF-1 is evolutionarily conserved from insects to humans and is the main activator of metallothionein genes, which encode small cysteine-rich proteins that can scavenge toxic heavy metals and free radicals. MTF-1 has been suggested to act as an intracellular metal sensor but evidence for direct metal sensing was scarce. Here we review recent advances in our understanding of MTF-1 regulation with a focus on the mechanism underlying heavy metal responsiveness and transcriptional activation mediated by mammalian or Drosophila MTF-1. This article is part of a Special Issue entitled: Cell Biology of Metals.

Zinc transporter expression in zebrafish (Danio rerio) during development

Zinc is a micronutrient important in several biological processes including growth and development. We have limited knowledge on the impact of maternal zinc deficiency on zinc and zinc regulatory mechanisms in the developing embryo due to a lack of in vivo experimental models that allow us to directly study the effects of maternal zinc on embryonic development following implantation. To overcome this barrier, we have proposed to use zebrafish as a model organism to study the impact of zinc during development. The goal of the current study was to profile the mRNA expression of all the known zinc transporter genes in the zebrafish across embryonic and larval development and to quantify the embryonic zinc concentrations at these corresponding developmental time points. The SLC30A zinc transporter family (ZnT) and SLC39A family, Zir-,Irt-like protein (ZIP) zinc transporter proteins were profiled in zebrafish embryos at 0, 2, 6, 12, 24, 48 and 120 h post fertilization to capture expression patterns from a single cell through full development. We observed consistent embryonic zinc levels, but differential expression of several zinc transporters across development. These results suggest that zebrafish is an effective model organism to study the effects of zinc deficiency and further investigation is underway to identify possible molecular pathways that are dysregulated with maternal zinc deficiency.

The role of small ubiquitin-like modifier-interacting motif in the assembly and regulation of metal-responsive transcription factor 1

Metal-responsive transcription factor 1 (MTF-1) is an essential protein required for mouse embryonic development. We report here the occurrence of sumoylation on MTF-1. Mutational studies demonstrated that sumoylation occurs on the lysine residue at position 627 (Lys(627)) of mouse MTF-1. Small ubiquitin-like modifier (SUMO)-1 was fused to the C terminus of MTF-1 to mimic the sumoylated form of the protein and it suppressed the transcriptional activity of MTF-1. The nuclear translocation activity, DNA-binding activity, and protein stability of SUMO-fused MTF-1 are similar to that of wild type MTF-1. The level of sumoylation was reduced by metal in a dose- and time-dependent manner. The fact that zinc reduces MTF-1 sumoylation makes the suppressive role of sumoylated MTF-1 in transcription physiologically less significant because the SUMO moiety of MTF-1 is removed when MTF-1 translocates into nucleus. We further identified a SUMO-interacting motif (SIM) on MTF-1. Remarkably, MTF-1 binds sumoylated MTF-1 and/or other cellular factors in a SIM-dependent manner. This interaction was disrupted by treating cells with zinc. Gel permeation chromatography demonstrated that MTF-1 forms SIM-dependent complexes. This cross-interaction transpires in the cytoplasm and markedly reduces upon nuclear translocation. It can therefore be concluded that SUMO conjugation and the SIM on MTF-1 do not play a critical role in suppressing transcriptional activity. Instead, MTF-1 forms complexes with cellular factors through SIM and SUMO moiety in the cytoplasm. The result explores a new understanding for the mode of MTF-1 assembly and regulation in cells.

The parkin mutant phenotype in the fly is largely rescued by metal-responsive transcription factor (MTF-1)

The gene for Parkin, an E3 ubiquitin ligase, is mutated in some familial forms of Parkinson's disease, a severe neurodegenerative disorder. A homozygous mutant of the Drosophila ortholog of human parkin is viable but results in severe motoric impairment including an inability to fly, female and male sterility, and a decreased life span. We show here that a double mutant of the genes for Parkin and the metal-responsive transcription factor 1 (MTF-1) is not viable. MTF-1, which is conserved from insects to mammals, is a key regulator of heavy metal homeostasis and detoxification and plays additional roles in other stress conditions, notably oxidative stress. In contrast to the synthetic lethality of the double mutant, elevated expression of MTF-1 dramatically ameliorates the parkin mutant phenotype, as evidenced by a prolonged life span, motoric improvement including short flight episodes, and female fertility. At the cellular level, muscle and mitochondrial structures are substantially improved. A beneficial effect is also seen with a transgene encoding human MTF-1. We propose that Parkin and MTF-1 provide complementary functions in metal homeostasis, oxidative stress and other cellular stress responses. Our findings also raise the possibility that MTF-1 gene polymorphisms in humans could affect the severity of Parkinson's disease.

Characterization and localization of metal-responsive-element-binding transcription factors from tilapia

Two isoforms of MTF-1, MTF-1L (long form) and MTF-1S (short form), were cloned in tilapia (Ti) and characterized in a tilapia liver cell line, Hepa-T1. The cloned tiMTF-1L has the characteristics of all of the tiMTF-1S identified so far with the zinc finger domain having six fingers, the acidic-rich, proline-rich, and serine/threonine-rich domains; however, the short form encodes for the zinc finger domain with five zinc fingers only and no other domains. The transient transfection of tiMTF-1L into human HepG2 cells showed both constitutive and zinc-induced metal-responsive-element (MRE)-driven reporter gene expression. However, the transfection of tiMTF-1S (which lacks all three transactivation domains) into a human cell line showed reduced transcriptional activities compared with an endogenous control in both basal- and Zn(2+)-induced conditions. The tiMTF-1 isoforms were tagged with GFP and transfected into Hepa-T1 cells (tilapia hepatocytes). The nuclear translocation of tiMTF-1L was observed when the cells were exposed to a sufficient concentration of metals for 6h. However, tiMTF-1S, was localized in the nucleus with or without metal treatment. Electrophoretic mobility shift assay (EMSA) confirmed that both of the isoforms were able to bind to the MRE specifically in vitro. Tissue distribution studies showed that tiMTF-1L was more abundant than tiMTF-1S in all of the tissues tested.

Pre-gastrula expression of zebrafish extraembryonic genes

Background

Many species form extraembryonic tissues during embryogenesis, such as the placenta of humans and other viviparous mammals. Extraembryonic tissues have various roles in protecting, nourishing and patterning embryos. Prior to gastrulation in zebrafish, the yolk syncytial layer - an extraembryonic nuclear syncytium - produces signals that induce mesoderm and endoderm formation. Mesoderm and endoderm precursor cells are situated in the embryonic margin, an external ring of cells along the embryo-yolk interface. The yolk syncytial layer initially forms below the margin, in a domain called the external yolk syncytial layer (E-YSL).

Results

We hypothesize that key components of the yolk syncytial layer's mesoderm and endoderm inducing activity are expressed as mRNAs in the E-YSL. To identify genes expressed in the E-YSL, we used microarrays to compare the transcription profiles of intact pre-gastrula embryos with pre-gastrula embryonic cells that we had separated from the yolk and yolk syncytial layer. This identified a cohort of genes with enriched expression in intact embryos. Here we describe our whole mount in situ hybridization analysis of sixty-eight of them. This includes ten genes with E-YSL expression (camsap1l1, gata3, znf503, hnf1ba, slc26a1, slc40a1, gata6, gpr137bb, otop1 and cebpa), four genes with expression in the enveloping layer (EVL), a superficial epithelium that protects the embryo (zgc:136817, zgc:152778, slc14a2 and elovl6l), three EVL genes whose expression is transiently confined to the animal pole (elovl6l, zgc:136359 and clica), and six genes with transient maternal expression (mtf1, wu:fj59f04, mospd2, rftn2, arrdc1a and pho). We also assessed the requirement of Nodal signaling for the expression of selected genes in the E-YSL, EVL and margin. Margin expression was Nodal dependent for all genes we tested, including the concentrated margin expression of an EVL gene: zgc:110712. All other instances of EVL and E-YSL expression that we tested were Nodal independent.

Conclusion

We have devised an effective strategy for enriching and identifying genes expressed in the E-YSL of pre-gastrula embryos. To our surprise, maternal genes and genes expressed in the EVL were also enriched by this strategy. A number of these genes are promising candidates for future functional studies on early embryonic patterning.

Patched 1 is a crucial determinant of asymmetry and digit number in the vertebrate limb

The vertebrate hedgehog receptor patched 1 (Ptc1) is crucial for negative regulation of the sonic hedgehog (Shh) pathway during anterior-posterior patterning of the limb. We have conditionally inactivated Ptc1 in the mesenchyme of the mouse limb using Prx1-Cre. This results in constitutive activation of hedgehog (Hh) signalling during the early stages of limb budding. Our data suggest that variations in the timing and efficiency of Cre-mediated excision result in differential forelimb and hindlimb phenotypes. Hindlimbs display polydactyly (gain of digits) and a molecular profile similar to the Gli3 mutant extra-toes. Strikingly, forelimbs are predominantly oligodactylous (displaying a loss of digits), with a symmetrical, mirror-image molecular profile that is consistent with re-specification of the anterior forelimb to a posterior identity. Our data suggest that this is related to very early inactivation of Ptc1 in the forelimb perturbing the gene regulatory networks responsible for both the pre-patterning and the subsequent patterning stages of limb development. These results establish the importance of the downstream consequences of Hh pathway repression, and identify Ptc1 as a key player in limb patterning even prior to the onset of Shh expression.

Mammalian zinc transporters: nutritional and physiologic regulation

Research advances defining how zinc is transported into and out of cells and organelles have increased exponentially within the past five years. Research has progressed through application of molecular techniques including genomic analysis, cell transfection, RNA interference, kinetic analysis of ion transport, and application of cell and animal models including knockout mice. The knowledge base has increased for most of 10 members of the ZnT family and 14 members of the Zrt-, Irt-like protein (ZIP) family. Relative to the handling of dietary zinc is the involvement of ZnT1, ZIP4, and ZIP5 in intestinal zinc transport, involvement of ZIP10 and ZnT1 in renal zinc reabsorption, and the roles of ZIP5, ZnT2, and ZnT1 in pancreatic release of endogenous zinc. These events are major factors in regulation of zinc homeostasis. Other salient findings are the involvement of ZnT2 in lactation, ZIP14 in the hypozincemia of inflammation, ZIP6, ZIP7, and ZIP10 in metastatic breast cancer, and ZnT8 in insulin processing and as an autoantigen in diabetes.

Identification of a splice variant of the metal-responsive transcription factor MTF-1 in common carp

Transactivation of the expression of metallothionein genes involves the Metal-responsive Transcription Factor (MTF-1). We report here the identification of mtf-1.1a, the first known splice variant of mtf-1.1 mRNA, in common carp (Cyprinus carpio). The lack of a 103 nt internal segment results in a frame shift, causing the early termination of translation. mtf-1.1a mRNA encodes a protein consisting of the first 349 amino acids of MTF-1.1 plus an additional 64 amino acids, with no significant similarity to any of the proteins in the databases. The predicted MTF-1.1a protein carries the Zn-finger domain and the nuclear exporting and nuclear localization signals, and lacks the transcription activation domains. mtf-1.1a was detected in all tissues examined but the liver, with the highest level in the brain. Arsenic alters the levels of both mtf-1.1 and mtf-1.1a transcripts, in an isoform- and tissue-specific manner.

Zebrafish (Danio rerio) as a model for studying the genetic basis of copper toxicity, deficiency, and metabolism

Unicellular eukaryotes and cultured cells from several animal species were invaluable in discovering the mechanisms that govern incorporation, handling, and excretion of copper at the cellular level. However, understanding the systemic regulation of copper availability and distribution among the different tissues of an intact multicellular organism has proven to be more challenging. This analysis is made even more difficult if the genetic variability among organisms is taken into account. The zebrafish has long been considered a powerful animal model because of its tractable genetics and embryology, but it has more recently become a player in environmental studies, pharmaceutical screening, and physiologic analysis. In particular, the use of the larvae, small enough to fit into a microtiter well, but developed enough to have full organ functionality, represents a convenient alternative for studies that aim to establish effects of environmental agents on the intact, living organism. Studies by our group and others have characterized absorption and tissue distribution of copper and have described the acute effects of the metal on larvae in terms of survival, organ stress, and functionality of sensory organs. A large body of work has shown that there is strong conservation in mechanisms and genes between fish and mammals, opening the possibility for genetic or small molecule screens or for generating fish models of human diseases related to copper metabolism. The variability within humans in terms of tolerance to copper excess or deficiency requires a genetic approach to be taken to understand the behavior of populations, because markers and vulnerabilities need to be identified. The zebrafish could represent a unique tool to move in this direction.

Identification and characterization of two mtf-1 genes in common carp

The Metal-responsive Transcription Factor (MTF-1) serves as an essential regulator of Zn(2+) homeostasis via the activation of metallothionein gene expression. Only a single mtf-1 gene has been identified in any organism investigated previously. We report here the first evidence of the existence of two genes encoding MTF-1 proteins (mtf-1.1 and mtf-1.2). The expression patterns were followed in the liver, kidney, muscle, brain and heart by means of Northern hybridization and reverse transcription coupled polymerase chain reactions (RT-PCR). mtf-1.1 mRNA was detected in all tissues examined, with the highest level in the brain, and the lowest in the kidney and the liver. mtf-1.2 expression was detected exclusively in the brain. Cold shock and Cd(2+) exposure influence the gene expression at the transcriptional level, in a stress-specific manner.

Copper sensing function of Drosophila metal-responsive transcription factor-1 is mediated by a tetranuclear Cu(I) cluster

Drosophila melanogaster MTF-1 (dMTF-1) is a copper-responsive transcriptional activator that mediates resistance to Cu, as well as Zn and Cd. Here, we characterize a novel cysteine-rich domain which is crucial for sensing excess intracellular copper by dMTF-1. Transgenic flies expressing mutant dMTF-1 containing alanine substitutions of two, four or six cysteine residues within the sequence (547)CNCTNCKCDQTKSCHGGDC(565) are significantly or completely impaired in their ability to protect flies from copper toxicity and fail to up-regulate MtnA (metallothionein) expression in response to excess Cu. In contrast, these flies exhibit wild-type survival in response to copper deprivation thus revealing that the cysteine cluster domain is required only for sensing Cu load by dMTF-1. Parallel studies show that the isolated cysteine cluster domain is required to protect a copper-sensitive S. cerevisiae ace1Delta strain from copper toxicity. Cu(I) ligation by a Cys-rich domain peptide fragment drives the cooperative assembly of a polydentate [Cu(4)-S(6)] cage structure, characterized by a core of trigonally S(3) coordinated Cu(I) ions bound by bridging thiolate ligands. While reminiscent of Cu(4)-L(6) (L = ligand) tetranuclear clusters in copper regulatory transcription factors of yeast, the absence of significant sequence homology is consistent with convergent evolution of a sensing strategy particularly well suited for Cu(I).

Understanding the mechanisms of zinc-sensing by metal-response element binding transcription factor-1 (MTF-1)

The regulation of divalent zinc has been observed in a wide range of organisms. Since this metal is an essential nutrient, but also toxic in excess, zinc homeostasis is crucial for normal cellular functioning. The metal-responsive-element-binding transcription factor-1 (MTF-1) is a key regulator of zinc in higher eukaryotes ranging from insects to mammals. MTF-1 controls the expression of metallothioneins (MTs) and a number of other genes directly involved in the intracellular sequestration and transport of zinc. Although the diverse functions of MTF-1 extend well beyond zinc homeostasis to include stress-responses to heavy metal toxicity, oxidative stress, and selected chemical agents, in this review we focus on the recent advances in understanding the mechanisms whereby MTF-1 regulates MT gene expression to protect the cell from fluctuations in environmental zinc. Particular emphasis is devoted to recent studies involving the Cys2His2 zinc finger DNA-binding domain of MTF-1, which is an important contributor to the zinc-sensing and metal-dependent transcriptional activation functions of this protein.

Expression pattern of metallothionein, MTF-1 nuclear translocation, and its dna-binding activity in zebrafish (Danio rerio) induced by zinc and cadmium

Metallothionein is a small (6-kDa), cysteine-rich protein expressed by a six-zinc finger protein called metal-responsive element-binding transcription factor-1 (MTF-1) in response to Zn and Cd. Our previous reports have shown the basal expression of metallothionein (mt) and MTF-I (mtf-1) genes in embryo and early larval stages of zebrafish (Danio rerio). In the present study, we investigated the mt expression in zebrafish early larvae induced by exposure to Cd and Zn (48, 72, 96, and 120 h postfertilization). Whole-mount in situ hybridization showed that Zn induced mt expression in the olfactory pit, cerebellum, ceratobranchials, liver, chloride cells, and neuromasts of the lateral line. Cadmium also induced mt expression in all the above regions except the cerebellum. Using fluorescence techniques, we have shown that Zn and Cd mediate cytoplasmic and nuclear translocation of MTF- 1-enhanced green fluorescent protein fusion protein in zebrafish liver cell line. The MTF-1 protein was produced recombinantly by inserting zebrafish mtf-1 cDNA (1.8 kb) into pET-20b(+) expression vector and expressing in Escherichia coli BL21 (DE3) pLysS host strain competent cell on induction with isopropyl-beta-D-thiogalactopyranoside. The protein was then purified by affinity chromatography on a nickel-nitrilotriacetic acid column. Electrophoretic mobility shift assay revealed binding of the recombinant MTF-1 in response to Zn and Cd at the putative metal-responsive elements (MREs) in the promoter region of the mt gene. Taken together, these results suggest that Zn and Cd are efficiently involved with mt expression induced in zebrafish embryos and with MTF-1 nuclear translocation and that this induction is achieved through the activation of MTF-1 binding at the MREs.

Cloning of hif-1alpha and hif-2alpha and mRNA expression pattern during development in zebrafish

Hypoxia-inducible factors (HIFs) regulate gene expression in response to hypoxia and in vertebrates they are known to participate in several developmental processes, including angiogenesis, vasculogenesis, heart and central nervous system development. Over the last decade, major progress in unraveling the molecular mechanisms that mediate regulation of HIF proteins by oxygen tension has been reported, but our knowledge on their developmental regulation during embryogenesis in model organisms is limited. Expression of hif-1alpha and hif-2alpha genes has been characterized during normal mouse development and they were found to be expressed from stages E7.5, later in E9.5 and E15.5 in several different tissues such as the brain, heart and blood vessels. However, there is no detailed temporal information on their expression at other embryonic stages, even though orthologous genes have been described in several different vertebrate species. In this study, we describe the cloning and detailed expression pattern of zebrafish hif-1alpha and hif-2alpha genes. Sequence analysis revealed that zebrafish Hif proteins are highly homologous to other vertebrate orthologues. Zebrafish hif-1alpha and hif-2alpha are both expressed throughout development in discrete territories in a dynamic pattern. Interestingly, in the notochord the expression of hif-1alpha is switched off, while hif-2alpha transcription is turned on, signifying that the two genes might have partially overlapping, although non-redundant functions in development. This is the first time that a detailed comparison of the expression of hif-1alpha and hif-2alpha is directly assessed in a vertebrate model system throughout development.

Cloning and characterization of zebra fish SPATA4 gene and analysis of its gonad specific expression

The spermatogenesis associated 4 gene (SPATA4, previously named TSARG2) was first cloned in human tissues and was reported to be a candidate spermatocyte apoptosis-related gene that is expressed specifically in testis. Analysis of SPATA4 expression and regulation in zebra fish may provide insight into the understanding of the complicated process of gonadogenesis. In this study, we cloned and characterized the SPATA4 gene from zebra fish (Danio rerio), which is homologous to human and mouse SPATA4. Zebra fish SPATA4 consists of six exons separated by five introns, as all SPATA4 genes in vertebrates. A promoter region was predicted using homologous blast and cloned for further study, and possible transcription factors were analyzed in this region. The putative protein encoded by this gene was analyzed using bioinformatics methods. Multi-tissue RT-PCR results demonstrated that the zebra fish SPATA4 gene is expressed specifically in testis and slightly in ovary. Analysis of the SPATA4 sequence and its spatial expression pattern indicate that this gene is highly conserved and may play an important role in the process of zebra fish gonadogenesis.

Two major branches of anti-cadmium defense in the mouse: MTF-1/metallothioneins and glutathione

Metal-responsive transcription factor 1 (MTF-1) regulates expression of its target genes in response to various stress conditions, notably heavy metal load, via binding to metal response elements (MREs) in the respective enhancer/promoter regions. Furthermore, it serves a vital function in embryonic liver development. However, targeted deletion of Mtf1 in the liver after birth is no longer lethal. For this study, Mtf1 conditional knockout mice and control littermates were both mock- or cadmium-treated and liver-specific transcription was analyzed. Besides the well-characterized metallothionein genes, several new MTF-1 target genes with MRE motifs in the promoter region emerged. MTF-1 is required for the basal expression of selenoprotein W, muscle 1 gene (Sepw1) that encodes a glutathione-binding and putative antioxidant protein, supporting a role of MTF-1 in the oxidative stress response. Furthermore, MTF-1 mediates the cadmium-induced expression of N-myc downstream regulated gene 1 (Ndrg1), which is induced by several stress conditions and is overexpressed in many cancers. MTF-1 is also involved in the cadmium response of cysteine- and glycine-rich protein 1 gene (Csrp1), which is implicated in cytoskeletal organization. In contrast, MTF-1 represses the basal expression of Slc39a10, a putative zinc transporter. In a pathway independent of MTF-1, cadmium also induced the transcription of genes involved in the synthesis and regeneration of glutathione, a cadmium-binding antioxidant. These data provide strong evidence for two major branches of cellular anti-cadmium defense, one via MTF-1 and its target genes, notably metallothioneins, the other via glutathione, with an apparent overlap in selenoprotein W.

The six zinc fingers of metal-responsive element binding transcription factor-1 form stable and quasi-ordered structures with relatively small differences in zinc affinities

Six Cys(2)His(2) zinc fingers (F1-6) comprise the DNA binding domain of metal-responsive element binding transcription factor-1 (MTF-1). F1-6 is necessary for basal and zinc-induced expression of metallothionein genes. Analysis of NMR structural and dynamic data for an F1-6 protein construct demonstrates that each zinc finger adopts a stable betabetaalpha fold in the presence of stoichiometric Zn(II), provided that all cysteine ligands are in a reduced state. Parallel studies of protein constructs spanning the four N-terminal core DNA binding fingers (F1-4) and two C-terminal low DNA affinity fingers (F5-6) reveal similar stable zinc finger structures. In both the F1-6 and F5-6 proteins, the finger 5 cysteines were found to readily oxidize at neutral pH. Detailed spectral density and hydrodynamic analysis of (15)N relaxation data revealed quasi-ordered anisotropic rotational diffusion properties of the six F1-6 zinc fingers that could influence MTF-1 DNA binding function. A more general effect on the rotational diffusion properties of Cys(2)His(2) zinc fingers was also uncovered that is dependent upon the position of each finger within multifinger domains. Analysis of NMR (1)H-(15)N-heteronuclear single quantum coherence spectral peak intensities measured as a function of added Zn(II) in conjunction with Zn(II) binding modeling studies indicated that the Zn(II) affinities of all MTF-1 zinc fingers are within approximately 10-50-fold. These analyses further suggested that metal sensing by MTF-1 in eukaryotic cells involves multiple zinc fingers and occurs over a 100-fold or less range of accessible Zn(II) concentration.

C. elegans metallothioneins: new insights into the phenotypic effects of cadmium toxicosis

Metallothioneins are considered to be the primary player in the detoxification of and protection from cadmium, a teratogen, mutagen and potentially lethal heavy metal. The nematode Caenorhabditis elegans has only two metallothioneins, mtl-i and mtIl-2, thus making it an ideal organism to investigate the phenotypic effects of cadmium toxicosis. The functional importance of metallothioneins in cadmium trafficking was highlighted through the generation of viable green fluorescent protein (GFP) expressing transgenes, a metallothionein null allele, as well as RNAi mediated metallothionein knock-downs. A highly sensitive dose and temporal transcriptional response to cadmium, but not copper or zinc, was shown to be equally prevalent in both isoforms. No measurable compensatory up-regulation of mtl-l could be observed in the null allele of mtl-2, suggesting that both isoforms are independent and not synergistic in their mode of action. Exposure to cadmium affected all demographic indices measured, manifested by a reduction in body size, generation time, brood size and lifespan. These effects were magnified in the knock-out or wild-type subjected to a knock down by RNAi, however, only in the presence of cadmium. This substantiates the notion that metallothioneins play a pivotal role in the protection from cadmium toxicosis. Finally, an earthworm metallothionein-GFP construct could be activated in C. elegans upon exposure to cadmium, the results providing further evidence that the transcriptional control of metallothioneins is fundamentally divergent in lower invertebrates and not mediated via MTF-1 as in more complex organisms.

Expression of metallothionein gene during embryonic and early larval development in zebrafish

Metallothionein (Mt) has been considered as a molecular marker of metal pollution in aquatic ecosystems. Less is known about the expression of mt gene during embryogenesis. Here, we report the cloning, sequencing, and the expression pattern of mt gene during developmental stages in zebrafish. The zebrafish embryogenesis when takes place in a medium containing a dosage of 1000 microM zinc resulted in high mortality, indicating the deleterious effect of zinc on development. The zebrafish mt gene consists of three exons encoding 60 amino acids with 20 conserved cysteine residues. RT-PCR result indicates the maternal contribution of Mt transcripts. Using digoxigenin (DIG)-labeled anti-sense RNA probe, whole-mount in situ hybridization was performed to observe the expression pattern of zebrafish mt gene during embryonic and early larval stages. Stronger as well as ubiquitous expression of mt gene during early embryonic stages narrowed to specific expression after hatching. The mt promoter region contains seven copies of putative metal-responsive elements (MREs), which are shown to be important for the high level activity by deletion analysis. The expression of mt gene during embryogenesis implies its significant role on development.

Cloning, expression pattern and essentiality of the high-affinity copper transporter 1 (ctr1) gene in zebrafish

The high-affinity copper transporter 1 (Ctr1) is a highly conserved transmembrane protein that mediates the internalization of copper ions from the extracellular medium. In this study, we have isolated the zebrafish ctr1 gene. The zebrafish ctr1 cDNA encodes a protein with 69% identity to the human orthologue and shows conservation of specific amino acid residues involved in copper transport. We find only a single ctr1 gene in the zebrafish genome which maps to linkage group 5. The genomic structure of the zebrafish gene shows that it consists of five exons and that exon-intron boundaries are absolutely conserved with the mammalian ctr1 genes. Expression in embryos was analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and by in situ hybridization. Zebrafish ctr1 is maternally loaded, and transcripts can be detected throughout development and in adult fish. Distribution of ctr1 message appears ubiquitous during early stages becoming restricted to the brain and ventral tissues by 24 h post fertilization (hpf). Beginning at 3 days post fertilization (dpf), expression is found mainly in the developing intestine. Specific knockdown of ctr1 by antisense morpholino oligonucleotides (MOs) causes early larval lethality. Defects include cell death in tissues where ctr1 is most heavily expressed, a finding similar to that described for a mouse knockout of mCtr1. Despite the existence of at least one other copper transport mechanism in the fish, our studies show that zebrafish ctr1 is an essential gene for development.

Cloning and characterization of metallothionein gene in ayu Plecoglossus altivelis

Metallothionein (MT) has been used widely as a potential molecular marker to detect the deleterious effects of heavy metals in aquatic ecosystem. Here we exposed ayu, Plecoglossus altivelis, to zinc (Zn) and tested the distribution as well as the induction of MT in various tissues such as liver, kidney, intestine and stomach. MT induction was significant in liver tissue, followed by kidney and intestine, whereas no induction was detected in stomach. The gene encoding ayu MT was successfully cloned and characterized. Complete nucleotide sequencing and analysis of the 4.5 kb DNA fragment containing the ayu MT gene revealed that the gene has three exons interrupted by two introns, a 5'-flanking region of about 2.5 kb and about 1.6 kb of 3'-flanking region. In grouper heart and kidney cells, the 2.5 kb promoter containing eight metal responsive elements (MREs), two hepatic nuclear factor 5 responsive elements (HNF5REs) and one cAMP responsive element (CRE) had the highest reporter activity.

A novel cysteine cluster in human metal-responsive transcription factor 1 is required for heavy metal-induced transcriptional activation in vivo

Metal-responsive transcription factor 1 (MTF-1) specifically binds to metal response elements (MREs) associated with a number of metal- and stress-responsive genes. Human MTF-1 contains a cysteine-rich cluster, -632Cys-Gln-Cys-Gln-Cys-Ala-Cys638-, conserved from pufferfish to humans far removed from the MRE-binding zinc finger domain and just C-terminal to a previously mapped serine/threonine-rich transcriptional activation domain. MTF-1 proteins containing two Cys-->Ala substitutions (C632A/C634A) or a deletion in this region altogether (Delta(632-644)) are significantly impaired in their ability to induce Zn(II)- and Cd(II)-responsive transcription of a MRE-linked reporter gene in transiently transfected mouse dko7 (MTF-1-/-) cells in culture under moderate metal stress but retain the ability to drive basal levels of transcription in a MRE-dependent manner in vivo and in vitro. In addition, the mutated proteins respond to induction by Zn(II) or Cd(II) with nuclear translocation and MRE binding activities comparable with wild-type MTF-1. Attempts to rescue the Delta(632-644) deletion mutant phenotype by inserting similar Cys-rich sequences from Drosophila MTF-1 were unsuccessful, suggesting that the structure of this motif within intact human MTF-1, rather than the simple presence of multiple closely spaced Cys residues, is required for function. This cysteine cluster therefore functions at a step subsequent to nuclear translocation and MRE-binding DNA to naked promoter-containing DNA and appears to be specifically required for MTF-1 to activate transcription in the presence of inducing heavy metal ions.

Stressor-dependent regulation of the heat shock response in zebrafish, Danio rerio

Heat shock transcription factors (HSFs) regulate expression of heat shock proteins (Hsps). We have previously shown that in zebrafish a unique isoform, zHSF1b, disappears concomitant with heat shock-induced Hsp70 expression. To characterize the role of zHSF1a and zHSF1b isoforms in the regulation of the stress response in vivo, we have carried out cadmium (10-100 microM) and copper (10-30 microM) exposures in order to specify whether the disappearance of HSF1b is specific for heat stress. After 4-h metal exposures we analyzed the expression of hsp70, zHSF1a, zHSF1b and metallothionein (MT) by reverse transcriptase polymerase chain reaction in zebrafish liver, gonads and gills. Although cadmium is a known inducer of Hsps, it did not affect hsp70 expression significantly in the studied tissues. Induction of hsp70 was observed upon copper exposure in liver and gonads, but not in gills. Neither metal affected the zHSF1a/b ratio. Both cadmium and copper exposure caused upregulation of MT, regulator of metal homeostasis and detoxification, confirming that the tissues were subjected to metal loads. Thus, hsp70 appears to be more weakly induced upon metal exposure than in response to heat shock and HSF1 isoforms may participate in stressor-specific regulation of hsp70.