Cloning and characterization of a gene coding for a novel metallothionein in the Pacific oyster Crassostrea gigas (CgMT2): a case of adaptive response to metal-induced stress?

Testing combined effects of environmental trace metals/arsenic and marine trophic status on the bioaccumulation in Pacific oysters: Insights from 22-site field samplings

Pacific oysters were sampled from 22 human-impacted sites in northeastern Japan to measure Cr, Cu, Zn, Pb, Cd, and As. The hazard quotient was slightly >1 for Cu and/or As at two sites, but <1 for all metal species and As at the other sites, indicating low human health risks. Oysters' Cu, Zn, and Pb contents were positively related to their concentrations in the sediment, while Cr and As were not. Oysters' Cu and Zn were negatively related to the inorganic nitrogen in seawater, while oysters' Pb and As showed positive relationships with the particulate organic carbon. These findings suggest that marine trophic status affects oysters' metal uptake differently among the metal species. Furthermore, oysters' Cr, Cu, Zn, and Pb contents were negatively related to their eicosapentaenoic acid content and condition index. Therefore, the nutritional conditions of oysters may influence their elimination or accumulation of these metals.

Expression and Functional Analysis of the Metallothionein and Metal-Responsive Transcription Factor 1 in Phascolosoma esculenta under Zn Stress

Metallothioneins (MTs) are non-enzymatic metal-binding proteins widely found in animals, plants, and microorganisms and are regulated by metal-responsive transcription factor 1 (MTF1). MT and MTF1 play crucial roles in detoxification, antioxidation, and anti-apoptosis. Therefore, they are key factors allowing organisms to endure the toxicity of heavy metal pollution. Phascolosoma esculenta is a marine invertebrate that inhabits intertidal zones and has a high tolerance to heavy metal stress. In this study, we cloned and identified MT and MTF1 genes from P. esculenta (designated as PeMT and PeMTF1). PeMT and PeMTF1 were widely expressed in all tissues and highly expressed in the intestine. When exposed to 16.8, 33.6, and 84 mg/L of zinc ions, the expression levels of PeMT and PeMTF1 in the intestine increased first and then decreased, peaking at 12 and 6 h, respectively, indicating that both PeMT and PeMTF1 rapidly responded to Zn stress. The recombinant pGEX-6p-1-MT protein enhanced the Zn tolerance of Escherichia coli and showed a dose-dependent ABTS free radical scavenging ability. After RNA interference (RNAi) with PeMT and 24 h of Zn stress, the oxidative stress indices (MDA content, SOD activity, and GSH content) and the apoptosis indices (Caspase 3, Caspase 8, and Caspase 9 activities) were significantly increased, implying that PeMT plays an important role in Zn detoxification, antioxidation, and anti-apoptosis. Moreover, the expression level of PeMT in the intestine was significantly decreased after RNAi with PeMTF1 and 24 h of Zn stress, which preliminarily proved that PeMTF1 has a regulatory effect on PeMT. Our data suggest that PeMT and PeMTF1 play important roles in the resistance of P. esculenta to Zn stress and are the key factors allowing P. esculenta to endure the toxicity of Zn.

Identification of three metallothioneins in the black soldier fly and their functions in Cd accumulation and detoxification

The black soldier fly (BSF) Hermetia illucens has a strong tolerance to cadmium stress. This helps to use BSF in entomoremediation of heavy metal pollution. Rich metallothionein (MT) proteins were thought to be important for some insects to endure the toxicity of heavy metal. We identified and characterized three MTs genes in BSF (BSFMTs), including BSFMT1, BSFMT2A, and BSFMT2B. Molecular modeling was used to predict metal binding sites. Phylogenetic analysis was used to identify gene families. Overexpression of the recombinant black soldier fly metallothioneins was found to confer Cd tolerance in Escherichia coli. Finally, functions of BSFMTs in BSF were explored through RNA interference (RNAi). RNAi results of BSFMT2B showed that the larval fresh weight decreased significantly, and the larvae mortality increased significantly. This study suggests that BSFMTs have important properties in Cd detoxification and tolerance in BSF. Further characterization analyses of physiological function about metallothioneins are necessary in BSF and other insects.

Tunicates Illuminate the Enigmatic Evolution of Chordate Metallothioneins by Gene Gains and Losses, Independent Modular Expansions, and Functional Convergences

To investigate novel patterns and processes of protein evolution, we have focused in the metallothioneins (MTs), a singular group of metal-binding, cysteine-rich proteins that, due to their high degree of sequence diversity, still represents a "black hole" in Evolutionary Biology. We have identified and analyzed more than 160 new MTs in nonvertebrate chordates (especially in 37 species of ascidians, 4 thaliaceans, and 3 appendicularians) showing that prototypic tunicate MTs are mono-modular proteins with a pervasive preference for cadmium ions, whereas vertebrate and cephalochordate MTs are bimodular proteins with diverse metal preferences. These structural and functional differences imply a complex evolutionary history of chordate MTs-including de novo emergence of genes and domains, processes of convergent evolution, events of gene gains and losses, and recurrent amplifications of functional domains-that would stand for an unprecedented case in the field of protein evolution.

Modularity in Protein Evolution: Modular Organization and De Novo Domain Evolution in Mollusk Metallothioneins

Metallothioneins (MTs) are proteins devoted to the control of metal homeostasis and detoxification, and therefore, MTs have been crucial for the adaptation of the living beings to variable situations of metal bioavailability. The evolution of MTs is, however, not yet fully understood, and to provide new insights into it, we have investigated the MTs in the diverse classes of Mollusks. We have shown that most molluskan MTs are bimodular proteins that combine six domains—α, β1, β2, β3, γ, and δ—in a lineage-specific manner. We have functionally characterized the Neritimorpha β₃β₁ and the Patellogastropoda γβ₁ MTs, demonstrating the metal-binding capacity of the new γ domain. Our results have revealed a modular organization of mollusk MT, whose evolution has been impacted by duplication, loss, and de novo emergence of domains. MTs represent a paradigmatic example of modular evolution probably driven by the structural and functional requirements of metal binding.

Metal binding characterization of heterologously expressed metallothionein of the freshwater crab Sinopotamon henanense

We characterized the metal tolerance of recombinant strains harboring metallothionein from the freshwater crab Sinopotamon henanense (ShMT) in vivo and metal binding properties of ShMT purified in vitro. The recombinant strains harboring ShMT were exposed to 0.1 mM Cd²⁺, 0.3 mM Cu²⁺, 0.5 mM Pb²⁺, and 0.8 mM Zn²⁺. The growth curves and spot assays of recombinant strains and the contents of heavy metal ions were analysed in the media supplemented with above metal ions provided to recombinant E. coli synthesis. The structural characteristics of the Cd-, Cu-, Pb-, and Zn-ShMT were determined through ultraviolet spectroscopy (UV-vis), circular dichroism (CD), and isothermal titration calorimetry (ITC). The in vivo results showed that, compared to control strains, recombinant strains tolerated Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺. Furthermore, the contents of Cd²⁺ and Pb²⁺ in media decreased substantially. In vitro and the Cd-ShMT had a higher degree of folding compactness in solution. 5,5'-Dithiobis-(2-nitrobenzoic) acid (DTNB) reaction and ITC results demonstrated that ShMT yielded Cd₆-, Cu₇-, and Pb₆-ShMT. The binding stability order was Cu-ShMT > Cd-ShMT > Pb-ShMT > Zn-ShMT. Overall, ShMT is a canonical crustacean MT and is defined as a Cd-specific MT isoform that functions mainly in a detoxifying Cd²⁺ and Pb²⁺ and in regulating Zn²⁺ homeostasis in S. henanense. This research on the metal binding properties of ShMT provides a better understanding of the physiological function of ShMT reducing heavy metal bioavailability and by regulating essential trace metals.

Characterisation of the metallothionein gene in the Sydney rock oyster and its expression upon metal exposure in oysters with different prior metal exposure histories

The metal-binding protein metallothionein (MT) is widely used as a biomarker of metal contamination. In this study, we cloned a MT gene (sgMT) from the Sydney rock oyster Saccostrea glomerata. The gene encodes a MT-I protein with a classical αβ domain structure and is expressed as two transcripts resulting from alternative polyadenylation. The gene promoter contains two putative metal-responsive elements (MREs) which are known to be required for metal-inducible transcription. A specific and efficient qPCR assay was developed to quantify sgMT mRNA expression. Further, we assessed whether prior metal exposure history influences sgMT mRNA expression upon subsequent metal exposure. Oysters with varying prior metal exposure histories (contaminated and reference) were exposed to Cu, Cd and Zn. Expression of sgMT generally increased with metal dose, and oysters with an elevated past metal exposure history exhibited higher sgMT expression under Cd and Zn stress, representing a potential acclimatory response to prior metal exposure.

A preliminary integrated genetic map distinguishes every chromosome pair and locates essential genes related to abiotic adaptation of Crassostrea angulata/gigas

BACKGROUND

The re-sequencing of C. angulata has revealed many polymorphisms in candidate genes related to adaptation to abiotic stress that are not present in C. gigas; these genes, therefore, are probably related to the ability of this oyster to retain high concentrations of toxic heavy metals. There is, in addition, an unresolved controversy as to whether or not C. angulata and C. gigas are the same species or subspecies. Both oysters have 20 metacentric chromosomes of similar size that are morphologically indistinguishable. From a genomic perspective, as a result of the great variation and selection for heterozygotes in C. gigas, the assembly of its draft genome was difficult: it is fragmented in more than seven thousand scaffolds.

RESULTS

In this work sixty BAC sequences of C. gigas downloaded from NCBI were assembled in BAC-contigs and assigned to BACs that were used as probes for mFISH in C. angulata and C. gigas. In addition, probes of H3, H4 histone, 18S and 5S rDNA genes were also used. Hence we obtained markers identifying 8 out the 10 chromosomes constituting the karyotype. Chromosomes 1 and 9 can be distinguished morphologically. The bioinformatic analysis carried out with the BAC-contigs annotated 88 genes. As a result, genes associated with abiotic adaptation, such as metallothioneins, have been positioned in the genome. The gene ontology analysis has also shown many molecular functions related to metal ion binding, a phenomenon associated with detoxification processes that are characteristic in oysters. Hence the provisional integrated map obtained in this study is a useful complementary tool for the study of oyster genomes.

CONCLUSIONS

In this study 8 out of 10 chromosome pairs of Crassostrea angulata/gigas were identified using BAC clones as probes. As a result all chromosomes can now be distinguished. Moreover, FISH showed that H3 and H4 co-localized in two pairs of chromosomes different that those previously escribed. 88 genes were annotated in the BAC-contigs most of them related with Molecular Functions of protein binding, related to the resistance of the species to abiotic stress. An integrated genetic map anchored to the genome has been obtained in which the BAC-contigs structure were not concordant with the gene structure of the C. gigas scaffolds displayed in the Genomicus database.

Trace metals in oysters: molecular and cellular mechanisms and ecotoxicological impacts

Oysters are important benthic bivalves in coastal and estuarine environments. They are widely farmed due to their rapid growth and taste; they are also widely applied in environmental monitoring of coastal pollution due to their accumulation of contaminants. Most importantly, oysters are among the few marine organisms that are considered to be hyper-accumulators of many toxic metals, such as cadmium, copper and zinc. As such, there is a tremendous call to study the interactions between metals and oysters, especially due to the increasing metal pollution in many coastal and estuarine waters. Over the past decades, many studies have focused on metal accumulation in oysters as well as the ecotoxicological effects of metals on oysters. In this review, we summarize the recent progress in our understanding of the molecular and cellular mechanisms of metal accumulation, sequestration and toxicity in oysters. Applications of modern technologies such as omics and nanoscale imaging have added significantly to our knowledge of metal biology in oysters. Variations between different metals also demonstrate the diversity of the interactions between oysters and metals. Despite this recent progress, however, there is a need for further study of the molecular mechanisms of metal uptake and toxicity as well as the joint effects of metal mixtures on oyster populations. Oysters have higher numbers of stress responsive genes than most animals, which may have been induced by gene duplication during the evolution of their intertidal environmental adaptations. The divergent expression of stress responsive genes may explain the different tolerances for metals among different species. These fundamental studies may eventually provide promising solutions for reducing toxic metal concentrations in oysters for safe consumption by humans. To conclude, the complexity of life history and metal chemistry of oysters coupled with emerging pollution and application of modern techniques represents an important and exciting research area in modern ecotoxicology.

Functional comparision between truncated MTT1 and truncated MTT2 from Tetrahyemna thermophila

Metallothioneins (MTs) are low-molecular-weight proteins with high Cys content and high metal-chelating ability. CdMT and CuMT subfamilies present different characteristics in Tetrahymena. To explore the effect of the cysteine arrangement and sequence length of MTs for binding different metal ions, MTT1, truncated MTT1 (TM1), MTT2, and truncated MTT2 (TM2) were expressed in E. coli. The half-maximal inhibiting concentrations (IC50) of Cd2+ and Cu+ for the recombinant strains were different. Furthermore, E. coli cells expressing MTT1 and TM1 exhibited higher accumulating ability for Cd2+ than cells expressing MTT2 and TM2. However, the opposite is true for Cu+. The binding ability of the different recombinant proteins to Cd2+ and Cu+ were also different. MTT1 and truncated mutant TM1 were the preference for Cd2+, whereas MTT2 and truncated mutant TM2 were the preference for Cu+ coordination. These results showed that metal ion tolerance and accumulation ability not only depended on cysteine arrangement pattern but also on sequence length of MT in Tetrahymena.

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.

The oyster immunity

Oysters, the common name for a number of different bivalve molluscs, are the worldwide aquaculture species and also play vital roles in the function of ecosystem. As invertebrate, oysters have evolved an integrated, highly complex innate immune system to recognize and eliminate various invaders via an array of orchestrated immune reactions, such as immune recognition, signal transduction, synthesis of antimicrobial peptides, as well as encapsulation and phagocytosis of the circulating haemocytes. The hematopoietic tissue, hematopoiesis, and the circulating haemocytes have been preliminary characterized, and the detailed annotation of the Pacific oyster Crassostrea gigas genome has revealed massive expansion and functional divergence of innate immune genes in this animal. Moreover, immune priming and maternal immune transfer are reported in oysters, suggesting the adaptability of invertebrate immunity. Apoptosis and autophagy are proved to be important immune mechanisms in oysters. This review will summarize the research progresses of immune system and the immunomodulation mechanisms of the primitive catecholaminergic, cholinergic, neuropeptides, GABAergic and nitric oxidase system, which possibly make oysters ideal model for studying the origin and evolution of immune system and the neuroendocrine-immune regulatory network in lower invertebrates.

Preventive Effect of 3,5-dihydroxy-4-methoxybenzyl Alcohol (DHMBA) and Zinc, Components of the Pacific Oyster Crassostrea gigas, on Glutamatergic Neuron Activity in the Hippocampus

The effects of 3,5-dihydroxy-4-methoxybenzyl alcohol (DHMBA), and zinc--both components of the Pacific oyster Crassostrea gigas--were examined by glutamatergic neuron activity in rats in an in vivo microdialysis experiment and an in vitro brain slice experiment. The basal concentration of extracellular glutamate in the hippocampus was decreased under hippocampal perfusion with DHMBA (1 mmol l(-1)) or ZnCl2 (μmol l(-1)), indicating that DHMBA and Zn(2+) suppress glutamatergic neuron activity under basal (static) conditions. To assess the preventive effect of DHMBA and Zn(2+) on glutamate release from neuron terminals, brain slices were pretreated with DHMBA (1 mmol l(-1)) or ZnCl2 (100 nmol l(-1)) for 1 h, then stimulated with high K(+). A high, K(+)-induced increase in extracellular Zn(2+) level, an index of glutamate release, was suppressed with pretreatment with DHMBA or zinc. A high, K(+)-induced increase in intracellular Ca(2+) level was also suppressed with pretreatment with DHMBA or Zn(2+). These results suggest that DHMBA and Zn(2+), previously taken up in the hippocampal cells, suppress high, K(+)-induced glutamate release in the hippocampus, probably via presynaptic suppression of intracellular Ca(2+) signaling. It is likely that Zn(2+) and DHMBA play a preventive role in suppressing excess glutamatergic neuron activity in rats and mice.

The polymorphisms in the promoter of HSP90 gene and their association with heat tolerance of bay scallop

The heat shock protein 90 (HSP90) is a highly abundant and ubiquitous molecular chaperone which plays essential roles in many cellular processes. In the present study, the messenger RNA (mRNA) expressions of HSP90 after acute heat stress were investigated in two bay scallop populations (Argopecten irradians irradians and Argopecten irradians concentricus). The heat-resistant scallop A. i. concentricus, which is distributed in Zhanjiang, China, exhibited significantly higher induction of HSP90 compared with that of the heat-sensitive scallop A. i. irradians, which is distributed in Qinhuangdao, China. The promoter sequence of HSP90 gene from bay scallop (AiHSP90) was cloned, and the polymorphisms within this region were investigated by sequencing to analyze their association with heat tolerance. A total of six single nucleotide polymorphisms (SNPs), including -1167 T-C, -1023 A-C, -799 C-T, -774 A-G, -686 C-T, and -682 A-C, were identified in the amplified promoter region, and most of them affected the putative transcription factor binding sites except for locus -1167. All the six SNP sites were found to be associated with heat tolerance after Hardy-Weinberg equilibrium (HWE) and association analysis. Moreover, haplotypes CACACC and TCTATC were also found to be associated with heat tolerance based on the result of linkage disequilibrium and association analysis. The results provided insights into the molecular mechanisms underlying the thermal adaptation of different congener endemic bay scallops, which suggested that the increased heat tolerance of A. i. concentricus (compared with A. i. irradians) was associated with the higher expression of AiHSP90. Meanwhile, the six genotypes (-1167 TT, -1023 CC, -799 TT, -774 GG, -686 CC, and -682 AA) and two haplotypes (CACACC and TCTATC) could be used as potential markers for scallop selection breeding with higher heat tolerance.

Transcriptomic response of the hydrothermal mussel Bathymodiolus azoricus in experimental exposure to heavy metals is modulated by the Pgm genotype and symbiont content

Hydrothermal vent mussels belonging to the genus Bathymodiolus dominate communities at hydrothermal sites of the Mid-Atlantic Ridge. The mussel Bathymodiolus azoricus harbors thiotrophic and methanotrophic symbiotic bacteria in its gills and evolves in naturally highly metal contaminated environments. In the context of investigations on metal tolerance/effect in B. azoricus, we focused our work on the short-term adaptive response (15days) of mussels to different metals exposure at a molecular level using metal concentrations chosen to mimic natural situations at three vents sites. The expression of a set of 38 genes involved in different steps of the metal uptake, detoxication and various metabolisms was analysed by qPCR. Mussels were also genotyped at 10 enzyme loci to explore the relationships among natural genetic variation and gene expression. Relation between symbiont content (both sulfur-oxidizing and methanogen bacteria) and gene expression was also analysed. Our study demonstrated the influence of metal cocktail composition and time exposure on the transcriptome regulation with a specific pattern of regulation observed for the three metal cocktail tested. We also evidenced the significant influence of some specific Pgm genotype on the global gene expression in our experimental populations and a general trend of a higher gene expression in individuals carrying a high symbiont content.

The polymorphism in the promoter of HSP70 gene is associated with heat tolerance of two congener endemic bay scallops (Argopecten irradians irradians and A. i. concentricus)

Background

The heat shock protein 70 (HSP70) is one kind of molecular chaperones, which plays a key role in protein metabolism under normal and stress conditions.

Methodology

In the present study, the mRNA expressions of HSP70 under normal physiological condition and after acute heat stress were investigated in gills of two bay scallop populations (Argopecten irradians irradians and A. i. concentricus). The heat resistant scallops A. i. concentricus showed significantly lower basal level and higher induction of HSP70 compared with that of the heat sensitive scallops A. i. irradians. The promoter sequence of HSP70 gene from bay scallop (AiHSP70) was cloned and the polymorphisms within this region were investigated to analyze their association with heat tolerance. Totally 11 single nucleotide polymorphisms (SNPs) were identified, and four of them (−967, −480, −408 and −83) were associated with heat tolerance after HWE analysis and association analysis. Based on the result of linkage disequilibrium analysis, the in vitro transcriptional activities of AiHSP70 promoters with different genotype were further determined, and the results showed that promoter from A. i. concentricus exhibited higher transcriptional activity than that from A. i. irradians (P<0.05).

Conclusions

The results provided insights into the molecular mechanisms underlying the thermal adaptation of different congener endemic bay scallops, which suggested that the increased heat tolerance of A. i. concentricus (compared with A. i. irradians) was associated with the higher expression of AiHSP70. Meanwhile, the −967 GG, −480 AA, −408 TT and −83 AG genotypes could be potential markers for scallop selection breeding with higher heat tolerance.

Metallothioneins, unconventional proteins from unconventional animals: a long journey from nematodes to mammals

Metallothioneins (MTs) are ubiquitous low molecular weight cysteine-rich proteins characterized by high affinity for d10 electron configuration metals, including essential (Zn and Cu) and non-essential (Cd and Hg) trace elements. The biological role of these ancient and well-conserved multifunctional proteins has been debated since MTs were first discovered in 1957. Their main hypothesized functions are: (1) homeostasis of Zn and Cu; (2) detoxification of Cd, and Hg; and (3) free radical scavenging. This review will focus on MTs in unconventional animals, those not traditionally studied in veterinary medicine but of increasing interest in this field of research. Living in different environments, these animals represent an incredible source of physiological and biochemical adaptations still partly unexplored. The study of metal-MT interactions is of great interest for clinicians and researchers working in veterinary medicine, food quality and endangered species conservation.

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.

Tissue-specificity and phylogenetics of Pl-MT mRNA during Paracentrotus lividus embryogenesis

Metallothioneins (MTs) constitute a family of cysteine-rich, low molecular weight proteins, which generally provide protection against metal toxicity and oxidative stress counteracting the cell damage caused by essential and non-essential heavy metals. Equally important is the physiological role of MTs in the homeostasis of essential metals, which are involved in a wide variety of cellular processes. The aim of this work was to investigate the expression and the territorial localization of Paracentrotus lividus MT (Pl-MT) mRNA during sea urchin development by Quantitative Polymerase Chain Reaction (QPCR) and Whole Mount In Situ Hybridization (WMISH), as well as the phylogenetic comparison with selected MT homologs present in different phyla. We found that Pl-MT mRNA is accumulated in unfertilized eggs and constitutively expressed during development, with very low levels of maternal mRNA at cleavage stages, followed by a significant rise during gastrulation with a peak at the prism stage. Pl-MT mRNA was expressed in the vegetative plate at mesenchyme blastula, later restricted to the endoderm of gastrula embryos and finally to the gut of plutei. Indirect immunofluorescence (IF) using a specific antibody for the endoderm marker Endo1 demonstrated a co-localization with the Pl-MT transcripts in the midgut and hindgut after the intestine differentiation occurs and when larval feeding begins. Our results show for the first time the constitutive temporal and tissue-specific expression of MT in P. lividus embryos, providing new information for studies on the mechanisms controlling basal and induced MT gene expression. The analysis of the phylogenetic relationship of Pl-MT with homologs from different phyla, ranging from yeast to vertebrates, suggests the evolutionary process of these proteins, which could have been selected not only on the basis of their ability to bind metals but also by their tissue-specificity.

Effects of heavy metals on the expression of a zinc-inducible metallothionein-III gene and antioxidant enzyme activities in Crassostrea gigas

Sequestration by metallothioneins and antioxidant defense are two kinds of important defense mechanisms employed by mollusks to minimize adverse effects caused by heavy metal contaminants in marine environment. In the present study, a novel metallothionein gene, CgMT-III, was cloned from Crassostrea gigas, consisting of eighteen conserved cysteine residues and encoding a MT III-like protein with two tandem β domains. The expression level of CgMT-III transcript induced by zinc was much higher than that induced by cadmium exposure. It suggested that CgMT-III was perhaps mainly involved in homeostatic control of zinc metabolism, which was distinct from previously identified MTs in C. gigas. Among the tested antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), SOD and GPx showed varying up-regulations in a tissue-specific manner, while CAT activities were inhibited in both gill and hepatopancreas from C. gigas exposed to heavy metals. It can be inferred that CgMT-III was mainly involved in zinc homeostasis, and CgMT-III gene together with CAT enzyme could be potential biomarkers to indicate heavy metal, especially zinc pollution in marine organisms.

Responses of Pacific oyster Crassostrea gigas populations to abiotic stress in environmentally contrasted estuaries along the Atlantic coast of France

Genetic and ecophysiological responses of oyster, Crassostrea gigas, populations to environmental stress were investigated in three highly contaminated French estuaries (the Gironde, Loire and Vilaine) and compared to a control, the Belon estuary. A strong response in both metallothionein CgMT4 mRNA expression, as determined by semi-quantitative RT-PCR, and amount of protein, as determined by ELISA tests, was demonstrated in estuarine populations subjected to differential contamination, with an inhibition in the area most highly contaminated with metals. In these same estuarine populations, we found polymorphisms of the metallothionein CgMT4 gene and three other genes (glutamine synthetase--GS, delta-9 desaturase--D9 and phosphoglucomutase--PGM) involved in stress response of C. gigas. We showed that genetic differentiation was observed for MT4 and PGM genes in the Gironde estuary which is highly contaminated with metals. A strong seasonal effect was observed. Phenotype-genotype coupling revealed that one particular MT4 allele and one PGM allele seemed to be associated with metal sensitivity expressed as lower detoxification efficiency and higher metal bioaccumulation. The MT4 gene is a good physiological and genetic marker of stress response and susceptibility.

Isolation and characterization of a phenolic antioxidant from the Pacific oyster (Crassostrea gigas)

Using an oxygen radical absorbance capacity (ORAC) assay, antioxidant activity was detected in the ethanol extract of the Pacific oyster, which was purified by sequential extraction with organic solvents. The ethyl acetate fraction showed the strongest antioxidant activity and was further purified, yielding a single compound [as assessed by thin-layer chromatography (TLC) and reverse-phase high-performance liquid chromatography (HPLC)]. This compound was identified as 3,5-dihydroxy-4-methoxybenzyl alcohol on the basis of (1)H and (13)C nuclear magnetic resonance (NMR), heteronuclear multiple-bond correlation (HMBC), and electrospray ionization-mass spectrometry (ESI-MS) spectral analyses, a conclusion that was confirmed by chemical synthesis. The concentration of the compound was 6.7 mg/100 g of whole oyster meat wet weight. This amphiphilic antioxidant retarded the copper-mediated oxidation of low-density lipoproteins (LDLs) and the generation of thiobarbituric acid reactive substances. Furthermore, the compound showed substantial antioxidant activity using the ORAC and 2,2-diphenyl-1-picrylhydrazyl (DPPH) assays compared to natural antioxidants. Although the same compound was previously found in brown algae, its presence in other organisms and antioxidant activity are reported here for the first time.

Characterisation and genetic polymorphism of metallothionein gene CgMT4 in experimental families of Pacific oyster Crassostrea gigas displaying summer mortality

Summer mortality events have been observed in Pacific oyster Crassostrea gigas for several decades. This paper examines the selective pressure exerted by summer mortality on the polymorphism of a newly identified oyster metallothionein gene. CgMT4 cDNA and genomic sequences were obtained. CgMT4 was studied in two generations of oysters reared in three sites on the French Atlantic coast, using single strand conformation polymorphism analysis. Four alleles were detected. Individuals carrying genotype MT4-CD seem to have higher susceptibility to summer risk conditions. The MT4 gene could be a potential new genetic marker for susceptibility; further validation studies are recommended.

Metallothionein cDNA cloning, metallothionein expression and heavy metals in Scapharca inaequivalvis along the Northern Adriatic coast of Italy

The aims of this work were: (1) identification of the metallothionein (MT) gene coding sequence in order to prepare an MT probe in Scapharca inaequivalvis and (2) quantification of Cd, Zn, Cu, MT and MTmRNA expression in tissues of molluscs from three areas along the Northern Adriatic coast of Italy. By RT-PCR we cloned the MTcDNA of S. inaequivalvis using the RNA extracted from hepatopancreas of specimens exposed to Cd. The 61 amino acids sequence of MT was deduced and was 70% identical to S. brughtonii MT. Cd concentration in molluscs from the wild was significantly higher in gills from specimens sampled near Ravenna. Zn concentration in the same tissue was significantly higher in Ravenna with respect to Porto Garibaldi while no difference with respect to Cesenatico was detected. Cu levels showed significant differences among sites in gills and mantle whereas values in the hepatopancreas were similar in all sites. The low MT levels were indicative of a low metal exposure; few differences were found in MTmRNA concentrations, which resulted significantly higher in hepatopancreas of molluscs from Porto Garibaldi.

The metallothionein genes of Mytilus galloprovincialis: genomic organization, tissue expression and evolution

Recently, increasing interest has been directed to the study of metallothioneins (MTs), which are small proteins that are able to bind metal ions. The induction of MT synthesis after exposure to metal or other environmental contaminants in a large number of aquatic invertebrates makes these proteins good biomarkers in water monitoring programs. Within bivalves, the species Mytilus galloprovincialis and Mytilus edulis represent model organisms for these types of studies, as well as for molecular studies regarding the expression and characterization of MT encoding genes. In the present paper, we focused on the genomic characterization, evolutionary, and tissue-expression analyses of the MT-10, MT-10 Intronless, and MT-20 genes in M. galloprovincialis. The comparison of the genomic sequences showed the presence of long nucleotide stretches within the introns of the MT genes that are conserved between M. galloprovincialis and M. edulis. These non-coding conserved sequences may contain regulatory motifs. Real-Time RT-PCR experiments revealed that, at the basal conditions, the MT-10 and MT-10 Intronless genes are expressed at levels considerably higher than the MT-20 gene, mainly in the digestive gland and gill tissue. The strong induction of the MT-20 gene expression detected in a field-collected sample is associated with the up-regulation of both the MT-10 and MT-10 Intronless genes. Evolutionary analysis revealed signals of localized positive selection that, together with the tissue-expression data, support a possible functional diversification between the MTs encoded by the MT-10 and MT-10 Intronless genes.

Shaping mechanisms of metal specificity in a family of metazoan metallothioneins: evolutionary differentiation of mollusc metallothioneins

Background

The degree of metal binding specificity in metalloproteins such as metallothioneins (MTs) can be crucial for their functional accuracy. Unlike most other animal species, pulmonate molluscs possess homometallic MT isoforms loaded with Cu⁺ or Cd²⁺. They have, so far, been obtained as native metal-MT complexes from snail tissues, where they are involved in the metabolism of the metal ion species bound to the respective isoform. However, it has not as yet been discerned if their specific metal occupation is the result of a rigid control of metal availability, or isoform expression programming in the hosting tissues or of structural differences of the respective peptides determining the coordinative options for the different metal ions. In this study, the Roman snail (Helix pomatia) Cu-loaded and Cd-loaded isoforms (HpCuMT and HpCdMT) were used as model molecules in order to elucidate the biochemical and evolutionary mechanisms permitting pulmonate MTs to achieve specificity for their cognate metal ion.

Results

HpCuMT and HpCdMT were recombinantly synthesized in the presence of Cd²⁺, Zn²⁺ or Cu²⁺ and corresponding metal complexes analysed by electrospray mass spectrometry and circular dichroism (CD) and ultra violet-visible (UV-Vis) spectrophotometry. Both MT isoforms were only able to form unique, homometallic and stable complexes (Cd₆-HpCdMT and Cu₁₂-HpCuMT) with their cognate metal ions. Yeast complementation assays demonstrated that the two isoforms assumed metal-specific functions, in agreement with their binding preferences, in heterologous eukaryotic environments. In the snail organism, the functional metal specificity of HpCdMT and HpCuMT was contributed by metal-specific transcription programming and cell-specific expression. Sequence elucidation and phylogenetic analysis of MT isoforms from a number of snail species revealed that they possess an unspecific and two metal-specific MT isoforms, whose metal specificity was achieved exclusively by evolutionary modulation of non-cysteine amino acid positions.

Conclusion

The Roman snail HpCdMT and HpCuMT isoforms can thus be regarded as prototypes of isoform families that evolved genuine metal-specificity within pulmonate molluscs. Diversification into these isoforms may have been initiated by gene duplication, followed by speciation and selection towards opposite needs for protecting copper-dominated metabolic pathways from nonessential cadmium. The mechanisms enabling these proteins to be metal-specific could also be relevant for other metalloproteins.

Isolation and characterization of a novel copper-inducible metallothionein gene of a ciliate, Tetrahymena tropicalis lahorensis

The two isoforms of copper metallothionein (CuMT) gene of a copper resistant ciliate, Tetrahymena tropicalis lahorensis (Ttl), have been isolated and characterized. The molecular cloning and nucleotide sequencing of cDNAs coding for the two CuMT isoforms revealed that TtlCuMT1 gene has 300, while TtlCuMT2 has 327 nucleotides, both with ATG as the initiation codon and TGA as the translational termination codon. TAG codes for glutamine in TtlCuMT2 gene which is peculiar to Tetrahymena. The deduced or translated TtlCuMT1 and TtlCuMT2 peptide sequences contain 100 and 108 amino acid residues including 28 and 32 cysteine residues, respectively. The amino acid sequences of TtlCuMT1 and TtlCuMT2 have special features of two and three CXCXXCXCXXCXC intragenic tandem repeats with a conserved structural pattern of cysteine, respectively. The predicted tertiary structures of these two isoforms indicate two domains. Domain I and the initial part of domain II showed >98% homology with other Tetrahymena CuMT. On the basis of the differences in the domain II, the metallothionein subfamily 7b can be divided into two groups, one (TtlCuMT1) comprising >100 amino acids and the other (TtlCuMT2) comprising <100 amino acids. This is a novel finding of the present study as no such report on this type of classification exists at the moment. TtlCuMT1 has 95%, while TtlCuMT2 has 97% resemblance with the previously reported CuMT genes of Tetrahymena spp. SDS-PAGE analysis using fluorescent probe as well as coomassie brilliant blue staining also confirmed the presence of metallothionein.

Short-term metallothionein inductions in the edible cockle Cerastoderma edule after cadmium or mercury exposure: discrepancy between mRNA and protein responses

Metallothioneins (MT) are essential metal binding proteins involved in metal homeostasis and detoxification in living organisms. Numerous studies have focused on MT response to metal exposure and showed an important variability according to species, metal, concentration and time of exposure. In this study, the expression of one isoform of MT gene (Cemt1) and associated MT protein synthesis were determined after 1, 3, 9, 24, 72 and 168h of cadmium (Cd) or mercury (Hg) exposures in gills of the cockle Cerastoderma edule. This experiment, carried out in laboratory conditions, revealed that in Cd-exposed cockles, induction of Cemt1 is time-dependent following a "pulse-scheme" with significant upregulation at 24h and 168h intersected by time point (72h) with significant downregulation. MT protein concentration increases with time in gills of exposed cockles in relation with the progressive accumulation of Cd in soluble fraction. On contrary, Hg exposure does not lead to any induction of Cemt1 mRNA expression or MT protein synthesis compared to control, despite a higher accumulation of this metal in gills of cockles compared to Cd. The localization of Hg (85-90%) is in insoluble fraction, whereas MT was located in the cytoplasm of cells. This gives us a first clue to understand the inability of Hg to activate MT synthesis. However, other biochemical processes probably occur in gills of C. edule since the remaining soluble fraction of Hg exceeds MT sequestration ability. Finally, since one of the first main targets of metal toxicity in cells was the mitochondria, some genes involved in mitochondria metabolism were also analyzed in order to assess potential differences in cellular damages between two metal exposures. Indeed, until T(168), no impact on mitochondrial genes was shown following Hg exposure, despite the complete lack of MT response. This result indicated the presence of other effective cellular ligands which sequester the cytosolic fraction of this metal and consequently inhibit metal reactivity. Such competition mechanisms with other cytosolic ligands more sensitive to Hg were particularly argued in the discussion.

Effects of acid mine drainage on the genetic diversity and structure of a natural population of Daphnia longispina

The increase in resistance to contaminants can result in the loss of genetic diversity of impacted populations. In this work, the effects of acid mine drainage (AMD) on the genetic diversity and structure of a historically exposed population of Daphnia longispina were evaluated using amplified fragment length polymorphism (AFLP) analysis. Individual sensitivity to acute copper exposure was determined in order to characterize the populations in terms of metal tolerance and in an attempt to identify possible contaminant indicative bands (CIB). No reduction in genetic diversity was found in the AMD impacted site population, in comparison to two reference populations. However, the analysis of molecular variance indicated a significant genetic differentiation from the two reference populations and a significant correlation between individual genetic distance and tolerance. The different average tolerance of individuals presenting one specific AFLP band indicated the existence of one putative CIB.

Effects of elevated temperature and cadmium exposure on stress protein response in eastern oysters Crassostrea virginica (Gmelin)

Stress proteins such as heat shock proteins (HSPs) and metallothioneins (MTs) play a key role in cellular protection against environmental stress. Marine ectotherms such as eastern oysters Crassostrea virginica are commonly exposed to multiple stressors including temperature and pollution by metals such as cadmium (Cd) in estuaries and coastal zones; however, the combined effects of these stressors on their cellular protection mechanisms are poorly understood. We acclimated C. virginica from populations adapted to different thermal regimes (Washington, North Carolina and Texas) at a common temperature of 12 degrees C, and analyzed their expression of MTs and HSPs (cytosolic HSP69, HSC72-77, HSP90 and mitochondrial HSP60) in response to the combined acute temperature stress and long-term Cd exposure. Overall, HSP and MT induction patterns were similar in oysters from the three studied geographically distant populations. HSP69 and MTs were significantly up-regulated by Cd and temperature stress implying their important role in cellular stress protection. In contrast, HSC72-77, HSP60 and HSP90 were not consistently induced by either acute heat or Cd exposure. The induction temperature for MTs was higher than for HSP69 (>28 degrees C vs. 20 degrees C, respectively), and MTs were more strongly induced by Cd than by temperature stress (to up to 38-94-fold compared by 3.5-7.5-fold, respectively) consistent with their predominant role in metal detoxification. Notably, heat stress did not result in an additional increase in metallothionein expression in Cd-exposed oysters suggesting a capacity limitation during the combined exposure to Cd and temperature stress. Levels of HSP69 and in some cases, HSC72-77 and HSP90 were lower in Cd-exposed oysters as compared to their control counterparts during heat stress indicating that simultaneous exposure to these two stressors may have partially suppressed the cytoprotective upregulation of molecular chaperones. These limitations of stress protein response may contribute to the reduced thermotolerance of oysters from metal-polluted environments.

Metallothioneins in Aquatic Organisms: Fish, Crustaceans, Molluscs, and Echinoderms

Metallothioneins (MTs) have been described in a wide range of organisms, from bacteria to mammals, thus representing an interesting example of evolutionary molecular adaptation. If the moderate variability of MTs across phylogenetically distant organisms reflects their highly conserved function, the specific environmental requirements may explain the multiplicity of isoforms also in the same organism. The MT polymorphism is particularly important in invertebrates with respect to vertebrates. This review is an attempt to summarize the knowledge about MTs from aquatic animals, both vertebrates and invertebrates, to gain new insights into the structure-function relationship of this class of proteins. The large and increasing literature on MTs indicates that MTs from aquatic vertebrates are rather similar to mammalian counterparts, whereas a variety of structures have been described in invertebrates. Although the prototypical αβ-domain organization of vertebrate MTs has been observed in most invertebrate isoforms, some invertebrate MTs display alternative structures in which the canonical organization has been modified, such as the ββ-domain, the αββ-domain, and the multiple α-domain structures of oyster MTs, and the inverted βα-domain organization of sea urchin MTs. In this review we emphasize three major taxa of aquatic invertebrates, the molluscs, the crustaceans and the echinoderms, although some data have been reported for other invertebrates.

Alteration of metallothionein mRNA in bay scallop Argopecten irradians under cadmium exposure and bacteria challenge

Metallothionein (MT) is a superfamily of cysteine-rich proteins contributing to metal metabolism, detoxification of heavy metals, and immune response such as protecting against ionizing radiation and antioxidant defense. A metallothionein (designated AiMT2) gene was identified and cloned from bay scallop, Argopecten irradians. The full length cDNA of AiMT2 consisted of an open reading frame (ORF) of 333 bp encoding a protein of 110 amino acids, with nine characteristic Cys-X-Cys, five Cys-X-X-Cys, five Cys-X-X-X-Cys and two Cys-Cys motif arrangements and a conserved structural pattern Cys-x-Cys-x(3)-Cys-Tyr-x(3)-Cys-x-Cys-x(3)-Cys-x-Cys-Arg at the C-terminus. The cloned AiMT showed about 50% identity in the deduced amino acid sequence with previously published MT sequences of mussels and oysters. The conserved structural pattern and the close phylogenetic relationship of AiMT2 shared with MTs from other mollusc especially bivalves indicated that AiMT2 was a new member of molluscan MT family. The mRNA transcripts in hemolymph of AiMT2 under cadmium (Cd) exposure and bacteria challenge were examined by real-time RT-PCR. The mRNA expression of AiMT2 was up-regulated to 3.99-fold at 2 h after Listonella anguillarum challenge, and increased drastically to 66.12-fold and 126.96-fold at 16 and 32 h post-challenge respectively. Cadmium ion exposure could induce the expression of AiMT2, and the expression level increased 2.56-fold and 6.91-fold in hemolymph respectively after a 10-day exposure of 100 microg L(- 1) and 200 microg L(- 1) CdCl(2). The sensitivity of AiMT2 to bacteria challenge and cadmium stress indicated it was a new Cd-dependent MT in bay scallop and also regulated by an immune challenge. The changes in the expression of AiMT2 could be used as an indicator of exposure to metals in pollution monitoring programs and oxidative stress, and bay scallop as a potential sentinel organism for the cadmium contamination in aquatic environment.

Effects of cadmium on cellular protein and glutathione synthesis and expression of stress proteins in eastern oysters, Crassostrea virginica Gmelin

Cadmium (Cd) is an important toxicant in estuarine and coastal environments that can strongly affect energy balance of aquatic organisms by increasing the organism's basal energy demand and reducing its aerobic capacity. Mechanisms of cadmium-induced increase in basal metabolic costs are not well understood and may involve elevated detoxification costs due to the synthesis of cellular protective proteins and glutathione. We studied the short-term effects of cadmium exposure (4 h) on protein and glutathione (GSH) synthesis and expression of stress proteins (heat shock proteins HSP60, HSP70 and HSP90) and metallothioneins in isolated gill and hepatopancreas cells of the eastern oyster, Crassostrea virginica. Our study showed that exposure to cadmium resulted in a dose-dependent increase in the rate of protein synthesis in oyster cells, which reached 150% of the control at the highest tested Cd level (2000 micromol l(-1)). GSH synthesis was significantly inhibited by the highest Cd concentrations, especially in hepatopancreas, which resulted in a slight but significant decrease in the total GSH concentrations. Elevated protein synthesis was associated with the increased expression of metallothioneins and heat shock proteins. Interestingly, stress protein response differed considerably between gill and hepatopancreas cells. In hepatopancreas, expression of metallothionein mRNA (measured by real-time PCR) increased 2-8-fold in response to Cd exposure, whereas no significant increase in metallothionein expression was found in Cd-exposed gill cells. By contrast, HSP60 and HSP70 protein levels increased significantly in Cd-exposed gill cells (by 1.5-2-fold) but not in hepatopancreas. No change in HSP90 expression was detected in response to Cd exposure in oyster cells. These data indicate that metallothionein expression may provide sufficient protection against Cd-induced damage to intracellular proteins in hepatopancreas, alleviating the need for overexpression of molecular chaperones. By contrast, Cd detoxification mechanisms such as inducible metallothioneins and GSH appear to be insufficient to fully prevent protein damage in gill cells, thus necessitating induction of HSPs as a secondary line of cellular defense. Therefore, gills are likely to be among the most Cd-sensitive tissues in oysters, which may have important implications for impaired oxygen uptake contributing to energy misbalance and reduced aerobic scope in Cd-exposed oysters.

Identification and cloning of first cadmium metallothionein like gene from locally isolated ciliate, Paramecium sp

First cadmium metallothionein like gene PMCd1 of a ciliate, Paramecium sp., isolated from industrial wastewater has been cloned and sequenced. PMCd1 is an intronless gene, encoding 612 nucleotides, with TAA coding for glutamine. The coding region of PMCd1 comprises 203 amino acids, including 37 cysteine residues with a conserved structural pattern in the form of recurring structural motifs, arranged in 17 x-cys-x-y-cys-x, 1 x-cys-cys-x and x-cys-x contexts. Both, the deduced amino acids and nucleotide sequence differ, not only from other animal metallothioneins (MTs), but also from the previously characterized Tetrahymena Cu and Cd-MTs. The translated protein of PMCd1 contains conserved cysteine residues, peculiar characteristic of stress inducible metallothionein genes of ciliates and other groups of organisms.

Metal binding and antioxidant properties of chimeric tri- and tetra-domained metallothioneins

An unusual tri-domained (alpha-beta-beta) natural oyster metallothionein (MT) is known, and non-oxidative MT dimers occur in vivo in mollusk species and in mammals. To assess the respective role of the MT domains, two chimeric MTs were constructed: a tetra-domained oyster MT corresponding to the alpha-beta-alpha-beta structure, in order to mimic the natural non-oxidative dimeric form, and a tri-domained alpha-beta-alpha oyster MT. Metal binding and putative antioxidant properties of these two chimeric MTs were investigated using expression of the related genes in the bacteria Escherichia coli. In a wild-type strain these MTs could efficiently bind Cd. In a superoxide dismutase (sodA sodB) null mutant, the tri-domained MT was found to exacerbate Cd toxicity whereas the tetra-domained MT efficiently protected bacteria from Cd. The paradoxical toxicity displayed by the tri-domained MT upon Cd contamination was linked to the generation of superoxide radicals generated by a mechanism which most probably involves a copper-redox cycling reaction, since a Cd-contaminated sodA sodB strain expressing this MT produced 4 times more O2(-) than the control bacteria, and MT toxicity disappeared in the presence of bathocuproine disulfonic acid, a copper chelator. In contrast, the tetra-domained form did not. Interestingly, in bacteria producing superoxide dismutase but hypersensitive to oxidative stress due to either mutations in thioredoxin and glutathione reductase pathways (WM104 mutant) or to a lack of gamma-glutamylcysteine synthetase (gshA mutant), both chimeric MTs were protecting against Cd toxicity. However, an unexpected lack of antioxidant function was observed for both chimeric MTs, which were found to enhance the toxicity of hydrogen peroxide in WM104, or that of menadione in QC1726. Altogether, our results suggest that superoxide dismutase activity counteracts the potential prooxidative effect of the tri-domained MT mediated by Cu ions and that the tetra-domained form is a very efficient protector against metal toxicity in vivo.

Role of metallothioneins in superoxide radical generation during copper redox cycling: Defining the fundamental function of metallothioneins

In order to demonstrate the in vivo antioxidant properties of metallothioneins (MTs), the bacteria Escherichia coli was used as a cell reactor in which we compared the metal binding and antioxidative functions of MTs from different species, with different structures and polypeptide lengths. No protective effects of cytoplasmic MTs from cadmium (Cd) or zinc (Zn) contamination were observed in a wild-type E. coli strain, although these MTs can efficiently bind both Cd and Zn. To test their antioxidant properties, MTs were expressed within the cytoplasm of a sodA sodB deficient mutated strain (QC1726). However, a paradoxical MT toxicity was found when this strain was contaminated with Cd and Zn, suggesting that in a wild-type strain, superoxide dismutase counteracts MT toxicity. The most toxic MT was the one with the strongest Cd and Zn binding capacities. This toxic effect was linked to the generation of superoxide radicals, since a Cd-contaminated QC1726 strain expressing oyster MT isoforms produced 75-85% more O(2)*(-) than the control QC1726 strain. Conversely, under anaerobiosis or in the presence of a copper chelator, MTs protected QC1726 strain from Cd and Zn contamination. A model is proposed to explain the observed MT toxicity.

Identification, cloning and sequencing of a novel stress inducible metallothionein gene from locally isolated Tetrahymena tropicalis lahorensis

A novel cadmium inducible metallothionein (TMCd1) gene has been identified and sequenced from the locally isolated ciliate, Tetrahymena tropicalis lahorensis from industrial effluents. The TMCd1 gene encodes 471 nucleotides, with TGA as the stop codon and TAA coding for glutamine. This new gene is quite different from the previously reported MT genes in Tetrahymena pyriformis and Tetrahymena pigmentosa. However, it shows 78% homology with four different Cd-MT genes reported from Tetrahymena thermophila. A TATA box is located in the 5' flanking region at nucleotide 34-38 upstream region of ATG. The TMCd1 gene is intronless like many other genes isolated from Tetrahymena species. The amino acids sequence of TMCd1 has a special feature of three CCCX(6)CCX(6)CCCX(6)CC and two CCX(6)CXCX(2)CXCC intragenic tandem repeats with a conserved structural pattern of cysteine. The translated protein of TMCd1 contains 30.12% cysteine residues, which is a characteristic of a typical Tetrahymena Cd inducible MT genes. On the basis of 78% homology of nucleotide sequence of genomic DNA and its cDNA, TMCd1 has been considered as a new gene being reported from Tetrahymena tropicalis from this part of the world.

Recombinational micro-evolution of functionally different metallothionein promoter alleles from Orchesella cincta

Background

Metallothionein (mt) transcription is elevated in heavy metal tolerant field populations of Orchesella cincta (Collembola). This suggests that natural selection acts on transcriptional regulation of mt in springtails at sites where cadmium (Cd) levels in soil reach toxic values This study investigates the nature and the evolutionary origin of polymorphisms in the metallothionein promoter (pmt) and their functional significance for mt expression.

Results

We sequenced approximately 1600 bp upstream the mt coding region by genome walking. Nine pmt alleles were discovered in NW-European populations. They differ in the number of some indels, consensus transcription factor binding sites and core promoter elements. Extensive recombination events between some of the alleles can be inferred from the alignment. A deviation from neutral expectations was detected in a cadmium tolerant population, pointing towards balancing selection on some promoter stretches. Luciferase constructs were made from the most abundant alleles, and responses to Cd, paraquat (oxidative stress inducer) and moulting hormone were studied in cell lines. By using paraquat we were able to dissect the effect of oxidative stress from the Cd specific effect, and extensive differences in mt induction levels between these two stressors were observed.

Conclusion

The pmt alleles evolved by a number of recombination events, and exhibited differential inducibilities by Cd, paraquat and molting hormone. In a tolerant population from a metal contaminated site, promoter allele frequencies differed significantly from a reference site and nucleotide polymorphisms in some promoter stretches deviated from neutral expectations, revealing a signature of balancing selection. Our results suggest that the structural differences in the Orchesella cincta metallothionein promoter alleles contribute to the metallothionein -over-expresser phenotype in cadmium tolerant populations.

Tetrahymena metallothioneins fall into two discrete subfamilies

BACKGROUND

Metallothioneins are ubiquitous small, cysteine-rich, multifunctional proteins which can bind heavy metals.

METHODOLOGY/PRINCIPAL FINDINGS

We report the results of phylogenetic and gene expression analyses that include two new Tetrahymena thermophila metallothionein genes (MTT3 and MTT5). Sequence alignments of all known Tetrahymena metallothioneins have allowed us to rationalize the structure of these proteins. We now formally subdivide the known metallothioneins from the ciliate genus Tetrahymena into two well defined subfamilies, 7a and 7b, based on phylogenetic analysis, on the pattern of clustering of Cys residues, and on the pattern of inducibility by the heavy metals Cd and Cu. Sequence alignment also reveals a remarkably regular, conserved and hierarchical modular structure of all five subfamily 7a MTs, which include MTT3 and MTT5. The former has three modules, while the latter has only two. Induction levels of the three T. thermophila genes were determined using quantitative real time RT-PCR. Various stressors (including heavy metals) brought about dramatically different fold-inductions for each gene; MTT5 showed the highest fold-induction. Conserved DNA motifs with potential regulatory significance were identified, in an unbiased way, upstream of the start codons of subfamily 7a MTs. EST evidence for alternative splicing in the 3' UTR of the MTT5 mRNA with potential regulatory activity is reported.

CONCLUSION/SIGNIFICANCE

The small number and remarkably regular structure of Tetrahymena MTs, coupled with the experimental tractability of this model organism for studies of in vivo function, make it an attractive system for the experimental dissection of the roles, structure/function relationships, regulation of gene expression, and adaptive evolution of these proteins, as well as for the development of biotechnological applications for the environmental monitoring of toxic substances.

Challenging the model for induction of metallothionein gene expression

Metallothioneins (MTs) are low-molecular-weight, cysteine-rich metal-binding proteins found in a wide variety of organisms including bacteria, fungi and all eukaryotic plant and animal species. MTs bind essential and non-essential heavy metals. In mammalian cells MT genes are highly inducible by many heavy metals including Zn, Cd, Hg, and Cu. Aquatic systems are contaminated by different pollutants, including metals, as a result of man's activities. Bivalve molluscs are known to accumulate high concentrations of heavy metals in their tissue and are widely used as bioindicators for pollution in marine and freshwater environments, with MTs frequently used as a valuable marker of metal contamination. We here describe the MT isoform gene expression patterns of marine and freshwater molluscs and fish species after Cd or Zn contamination. Contamination was carried out at a river site polluted by a zinc ore extraction plant or in the laboratory at low, environmentally relevant metal concentrations. A comparison for each species based on the accumulated MT protein levels often shows discrepancies between gene expression and protein level. In addition, several differences observed in the pattern of MT gene expression between mollusc and mammalian species enable us to discuss and challenge a model for the induction of MT gene expression.

Regulation of metallothionein genes in the American oyster (Crassostrea virginica): Ontogeny and differential expression in response to different stressors

Metallothioneins (MTs) are typically low molecular weight (6-7 kDa), metal-binding proteins with characteristic repeating cysteine motifs (Cys-X-Cys or Cys-Xn-Cys) and a prolate ellipsoid shape containing single alpha- and beta-domains. While functionally diverse, they play important roles in metals homeostasis, detoxification and the stress response. The present study, combined with previous observations (e.g., Jenny et al., Eur. J. Biochem. 2005; 271:1702-1712) defines an unprecedented diversity of MT primary structure and domain organization in the American oyster, Crassostrea virginica. Two novel molluscan MT families are described. One of these (CvMT-III) is characterized by the presence of two beta-domains and the absence of alpha-domains. This family exhibits constitutive expression during larval development and is the dominant CvMT isoform expressed in larvae. CvMT-III displays low basal levels of expression in adult tissues and only moderate responsiveness to metal challenges in both larvae and adults. A second novel MT isoform (CvMT-IV) was isolated from hemocytes by subtractive hybridization techniques following a 4-hour immune challenge with heat-killed bacteria (Vibrio, Bacillus, Micrococcus spp. mixture). Based on conservation of the cysteine motifs, this isoform appears to be a sub-family related to the molluscan alphabeta-domain MTs. A series of amino acid substitutions has resulted in four additional cysteines which give rise to a Cys-Cys motif and three Cys-Cys-Cys motifs. Northern blot analyses demonstrate that CvMT-IV is down-regulated upon sterile wounding and immune challenge, displays moderate expression in larvae and adults and differential gene induction in response to metals exposure.

New resources for marine genomics: bacterial artificial chromosome libraries for the Eastern and Pacific oysters (Crassostrea virginica and C. gigas)

Large-insert genomic bacterial artificial chromosome (BAC) libraries of two culturally and economically important oyster species, Crassostrea virginica and C. gigas, have been developed as part of an international effort to develop tools and reagents that will advance our ability to conduct genetic and genomic research. A total of 73,728 C. gigas clones with an average insert size of 152 kb were picked and arrayed representing an 11.8-fold genome coverage. A total of 55,296 clones with an average insert size of 150 kb were picked and arrayed for C. virginica, also representing an 11.8-fold genome coverage. The C. gigas and C. virginica libraries were screened with probes derived from selected oyster genes using high-density BAC colony filter arrays. The probes identified 4 to 25 clones per gene for C. virginica and 5 to 50 clones per gene for C. gigas. We conducted a preliminary analysis of genetic polymorphism represented in the C. gigas library. The results suggest that the degree of divergence among similar sequences is highly variable and concentrated in intronic regions. Evidence supporting allelic polymorphism is reported for two genes and allelic and/or locus specific polymorphism for several others. Classical inheritance studies are needed to confirm the nature of these polymorphisms. The oyster BAC libraries are publicly available to the research community on a cost-recovery basis at (www.genome.clemson.edu).

Cloning and Characterization of a New Multi-Stress Inducible Metallothionein Gene in Tetrahymena pyriformis

A new multi-stress-inducible metallothionein (MT) gene isoform has been cloned and characterized from the ciliate Tetrahymena pyriformis. Both the 5'- and 3'-UT regions of the Tp-MT2 gene are very different from the previously reported Tp-MT1 isoform in this organism and from other described MT genes in Tetrahymena pigmentosa and Tetrahymena thermophila. The putative protein sequence of Tp-MT2 contains cysteine clusters with characteristics of the typical Tetrahymena Cd-inducible MT genes. However, the sequence has a special feature of four intragenic tandem repeats within its first half, with a conserved structural pattern x(5/8)CCCx(6)CCx(6)CxCxNCxCCK. To investigate the transcriptional activities of both Tp-MT2 and Tp-MT1 genes toward heavy metals (Cd, Hg, Cu, Zn) and H(2)O(2), the mRNA levels of these two isoforms were evaluated by means of real-time quantitative PCR. Results showed that Tp-MT2 had a higher basal expression level than Tp-MT1 and both genes were induced by Cd, Hg, Cu, and Zn ions after short exposure (1h), although to different extents. Cd was the most effective metal inducer of both two isoforms, but the relative expression level of Tp-MT2 was much lower than that of Tp-MT1. Different expression patterns were also shown between the two genes when treated with Cd over a period of 24h. We suggest that TpMT-1 plays the role of a multi-inducible stress gene, while TpMT-2 may have a more specific function in basal metal homeostasis although it may have undergone a functional differentiation process. The putative functional significance and evolutionary mode of the TpMT-2 isoform are discussed.

Response of the Pacific oyster Crassostrea gigas to hypoxia exposure under experimental conditions

The molecular response to hypoxia stress in aquatic invertebrates remains relatively unknown. In this study, we investigated the response of the Pacific oyster Crassostrea gigas to hypoxia under experimental conditions and focused on the analysis of the differential expression patterns of specific genes associated with hypoxia response. A suppression subtractive hybridization method was used to identify specific hypoxia up- and downregulated genes, in gills, mantle and digestive gland, after 7-10 days and 24 days of exposure. This method revealed 616 different sequences corresponding to 12 major physiological functions. The expression of eight potentially regulated genes was analysed by RT-PCR in different tissues at different sampling times over the time course of hypoxia. These genes are implicated in different physiological pathways such as respiration (carbonic anhydrase), carbohydrate metabolism (glycogen phosphorylase), lipid metabolism (delta-9 desaturase), oxidative metabolism and the immune system (glutathione peroxidase), protein regulation (BTF3, transcription factor), nucleic acid regulation (myc homologue), metal sequestration (putative metallothionein) and stress response (heat shock protein 70). Stress proteins (metallothioneins and heat shock proteins) were also quantified. This study contributes to the characterization of many potential genetic markers that could be used in future environmental monitoring, and could lead to explore new mechanisms of stress tolerance in marine mollusc species.