Metallothionein-like proteins turnover, Cd and Zn biokinetics in the dietary Cd-exposed scallop Chlamys nobilis

Molecular Identification, Characterization, and Expression Analysis of a Metallothionein Gene from Septifer virgatus

This study provides a preliminary characterization of a metallothionein (MT) gene in Septifer virgatus and highlights its potential use in biomonitoring. The full-length SvMT cDNA and the complete sequence of the SvMT gene were identified using reverse transcriptase PCR coupled with the rapid amplification of cDNA ends and the primer walking method. The SvMT cDNA encodes a protein of 72 amino acids having nine classical Cys-X-Cys motifs. Moreover, the deduced amino acids contained the conserved motif (Cys-x-Cys-x(3)-Cys-Thr-Gly-x(3)-Cys-x-Cys-x(3)-Cys-x-Cys-Lys) of MT family 2. Its molecular mass and isoelectric point were estimated to be 7.01 kDa and 7.00, respectively. BLAST-based searching indicated that SvMT shared 81.0% amino acid sequence identity with Mytilus edulis MT-20-II. The SvMT gene has three coding exons and two introns. After exposure to 1 mg/L cadmium chloride, the expression of SvMT increased 15-fold by 3 days (d), with a maximum expression of 27-fold by 5 d compared with the pre-exposure level. After exposure to 2 mg/L zinc chloride, the expression of SvMT increased 2.5-fold by 3 d and 4.7-fold by 5 d compared with the pre-exposure level. A significant increase in the expression level of SvMT mRNA was observed after the exposure of S. virgatus to the combination of 0.003 mg/L cadmium chloride and 0.2 mg/L zinc chloride compared with the pre-exposure level. Our work indicates that the SvMT gene is associated with stress responses and could be a potential biomarker for marine pollution.

Changes in metallothionein transcription levels in the mussel Mytilus galloprovincialis exposed to CdTe quantum dots

Quantum dots (QDs) are a class of engineered nanoparticles (ENPs) with several biomedical, industrial and commercial applications. However, their metabolism and detoxification process in aquatic invertebrates and environmental health hazards remain unclear. This study investigate the transcriptional changes of metallothioneins (MTs) isoforms (mt10IIIa and mt20IV) induced by CdTe QDs, in comparison with its dissolved counterpart, in the marine mussel Mytilus galloprovincialis. Mussels were exposed to CdTe QDs and to the same Cd concentration (10 µg Cd L^-1) of dissolved Cd for 14 days and mt transcription levels were measured by real time quantitative PCR (qPCR). Tissue specific mt transcription patterns were observed in mussels exposed to both Cd forms, wherein the gills were a more sensitive organ compared to the digestive gland. No significant changes were observed in mt10IIIa transcription levels in mussels exposed to both Cd forms. In contrast, transcription of mt20IV was tissue and exposure time dependent, with higher mt20IV mRNA levels in mussels exposed to QDs and dissolved Cd when compared to unexposed mussels. Multivariate analysis indicates particle-specific effects after 14 days of exposure and a dual role of MTs in the QD metabolism and in the protection against oxidative stress in mussels exposed to Cd-based ENPs.

Uniquely high turnover of nickel in contaminated oysters Crassostrea hongkongensis: Biokinetics and subcellular distribution

Despite the environmental concerns regarding nickel (Ni) especially in China, it has received little attention in aquatic animals due to its comparatively weak toxicity. In the present study, we explored the bioaccumulation, biokinetics, and subcellular distribution of Ni in an estuarine oyster Crassostrea hongkongensis. We demonstrated that Ni represented a new pattern of bioaccumulation in oysters characterized by rapid elimination and low dissolved uptake. The waterborne uptake rate constant and dietary assimilation efficiency were 0.036L/g/h and 28%, respectively, and dissolved uptake was the predominant exposure route. The efflux rate constant was positively related to tissue Ni concentration, with the highest efflux of 0.155d^-1. Such high elimination resulted in a high Ni turnover and steady-state condition reached rapidly, as shown with a 4-week waterborne exposure experiment at different Ni concentrations. Ni in oysters was mainly sequestered in metallothionein-like protein (MTLP), metal-rich granule, and cellular debris. MTLP was the most important binding fraction during accumulation and depuration, and played a dynamic role leading to rapid Ni elimination. Pre-exposure to Ni significantly reduced the dissolved uptake, probably accompanied by depressed filtration activity. Overall, the high turnover and regulation of Ni in oysters were achieved by enhanced efflux, suppressed uptake, and sequestration of most Ni into the detoxified pool.

Embryo-larvae and juvenile toxicity of Pb and Cd in Northern Chilean scallop Argopecten purpuratus

This study aimed to investigate the effects of Cd and Pb on earlier stage of the commercially important scallop (Argopecten purpuratus) in the contamination context of Northern Chile where this specie is farmed, through acute exposure bioassays in embryo-larvae measuring cumulative abnormality (EC₅₀) and juvenile cumulative mortality (LC₅₀) as endpoints, based on environmentally detected concentrations and available toxicological data from similar species. Embryo-larvae exposure indicates 48 h EC₅₀ of 1.55 mg/L Cd, and 0.044 mg/L Pb. On the contrary, 96 h LC₅₀ in juvenile scallops was 0.48 mg/L Cd and 1.47 mg/L Pb. Our results demonstrated differential toxicity between embryo and juvenile scallops that might relate to different primary defense mechanisms or effect in morphological development of individuals in each ontogenetic stage. Compared to similar bivalve metal toxicity tests, this study demonstrated that A. purpuratus embryos are more sensitive to Pb than most other bivalve species. Our results indicate that maximum permitted levels of Pb in marine waters and estuaries (according to Chilean regulation) could pose a risk for scallops' first stage of life (embryo-larvae) development, and needs to be reviewed. Furthermore, Chilean environmental regulations do not have quality standards for marine sediments (currently under discussion), where high levels of metals have been continuously reported.

Organ-specific accumulation, transportation, and elimination of methylmercury and inorganic mercury in a low Hg accumulating fish

Low mercury (Hg) concentrations down to several nanograms Hg per gram of wet tissue are documented in certain fish species such as herbivorous fish, and the underlying mechanisms remain speculative. In the present study, bioaccumulation and depuration patterns of inorganic Hg(II) and methylmercury (MeHg) in a herbivorous rabbitfish Siganus canaliculatus were investigated at organ and subcellular levels following waterborne or dietary exposures. The results showed that the efflux rate constants of Hg(II) and MeHg were 0.104 d(-1) and 0.024 d(-1) , respectively, and are probably the highest rate constants recorded in fish thus far. The dietary MeHg assimilation efficiency (68%) was much lower than those in other fish species (∼90%). The predominant distribution of MeHg in fish muscle was attributable to negligible elimination of MeHg from muscle (< 0) and efficient elimination of MeHg from gills (0.12 d(-1) ), liver (0.17 d(-1) ), and intestine (0.20 d(-1) ), as well as efficient transportation of MeHg from other organs into muscle. In contrast, Hg(II) was much more slowly distributed into muscle but was efficiently eliminated by the intestine (0.13 d(-1) ). Subcellular distribution indicated that some specific membrane proteins in muscle were the primary binding pools for MeHg, and both metallothionein-like proteins and Hg-rich granules were the important components in eliminating both MeHg and Hg(II). Overall, the present study's results suggest that the low tissue Hg concentration in the rabbitfish was partly explained by its unique biokinetics. Environ Toxicol Chem 2016;35:2074-2083. © 2016 SETAC.

Subcellular partitioning kinetics, metallothionein response and oxidative damage in the marine mussel Mytilus galloprovincialis exposed to cadmium-based quantum dots

The environmental health impact of metal-based nanomaterials is of emerging concern, but their metabolism and detoxification pathways in marine bioindicator species remain unclear. This study investigated the role of subcellular partitioning kinetics, metallothioneins (MTs) response and oxidative damage (lipid peroxidation - LPO) in the marine mussel Mytilus galloprovincialis exposed to CdTe quantum dots (QDs) in comparison with its dissolved counterpart. Mussels were exposed to QDs and dissolved Cd for 21 days at 10 μg Cd L(-1) followed by a 50 days depuration. Higher Cd concentrations were detected in fractions containing mitochondria, nucleus and lysosomes, suggesting potential subcellular targets of QDs toxicity in mussel tissues. Tissue specific metabolism patterns were observed in mussels exposed to both Cd forms. Although MT levels were directly associated with Cd in both forms, QDs subcellular partitioning is linked to biologically active metal (BAM), but no increase in LPO occurred, while in the case of dissolved Cd levels are in the biologically detoxified metal (BDM) form, indicating nano-specific effects. Mussel gills showed lower detoxification capability of QDs, while the digestive gland is the major tissue for storage and detoxification of both Cd forms. Both mussel tissues were unable to completely eliminate the Cd accumulated in the QDs form (estimated half-life time>50 days), highlighting the potential source of Cd and QDs toxicity for human and environmental health. Results indicate tissue specific metabolism patterns and nano-specific effects in marine mussel exposed to QDs.

Bioavailability of cadmium and biochemical responses on the freshwater bivalve Corbicula fluminea--the role of TiO₂ nanoparticles

The increasing and widespread applications of TiO2 engineered nanoparticles (nTiO2) led to the release of these materials into aquatic environments and consequently a change on the assessment of the environmental risk of trace metals. In this work, the role of two commercial nTiO2 with distinct crystalline phases and sizes (nTiO2-P25: 80% anatase+20% rutile, d=20nm; nTiO2-NA: 100% anatase, d=5 nm; 0.1 and 1.0 mg L(-1)) on Cd (112 μg L(-1)) speciation, biouptake and toxicity for the freshwater bivalve Corbicula fluminea was evaluated. The electroanalytical technique 'absence of gradients and Nernstian equilibrium stripping (AGNES)' was used to quantify the free Cd concentrations in the exposure medium in presence of both particles. Despite ca. 30-40% decrease of free Cd in the medium in presence of nTiO2, Cd uptake by C. fluminea was similar in the absence and presence of either of the particles. Superoxide dismutase and glutathione-S-transferase activities remained unchanged for Cd in absence and presence of nTiO2, whereas a significant increase of the catalase activity was obtained at the third day for Cd in presence of both nTiO2. Despite lipid peroxidation data shows that the presence of both nTiO2 seems to exert cells damage, a more quantitative description is not possible with the obtained data. The lack of clear-cut responses by the studied biomarkers, even when only in presence of Cd, do not allow insights into the effect of the presence of nTiO2 on the Cd toxicity to the bivalves. Notwithstanding, morphological changes in the digestive gland were clearly obtained in the presence of Cd, nTiO2 and Cd+nTiO2 indicating an inflammatory response.

The metallothionein gene from the white shrimp Litopenaeus vannamei: characterization and expression in response to hypoxia

Aquatic animals encounter variation in oxygen tension that leads to the accumulation of reactive oxygen species (ROS) that can harm the organisms. Under these circumstances some organisms have evolved to tolerate hypoxia. In mammals, metallothioneins (MTs) protect against hypoxia-generated ROS. Here we report the MT gene from the shrimp Litopenaeus vannamei (LvMT). LvMT is differentially expressed in hemocytes, intestine, gills, pleopods, heart, hepatopancreas and muscle, with the highest levels in hepatopancreas and heart. LvMT mRNA increases during hypoxia in hepatopancreas and gills after 3 h at 1.5 mg L(-1) dissolved oxygen (DO). This gene structure resembles the homologs from invertebrates and vertebrates possessing three exons, two introns and response elements for metal response transcription factor 1 (MTF-1), hypoxia-inducible factor 1 (HIF-1) and p53 in the promoter region. During hypoxia, HIF-1/MTF-1 might participate inducing MT to contribute towards the tolerance to ROS toxicity. MT importance in aquatic organisms may include also ROS-detoxifying processes.

Different responses of abalone Haliotis discus hannai to waterborne and dietary-borne copper and zincexposure

To investigate the potential influence of the contamination of copper (Cu) or zinc (Zn), the abalone Haliotis discus hannai was exposed to waterborne or macroalgae-borne Cu or Zn over a period of 8weeks. Both Cu and Zn were effectively accumulated by the abalones from water or macroalgae, but their concentration factors and trophic transfer factors were low due to the regulation of Cu and Zn accumulation. Following waterborne or dietary exposure, the abalones exhibited different accumulation patterns of Cu and Zn. The tissue Zn burden decreased quickly after the initial accumulation, and the incoming Zn was mainly deposited in the viscera. In contrast, the tissue Cu burden increased rapidly and gradually reached a steady state. The abalone muscle exhibited a comparable storage capacity of Cu as the viscera and the accumulated Cu in muscle was mainly derived from the dissolved phase instead of trophic transfer. The feeding and growth of the abalone were not influenced in all the exposure regimes. Moreover, the significant induction of metallothionein indicated that the bioaccumulated metals were actively detoxified. In the metal-exposed abalones, more Cu was distributed into the biologically detoxified fractions (metallothionein-like protein and/or metal-rich granule), whereas no significant subcellular redistribution of Zn was observed. Our study suggested that the abalone may have high endurance to the contamination of Cuor Zn.

Facilitated bioaccumulation of cadmium and copper in the oyster Crassostrea hongkongensis solely exposed to zinc

Exposure to one metal might have significant effects on the bioaccumulation of other metals. In the present study, we examined the possible effects of Zn exposure on the bioaccumulation of Cd and Cu in three populations of the oyster Crassostrea hongkongensis. We found that Zn exposure significantly enhanced the tissue concentrations of Cd and Cu in all populations, and the tissue concentrations of Cd and Cu were highly and positively related to the tissue Zn concentration. Furthermore, the enhanced bioaccumulation of Cd and Cu resulted mainly from their increasing accumulation and distribution in two subcellular fractions (i.e., metallothionein-like proteins and metal-rich granules). Tissue concentrations of Cd and Cu in the natural Zn-contaminated oysters also covaried with tissue Zn concentration, and prediction analyses revealed that Zn exposure was a significant contributor to tissue Cd and Cu concentrations. Therefore, we concluded that the increased Zn bioavailability in ambient waters not only increased the tissue Zn concentration but also enhanced the overall bioaccumulation of Cd and Cu. This study strongly demonstrates that contamination of metals in oysters may result from concurrent exposure to other metals. Thus, environmental managers should consider the possible exposure to other metals such as Zn in order to interpret/predict the tissue concentrations of toxic metals in oysters.

Metal bioavailability from different natural prey to a marine predator Nassarius siquijorensis

Gastropods are often the top predators in marine benthic environments, and trophic transfer is the predominant route by which metals are accumulated in these predators. In the present study, the potential influences of prey composition on the trophic transfer, accumulation, subcellular distribution and metallothionein induction of six metals (Ag, As, Cd, Cu, Pb and Zn) in a predator Nassarius siquijorensis were investigated. The snails were fed venerid clams Ruditapes philippinarum, mussels Perna viridis, oysters Crassostrea angulata or barnacles Fistulobalanus albicostatus, each differing greatly in their metal accumulation and handling patterns. N. siquijorensis showed prey-specific bioaccumulation and trophic transfer of the six metals. In general, the body burdens of metals in the viscera and muscles of N. siquijorensis increased with increasing exposure period and metal concentration in the four prey. The calculated trophic transfer factors (TTFs) of the metals in different prey varied and were the highest for clams and mussels prey, indicating that metal bioavailability from these prey was higher than that from barnacles and oysters. All the studied metals except Pb were enriched during transfer to the snails. The subcellular metal distribution in the viscera was affected by prey composition. Exposure to the four natural prey induced MTs, which may be used as a better biomarker for muscle than for viscera for metal stress. Our results imply that metals from different natural prey have different bioavailability and may help better understand the trophic transfer of metals in marine benthic food chain.

Proteome pattern in oysters as a diagnostic tool for metal pollution

The present study investigated whether proteome pattern of an oyster Crassostrea hongkongensis could be used as a diagnostic tool for contamination and toxicity of metals/metalloids in a real multiple metal-contaminated estuary. We collected oysters along a pollution gradient from highly contaminated to relatively clean sites. The oysters showed distinct contamination gradients of Cu, Zn and Cd. Proteomic analysis of the oyster gills as one of major metal targets identified a proteome pattern composed of 13 commonly altered proteins in the contaminated oysters. The discovered proteome pattern completely segregated the contaminated from the clean individuals, and the pattern achieved clear classification of the oysters with different contamination levels. Importantly, the integrated changes of gill proteome were linearly related to the integrated contamination of the metal mixtures present in oyster tissues. It is suggested that proteome pattern is a promising diagnostic tool for metal pollution assessment in environmental monitoring programs.

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.

Cadmium-induced changes in trace element bioaccumulation and proteomics perspective in four marine bivalves

Bivalves are employed widely as biomonitors of metal pollution and proteomics has increasingly been applied to solve ecotoxicological issues. This study aimed to investigate the effects of Cd exposure on the bioaccumulation of other trace elements and reveal the molecular mechanisms using proteomics technologies. The results showed that Cd exposure resulted in remarkable changes in body concentrations of Zn, Cu, Ag, Co, Ni, Pb, and Se in four marine bivalves (scallop Chlamys nobilis, clam Ruditapes philippinarum, mussel Perna viridis, and oyster Saccostrea cucullata). Generally, the bivalves exposed to higher Cd concentration accumulated higher concentrations of Zn, Cu, and Se, but a lower concentration of Co. The accumulation of Ag, Ni, and Pb was specific for different species. The data strongly suggest that the influences of one metal exposure on the bioaccumulation of other metals/metalloids need to be considered in interpreting body concentrations of the elements in the biomonitors. Cd exposure had little effect on bivalve proteomes, and the identified proteins were insufficient to explain the observed disruption of trace element metabolism. However, protein expression signatures composed of the altered proteins could distinguish the clams and the mussels with different body Cd levels. The strong up-regulation of galectin in Cd-exposed oysters indicated the protein as a novel biomarker in environmental monitoring.