Mechanisms of gastrointestinal copper absorption in the African walking catfish: copper dose-effects and a novel anion-dependent pathway in the intestine

Cu(II)-Dependent Spine Development Injury in Zebrafish (Danio rerio) with Organ Heterogeneous Cu Imbalance

Growing evidence suggests that the imbalance of Cu leads to multiorgan diseases or other adverse effects, but the underlying mechanisms remain largely unknown. Herein, we used zebrafish to uncover the mystery of organ heterogeneous responses to Cu stress and Cu(II)-dependent spine developmental injury in the early organogenesis stage. We first demonstrated that Cu(I) was distributed in the entire body, but high contents of Cu(II) were accumulated in the yolk sac and eye in normal zebrafish larvae. Cu exposure from birth to 144 hpf caused no obvious damage to Cu-metabolizing organs (liver and intestine), despite the elevated Cu(I) and Cu(II) levels. However, the spine was more sensitive to the Cu exposure. In the spine region, the Cu(I) level remained stable, whereas the level of Cu(II) significantly increased, which was highly associated with spine development injury. A significant negative correlation between Cu(II) and the spine-related parameters was identified. Moreover, cuproptosis caused spine development deformation during the early embryogenesis stage. Spine-related pathways such as somitegenesis significantly changed in the early embryogenesis period, and 5 spine-related pathways were significantly altered in the larval stage at 96 hpf. Our study suggested that Cu stress induced organ heterogeneous Cu imbalance and Cu(II)-dependent spine development injury in zebrafish.

Copper redox state in cells and aquatic organisms: Implication for toxicity

Copper (Cu) redox state has been an important issue in biology and toxicology research, but many research gaps remain to be explored due to the limitations in the detecting techniques. Herein, the regulation of Cu homeostasis, including absorption, translocation, utilization, storage, and elimination behavior is discussed. Cuproptosis, a newly identified type of cell death caused by excessive Cu accumulation, which results in the aggregation of DLAT protein or the loss of Fe-S cluster and finally proteotoxic stress, is reviewed. Several longstanding mysteries of diseases such as Wilson disease and toxic effects, may be attributed to cuproptosis. Furthermore, we review the advanced detection methods and application of Cu(I) and Cu(II), especially the in-situ imaging techniques such as XANES, and chemosensors. Most of the existing studies using these detection techniques focus on the bioaccumulation and toxicity of Cu(I) and Cu(II) in cells and aquatic organisms. Finally, it will be important to identify the roles of Cu(I) and Cu(II) in the growth, development, and diseases of organisms, as well as the relationship between bioaccumulation and toxicity of Cu(I) and Cu(II) in cellular and aquatic toxicology.

Interactive Effects of Copper-Silver Mixtures at the Intestinal Epithelium of Rainbow Trout: An In Vitro Approach

While metals are present in mixture in the environment, metal toxicity studies are usually conducted on an individual metal basis. There is a paucity of data in the existing literature regarding specific metal-metal interactions and their effect on metal toxicity and bioavailability. We studied interactions of a silver (Ag)-copper (Cu) mixture at the intestinal epithelium using an intestinal cell line derived from rainbow trout (Oncorhynchus mykiss), the RTgutGC. Exposures were conducted in media containing different chloride concentrations (low chloride, 1 mM; high chloride, 146 mM), thus resulting in different metal speciation. Cytotoxicity was evaluated based on two endpoints, cell metabolic activity and cell membrane integrity. The Ag-Cu mixture toxicity was assessed using two designs: independent action and concentration addition. Metal mixture bioavailability was studied by exposing cells to 500 nM of Ag or Cu as a single metal or a mixture (i.e., 500 nM of Cu plus 500 nM of Ag). We found an antagonistic effect in the low-chloride medium and an additive/synergistic effect in the high-chloride medium. We found that Cu dominates over Ag toxicity and bioavailability, indicating a competitive inhibition when both metals are present as free metal ions in the exposure media, which supports our hypothesis. Our study also suggests different mechanisms of uptake of free metal ions and metal complexes. The study adds valuable information to our understanding of the role of metal speciation on metal mixture toxicity and bioavailability. Environ Toxicol Chem 2024;43:105-114. © 2023 SETAC.

Role of the luminal composition on intestinal metal toxicity, bioavailability and bioreactivity: An in vitro approach based on the cell line RTgutGC

The bioavailability of metal complexes is poorly understood. To evaluate bioavailability and toxicity of neutral and charged complexes as well as free metal ions, Visual Minteq, a chemical equilibrium model, was used to design media containing different metal species. Two non-essential (silver and cadmium) and two essential (copper and zinc) metals were selected. The rainbow trout (Oncorhynchus mykiss) gut cell line (RTgutGC) was used to investigate bioavailability, bioreactivity and toxicity of the different metal species. Toxicity was measured using a multiple endpoint cytotoxicity assay, bioavailability by measuring intracellular metal concentration, and bioreactivity by quantification of mRNA level of the metal responsive genes, metallothionein (MT), glutathione reductase (GR) and zinc transporter 1 (ZnT1). Speciation calculations showed that silver and cadmium preferentially bind chloride, copper phosphate and bicarbonate, and zinc remained primarily as a free ion. Cysteine avidly complexed with all metals reducing toxicity, bioavailability and bioreactivity. Silver and copper toxicity was not affected by inorganic metal speciation, whereas cadmium and zinc toxicity was decreased by chloride complexation. Moreover, reduction of calcium concentration in the medium increased toxicity and bioavailability of cadmium and zinc. Bioavailability of silver and zinc was reduced by low chloride while cadmium bioavailability was increased by low chloride and in presence of bicarbonate. Copper bioavailability was not affected by the medium composition. Cadmium and silver were more bioreactive, independently from the medium composition, in comparison to copper and zinc (i.e., higher induction of MT and GR). Cadmium was the only metal able to induce MT in presence of cysteine. ZnT1 was induced by cadmium in low-chloride, by zinc in low-chloride low-calcium and by cadmium and copper in the bicarbonate media. Overall, this study demonstrates that metal complexation alone is not sufficient to explain metal toxicity, and that anion exchange mechanisms play a role in metal uptake and bioreactivity.

What happens to Hoplias malabaricus fed on live prey (Astyanax altiparanae) previously exposed to copper? A multiple biomarker approach

Copper waterborne toxicity is well understood in aquatic organisms. However, the dietary copper effects are much less known, especially in tropical fish. The toxicity of copper via the trophic route could be influenced by the composition of the food, and diets naturally impregnated with copper seem to have greater toxicity at lower concentrations than artificially impregnated ones. Thus, our objective was to investigate the effects of copper on juveniles of the Neotropical fish Hoplias malabaricus fed on live prey (Astyanax altiparanae) previously exposed to the metal (20 µg L ^- 1) for 96 h. The prey fish were given to H. malabaricus every 96 h, totaling 10 doses at the end of the experiment. Thus, after 40 days fish were killed and tissues were sampled. Blood showed to be the only tissue in which copper accumulated. Anemia was found and there was damage to the DNA of erythrocytes. Furthermore, ionic imbalances were observed in plasma. There was an increase in the concentration of Na⁺ and Cl^- and a decrease in Ca²⁺, which were associated with increased copper uptake in the gastrointestinal tract of fish fed on copper exposed prey. All the antioxidant enzymes evaluated in the gills showed decreased activity compared to the control group. Copper seems to have interfered in the energy metabolism of H. malabaricus, since a lower condition factor and feed conversion efficiency rate were observed in fish fed with copper diet. The present study confirms the trophic route as an important copper toxicity pathway for H. malabaricus and reinforces the idea that metal toxicity can be increased when it is naturally impregnated in the prey tissues, even if the prey has been exposed to the metal only for a short period of time.

Effect of chloride concentration on the cytotoxicity, bioavailability, and bioreactivity of copper and silver in the rainbow trout gut cell line, RTgutGC

Chloride (Cl^-) influences the bioavailability and toxicity of metals in fish, but the mechanisms by which it influences these processes is poorly understood. Here, we investigated the effect of chloride on the cytotoxicity, bioavailability (i.e., accumulation) and bioreactivity (i.e., induction of mRNA levels of metal responsive genes) of copper (Cu) and silver (Ag) in the rainbow trout gut cell line (RTgutGC). Cells were exposed to metals in media with varying Cl^- concentrations (0, 1, 5 and 146 mM). Metal speciation in exposure medium was analyzed using Visual MINTEQ software. Cytotoxicity of AgNO₃ and CuSO₄ was measured based on two endpoints: metabolic activity and membrane integrity. Cells were exposed to 500 nM of AgNO₃ and CuSO₄ for 24 h in respective media to determine metal bioavailability and bioreactivity. Ag speciation changes from free ionic (Ag⁺) to neutral (AgCl), to negatively charged chloride complexes (AgCl₂^-, AgCl₃^-) with increasing Cl^- concentration in exposure media whereas Cu speciation remains in two forms (Cu²⁺ and CuHPO₄) across all media. Chloride does not affect Ag bioavailability but decreases metal toxicity and bioreactivity. Cells exposed to Ag expressed significantly higher metallothionein mRNA levels in low Cl^- media (0, 1, and 5 mM) than in high Cl^- medium (146 mM). This suggests that chloride complexation reduces silver bioreactivity and toxicity. Conversely, Cu bioavailability and toxicity were higher in the high chloride medium (146 mM) than in the low Cl^- (0, 1, and 5 mM) media, supporting the hypothesis that Cu uptake may occur via a chloride dependent mechanism. CLINICAL TRIALS REGISTRATION: This study did not require clinical trial registration.

Functional significance and physiological regulation of essential trace metals in fish

Trace metals such as iron, copper, zinc and manganese play essential roles in various biological processes in fish, including development, energy metabolism and immune response. At embryonic stages, fish obtain essential metals primarily from the yolk, whereas in later life stages (i.e. juvenile and adult), the gastrointestine and the gill are the major sites for the acquisition of trace metals. On a molecular level, the absorption of metals is thought to occur at least in part via specific metal ion transporters, including the divalent metal transporter-1 (DMT1), copper transporter-1 (CTR1), and Zrt- and Irt-like proteins (ZIP). A variety of other proteins are also involved in maintaining cellular and systemic metal homeostasis. Interestingly, the expression and function of these metal transport- and metabolism-related proteins can be influenced by a range of trace metals and major ions. Increasing evidence also demonstrates an interplay between the gastrointestine and the gill for the regulation of trace metal absorption. Therefore, there is a complex network of regulatory and compensatory mechanisms involved in maintaining trace metal balance. Yet, an array of factors is known to influence metal metabolism in fish, such as hormonal status and environmental changes. In this Review, we summarize the physiological significance of iron, copper, zinc and manganese, and discuss the current state of knowledge on the mechanisms underlying transepithelial metal ion transport, metal-metal interactions, and cellular and systemic handling of these metals in fish. Finally, we identify knowledge gaps in the regulation of metal homeostasis and discuss potential future research directions.

Establishment of a striped catfish skin explant model for studying the skin response in Aeromonas hydrophila infections

Teleost fish skin serves as the first line of defense against pathogens. The interaction between pathogen and host skin determines the infection outcome. However, the mechanism(s) that modulate infection remain largely unknown. A proper tissue culture model that is easier to handle but can quantitatively and qualitatively monitor infection progress may shed some lights. Here, we use striped catfish (Pangasius hypophthalmus) to establish an ex vivo skin explant tissue culture model to explore host pathogen interactions. The skin explant model resembles in vivo skin in tissue morphology, integrity, and immune functionality. Inoculation of aquatic pathogen Aeromonas hydrophila in this model induces epidermal exfoliation along with epithelial cell dissociation and inflammation. We conclude that this ex vivo skin explant model could serve as a teleost skin infection model for monitoring pathogenesis under various infection conditions. The model can also potentially be translated into a platform to study prevention and treatment of aquatic infection on the skin in aquaculture applications.

Nutrition and Metabolism of Minerals in Fish

Simple Summary

Our aim is to introduce the mineral nutrition of fish and explain the complexity of determining requirements for these elements, which are absorbed and excreted by the fish into the surrounding water. To date, only the requirements for nine minerals have been investigated. The review is focused on the absorption and the dietary factors that reduce their absorption from feed ingredients of plant and animal origin. Some diseases, such as cataracts, anemia and bone deformity, have been linked to dietary deficiency of minerals.

Abstract

Aquatic animals have unique physiological mechanisms to absorb and retain minerals from their diets and water. Research and development in the area of mineral nutrition of farmed fish and crustaceans have been relatively slow and major gaps exist in the knowledge of trace element requirements, physiological functions and bioavailability from feed ingredients. Quantitative dietary requirements have been reported for three macroelements (calcium, phosphorus and magnesium) and six trace minerals (zinc, iron, copper, manganese, iodine and selenium) for selected fish species. Mineral deficiency signs in fish include reduced bone mineralization, anorexia, lens cataracts (zinc), skeletal deformities (phosphorus, magnesium, zinc), fin erosion (copper, zinc), nephrocalcinosis (magnesium deficiency, selenium toxicity), thyroid hyperplasia (iodine), muscular dystrophy (selenium) and hypochromic microcytic anemia (iron). An excessive intake of minerals from either diet or gill uptake causes toxicity and therefore a fine balance between mineral deficiency and toxicity is vital for aquatic organisms to maintain their homeostasis, either through increased absorption or excretion. Release of minerals from uneaten or undigested feed and from urinary excretion can cause eutrophication of natural waters, which requires additional consideration in feed formulation. The current knowledge in mineral nutrition of fish is briefly reviewed.

Copper chloride inhibits brush border membrane enzymes, alters antioxidant and metabolic status and damages DNA in rat intestine: a dose-dependent study

Copper (Cu) is an extensively used heavy metal and an indispensible micronutrient for living beings. However, Cu is also toxic and exerts multiple adverse health effects when humans are exposed to high levels of this metal. We have examined the effect of single acute oral dose of copper chloride (CuCl₂) on parameters of oxidative stress, cellular metabolism, membrane and DNA damage in rat intestine. Adult male Wistar rats were divided into four groups and separately administered a single oral dose of 5, 15, 30 and 40 mg CuCl₂/kg body weight. Rats not administered CuCl₂ served as the control. Oral administration of CuCl₂ led to significant alterations in the activities of metabolic and membrane-bound enzymes; brush border enzymes were inhibited by 45-75% relative to the control set. Inhibition of antioxidant enzymes diminished the metal-reducing and free radical quenching ability of the cells. Oxidative damage caused cellular oxidation of thiols, proteins and lipids. Diphenylamine and comet assays showed that CuCl₂ treatment enhanced DNA damage while DNA-protein crosslinking was also increased in the intestinal cells. Examination of stained sections showed that CuCl₂ treatment led to marked histological changes in the intestine. All the changes seen were in a CuCl₂ dose-dependent manner with more prominent alterations at higher doses of CuCl₂. These results clearly show that oral administration of CuCl₂ results in oxidative damage to the intestine which can impair its digestive and absorptive functions.

Dietary uptake and effects of copper in Sticklebacks at environmentally relevant exposures utilizing stable isotope-labeled 65CuCl2 and 65CuO NPs

Copper oxide nanoparticles (CuO NPs) accumulating in sediment can be taken up by invertebrates that serve as prey for fish. Thus, it is likely that the latter are exposed to CuO NPs via the gut. However, to this day it is unknown if CuO NPs can be taken up via the gastrointestinal tract and if and in which tissues/organs they accumulate. To address this knowledge gap, we synthesized CuO NPs enriched in the stable isotope ⁶⁵Cu and incorporated them at low concentration (5 μg ⁶⁵Cu g^-1 ww food) into a practical diet prepared from worm homogenate, which was then fed to Three-spined Stickleback (Gasterosteus aculeatus) for 16 days. For comparison, fish were exposed to a diet spiked with a ⁶⁵CuCl₂ solution. Background Cu and newly taken up ⁶⁵Cu in fish tissues/organs including gill, stomach, intestine, liver, spleen, gonad and carcass and feces were quantified by ICP-MS. In addition, expression levels of genes encoding for proteins related to Cu uptake, detoxification and toxicity (ctr-1, gcl, gr, gpx, sod-1, cat, mta and zo-1) were measured in selected tissues using RT-qPCR. The obtained results showed that feces of fish fed ⁶⁵CuO NP-spiked diet contained important amounts of ⁶⁵Cu. Furthermore, there was no significant accumulation of ⁶⁵Cu in any of the analyzed internal organs, though ⁶⁵Cu levels were slightly elevated in liver. No significant modulation in gene expression was measured in fish exposed to ⁶⁵CuO NP-spiked diet, except for metallothionein, which was significantly upregulated in intestinal tissue compared to control fish. Altogether, our results suggests that dietary absorption efficiency of CuO NPs, their uptake across the gastrointestinal barrier into the organism, and effects on Cu-related genes is limited at low, environmentally relevant exposure doses (0.2 μg ⁶⁵Cu ^-1 fish ww day^-1).

Dietary bioaccumulation potential of silver nanomaterials compared to silver nitrate in wistar rats using an ex vivo gut sac technique

Chronic dietary bioaccumulation tests with rodents are required for new substances, including engineered nanomaterials (ENMs), in order to provide information on the potential hazards to human health. However, screening tools are needed to manage the diversity of ENMs and alternative methods are desirable with respect to animal welfare. Here, an ex vivo gut sac method was used to estimate the dietary bioaccumulation potential of silver nanomaterials. The entire gastrointestinal tract (except the caecum) was removed and filled with a gut saline containing 1 mg L^-1 of Ag as either AgNO₃, silver nanoparticles (Ag NPs) or silver sulphide nanoparticles (Ag₂S NPs), and compared to controls with no added Ag. The gut sacs were incubated for 4 h, rinsed to remove excess media, and the total Ag determined in the mucosa and muscularis. There was no detected Ag in the control treatments. Within the Ag treatments, 1.4-22% of the exposure dose was associated with the tissues and serosal saline. Within the mucosa of the AgNO₃ treatment, the highest Ag concentration was associated with the intestinal regions (3639-7087 ng g^-1) compared to the stomach (639 ± 128 ng g^-1). This pattern was also observed in the Ag NP and Ag₂S NP treatments, but there was no significant differences between any Ag treatments for the mucosa. However, differences between treatments were observed in the muscularis concentration. For example, both the Ag NP (907 ± 284 ng g ^-1) and Ag₂S NP (1482 ± 668 ng g^-1) treatments were significantly lower compared to the AgNO₃ treatment (2514 ± 267 ng g^-1). The duodenum demonstrated serosal accumulation in both the AgNO₃ (~10 ng mL^-1) and Ag NP (~3 ng mL^-1) treatments. The duodenum showed some of the highest Ag accumulation with 41, 61 and 57% of the total Ag in the mucosa compared to the muscularis for the AgNO₃, Ag NP and Ag₂S NP treatments, respectively. In conclusion, the ex vivo gut sac method demonstrates the uptake of Ag in all Ag treatments, with the duodenum the site of highest accumulation. Based on the serosal saline accumulation, the ranked order of accumulation is AgNO₃ > Ag NPs > Ag₂S NPs.

Quantification of Mg2+, Ca2+ and H+ transport by the gastrointestinal tract of the goldfish, Carassius auratus, using the Scanning Ion-selective Electrode Technique (SIET)

An in vitro gut-sac technique and the scanning ion-selective electrode technique (SIET) were used to characterize Mg²⁺, Ca²⁺, and H⁺ transport at both the mucosal and serosal surfaces of non-everted and everted gastrointestinal tissues obtained from Carassius auratus. As part of the study, two magnesium ionophores were compared (II vs. VI). Unfed animals displayed uniform transport of all ions along the intestine. Feeding resulted in elevated Mg²⁺ and Ca²⁺ transport when the gut lumen contained chyme however, under symmetrical conditions this increased transport rate was absent. Furthermore, zonation of divalent cation transport was present for both Ca²⁺ and Mg²⁺ under non-symmetrical conditions while the zonation remained for Ca²⁺ alone under symmetrical conditions. High dietary Mg²⁺ decreased absorption and induced secretion of Mg²⁺ in the posterior intestine. Uptake kinetics in the esophagus suggest large diffusive and/or convective components based on a linear relationship between Mg²⁺ transport and concentration and lack of inhibition by ouabain, an inhibitor of Na⁺-K⁺-ATPase. In contrast, kinetics in the rectum were suggestive of a low affinity, saturable carrier-mediated pathway. A decrease in Mg²⁺ and Ca²⁺ transport was observed in the posterior intestine (both at the mucosal and serosal surfaces) in response to ouabain. This impact was greatest for Ca²⁺ transport and when applied to the mucosal fluid and measured in everted preparations. In contrast a putative Mg²⁺ transport inhibitor, cobalt(III)hexamine-chloride, did not affect Mg²⁺ transport. This is the first study to use SIET approaches to study ion transport in the gut of teleost fish. This is also the first study to provide characterization of Mg²⁺ transport in the gut of C. auratus. Due to the limited selectivity of Magnesium ionophore II, subsequent studies of tissues bathed in physiological saline should be made using Magnesium Ionophore VI.

Identification of eight copper (Cu) uptake related genes from yellow catfish Pelteobagrus fulvidraco, and their tissue expression and transcriptional responses to dietborne Cu exposure

The present working hypothesis is that absorption of dietary Cu is related to mRNA expressions of genes involved in Cu uptake and transport of the intestine in fish. To this end, the full-length cDNA sequences of eight Cu uptake related genes, including two isoforms of copper transporter genes (ctr1 and ctr2), three copper chaperone genes (atox1, ccs and cox17), two Cu-ATPase genes (atp7a and atp7b) and divalent metal ion transporter 1 (dmt1), were cloned and characterized in yellow catfish P. fulvidraco, respectively. Their mRNA tissue expression and transcriptional responses to dietborne Cu exposure were investigated. Compared to the corresponding members of mammals, all of these members in P. fulvidraco shared the similar conserved domain structures. Their mRNAs were expressed in a wide range of tissues (including liver, muscle, spleen, brain, gill, intestine, heart and kidney), but at variable levels. In anterior intestine, mRNA levels of ctr1, cox17, dmt1 and atp7a declined with increasing dietary Cu levels. The mRNA levels of ctr2 and mt were the highest for excess dietary Cu group and showed no significant differences between other two treatments. Atox1 mRNA levels were the highest for Cu-deficient group and showed no significant differences between other two treatments. The mRNA levels of ccs were the highest for Cu-deficient group, followed by Cu-excess group and the lowest for adequate-Cu group. In contrast, atp7b mRNA levels were the highest for Cu-excess group and the lowest for adequate Cu group. In the mid-intestine, mRNA levels of ctr1, ctr2, atox1, ccs, cox17, dmt1 and atp7a declined with increasing dietary Cu levels. Atp7b mRNA levels were the lowest for adequate Cu group and showed no significant differences between other two treatments. Mt mRNA levels were the lowest for adequate Cu group and highest for Cu-excess group. For the first time, our study cloned and characterized ctr1, ctr2, atox1, ccs, cox17, atp7a, atp7b and dmt1 genes in P. fulvidraco and determined their tissue-specific expression, and transcriptional responses in the anterior and mid-intestine of yellow catfish under dietborne Cu exposure, which shed new light on the Cu uptake system and help to understand the molecular mechanisms of Cu homeostasis in fish.

Application of the rainbow trout derived intestinal cell line (RTgutGC) for ecotoxicological studies: molecular and cellular responses following exposure to copper

There is an acknowledged need for in vitro fish intestinal model to help understand dietary exposure to chemicals in the aquatic environment. The presence and use of such models is however largely restrictive due to technical difficulties in the culturing of enterocytes in general and the availability of appropriate established cell lines in particular. In this study, the rainbow trout (Oncorhynchus mykiss) intestinal derived cell line (RTgutGC) was used as a surrogate for the "gut sac" method. To facilitate comparison, RTgutGC cells were grown as monolayers (double-seeded) on permeable Transwell supports leading to a two-compartment intestinal model consisting of polarised epithelium. This two-compartment model divides the system into an upper apical (lumen) and a lower basolateral (portal blood) compartment. In our studies, these cells stained weakly for mucosubstances, expressed the tight junction protein ZO-1 in addition to E-cadherin and revealed the presence of polarised epithelium in addition to microvilli protrusions. The cells also revealed a comparable transepithelial electrical resistance (TEER) to the in vivo situation. Importantly, the cell line tolerated apical saline (1:1 ratio) thus mimicking the intact organ to allow assessment of uptake of compounds across the intestine. Following an exposure over 72 h, our study demonstrated that the RTgutGC cell line under sub-lethal concentrations of copper sulphate (Cu) and modified saline solutions demonstrated uptake of the metal with saturation levels comparable to short term ex situ gut sac preparations. Gene expression analysis revealed no significant influence of pH or time on mRNA expression levels of key stress related genes (i.e. CYP3A, GST, mtA, Pgp and SOD) in the Transwell model. However, significant positive correlations were found between all genes investigated suggesting a co-operative relationship amongst the genes studied. When the outlined characteristics of the cell line are combined with the division of compartments, the RTgutGC double seeded model represents a potential animal replacement model for ecotoxicological studies. Overall, this model could be used to study the effects and predict aquatic gastrointestinal permeability of metals and other environmentally relevant contaminants in a cost effective and high throughput manner.

A fish intestinal epithelial barrier model established from the rainbow trout (Oncorhynchus mykiss) cell line, RTgutGC

The intestine of fish is a multifunctional organ: lined by only a single layer of specialized epithelial cells, it has various physiological roles including nutrient absorption and ion regulation. It moreover comprises an important barrier for environmental toxicants, including metals. Thus far, knowledge of the fish intestine is limited largely to in vivo or ex vivo investigations. Recently, however, the first fish intestinal cell line, RTgutGC, was established, originating from a rainbow trout (Oncorhynchus mykiss). In order to exploit the opportunities arising from RTgutGC cells for exploring fish intestinal physiology and toxicology, we present here the establishment of cells on commercially available permeable membrane supports and evaluate its suitability as a model of polarized intestinal epithelia. Within 3 weeks of culture, RTgutGC cells show epithelial features by forming tight junctions and desmosomes between adjacent cells. Cells develop a transepithelial electrical resistance comparable to in vivo measured values, reflecting the leaky nature of the fish intestine. Immunocytochemistry reveals evidence of polarization, such as basolateral localization of Na⁺/K⁺-ATPase (NKA) and apical localization of the tight junction protein ZO-1. NKA mRNA abundance was induced as physiological response toward a saltwater buffer, mimicking the migration of rainbow trout from fresh to seawater. Permeation of fluorescent molecules proved the barrier function of the cells, with permeation coefficients being comparable to those reported in fish. Finally, we demonstrate that cells on permeable supports are more resistant to the toxicity elicited by silver ions than cells grown the conventional way, likely due to improved cellular silver excretion.

Homeostatic regulation of copper in a marine fish simulated by a physiologically based pharmacokinetic model

Copper (Cu) is an essential yet potentially toxic metal, thus delicate homeostatic controls are developed in the fish. In this study, a physiologically based pharmacokinetic (PBPK) model was developed to simulate the homeostatic regulation of Cu in a marine fish (Terapon jarbua) under dietary and waterborne exposures. In this model, fish were schematized as a six-compartment model, with the intestine being divided into two sub-compartments (chyme and gut wall). The blood was assumed to be the "carrier" distributing Cu into different compartments. The transfer rates between different compartments were determined in fish during Cu exposure (20 d) and depuration (20 d). The differences in Cu transfer from chyme to gut wall between dietary and waterborne treatments suggested that the intestine regulated the dietary uptake and re-absorption of Cu from the chyme. The extremely low uptake rate constant (0.0013 d^-1) for gills under waterborne exposure indicated that gills strongly restricted Cu uptake from the ambient water. For both treatments, the liver had considerable input rate through the enterohepatic circulation and comparably high exchange rate with the blood, suggesting that the liver can efficiently accumulate newly absorbed Cu. The differences in Cu output from the liver between dietary and waterborne treatments suggested that it can effectively regulate the redistribution of Cu. All of these observations demonstrated that the liver played the central role in Cu homeostasis by serving as the main depository and distributing center. Modeling results also indicated that renal and branchial excretion was of minor importance, whereas biliary excretion combined with defecation played the most important role in whole-body Cu elimination in marine fish. The effective regulation by the "Blood-Liver-Intestine" cycle could be the main reason for the relatively low levels of Cu in fish.

Effects of waterborne Cu exposure on intestinal copper transport and lipid metabolism of Synechogobius hasta

The present study was conducted to explore the effects of waterborne Cu exposure on intestinal Cu transport and lipid metabolism of Synechogobius hasta. S. hasta were exposed to 0, 0.4721 and 0.9442μM Cu, respectively. Sampling occurred on days 0, 21 and 42, respectively. Growth performance, intestinal lipid deposition, Cu content, and activities and mRNA expression of enzymes and genes involved in Cu transport and lipid metabolism were analyzed. Cu exposure decreased WG and SGR on days 21 and 42. Cu exposure increased intestinal Cu and lipid contents. Increased Cu accumulation was attributable to increased enzymatic activities (Cu-ATPase and Cu, Zn-SOD) and genes' (CTR1, CTR2, DMT1, ATP7a, ATP7b, MT1 and MT2) expression involved in Cu transport. Waterborne Cu exposure also increased activities of lipogenic enzymes (6PGD and ICDH on both days 21 and 42, ME on day 42), up-regulated mRNA levels of lipogenic genes (G6PD, 6PGD, ME, ICDH, FAS and ACCa), lipolytic genes (ACCb, CPT I and HSLa) and genes involved in intestinal fatty acid uptake (IFABP and FATP4) on both days 21 and 42. The up-regulation of lipolysis may result from the increased metabolic expenditure for detoxification and maintenance of the normal body functions in a response to Cu exposure. Meantime, Cu exposure increased lipogenesis and fatty acid uptake, leading to net lipid accumulation in the intestine despite increased lipolysis. To our knowledge, this is the first report involved in intestinal lipid metabolism in combination with intestinal Cu absorption following waterborne Cu exposure, which provides new insights and evidence into Cu toxicity in fish.

Sublethal effects of copper sulphate compared to copper nanoparticles in rainbow trout (Oncorhynchus mykiss) at low pH: physiology and metal accumulation

A few studies have investigated the interaction between copper toxicity and water pH in fishes, but little is known about the effects of acidic pH on the toxicity of copper nanoparticles (Cu-NPs). This study aimed to describe the sub-lethal toxic effects of Cu-NPs compared to CuSO4 at neutral and acidic water pH values in juvenile rainbow trout. Fish were exposed in triplicate (3 tanks/treatment) to control (no added Cu), or 20μgl(-1) of either Cu as CuSO4 or Cu-NPs, at pH 7 and 5 in a semi-static aqueous exposure regime for up to 7 days. Acidification of the water altered the mean primary particle size (at pH 7, 60±2nm and pH 5, 55±1nm) and dialysis experiments to measure dissolution showed an increased release of dissolved Cu from Cu-NPs at pH 5 compared to pH 7. Copper accumulation was observed in the gills of trout exposed to CuSO4 and Cu-NPs at pH 7 and 5, with a greater accumulation from the CuSO4 treatment than Cu-NPs at each pH. The liver also showed Cu accumulation with both Cu treatments at pH 7 only, whereas, the spleen and kidney did not show measurable accumulation of Cu at any of the water pH values. Exposure to acid water caused changes in the ionoregulatory physiology of control fish and also altered the observed effects of Cu exposure; at pH 5, branchial Na(+)/K(+)-ATPase activity was greater than at pH 7 and the inhibition of Na(+)/K(+)-ATPase activity caused by exposure to CuSO4 at pH 7 was also not observed. There were some changes in haematology and depletion of plasma Na(+) at pH 7 and 5 due to Cu exposure, but there were few material-type or pH effects. Overall, the data show that the accumulation of Cu is greater from CuSO4 than Cu-NPs; however, understanding of the effects of low pH on bioavailability of CuSO4 may not be directly transferred to Cu-NPs without further consideration of the physico-chemical behaviour of Cu-NPs in acid water.

Feeding reduces waterborne Cu bioaccumulation in a marine rabbitfish Siganus oramin

Waterborne metal uptake has been extensively studied and dietary metal assimilation is increasingly recognized in fish, whilst the interaction between the two uptake routes is largely overlooked. This study compared the waterborne Cu bioaccumulation ((65)Cu as tracer) in a juvenile rabbitfish at different feeding regimes (starvation (SG), feeding normal diet (NDG) or diet supplemented with extra Cu (DCG)) to test the hypothesis that feeding can influence waterborne metal uptake in marine fish. NDG and DCG diet was fed as a single meal and then all fish were exposed to waterborne (65)Cu for 48 h, during which the time course sampling was conducted to determine (65)Cu bioaccumulation, chyme flow and dietary Cu assimilation. The results revealed that SG fish accumulated the highest (65)Cu, followed by NDG (61% of SG), whilst DCG fish accumulated the lowest (65)Cu (34% of SG). These results suggested a protective effect of feeding against waterborne Cu bioaccumulation. This effect was most notable between 10 min and 16 h when there was chyme in gastrointestinal tract (GT). Dietary Cu assimilation mainly occurred before 16 h after feeding. Waterborne (65)Cu influx rate in the GT was positively correlated with (65)Cu contents of chyme in NDG, whereas it was largely negatively correlated with (65)Cu contents of chyme in DCG. The waterborne Cu uptake in the GT was mainly influenced by the chyme flow and dietary Cu assimilation. Overall, our findings suggested that feeding has an important effect on waterborne metal uptake and that both the feeding status of the fish and the relative metal exposure through water and food should be considered in prediction of the metal bioaccumulation and biomonitoring programs.

Trophic transfer of (110m)Ag in the turbot Scophthalmus maximus through natural prey and compounded feed

Industrial incidents can result in radionuclide release in the environment, among which (110m)Ag. Indeed, under particular circumstances, non-negligible amounts of (110m)Ag have been measured in the marine environment (as observed in Fukushima Dai-ichi incident). This element can therefore be accumulated by aquatic organisms through different pathways including the trophic transfer. The present study aimed at examining the variation of (110m)Ag assimilation efficiency (AE) by turbots, Scophthalmus maximus, when exposed through different feeds. Pulse-chase feeding experiments were carried out in mesocosms, using radiolabelled feeds (natural prey and commercial pellets). Depuration kinetics of (110m)Ag over 21 days were generally fitted by a two-component exponential model; the ingested radioelement was poorly assimilated by turbots regardless of the food item that was used (AE always <3%). Concentration and subcellular distribution of (110m)Ag in prey did not seem to influence its assimilation by turbot. These results suggest that physiological mechanisms could occur in fish that would prevent the transfer of (110m)Ag from gut lumen to internal organs (e.g. (110m)Ag neutralization in the lumen of the stomach, detoxification mechanisms occurring in the gut).

Uptake epithelia behave in a cell-centric and not systems homeostatic manner in response to zinc depletion and supplementation

Global transcriptomic analysis, non-invasive real-time flux, nutritional profiling and metallomics reveal cell-centric response to zinc supplementation/depletion in zebrafish uptake epithelia.

Much remains to be understood about systemic regulation of zinc uptake in vertebrates, and adequate zinc status is far from always achieved in animals or human. In addition to absorbing zinc from the diet, fish are able to take up zinc directly from the water with the gills. This provides an elegant system to study zinc uptake, how it relates to zinc status, and the expression of genes for proteins involved in zinc acquisition. A 21-day experiment was conducted in which zebrafish were acclimated to deficient, control or excess zinc concentrations in the water and feed. Deficient provision of zinc reduced whole body zinc, potassium, sodium and calcium levels whilst zinc concentrations in the uptake epithelia (gills and gut) remained unchanged. Excess levels of zinc caused accumulation of zinc in the gills, intestine and carcass, but impaired whole body iron, sodium and calcium concentrations. Fish subjected to zinc deficiency had, surprisingly, a reduced zinc influx across the gill epithelium, even when tested at a high concentration of zinc in the water. Zinc influx in the excess group was indistinct from the control. Expression of genes for metallothionein-2 (Mt2) and zinc transporters-1, -2, and -8 (Znt1, Znt2, Znt8) in uptake epithelia showed in general a direct relationship with zinc supply, while mRNA for Zip4 was inversely related to zinc supply. Transcripts for the epithelial calcium channel (Ecac/Trpv6) showed time-dependent increased expression in the gills of the deficiency group, and a transient decrease of expression during zinc excess. Transcriptome profiling by microarrays showed that in both gills and intestine, the most markedly affected biological functions were those related to cell growth, proliferation and cancer, closely followed by processes of gene transcription and protein synthesis in general. Whilst changes in zinc supply had profound effects in the intestine on genes associated with uptake and metabolism of macronutrients, many of the unique categories of genes preferentially regulated in the gill could be mapped onto signalling pathways. This included pathways for PPAR/RXR, LXR/RXR, ATM, chemokine, and BMP signalling. Overall, the responses of epithelial tissue to zinc deficiency and excess are best explained by local epithelial homeostasis with no evidence of systemic control.

Divalent metal transporter 1 (Dmt1) mediates copper transport in the duodenum of iron-deficient rats and when overexpressed in iron-deprived HEK-293 cells

Intracellular copper-binding proteins (metallothionein I/II) and a copper exporter (Menkes copper-transporting ATPase) are upregulated in duodenal enterocytes from iron-deficient rats, consistent with copper accumulation in the intestinal mucosa. How copper enters enterocytes during iron deficiency is, however, not clear. Divalent metal transporter 1 (Dmt1), the predominant iron importer in the mammalian duodenum, also transports other metal ions, possibly including copper. Given this possibility and that Dmt1 expression is upregulated by iron deprivation, we sought to test the hypothesis that Dmt1 transports copper during iron deficiency. Two model systems were utilized: the Belgrade (b) rat, expressing mutant Dmt1, and an inducible Dmt1-overexpression cell culture system. Mutant rats (b/b) were fed a semipurified, AIN93G-based control diet and phenotypically normal littermates (+/b) were fed control or iron-deficient diets for ~14 wk. An everted gut sleeve technique and a colorimetric copper quantification assay were utilized to assess duodenal copper transport. The control diet-fed +/b rats had normal hematological parameters, whereas iron-deprived +/b and b/b rats were iron deficient and Dmt1 mRNA and protein levels increased. Importantly, duodenal copper transport was similar in the control +/b and b/b rats; however, it significantly increased (~4-fold) in the iron-deprived +/b rats. Additional experiments in Dmt1 overexpressing HEK-293 cells showed that copper ((64)Cu) uptake was stimulated (∼3-fold) in the presence of an iron chelator. Dmt1 transcript stabilization due to a 3' iron-responsive element was also documented, likely contributing to increased transport activity. In summary, these studies suggest that Dmt1 enhances copper uptake into duodenal enterocytes during iron deprivation.

A simplified method for determining titanium from TiO2 nanoparticles in fish tissue with a concomitant multi-element analysis

The reliable detection of nanoparticles (NPs) in fish tissue is required to support ecotoxicological research and food safety investigations. Therefore the current work aimed to develop a simple method to determine Ti from TiO2 NPs in fish tissue whilst simultaneously measuring other elements in the sample. Spike recovery tests showed no differences when digestion was conducted in glass or plastic vials, there was stirring or sonication of the samples, or when sodium dodecyl sulfate was added. However, the addition of 2% Triton X-100 and sonicating and then vortexing of samples immediately prior to analysis did improve recovery (approximately 20% to >90% in trout gill and muscle samples). Method precision and accuracy were good with coefficients of variation <7%. Copper spike recovery results showed that the method is also suitable for multi-element analysis in the same samples. This improved method is simple with high throughput and represents a marked improvement for routine determination Ti from TiO2 NPs in fish tissues.

Bioavailability of silver nanoparticles and ions: from a chemical and biochemical perspective

Owing to their antimicrobial properties, silver nanoparticles (NPs) are the most commonly used engineered nanomaterial for use in a wide array of consumer and medical applications. Many discussions are currently ongoing as to whether or not exposure of silver NPs to the ecosystem (i.e. plants and animals) may be conceived as harmful or not. Metallic silver, if released into the environment, can undergo chemical and biochemical conversion which strongly influence its availability towards any biological system. During this process, in the presence of moisture, silver can be oxidized resulting in the release of silver ions. To date, it is still debatable as to whether any biological impact of nanosized silver is relative to either its size, or to its ionic constitution. The aim of this review therefore is to provide a comprehensive, interdisciplinary overview--for biologists, chemists, toxicologists as well as physicists--regarding the production of silver NPs, its (as well as in their ionic form) chemical and biochemical behaviours towards/within a multitude of relative and realistic biological environments and also how such interactions may be correlated across a plethora of different biological organisms.

Practical considerations for conducting ecotoxicity test methods with manufactured nanomaterials: what have we learnt so far?

This review paper reports the consensus of a technical workshop hosted by the European network, NanoImpactNet (NIN). The workshop aimed to review the collective experience of working at the bench with manufactured nanomaterials (MNMs), and to recommend modifications to existing experimental methods and OECD protocols. Current procedures for cleaning glassware are appropriate for most MNMs, although interference with electrodes may occur. Maintaining exposure is more difficult with MNMs compared to conventional chemicals. A metal salt control is recommended for experiments with metallic MNMs that may release free metal ions. Dispersing agents should be avoided, but if they must be used, then natural or synthetic dispersing agents are possible, and dispersion controls essential. Time constraints and technology gaps indicate that full characterisation of test media during ecotoxicity tests is currently not practical. Details of electron microscopy, dark-field microscopy, a range of spectroscopic methods (EDX, XRD, XANES, EXAFS), light scattering techniques (DLS, SLS) and chromatography are discussed. The development of user-friendly software to predict particle behaviour in test media according to DLVO theory is in progress, and simple optical methods are available to estimate the settling behaviour of suspensions during experiments. However, for soil matrices such simple approaches may not be applicable. Alternatively, a Critical Body Residue approach may be taken in which body concentrations in organisms are related to effects, and toxicity thresholds derived. For microbial assays, the cell wall is a formidable barrier to MNMs and end points that rely on the test substance penetrating the cell may be insensitive. Instead assays based on the cell envelope should be developed for MNMs. In algal growth tests, the abiotic factors that promote particle aggregation in the media (e.g. ionic strength) are also important in providing nutrients, and manipulation of the media to control the dispersion may also inhibit growth. Controls to quantify shading effects, and precise details of lighting regimes, shaking or mixing should be reported in algal tests. Photosynthesis may be more sensitive than traditional growth end points for algae and plants. Tests with invertebrates should consider non-chemical toxicity from particle adherence to the organisms. The use of semi-static exposure methods with fish can reduce the logistical issues of waste water disposal and facilitate aspects of animal husbandry relevant to MMNs. There are concerns that the existing bioaccumulation tests are conceptually flawed for MNMs and that new test(s) are required. In vitro testing strategies, as exemplified by genotoxicity assays, can be modified for MNMs, but the risk of false negatives in some assays is highlighted. In conclusion, most protocols will require some modifications and recommendations are made to aid the researcher at the bench.

Unifying prolonged copper exposure, accumulation, and toxicity from food and water in a marine fish

The link between metal exposure and toxicity is complicated by numerous factors such as exposure route. Here, we exposed a marine fish (juvenile blackhead seabream Acanthopagrus schlegelii schlegelii) to copper either in a commercial fish diet or in seawater. Copper concentrations in intestine/liver were correlated linearly with influx rate, but appeared to be less influenced by uptake pathway (waterborne or dietary exposure). Influx rate best predicted Cu accumulation in the intestine and liver. However, despite being a good predictor of mortality within each pathway, influx rate was not a good predictor of mortality across both exposure pathways, as waterborne Cu caused considerably higher mortality than dietary Cu at a given influx rate. We show that the use of gill Cu accumulation irrespective of the exposure route as a model for observed fish mortality provided a clear relationship between accumulation and toxicity. Investigation of gill Cu accumulation may shed light on the different accumulation strategies from the two exposure pathways. This correlation offers potential for the use of branchial Cu concentration as an indicator of long-term Cu toxicity, allowing for differences in the relative importance of the uptake pathways in different field situations.

Physiological effects of nanoparticles on fish: a comparison of nanometals versus metal ions

The use of nanoscale materials is growing exponentially, but there are also concerns about the environmental hazard to aquatic biota. Metal-containing engineered nanoparticles (NPs) are an important group of these new materials, and are often made of one metal (e.g., Cu-NPs and Ag-NPs), metal oxides (e.g., ZnO and TiO(2) NPs), or composite of several metals. The physiological effects and toxicity of trace metals in the traditional dissolved form are relatively well known and the overall aim of this review was to use our existing conceptual framework of metal toxicity in fish to compare and contrast the effects of nanometals. Conceptually, there are some fundamental differences that relate to bioavailability and uptake. The chemistry and behaviour of nanometals involves dynamic aspects of aggregation theory, rather than the equilibrium models traditionally used for free metal ions. Some NPs, such as Cu-NPs, may also release free metal ions from the surface of the particle. Biological uptake of NPs is not likely via ion transporters, but endocytosis is a possible uptake mechanism. The body distribution, metabolism, and excretion of nanometals is poorly understood and hampered by a lack of methods for measuring NPs in tissues. Although data sets are still limited, emerging studies on the acute toxicity of nanometals have so far shown that these materials can be lethal to fish in the mg-μgl(-1) range, depending on the type of material. Evidence suggests that some nanometals can be more acutely toxic to some fish than dissolved forms. For example, juvenile zebrafish have a 48-h LC(50) of about 0.71 and 1.78mgl(-1) for nano- and dissolved forms of Cu respectively. The acute toxicity of metal NPs is not always explained, or only partly explained, by the presence of free metal ions; suggesting that other novel mechanisms may be involved in bioavailability. Evidence suggests that nanometals can cause a range of sublethal effects in fish including respiratory toxicity, disturbances to trace elements in tissues, inhibition of Na(+)K(+)-ATPase, and oxidative stress. Organ pathologies from nanometals can be found in a range of organs including the gill, liver, intestine, and brain. These sublethal effects suggest some common features in the sublethal responses to nanometals compared to metal salts. Effects on early life stages of fish are also emerging, with reports of nanometals crossing the chorion (e.g., Ag-NPs), and suggestions that the nano-forms of some metals (Cu-NPs and ZnO NPs) may be more toxic to embryos or juveniles, than the equivalent metal salt. It remains possible that nanometals could interfere with, and/or stimulate stress responses in fish; but data has yet to be collected on this aspect. We conclude that nanometals do have adverse physiological effects on fish, and the hazard for some metal NPs will be different to the traditional dissolved forms of metals.

Dynamic transcriptomic profiles of zebrafish gills in response to zinc supplementation

Background

Dietary zinc supplementation may help to promote growth, boost the immune system, protect against diabetes, and aid recovery from diarrhoea. We exploited the zebrafish (Danio rerio) gill as a unique vertebrate ion transporting epithelium model to study the time-dependent regulatory networks of gene-expression leading to homeostatic control during zinc supplementation. This organ forms a conduit for zinc uptake whilst exhibiting conservation of zinc trafficking components.

Results

Fish were maintained with either zinc supplemented water (4.0 μM) and diet (2023 mg zinc kg^-1) or water and diet containing Zn²⁺ at 0.25 μM and 233 mg zinc kg^-1, respectively. Gill tissues were harvested at five time points (8 hours to 14 days) and transcriptome changes analysed in quintuplicate using a 16 K microarray with results anchored to gill Zn²⁺ influx and whole body nutrient composition (protein, carbohydrate, lipid, elements). The number of regulated genes increased up to day 7 but declined as the fish acclimated. In total 525 genes were regulated (having a fold-change more than 1.8 fold change and an adjusted P-value less than 0.1 which is controlling a 10% False discovery rate, FDR) by zinc supplementation, but little overlap was observed between genes regulated at successive time-points. Many genes displayed cyclic expression, typical for homeostatic control mechanisms. Annotation enrichment analysis revealed strong overrepresentation of "transcription factors", with specific association evident with "steroid hormone receptors". A suite of genes linked to "development" were also statistically overrepresented. More specifically, early regulation of genes was linked to a few key transcription factors (e.g. Mtf1, Jun, Stat1, Ppara, Gata3) and was followed by hedgehog and bone morphogenic protein signalling.

Conclusions

The results suggest that zinc supplementation reactivated developmental pathways in the gill and stimulated stem cell differentiation, a response likely reflecting gill remodelling in response to its altered environment. This provides insight to the role of zinc during cell differentiation and illustrates the critical nature of maintaining zinc status. The study also highlights the importance of temporal transcriptomics analysis in order resolve the discrete elements of biological processes, such as zinc acclimation.

Post-prandial metabolic alkalosis in the seawater-acclimated trout: the alkaline tide comes in

The consequences of feeding and digestion on acid-base balance and regulation in a marine teleost (seawater-acclimated steelhead trout; Oncorhynchus mykiss) were investigated by tracking changes in blood pH and [HCO3-], as well as alterations in net acid or base excretion to the water following feeding. Additionally the role of the intestine in the regulation of acid-base balance during feeding was investigated with an in vitro gut sac technique. Feeding did not affect plasma glucose or urea concentrations, however, total plasma ammonia rose during feeding, peaking between 3 and 24 h following the ingestion of a meal, three-fold above resting control values (approximately 300 micromol ml(-1)). This increase in plasma ammonia was accompanied by an increase in net ammonia flux to the water (approximately twofold higher in fed fish versus unfed fish). The arterial blood also became alkaline with increases in pH and plasma [HCO3-] between 3 and 12 h following feeding, representing the first measurement of an alkaline tide in a marine teleost. There was no evidence of respiratory compensation for the measured metabolic alkalosis, as Pa CO2 remained unchanged throughout the post-feeding period. However, in contrast to an earlier study on freshwater-acclimated trout, fed fish did not exhibit a compensating increase in net base excretion, but rather took in additional base from the external seawater, amounting to approximately 8490 micromol kg(-1) over 48 h. In vitro experiments suggest that at least a portion of the alkaline tide was eliminated through increased HCO3- secretion coupled to Cl- absorption in the intestinal tract. This did not occur in the intestine of freshwater-acclimated trout. The marked effects of the external salinity (seawater versus freshwater) on different post-feeding patterns of acid-base balance are discussed.

In vitro examination of interactions between copper and zinc uptake via the gastrointestinal tract of the rainbow trout (Oncorhynchus mykiss)

An in vitro gut sac technique was used to investigate whether reciprocal inhibitory effects occurred between Cu and Zn uptake in the gastrointestinal tract of the rainbow trout and, if so, whether there was regional variation among the stomach, anterior intestine, mid intestine, and posterior intestine in the phenomena. Metal accumulation in surface mucus and in the mucosal epithelium and transport into the blood space were assayed using radiolabeled Cu or Zn at environmentally realistic concentrations of 50 micromol L(-1) in the luminal saline, with 10-fold higher levels of the other metal (nonradioactive) as a potential inhibitor. Zn transport rates were generally higher than Cu transport rates in all compartments except the stomach, where they were lower. High [Zn] reduced Cu transport into the blood space in the mid and posterior intestines by 67% and 33%, respectively, whereas high [Cu] reciprocally reduced Zn transport into the blood space in these same sections by 54% and 78%. No inhibitions occurred in either the anterior intestine or the stomach. In these segments, elevated concentrations of the other metal stimulated Cu and Zn transport into the blood space and/or the mucosal epithelium by 50-100%, possibly by displacement from intracellular binding sites, thereby raising local concentrations at other transport sites. None of the treatments affected metal accumulation in surface mucus. The results indicate that one or more shared high-affinity pathways (possibly DMT1) occur in the mid and posterior intestine, which transport both Cu and Zn. These pathways appear to be absent from the stomach and anterior intestine, where other transport mechanisms may occur.

Manufactured nanoparticles: their uptake and effects on fish--a mechanistic analysis

There is an emerging literature reporting toxic effects of manufactured nanomaterials (NMs) and nanoparticles (NPs) in fish, but the mechanistic basis of both exposure and effect are poorly understood. This paper critically evaluates some of the founding assumptions in fish toxicology, and likely mechanisms of absorption, distribution, metabolism and excretion (ADME) of NPs in fish compared to other chemicals. Then, using a case study approach, the paper compares these assumptions for two different NPs; TiO2 and C60 fullerenes. Adsorption of NPs onto the gill surface will involve similar processes in the gill microenvironment and mucus layer to other substances, but the uptake mechanisms for NPs by epithelial cells are more likely to occur via vesicular processes (e.g., endocytosis) than uptake on membrane transporters or by diffusion through the cell membranes. Target organs may include the gills, gut, liver and sometimes the brain. Information on metabolism and excretion of NPs in fish is limited; but hepatic excretion into the bile seems a more likely mechanism, rather than mainly by renal or branchial excretion. TiO2 and C60 share some common chemical properties that appear to be associated with some similar toxic effects, but there are also differences, that highlight the notion that chemical reactivity can inform toxic effect of NPs in a fundamentally similar way to other chemicals. In this paper we identify many knowledge gaps including the lack of field observations on fish and other wildlife species for exposure and effects of manufactured NMs. Systematic studies of the abiotic factors that influence bioavailability, and investigation of the cell biology that informs on the mechanisms of metabolism and excretion of NMs, will greatly advance our understanding of the potential for adverse effects. There are also opportunities to apply existing tools and techniques to fundamental studies of fish toxicology with NPs, such as perfused organs and fish cell culture systems.

Copper transporter 1, metallothionein and glutathione reductase genes are differentially expressed in tissues of sea bream (Sparus aurata) after exposure to dietary or waterborne copper

The high affinity copper transporter 1 (Ctr1), metallothionein (MT) and glutathione reductase (GR) are essential for copper uptake, sequestration and defense respectively. Following rearing on a normal commercial diet (12.6+/-0.2 mg kg(-1) Cu), sea bream were fed an experimental control diet lacking mineral mix (7.7+/-0.3 mg kg(-1) Cu), an experimental diet enhanced with Cu (135+/-4 mg kg(-1) Cu) or an experimental diet (7.7+/-0.3 mg kg(-1) Cu) whilst exposed to Cu in water (0.294+/-0.013 mg L(-1)). Fish were sampled at 0, 15 and 30 days after exposures. Fish fed the Cu-enhanced experimental diet showed lower levels of expression of Ctr1 in the intestine and liver compared to fish fed control experimental diets, whilst Ctr1 expression in the gill and kidney was unaffected by excess dietary Cu exposure. Waterborne-Cu exposure increased Ctr1 mRNA levels in the intestine and the kidney compared to experimental controls. Excess dietary Cu exposure had no effect on levels of metallothionein (MT) mRNA, and the only effect of dietary excess Cu on glutathione reductase (GR) mRNA was a decrease in the intestine. Both MT mRNA and GR were increased in the liver and gill after waterborne-Cu exposure, compared to levels in fish fed experimental control low Cu diets. Thus, Ctr1, MT and GR mRNA expression in response to excess Cu is dependent on the route of exposure. Furthermore, the tissue expression profile of sea bream Ctr1 is consistent with the known physiology of copper exposure in fish and indicates a role both in essential copper uptake and in avoidance of excess dietary and waterborne copper influx.

Absorption of copper and copper–histidine complexes across the apical surface of freshwater rainbow trout intestine

Bioavailability is integral in mediating the delicate balance between nutritive and potentially toxic levels of copper in fish diets. Brush-border membrane vesicles isolated from freshwater rainbow trout intestine were used to characterise apical copper absorption, and to examine the influence of the amino acid histidine on this process. In the absence of histidine, a low affinity, high capacity copper uptake mechanism was described. However, when expressed as a function of ionic copper (Cu(2+)), absorption was linear, rather than saturable, suggesting that the saturable curve was an artifact of copper speciation. Conversely, in the presence of L: -histidine (780 microM) saturable uptake was characterised. The uptake capacity discerned (J (max) of 354 +/- 81 nmol mg protein(-1) min(-1)) in the presence of histidine indicated a significantly reduced capacity for copper transport than that in the absence of histidine. To determine if copper uptake was achievable through putative histidine uptake pathways, copper and histidine were incubated in the presence of tenfold greater concentrations of amino acids proposed to block histidine transporters. Accounting for changes in copper speciation, significant inhibition of uptake by glycine and lysine were noted at copper levels of 699 and 1,028 microM. These results suggest that copper-histidine complexes may be transportable via specific amino acid-transporters in the brush-border membrane.

Dietary copper exposure in the African walking catfish, Clarias gariepinus: transient osmoregulatory disturbances and oxidative stress

There are few dietary Copper (Cu) toxicity studies on warm water fish. We used the African walking catfish (Clarias gariepinus) to perform the first in vivo dietary Cu toxicity study on this species. We measured end points normally associated with metal toxicity (nutritional performance, haematology, histology, tissue Cu, Na(+), and K(+), Na(+)K(+)-ATPase activity) and add to the limited data on oxidative stress during dietary Cu exposure (thiobarbituric acid reactive substances or TBARS, and total glutathione). Clarias gariepinus were fed to satiation on a Cu-loaded diet (1500 mg Cu kg(-1)dw feed), or a control diet (15 mg Cu kg(-1) dw feed), for 30 days. Dietary copper exposure caused elevated Cu concentrations in the intestine (20 fold), liver (5 fold) and gills (4 fold) of Cu-exposed fish compared to controls after 30 days (ANOVA, P<0.05). Copper-exposed fish showed a reduction in food intake and specific growth rate (SGR), but only very modest reductions in mean body mass at the end of the experiment (the latter not statistically significant). There were no treatment-dependent effects on food conversion ratio or proximate composition, and only transient disturbances to tissue electrolytes and Na(+)K(+)-ATPase activity. Haematology was normal throughout the experiment. Cu-exposed fish showed an increase in TBARS in the gill (1.5 fold) and intestine (2 fold increase) compared to the controls (ANOVA or Kruskal-Wallis, P<0.05). Total glutathione content in the intestine of Cu-exposed fish doubled by the end of the experiment compared to controls, reaching 12.7+/-2.85 micromol g(-1) wet weight (mean+/-S.E.M., n=6, Student's t-test, P<0.05). The liver showed some glycogen depletion consistent with reduced food intake, but no overt pathologies in the gills, liver or intestine were observed.

In vitro analysis of the bioavailability of six metals via the gastro-intestinal tract of the rainbow trout (Oncorhynchus mykiss)

An in vitro gut sac technique was used to compare the uptake rates of essential (copper, zinc and nickel) and non-essential metals (silver, cadmium and lead) at 50 micromol L(-1) each (a typical nutritive level in solution in chyme) in the luminal saline in four sections of the gastro-intestinal tract (stomach, anterior, mid and posterior intestines) of the freshwater rainbow trout. Cu, Zn, Cd and Ag exhibited similar regional patterns: on an area-specific basis, uptake rates for these metals were highest in the anterior intestine, lowest in the stomach, and approximately equal in the mid and posterior intestinal segments. When these rates were converted to a whole animal basis, the predominance of the anterior intestine increased because of its greater area, while the contribution of the stomach rose slightly to approach those of the mid and posterior intestines. However, for Pb and Ni, area-specific and whole organism transport rates were greatest in the mid (Pb) and posterior (Ni) intestines. Surprisingly, total transport rates did not differ appreciably among the essential and non-essential metals, varying only from 0.025 (Ag) to 0.050 nmol g(-1)h(-1) (Ni), suggesting that a single rate constant can be applied for risk assessment purposes. These rates were generally comparable to previously reported uptake rates from waterborne exposures conducted at concentrations 1-4 orders of magnitude lower, indicating that both routes are likely important, and that gut transporters operate with much lower affinity than gill transporters. Except for Ni, more metal was bound to mucus and/or trapped in the mucosal epithelium than was transported into the blood space in every compartment except the anterior intestine, where net transport predominated. Overall, mucus binding was a significant predictor of net transport rate for every metal except Cd, and the strongest relationship was seen for Pb.

Mechanisms of dietary Cu uptake in freshwater rainbow trout: evidence for Na-assisted Cu transport and a specific metal carrier in the intestine

Copper (Cu) is both a vital nutrient and a potent toxicant. The objective of this study was to analyze the mechanistic nature of intestinal Cu transport in rainbow trout using radiolabeled Cu (64Cu) and an in vitro gut sac technique. Reduction of mucosal NaCl levels inhibited Cu transport while increase caused stimulation; Na(2)SO(4) had an identical effect, implicating Na(+) rather than the anion. These responses were unrelated to solvent drag, osmotic pressure or changes in transepithelial potential. The presence of elevated luminal Ag stimulated Cu and Na(+) uptake. Phenamil caused a partial inhibition of both Cu and Na(+) uptake while hypercapnia stimulated Na(+) and Cu transport. Cu uptake was sensitive to luminal pH and inhibited by a tenfold excess of Fe and Zn. These factors had no effect on Na(+ )uptake. On the basis of these results we propose a novel Na(+)-assisted mechanism of Cu uptake wherein the Na(+) gradient stimulates an increase in the H(+) concentration of the brushborder creating a suitable microenvironment for the effective transport of Cu via either DMT1 or Ctr1.

Gastrointestinal assimilation of Cu during digestion of a single meal in the freshwater rainbow trout (Oncorhynchus mykiss)

Gastrointestinal processing and assimilation of Cu in vivo was investigated by sequential chyme analysis over a 72 h period following ingestion of a single satiation meal (3% body weight) of commercial trout food (Cu content=0.42 micromol g(-1)) by adult rainbow trout. Leaded glass ballotini beads incorporated into the food and detected by X-ray radiography were employed as an inert marker in order to quantify net Cu absorption or secretion in various parts of the tract. Cu concentrations in the supernatant (fluid phase) fell from about 0.06 micromol mL(-1) (63 microM) in the stomach at 2 h to about 0.003 micromol mL(-1) (3 microM) in the posterior intestine at 72 h. Cu concentrations in the solid phase were 10 to 30-fold higher than in the fluid phase, and increased about 4-fold from the stomach at 2 h to the posterior intestine at 72 h. By reference to the inert marker, overall net Cu absorption from the ingested food by 72 h was about 50%. The mid-intestine, and posterior intestine emerged as important sites of net Cu and water absorption and a potential role for the stomach in this process was also indicated. The anterior intestine was a site of large net Cu addition to the chyme, probably due to large net additions of Cu-containing fluids in the form of bile and other secretions in this segment. The results provide valuable information about sites of Cu absorption and realistic concentrations of Cu in chyme fluid for future in vitro mechanistic studies on Cu transport in the trout gastrointestinal tract.

Physical characterization of high-affinity gastrointestinal Cu transport in vitro in freshwater rainbow trout Oncorhynchus mykiss

This study investigated the transport of copper (Cu) in the gut of trout. Examination of the spatial distribution of Cu along the digestive tract and a physical characterization of the uptake process was carried out using an in vitro gut sac technique and (64)Cu as a tracer. Unidirectional Cu uptake was highest in the anterior intestine followed in decreasing order by the posterior intestine, mid intestine and the stomach. Cu uptake was resistant to hypoxia and appeared to be fueled equally well by Cu(II) or Cu (I) at Cu concentrations typically found in the fluid phase of the chyme in vivo in the trout intestine. Transport demonstrated saturation kinetics (e.g. K (m) = 31.6 microM, J (max) = 17 pmol cm(-2) h(-1), in mid intestine) at low Cu levels representative of those measured in the chyme in vivo, with a diffusive component at higher Cu concentrations. Q (10) analysis indicated Cu uptake is via diffusion across the apical membrane and biologically mediated across the basolateral membranes of enterocytes. The presence of L-histidine but not D-histidine stimulated both Cu and Na uptake suggesting a common pathway for the transport of Cu/Na with L-histidine.

The influence of dietary Na on Cu accumulation in juvenile rainbow trout exposed to combined dietary and waterborne Cu in soft water

Fish inhabiting metal-contaminated environments can take up metals such as Cu via the gills as well as via the gut. Previous research on rainbow trout (Oncorhynchus mykiss) has indicated that dietary Na can reduce the accumulation of waterborne Cu; however, in hard water, dietary Na does not reduce the accumulation of dietary Cu. In this study, we exposed juvenile rainbow trout in soft water with slightly elevated [Cu] for 28 days to control or high levels of dietary Cu (6 and 580 microg Cu/g food, respectively) at low (1.5%), intermediate (3%), or high (4.5%) levels of dietary Na, for a total of six experimental groups. A separate gastrointestinal sampling experiment demonstrated that these levels resulted in moderately elevated Na concentrations in the gastrointestinal fluid, which declined between 6 h and 12 h post-feeding. Growth and condition indices were not affected by the dietary Cu and Na exposure. Among the control dietary Cu groups, those that received the highest amount of dietary Na had significantly higher whole-body [Cu] on days 18 and 28. In contrast, among the high-Cu groups, fish that were fed the highest amount of Na tended to have significantly lower whole-body [Cu] on days 9 and 18. Tissue Na concentrations did not differ among any of the groups, and unidirectional Na flux measurements demonstrated that Na homeostasis was not impaired by dietary or waterborne Cu. Our results suggest that elevated dietary Na stimulates Cu uptake via the gut under low-Cu conditions, thereby increasing whole-body [Cu], whereas under Cu-loaded conditions, downregulation of Cu uptake at the gills, and/or competitive inhibition of gut Cu uptake as a result of increased dietary Na, leads to decreased whole-body [Cu].

Dose-dependent inorganic mercury absorption by isolated perfused intestine of rainbow trout, Oncorhynchus mykiss, involves both amiloride-sensitive and energy-dependent pathways

The trophic transfer and nutritional toxicity of mercury (Hg) in aquatic food chains is well known, but there is limited information on the mechanism of mercury uptake across the gut. In this study, isolated whole gut sacs from rainbow trout were used to identify the regions of the gut involved in Hg absorption, and then perfused intestines were used to investigate Hg uptake. Exposure of whole gut sacs to 100 micromol l(-1) Hg as HgCl2 in the luminal solution caused Hg accumulation primarily in the mucosa (78% or more), with the intact mid and hind gut supporting 59% of the accumulated Hg. Luminal exposure to [Hg] between 0 and 100 micromol l(-1) for 4 h in perfused trout intestines showed a non-linear dose-dependent accumulation with a maximum Hg uptake rate of about 103 nmol g(-1) h(-1), and suggests carrier mediated transport into the gut cells and the blood. Additions of 2 mmol l(-1) amiloride depressed Hg accumulation by the mid and hind gut by 40-50%, whilst additions of the Ca chelator 1 mmol l(-1) EGTA increased Hg levels in the tissue. Symmetrical additions of 10 mmol l(-1) cyanide did not prevent tissue accumulation of Hg, but caused a 3.4-fold decline in net Hg flux to the serosal compartment. We conclude that Hg absorption across the gut is partly carrier mediated and involves both amiloride sensitive, and energy-dependent pathways.

Sodium-sensitive and -insensitive copper accumulation by isolated intestinal cells of rainbow troutOncorhynchus mykiss

The pathway for copper (Cu) uptake across the mucosal membrane into intestinal cells has not been elucidated in fish. Copper accumulation in freshly isolated intestinal cells from rainbow trout Oncorhynchus mykiss was measured after exposure to 0–800 μmol l–1 CuSO4 for 15 min. With external Cu concentration (Cuo) of 800 μmol l–1, the rate of Cu accumulation by cells was 1.88±0.52 nmol Cu mg–1cell protein h–1 compared to 0.05±0.01 nmol Cu mg–1 cell protein h–1 with no added Cuo (means ± s.e.m., N=6). Deduction of a rapid Cu accumulation measured on/in cells at time zero (about 12% of the total Cu uptake when Cuo was 800 μmol l–1)revealed a saturable uptake curve, which reached a plateau at 400 μmol l–1 Cuo (Km=216 μmol l–1 Cuo; Vmax=1.09 nmol Cu mg–1 cell protein h–1; 140 mmol l–1 NaCl throughout). Incubation of cells at 4°C did not prevent Cu accumulation. Lowering external [Na+] to 11 mmol l–1 (low Na+o) generally did not alter the rate of Cu accumulation into the cells over a 15 min period. Under low Na+o conditions Cu accumulation was exponential(non-saturable). Na+-insensitive Cu accumulation dominated (59% of total Cu accumulation) when Cuo was 400 μmol l–1 or less. At high Cuo (800 μmol l–1), removal of Na+ caused a 45% increase in Cu accumulation. Pre-incubation of cells with blocking agents of epithelial Na+ channel (ENaC) for 15 min (normal [NaCl] throughout) caused Cu accumulation rates to increase by 40-fold (100 μmol l–1phenamil), 21-fold (10 μmol l–1 CDPC) or 12-fold (2 mmol l–1 amiloride) when Cuo was 800 μmol l–1 compared to those in drug-free controls. Lowering the external chloride concentration [Cl–]o from 131.6 to 6.6 mmol l–1 (replaced by sodium gluconate) caused the rate of Cu accumulation to increase 11-fold when Cuo was 800μmol l–1. Application of 0.1 mmol l–1DIDS (normal Cl–o) caused a similar effect. Lowering external pH from 7.4 to pH 5.5 produced a 17-fold, saturable,increase in Cu accumulation rate, which was not explained by increased instantaneous Cu accumulation on/in cells at low pH. We conclude that Cu accumulation by intestinal cells is mainly Na+-insensitive and more characteristic of a pH- and K+-sensitive Ctr1-like pathway than Cu uptake through ENaCs.

Responses of hybrid striped bass to waterborne and dietary copper in freshwater and saltwater

Mechanisms of copper toxicity and consequences of exposure vary due to uptake route and ionoregulatory status. The goal of this research was to develop a model fish system to assess the influence of different Cu exposure routes (waterborne or dietary) on bioavailability, uptake, and effects in hybrid striped bass (Morone chrysops x Morone saxatilis) acclimated to fresh- or saltwater. Initially, hybrid striped bass were exposed to dietary Cu concentrations of 571, 785, and 1013 mug Cu/g, along with a control (approximately 5 microg Cu/g), for 14 days in saltwater. Intestinal and liver Cu accumulated in a dose-dependent manner in fish exposed to increasing levels of dietary Cu. Chronic (42 days) experiments were then conducted to determine sub-lethal effects of aqueous, dietary, and combined aqueous and dietary Cu exposures to both freshwater- and saltwater-acclimated hybrid striped bass. Growth and Cu accumulation in the gill, intestine, and liver were measured. Although no significant effects were observed in fish exposed to waterborne Cu, those exposed through the diet accumulated significant liver and intestinal Cu but showed no significant change in growth. Overall, these results suggest that at the levels tested, exposure to elevated waterborne Cu did not cause significant long-term tissue Cu accumulation, whereas dietary Cu exposure caused significant liver and intestinal Cu accumulation in hybrid striped bass which was comparable in both freshwater and saltwater (15 g/L).