Metal redistribution in largemouth bass (Micropterus salmoides) in response to restrainment stress and dietary cadmium: Role of metallothionein and other metal-binding proteins

Chronic exposure of Oreochromis niloticus to sub-lethal copper concentrations: Effects on growth, antioxidant, non-enzymatic antioxidant, oxidative stress and non-specific immune responses

BACKGROUND

Trace elements of copper (Cu) are one of the main forms of ecological noxious waste in freshwater systems that affect the survival and development of organisms. The objective of the current study was to investigate the effects of chronic exposure to Cu on the growth, oxidative stress, immune and biochemical response in the Nile tilapia, Oreochromis niloticus.

METHODS

Three groups of O. niloticus were tested as follows; the first group was used as the control (not treated with Cu in water), while the 2nd and 3rd groups were exposed to (low) 40 μg L^-1 and (high) 400 μg L^-1 concentrations of Cu added to water, respectively. The duration of the experiment, which was conducted in triplicate, was 60 d. End points were evaluated on days 30 and 60. Following 30 d and 60 d of exposure to Cu, the fish were removed from experimental tanks to determine growth. Consequently, blood samples were collected from caudal veins at the end of the trial period (30 d and 60 d) and serum was separated to evaluate different immunological parameters, such as lysozymes (LYZ), respiratory burst activity (RBA) and myeloperoxidase (MPO). Gill and liver tissues were collected for evaluation of Cu and certain biochemical parameters as follows: antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione-S-transferase (GST); non-enzymatic antioxidants such as glutathione (GSH) and metallothionein (MT), and oxidative stress indicators such as malondialdehyde (MDA) and protein carbonyl (PCO). The results pertaining to treatments and the control were compared using two-way ANOVA and Tukey's HSD test. The level of significance was set at P ≤ 0.05. Data were expressed as mean ± SD.

RESULTS

Chronic exposure to Cu did not induce any mortality in fish during the test period. However, following exposure to Cu, growth of fish in the exposed groups was affected more than that in the control group (unexposed to Cu). In addition, accumulation of Cu in the liver tissue was higher than that in the gill tissues of fish exposed to Cu, compared to that in the control. Gill and liver tissues of Cu-exposed fish showed a significant (P ≤ 0.05) reduction in the activities of the antioxidant enzymes, SOD, CAT, GPx, and GST, compared to those of unexposed fish. Non-enzymatic antioxidants, GSH and MT, in gill and liver tissues were significantly increased (P ≤ 0.05) in fish exposed to both concentrations of Cu, compared to those in unexposed fish. Oxidative stress indicators, MDA and PCO in gills and liver of Cu-exposed fish was significantly (P ≤ 0.05) at both tested concentrations, when compared to control group. Non-specific immune response of LYZ, RBA, and MPO activity in serum decreased significantly (P ≤ 0.05) in Cu-exposed fish, compared with that of unexposed fish.

CONCLUSION

Overall, the present results highlighted that chronic exposure to Cu ions may exert a strong effect on the antioxidant and immune responses of O. niloticus. Changes in antioxidant enzymes, oxidative stress effects and immune parameters during post-chronic metal exposure may indicate the potential of these parameters as biomarkers of metal toxicity in aquatic ecosystems.

Relevance of biotic parameters in the assessment of the spatial distribution of gastrointestinal metal and protein levels during spawning period of European chub (Squalius cephalus L.)

The present field study, conducted during the spawning period (April/May) of European chub (Squalius cephalus L.) from the Sava River in Croatia, indicates that seasonal changes of fish physiological state might cause variability in gastrointestinal metal (Cd, Cu, Fe, Mn and Zn), total cytosolic protein and metallothionein (MT) levels. During the period of fish spawning and increased metabolic activity, a significant relationship with chub hepatosomatic index was evident for Fe and Mn in gastrointestinal tissue (r = 0.35 and 0.26, respectively) and in cytosolic fraction (r = 0.32 and 0.41, respectively) and for Zn and Fe in the gut content (r = 0.36 and 0.31, respectively). Total cytosolic protein and MT concentrations followed the same spatial distribution as Fe and Mn in all gastrointestinal fractions and as Zn in the sub-cellular fractions, with higher levels at upstream locations. Due to the role of essential metals in metabolic processes and gonad development, increased feeding and spawning activity in April/May resulted in higher gastrointestinal essential metal (Fe, Mn and Zn) and MT concentrations, which probably follow an increase in Zn concentrations, known as the primary MT inducer. Therefore, biotic factors should be considered as important confounding factors in metal exposure assessment, while their influence on gastrointestinal metal and protein levels should be interpreted depending on the season studied.

Metal accumulation and oxidative stress biomarkers in liver of freshwater fish Carassius auratus following in vivo exposure to waterborne zinc under different pH values

In this study, laboratory experiments were conducted to investigate the combined effect of zinc and pH on metal accumulation and oxidative stress biomarkers in Carassius auratus. Fish were exposed to 0.1 and 1.0mg Zn/L at three pH values (5.0, 7.25, 9.0) for 3, 12, and 30 d. After each exposure, the contents of three trace elements (Zn, Fe and Cu) were determined in liver. Generally, longer exposure to zinc (12d and 30 d) increased hepatic Zn and Cu deposition, but decreased Fe content. Increasing accumulation of Zn in the tissue was also observed with increasing zinc concentration in the exposure medium. Moreover, hepatic antioxidant enzyme activities including superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx), together with the level of glutathione (GSH) were measured to evaluate the oxidative stress status. The decreases in the four measured biochemical parameters after 3d exposure might reflect the failure of the antioxidant defense system in neutralizing the ROS generated during the metabolic process, while the recovery of the antioxidants at days 12 and 30 suggested a possible shift toward a detoxification mechanism. With regard to the influence of pH on zinc toxicity, the general observation was that the living environment became more stressful when the water conditions changed from an acidic state toward a near-neutral or alkaline state.

Antioxidant enzymes in the liver of Chelidonichthys obscurus from the Montenegrin coastline

The activities of antioxidant defence enzymes — total, manganese and copper zinc containing superoxide dismutase (Tot SOD, Mn SOD, CuZn SOD), catalase (CAT), glutathione peroxidase (GSH-Px), glutathione reductase (GR) and biotransformation phase II enzyme glutathione-S-transferase (GST) — in the liver of longfin gurnard (Chelidonichthys obscurus) from the Montenegrin coastline (Adriatic sea) were investigated. The specimens were collected in winter (February) and late spring (May) at two localities: Platamuni (PL, potentially unpolluted) and the Estuary of the River Bojana (EB, potentially polluted). The obtained results show that the activities of Mn SOD, CAT, GSH-Px and GST in winter were significantly lower at EB than at PL. In spring, the activities of CAT and GST were decreased, while GR activity was increased at EB in comparison to PL. The activities of Mn SOD and GST at PL were decreased and GSH-Px, GR and GST activities at EB were increased in spring compared to winter. Our work represents the first study of liver antioxidant enzymes of longfin gurnard from the Montenegrin coastline and reveals that locality, as a variable, has a greater influence on antioxidant enzymes and biotransformation phase II enzyme GST activities compared to season.

The yellow catfish, Pelteobagrus fulvidraco (Siluriformes) metallothionein cDNA: molecular cloning and transcript expression level in response to exposure to the heavy metals Cd, Cu, and Zn

Metallothionein (MT) has been used extensively as a potential molecular biomarker to detect heavy metal pollution in aquatic organisms. In order to investigate the modulation effect of heavy metals and to establish suitable biomarkers for the monitoring of heavy metal pollution, Pelteobagrus fulvidraco metallothionein gene was characterized as the first report in the family Bagridae. Pf-MT transcript was detected at high levels in liver, gonad, kidney, and brain compared to other tissues. A time-course study in response to waterborne Cd (5 ppm) revealed that a significant increase in the Pf-MT transcript abundance was observed at 6 h in gill, kidney, and liver. These elevated levels were kept for 96 h, implying that Cd distributed fast into different organs and was involved in the tissue-specific induction pattern. We observed a significant Pf-MT transcript increase in liver tissues at 48 h, followed by gill at 12 h and intestine at 48 h after Cd exposure. This indicates hepatic MT expression as a potential biomarker of acute Cd exposure in this species. Cd-binding ability of recombinant Pf-MT protein provided evidence for sensitivity to Cd and other heavy metal exposure. In the case of Zn exposure (1 ppm), a significant increase in Pf-MT transcript abundance was observed at 12 h, and a peak induction level reaching sixfold at 24 h was kept until 48 h, showing similar transcript induction patterns with Cd. A high level of Pf-MT mRNA after exposure to Cu (1 ppm) was observed at 12 h that gradually increased until 96 h with a 12-fold induction, revealing a long-lasting induction and somewhat dissimilar pattern compared to other metals in liver. Our results demonstrate that Pf-MT can be induced by heavy metals in a tissue-specific and metal-specific manner and plays probably a conserved role in metal detoxification. This study provides new information on P. fulvidraco metallothionein gene for the use of biomarkers indicating metal pollution in fish.

Metal-metal interactions of dietary cadmium, copper and zinc in rainbow trout, Oncorhynchus mykiss

The influence of metal-metal interactions on uptake, accumulation, plasma transport and chronic toxicity of dietary Cu, Cd and Zn in rainbow trout (Oncorhynchus mykiss) was explored. Juvenile rainbow trout were fed diets supplemented with (μg/g) 500 Cu, 1000 Zn and 500 Cd singly and as a ternary mixture at 2.5% body weight daily ration for 28 days. Complex interactions among the metals dependent on the tissue/organ, metals ratios and duration of exposure were observed. While Zn did not accumulate, whole-body Cd and Cu concentrations increased following linear and saturation patterns, respectively. Early enhanced whole-body Cu accumulation in fish exposed to the metals mixture was correlated with reduced Cd concentration whereas late enhancement of Cd accumulation corresponded with elevated Cd concentration. This suggests early mutual antagonism and late cooperation between Cd and Cu probably due to interactions at temporally variable metal accumulation sites. At the level of uptake, Cd and Cu were either antagonistic or mutually increased the concentrations of each other depending on the duration of exposure and section of the gut. At the level of transport, enhanced Cd accumulation in plasma was closely correlated with reduced concentrations of both Zn and Cu indicating competitive binding to plasma proteins and/or antagonism at uptake sites. Compared to the Cu alone exposure, Cu concentrations were either lower (gills and carcasses) or higher (liver and kidney) in fish exposed to the metals mixture. On the other hand, Cd accumulation was enhanced in livers and carcasses of fish exposed to the mixture compared to those exposed to Cd alone, while Zn stimulated Cu accumulation in gills. Chronic toxicity was demonstrated by elevated malondialdehyde levels in livers and reduced concentrations of Zn and Cu in plasma. Overall, interactions of Cd, Cu and Zn are not always consistent with the isomorphous competitive binding theory.

Seasonal variations of the activity of antioxidant defense enzymes in the red mullet (Mullus barbatus l.) from the Adriatic Sea

This study investigated seasonal variations of antioxidant defense enzyme activities: total, manganese, copper zinc containing superoxide dismutase (Tot SOD, Mn SOD, CuZn SOD), catalase (CAT), glutathione peroxidase (GSH-Px), glutathione reductase (GR) and biotransformation phase II enzyme glutathione-S-transferase (GST) activity in the liver and white muscle of red mullet (Mullus barbatus). The investigations were performed in winter and spring at two localities: Near Bar (NB) and Estuary of the River Bojana (EB) in the Southern Adriatic Sea. At both sites, Mn SOD, GSH-Px, GR and GST activities decreased in the liver in spring. In the white muscle, activities of Mn SOD, GSH-Px, GR and GST in NB decreased in spring. GR decreased in spring in EB, while CAT activity was higher in spring at both sites. The results of Principal Component Analysis (PCA) based on correlations indicated a clear separation of various sampling periods for both investigated tissues and a marked difference between two seasons. Our study is the first report on antioxidant defense enzyme activities in the red mullet in the Southern Adriatic Sea. It indicates that seasonal variations of antioxidant defense enzyme activities should be used in further biomonitoring studies in fish species.

Opercular epithelial cells: a simple approach for in vitro studies of cellular responses in fish

This study evaluated the efficacy of fish opercular external (skin) and inner (opercular membrane) epithelium as an in vitro model for toxic and other substances studies. The rainbow trout (Oncorhynchus mykiss) operculum was cultured in 12-well dishes containing sterile Leibovitz 15 (L-15) supplemented with glutamine medium during 24h at 9 degrees C, and the effect of copper, a toxic agent, and/or cortisol, an endogenous agent, on the epithelial cells was analyzed using light microscopy techniques. The opercula were submitted to four treatments: (i) control (Cont), L-15 medium only, (ii) 0.28 microM cortisol (Cort), (iii) 100 microM CuSO(4) (Cu), and (iv) 0.28 microM cortisol+100 microM CuSO(4) (Cort-Cu). The tissue condition after 24h incubation was analyzed by staining the mucous cells for neutral and acid mucosubstances. Cellular necrosis was evaluated by measuring the lactate dehydrogenase (LDH) leakage at 12 and 24h incubation. Cellular proliferation, apoptosis, metallothionein (MT) and glucocorticoid receptor (GR) expression were evaluated by immunohistochemistry. The LDH leakage was higher and the proliferating cell nuclear antigen (PCNA) positive-stained cells were lower in Cu treatment in both, epidermis and opercular membrane. Apoptotic cells in the opercular membrane were higher in the Cort and Cort-Cu treatments while, in the epidermis, they were higher in Cu and Cort-Cu treatments. GR-positive stained cells decreased significantly in all treatments in both epithelia and the MT-positive cells increased in the Cu and Cort-Cu treated groups. Copper showed to be a potent toxic stressor killing the cells via necrosis, decreasing the number of PCNA-positive cells and inducing MT synthesis while cortisol did not affect the MT synthesis, although it might stimulate apoptosis. The results are evidence that the opercular epithelia serve as a suitable model for studying in vitro effects of toxic agents, as well as endogenous factors on the cellular responses without interference of the physiological state of fish being useful to predict in vivo toxicity.

Age- and tissue-dependent metallothionein and cytosolic metal distribution in a native Mediterranean fish, Mullus barbatus, from the Eastern Adriatic Sea

The levels of metallothionein (MT), a biomarker of metal exposure, and of cytosolic metals (Zn, Cu, Cd), known as MT inducers, were investigated as variables of age (1 to 8 years) and tissue mass (liver, kidney, brain) of red mullet (Mullus barbatus). Within the age from 1 to 8 years the most significant increase is evident for cytosolic Cd in liver (43-fold) and in kidney (5-fold). MT and essential metals are constant with age or slightly increased. Over the growth period, statistically significant MT and metal increase is evident only between 1 and 6-8 years old specimens, while for Cd in liver and kidney cytosol significant increase already exists at 4 years old specimens. Metal distribution in all tissues follows the order: Zn>Cu>Cd, with even 500-800 times lower Cd levels than essential metal levels. Consequently, MTs follow the levels of essential metals, Zn and Cu, indicating MT involvement in homeostasis of essential metals. In contrast to kidney and brain, hepatic MT levels are not age-dependent. Inclusion of hepatic MT measurements and the associated cytosolic metals will be useful in the assessment of long-term metal effects in demersal fish M. barbatus.

Ultrastructural changes in various organs of the fish Puntius gonionotus fed cadmium-enriched cyanobacteria

The accumulation and toxicity of cadmium in Puntius gonionotus fish that consumed the cyanobacterium Spirulina platensis contaminated with cadmium were studied. Fish were fed cadmium-contaminated cells for 4 weeks, after which cadmium accumulation in various organs was determined. The highest cadmium content was found in the kidney (56.0 microg Cd/g wet weight). Cadmium was not detected in the gill during the entire 4 weeks of cadmium feeding. Histopathological alteration of cells was observed in the gill, kidney, and liver. The results showed that dietary cadmium caused hypertrophy and edema of gill filaments. Coagulative necrosis and karyolysis of the nucleus were observed in the kidney. Vacuoles and hyaline droplets had accumulated in the epithelial cells of the proximal tubule. In the liver vacuolation of the cytoplasm, infiltration of macrophages, and focal necrosis were found. The ultrastructural changes that occurred in the cells of different organs were similar. These included a proliferation of vacuoles and lysosomes, formation of myelin bodies, degranulation, vesiculation, and dilation of rough endoplasmic reticulum, as well as swelling of mitochondria with loss of cristae.

In vivo characterisation of intestinal zinc uptake in freshwater rainbow trout

Knowledge of the uptake mechanisms and metabolism of metals is essential for understanding the factors governing metal toxicity, discerning means by which acclimation and homeostasis may be achieved and characterising interactions between the metal of interest and other environmental moieties. Zinc is both an important aquatic contaminant and a vital micronutrient. The physiological characterisation of dietary zinc absorption in fish has, therefore, important implications for environmental protection and aquaculture. The present study aimed to elucidate the mechanism of intestinal zinc uptake in freshwater rainbow trout (Oncorhynchus mykiss), using an in vivo cannulation technique. Only a saturable component of zinc uptake, with a concentration giving half-maximal rate of accumulation (K0.5) of 309 μmol l–1, and a maximal rate of accumulation (Jmax) of 933 nmol kg–1 h–1, was described. This characterised the intestine as a low-affinity, high-capacity zinc absorption pathway. Physiological mechanisms appear to regulate zinc uptake. Intestinal mucus was one important regulatory locus, promoting zinc uptake at low concentrations yet buffering the animal against high luminal zinc loads. Regulatory mechanisms also seemed to limit subepithelial zinc accumulation. Experiments using ethylene glycol tetraacetic acid (EGTA) to wash the intestinal lumen following zinc perfusion exhibited a higher proportion of loosely associated zinc at higher perfused concentrations. This was attributed to saturation of the uptake process or efflux from the subepithelium. Two distinct pathways for passage of zinc across the epithelium were discerned, with post-intestinal transfer possibly mediated by sulphydryl groups, as illustrated by N-ethylmaleimide perfusion experiments. Putative roles of zinc transporters and/or intracellular-binding proteins are discussed.

Killifish metallothionein messenger RNA expression following temperature perturbation and cadmium exposure

Metallothionein (MT), a cysteine-rich metal binding protein, is considered to play an essential role in the regulation of intracellular metals. Induction of MT in mammalian and nonmammalian tissues following heavy metal exposure may serve as a defense mechanism and a biomarker of environmental exposure to chemical stressors such as toxic metals. In this study, MT messenger RNA (mRNA) expression was characterized in male and female nonspawning and spawning killifish (Fundulus heteroclitus) following an 8-day exposure to specific sublethal stressors, which included temperature perturbation (26 degrees C or 10 degrees C) and/or 6 ppb of waterborne cadmium chloride (CdCl2). Hepatic, gill, and intestinal MT mRNA, expressed as copy number per microgram of total RNA, was assessed by reverse transcriptase-polymerase chain reaction and electrochemiluminescence using winter flounder (Pleuronectes americanus) MT complementary DNA primers. Liver, gill, and intestine MT mRNA expression was significantly (P < 0.05) increased in nonspawning killifish exposed to 26 degrees C compared with those exposed to 19 degrees C (control). In addition, a significant (P < 0.05) increase in gill MT mRNA induction was observed in nonspawning killifish exposed to 6 ppb of waterborne CdCl2 compared with controls. The results of this study demonstrate significant MT mRNA induction in nonspawning killifish following short-term exposure to physiological and chemical stressors. Thus, further research may be necessary before the use of killifish MT mRNA induction as a biomarker of environmental chemical stress exposure alone.

Metallothionein and heavy metals in brown trout (Salmo trutta) and European eel (Anguilla anguilla): a comparative study

The levels and the cellular distribution of heavy metals, and the extent by which the metals binds to metallothionein (MT) in brown trout (Salmo trutta) and European eel (Anguilla anguilla), were analyzed in order to assess the natural conditions of MT and heavy metals in these two fish species. There were no differences in heavy metals and MT concentrations between males and females of brown trout in a nonreproductive status and between adult brown trout individuals. Brown trout presented higher Cu content than European eel. The cellular distribution of Cu was also different between the two fish species; while in brown trout most of the Cu was in the noncytosolic fraction, Cu was mainly located in the cytosol in European eel. However, the cellular distribution of Zn, Cd, and Pb was similar in the two fish species. There was also an important difference in the metal content of MT between both species. Whereas, in brown trout, Cu-binding MT represented 75% of total metal-binding MT, this value was 25% in European eel. The between-species differences found in this study are intrinsic characteristics not associated with environmental factors. These results establish the basis to use MT as a bioindicator.

Impaired adrenocortical response to stress by brown trout, Salmo trutta, living in metal-contaminated waters of the Eagle River, Colorado

Brown trout, Salmo trutta, were collected from two sites contaminated with cadmium (Cd) and zinc (Zn) and one uncontaminated site. These fish were subjected to a continuous confinement stressor in wire cages placed in the river (moderate stress) or in 5-gal. plastic buckets on land (severe stress). Plasma cortisol and corticotropin (ACTH) were determined for fish in buckets by radioimmunoassay after 0, 1, 3, 12, or 24 h of confinement. Plasma cortisol and ACTH levels of brown trout from both contaminated and uncontaminated sites initially were the same and increased with time. However, the rise in plasma cortisol was delayed significantly in fish residing in contaminated sites, even though ACTH secretion initially was elevated compared with control trout. Furthermore, secretion of cortisol and ACTH by these fish declined significantly between 3 and 24 h of confinement. Fish from the uncontaminated site responded more rapidly to confinement with increased cortisol secretion and elevated levels of ACTH and continued to exhibit elevated levels of both hormones up to 24 h of confinement. Caged fish examined after 0, 3, 12, and 24 h of confinement exhibited similar plasma cortisol responses regardless of previous exposure to metals. These results suggest that the overall response to severe, short-term confinement stress by the hypothalamo-pituitary-adrenocortical axis of fish chronically exposed to Cd and Zn was depressed and that these fish could not sustain the stress response as readily as fish living in uncontaminated water.