Characterization of dietary Ni uptake in the rainbow trout, Oncorhynchus mykiss

Making in vitro conditions more reflective of in vivo conditions for research on the teleost gastrointestinal tract

To date, the majority of in vitro or ex vivo fish gastrointestinal research has been conducted under unrealistic conditions. In a living fish, ionic conditions, as well as levels of ammonia, pH, HCO3- and PCO2 differ considerably between the different regions of the gastrointestinal tract. These factors also differ from those of the saline often used in gut research. Furthermore, the oxygen gradient from the serosa to the gut lumen is rarely considered: in contrast to the serosa, the lumen is a hypoxic/anoxic environment. In addition, the gut microbiome plays a significant role in gut physiology, increasing the complexity of the in vivo gut, but replicating the microbial community for in vitro studies is exceptionally difficult. However, there are ways in which we can begin to overcome these challenges. Firstly, the luminal chemistry and PO2 in each gut compartment must be carefully considered. Secondly, although microbiological culture techniques are improving, we must learn how to maintain the microbiome diversity seen in vivo. Finally, for ex vivo studies, developing mucosal (luminal) solutions that more closely mimic the in vivo conditions will better replicate physiological processes. Within the field of mammalian gut physiology, great advances in 'gut-on-chip' devices are providing the tools to better replicate in vivo conditions; adopting and adapting this technology may assist in fish gut research initiatives. This Commentary aims to make fish gut physiologists aware of the various issues in replicating the in vivo conditions and identifies solutions as well as those areas that require further improvement.

Assessing the impact of benzo[a]pyrene with the in vitro fish gut model: An integrated approach for eco-genotoxicological studies

In vitro models are emerging tools for reducing reliance on traditional toxicity tests, especially in areas where information is sparse. For studies of fish, this is especially important for extrahepatic organs, such as the intestine, which, until recently, have been largely overlooked in favour of the liver or gill. Considering the importance of dietary uptake of contaminants, the rainbow trout (Oncorhynchus mykiss) intestine-derived cell line RTgutGC was cultured, to test its suitability as a high-throughput in vitro model. Benzo[a]pyrene (B[a]P) is an important contaminant and a model polycyclic aromatic hydrocarbon (PAH). Over 48 h exposure, a range of endpoints and xenobiotic metabolism rates were examined at three different pH levels indicative of the in vitro (pH 7.5) and in vivo mid-gut (pH 7.7) and hind-gut (pH 7.4) regions as a function of time. These endpoints included (i) cell viability: acid phosphatase (APH) and lactate dehydrogenase (LDH) assays; (ii) glucose uptake; (iii) cytochrome P450 enzyme activity: 7-ethoxyresoorufin-O-deethylase (EROD) assay; (iv) glutathione transferase (GST) activity; (v) genotoxic damage determined using the comet assay. Absence of cell viability loss, in parallel with decrease in the parent compound (B[a]P) in the medium and its subsequent increase in the cells suggested active sequestration, biotransformation, and removal of this representative PAH. With respect to genotoxic response, significant differences were observed at both the sampling times and the two highest concentrations of B[a]P. No significant differences were observed for the different pH conditions. Overall, this in vitro xenobiotic metabolism system appears to be a robust model, providing a basis for further development to evaluate metabolic and toxicological potential of contaminants without use of animals.

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.

Mechanisms of nickel toxicity to fish and invertebrates in marine and estuarine waters

In freshwater settings the toxicity of the trace metal nickel (Ni) is relatively well understood. However, until recently, there was little knowledge regarding Ni toxicity in waters of higher salinity, where factors such as water chemistry and the physiology of estuarine and marine biota would be expected to alter toxicological impact. This review summarizes recent literature investigating Ni toxicity in marine and estuarine invertebrates and fish. As in freshwater, three main mechanisms of Ni toxicity exist: ionoregulatory impairment, inhibition of respiration, and promotion of oxidative stress. However, unlike in freshwater biota, where mechanisms of toxicity are largely Class-specific, the delineation of toxic mechanisms between different species is less defined. In general, despite changes in Ni speciation in marine waters, organism physiology appears to be the main driver of toxic impact, a fact that will need to be accounted for when adapting regulatory tools (such as bioavailability normalization) from freshwater to estuarine and marine environments.

Making sense of nickel accumulation and sub-lethal toxic effects in saline waters: Fate and effects of nickel in the green crab, Carcinus maenas

In freshwater, invertebrates nickel (Ni) is considered an ionoregulatory toxicant, but its mechanism of toxicity in marine settings, and how this varies with salinity, is poorly understood. This study investigated Ni accumulation and physiological mechanisms of sub-lethal Ni toxicity in the euryhaline green crab Carcinus maenas. Male crabs were exposed to 8.2μg/L (the US EPA chronic criterion concentration for salt waters) of waterborne Ni (radiolabelled with (63)Ni) at three different salinities, 20%, 60% and 100% SW for 24h. Whole body Ni accumulation in 20% SW was 3-5 fold greater than in 60% or 100% SW, and >80% of accumulated Ni was in the carapace at all salinities. Ni also accumulated in posterior gill 8, which showed a higher accumulation in 20% SW than in other salinities, a pattern also seen at higher exposure concentrations of Ni (500 and 3000μg/L). Gill perfusion experiments revealed that Ni was taken up by both anterior and posterior gills, but in 20% SW the posterior gill 8, which performs ionoregulatory functions, accumulated more Ni than the anterior gill 5, which primarily has a respiratory function. The sub-lethal consequences of Ni exposure were investigated by placing crabs in Ni concentrations of 8.2, 500, and 3000μg/L at 20, 60 or 100% SW for 24h. In 20% SW, haemolymph Ca levels were significantly decreased by exposure to Ni concentrations of 8.2μg/L or higher, whereas Na concentrations were depressed only at 3000μg/L. Na(+)/K(+)-ATPase activity was inhibited at both 500 and 3000μg/L in gill 8, but only in 20% SW. Haemolymph K, Mg, and osmolality were unaffected throughout, though all varied with salinity in the expected fashion. These data suggest that Ni impacts ionoregulatory function in the green crab, in a gill- and salinity-dependent manner.

Novel route of toxicant exposure in an ancient extant vertebrate: nickel uptake by hagfish skin and the modifying effects of slime

Utilizing an in vitro technique, the skin of Pacific hagfish (Eptatretus stouti) was shown to take up nickel from the water via a high affinity, low capacity transport pathway. Uptake was biphasic, with saturation occurring at low nickel exposure concentrations, superseded by linear, diffusive uptake at levels greater than 50 μM. In vivo exposures showed that nickel accumulated mainly in the gill, heart, and brain, representing a tissue distribution distinct from that found in teleosts. Slime on the epidermal surface was shown to significantly reduce the uptake of low concentrations (10 μM) of the metals zinc and nickel, but slime had no effect on organic nutrient (the amino acid l-alanine) absorption. At a higher metal exposure concentration (1 mM), slime was no longer protective, indicating saturation of metal-binding sites. This is the first study to show that metals can be taken up by the integument of hagfish. The ability of the skin to act as a transport epithelium may be of particular importance for a burrowing, benthic scavenger, such as hagfish, which are likely to be exposed to relatively enriched levels of metal toxicants through their habitat and lifestyle, and this may have consequences for human health through hagfish consumption.

Effects of exposure to high concentrations of waterborne Tl on K and Tl concentrations in Chironomus riparius larvae

Thallium (Tl) is a non-essential metal which is released into the environment primarily as the result of anthropogenic activities such as fossil fuel burning and smelting of ores. The ionic radius of monovalent Tl⁺ is similar to that of K⁺ and Tl⁺ may thus interfere with K⁺-dependent processes. We determined that the acute (48 h) lethal concentration where 50% of the organisms do not survive (LC₅₀) of Tl for 4th instar Chironomus riparius larvae was 723 μmol L⁻¹. Accumulation of Tl by the whole animal was saturable, with a maximum accumulation (Jmax) of 4637 μmol kg⁻¹ wet mass, and K(D) of 670 μmol Tl l⁻¹. Tl accumulation by the gut appeared saturable at the lowest four Tl concentrations, with a Jmax of 2560 μmol kg⁻¹ wet mass and a K(D) of 54.5 μmol Tl l⁻¹. The saturable accumulation at the gut may be indicative of a limited capacity for intracellular detoxification, such as storage in lysosomes or complexation with metal-binding proteins. Tl accumulation by the hemolymph was found to be linear and Tl concentrations in the hemolymph were ~75% of the exposure concentration at Tl exposures >26.9 μmol L⁻¹. There was not a significant decrease in whole animal, gut or hemolymph K during exposure to waterborne Tl at any of the concentrations tested (up to 1500 μmol L⁻¹). The avoidance of hypokalemia by C. riparius larvae may contribute to survival during acute waterborne exposures to Tl.

Chronic nickel bioaccumulation and sub-cellular fractionation in two freshwater teleosts, the round goby and the rainbow trout, exposed simultaneously to waterborne and dietborne nickel

Rainbow trout and round goby were exposed for 30 days to waterborne and dietary Ni in combination at two waterborne concentration ranges (6.2-12 μmol/L, 68-86 μmol/L), the lower of which is typical of contaminated environments. The prey (black worms; Lumbriculus variegatus) were exposed for 48 h in the effluent of the fish exposure tanks before being fed to the fish (ration=2% body weight/day). Ni in gills, gut, and prey was fractionated into biologically inactive metal [BIM=metal-rich granules (MRG) and metallothionein-like proteins (MT)] and biologically active metal [BAM=organelles (ORG) and heat-denaturable proteins (HDP)]. Gobies were more sensitive than trout to chronic Ni exposure. Possibly, this greater sensitivity may have been due to the goby's pre-exposure to pollutants at their collection site, as evidenced by ∼2-fold greater initial Ni concentrations in both gills and gut relative to trout. However, this was followed by ∼2-16× larger bioaccumulation in both the gills and the gut during the experimental exposure. On a subcellular level, ∼3-40× more Ni was associated with the BAM fraction of goby in comparison to trout. Comparison of the fractional distribution of Ni in the prey versus the gut tissue of the predators suggested that round goby were more efficient than rainbow trout in detoxifying Ni taken up from the diet. Assessing sub-cellular distribution of Ni in the gills and gut of two fish of different habitat and lifestyles revealed two different strategies of Ni bioaccumulation and sub-cellular distribution. On the one hand, trout exhibited an ability to regulate gill Ni bioaccumulation and maintain the majority of the Ni in the MT fraction of the BIM. In contrast goby exhibited large Ni spillovers to both the HDP and ORG fractions of the BAM in the gill. However, the same trend was not observed in the gut, where the potential acclimation of goby to pollutants from their collection site may have aided their ability to regulate Ni spillover to the BAM more so than in trout. Overall, chronic mortality observed in goby may be associated more with Ni bioaccumulation in gills than in gut; the former at either 4-d or 30-d was predictive of chronic Ni toxicity. BIM and BAM fractions of the goby gills were equally predictive of chronic (30-d) mortality. However, critical body residue (CBR50) values of the BIM fraction were ∼2-4× greater than CBR50 values of the BAM fraction, suggesting that goby are more sensitive to Ni bioaccumulation in the BAM fraction. There was insufficient mortality in trout to assess whether Ni bioaccumulation was predictive of chronic mortality.

Reproductive impacts and physiological adaptations of zebrafish to elevated dietary nickel

Nickel (Ni) concentrations in the environment can rise due to human industrial activities. The toxicity of waterborne Ni to aquatic animals has been examined in a number of previous studies; however, little is known about the impacts of elevated dietary Ni. In the present study, zebrafish were chronically fed diets containing two concentrations of Ni [3.7 (control) and 116 μg Ni/g diet]. Ni-exposed males, but not females, were significantly smaller (26%) compared to controls at 80 days. In addition, total egg production was decreased by 65% in the Ni treatment at 75-78 days of the experiment. Ni was ubiquitously distributed in control animals (similar to previous studies), and concentrations varied between tissues by 15-fold. Ni exposure resulted in modest but significant Ni accumulation in some tissues (increases were highest in brain, vertebrae and gut; 44%, 34% and 25%, respectively), an effect observed only at 80 days. The limited Ni accumulation may be due to (1) the lack of an acidified stomach in zebrafish and/or (2) the efficient upregulation of Ni transport and excretion mechanisms, as indicated by the 4.5-fold increase in waterborne (63)Ni uptake by Ni-exposed fish. Eggs from Ni-exposed adults had Ni concentrations that were 5.2-fold higher than controls. However, by 4 days post fertilization, larvae had similar Ni concentrations as controls, demonstrating a capacity for rapid Ni depuration. Larvae from Ni-exposed adults were also more resistant to waterborne Ni (35% increase in the 96-h LC50 over controls). In conclusion, elevated dietary Ni significantly affected zebrafish reproduction despite only modest tissue Ni accumulation. There were also indications of adaptation, including increased Ni uptake rates and increased Ni tolerance of offspring from Ni-exposed adults. Ni concentrations were particularly elevated in the brain with exposure; possible relations to growth and reproductive impacts require further study.

In vitro characterization of calcium transport along the gastrointestinal tract of freshwater rainbow trout Oncorhynchus mykiss

Using an in vitro gut-sac technique, this study examined the mechanisms of calcium (Ca) uptake along the gastrointestinal tract (GIT) of rainbow trout Oncorhynchus mykiss. Ca uptake into three different compartments (mucous-bound, mucosal epithelium and blood space) of four distinct GIT segments (stomach, anterior intestine, mid intestine and posterior intestine) was monitored after luminal exposure to 10 mM Ca saline (radiolabelled with (45) Ca). Ca transport was determined to be both time-dependent and concentration-dependent. The concentration-dependent kinetics of Ca uptake was investigated using varying luminal concentrations of Ca (1, 10, 30, 60 and 100 mM). In the blood-space compartment, Ca uptake was saturable at high Ca concentrations in the mid intestine (suggesting mediated transport), while linear uptake was found in the other gut segments. In the mucous-bound and mucosal epithelium compartments, however, saturation kinetics were found for most GIT segments, also suggesting mediated transport. Manipulation of serosal saline osmotic pressure with mannitol demonstrated that Ca uptake was not greatly affected by solvent drag. Elevated mucosal cadmium (Cd) did not appear to inhibit Ca uptake into the blood space in any of the GIT sections, and Ca uptake did not appear to be sodium dependent. Maximum transport capacities for Ca and Cd were found to be comparable between the gills and gut, but affinities were much higher at the gills (up to 3000 times).

The effects of salinity on acute and chronic nickel toxicity and bioaccumulation in two euryhaline crustaceans: Litopenaeus vannamei and Excirolana armata

We investigated the influence of salinity (5 ppt versus 25 ppt) on acute (96-h LC(50)) and chronic toxicity (15-30 day LC(50)) of Ni in two euryhaline crustaceans, the shrimp (Litopenaeus vannamei) and the isopod (Excirolana armata). 96-h LC50 values were 41 μmolL(-1) and 362 μmolL(-1) for L. vannamei and 278 μmolL(-1) and >1000 μmolL(-1) for E. armata at 5 ppt and 25 ppt, respectively. Speciation analysis demonstrated that complexation with anions such as SO(4)(2-), HCO(3)(-) and Cl(-) at 25 ppt had a negligible effect on reducing the free Ni(2+) ion component in comparison to 5 ppt. The salinity-dependent differences in acute Ni toxicity could not be explained by differences in Ni bioaccumulation. Therefore, differences in physiology of the organisms at the two salinities may be the most likely factor contributing to differences in acute Ni toxicity. Chronic LC(50) values (2.7-23.2 μmolL(-1)) were similar in the two species, but salinity had no significant effect, indicating that water chemistry and osmoregulatory strategy do not influence chronic toxicity. However chronic (15-day) mortality in both species could be predicted by acute (96-h) Ni bioaccumulation patterns.