Identification and response to metals of metallothionein in two ancient fishes: white sturgeon (Acipenser transmontanus) and lake sturgeon (Acipenser fulvescens)

Cadmium-109 Internal Kinetics in Diamond Sturgeon Acipenser gueldenstaedtii are Strongly Influenced by Salinity, Exposure Pathway and History

Cadmium-109 whole-body and internal biokinetics were experimentally investigated in critically endangered diamond sturgeon Acipenser gueldenstaedtii after uptake from water or food, in fresh (FW) and brackish (BW; 9‰) salinities typical of the Caspian Sea. Whole-body rates of uptake of 109Cd from water and subsequent depuration were quantified over 14 and 28 days, respectively. Uptake was greater in FW than BW by a factor of 2.4, but depuration rates were similar in both salinities. In contrast, for the dietary (chironomid) exposure pathway 109Cd assimilation efficiencies (AEs) were higher in BW (13%) compared to FW (9.5%). Head (including gills) or digestive tract were major repositories of 109Cd following aqueous and dietary exposures, respectively, including both uptake and depuration phases. The point-of-entry of 109Cd into the body was also a major and persistent determiner of its subsequent internal distribution. For aqueous exposures, the internal distributions of 109Cd changed appreciably during depuration with increased activity concentrations in some body components, which again varied with salinity. Increased salinity appreciably enhanced the percentage distributions and activity concentrations of 109Cd in the liver, kidney and digestive tract, which are typically most pathologically altered by elevated Cd exposure. For dietary exposure, increased salinity also enhanced 109Cd activity concentrations in most body components. The results repeatedly indicate the important role of salinity in the whole-body and internal biokinetics of 109Cd in A. gueldenstaedtii, a representative of both a phylogenetically distinct and most endangered family of fishes.

Toxicokinetic Models for Bioconcentration of Organic Contaminants in Two Life Stages of White Sturgeon (Acipenser transmontanus)

The white sturgeon (Acipenser transmontanus) is an endangered ancient fish species that is known to be particularly sensitive to certain environmental contaminants, partly because of the uptake and subsequent toxicity of lipophilic pollutants prone to bioconcentration as a result of their high lipid content. To better understand the bioconcentration of organic contaminants in this species, toxicokinetic (TK) models were developed for the embryo-larval and subadult life stages. The embryo-larval model was designed as a one-compartment model and validated using whole-body measurements of benzo[a]pyrene (B[a]P) metabolites from a waterborne exposure to B[a]P. A physiologically based TK (PBTK) model was used for the subadult model. The predictive power of the subadult model was validated with an experimental data set of four chemicals. Results showed that the TK models could accurately predict the bioconcentration of organic contaminants for both life stages of white sturgeon within 1 order of magnitude of measured values. These models provide a tool to better understand the impact of environmental contaminants on the health and the survival of endangered white sturgeon populations.

Crosstalk between heat shock proteins, NRF2, NF-κB and different endogenous antioxidants during lead-induced hepatotoxicity in Puntius ticto

Present study aims to investigate interaction of molecular chaperons (heat shock protein 70, heat shock protein 90) with transcriptional factors (nuclear factor kappa B/nuclear factor E2-related factor 2/Kelch-like ECH-associated protein 1) to evaluate their role during metal induced stress in fish hepatocytes. Adult Puntius ticto were exposed to lead nitrate at 0 mg/l (control), 1/50th (0.04 mg/l) and 1/20th (0.12 mg/l) of LC₅₀ for 30 days and sacrificed to collect liver tissues. Activity of selected liver enzymes, antioxidants and metallothionein were analyzed. Levels of heat shock protein 70, heat shock protein 90, nuclear factor kappa B, nuclear factor E2-related factor 2 and Kelch-like ECH-associated protein 1 were also measured. Liver enzymes showed a significant increase (p < 0.05) in both Pb exposed groups indicating that the liver might be at risk of damage. Increased level of lipid peroxidation due to metal stress was marked by significant increase (p < 0.05) in malondialdehyde level in fish exposed to the higher Pb concentration compared to control (+ 13.7 %). Significant increase (p < 0.05) in gluthathione reductase (+ 35 %, + 39.2 %), glutathione s-transferase (+ 22.4 %, + 50.4 %) activities and decrease in reduced glutathione level (- 6.75 %, - 12.25 %) in fish exposed to both lower and higher Pb concentration compared to control also indicated metal induced oxidative damage in fish liver. Super oxide dismutase and catalase activities increased significantly (p < 0.05) during exposure to lower Pb concentration, while decreased significantly (p < 0.05) during exposure to higher Pb concentration compared to those in control. Significant (P < 0.05) increase (+ 52.63 %, + 89.47 %) in metallothionein in Pb exposed groups confirmed its role in detoxification process of the metal. Heat shock protein 70 and heat shock protein 90 expression levels increased significantly (p < 0.05) during metal exposure indicating their role as modulator of stress-induced antioxidant protein remodelling. A positive correlation between nuclear factor kappa B/nuclear factor E2-related factor 2/Kelch-like ECH-associated protein 1 with gluthathione regulatory enzymes (gluthathione reductase and glutathione s-transferase) was noted. Current study effectively illuminates the critical role of different factors (heat shock proteins/nuclear factor kappa B/nuclear factor E2-related factor 2/Kelch-like ECH-associated protein 1) to influence the expression and synthesis of antioxidants and other functional enzymes in lead-exposed fish liver.

Molecular mechanisms of heavy metals resistance of Stenotrophomonas rhizophila JC1 by whole genome sequencing

In this study, a higher metal ions-resistant bacterium, Stenotrophomonas rhizophila JC1 was isolated from contaminated soil in Jinchang city, Gansu Province, China. The Pb²⁺ (120 mg/L) and Cu²⁺ (80 mg/L) removal rate of the strain reached at 76.9% and 83.4%, respectively. The genome comprises 4268161 bp in a circular chromosome with 67.52% G + C content and encodes 3719 proteins. The genome function analysis showed czc operon, mer operon, cop operon, arsenic detoxification system in strain JC1 were contributed to the removal of heavy metals. Three efflux systems (i.e., RND, CDF, and P-ATPase) on strain JC1 genome could trigger the removal of divalent cations from cells. cAMP pathway and ABC transporter pathway might be involved in the transport and metabolism of heavy metals. The homology analysis exhibited multi-gene families such as ABC transporters, heavy metal-associated domain, copper resistance protein, carbohydrate-binding domain were distributed across 410 orthologous groups. In addition, heavy metal-responsive transcription regulator, thioredoxin, heavy metal transport/detoxification protein, divalent-cation resistance protein CutA, arsenate reductase also played important roles in the heavy metals adsorption and detoxification process. The complete genome data provides insight into the exploration of the interaction mechanism between microorganisms and heavy metals.

Differences in the subcellular distribution of cadmium and copper in the gills and liver of white sturgeon (Acipenser transmontanus) and rainbow trout (Oncorhynchus mykiss)

Recent studies have shown that white sturgeon (Acipenser transmontanus) are more resistant to cadmium (Cd) compared to rainbow trout (Oncorhynchus mykiss), whereas they are more sensitive than rainbow trout when exposed to copper (Cu). Differences in the subcellular distribution of metals among species could be one of the factors responsible for the differences in the sensitivity to metals. Although, subcellular distribution has been studied extensively in many species with many metals, its direct role in species-specific differences in the sensitivity has not been well studied. The objective of this study was to evaluate the role of subcellular distribution of metals in species-specific differences in the sensitivity to metals between sturgeon and trout. We compared the subcellular distribution of metals Cd and Cu in the cellular debris, heat-stable proteins, heat-denatured fraction, metal-rich granules, and organelles fractions from the gills and liver after exposure of juveniles of both species to 1.25 and 20 μg/L Cd and Cu for 8 days, respectively. Sturgeon diverted a higher amount of Cd towards biologically inactive metal pool (BIM) and a lower amount towards the biologically active metal pool (BAM) compared to trout in both tissues. This explained why sturgeon are able to tolerate a relatively higher exposure level to Cd compared to trout. For Cu, there was no statistically significant species-specific differences in the amounts diverted towards either BAM or BIM; hence, white sturgeon's greater sensitivity to Cu was not explained by its subcellular distribution strategies.

Synthetic fish metallothionein design as a potential tool for monitoring toxic metals in water

The diversity of aquatic ecosystems impacted by toxic metals is widely distributed throughout the world. The application of metallothionein (MT) as an early warning sign of metal exposure in freshwater fish is important in biomonitoring, but a more accessible, sensitive, safe, and efficient new methodological strategy is necessary. On this way, a fish MT synthetic gene from Oreochromis aureos was expressed in Escherichia coli to produce polyclonal antibodies against the protein. In the validation assays, these antibodies were able to detect hepatic MT from freshwater fishes Oreochromis niloticus, Pimelodus maculatus, Prochilodus lineatus, and Salminus brasiliensis showing a potential tool for toxic metals biomarker in biomonitoring of aquatic ecosystems. The current results showed the applicability of this molecule in quantitative immunoassays as a sensor for monitoring aquatic environments impacted by toxic metals. Due to the lack of methods focusing on metal pollution diagnostics in aquatic ecosystems, the current proposal revealed a promising tool to applications in biomonitoring programs of water resources, mainly in Brazil where the mining activity is very developed.

The Last Half Century of Fish Explant and Organ Culture

Explants are three-dimensional tissue fragments maintained outside the organism. The goals of this article are to review the history of fish explant culture and discuss applications of this technique that may assist the modern zebrafish laboratory. Because most zebrafish workers do not have a background in tissue culture, the key variables of this method are deliberately explained in a general way. This is followed by a review of fish-specific explantation approaches, including presurgical husbandry, aseptic dissection technique, choice of media and additives, incubation conditions, viability assays, and imaging studies. Relevant articles since 1970 are organized in a table grouped by organ system. From these, I highlight several recent studies using explant culture to study physiological and embryological processes in teleosts, including circadian rhythms, hormonal regulation, and cardiac development.

Copper Concentrations in the Upper Columbia River as a Limiting Factor in White Sturgeon Recruitment and Recovery

Currently there is little natural recruitment of white sturgeon (Acipenser transmontanus) in the Upper Columbia River located in British Columbia, Canada and Washington, USA. This review of life history, physiology, and behavior of white sturgeon, along with data from recent toxicological studies, suggest that trace metals, especially Cu, affect survival and behavior of early life stage fish. Sturgeon free embryos, first feeding embryos, and mixed feeding embryos utilize interstitial spaces between gravel. Although concentrations of Cu in the water column of the Upper Columbia River are typically less than US water quality criteria defined to protect aquatic life, samples at the sediment-water interface were as large as 24 µg/L and exceed the criteria. Toxicological studies reviewed here demonstrate mortality, loss of equilibrium, and immobility at Cu concentrations of 1.5 to <16 µg/L and reduced swimming activity was documented at 0.88 to 7 μg/L. Contaminated invertebrates and slag particles provide other routes of exposure. These additional routes of exposure can cause indirect effects from starvation due to potential lack of prey items and ingestion of contaminated prey or slag particles. The lack of food in stomachs during these critical early life stages may coincide with a threshold "point of no return" at which sturgeon will be unable to survive even if food becomes available following that early time frame. These findings become especially important as work progresses to enhance white sturgeon recruitment in the Upper Columbia River. To date, decisions against including trace metals as a factor in sturgeon recovery have focused on surface-water concentrations and measurements of lethality (LC50) to establish threshold concentrations for sturgeon sensitivity. However, information provided here suggests that measurements from the sediment-water interface and effect concentrations (EC50) be considered with white sturgeon life history characteristics. These data support minimizing Cu exposure risk to enhance a successful white sturgeon recovery effort. Integr Environ Assess Manag 2020;16:378-391. Published 2020. This article is a US Government work and is in the public domain inthe USA.

Experimental modeling of the acute toxicity and cytogenotoxic fate of composite mixtures of chromate, copper and arsenate oxides associated with CCA preservative using Clarias gariepinus (Burchell 1822)

Concurrent occurrence of chromium (Cr), copper (Cu) and arsenic (As) from chromated copper arsenate (CCA) wood preservative in aquatic ecosystems demands that their joint-actions in eliciting toxic effects be assessed for adequate understanding of the health risk they may pose to biota. Clarias gariepinus was exposed to As₂O₃ , CrO₃ and CuO and their composite mixtures (1:1 and 1:1:1) at various concentrations (0 – 600 mg/L) for 96-h to determine the acute toxicity using OECD (1992) protocol. C. gariepinus was then exposed to sub-lethal concentrations corresponding to 6.25, 12.5, 25.0, 50.0 and 100% of the 96-h LC₅₀ for 7 days to assess the cytogenotoxic effects using piscine micronucleus (MN) test. The 96-h LC₅₀ showed that the metals/metalloid demonstrated differential interactions in a concentration dependent pattern. The 96-h LC₅₀ showed that Cr was the most toxic while Cu and As:Cu were indeterminate (Cr > Cr:Cu > As:Cr > As > As:Cr:Cu > Cu = As:Cu indeterminate). Isobologram and synergistic ratio (SR) models predicted antagonistic interaction between Cu:Cr and As:Cr and synergism between As:Cu in the causation of morbidity and mortality of C. gariepinus. Interaction factor model predicted antagonism as common interactive mechanism among the metal/metalloid mixtures in the induction of MN and abnormal nuclear erythrocytes in C. gariepinus. Predicted interactions among the three metals/ metalloid were largely antagonism and synergism towards the induction of acute toxicity and cytogenotoxicity. The models employed herein may be useful in establishing environmental safe limits for mixtures of metals/metalloids against the induction of acute toxicity and DNA damage in lower aquatic vertebrates.

Species and life-stage specific differences in cadmium accumulation and cadmium induced oxidative stress, metallothionein and heat shock protein responses in white sturgeon and rainbow trout

Understanding the mechanistic basis of differences in the sensitivity of fishes to metals is important for developing informed ecological risk assessment approaches for metals. Whole body metal accumulation, metallothionein induction, oxidative stress and associated antioxidant response, as well as heat shock proteins (mainly HSP70) are known to play important roles in determining the toxicity of metals in fish. Hence, in this study we have cross-evaluated these parameters as a function of Cd exposure across different life-stages of two evolutionarily distinct fish species, namely rainbow (Oncorhynchus mykiss) trout and white sturgeon (Acipenser transmontanus). These two species have been shown to differ significantly in their physiological and apical responses to Cd exposure. The findings of the present study suggest that species-specific differences in the sensitivity to Cd could partially be explained by HSP70 gene response and oxidative damage biomarkers. However, not all the parameters studied in this study could explain the life-stage specific differences universally and were limited to only some life-stages. Based on the observations in the present study and other recent studies, it is apparent that species- and life-stage specific differences in the sensitivity to Cd and possibly other metals is a complex phenomenon and could be driven by multiple toxicokinetic and toxicodynamic factors.

Sub-lethal effects of waterborne copper in early developmental stages of rainbow trout (Oncorhynchus mykiss)

The aim of this work was to study the impact of copper during a sub-chronic exposure to environmental concentrations in the early life stages of rainbow trout (Oncorhynchus mykiss). Eyed-stage embryos of rainbow trout, at 265 °D, were exposed in semi-static conditions to sub-lethal concentrations of CuSO₄ up to the larval stage (528 °D) under laboratory-controlled conditions. During 3 weeks, they were exposed to the environmentally-realistic concentration of 2 µg/L Cu and to a 10-fold higher concentration, 20 µg/L Cu. Several biological (survival, hatching success, malformation, growth) and behavioral (swimming activity) and molecular endpoints (genotoxicity and gene transcription) were studied. Exposure to 20 µg/L Cu had an inhibitory effect on hatching and increased half-hatched embryos (25%). At the end of the exposure, no significant differences were observed in growth of the larvae exposed to the highest Cu concentration. However, larvae exposed to 2 µg/L Cu exhibited increased growth in comparison with non-exposed larvae. The percentage of malformed larvae was significantly higher for both copper conditions, with skeletal malformations being the most observed. Expression of several genes was evaluated in whole larvae using quantitative real-time PCR. Genes involved in detoxification (gst, mt1 and mt2) and in cell cycle arrest (p53) were significantly repressed in both copper conditions when compared to control. In addition, potential genotoxic effects on larvae were investigated by the comet assay on blood cells, but this test did not demonstrate any significant DNA damage on larvae exposed to copper. This study confirms the adverse effects of copper on early life stages of rainbow trout even at the lowest environmentally relevant tested concentration.

Investigating the Role of Ionoregulatory Processes in the Species- and Life-Stage-Specific Differences in Sensitivity of Rainbow Trout and White Sturgeon to Cadmium

There are huge variations in life-stage- and species-specific sensitivities among the fishes to the exposure with metals; however, the physiological mechanisms underlying these differences are not well understood to date. This study revealed significant life-stage-specific (larval, swim-up, and juvenile) and species-specific differences between two evolutionary distant species of fishes, rainbow trout ( Oncorhynchus mykiss) and white sturgeon ( Acipenser transmontanus), following acute exposures to Cd. Although the 96 h LC50 of Cd was similar in both species at the larval stage, trout demonstrated an increased sensitivity to Cd at later life stages as compared to sturgeon. Moreover, exposure to Cd disrupted calcium (Ca) uptake and whole body Ca levels in trout by a greater degree relative to that in sturgeon regardless of life stage. Finally, white sturgeon demonstrated a lower affinity for Cd uptake relative to the more sensitive rainbow trout. This infers a differential nature of the interaction between Cd and Ca transport pathways in the two species and partially explains the differences in Cd sensitivity between rainbow trout and white sturgeon described previously. Overall, our results suggest that species- and life-stage-specific differences in sensitivity to waterborne Cd in fish are likely a function of the interplay between Cd uptake and Cd-induced disruption of Ca homeostasis.

Expression stability and selection of optimal reference genes for gene expression normalization in early life stage rainbow trout exposed to cadmium and copper

Gene expression analysis represents a powerful approach to characterize the specific mechanisms by which contaminants interact with organisms. One of the key considerations when conducting gene expression analyses using quantitative real-time reverse transcription-polymerase chain reaction (qPCR) is the selection of appropriate reference genes, which is often overlooked. Specifically, to reach meaningful conclusions when using relative quantification approaches, expression levels of reference genes must be highly stable and cannot vary as a function of experimental conditions. However, to date, information on the stability of commonly used reference genes across developmental stages, tissues and after exposure to contaminants such as metals is lacking for many vertebrate species including teleost fish. Therefore, in this study, we assessed the stability of expression of 8 reference gene candidates in the gills and skin of three different early life-stages of rainbow trout after acute exposure (24h) to two metals, cadmium (Cd) and copper (Cu) using qPCR. Candidate housekeeping genes were: beta actin (b-actin), DNA directed RNA polymerase II subunit I (DRP2), elongation factor-1 alpha (EF1a), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), glucose-6-phosphate dehydrogenase (G6PD), hypoxanthine phosphoribosyltransferase (HPRT), ribosomal protein L8 (RPL8), and 18S ribosomal RNA (18S). Four algorithms, geNorm, NormFinder, BestKeeper, and the comparative ΔCt method were employed to systematically evaluate the expression stability of these candidate genes under control and exposed conditions as well as across three different life-stages. Finally, stability of genes was ranked by taking geometric means of the ranks established by the different methods. Stability of reference genes was ranked in the following order (from lower to higher stability): HPRT<GAPDH<EF1a<G6PD<RPL8<DRP2<b-actin in gills of fish exposed to Cd; b-actin<GAPDH<G6PD<DRP2<RPL8<HPRT<EF1a in gills of fish exposed to Cu; RPL8<HPRT<GAPDH<G6PD<EF1a<DRP2<b-actin in the skin of fish exposed to Cd; and EF1a<GAPDH<RPL8<HPRT<G6PD<b-actin<DRP2 in the skin of fish exposed to Cu. The results demonstrated that the stability of reference genes depended on the metal, life-stage and/or organ in question. Thus, attention should be paid to these factors before selection of reference gene for relative quantification of the gene expressions.

Linking Oxidative Stress and Magnitude of Compensatory Responses with Life-Stage Specific Differences in Sensitivity of White Sturgeon (Acipenser transmontanus) to Copper or Cadmium

Sensitivity of white sturgeon (Acipenser transmontanus) to copper (Cu) or cadmium (Cd) has been shown to significantly differ as a function of life-stage. This study investigated oxidative stress, metal homeostasis, and associated compensatory responses as potential mechanisms of this sensitivity pattern in three early life-stages. Sturgeon were most sensitive to Cu at 15 days post hatch (dph), which was accompanied by a significant increase in lipid peroxidation (LPO). Genes involved with amelioration of oxidative stress were significantly less inducible at this stage than in older, less sensitive fry. At 48 dph, acute lethality of sturgeon exposed to Cd was greatest and body LPO was significantly induced by 3.5-fold at 5 μg Cd/L. Moreover, there was a small but significant increase in antioxidative responses. At 139 dph, sturgeon were most tolerant to Cu and Cd and accumulation of these metals was least. Also, expression of metallothionein (MT) and apoptotic genes were greatest while expression of metal transporters was reduced and concentration of LPO was not different from controls. Our results suggest that life-stage specific sensitivity of white sturgeon to metals is complex, encompassing differences in the ability to mount compensatory responses important for metal homeostasis and combating oxidative stress and concomitant damages.

Stability evaluation of reference genes for real-time PCR in zebrafish (Danio rerio) exposed to cadmium chloride and subsequently infected by bacteria Aeromonas hydrophila

Environmental and occupational cadmium (Cd) toxicity is a global concern, and the model organism zebrafish is an ideal species to investigate Cd toxicity. Among various detecting techniques, quantitative real-time PCR (qPCR) is a sensitive and efficient tool. Stable reference genes are critical for relative qPCR analysis. However, accumulated evidence shows that conventional reference genes can vary significantly under different experimental setups. Here we evaluated the stability of eight candidate reference genes of zebrafish with or without exposure to different concentrations of Cd. The results showed that the best four suitable reference genes in the five selected organs were: (1) spleen: β-actin>gapdh>ef1α>rpl13α; (2) kidney: rplp2>rpl7>β-actin>ef1α; (3) liver: rpl7>rpl13α>β-actin>ef1α; (4) gills: rplp2>gapdh>rnf7>ef1α; (5) intestine: ef1α>rnf7>rplp2>rpl13α. Moreover, we further assessed the expression stability of the four reference genes for Cd immunotoxicology studies in zebrafish. The expression profiles showed that ef1α in spleen and kidney, rpl13a in liver and rplp2 in intestine were the most suitable reference genes at 12h and 9 days after the injection with Aeromonas hydrophila following Cd exposure. In gills, the expression of gapdh was more stable than ef1α after 9 days of bacteria challenge while ef1α showed a higher stability than gapdh at 12h after bacteria injection. In conclusion, this study has demonstrated that different tissues of zebrafish have different suitable reference genes after Cd exposure and the subsequently pathogenic insults for qPCR. It emphasized the importance of reference gene evaluation for studies using qPCR, in particular when investigations involve factors not explored previously.