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

Trace metals in the teleost fish gill: biological roles, uptake regulation, and detoxification mechanisms

In fish, the gill plays a vital role in regulating the absorption of trace metals and is also highly susceptible to metal toxicity. Trace metals such as iron (Fe), copper (Cu), zinc (Zn), and manganese (Mn) are involved in various catalytic activities and molecular binding within the gill, thereby supporting a range of physiological processes in this organ. While beneficial at normal levels, these metals can become toxic when present in excess. Conversely, nonessential metals like cadmium (Cd) and lead (Pb) can gain entry into gill cells through similar metal transport pathways, potentially interfering with various cellular processes. The transepithelial transport of these metals across the gill epithelium is governed by a variety of metal transport and metal binding proteins. These include the Cu transporter 1 (CTR1), divalent metal transporter 1 (DMT1), and members of the Zrt-/Irt-like protein (ZIP) and zinc transport (ZnT) families. Additionally, some of these metals can compete with major ions (e.g., calcium, sodium) for absorption sites in the gill. This complex crosstalk suggests an interdependent mechanism that balances metal uptake to meet physiological needs while preventing excessive accumulation. In this article, I review the roles of trace metals in proteins/enzymes that support the different functions in the gill of teleost fish. I also discuss current understanding of the pathways involved in regulating the branchial uptake of metals and their influence on ionic regulation, and the potential detoxification mechanisms in the gill. Finally, I summarize knowledge gaps and potential areas for further investigation.

Dynamics of copper regulation in a marine clam Sinonovacula constricta at the organ level: Insight from a physiologically based pharmacokinetic model

Copper (Cu) is a common pollutant in estuaries and has received considerable attention worldwide. To gain an insight into the physiological mechanisms of waterborne Cu absorption, tissue distribution, storage, metabolism, and excretion in an estuarine razor clam Sinonovacula constricta, we developed a physiologically-based pharmacokinetic model based on prolonged Cu exposure with two exposure treatments. The tissues of S. constricta were divided into four parts: blood, digestive gland, gill, and other tissues. Our results showed that the waterborne Cu entered and exchanged with the gills and digestive gland, whereas digestive gland and other tissues were the main storage sites for Cu. Gills of S. constricta were able to maintain their Cu concentrations under both exposure treatments. Additionally, the gills exhibited a remarkable ability to remove Cu from water, with a transfer rate constant of 1.73 d-1 from the gills to water, while restricting its transfer from the blood with a transfer rate constant of 0.0131 d-1 from blood to gills. These results highlighted the crucial role of gills in regulating Cu levels in S. constricta as well as the detoxification and maintenance of metal homeostasis. Cu uptake rate constant in gill from waterborne was similar to that of digestive gland (0.294 vs. 0.364 L g-1 d-1), thus water entering the digestive tract was considered as another route of waterborne Cu absorption in bivalves. A significant amount of Cu in the blood was transferred to the digestive glands. These two factors explained the relatively higher Cu accumulation in the digestive glands than in other tissues in clams. The findings of this study enhanced our understanding of the homeostatic regulation and transportation mechanisms in marine bivalves.

What contributes to the metal-specific partitioning in the chub-acanthocephalan system?

Physiologically based pharmacokinetic (PBPK) models have been applied to simulate the absorption, distribution, metabolism, and elimination of various toxicants in fish. This approach allows for considering metal accumulation in intestinal parasites. Unlike "semi" physiologically-based models developed for metals, metal accumulation in fish was characterised based on metal-specific parameters (the fraction in blood plasma and the tissue-blood partition coefficient) and physiological characteristics of the fish (the blood flow and the tissue weight) in our PBPK model. In the model, intestinal parasites were considered a sink of metals from the host intestine. The model was calibrated with data for the system of the chub Squalius cephalus and the acanthocephalan Pomphorhynchus tereticolliis. Metal concentrations in this fish-parasite system were monitored in Ag and Co treatments in duplicate during a 48-day exposure phase (Ag and Co were added to tap water at concentrations of 1 and 2 µg/L, respectively) and a 51-day depuration phase. Their concentrations in the gills increased during the exposure phase and decreased in the depuration phase. A similar pattern was observed for Ag concentrations in other chub organs, while a relatively stable pattern for Co indicates regulations in the accumulation of essential metals by chubs. The metals were taken up by the acanthocephalans at similar rate constants. These results indicate that metal availability to parasites, which is determined by the internal distribution and fate, is critical to metal accumulation in the acanthocephalans. The high concentration of Ag in the liver as well as the high rate of Ag excretion from the liver to the intestine might contribute to higher concentrations of metals in the bile complexes in the intestine, which are available to the parasites, but not to the reabsorption by the host intestine. The opposite pattern might explain the lower availability of Co to the acanthocephalans.

Responsive fluorescence enhancement for in vivo Cu(II) monitoring in zebrafish larvae

Several neurodegenerative diseases are ascribed to disorders caused by the secretion of Cu ions. However, a majority of the current techniques for copper ion detection are restricted to in vivo monitoring and nonspecific interactions. Their methods are limited to the systematic analysis of Cu ions in living organisms. Thus, a synthetic molecular fluorophore, 5-amino 2,3-dihydroquinolinimine (NDQI), has been developed and successfully utilized in in vivo monitoring of the distribution of Cu(II) in zebrafish larvae. The reversible formation of the NDQI-Cu complex allows its use with high metal concentrations and in oxidative stress conditions. The NDQI-directed strategy developed here can quantitatively differentiate cells with different Cu(II) concentrations. Remarkably, dynamic distribution of Cu(II) in the intestine and liver can be observed.

Expression and Function of ABC Proteins in Fish Intestine

In fish, the intestine is fundamental for digestion, nutrient absorption, and other functions like osmoregulation, acid-base balance, and excretion of some metabolic products. These functions require a large exchange surface area, which, in turn, favors the absorption of natural and anthropogenic foreign substances (xenobiotics) either dissolved in water or contained in the food. According to their chemical nature, nutrients, ions, and water may cross the intestine epithelium cells' apical and basolateral membranes by passive diffusion or through a wide array of transport proteins and also through endocytosis and exocytosis. In the same way, xenobiotics can cross this barrier by passive diffusion or taking advantage of proteins that transport physiological substrates. The entry of toxic substances is counterbalanced by an active efflux transport mediated by diverse membrane proteins, including the ATP binding cassette (ABC) proteins. Recent advances in structure, molecular properties, and functional studies have shed light on the importance of these proteins in cellular and organismal homeostasis. There is abundant literature on mammalian ABC proteins, while the studies on ABC functions in fish have mainly focused on the liver and, to a minor degree, on the kidney and other organs. Despite their critical importance in normal physiology and as a barrier to prevent xenobiotics incorporation, fish intestine's ABC transporters have received much less attention. All the ABC subfamilies are present in the fish intestine, although their functionality is still scarcely studied. For example, there are few studies of ABC-mediated transport made with polarized intestinal preparations. Thus, only a few works discriminate apical from basolateral transport activity. We briefly describe the main functions of each ABC subfamily reported for mammals and other fish organs to help understand their roles in the fish intestine. Our study considers immunohistochemical, histological, biochemical, molecular, physiological, and toxicological aspects of fish intestinal ABC proteins. We focus on the most extensively studied fish ABC proteins (subfamilies ABCB, ABCC, and ABCG), considering their apical or basolateral location and distribution along the intestine. We also discuss the implication of fish intestinal ABC proteins in the transport of physiological substrates and aquatic pollutants, such as pesticides, cyanotoxins, metals, hydrocarbons, and pharmaceutical products.

Physiologically based pharmacokinetic model revealed the distinct bio-transportation and turnover of arsenobetaine and arsenate in marine fish

Arsenobetaine (AsB) is the major form of arsenic in marine fish; however, its biodynamics within the fish tissues is not well understood. This study simulated the biodynamics and biotransportation (absorption, distribution, and elimination) of dietary AsB and arsenate [As(V)] in the marine grouper Epinephelus fuscoguttatus, by constructing a physiologically based pharmacokinetic (PBPK) model. The transfer rates between different compartments (gill, intestine, liver, heart, kidney, and muscle) and blood were modeled during exposure (14 d) and depuration (20 d). The model showed that AsB had a weak ability to cross the intestinal membranes and circulated slowly in the blood. The newly AsB absorbed from the blood did not enter the hepatointestinal circulation for elimination, but was effectively distributed in liver. Thereafter, it was slowly absorbed and finally stored in the muscle, the most important organ for AsB deposition, at a constant rate of 63.5 d^-1. In contrast, As(V) displayed a dynamic behavior, including rapid crossing through the intestinal membranes, quick circulation in the blood and transportation to other tissues, and elimination. Biodynamics coupled with biotransformation illustrated, for the first time, the unique strategies of dietary AsB that passed slowly through the fish intestine with the highest deposition rate in the muscle, thereby contributing to the high AsB bioaccumulation in the muscle tissue of marine fish. CAPSULE: AsB displayed a weaker ability to cross the intestine membranes, slowly absorbed and finally stored in muscle, whereas As(V) displayed rapid crossing the intestine membranes, quick transportation, and elimination.

Tissue-Specific Accumulation, Biotransformation, and Physiologically Based Toxicokinetic Modeling of Benzotriazole Ultraviolet Stabilizers in Zebrafish (Danio rerio)

Benzotriazole ultraviolet stabilizers (BUVSs) are high-production-volume chemicals with ubiquitous occurrence in the aquatic environment. However, little is known about their bioconcentration and biotransformation, and physiologically based toxicokinetic (PBTK) models for BUVSs are lacking. This study selected six BUVSs for which experiments were performed with zebrafish (Danio rerio) exposed to two different levels (0.5 and 10 μg·L^-1). Higher kinetic bioconcentration factors (BCFs) were observed at the lower exposure level with environmental relevance, with BCF of 3.33 × 10³ L·kg^-1 for 2-(2-hydroxy-3,5-di-tert-butylphenyl)-5-chlorobenzotriazole (UV-327). This phenomenon was interpreted by a nonlinear adsorption mechanism, where binding with specific protein sites contributes to bioconcentration. Muscle exhibited the lowest accumulation, in which depuration half-life of UV-327 was 19.5 d. In kidney, muscle, ovary, gill, and skin, logBCF increased with increase in log K_OW of the BUVSs until log K_OW was ca. 6.5, above which logBCF decreased. However, the trend was not observed in the liver and intestine. Six biotransformation products were identified and mainly accumulated in the liver and intestine. Considering the nonlinear adsorption mechanism in the PBTK model, the prediction accuracy of the model was improved, highlighting the binding of xenobiotics with specific protein sites in assessing the bioconcentration of chemicals for their risk assessment.

Physiologically Based Pharmacokinetic Model for the Biotransportation of Arsenic in Marine Medaka (Oryzias melastigma)

The toxicity of arsenic (As) targets specific tissues of organisms, while the biotransportation of As among the tissues of fish remains poorly understood. In the present study, radiotracer techniques followed by a physiologically based pharmacokinetic (PBPK) modeling were applied to simulate the biotransportation (absorption, distribution, and elimination) of ⁷³As(V) and biotransformation of As(V) in the marine medaka Oryzias melastigma after waterborne As exposure. Fish were simulated by a six-compartment model by assuming that blood was the intermediate exchange among different compartments (gill, intestine, liver, head, and carcass). Modeling suggested that intestine and gill were the uptake, exchange, as well as elimination sites of waterborne As, while carcass and head were the main storage sites. Intestine played a vital role in the metabolism of As(V) by biotransforming inorganic As into arsenobetaine (AsB), possibly because of the important role of gut microbiota. The correlation between the PBPK model constants and the As speciation (e.g., AsB %, inorganic As %, and methylated As %) indicated that AsB tended to be stored in the tissues rather than being depurated, while inorganic and methylated As were more easily transferred from tissues to the blood and eliminated. Modeling simulation coupling with biotransformation for the first time demonstrated that the fish intestine was the main metabolic site, and synthesis of AsB as mediated by the microbiota in the intestine contributed to the high As bioaccumulation in marine fish.

Determination of the Low Hg Accumulation in Rabbitfish (Siganus canaliculatus) by Various Elimination Pathways: Simulation by a Physiologically Based Pharmacokinetic Model

Mercury (Hg) in fish poses a great threat to human health. Consumption of low-Hg-level fish species (e.g., rabbitfish, Siganus canaliculatus) could be one possible solution to balance the nutrient benefits and Hg exposure. However, the underlying mechanisms for the low Hg accumulation in rabbitfish remain unclear. This study quantitatively described the disposition of inorganic Hg(II) and methylmercury (MeHg) in rabbitfish under different exposure routes by constructing a physiologically based pharmacokinetic (PBPK) model. The results strongly suggested that effective elimination (estimated rate constant of 0.060, 0.065, and 0.020 d^-1 for waterborne Hg(II)-, dietary Hg(II)-, and MeHg-exposed fish, respectively) was the main reason for the low Hg accumulation in rabbitfish. By quantifying the possible pathways for Hg elimination, our study revealed that biliary coupled with fecal excretion played an important role in the elimination of dietary Hg. Although the biliary excretion rate for MeHg was remarkable (6.8 ± 2.2 d^-1) and the excreted amount per day could reach up to 790 ng, most of the MeHg in the bile was reabsorbed by the intestine and transferred back to the liver through enterohepatic circulation, leading to a prolonged retention time in the fish body. Moreover, branchial excretion dominated the Hg(II) elimination following aqueous exposure, suggesting a flexible alteration on elimination pathways against different exposure scenarios. The present study provided important understanding of the unique strategies adopted by rabbitfish to maintain the low Hg levels.

Applications of dynamic models in predicting the bioaccumulation, transport and toxicity of trace metals in aquatic organisms

This review evaluates the three dynamic models (biokinetic model: BK, physiologically based pharmacokinetic model: PBPK, and toxicokinetic-toxicodynamic model: TKTD) in our understanding of the key questions in metal ecotoxicology in aquatic systems, i.e., bioaccumulation, transport and toxicity. All the models rely on the first-order kinetics principle of metal uptake and elimination. The BK model basically treats organisms as a single compartment, and is both physiologically and geochemically based. With a good understanding of each kinetic parameter, bioaccumulation of metals in any aquatic organisms can be studied holistically and mechanistically. Modeling efforts are not merely restrained from the prediction of metal accumulation in the tissues, but instead provide the direction of the key processes that need to be addressed. PBPK is more physiologically based since it mainly addresses the transportation, transformation and distribution of metals in the organisms. It can be treated conceptually as a multi-compartmental kinetic model, whereas the physiology is driving the development of any good PBPK model which is no generic for aquatic animals and contaminants. There are now increasingly applications of the PBPK modeling specifically in metal studies, which reveal many important processes that are impossible to be teased out by direct experimental measurements without adequate modeling. TKTD models further focus on metal toxicity in addition to metal bioaccumulation. The TK part links exposure and bioaccumulation, while the TD part links bioaccumulation and toxic effects. The separation of TK and TD makes it possible to model processes, e.g., toxicity modification by environmental factors, interaction between different metals, at both the toxicokinetic and toxicodynamic levels. TKTD models provide a framework for making full use of metal toxicity data, and thus provide more information for environmental risk assessments. Overall, the three models reviewed here will continue to provide guiding principles in our further studies of metal bioaccumulation and toxicity in aquatic organisms.

The Change of Metallothionein and Oxidative Response in Gills of the Oreochromis niloticus after Exposure to Copper

Simple Summary

Copper is an essential element for the aquatic organisms for a number of biological processes. However, it may be toxic at high concentrations. The present study revealed that the levels of Cu in gills of all Cu-exposed tilapia significantly increased during the first few days, and then gradually decreased, matching the control at D4-D5. The concentration of metallothionein (MT) and the activities of superoxide dismutase (SOD) and catalase (CAT) in the gills of Cu-exposed fish were in line with the accumulated Cu. The increase of MT, SOD, and CAT during the first few days might be the adaptive response of the animal to Cu toxicity. MT binds the elevated Cu, while SOD and CAT scavenge the increased free radicals due to the increasing level of Cu. Cu does not affect the malondialdehyde (MDA) concentration in gills of fish, which suggests the SOD, CAT and MT as antioxidant defense systems were able to completely scavenge the increased free radicals.

Abstract

In the present study, we investigated the effects of waterborne copper (Cu) on the levels of metallothionein (MT) and malondialdehyde (MDA), as well as activities of superoxide dismutase (SOD) and catalase (CAT) in gills of cichlid fish Oreochromis niloticus. The Cu concentrations in gills were measured using an atomic absorption spectrometer. The sandwich-ELISA was used to measure MT, SOD, CAT, and MDA. The Cu concentrations in gills of fish that were exposed to 1, 5, and 10 mg Cu/L were significantly increased at day 1 (D1), then gradually decreased starting from D2, and reaches the similar value with the controls at D5. A similar tendency has been observed in the MT levels in the gills. All of the Cu-exposed fish showed the highest level of MT on D1, and then decreased at D3 and a plateau at D4 and D5. The levels of SOD and CAT in gills in all Cu-exposed fish showed a similar pattern: increased significantly at D1, then gradually decreased starting from D2, and increased again at D4 and D5. The levels of MDA in gills of all Cu-exposed fish showed no significant difference. The indifference levels of MDA in gills of all Cu-exposed fish suggested the antioxidant defense systems (SOD and CAT) combined with the induction of MT were able to completely scavenge the increased ROS.

Physiologically based toxicokinetic and toxicodynamic (PBTK-TD) modelling of Cd and Pb exposure in adult zebrafish Danio rerio: Accumulation and toxicity

Accurately predicting the accumulation and toxicity of metals in organisms is a challenging work in ecotoxicology. Here, we developed and validated a physiologically based toxicokinetic and toxicodynamic (PBTK-TD) model for adult zebrafish exposed to Cd and Pb. The model included the gill, liver, intestine, gonad, carcass, and brain, which were linked by blood circulation in the PBTK process and by dynamic relationships between the target organ concentrations and mortality in the TD process. Results showed that the PBTK sub-model can accurately describe and predict the uptake, distribution and disposition kinetics of Cd and Pb in zebrafish. The exchange rates and the accumulation of the metals in the organs were significantly different. For Cd, the highest exchange rate was between blood and liver, and the greatest accumulation of Cd occurred in the liver. For Pb, the greatest accumulation occurred in the gill. The TD sub-model further indicated that metal concentrations in the gill may effectively act as more suitable indicator of Cd and Pb toxic effect than whole body or other organs. The proposed PBTK-TD model is helpful to understanding the fundamental processes by which zebrafish regulate the uptake and disposition of metal and to quantitatively predicting metal toxicity.

Management of Recurrent and De Novo NAFLD/NASH After Liver Transplantation

Nonalcoholic steatohepatitis (NASH) is a growing indication for liver transplant whether the primary or secondary cause of liver disease, and it is expected to be the leading indication in the years to come. Nonalcoholic steatohepatitis recurs after transplant but the impact of the recurrence on allograft and patient outcomes is unclear. A group of multidisciplinary transplant practice providers convened at the International Liver Transplantation Society NASH consensus conference with the purpose of determining the current knowledge and future directions for understanding the recurrence rates, risk and management of NASH in the transplant allograft. Specific questions relating to posttransplant NASH were proposed and reviewed in detail with recommendations on future actions to fill the knowledge gaps.

Physiologically based toxicokinetics (PBTK) models for pharmaceuticals and personal care products in wild common carp (Cyprinus carpio)

Pharmaceutical and personal care products (PPCPs) are regarded as "pseudo-persistent" pollutants due to their being continuously loaded into the aquatic environment. Physiologically based toxicokinetics (PBTK) models that can quantitatively describe absorption, distribution, metabolism and excretion processes of chemicals in biota are of importance to predict internal exposure (e.g. doses at specific target tissues/organs) from external exposure concentrations. In this study, PBTK models with up to six compartments including brain, liver, kidney, gills, richly perfused tissues and poorly perfused tissues, were developed for predicting internal distribution of 10 PPCPs in wild common carp (Cyprinus carpio). The PBTK predicted concentrations were close to the measured ones, with deviations less than 1 log unit for most of PPCPs. Sensitivity analysis showed that various partition coefficients of the chemicals exerted significant influence on model outputs.

Distribution of Al, Cd, Cu, Fe, Mn, Pb and Zn in Liver, Gills and Muscle of Early Life Stages of Atlantic Salmon (Salmo salar)

The aim of this study was to determine metal distribution in different tissues in early life stages of Atlantic salmon. The concentrations of the metals Al, Cd, Cu, Fe, Mn, Pb and Zn in liver, gills, muscle of fish and water from a salmon farm in the Region de los Lagos of southern Chile were determined. Results show that Cd and Pb had the lowest concentrations, while Zn the highest concentration, followed by Cu, Fe and Mn. Cu and Fe had the highest concentrations in liver, while Mn and Zn were the highest in gills. Cu and Fe in liver were higher in pre-smolts than in fingerlings. Cu had the highest bioaccumulation factor in pre-smolts, both in liver and gills. In conclusion the metals tend to bioaccumulate selectively in different tissues of Salmo salar. Liver is the most relevant for Cu and Fe, and liver and gills for Zn.

Selenium induces the demethylation of mercury in marine fish

The antagonistic effect of selenium (Se) on mercury (Hg) toxicity has been known for decades. Earlier studies mainly focused on Hg-Se interaction based on biokinetics and bioaccumulation, but the influences of Se on in vivo biotransformation of methylmercury (MeHg) have not been well understood. We conducted a 42-day exposure study to investigate the dynamic changes of MeHg and its primary degradation product - inorganic mercury (IHg) - in different organs of black seabream (Acanthopagrus schlegeli) exposed to different dietary Se levels. A physiologically based pharmacokinetic (PBPK) model was then developed to describe the biotransformation and disposition of MeHg under the influence of Se. Our results demonstrated that Se significantly increased the transformation from MeHg into IHg, thereby decreasing the accumulation of MeHg. The simulation further showed that the intestine was the major site for demethylation, with an estimated rate 1.5-fold higher in high Se treatment than in low Se treatment. However, the hepatic demethylation rate was extremely low and comparable between the two treatments (0.012-0.015 d^-1). These results strongly suggested that the intestine instead of the commonly assumed liver was the major site for Hg-Se interaction. Furthermore, Se did not show significant influences on the distribution and elimination of MeHg, but promoted the uptake and elimination of the generated IHg from demethylation. Therefore, Se-induced demethylation especially in the intestine played an important role in mitigating the MeHg accumulation. This study provided new sight to elucidate the Hg-Se interaction in fish.

In Vivo Mercury Demethylation in a Marine Fish (Acanthopagrus schlegeli)

Mercury (Hg) in fish has attracted public attention for decades, and methylmercury (MeHg) is the predominant form in fish. However, the in vivo MeHg demethylation and its influence on Hg level in fish have not been well-addressed. The present study investigated the in vivo demethylation process in a marine fish (black seabream, Acanthopagrus schlegeli) under dietary MeHg exposure and depuration and quantified the biotransformation and interorgan transportation of MeHg by developing a physiologically based pharmacokinetic (PBPK) model. After exposure, we observed a 2-fold increase of the whole-body inorganic Hg (IHg), indicating the existence of an in vivo demethylation process. The results strongly suggested that the intestine played a predominant role in MeHg demethylation with a significant rate (6.6 ± 1.7 day^-1) during exposure, whereas the hepatic demethylation appeared to be an extremely slow (0.011 ± 0.001 day^-1) process and could hardly affect the whole-fish Hg level. Moreover, demethylation in the intestine served as an important pathway for MeHg detoxification. Our study also pointed out that in vivo MeHg demethylation could influence Hg level and speciation in fish although food is the major pathway for Hg accumulation. Enhancing in vivo MeHg biotransformation (especially in the intestine) could be a potential key solution in minimizing Hg contamination in fish. The related factors involved in intestinal demethylation deserve more attention in the future.