Effects of chronic exposure to dietary selenomethionine on the physiological stress response in juvenile white sturgeon (Acipenser transmontanus)

Cross-species apical microinjected selenomethionine toxicity in embryo-larval fishes

Selenium (Se) is an essential micronutrient that becomes toxic when exposures minimally exceed those that are physiologically required. Studies on Se contaminated aquatic environments have identified that embryo-larval fishes are at particular risk of Se toxicity, primarily due to maternal Se transfer to developing eggs during oogenesis. This study emulated these exposures in embryo-larval fathead minnow (FHM), rainbow trout (RBT), white sucker (WSu), and white sturgeon (WSt) using embryonic selenomethionine (SeMet) microinjections. Adverse Se-outcomes observed across these species included spinal and edematous deformities, total individuals deformed, and reduced survival. Spinal deformity was the most sensitive sublethal endpoint and developed at the lowest concentrations in WSt (10 % effects concentration (EC10) = 12.42 μg (total) Se/g dry weight (d.w.)) followed by WSu (EC10 = 14.49 μg Se/g d.w.) and FHM (EC10 = 18.10 μg Se/g d.w.). High mortality was observed in RBT, but SeMet influences were confounded by the species' innate sensitivity to the microinjections themselves. 5 % hazardous concentrations derived across exposure type data subsets were ∼49 % higher when derived from within-species maternal transfer exclusive data as opposed to all, or within-species microinjection exclusive, data. These results support the current exclusion of SeMet microinjections during regulatory guideline derivation and their inclusion when studying mechanistic Se toxicity across phylogenetically distant fishes.

Prolonged maternal exposure to glucocorticoids alters selenoprotein expression in the developing brain

Aberrant activation of the stress-response system in early life can alter neurodevelopment and cause long-term neurological changes. Activation of the hypothalamic-pituitary-adrenal axis releases glucocorticoids into the bloodstream, to help the organism adapt to the stressful stimulus. Elevated glucocorticoid levels can promote the accumulation of reactive oxygen species, and the brain is highly susceptible to oxidative stress. The essential trace element selenium is obtained through diet, is used to synthesize antioxidant selenoproteins, and can mitigate glucocorticoid-mediated oxidative damage. Glucocorticoids can impair antioxidant enzymes in the brain, and could potentially influence selenoprotein expression. We hypothesized that exposure to high levels of glucocorticoids would disrupt selenoprotein expression in the developing brain. C57 wild-type dams of recently birthed litters were fed either a moderate (0.25 ppm) or high (1 ppm) selenium diet and administered corticosterone (75 μg/ml) via drinking water during postnatal days 1 to 15, after which the brains of the offspring were collected for western blot analysis. Glutathione peroxidase 1 and 4 levels were increased by maternal corticosterone exposure within the prefrontal cortex, hippocampus, and hypothalamus of offspring. Additionally, levels of the glucocorticoid receptor were decreased in the hippocampus and selenoprotein W was elevated in the hypothalamus by corticosterone. Maternal consumption of a high selenium diet independently decreased glucocorticoid receptor levels in the hippocampus of offspring of both sexes, as well as in the prefrontal cortex of female offspring. This study demonstrates that early life exposure to excess glucocorticoid levels can alter selenoprotein levels in the developing brain.

Selenium in Bodily Homeostasis: Hypothalamus, Hormones, and Highways of Communication

The ability of the body to maintain homeostasis requires constant communication between the brain and peripheral tissues. Different organs produce signals, often in the form of hormones, which are detected by the hypothalamus. In response, the hypothalamus alters its regulation of bodily processes, which is achieved through its own pathways of hormonal communication. The generation and transmission of the molecules involved in these bi-directional axes can be affected by redox balance. The essential trace element selenium is known to influence numerous physiological processes, including energy homeostasis, through its various redox functions. Selenium must be obtained through the diet and is used to synthesize selenoproteins, a family of proteins with mainly antioxidant functions. Alterations in selenium status have been correlated with homeostatic disturbances in humans and studies with animal models of selenoprotein dysfunction indicate a strong influence on energy balance. The relationship between selenium and energy metabolism is complicated, however, as selenium has been shown to participate in multiple levels of homeostatic communication. This review discusses the role of selenium in the various pathways of communication between the body and the brain that are essential for maintaining homeostasis.

Effects of lithium and selenium in the tail muscle of American bullfrog tadpoles (Lithobates catesbeianus) during premetamorphosis

The amphibian populations have faced a drastic decline over the past decades. This decline has been associated with the presence of contaminants in the environment, among other environmental stressors. The present study tested the responses following the exposure to lithium (2.5 mg L^-1) and selenium (10μg L^-1), both isolated and as a mixture, on the metabolic status of the tail muscle of premetamorphic American bullfrog (Lithobates catesbeianus) through the assessment of the total protein content, mobilization of glucose and triglycerides, and the activity of lactate dehydrogenase (LDH). The exposure followed a 21-day assay with two sampling periods (on the 7th and 21st day after the onset of exposure) to evaluate the effects over time. The group exposed to the mixture presented a statistically decreased LDH activity (P < 0.05) in both sampling periods. The presence of selenium elicited a statistically significant increase (P < 0.05) in the glucose mobilization after 7 days of exposure. After 21 days, the animals exposed to selenium presented levels of glucose mobilization comparable to the control group. The mobilization of glucose and triglycerides remained similar to the control group for the animals exposed to lithium and to the mixture in both periods of sampling (P > 0.05). The total protein content did not show any statistical difference in the treated groups throughout the experiment (P > 0.05). The presented results highlight the importance of the assessment of mixtures that can occur in the environment, since the combination of contaminants may elicit distinct toxicity compared with the effects triggered by the chemicals isolated.

The lampricide 3-trifluoromethyl-4-nitrophenol causes temporary metabolic disturbances in juvenile lake sturgeon (Acipenser fulvescens): implications for sea lamprey control and fish conservation

The pesticide 3-trifluoromethyl-4-nitrophenol (TFM) is applied to rivers and streams draining into the Laurentian Great Lakes to control populations of invasive sea lamprey (Petromyzon marinus), which are ongoing threats to fisheries during the lamprey's hematophagous, parasitic juvenile life stage. While TFM targets larval sea lamprey during treatments, threatened populations of juvenile lake sturgeon (Acipenser fulvescens), particularly young-of-the-year (<100 mm in length), may be adversely affected by TFM when their habitats overlap with larval sea lamprey. Exposure to TFM causes marked reductions in tissue glycogen and high energy phosphagens in lamprey and rainbow trout (Oncorhynchus mykiss) by interfering with oxidative ATP production in the mitochondria. To test that environmentally relevant concentrations of TFM would similarly affect juvenile lake sturgeon, we exposed them to the larval sea lamprey minimum lethal concentration (9-h LC99.9), which mimicked concentrations of a typical lampricide application and quantified energy stores and metabolites in the carcass, liver and brain. Exposure to TFM reduced brain ATP, PCr and glycogen by 50-60%, while lactate increased by 45-50% at 6 and 9 h. A rapid and sustained depletion of liver glucose and glycogen of more than 50% was also observed, whereas the respective concentrations of ATP and glycogen were reduced by 60% and 80% after 9 h, along with higher lactate and a slight metabolic acidosis (~0.1 pH unit). We conclude that exposure to environmentally relevant concentrations of TFM causes metabolic disturbances in lake sturgeon that can lead to impaired physiological performance and, in some cases, mortality. Our observations support practices such as delaying TFM treatments to late summer/fall or using alternative TFM application strategies to mitigate non-target effects in waters where lake sturgeon are present. These actions would help to conserve this historically and culturally significant species in the Great Lakes.

The adverse effects of selenomethionine on skeletal muscle, liver, and brain in the steelhead trout (Oncorhynchus mykiss)

Juvenile Oncorhynchus mykiss (average weight: 22.3 g) were fed one of five selenomethionine diets (1.09, 8.79, 15.37, 30.79, or 61.58 mg Se/kg diet). After 4 weeks, hepatic catalase activity over 15.37 mg Se/kg diets was significantly decreased, and the glutathione peroxidase activity over 30.79 mg Se/kg diets was elevated compared to the controls. In the brain, the dopamine levels at 61.58 mg Se/kg diet and the serotonin levels over 15.37 mg Se/kg diets were significantly increased, whereas the 3,4-dihydroxyphenylacetic acid, homovanillic acid, and dopamine turnover, and the 5-hydroxyindoleacetic acid and serotonin turnover over 30.79 mg Se/kg diets were decreased. In muscle, the 3-nitrotyrosine level over 15.37 mg Se/kg diets, acetylcholine esterase activity over 30.79 mg Se/kg diets, and histological alterations over 8.79 mg Se/kg diets were increased. Our current results showed that selenomethionine disrupted dopamine and serotonin metabolism in the brain and damaged the neuromuscular system in skeletal muscle.

Examining the Effects of Chronic Selenium Exposure on Traditionally Used Stress Parameters in Juvenile American Alligators (Alligator mississippiensis)

Environmental contaminants, such as the trace element selenium (Se), are a continuing concern to species worldwide due to their potential pathophysiological effects, including their influence on the stress response mediated through glucocorticoids (GCs; stress hormones). Environmental concentrations of Se are increasing due to anthropogenic activities, including the incomplete combustion of coal and subsequent disposal of coal combustion wastes. However, most studies examining how Se affects GCs have been focused on lower trophic organisms. The objectives of this study were to investigate the effects of long-term Se exposure on traditionally used stress parameters and to identify which of these parameters best indicate Se accumulation in liver and kidney of the American alligator (Alligator mississippiensis), a top trophic carnivore found in the southeastern United States and known to inhabit Se-containing areas. Alligators were divided into three dietary treatments and fed prey spiked with 1000 or 2000 ppm of selenomethionine (SeMet) or deionized water (control treatment) for 7 weeks. Following the 7-week treatment protocol, blood and tissue samples were obtained to measure plasma corticosterone (CORT; the main crocodilian GC), tail scute CORT, the ratio of peripheral blood heterophils (H) to lymphocytes (L) as H/L ratio, and body condition. To evaluate which parameter best indicated Se accumulation in the liver and kidney, principal component and discriminant analyses were performed. The only parameter significantly correlated with liver and kidney Se concentrations was scute CORT. Our results suggest that measurement of CORT in tail scutes compared with plasma CORT, H/L ratios, and body condition is the best indicator of Se-exposure and accumulation in crocodilians.

Influences of dietary selenomethionine exposure on tissue accumulation, blood biochemical profiles, gene expression and intestinal microbiota of Carassius auratus

A 30-days feeding trail was conducted to determine the sensitivity of Carassius auratus to the toxicological effects of elevated dietary Selenomethionine (Se-Met). C. auratus averaging 23.56 ± 1.82 g were exposed to four Se-Met concentrations (mg Se/kg): 0 (Se-Met0), 5 (Se-Met5), 10 (Se-Met10) and 20 (Se-Met20) to estimate the effects on tissue selenium (Se) accumulation, blood biochemical profiles, transcript expression and intestinal microbiota. Se accumulated in the kidney, liver and muscle in a dose-dependent manner and followed this order: kidney > liver > muscle, the highest accumulation were obtained in kidney of Se-Met20 diet after 30 days of feeding. Serum contents of alanine transaminase (ALT), aspartate transaminase (AST) and alkaline phosphatase (ALP) in fish exposed to Se-Met20 group was significantly highest among Se-Met exposure groups. Hydrogen peroxide (H₂O₂) concentrations in liver were affected by dietary Se-Met exposures. Liver contents of total antioxidant capacity (TAC), catalase (CAT), glutathione peroxidase (GPx) and malondialdehyde (MDA) in fish exposure to Se-Met5 group was significantly highest among Se-Met exposure groups. Growth hormone receptor (GHR), insulin-like growth factor 1 (IGF-1) and antioxidant enzyme related genes including glutathione peroxidase (GPx), catalase (CAT) and glutathione S-transferase (GST) expression in liver were down-regulated with the concentration of Se-Met exposure groups. The results of high-throughput sequencing showed that gut microbial communities and hierarchy cluster heatmap analysis were significantly affected by Se-Met exposure. The abundances of Cetobacterium and Vibrio increased while fish exposed to Se-Met20 group. The abundance of Ralstonia increased when the Se-Met exposure dose reached 10 mg Se kg^-1. The results suggested that the exposure to elevated dietary Se-Met may result toxic effects in C. auratus.

Transcriptomic and cortisol analysis reveals differences in stress alleviation by different methods of anesthesia in Crucian carp (Carassius auratus)

Stress response has negative effect on fish in aquaculture and research, which can be alleviated with anesthetic. To determine the optimal anesthetic, we investigated the physiological response of crucian carp (Carassius auratus) treated with three different anti-stress treatments: MS-222, eugenol and percussive stunning. Stress responses were evaluated by analyzing serum cortisol level and gene expression in blood. We determined the optimal concentrations of MS-222 (100 mg L^-1) and eugenol (20 mg L^-1) by dose selection. We found that the control group had significantly higher cortisol levels (172.78 ± 19.95 ng mL^-1) compared to the MS-222 treated group (46.85 ± 3.22 ng mL^-1), the eugenol treated group (72.78 ± 9.07 ng mL^-1), and the stunning treatment group (82.78 ± 8.16 ng mL^-1). Transcriptome analysis revealed 1572 differentially expressed genes (DEGs), including 155 DEGs related to the stress response, mainly involved in oxidative-stress response, heat shock proteins, and cold shock domain-containing protein. The heat shock protein genes were the primary DEGs in response to stress. RT-qPCR analysis confirmed differential expression of Hsps. We analyzed the function of the DEGs, which were enriched in genes involved in cellular response to stress and antigen processing and presentation. Combining the results from biochemical, transcriptome, and gene expression analysis, our data suggest that eugenol is more effective than MS-222 and percussive stunning in alleviating stress in crucian carp.

Deoxynivalenol decreased the growth performance and impaired intestinal physical barrier in juvenile grass carp (Ctenopharyngodon idella)

Deoxynivalenol (DON) is one of the most common mycotoxin contaminants of animal feed worldwide and brings significant threats to the animal production. However, studies concerning the effect of DON on fish intestine are scarce. This study explored the effects of DON on intestinal physical barrier in juvenile grass carp (Ctenopharyngodon idella). A total of 1440 juvenile grass carp (12.17 ± 0.01 g) were fed six diets containing graded levels of DON (27, 318, 636, 922, 1243 and 1515 μg/kg diet) for 60 days. This study for the first time documented that DON caused body malformation in fish, and histopathological lesions, oxidative damage, declining antioxidant capacity, cell apoptosis and destruction of tight junctions in the intestine of fish. The results indicated that compared with control group (27 μg/kg diet), DON: (1) increased the reactive oxygen species (ROS), malondialdehyde (MDA) and protein carbonyl (PC) content, and up-regulated the mRNA levels of Kelch-like-ECH-associated protein 1 (Keap1: Keap1a but not Keap1b), whereas decreased glutathione (GSH) content and antioxidant enzymes activities, and down-regulated the mRNA levels of antioxidant enzymes (except GSTR in MI) and NF-E2-related factor 2 (Nrf2), as well as the protein levels of Nrf2 in fish intestine. (2) up-regulated cysteinyl aspartic acid-protease (caspase) -3, -7, -8, -9, apoptotic protease activating factor-1 (Apaf-1), Bcl2-associated X protein (Bax), Fas ligand (FasL) and c-Jun N-terminal protein kinase (JNK) mRNA levels, whereas down-regulated B-cell lymphoma-2 (bcl-2) and myeloid cell leukemia-1 (Mcl-1) mRNA levels in fish intestine. (3) down-regulated the mRNA levels of ZO-1, ZO-2b, occludin, claudin-c, -f, -7a, -7b, -11 (except claudin-b and claudin-3c), whereas up-regulated the mRNA levels of claudin-12, -15a (not -15b) and myosin light chain kinase (MLCK) in fish intestine. All above data indicated that DON caused the oxidative damage, apoptosis and the destruction of tight junctions via Nrf2, JNK and MLCK signaling in the intestine of fish, respectively. Finally, based on PWG, FE, PC and MDA, the safe dose of DON for grass carp were all estimated to be 318 μg/kg diet.

Dietary Selenomethionine Administration and Its Effects on the American Alligator (Alligator mississippiensis): Oxidative Status and Corticosterone Levels

Selenium (Se) is an essential nutrient which in excess causes toxicity. The disposal of incompletely combusted coal, which often is rich in Se, into aquatic settling basins is increasing the risk of Se exposure worldwide. However, very few studies have looked at the physiological effects of Se exposure on long-lived, top trophic vertebrates, such as the American alligator (Alligator mississippiensis). During a 7-week period, alligators were fed one of three dietary treatments: mice injected with deionized water or mice injected with water containing 1000 or 2000 ppm selenomethionine (SeMet). One week after the last feeding alligators were bled within 3 min of capture for plasma corticosterone (CORT). A few days later, all alligators were euthanized and whole blood and tail tissue were harvested to measure oxidative damage, an antioxidant-associated transcription factor, and antioxidant enzymes [glutathione peroxidase-1 (GPX1), superoxide dismutase-1 (SOD1), and SOD2] by Western blotting. There was a dose-dependent increase in baseline CORT levels in alligators administered SeMet. Except for blood SOD2 levels, SeMet treatment had no effect (p > 0.05 for all) on oxidative status: oxidative damage, GPX1, SOD1, and muscle SOD2 levels were similar among treatments. Our results illustrate that high levels of Se may act as a stressor to crocodilians. Future studies should investigate further the physiological effects of Se accumulation in long-lived, top-trophic vertebrates.

Bioaccumulation of mercury and other metal contaminants in invasive lionfish (Pterois volitans/miles) from Curaçao

A wide range of ecological and environmental factors influence metal bioaccumulation in fish. Studies of mercury and other metal contaminants in invasive Indo-Pacific lionfish are limited, yet consumption of the invasive predator is increasingly utilized as a management strategy. In this study, we examined the effects of body size, body condition, sex, trophic level, carbon source, diet, depth and capture location on mercury concentrations in lionfish collected from Curaçao. In addition, we examined whether or not a local petroleum refinery is the source of metal contamination in lionfish. Mercury concentrations ranged from 0.008 to 0.106 mg/kg and we found no effect of the petroleum refinery on metal bioaccumulation in lionfish. Low concentrations of metal contaminants indicate lionfish from Curaçao are safe for human consumption.

Dietary supplementation of biofloc influences growth performance, physiological stress, antioxidant status and immune response of juvenile sea cucumber Apostichopus japonicus (Selenka)

Bioflocs are rich in various probiotics and bioactive compounds, which play an important role in improving growth and health status of aquatic organisms. A 60-day experiment was conducted to investigate the effects of dietary supplementation of biofloc on growth performance, digestive enzyme activity, physiological stress, antioxidant status, expression of immune-related genes and disease resistance of sea cucumber Apostichopus japonicus. Juvenile sea cucumbers were fed five experimental diets containing graded levels of biofloc from 0% to 20% (referred as B0, B5, B10, B15 and B20, respectively). The results showed that the sea cucumbers at dietary supplementation levels of 10%-15% biofloc had significantly higher specific growth rate (SGR) compared to control group (diet B0). Digestive enzyme activity increased with the increasing of dietary biofloc level, while no significant difference was found between diets B15 and B20. Dietary supplementation of biofloc also had significant influences on physiological stress parameters except for lactate. There was no significant discrepancy in total coelomocytes counts (TCC) in coelomic fluid of sea cucumber between the treatments. Phagocytosis and respiratory burst of cellular immune at 15% and 20% biofloc levels were significantly higher than those of control group. Significant increases in superoxide dismutase (SOD), total nitric oxide synthase (T-NOS), lysozyme (LSZ), acid phosphatase (ACP) and alkaline phosphatase (AKP) activities of sea cucumber were found at highest dietary supplementation level of 20% biofloc. The expression patterns of immune-related genes (i.e., Hsp90, Hsp70, p105, Rel, NOS and LSZ) in tissues of sea cucumber were analyzed between the experimental diets, and a general trend of up-regulation was observed at higher biofloc levels. Furthermore, dietary 10%-20% biofloc significantly reduced cumulative mortality of sea cucumber after being challenged with Vibrio splendidus. In conclusion, dietary supplementation of biofloc could improve growth performance of A. japonicus, by increasing digestive enzyme activity, releasing physiological stress, enhancing immune response and disease resistance of sea cucumber. The suitable supplemental level of approximately 15% biofloc was recommended in the present study.