Temporal control of responses to chemically induced oxidative stress in the gill mucosa of Atlantic salmon (Salmo salar)

Intracellular metabolome elucidates the time-of-day-dependent response to hydrogen peroxide in salmonid gill epithelial cells

The internal timekeeping system regulates the daily cycle of physiological and behavioural changes in living organisms. This rhythmic phenomenon also influences cellular responses to reactive oxygen species, such as hydrogen peroxide (H2O2). However, the temporal interaction between H2O2 and fish mucosal cells is not well understood. This study examined the temporal variations of immunological and physiological responses to H2O2 in salmonid gill cells using the RTgill-W1 cell line. The results showed that gene expression levels varied during a 24-h cycle but did not exhibit rhythmicity. The presence of a 12-h light-dark cycle (12L:12D) signal increased gene expression levels compared to a 24-h dark cycle (0L:24D). To investigate whether the time of day affects the defences in gills, cells were exposed to H2O2 at two different times (Zeitgebertime 2, ZT2, or ZT14). Although significant expression changes were observed in genes related to stress and NF-κB signalling, only a limited time-dependent pattern of response to H2O2 was observed. The intracellular metabolome of gill cells was primarily composed of organic acid and derivatives, organoheterocyclic compounds, benzoids, organic oxygen and nitrogen compounds. Exposure to H2O2 at ZT2 led to significant changes in the metabolome compared to the control group, while no such changes were observed at ZT14. Within the control groups, the concentrations of 11 metabolites significantly varied between ZT2 and ZT14, with higher levels at ZT14. These metabolites were involved in arginine biosynthesis, amino acid metabolism, and nitrogen metabolism. In contrast, the level of 26 metabolites significantly varied between ZT2 and ZT14 in H2O2-exposed groups, with lower levels at ZT14. Comparing control and H2O2-exposed groups at ZT2, 38 metabolites were affected, primarily organic acid and derivatives and organic oxygen compounds. Functional annotation revealed that these altered metabolites were involved in 15 different pathways, with valine, leucine, and isoleucine biosynthesis being the most affected. This study reveals the presence of a time-dependent response to H2O2 in salmonid gill cells, which is reflected in the intracellular metabolome. The findings provide new insights into the temporal regulation of mucosal defences in fish.

Can photoperiod improve growth performance and antioxidant responses of pacu (Piaractus mesopotamicus) reared in recirculation aquaculture systems?

The present study investigated the best photoperiod for culturing pacu (Piaractus mesopotamicus) in recirculation aquaculture systems (RAS) based on its growth performance and hematological and oxidative stress responses. Juveniles (∼ 5 g) were subjected to five treatments (in triplicate): 24 L (light):0D (dark), 15 L: 09D, 12 L:12D, 9 L:15D, and 0 L:24D for 45 days. A total of 225 pacu individuals were randomly distributed among 15 tanks of 210 L (n = 15 per tank). Zootechnical, hematological (glucose, lactate, hematocrit, and hemoglobin), and antioxidant and oxidative stress parameters (glutathione S-transferase (GST), total antioxidant capacity against peroxyl radicals (ACAP), and lipid peroxidation (LPO) were analyzed. The zootechnical parameters (e.g., weight gain, Fulton's condition factor, and specific growth rate) were better and worse with 9 L:15D and 24 L:0D photoperiods, respectively. The hepatosomatic index was higher and lower in the 0 L:24D and 9 L:15D photoperiods. Blood lactate levels and antioxidant and oxidative stress responses were increased in the longest photoperiods (15 L:9D and 24 L:0D). In contrast, the treatments that showed lower oxidative damage (liver, gills, brain, and muscle) were 9 L:15D and 12 L:12D. In conclusion, manipulating artificial light is one way to improve fish growth and health, where the best photoperiod for pacu farming in RAS is 9 L:15D.

Embracing prospects for reducing the numbers of animals used in aquaculture research

The principles of three Rs-REPLACEMENT, REDUCTION, and REFINEMENT-govern the protection and use of animals, including fish, for research purposes in the European Union and Norway. In this paper, we discuss some straightforward steps to simplify the delivery of these principles at the idea stage and adapt some of these examples for conducting fish trials related to health and welfare. Although some of the approaches are well established in other animal science arenas, we believe there can be a timely recap of their key facets. We discuss a number of simple strategies to emphasize how a reduction in fish numbers can be achieved from initial project conception to implementation, highlighting not only their advantages but also their limitations. We also highlight the role that funding agencies can play in the implementation of the 3R principles in aquaculture research. These simple points can be used in frameworks to initiate a broader and dynamic intersectoral dialogue among stakeholders of aquaculture research on how to promote ethics and embrace opportunities for this within the tenets of the 3Rs.

Superoxidase dismutases (SODs) in the European eel: Gene characterization, expression response to temperature combined with hormonal maturation and possible migratory implications

Superoxide dismutases (SODs) are antioxidant enzymes that protect cells from oxidation. Three SODs have been identified in mammals, but there is limited information in teleosts. This study investigates SODs in the European eel and their expression patterns during testis maturation. Phylogenetic and synteny analyses revealed SODs paralogs and their evolution in vertebrates. The eel possesses one SOD1 and two SOD2/3 (a and b), indicating SOD2 and SOD3 duplication in elopomorphs. SODs expression were then evaluated in various male and female tissues. SOD1 is more expressed in females, while SOD2a and SOD2b dominate brain-pituitary-gonad tissues in both sexes. SOD3a showed predominant expression in the ovary and the male livers, whereas SOD3b was found in the pituitary and brain of both sexes. The effects of different maturation protocols (standard hormonal treatment vs. same protocol preceded with cold seawater pre-treatment) on SODs expression during testis maturation were evaluated. Salinity increase at the onset of standard treatment at 20 °C, simulating early migration, upregulated SOD1, SOD2a, and SOD2b, coinciding with spermatogonia type A differentiated cells dominance. Thereafter, SOD2a and SOD3a decreased, while SOD2b increased during hormonal treatment-induced spermatogenesis. Pre-treatment with seawater at 10 °C, mimicking the conditions at the beginning of the seawater migration, downregulated SOD1 but increased SOD3a expression. Finally, the standard hormonal treatment, replicating spawning at higher temperatures, downregulated SOD1 in eels without any pre-treatment while SOD2a expression increased in pre-treated eels. This study revealed tissue-specific, sex-dependent, and maturation-related SOD expression patterns, predicting SODs dynamic expression profiles during their reproductive migration.

Immunonutrition: facilitating mucosal immune response in teleost intestine with amino acids through oxidant-antioxidant balance

Comparative animal models generate fundamental scientific knowledge of immune responses. However, these studies typically are conducted in mammals because of their biochemical and physiological similarity to humans. Presently, there has been an interest in using teleost fish models to study intestinal immunology, particularly intestinal mucosa immune response. Instead of targeting the pathogen itself, a preferred approach for managing fish health is through nutrient supplementation, as it is noninvasive and less labor intensive than vaccine administrations while still modulating immune properties. Amino acids (AAs) regulate metabolic processes, oxidant-antioxidant balance, and physiological requirements to improve immune response. Thus, nutritionists can develop sustainable aquafeeds through AA supplementation to promote specific immune responses, including the intestinal mucosa immune system. We propose the use of dietary supplementation with functional AAs to improve immune response by discussing teleost fish immunology within the intestine and explore how oxidative burst is used as an immune defense mechanism. We evaluate immune components and immune responses in the intestine that use oxidant-antioxidant balance through potential selection of AAs and their metabolites to improve mucosal immune capacity and gut integrity. AAs are effective modulators of teleost gut immunity through oxidant-antioxidant balance. To incorporate nutrition as an immunoregulatory means in teleost, we must obtain more tools including genomic, proteomic, nutrition, immunology, and macrobiotic and metabonomic analyses, so that future studies can provide a more holistic understanding of the mucosal immune system in fish.

Sulphide donors affect the expression of mucin and sulphide detoxification genes in the mucosal organs of Atlantic salmon (Salmo salar)

Hydrogen sulphide (H2S) is a gas that affects mucosal functions in mammals. However, its detrimental effects are less understood in fish despite being known to cause mass mortality. Here we used explant models to demonstrate the transcriptional responses of Atlantic salmon (Salmo salar) mucosa to the sulphide donor sodium hydrosulphide (NaHS). The study focused on two groups of genes: those encoding for sulphide detoxification and those for mucins. Moreover, we performed pharmacological studies by exposing the organ explants to mucus-interfering compounds and consequently exposed them to a sulphide donor. Exposure to NaHS significantly affected the expression of sulphide:quinone oxidoreductase (sqor1, sqor2) and mucin-encoding genes (muc5ac, muc5b). The general profile indicated that NaHS upregulated the expression of sulphide detoxification genes while a significant downregulation was observed with mucins. These expression profiles were seen in both organ explant models. Pharmacological stimulation and inhibition of mucus production used acetylcholine (ACh) and niflumic acid (NFA), respectively. This led to a significant regulation of the two groups of marker genes in the gills and olfactory rosette explants. Treatment of the mucosal organ explants with the mucus-interfering compounds showed that low dose NFA triggered more substantial changes while a dose-dependent response could not be established with ACh. Pharmacological interference demonstrated that mucins played a crucial role in mucosal protection against H2S toxicity. These results offer insights into how a sulphide donor interfered with mucosal responses of Atlantic salmon and are expected to contribute to our understanding of the least explored H2S-fish interactions-particularly at the mucosa.

Mode of Application of Peracetic Acid-Based Disinfectants has a Minimal Influence on the Antioxidant Defences and Mucosal Structures of Atlantic Salmon (Salmo salar) Parr

Peracetic acid (PAA) is an oxidative disinfectant with a broad spectrum of antimicrobial activity and low environmental impact. In this study, we investigated the physiological impacts of PAA application in Atlantic salmon (Salmo salar) parr reared in freshwater recirculating aquaculture systems over a 4-week period. PAA at a target concentration of 1 mg/L was administered either in pulse (every 3 days) or continuous. The group that did not receive PAA served as a control. Fish tissue samples were collected for histology, gene expression, and biochemical analyses at day 0 and after 2 and 4 weeks of exposure. The expression of genes encoding for antioxidant defence in the olfactory organs, skin, and gills changed during the trial, but the temporal effects were more pronounced than inter-treatment impacts. The glutathione group of antioxidant genes was more responsive to PAA. In most cases, an upregulation was observed. Significantly lower levels of reactive oxygen species were identified in the plasma and skin mucus of the two PAA-exposed groups at week 4; nonetheless, significantly increased levels of total antioxidant capacity were only observed in the skin mucus of fish from the continuous treatment group. Additional markers of oxidative stress (i.e., 8-oxo-2'-deoxyguanosine and o,o'-dityrosine) were analysed in the skin, gills, liver, and dorsal fins. These markers were unaffected by the two PAA treatments. Sporadic reversible structural alterations were observed in the three mucosal organs; the changes were time-dependent, and the effects of PAA treatment were minimal. The number of mucous cells varied over time but not within treatments except in the skin of the pulse group at week 4 where a reduction was observed. The ratio of acidic and neutral mucous cells in the skin and gills were affected by PAA treatments especially in the pulse group. Overall, this study revealed that Atlantic salmon parr mobilised mucosal and systemic antioxidant defences against the oxidative disinfectant PAA, but it was evident that the mode of application did not impose a strong influence. The minimal effects of PAA application on the indicators of health and welfare underscore the potential use of PAA as a routine disinfectant in recirculating aquaculture systems.

The fasted and post-prandial physiological responses of the Patagonian blennie Eleginops maclovinus

Eleginops maclovinus is a native species with potential for Chilean aquaculture. Understanding the variations between the post-prandial and fasted metabolic responses can contribute to improving the aquaculture of this species. This study aimed to characterize variations in intermediate metabolism during the course of the day in the liver, serum, and gills of fed and unfed fish. For this, 72 fish were assigned to two experimental groups, "fed" and "fasted". The first group was fed "ad libitum" at 8.30, while the fasted group was not fed for 24 h. Samples were taken from both groups at 9:00, and every 2 h: 11:00, 13:00, 15:00, 17:00, and 19:00. In the fed group, food spent a long time in the gastrointestinal tract, with a large increase in stomach size and without evidence of complete emptying of the stomach at 19:00 (10.5 h post-feeding). In serum, the levels of amino acids, glucose, and triglycerides presented significant differences with peak levels at different times of day in the fed group. The cortisol in the fasted group presented a diurnal pattern with high levels during the morning and very low levels after 13:00, while in the fed group, the high cortisol variability did not allow a clear pattern to be established. In the liver, the effect of time on the enzymatic activity of the intermediary metabolism was greater compared to the effect of feeding. In the liver, enzyme activity decreased at later hours of the day, while glycogen levels increased at later hours of the day in both groups: but its levels were higher in the fed group. In gills, as well as in the liver, time had a greater effect than feeding on intermediate metabolism, since feeding only had a significant effect on the levels of hexokinase, lactate, and amino acids, suggesting an effect on carbohydrate metabolism. Meanwhile, time significantly affected the levels of Na⁺, K⁺-ATPase, glutamate dehydrogenase, aspartate aminotransferase, amino acids, and proteins, suggesting an effect on amino acid metabolism. In conclusion, the intermediate metabolism of E. maclovinus presents variations according to the time of day, with an increased metabolism during the morning and decreased metabolism as the day progresses, especially at the hepatic level. The gill tissue, despite not being a metabolic organ, presents feeding-dependent variations in its metabolism. Additional studies will be required to corroborate if coordinating a feeding strategy during the first hours of the day when metabolism is greater would improve the growth of E. maclovinus.

Intermittent administration of peracetic acid is a mild environmental stressor that elicits mucosal and systemic adaptive responses from Atlantic salmon post-smolts

Background

Fish encounter oxidative stress several times during their lifetime, and it has a pervasive influence on their health and welfare. One of the triggers of oxidative stress in fish farming is the use of oxidative disinfectants to improve rearing conditions, especially in production systems employing recirculation technology. Here we report the physiological and morphological adaptive responses of Atlantic salmon (Salmo salar L.) post-smolts to intermittent exposure to a potent oxidative agent peracetic acid (PAA). Fish reared in semi-commercial scale brackish water recirculating aquaculture system (RAS) were exposed to 1 ppm PAA every 3 days over 6 weeks. Mucosal and systemic responses were profiled before exposure, 22 and 45 days during the intermittent PAA administration.

Results

Oxidative stress was likely triggered as plasma antioxidant capacity increased significantly during the exposure period. Adaptive stress response to the periodic oxidant challenge was likewise demonstrated in the changes in plasma glucose and lactate levels. PAA-induced alterations in the transcription of antioxidants, cytokines, heat shock proteins and mucin genes showed a tissue-specific pattern: downregulation was observed in the gills and olfactory rosette, upregulation occurred in the skin, and no substantial changes in the liver. Further, PAA exposure resulted in histological changes in key mucosal organs (i.e. olfactory rosette, skin and gills); pathological alterations were predominant in the gills where cases of epithelial lifting, hypertrophy and clubbing were prevalent. In addition, intermittent PAA administration resulted in an apparent overproduction of mucus in the nasal mucosa. Lastly, PAA did not dramatically alter the ability of salmon to mount a physiological stress response in the presence of a secondary stressor, though some subtle interference was documented in the kinetics and magnitude of plasma cortisol and glucose response post-stress.

Conclusions

The present study collectively demonstrated that intermittent oxidant exposure was a mild environmental stressor that salmon could mount strong adaptive responses at systemic and mucosal levels. The results will be valuable in optimising the rearing conditions of post-smolts in RAS, especially in adopting water treatment strategies that do not considerably interfere with fish health and welfare.

Environmental chemicals affect circadian rhythms: An underexplored effect influencing health and fitness in animals and humans

Circadian rhythms control the life of virtually all organisms. They regulate numerous aspects ranging from cellular processes to reproduction and behavior. Besides the light-dark cycle, there are additional environmental factors that regulate the circadian rhythms in animals as well as humans. Here, we outline the circadian rhythm system and considers zebrafish (Danio rerio) as a representative vertebrate organism. We characterize multiple physiological processes, which are affected by circadian rhythm disrupting compounds (circadian disrupters). We focus on and summarize 40 natural and anthropogenic environmental circadian disrupters in fish. They can be divided into six major categories: steroid hormones, metals, pesticides and biocides, polychlorinated biphenyls, neuroactive drugs and other compounds such as cyanobacterial toxins and bisphenol A. Steroid hormones as well as metals are most studied. Especially for progestins and glucocorticoids, circadian dysregulation was demonstrated in zebrafish on the molecular and physiological level, which comprise mainly behavioral alterations. Our review summarizes the current state of knowledge on circadian disrupters, highlights their risks to fish and identifies knowledge gaps in animals and humans. While most studies focus on transcriptional and behavioral alterations, additional effects and consequences are underexplored. Forthcoming studies should explore, which additional environmental circadian disrupters exist. They should clarify the underlying molecular mechanisms and aim to better understand the consequences for physiological processes.

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.

Comparison of Circulating Markers and Mucosal Immune Parameters from Skin and Distal Intestine of Atlantic Salmon in Two Models of Acute Stress

Ensuring salmon health and welfare is crucial to maximize production in recirculation aquaculture systems. Healthy and robust mucosal surfaces of the skin and intestine are essential to achieve this goal because they are the first immunological defenses and are constantly exposed to multistressor conditions, such as infectious diseases, suboptimal nutrition, and environmental and handling stress. In this work, Atlantic salmon, split from a single cohort, were subjected to acute hypoxia stress or 15-min crowding stress and observed over a 24-h recovery period. Samples were collected from fish at 0, 1, 3, 6, 12 and 24 h post-stress to analyze plasma-circulating markers of endocrine function (cortisol), oxidative stress (glutathione peroxidase) and immune function (interleukin 10 (IL-10), annexin A1). In addition, mucosal barrier function parameters were measured in the skin mucus (Muc-like protein and lysozyme) and distal intestine (simple folds, goblet cell size and goblet cell area). The results showed that both acute stress models induced increases of circulating cortisol in plasma (1 h post-stress), which then returned to baseline values (initial control) at 24 h post-stress. Moreover, the hypoxia stress was mostly related to increased oxidative stress and IL-10 production, whereas the crowding stress was associated with a higher production of Muc-like protein and lysozyme in the skin mucus. Interestingly, in the distal intestine, smaller goblet cells were detected immediately and one hour after post-hypoxia stress, which could be related to rapid release of the cellular content to protect this organ. Finally, the correlation of different markers in the hypoxic stress model showed that the circulating levels of cortisol and IL-10 were directly proportional, while the availability of Muc-like proteins was inversely proportional to the size of the goblet cells. On the other hand, in the crowding stress model, a proportional relationship was established between plasma cortisol levels and skin mucus lysozyme. Our results suggest key differences in energy partitioning between the two acute stress models and support the need for further investigation into the interplay of multistressor conditions and strategies to modulate immunological aspects of mucosal surfaces.

The Effects of Ozone on Atlantic Salmon Post-Smolt in Brackish Water-Establishing Welfare Indicators and Thresholds

Ozone is a strong oxidant, and its use in aquaculture has been shown to improve water quality and fish health. At present, it is predominantly used in freshwater systems due to the high risk of toxic residual oxidant exposure in brackish water and seawater. Here, we report the effects of ozone on Atlantic salmon (Salmo salar) post-smolts (~100 g), in a brackish water (12 ppt) flow-through system. Salmon were exposed to oxidation reduction potential concentrations of 250 mV (control), 280 mV (low), 350 mV (medium), 425 mV (high) and 500 mV (very high). The physiological impacts of ozone were characterized by blood biochemical profiling, histopathologic examination and gene expression analysis in skin and gills. Fish exposed to 425 mV and higher showed ≥33% cumulative mortality in less than 10 days. No significant mortalities were recorded in the remaining groups. The skin surface quality and the thickness of the dermal and epidermal layers were not significantly affected by the treatments. On the other hand, gill histopathology showed the adverse effects of increasing ozone doses and the changes were more pronounced in the group exposed to 350 mV and higher. Cases of gill damages such as necrosis, lamellar fusion and hypertrophy were prevalent in the high and very high groups. Expression profiling of key biomarkers for mucosal health supported the histology results, showing that gills were significantly more affected by higher ozone doses compared to the skin. Increasing ozone doses triggered anti-oxidative stress and inflammatory responses in the gills, where transcript levels of glutathione reductase, copper/zinc superoxide dismutase, interleukin 1β and interleukin were significantly elevated. Heat shock protein 70 was significantly upregulated in the skin of fish exposed to 350 mV and higher. Bcl-2 associated x protein was the only gene marker that was significantly upregulated by increasing ozone doses in both mucosal tissues. In conclusion, the study revealed that short-term exposure to ozone at concentrations higher than 350 mV in salmon in brackish water resulted in significant health and welfare consequences, including mortality and gill damages. The results of the study will be valuable in developing water treatment protocols for salmon farming.