Epigenetic changes in pyloric caeca of Atlantic salmon fed diets containing increasing levels of lipids and choline

An earlier study of ours investigating the effect of dietary lipid levels on the choline requirement of Atlantic salmon showed increasing severity of intestinal steatosis with increasing lipid levels. As choline is involved in epigenetic regulation by being the key methyl donor, pyloric caeca samples from the study were analysed for epigenetic effects of dietary lipid and choline levels. The diets varied in lipid levels between 16% and 28%, and choline levels between 1.9 and 2.3 g/kg. The diets were fed for 8 weeks to Atlantic salmon of 25 g of initial weight. Using reduced representation bisulfite sequencing (RRBS), this study revealed that increasing dietary lipid levels induced methylation differences in genes involved in membrane transport and signalling pathways, and in microRNAs important for the regulation of lipid homoeostasis. Increasing choline levels also affected genes involved in fatty acid biosynthesis and transport, lipolysis, and lipogenesis, as well as important immune genes. Our observations confirmed that choline is involved in epigenetic regulation in Atlantic salmon, as has been reported for higher vertebrates. This study showed the need for the inclusion of biomarkers of epigenetic processes in studies that must be conducted to define optimal choline levels in diets for Atlantic salmon.

Intestinal strangulation in farmed Atlantic cod (Gadus morhua): Pathological changes and possible predisposing anatomical features

During recent years, there has been a renewed interest in establishing farming of Atlantic cod (Gadus morhua) in Norway. However, a fatal abdominal disorder compromises animal welfare and causes economic losses. A similar problem was present during a previous attempt to establish Atlantic cod farming more than a decade ago. In this paper, we provide the first in-depth description of this intestinal disorder, which is correctly denoted 'strangulating obstruction'. In affected fish, part of the intestine is permanently entrapped (incarcerated) under fibrous strands in the mesentery. The entrapment interferes with blood flow and physically blocks the intestine, causing a strangulating obstruction with severe venous congestion and ischemia of the intestinal wall. Furthermore, comparison of macroscopical and histological anatomy of farmed and wild Atlantic cod is presented and risk factors associated with the anatomical differences are discussed.

A comprehensive transcriptional body map of Atlantic salmon unveils the vital role of the intestine in the immune system and highlights functional specialization within its compartments

The intestine is a barrier organ that plays an important role in the immune system of Atlantic salmon. The immune functions are distributed among the diffuse gut lymphoid tissue containing diverse immune cells, and other cell types. Comparison of intestinal transcriptomes with those of other organs and tissues offers an opportunity to elucidate the specific roles of the intestine and its relationship with other parts of the body. In this work, a meta-analysis was performed on a large volume of data obtained using a genome-wide DNA oligonucleotide microarray. The intestine ranks third by the expression level of immune genes after the spleen and head kidney. The activity of antigen presentation and innate antiviral immunity is higher in the intestine than in any other tissue. By comparing transcriptome profiles, intestine shows the greatest similarity with the gill, head kidney, spleen, epidermis, and olfactory rosette (descending order), which emphasizes the integrity of the peripheral mucosal system and its strong connections with the major lymphoid organs. T cells-specific genes dominate among the genes co-expressed in these tissues. The transcription signature of CD8+ (86 genes, r > 0.9) includes a master gene of immune tolerance foxp3 and other negative regulators. Different segments of the intestine were compared in a separate experiment, in which expression gradients along the intestine were found across several functional groups of genes. The expression of luminal and intracellular (lysosome) proteases is markedly higher in pyloric caeca and distal intestine respectively. Steroid metabolism and cytochromes P450 are highly expressed in pyloric caeca and mid intestine while the distal intestine harbors genes related to vitamin and iron metabolism. The expression of genes for antigen presenting proteins and immunoglobulins shows a gradual increase towards the distal intestine.

Exploring gut microbiota in adult Atlantic salmon (Salmo salar L.): Associations with gut health and dietary prebiotics

BACKGROUND

The importance of the gut microbiota for physiological processes in mammals is well established, but the knowledge of their functional roles in fish is still limited. The aims of this study were to investigate associations between variation in taxonomical composition of the gut microbiota and gut health status in Atlantic salmon and to explore possible modulatory effects of dietary prebiotics in one net-pen farm in open water. The fish with initial mean body weight of around 240 g were fed diets based on the same basal composition, either without (Ref diet) or with (Test diet) yeast cell wall based-prebiotics, during the marine production phase from December to September the following year. Sampling was conducted at three sampling time points: January, April, and September, with average water temperature of 3.9 ℃, 3.4 ℃ and 9.6 ℃, respectively.

RESULTS

As the fish progressed towards September, growth, brush border membrane enzyme activities, and the expression in the gut of most of the observed genes involved in immune (e.g., il8, cd4a, myd88, il1b, gilt, tgfb, cd8b and cd3), barrier (e.g., zo1, occludin, ecad, claudin25b and claudin15), and metabolism increased significantly. Lipid accumulation in pyloric enterocytes decreased remarkably, suggesting improvement of gut health condition. The growth of the fish did not differ between dietary treatments. Further, dietary prebiotics affected the gut health only marginally regardless of duration of administration. Regarding gut microbiota composition, a decrease in alpha diversity (Observed species, Pielou and Shannon) over time was observed, which was significantly associated with an increase in the relative abundance of genus Mycoplasma and decrease in 32 different taxa in genus level including lactic acid bacteria (LAB), such as Lactobacillus, Leuconostoc, and Lactococcus. This indicates that developmental stage of Atlantic salmon is a determinant for microbial composition. Multivariate association analysis revealed that the relative abundance of Mycoplasma was positively correlated with gut barrier gene expression, negatively correlated with plasma glucose levels, and that its relative abundance slightly increased by exposure to prebiotics. Furthermore, certain LAB (e.g., Leuconostoc), belonging to the core microbiota, showed a negative development with time, and significant associations with plasma nutrients levels (e.g., triglyceride and cholesterol) and gene expression related to gut immune and barrier function.

CONCLUSIONS

As Atlantic salmon grew older under large-scale, commercial farm settings, the Mycoplasma became more prominent with a concomitant decline in LAB. Mycoplasma abundance correlated positively with time and gut barrier genes, while LAB abundance negatively correlated to time. Dietary prebiotics affected gut health status only marginally.

Soya saponins and prebiotics alter intestinal functions in Ballan wrasse (Labrus bergylta)

A 5-week feeding trial was conducted in the cleaner fish Ballan wrasse (Labrus bergylta) for a better understanding of the basic biology of the intestinal functions and health in this stomach less species. During the trial, Ballan wrasse was fed either a reference diet, the reference diet supplemented with (i) a commercial prebiotic (Aquate™ SG, 0·4 %) expected to have beneficial effects, (ii) soya saponins (0·7 %) expected to induce inflammation or (iii) a combination of the prebiotics and the soya saponins to find a remedy for gut inflammation. Blood, intestinal tissue and gut content from four consecutive intestinal segments (IN1 - IN4) were collected. No significant differences in fish growth were observed between the four dietary groups. Saponin supplementation, both alone and in combination with prebiotics, increased weight index of IN2 and IN3 and decreased blood plasma glucose, cholesterol and total protein. Dry matter of intestinal content and activity of digestive enzymes were not affected by diet. Histomorphological analyses revealed a progressing inflammation with increased infiltration by immune cells particularly into the distal parts of the intestine in fish fed diets with saponins, both alone and in combination with prebiotics. Gene expression profiles obtained by RNA sequencing and quantitative PCR mirrored the histological and biochemical changes induced by the saponin load. The study demonstrated that Ballan wrasse gut health and digestive function may be markedly affected by feed ingredients containing antinutrients.

Development of a Rainbow Trout (Oncorhynchus mykiss) Intestinal In Vitro Platform for Profiling Amino Acid Digestion and Absorption of a Complete Diet

The ever-increasing number and variation of raw materials utilized to provide alternative feed formulations continues to allow for a more sustainable and flexible approach. Testing all these options in vivo is still the most robust and reliable manner to pick the best raw material candidates, but it requires the use of large numbers of animals and is time-consuming and expensive. Therefore, we are developing an in vitro platform that can provide a reliable evaluation of new ingredients. The main aim of this work was to combine an in vitro digestion protocol of extruded, commercially relevant aquafeeds with the exposure of intestinal epithelial cells to the extracted bio-available fraction (BAF). The results show that 250,000 cells/cm² represents the optimal seeding density and that up to 50% BAF concentration for up to 24 h had no negative effects on the epithelial barrier morphology and function. It is possible to determine amino acid digestibility and bioavailability in all the experimental conditions (with and without BSA, at 25% and 50% dilution) and at all time points (0, 6, and 24 h). However, BAF concentration, the medium used for its dilution, and the length of exposure to the different epithelial cell lines can all influence the results and, therefore, must be selected according to the final aim of the experiment.

Establishment of an In Vitro Model to Study Viral Infections of the Fish Intestinal Epithelium

Viral infections are still a major concern for the aquaculture industry. For salmonid fish, even though breeding strategies and vaccine development have reduced disease outbreaks, viral diseases remain among the main challenges having a negative impact on the welfare of fish and causing massive economic losses for the industry. The main entry port for viruses into the fish is through mucosal surfaces including that of the gastrointestinal tract. The contradictory functions of this surface, both creating a barrier towards the external environment and at the same time being responsible for the uptake of nutrients and ion/water regulation make it particularly vulnerable. The connection between dietary components and viral infections in fish has been poorly investigated and until now, a fish intestinal in vitro model to investigate virus-host interactions has been lacking. Here, we established the permissiveness of the rainbow trout intestinal cell line RTgutGC towards the important salmonid viruses-infectious pancreatic necrosis virus (IPNV), salmonid alphavirus (subtype 3, SAV3) and infectious salmon anemia virus (ISAV)-and explored the infection mechanisms of the three different viruses in these cells at different virus to cell ratios. Cytopathic effect (CPE), virus replication in the RTgutGC cells, antiviral cell responses and viral effects on the barrier permeability of polarized cells were investigated. We found that all virus species infected and replicated in RTgutGC cells, although with different replication kinetics and ability to induce CPE and host responses. The onset and progression of CPE was more rapid at high multiplicity of infection (MOI) for IPNV and SAV3 while the opposite was true of ISAV. A positive correlation between the MOI used and the induction of antiviral responses was observed for IPNV while a negative correlation was detected for SAV3. Viral infections compromised barrier integrity at early time points prior to observations of CPE microscopically. Further, the replication of IPNV and ISAV had a more pronounced effect on barrier function than SAV3. The in vitro infection model established herein can thus provide a novel tool to generate knowledge about the infection pathways and mechanisms used to surpass the intestinal epithelium in salmonid fish, and to study how a virus can potentially compromise gut epithelial barrier functions.

Effects of dietary lipid level and environmental temperature on lipid metabolism in the intestine and liver, and choline requirement in Atlantic salmon (Salmo salar L) parr

Choline was recently established as an essential nutrient for Atlantic salmon at all life stages. Choline deficiency is manifested as an excessive accumulation of dietary fat within the intestinal enterocytes, a condition known as steatosis. Most of today's plant-based salmon feeds will be choline-deficient unless choline is supplemented. Choline's role in lipid transport suggests that choline requirement may depend on factors such as dietary lipid level and environmental temperature. The present study was therefore conducted to investigate whether lipid level and water temperature can affect steatosis symptoms, and thereby choline requirement in Atlantic salmon. Four choline-deficient plant-based diets were formulated differing in lipid level of 16, 20, 25 and 28 % and fed to salmon of 25 g initial weight in duplicate tanks per diet at two different environmental temperatures: 8 and 15 °C. After 8 weeks of feeding, samples of blood, tissue and gut content from six fish per tank were collected, for analyses of histomorphological, biochemical and molecular biomarkers of steatosis and choline requirement. Increasing lipid level did not affect growth rate but increased relative weight and lipid content of the pyloric caeca and histological symptoms of intestinal steatosis and decreased fish yield. Elevation of the water temperature from 8 to 15 °C, increased growth rate, relative weight of the pyloric caeca, and the histological symptoms of steatosis seemed to become more severe. We conclude that dietary lipid level, as well as environmental temperature, affect choline requirement to a magnitude of importance for fish biology and health, and for fish yield.

Effects of functional ingredients on gut inflammation in Atlantic salmon (Salmo salar L)

Functional feed ingredients are frequently used in feeds for Atlantic salmon, often claimed to improve immune functions in the intestine and reduce severity of gut inflammation. However, documentation of such effects is, in most cases, only indicative. In the present study effects of two packages of functional feed ingredients commonly used in salmon production, were evaluated employing two inflammation models. One model employed soybean meal (SBM) as inducer of a severe inflammation, the other a mixture of corn gluten and pea meal (CoPea) inducing mild inflammation. The first model was used to evaluate effects of two packages of functional ingredients: P1 containing butyrate and arginine, and P2 containing β-glucan, butyrate, and nucleotides. In the second model only the P2 package was tested. A high marine diet was included in the study as a control (Contr). The six diets were fed to salmon (average weight of 177g) in saltwater tanks (57 fish per tank), in triplicate, for 69 days (754 ddg). Feed intake was recorded. The growth rate of the fish was high, highest for the Contr (TGC: 3.9), lowest for SBM fed fish (TGC: 3.4). Fish fed the SBM diet showed severe symptoms of inflammation in the distal intestine as indicated by histological, biochemical, molecular, and physiological biomarkers. The number of differently expressed genes (DEG) between the SBM and Contr fed fish was 849 and comprised genes indicating alteration in immune functions, cellular and oxidative stress, and nutrient digestion, and transport functions. Neither P1 nor P2 altered the histological and functional symptoms of inflammation in the SBM fed fish importantly. Inclusion of P1 altered expression of 81 genes, inclusion of P2 altered 121 genes. Fish fed the CoPea diet showed minor signs of inflammation. Supplementation with P2 did not change these signs. Regarding composition of the microbiota in digesta from the distal intestine, clear differences regarding beta-diversity and taxonomy between Contr, SBM, and CoPea fed fish were observed. In the mucosa the microbiota differences were less clear. The two packages of functional ingredients altered microbiota composition of fish fed the SBM and the CoPea diet towards that of fish fed the Contr diet.

Effects of dietary supplementation with prebiotics and Pediococcus acidilactici on gut health, transcriptome, microbiota, and metabolome in Atlantic salmon (Salmo salar L.) after seawater transfer

BACKGROUND

Given the importance of gut microbiota for health, growth and performance of the host, the aquaculture industry has taken measures to develop functional fish feeds aiming at modulating gut microbiota and inducing the anticipated beneficial effects. However, present understanding of the impact of such functional feeds on the fish is limited. The study reported herein was conducted to gain knowledge on performance and gut health characteristics in post-smolt Atlantic salmon fed diets varying in content of functional ingredients. Three experimental diets, a diet containing fructo-oligosaccharides (FOS), a diet with a combination of FOS and Pediococcus acidilactici (BC) and a diet containing galacto-oligosaccharides (GOS) and BC, were used in a 10-weeks feeding trial. A commercial diet without functional ingredients was also included as a control/reference. Samples of blood plasma, mucosa and digesta were subjected to microbiota, transcriptome and metabolome profiling for evaluation of the diet effects.

RESULTS

No significant growth differences were observed between fish fed the supplemented diets, but FOS-BC fed fish showed significantly faster growth than the control fed fish. The microbiota results showed that the BC was present in both the digesta, and the mucosa samples of fish fed the FOS-BC and GOS-BC diets. Digesta-associated microbiota was altered, while mucosa-associated microbiota was relatively unaffected by diet. Replacing FOS with GOS increased the level of metabolites linked to phospholipid, fatty acid, carnitine and sphingolipid metabolism. Variation in metabolite levels between the treatments closely correlated with genera mainly belonging to Firmicutes and Actinobacteria phyla. The transcriptome analyses indicated diet effects of exchanging FOS with GOS on immune functions, oxidative defense and stress responses. No significant diet effect was observed on intestinal inflammation in the pyloric caeca or in the distal intestine, or on lipid accumulation in the pyloric caeca.

CONCLUSIONS

Dietary supplementation with BC induced moderate effects on the microbiota of the digesta, while the effects of replacing FOS with GOS were more marked and was observed also for nutrient metabolism. Our data indicates therefore that the quality of a prebiotic may be of great importance for the effects of a probiotic on gut microbiota, function, and health.

Dietary Fish Meal Level and a Package of Choline, β-Glucan, and Nucleotides Modulate Gut Function, Microbiota, and Health in Atlantic Salmon (Salmo salar, L.)

Steatosis and inflammation have been common gut symptoms in Atlantic salmon fed plant rich diets. Choline has recently been identified as essential for salmon in seawater, and β-glucan and nucleotides are frequently used to prevent inflammation. The study is aimed at documenting whether increased fishmeal (FM) levels (8 levels from 0 to 40%) and supplementation (Suppl) with a mixture of choline (3.0 g/kg), β-glucan (0.5 g/kg), and nucleotides (0.5 g/kg) might reduce the symptoms. Salmon (186 g) were fed for 62 days in 16 saltwater tanks before samples were taken from 12 fish per tank for observation of biochemical, molecular, metabolome, and microbiome indicators of function and health. Steatosis but no inflammation was observed. Lipid digestibility increased and steatosis decreased with increasing FM levels and supplementation, seemingly related to choline level. Blood metabolites confirmed this picture. Genes in intestinal tissue affected by FM levels are mainly involved in metabolic and structural functions. Only a few are immune genes. The supplement reduced these FM effects. In gut digesta, increasing FM levels increased microbial richness and diversity, and changed the composition, but only for unsupplemented diets. An average choline requirement of 3.5 g/kg was indicated for Atlantic salmon at the present life stage and under the present condition.

Bile components affect the functions and transcriptome of the rainbow trout intestinal epithelial cell line RTgutGC

The concomitant increase in cultivation of fish and decrease in supply of marine ingredients, have greatly increased the demand for new nutrient sources. This also regards so-called functional ingredients which may benefit health and welfare of the fish. In vitro cell line-based intestinal epithelial barrier models may serve as tools for narrowing down the broad range of ingredient options, to identify the most promising candidates before in vivo feeding trials are run. In vivo, differentiation of the various epithelial cells in the fish intestine, from the multipotent stem cells, takes place in the presence of a variety of substances from dietary and endogenous origin. Among these, bile salts have recently received attention as regulators of epithelial function in health and disease but have not, until now, been included in the medium when culturing fish gut epithelial cells in vitro. As bile salts are present at high levels in the chyme of the fish intestine, in particular in salmon and rainbow trout, mostly as taurocholate (>90%), their role for effects of diet ingredients on the in vitro gut cell model should be understood. With this study, we wanted to investigate whether inclusion of bile from rainbow trout or pure taurocholate in the culture media would modulate functions of the RTgutGC epithelial cells. Here, we demonstrated that the rainbow trout intestinal epithelial cell line RTgutGC responded significantly to the presence of bile components. Treatment with rainbow trout bile taken from the gall bladder (RTbile) or pure taurocholate (TC) at taurocholate concentrations of ≤0.5 mg/mL retained normal cell morphology, cell viability as in cell oxidation-reduction metabolic activity and membrane integrity, and barrier features, while high concentrations of bile salts (≥1 mg/mL) were cytotoxic to the cells. After long-term (4 days) bile treatment, transcriptome responses showed how bile salts play important roles in intestinal epithelial cell metabolism. qPCR data demonstrated that barrier function genes, brush border enzyme genes and immune genes were significantly affected. Although similar trends were seen, treatment with bile salt as a component of rainbow trout bile or pure taurocholate, induced somewhat different effects. In conclusion, this study clearly indicates that bile salts should be included in the cell medium when running in vitro studies of gut cell functions, not at least immune functions, preferably at the level of ∼0.5 mg/mL supplemented as pure taurocholate to ensure reproducibility.

Consistent changes in the intestinal microbiota of Atlantic salmon fed insect meal diets

BACKGROUND

Being part of fish's natural diets, insects have become a practical alternative feed ingredient for aquaculture. While nutritional values of insects have been extensively studied in various fish species, their impact on the fish microbiota remains to be fully explored. In an 8-week freshwater feeding trial, Atlantic salmon (Salmo salar) were fed either a commercially relevant reference diet or an insect meal diet wherein black soldier fly (Hermetia illucens) larvae meal comprised 60% of total ingredients. Microbiota of digesta and mucosa origin from the proximal and distal intestine were collected and profiled along with feed and water samples.

RESULTS

The insect meal diet markedly modulated the salmon intestinal microbiota. Salmon fed the insect meal diet showed similar or lower alpha-diversity indices in the digesta but higher alpha-diversity indices in the mucosa. A group of bacterial genera, dominated by members of the Bacillaceae family, was enriched in salmon fed the insect meal diet, which confirms our previous findings in a seawater feeding trial. We also found that microbiota in the intestine closely resembled that of the feeds but was distinct from the water microbiota. Notably, bacterial genera associated with the diet effects were also present in the feeds.

CONCLUSIONS

We conclude that salmon fed the insect meal diets show consistent changes in the intestinal microbiota. The next challenge is to evaluate the extent to which these alterations are attributable to feed microbiota and dietary nutrients, and what these changes mean for fish physiology and health.

Digestive and immune functions in the intestine of wild Ballan wrasse (Labrus bergylta)

This study was carried out to profile key characteristics of intestinal functions and health in wild-caught Ballan wrasse. To describe functional variation along the intestine, samples were collected from four intestinal segments, named from the proximal to the distal segment: IN1, IN2, IN3 and IN4. The sections showed quite similar structure, i.e. regarding mucosal fold height and branching, lamina propria and submucosal width and cellular composition and thickness of the muscle layers. Leucine aminopeptidase and maltase capacity decreased from IN1 to IN4, suggesting a predominant role of IN1 in digestion. Gene expression levels of vitamin C transporter (slc23a1) and fatty acid transporters (cd36 and fabp2) were higher in IN1 than in IN4, indicating a more important role of the proximal intestine regarding transport of vitamins and fatty acids. Higher expression of the gene coding for IgM heavy chain constant region (ighm) was found in IN4 than in IN1, suggesting an important immune function of the distal intestine. Other immune related genes il1b, il6, cd40, showed similar expression in the proximal and the distal part of the intestine. Parasite infection, especially the myxozoan parasite Enteromyxum leei, coincided with infiltration of lymphocytic and eosinophilic granular cells in the submucosa and lamina propria. The present study established reference information necessary for interpretation of results of studies of intestinal functions and health in cultured Ballan wrasse.

The plasma metabolome of Atlantic salmon as studied by 1H NMR spectroscopy using standard operating procedures: effect of aquaculture location and growth stage

INTRODUCTION

Metabolomics applications to the aquaculture research are increasing steadily. The use of standardized proton nuclear magnetic resonance (¹H NMR) spectroscopy can provide the aquaculture industry with an unbiased, reproducible, and high-throughput screening tool, which can help to diagnose nutritional and disease-related metabolic disorders in farmed fish.

OBJECTIVE

Standard operating procedures developed for analysing (human) plasma by ¹H NMR were applied to fingerprint the metabolome in plasma samples collected from Atlantic salmon. The aim was to explore the metabolome of salmon plasma in relation to growth stage and sampling site.

METHODS

A total of 72 salmon were collected from three aquaculture sites in Norway (Lat. 65, 67, and 70 °N) and over two sampling events (December 2017 and November 2018). Plasma drawn from each salmon was measured by ¹H NMR and metabolites were quantified using the SigMa software. The NMR data was analysed by principal component analysis (PCA) and ANOVA-simultaneous component analysis (ASCA).

RESULTS

Important metabolic differences were evidenced, with adult salmon having a much higher content of very low-density lipoproteins and cholesterol in their plasma, while smolts displayed significantly higher levels of propylene glycol. Overall, 24% of the metabolite variation was due to the growth stage, whereas 12% of the metabolite variation was related to the aquaculture site and practice (p < 0.001).

CONCLUSION

This study provides a baseline investigation of the plasma metabolome of the Atlantic salmon and demonstrates how ¹H NMR metabolomics can be used in future investigations for comparing aquaculture practices and their influence on the fish metabolome.

Microbiota in intestinal digesta of Atlantic salmon (Salmo salar), observed from late freshwater stage until one year in seawater, and effects of functional ingredients: a case study from a commercial sized research site in the Arctic region

BACKGROUND

The importance of the gut microbiota for health and wellbeing is well established for humans and some land animals. The gut microbiota is supposedly as important for fish, but existing knowledge has many gaps, in particular for fish in the Arctic areas. This study addressed the dynamics of Atlantic salmon digesta-associated gut microbiota assemblage and its associations with host responses from freshwater to seawater life stages under large-scale, commercial conditions in the Arctic region of Norway, and explored the effects of functional ingredients. The microbiota was characterized by 16S rRNA gene sequencing in distal intestinal digesta at four time points: 2 weeks before seawater transfer (in May, FW); 4 weeks after seawater transfer (in June, SW1); in November (SW2), and in April (SW3) the following year. Two series of diets were fed, varying throughout the observation time in nutrient composition according to the requirements of fish, one without (Ref diet), and the other with functional ingredients (Test diet). The functional ingredients, i.e. nucleotides, yeast cell walls, one prebiotic and essential fatty acids, were supplemented as single or mixtures based on the strategies from the feed company.

RESULTS

Overall, the fish showed higher microbial richness and lactic acid bacteria (LAB) abundance after seawater transfer, while Simpson's diversity decreased throughout the observation period. At SW1, the gut microbiota was slightly different from those at FW, and was dominated by the genera Lactobacillus and Photobacterium. As the fish progressed towards SW2 and SW3, the genera Lactobacillus and Mycoplasma became more prominent, with a corresponding decline in genus Photobacterium. The overall bacterial profiles at these time points showed a clear distinction from those at FW. A significant effect of functional ingredients (a mixture of nucleotides, yeast cell walls and essential fatty acids) was observed at SW2, where Test-fed fish showed lower microbial richness, Shannon's diversity, and LAB abundance. The multivariate association analysis identified differentially abundant taxa, especially Megasphaera, to be significantly associated with gut immune and barrier gene expressions, and plasma nutrients.

CONCLUSIONS

The gut microbiota profile varied during the observation period, and the Mycoplasma became the dominating bacteria with time. Megasphaera abundance was associated with gut health and plasma nutrient biomarkers. Functional ingredients modulated the gut microbiota profile during an important ongrowing stage.

Differential response of digesta- and mucosa-associated intestinal microbiota to dietary insect meal during the seawater phase of Atlantic salmon

Background

Intestinal digesta is commonly used for studying responses of microbiota to dietary shifts, yet evidence is accumulating that it represents an incomplete view of the intestinal microbiota. The present work aims to investigate the differences between digesta- and mucosa-associated intestinal microbiota in Atlantic salmon (Salmo salar) and how they may respond differently to dietary perturbations. In a 16-week seawater feeding trial, Atlantic salmon were fed either a commercially-relevant reference diet or an insect meal diet containing ~ 15% black soldier fly (Hermetia illucens) larvae meal. The digesta- and mucosa-associated distal intestinal microbiota were profiled by 16S rRNA gene sequencing.

Results

Regardless of diet, we observed substantial differences between digesta- and mucosa-associated intestinal microbiota. Microbial richness and diversity were much higher in the digesta than the mucosa. The insect meal diet altered the distal intestinal microbiota resulting in higher microbial richness and diversity. The diet effect, however, depended on the sample origin. Digesta-associated intestinal microbiota showed more pronounced changes than the mucosa-associated microbiota. Multivariate association analyses identified two mucosa-enriched taxa, Brevinema andersonii and Spirochaetaceae, associated with the expression of genes related to immune responses and barrier function in the distal intestine, respectively.

Conclusions

Our data show that salmon intestinal digesta and mucosa harbor microbial communities with clear differences. While feeding insects increased microbial richness and diversity in both digesta- and mucosa-associated intestinal microbiota, mucosa-associated intestinal microbiota seems more resilient to variations in the diet composition. To fully unveil the response of intestinal microbiota to dietary changes, concurrent profiling of digesta- and mucosa-associated intestinal microbiota is recommended whenever feasible. Specific taxa enriched in the intestinal mucosa are associated to gene expression related to immune responses and barrier function. Detailed studies are needed on the ecological and functional significance of taxa associated to intestinal microbiota dwelling on the mucosa.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42523-020-00071-3.

Anatomy, immunology, digestive physiology and microbiota of the salmonid intestine: Knowns and unknowns under the impact of an expanding industrialized production

Increased industrialized production of salmonids challenges aspects concerning available feed resources and animal welfare. The immune system plays a key component in this respect. Novel feed ingredients may trigger unwarranted immune responses again affecting the well-being of the fish. Here we review our current knowledge concerning salmon intestinal anatomy, immunity, digestive physiology and microbiota in the context of industrialized feeding regimes. We point out knowledge gaps and indicate promising novel technologies to improve salmonid intestinal health.

Gut immune functions and health in Atlantic salmon (Salmo salar) from late freshwater stage until one year in seawater and effects of functional ingredients: A case study from a commercial sized research site in the Arctic region

The present study was conducted to strengthen the knowledge on gut immune functions and health in Atlantic salmon under large scale, commercial conditions in the Arctic region of Norway. Two groups of fish were monitored, one fed a series of diets without functional ingredients (Ref) and the other diets with functional ingredients (Test). The nutritional composition of the two diet series varied in parallel according to the nutrient requirements of the fish during the observation time. The content of functional ingredients in the Test diets, i.e. nucleotides, yeast cell walls, a prebiotic and essential fatty acids, varied in accordance with a strategy developed by the feed company. The fish were observed at four sampling time points, the first (FW) in May 2016 two weeks before seawater transfer, the other three throughout the following seawater period until the fish reached a size of about 2 kg, i.e. in June, four weeks after seawater transfer (SW1); in November (SW2), and in April the following year (SW3). Gut health was assessed based on histopathological indicators of lipid malabsorption and gut inflammation, expression of gut immune, barrier and other health related genes, plasma biomarkers, somatic indices of intestinal sections, as well as biomarkers of digestive functions. Seawater transfer of the fish (SW1 compared to FW) caused a marked lowering of expression of genes related to immune and barrier functions in the distal intestine, i.e. cytokines (il1β, il10, tgfβ, ifnγ), T-cell markers (cd3γδ), myd88 and tight junction proteins (zo-1, claudin-15, claudin-25b), indicating suppressed immune and barrier functions. At SW2 and SW3, most of the immune biomarkers showed values similar to those observed at FW. The development of plasma cholesterol and triglyceride levels showed similar picture, with markedly lower levels after seawater transfer. Lipid malabsorption was observed in particular in fish from SW1 and SW2, as indicated by hyper-vacuolation of the pyloric caeca enterocytes with concurrently increased expression levels of plin2. Regarding effects of functional ingredients, significantly lower condition factor and plasma triglyceride level were observed for Test-fed fish at SW2, indicating a metabolic cost of use of a mixture of nucleotides, yeast cell walls and essential fatty acids. No clear effects of functional ingredients on expression of gut immune genes and other health indexes were observed through the observation period. The great, temporary lowering of expression of gut immune and barrier genes at SW1 is suggested to be an important factor underlying the increased vulnerability of the fish at this time point. Our findings regarding supplementation with functional ingredients raise questions whether some of these ingredients overall are beneficial or might come with a metabolic cost. Our results highlight the need for a better understanding of the cause and consequences of the suppression of gut immune functions of farmed Atlantic salmon just after seawater transfer, and the use of functional ingredients under commercial conditions.

Choline supplementation prevents diet induced gut mucosa lipid accumulation in post-smolt Atlantic salmon (Salmo salar L.)

Background

Various intestinal morphological alterations have been reported in cultured fish fed diets with high contents of plant ingredients. Since 2000, salmon farmers have reported symptoms indicating an intestinal problem, which we suggest calling lipid malabsorption syndrome (LMS), characterized by pale and foamy appearance of the enterocytes of the pyloric caeca, the result of lipid accumulation. The objective of the present study was to investigate if insufficient dietary choline may be a key component in development of the LMS.

Results

The results showed that Atlantic salmon (Salmo salar), average weight 362 g, fed a plant based diet for 79 days developed signs of LMS. In fish fed a similar diet supplemented with 0.4% choline chloride no signs of LMS were seen. The relative weight of the pyloric caeca was 40% lower, reflecting 65% less triacylglycerol content and histologically normal gut mucosa. Choline supplementation further increased specific fish growth by 18%. The concomitant alterations in intestinal gene expression related to phosphatidylcholine synthesis (chk and pcyt1a), cholesterol transport (abcg5 and npc1l1), lipid metabolism and transport (mgat2a and fabp2) and lipoprotein formation (apoA1 and apoAIV) confirmed the importance of choline in lipid turnover in the intestine and its ability to prevent LMS. Another important observation was the apparent correlation between plin2 expression and degree of enterocyte hyper-vacuolation observed in the current study, which suggests that plin2 may serve as a marker for intestinal lipid accumulation and steatosis in fish. Future research should be conducted to strengthen the knowledge of choline’s critical role in lipid transport, phospholipid synthesis and lipoprotein secretion to improve formulations of plant based diets for larger fish and to prevent LMS.

Conclusions

Choline prevents excessive lipid accumulation in the proximal intestine and is essential for Atlantic salmon in seawater.

Time- and concentration-dependent expression of immune and barrier genes in the RTgutGC fish intestinal model following immune stimulation

The fish intestine comprises an important environment-organism interface that is vital to fish growth, health and pathogen defense. Yet, knowledge about the physiology and defense mechanisms toward environmental stressors, such as bacterial or viral cues, is limited and depends largely on in vivo experiments with fish. On this background, we here explore the immune competence of a recently established in vitro intestinal barrier model based on the rainbow trout (Oncorhynchus mykiss) intestinal epithelial cell line, RTgutGC. We demonstrate that the RTgutGC cell barrier reacts to two immune stimuli, the bacterial lipopolysaccharide (LPS) from Escherichia coli and the viral Poly(I:C), by regulating the mRNA abundance of selected genes in a partly time- and concentration dependent manner. The immune stimuli activated the Myd88-and Ticam-dependent signalling cascades, which resulted in downstream activation of pro-inflammatory cytokines and interferon, comparable to the regulatory patterns known from in vivo. Stimuli exposure furthermore influenced the regulation of epithelial barrier markers and resulted in slightly impaired barrier functionality after long-term exposure to LPS. Collectively, we provide proof of the usefulness of this unique cell culture model to further gain basic understanding of the fish innate immune system and to apply it in various fields, such as fish feed development and fish health in aquaculture or the evaluation of immuno-toxicity of chemical contaminants.

Gut health and vaccination response in pre-smolt Atlantic salmon (Salmo salar) fed black soldier fly (Hermetia illucens) larvae meal

Limited availability of sustainable feed ingredients is a serious concern in salmon aquaculture. Insects may become an important, sustainable resource for expanding the raw material repertoire. Herein, we present data from an 8-week feeding trial with pre-smolt Atlantic salmon (initial body weight 49 ± 1.5 g) fed either a reference diet containing fish meal, soy protein concentrate and wheat gluten as protein sources, or a test diet wherein 85% of the protein was supplied by black soldier fly larvae meal. Possible diet effect on the systemic immune response was evaluated by measuring plasma antibody titers after vaccination against infectious pancreatic necrosis virus (IPNV). The gut health of fish was evaluated using endpoints including organ and tissue indices, histopathological parameters and gene expression. Both diets induced the same level of antibody responses against IPNV. In fish fed the reference diet, the histological examination of the pyloric caeca mucosa showed clear hyper-vacuolization suggestive of lipid accumulation in enterocytes, whereas this was less pronounced in the insect meal fed fish. Expression of genes relevant to lipid metabolism confirmed these histological findings. Immune and barrier-function gene expression profiles were both generally not affected by diet. However, the fish fed insect meal showed increased expression of genes indicative of stress response, immune tolerance and increased detoxification activity. In summary, our results showed no indications that dietary inclusion of insect meal affected the gut health of Atlantic salmon negatively. The insect meal based diet seemed to reduce excessive lipid deposition in the pyloric caeca and stimulate xenobiotic metabolism.

Rainbow Trout (Oncorhynchus Mykiss) Intestinal Epithelial Cells as a Model for Studying Gut Immune Function and Effects of Functional Feed Ingredients

The objective of this study was to evaluate the suitability of the rainbow trout intestinal epithelial cell line (RTgutGC) as an in vitro model for studies of gut immune function and effects of functional feed ingredients. Effects of lipopolysaccharide (LPS) and three functional feed ingredients [nucleotides, mannanoligosaccharides (MOS), and beta-glucans] were evaluated in RTgutGC cells grown on conventional culture plates and transwell membranes. Permeation of fluorescently-labeled albumin, transepithelial electrical resistance (TEER), and tight junction protein expression confirmed the barrier function of the cells. Brush border membrane enzyme activities [leucine aminopeptidase (LAP) and maltase] were detected in the RTgutGC cells but activity levels were not modulated by any of the exposures. Immune related genes were expressed at comparable relative basal levels as these in rainbow trout distal intestine. LPS produced markedly elevated gene expression levels of the pro-inflammatory cytokines il1b, il6, il8, and tnfa but had no effect on ROS production. Immunostaining demonstrated increased F-actin contents after LPS exposure. Among the functional feed ingredients, MOS seemed to be the most potent modulator of RTgutGC immune and barrier function. MOS significantly increased albumin permeation and il1b, il6, il8, tnfa, and tgfb expression, but suppressed ROS production, cell proliferation and myd88 expression. Induced levels of il1b and il8 were also observed after treatment with nucleotides and beta-glucans. For barrier function related genes, all treatments up-regulated the expression of cldn3 and suppressed cdh1 levels. Beta-glucans increased TEER levels and F-actin content. Collectively, the present study has provided new information on how functional ingredients commonly applied in aquafeeds can affect intestinal epithelial function in fish. Our findings suggest that RTgutGC cells possess characteristic features of functional intestinal epithelial cells indicating a potential for use as an efficient in vitro model to evaluate effects of bioactive feed ingredients on gut immune and barrier functions and their underlying cellular mechanisms.

Effects of dietary sodium selenite and organic selenium sources on immune and inflammatory responses and selenium deposition in growing pigs

The study was conducted to compare effects of different dietary Se sources (sodium selenite [NaSe], Se-enriched yeast [Se yeast] or L-selenomethionine [SeMet]) and one Se-deficient control diet on the expression of selected genes, hematological and clinical biochemical parameters, and muscle morphology in two parallel trials with finisher pigs. Se concentrations in blood plasma and tissues were also monitored. From the pigs in one of the parallel groups, muscle samples obtained from Musculus longissimus dorsi (LD) before and during the trial were examined. The pigs in the other parallel group were challenged once with lipopolysaccharide (LPS) intravenously. Transcriptional analyses of LD showed that selenogenes SelenoW and H were higher expressed in pigs fed Se-supplemented diets compared with control. Furthermore, the expression of interferon gamma and cyclooxygenase 2 was lower in the Se-supplemented pigs versus control. In whole blood samples prior to LPS, SelenoN, SelenoS and thioredoxin reductase 1 were higher expressed in pigs fed NaSe supplemented feed compared with the other groups, possibly indicating a higher level of oxidative stress. After LPS exposure glutathione peroxidase 1 and SelenoN were more reduced in pigs fed NaSe compared with pigs fed organic Se. Products of most above-mentioned genes are intertwined with the oxidant-antioxidant system. No significant effects of Se-source were found on hematologic parameters or microscopic anatomy. The Se-concentrations in various skeletal muscles and heart muscle were significantly different between the groups, with highest concentrations in pigs fed SeMet, followed by those fed Se yeast, NaSe, and control diet. Consistent with previous reports our results indicate that dietary Se at adequate levels can support the body's antioxidant system. Our results indicate that muscle fibers of pigs fed organic Se are less vulnerable to oxidative stress compared with the other groups.

Alternative Protein Sources in the Diet Modulate Microbiota and Functionality in the Distal Intestine of Atlantic Salmon (Salmo salar)

The present study aimed to investigate whether alternative dietary protein sources modulate the microbial communities in the distal intestine (DI) of Atlantic salmon, and whether alterations in microbiota profiles are reflected in modifications in host intestinal function and health status. A 48-day feeding trial was conducted, in which groups of fish received one of five diets: a reference diet in which fishmeal (diet FM) was the only protein source and four experimental diets with commercially relevant compositions containing alternative ingredients as partial replacements of fishmeal, i.e., poultry meal (diet PM), a mix of soybean meal and wheat gluten (diet SBMWG), a mix of soy protein concentrate and poultry meal (diet SPCPM), and guar meal and wheat gluten (diet GMWG). Samples were taken of DI digesta and mucosa for microbial profiling using high-throughput sequencing and from DI whole tissue for immunohistochemistry and expression profiling of marker genes for gut health. Regardless of diet, there were significant differences between the microbial populations in the digesta and the mucosa in the salmon DI. Microbial richness was higher in the digesta than the mucosa. The digesta-associated bacterial communities were more affected by the diet than the mucosa-associated microbiota. Interestingly, both legume-based diets (SBMWG and GMWG) presented high relative abundance of lactic acid bacteria in addition to alteration in the expression of a salmon gene related to cell proliferation (pcna). It was, however, not possible to ascertain the cause-effect relationship between changes in bacterial communities and the host's intestinal responses to the diets.IMPORTANCE The intestine of cultivated Atlantic salmon shows symptoms of compromised function, which are most likely caused by imbalances related to the use of new feed ingredients. Intestinal microbiota profiling may become in the future a valuable endpoint measurement in order to assess fish intestinal health status and effects of diet. The present study aimed to gain information about whether alternative dietary protein sources modulate the microbial communities in the Atlantic salmon intestine and whether alterations in microbiota profiles are reflected in alterations in host intestinal function and health status. We demonstrate here that there are substantial differences between the intestinal digesta and mucosa in the presence and abundance of bacteria. The digesta-associated microbiota showed clear dependence on the diet composition, whereas mucosa-associated microbiota appeared to be less affected by diet composition. Most important, the study identified bacterial groups associated with diet-induced gut dysfunction that may be utilized as microbial markers of gut health status in fish.

Intestinal Fluid Permeability in Atlantic Salmon (Salmo salar L.) Is Affected by Dietary Protein Source

In Atlantic salmon (Salmo salar L.), and also in other fish species, certain plant protein ingredients can increase fecal water content creating a diarrhea-like condition which may impair gut function and reduce fish growth. The present study aimed to strengthen understanding of the underlying mechanisms by observing effects of various alternative plant protein sources when replacing fish meal on expression of genes encoding proteins playing key roles in regulation of water transport across the mucosa of the distal intestine (DI). A 48-day feeding trial was conducted with five diets: A reference diet (FM) in which fish meal (72%) was the only protein source; Diet SBMWG with a mix of soybean meal (30%) and wheat gluten (22%); Diet SPCPM with a mix of soy protein concentrate (30%) and poultry meal (6%); Diet GMWG with guar meal (30%) and wheat gluten (14.5%); Diet PM with 58% poultry meal. Compared to fish fed the FM reference diet, fish fed the soybean meal containing diet (SBMWG) showed signs of enteritis in the DI, increased fecal water content of DI chyme and higher plasma osmolality. Altered DI expression of a battery of genes encoding aquaporins, ion transporters, tight junction and adherens junction proteins suggested reduced transcellular transport of water as well as a tightening of the junction barrier in fish fed the SBMWG diet, which may explain the observed higher fecal water content and plasma osmolality. DI structure was not altered for fish fed the other experimental diets but alterations in target gene expression and fecal water content were observed, indicating that alterations in water transport components may take place without clear effects on intestinal structure.

Bile components and lecithin supplemented to plant based diets do not diminish diet related intestinal inflammation in Atlantic salmon

BACKGROUND

The present study was undertaken to gain knowledge on the role of bile components and lecithin on development of aberrations in digestive functions which seemingly have increased in Atlantic salmon in parallel with the increased use of plant ingredients in fish feed. Post smolt Atlantic salmon were fed for 77 days one of three basal diets: a high fish meal diet (HFM), a low fishmeal diet (LFM), or a diet with high protein soybean meal (HPS). Five additional diets were made from the LFM diet by supplementing with: purified taurocholate (1.8 %), bovine bile salt (1.8 %), taurine (0.4 %), lecithin (1.5 %), or a mix of supplements (suppl mix) containing taurocholate (1.8 %), cholesterol (1.5 %) and lecithin (0.4 %). Two additional diets were made from the HPS diet by supplementing with: bovine bile salt (1.8 %) or the suppl mix. Body and intestinal weights were recorded, and blood, bile, intestinal tissues and digesta were sampled for evaluation of growth, nutrient metabolism and intestinal structure and function.

RESULTS

In comparison with fish fed the HFM diet fish fed the LFM and HPS diets grew less and showed reduced plasma bile salt and cholesterol levels. Histological examination of the distal intestine showed signs of enteritis in both LFM and HPS diet groups, though more pronounced in the HPS diet group. The HPS diet reduced digesta dry matter and capacity of leucine amino peptidase in the distal intestine. None of the dietary supplements improved endpoints regarding fish performance, gut function or inflammation in the distal intestine. Some endpoints rather indicated negative effects.

CONCLUSIONS

Dietary supplementation with bile components or lecithin in general did not improve endpoints regarding performance or gut health in Atlantic salmon, in clear contrast to what has been previously reported for rainbow trout. Follow-up studies are needed to clarify if lower levels of bile salts and cholesterol may give different and beneficial effects, or if other supplements, and other combinations of supplements might prevent or ameliorate inflammation in the distal intestine.

Ontogeny of the Digestive System of Atlantic Salmon (Salmo salar L.) and Effects of Soybean Meal from Start-Feeding

Despite a long history of rearing Atlantic salmon in hatcheries in Norway, knowledge of molecular and physiological aspects of juvenile development is still limited. To facilitate introduction of alternative feed ingredients and feed additives during early phases, increased knowledge regarding the ontogeny of the digestive apparatus in salmon is needed. In this study, we characterized the development of the gastrointestinal tract and accessory digestive organs for five months following hatch by using histological, biochemical and molecular methods. Furthermore, the effects of a diet containing 16.7% soybean meal (SBM) introduced at start-feeding were investigated, as compared to a fishmeal based control diet. Salmon yolk sac alevins and fry were sampled at 18 time points from hatch until 144 days post hatch (dph). Histomorphological development was investigated at 7, 27, 46, 54 and 144 dph. Ontogenetic expression patterns of genes encoding key digestive enzymes, nutrient transporters, gastrointestinal peptide hormones and T-cell markers were analyzed from 13 time points by qPCR. At 7 dph, the digestive system of Atlantic salmon alevins was morphologically distinct with an early stomach, liver, pancreas, anterior and posterior intestine. About one week before the yolk sac was internalized and exogenous feeding was started, gastric glands and developing pyloric caeca were observed, which coincided with an increase in gene expression of gastric and pancreatic enzymes and nutrient transporters. Thus, the observed organs seemed ready to digest external feed well before the yolk sac was absorbed into the abdominal cavity. In contrast to post-smolt Atlantic salmon, inclusion of SBM did not induce intestinal inflammation in the juveniles. This indicates that SBM can be used in compound feeds for salmon fry from start-feeding to at least 144 dph and/or 4-5 g body weight.

Soya Saponins Induce Enteritis in Atlantic Salmon (Salmo salar L.)

Soybean meal-induced enteritis (SBMIE) is a well-described condition in the distal intestine of salmonids, and saponins have been implicated as the causal agent. However, the question remains whether saponins alone cause SBMIE. Moreover, the dose-response relationship has not been described. In a 10 week feeding trial with Atlantic salmon, a highly purified (95%) soya saponin preparation was supplemented (0, 2, 4, 6, or 10 g/kg) to two basal diets, one containing fishmeal as the major protein source (FM) and the other 25% lupin meal (LP). Saponins caused dose-dependent increases in the severity of inflammation independent of the basal diet, with concomitant alterations in digestive functions and immunological marker expression. Thus, saponins induced inflammation whether the diet contained other legume components or not. However, responses were often the same or stronger in fish fed the corresponding saponin-supplemented LP diets despite lower saponin exposure, suggesting potentiation by other legume component(s).

Effects of dietary plant meal and soya-saponin supplementation on intestinal and hepatic lipid droplet accumulation and lipoprotein and sterol metabolism in Atlantic salmon (Salmo salar L.)

Altered lipid metabolism has been shown in fish fed plant protein sources. The present study aimed to gain further insights into how intestinal and hepatic lipid absorption and metabolism are modulated by plant meal (PM) and soya-saponin (SA) inclusion in salmon feed. Post-smolt Atlantic salmon were fed for 10 weeks one of four diets based on fishmeal or PM, with or without 10 g/kg SA. PM inclusion resulted in decreased growth performance, excessive lipid droplet accumulation in the pyloric caeca and liver, and reduced plasma cholesterol levels. Intestinal and hepatic gene expression profiling revealed an up-regulation of the expression of genes involved in lipid absorption and lipoprotein (LP) synthesis (apo, fatty acid transporters, microsomal TAG transfer protein, acyl-CoA cholesterol acyltransferase, choline kinase and choline-phosphate cytidylyltransferase A), cholesterol synthesis (3-hydroxy-3-methylglutaryl-CoA reductase) and associated transcription factors (sterol regulatory element-binding protein 2 and PPARγ). SA inclusion resulted in reduced body pools of cholesterol and bile salts. The hepatic gene expression of the rate-limiting enzyme in bile acid biosynthesis (cytochrome P450 7A1 (cyp7a1)) as well as the transcription factor liver X receptor and the bile acid transporter abcb11 (ATP-binding cassette B11) was down-regulated by SA inclusion. A significant interaction was observed between PM inclusion and SA inclusion for plasma cholesterol levels. In conclusion, gene expression profiling suggested that the capacity for LP assembly and cholesterol synthesis was up-regulated by PM exposure, probably as a compensatory mechanism for excessive lipid droplet accumulation and reduced plasma cholesterol levels. SA inclusion had hypocholesterolaemic effects on Atlantic salmon, accompanied by decreased bile salt metabolism.

Effects of oral Bt-maize (MON810) exposure on growth and health parameters in normal and sensitised Atlantic salmon, Salmo salar L

Responses to GM maize Bt-maize, MON810) expressing Cry1Ab protein from the soil bacterium Bacillus thuringiensis (Bt) in diets for both normal and immune-sensitised (with soyabean meal (SBM)-induced enteropathy) post-smolt Atlantic salmon were investigated following 33 and 97 d of exposure. Triplicate tanks of salmon were fed one of four diets, all containing 20% whole-kernel meal maize, either Bt-maize or its near-isogenic maternal line, without or with 15% extracted SBM inclusion. The fish fed Bt-maize utilised the feed less efficiently, as revealed by lower protein and mineral digestibilities and lower lipid and energy retention efficiencies. Higher intestinal weight, as well as increased interferon-γ and decreased sodium-glucose co-transporter mRNA expression, and a transient increase in T-helper cell presence, as measured by cluster of differentiation 4 (CD4) protein in the distal intestine (DI), may partly explain the lower nutrient digestibilities and retentions. The Bt-maize seemed to potentiate oxidative cellular stress in the DI of immune-sensitised fish, as indicated by increases in superoxide dismutase and heat shock protein 70 mRNA expression. The data suggest that Cry1Ab protein or other antigens in Bt-maize have local immunogenic effects in salmon DI. No systemic immune responses could be detected, as indicated by haematology, differential leucocyte counts, plasma clinical chemistry, as well as absence of Cry1Ab-specific antibodies and Cry1Ab protein in plasma. The responses to Bt-maize observed in the present study differed from results from earlier studies in salmon and other animals fed the same event Bt-maize. Longer-term experiments and more in-depth studies on intestinal physiology and immune responses are needed to evaluate health implications.

Transcriptional regulation of cholesterol and bile acid metabolism after dietary soyabean meal treatment in Atlantic salmon (Salmo salar L.)

Inclusion of plant protein sources such as soyabean meal (SBM) in aquafeeds is associated with decreased lipid digestibility, reduced bile acid levels and hypocholesterolaemia. The mechanism for these metabolic abnormalities is unknown. The present study aimed at gaining further insight into how cholesterol and bile acid metabolism is modulated by SBM feeding by quantifying a number of mRNA species corresponding to key proteins involved in cholesterol and bile acid metabolism using quantitative real-time PCR. A 21 d feeding trial with sequential sampling at ten time points following initiation of 20% SBM exposure was conducted on Atlantic salmon. A histological evaluation confirmed distal intestinal enteritis after 5 d of dietary exposure to the SBM, whereas diminished glycogen/lipid deposition was the only relevant finding observed in the liver. SBM inclusion resulted in reduced body pools of cholesterol and bile acids. Hepatic gene expression profiles revealed up-regulation of genes encoding rate-limiting enzymes in cholesterol (3-hydroxy-3-methyl-glutaryl-CoA reductase; HMGCR) and bile acid (cytochrome P4507A1 (CYP7A1)) biosynthesis, as well as up-regulation of their associated transcription factors (sterol regulatory element binding proteins 1 and 2, liver X receptor, farnesoid X receptor and PPAR isoforms). Hepatic gene expressions of cholesterol (ATP binding cassette G5 (ABCG5)) and bile acid (ATP binding cassette B11 (ABCB11)) transporters were, by and large, not influenced by the SBM, but distal intestinal expression patterns of ABCG5 and apical Na-dependent bile acid transporter indicated impaired cholesterol and bile acid reabsorption. In conclusion, hepatic gene expression profiles indicated that the capacity for cholesterol and bile acid synthesis was up-regulated, whereas the indicated impaired cholesterol and bile acid reabsorption probably occurred as a direct result of distal intestinal inflammation.

Gene expression and activity of digestive enzymes during the larval development of Asian seabass (Lates calcarifer)

Knowledge of digestive enzyme development during larval stages provides a better understanding of the digestive and nutritional physiology of fish larvae. This study characterized the ontogeny of key digestive enzymes in Asian seabass larvae from hatching to juvenile stage (30 days post hatch, dph) using molecular and biochemical approaches. Gene expression and activity of pepsinogen (pg), trypsinogen (try), chymotrypsinogen (ctr), bile salt-activated lipase (bal), α-amylase (amy), leucine aminopeptidase (lap) and alkaline phosphatase (alp) were determined. Gene expression and enzyme activity of all digestive enzymes were detectable from hatching. Pepsinogen mRNA levels were close to detection limit during 0-15 dph, but were highly expressed from 18 dph and onwards. This coincided with a marked increase in specific and individual pepsin activity, indicating complete development of digestive function. Expression levels of try, ctr, amy and bal were high between 3 and 15 dph and thereafter a decreasing trend was observed. Intestinal enzymes, lap and alp, showed highest expression levels during the yolk sac stage, and thereafter levels decreased. Activity of all digestive enzymes increased from around 18 dph and onwards. In conclusion, the development of main digestive enzymes in Asian seabass larvae shows a similar pattern to that of other marine fish species.

Early response of gene expression in the distal intestine of Atlantic salmon (Salmo salar L.) during the development of soybean meal induced enteritis

Plant products in general and soybeans in particular can challenge the function and health of the intestinal tract. Salmonids develop an intestinal inflammation when fed diets containing soybean meal (SBM) and certain other legume ingredients. In the present study a 44K oligonucleotide salmonid microarray, qPCR and histology were used to investigate early response mechanisms in the distal intestine of Atlantic salmon (Salmo salar L.) during the first week of oral exposure to a diet containing 20% extracted SBM. The distal intestine transcriptome was profiled on days 1, 2, 3, 5 and 7 and compared to a control group fed fishmeal as the sole protein source. Histological evaluation of the distal intestine revealed the first signs of inflammation on day 5. The most prominent gene expression changes were seen on days 3 and 5. Up-regulation in immune-related genes was observed during the first 5 days, including GTPase IMAP family members, NF-kB-related genes and regulators of T cell and B cell function. Many functional genes involved in lipid metabolism, proteolysis, transport, metabolism and detoxification were initially up-regulated on days 1-3, possibly as an attempt by the tissue to compensate for the initiating immune response. Cell repair and extracellular matrix remodeling genes were up-regulated (heparanase, collagenase) on days 3 and 5. Down regulation of genes related to endocytosis, exocytosis, detoxification, transporters and metabolic processes from day 5 indicated initiation of dysfunction of digestive and metabolic functions that may occur as a result of inflammation or as a response to the introduction of soybean meal in the diet. This is the first study conducting transcriptomic profiling to characterize early responses during the development of SBMIE. Switching Atlantic salmon from a fishmeal to a 20% SBM diet resulted in rapid changes to the intestinal transcriptome, indicating an immune reaction with subsequent impaired epithelial barrier function and other vital intestinal functions.

Intestinal fatty acid binding protein (fabp2) in Atlantic salmon (Salmo salar): Localization and alteration of expression during development of diet induced enteritis

In the present study full-length cDNAs corresponding to three isoforms of intestinal fatty acid binding protein (fabp2) in Atlantic salmon were cloned and characterized. Gene expression of fabp2 was observed in all tissues investigated, although differences were observed between isoforms. The highest fabp2a1, fabp2a2, and fabp2b expression was in the intestine. A 15kDa protein, corresponding to putative Fabp2 protein, was identified by immunoblotting using anti-human Fabp2 antibody. Immunoblotting and immunohistochemistry confirmed that Fabp2 protein was present in most Atlantic salmon tissues. Similar to gene expression, intestinal tissues had the highest Fabp2 protein levels, decreasing gradually from proximal to distal intestine. During development of distal intestinal inflammation caused by dietary soybean meal from 0 to 21days, Fabp2 decreased significantly on both transcriptional and protein levels. The reduction in Fabp2 was preceded by a down regulation of peroxisome proliferator activated receptor (ppar) alpha and gamma, fabp2's presumed regulatory proteins, and followed by a progressive increase in proliferating cell nuclear antigen (Pcna) staining. Results illustrate that the early decline of distal intestinal fabp2 was likely caused by a down regulation of their regulatory proteins, but at later time points reduced Fabp2 may largely be due to a less mature enterocyte population resulting from rapid cell turnover.

Dietary soyasaponin supplementation to pea protein concentrate reveals nutrigenomic interactions underlying enteropathy in Atlantic salmon (Salmo salar)

BACKGROUND

Use of plant ingredients in aquaculture feeds is impeded by high contents of antinutritional factors such as saponins, which may cause various pharmacological and biological effects. In this study, transcriptome changes were analyzed using a 21 k oligonucleotide microarray and qPCR in the distal intestine of Atlantic salmon fed diets based on five plant protein sources combined with soybean saponins.

RESULTS

Diets with corn gluten, sunflower, rapeseed or horsebean produced minor effects while the combination of saponins with pea protein concentrate caused enteritis and major transcriptome changes. Acute inflammation was characterised by up-regulation of cytokines, NFkB and TNFalpha related genes and regulators of T-cell function, while the IFN-axis was suppressed. Induction of lectins, complement, metalloproteinases and the respiratory burst complex parallelled a down-regulation of genes for free radical scavengers and iron binding proteins. Marked down-regulation of xenobiotic metabolism was also observed, possibly increasing vulnerability of the intestinal tissue. A hallmark of metabolic changes was dramatic down-regulation of lipid, bile and steroid metabolism. Impairment of digestion was further suggested by expression changes of nutrient transporters and regulators of water balance (e.g. aquaporin, guanylin). On the other hand, microarray profiling revealed activation of multiple mucosal defence processes. Annexin-1, with important anti-inflammatory and gastroprotective properties, was markedly up-regulated. Furthermore, augmented synthesis of polyamines needed for cellular proliferation (up-regulation of arginase and ornithine decarboxylase) and increased mucus production (down-regulation of glycan turnover and goblet cell hyperplasia) could participate in mucosal healing and restoration of normal tissue function.

CONCLUSION

The current study promoted understanding of salmon intestinal pathology and establishment of a model for feed induced enteritis. Multiple gene expression profiling further characterised the inflammation and described the intestinal pathology at the molecular level.

Peroxisome proliferator-activated receptors, estrogenic responses and biotransformation system in the liver of salmon exposed to tributyltin and second messenger activator

The mechanisms by which organotin compounds produce modulations of the endocrine systems and other biological responses are not fully understood. In this study, juvenile salmon were force-fed diet containing TBT (0: solvent control, 0.1, 1 and 10mg/kg fish) for 72 h. Subsequently, fish exposed to solvent control and 10mg TBT were exposed to waterborne concentration (200 microg/l) of the adenylate cyclase (AC) stimulator, forskolin for 2 and 4h. The overall aim of the study was to explore whether TBT endocrine disruptive effects involve second messenger activation. Liver was sampled from individual fish (n=8) at the end of the exposures. The transcription patterns of peroxisome proliferator-activated receptor (PPAR) isotype and acyl-coenzyme A oxidase 1 (ACOX1), aromatase isoform, estrogen receptor-alpha (ER alpha), pregnane X receptor (PXR), CYP3A and glutathione S-transferase (GST) genes were measured by quantitative polymerase chain reaction (qPCR). Our data showed a consistent increase in PPAR alpha, PPAR beta and PPAR gamma mRNA and protein expression after TBT exposure that were inversely correlated with ACOX1 mRNA levels. Forskolin produced PPAR isotype-specific mRNA and protein effects that were modulated by TBT. ACOX1 expression was decreased (at 2h) and increased (at 4h) by forskolin and the presence of TBT potentiated these effects. TBT apparently increased mRNA and protein levels of cyp19a, compared to the solvent control, whereas cyp19b mRNA levels were unaffected by TBT treatment. Combined TBT and forskolin exposure produced respective decrease and increase of mRNA levels of cyp19a and cyp19b, compared with control. TBT decreased ER alpha mRNA at low dose (1mg/kg) and forskolin exposure alone produced a consistent decrease of ER alpha mRNA levels that were not affected by the presence of TBT. Interestingly, PXR and CYP3A mRNA levels were differentially affected, either decreased or increased, after exposure to TBT and forskolin, singly and also in combination. GST mRNA was increased by TBT exposure. Exposure to forskolin alone increased GST expression with time, and combined exposure with TBT potentiated these respective effects. Overall, the present study demonstrates multiple biological effects of TBT given singly or in combination with cAMP activator. There are no studies known to us that have evaluated the endocrine disruptive effects of TBT in the presence of a second messenger activator, and our data suggest that TBT may exert endocrine, biotransformation and lipid peroxidative effects through modulation of cAMP/PKA second messenger signaling with overt physiological consequences.

Modulation of acute steroidogenesis, peroxisome proliferator-activated receptors and CYP3A/PXR in salmon interrenal tissues by tributyltin and the second messenger activator, forskolin

There are uncertainties regarding the role of sex steroids in sexual development and reproduction of gastropods, leading to the recent doubts as to whether organotin compounds do inhibit steroidogenic enzymes in these species. These doubts have led us to suspect that organotin compounds may affect other target molecules, particularly signal transduction molecules or secondary mediators of steroid hormone and lipid synthesis/metabolism. Therefore, we have studied the effects of TBT exposure through food on acute steroidogenesis, PPARs and CYP3A responses in the presence and absence of a cyclic AMP (cAMP) activator, forskolin. Two experiments were performed. Firstly, juvenile salmon were force-fed once with diet containing TBT doses (0.1, 1 and 10mg/kg fish) dissolved in ethanol and sampled after 72h. Secondly, fish exposed to solvent control and 10mg/kg TBT for 72h were transferred to new tanks and exposed to waterborne forskolin (200microg/L) for 2 and 4h. Our data show that juvenile salmon force-fed TBT showed modulations of multiple biological responses in interrenal tissues that include, steroidogenesis (cAMP/PKA activities; StAR and P450scc mRNA, and plasma cortisol), and mRNA for peroxisome proliferator-activated receptor (PPAR) isoforms (alpha, beta, gamma), acyl-CoA oxidase-1 (ACOX1) and CYP3A/PXR (pregnan X receptor). In addition, forskolin produced differential effects on these responses both singly and also in combination with TBT. Overall, combined forskolin and TBT exposure produced higher effects compared with TBT exposure alone, for most of the responses (cortisol, PPARbeta, ACOX1 and CYP3A). Interestingly, forskolin produced PPAR isoform-specific effects when given singly or in combination with TBT. Several TBT mediated toxicity in fish that includes thymus reduction, decrease in numbers of lymphocytes, inhibition of gonad development and masculinization, including the imposex phenomenon have been reported. When these effects are considered with the present findings, it suggests that studies on mechanisms of action or field studies may reveal endocrine, reproductive or other effects of TBT at lower concentrations than those reported to date from subchronic tests of fishes. Since the metabolic fate of organotin compounds may contribute to the toxicity of these chemicals, the present findings may represent some new aspects of TBT toxicity not previously reported.

Effects of tributyltin on salmon interrenal CYP11β, steroidogenic factor-1 and glucocorticoid receptor transcripts in the presence and absence of second messenger activator, forskolin

The mechanisms by which TBT produces modulations of the endocrine systems are not fully described. In this study, juvenile salmon were force-fed diet containing TBT (0: solvent control, 0.1, 1 and 10 mg/kg fish) for 72 h. Subsequently, fish exposed to solvent control and 10 mg/kg TBT were exposed to waterborne concentration of the adenyl cyclase stimulator forskolin (200 μg/L) for 2 and 4 h. Tissue and blood were sampled from individual fish (n=6). Gene expression patterns of CYP11β, steroidogenic factor-1 (SF-1), and glucocorticoid receptor (GlucR) were determined by qPCR. TBT generally decreased mRNA levels of CYP11β, GlucR and SF-1, compared to the solvent control and these effects were differentially modulated by the presence of forskolin. This study suggests that TBT may exert broader endocrine disrupting effects through possible modulation of cAMP/PKA second messenger systems.

Modulation of salmon ovarian steroidogenesis and growth factor responses by the xenoestrogen, 4-nonylphenol

Endocrine-disrupting chemicals are known to influence organismal reproductive processes, including the production and regulation of gonadal steroids. This study evaluated the effects of a xenoestrogen (nonylphenol: NP) on salmon ovarian steroidogenesis and growth factors using an in vitro organ culture system. Ovarian tissues were cultivated for 3 and 7d with different concentrations of NP (0 (control), 1, 10 and 50 microM) dissolved in ethanol (0.1%). The mRNA expressions of steroidogenic acute regulatory (StAR) protein, P450-mediated cholesterol side-chain cleavage (P450scc), aromatase isoforms, 3beta-hydroxysteroid dehydrogenase (3beta-HSD), Cyp11beta-, Cyp17 and 21-hydroxylase, and insulin-like growth factors (IGF-1 and IGF-2) and IGF1-receptor (IGF1-R) were quantified by real-time PCR. Tissue levels of estradiol-17beta (E2), testosterone (T) and 11-ketotestosterone (11-KT) were quantified using enzyme immunoassays. Our data show that nominal NP levels produced time- and concentration-specific effects on the expression of steroidogenesis- and IGF-related transcripts in salmon ovarian tissues. Tissue levels of ovarian E2, T and 11-KT were significantly modulated after NP exposure. Interestingly, elevated ovarian E2 levels after 10 microM NP exposure at day 3 paralleled P450Arom isoforms mRNA expression at the same time interval. The expression patterns of other steroidogenic protein and enzyme genes, such as StAR, P450scc, 3beta-HSD and Cyp17 inversely paralleled this pattern, displaying consistent decreased transcript levels. These findings show that NP (an ubiquitous environmental pollutant) can produce variations in gonadal steroidogenesis and growth regulating responses with potential consequences for overt fecundity in teleosts.

Neural aromatase transcript and protein levels in Atlantic salmon (Salmo salar) are modulated by the ubiquitous water pollutant, 4-nonylphenol

At present, there are no known direct occurrences of nonylphenol (NP) in nature. Therefore, its presence in nature is solely a consequence of human activities. NP is generated through degradation of alkylphenol ethoxylates released mainly from textile, metal working, institutional cleansing and laundry cleaning, but few data on the amount of the release is available. These compounds have been shown to affect several biological processes, including the endocrine systems, in a wide number of species. The cytochrome P450 aromatase (Cyp19) is the rate-limiting step in estrogen production, and is known to be a potential target for endocrine-disrupting chemicals (EDCs) such as NP. Teleost fish generally have a high brain aromatase activity, and the effects of EDCs in fish brain is not thoroughly investigated. In this study, juvenile Atlantic salmon (Salmo salar) were exposed to waterborne concentrations of the synthetic pharmaceutical and xenoestrogen 17alpha-ethynylestradiol (EE2; 5ng/L) and the xenoestrogen 4-nonylphenol (NP; 5 and 50microg/L) for 72h. Brain tissue and blood were sampled from individual fish. Gene expression patterns of Cyp19 isoforms were determined by quantitative PCR, aromatase protein immunoreactivity in the brain was evaluated by immunohistochemistry and immunoblotting, and aromatase activity was analyzed using the tritiated water-release assay. Plasma estradiol (E2) and testosterone (T) levels were measured by EIA. In the brain, EE2 increased the mRNA expression of Cyp19b almost threefold compared to the solvent control, whereas Cyp19a levels were unaffected by EE2 treatment. In contrast, both NP concentrations produced significant reduction of Cyp19a expression. Immunohistochemical aromatase protein reactivity was localized in several brain regions, but no apparent quantitative effects of the exposures were observed. Immunoblotting analysis showed that EE2 and NP produced a slight increase in brain immunoreactive aromatase protein band, compared with controls. Plasma levels of E2 increased twofold when treated with EE2 and 5microg NP/L, and threefold when exposed to 50microg NP/L. In general, the present study shows that the parallel biochemical, transcriptional and cellular detection of neural aromatase for endocrine-disrupting effects from EE2 and NP may be observed at specific levels of the biological organization.

Androgenic modulation of early growth of Atlantic cod (Gadus morhua L.) previtellogenic oocytes and zona radiata-related genes

Available evidence suggests that androgens play critical roles in early oocyte growth and development in fish. However, the molecular mechanisms underlying this important aspect of reproductive endocrinology have not yet been established. In this study the effects of androgens (11-ketotestosterone [11-KT] and testosterone [T]) were determined on gene expression patterns and growth of cod previtellogenic oocytes, using an in vitro oocyte culture technique. Previtellogenic ovarian tissue was cultured for 5 and 10 d at different concentrations of 11-KT and T (0, 1, or 1000 microM) dissolved in ethanol (0.3%). The androgen concentrations were selected as they represent physiological and supra-physiological concentrations, respectively. Quantitative polymerase chain reaction (PCR) demonstrated increased mRNA expression for five genes recently identified as androgen responsive in our subtracted cDNA library in previtellogenic cod ovary exposed in vitro to androgens. Quantitative histological analyses showed a consistent stereological validation of oocyte growth and development after exposure to androgens. In general, both 11-KT and T induced previtellogenic oocyte growth and development, and these effects were more pronounced with 11-KT exposure. Taken together, our study reveals some novel roles of androgens on the development of previtellogenic oocytes, indicating control of early follicular and oocyte growth in cod ovary. The potent effects of 11-KT on oocyte growth support our earlier hypothesis that non-aromatizable androgens play significant roles in regulating early oocyte growth with potential consequences for the fecundity process. Therefore, these novel roles of androgens as promoters of ovarian growth and development presented in this study may be useful for the aquaculture industry and for breeding of new captive and endangered species. From a toxicological point of view, the cod is a marine species and exposure to complex chemical mixtures that may exert androgenic and/or anti-androgenic effects represents an environmental issue of reasonable concern in the marine environment. Therefore, the findings in the present study represent a novel basis that can be used to determine the effects of xenoandrogens on oocyte development and fecundity in this important marine species.

Thyroid hormone-dependent gene expression as a biomarker of short-term 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE) exposure in European common frog (Rana temporaria) tadpoles

The effects on thyroid hormone-dependent gene biomarker responses of the persistent organochlorine pesticide metabolite 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE) were investigated after exposure of 4-week-old European common frog (Rana temporaria) (stage 36) tadpoles to two (0.001 and 0.01 ppm) DDE concentrations. Total body weight, total length, and tail length and width increased after 3-day exposure to DDE. Expression patterns of genes encoding for growth hormone, thyroid-stimulating hormone (TSHbeta) and thyroid hormone receptor (TRalpha and TRbeta) isoforms were evaluated in the head, body and tail regions using a validated real-time polymerase chain reaction (PCR) method. The mRNA expression of growth hormone in the body, and TSHbeta in the head showed significant DDE concentration-dependent decreases. While DDE caused variable effects on TRalpha mRNA steady-state, the expression of TRbeta was significantly decreased in the tail by DDE in a concentration-specific manner. The effect of DDE exposure on TRbeta mRNA expression showed a negative correlation with tail length and width during the exposure period. The unique pattern of a DDE-induced decrease of tail TRbeta expression probably reflects the significant role of this thyroid hormone receptor isoform in tail re-absorption and overall metamorphosis in anuran species. Therefore, the present study shows that the evaluation of thyroid hormone-dependent genes may represent quantitative biomarkers of acute exposure to organochlorine pesticides in anuran species during critical developmental periods such as metamorphosis. Given the widespread environmental levels of DDT and its metabolites, these pollutants will remain a subject of concern and their effects on anuran species should be studied in more detail.

Modulation of steroidogenesis and xenobiotic biotransformation responses in zebrafish (Danio rerio) exposed to water-soluble fraction of crude oil

The induction of CYP enzyme activities, particularly CYP1A1, through the aryl hydrocarbon receptor (AhR) in most vertebrate species is among the most studied biochemical response to planar and aromatic organic contaminant exposure. Since P450 families play central roles in the oxidative metabolism of a wide range of exogenous and endogenous compounds, interactions between the biotransformation processes and reproductive physiological responses are inevitable. Steroidogenesis is the process by which specialized cells in specific tissues, such as the gonad, brain (neurosteroids) and kidney, synthesize steroid hormones. In the present study, we evaluated the effects of water-soluble fraction (WSF) of crude oil on the xenobiotic biotransformation and steroidogenic processes in the head (brain) and whole-body tissue of a model species by transcript analysis using quantitative (real-time) polymerase chain reaction (qPCR), enzyme activities and steroid hormone (testosterone: T and 17beta-estradiol: E2) levels using enzyme immune assay (EIA). Our data showed that exposure of fish to WSF produced an apparent concentration-specific increase of AhR1, CYP1A1 and 3beta-hydroxysteroid dehydrogenase (3beta-HSD) mRNA levels, and decrease of AhR2. On the activity level, WSF produced concentration-specific increase of ethoxyresorufin O-deethylase (EROD), benzyloxyresorufin (BROD) methoxyresorufin (MROD) and pentoxyresorufin (PROD) activities in whole-body tissue. In the steroidogenic pathway, WSF exposure produced apparent concentration-specific decrease of ER* and ERbeta, steroidogenic acute regulatory (StAR) protein, cytochrome P450 side-chain cleavage (P450scc), P450aromA and P450aromB mRNA expression. For steroid hormones, while T levels decreased, E2 levels increased in an apparent WSF concentration-specific manner. In general, the xenobiotic biotransformation and estrogenic responses showed negative relationship after exposure of zebrafish to WSF, suggesting an interaction between these physiological pathways. The relationship between WSF mediated changes in brain StAR, P450scc, 3beta-HSD, ER*alpha, ERbeta, P450aromA, P450aromB and whole-body steroid hormone levels suggests that the experimental animals might be experiencing altered neurosteroidogenesis probably through increased activity level of the biotransformation system. Thus, these responses might represent sensitive diagnostic tools for short-term and acute exposure of fish or other aquatic organisms to WSF.

Effects of 17alpha-methyltestosterone exposure on steroidogenesis and cyclin-B mRNA expression in previtellogenic oocytes of Atlantic cod (Gadus morhua)

Steroid hormone (estrogens and androgens) synthesis and regulation involve a large number of enzymes and potential biochemical pathways. In the context of these biochemical pathways, it is believed that the true rate-limiting step in acute steroid production is the movement of cholesterol across the mitochondrial membrane by the steroidogenic acute regulatory (StAR) protein and the subsequent conversion to pregnenolone by cytochrome P450-mediated side-chain cleavage (P450scc) enzyme. Oocyte development is a complex process that is triggered by the maturation-promoting factor (MPF) involving cyclin-B as a regulatory factor. In the present study, we evaluated the endocrine effects of 17alpha-methyltestosterone (MT) on steroidogenic pathways of Atlantic cod (Gadus morhua), using an in vitro previtellogenic oocyte culture technique that is based on an agarose floating method. Tissue was cultured in a humidified incubator at 10 degrees C for 1, 5, 10 and 20 days with different concentrations of the synthetic androgen MT (0 (control), 1, 10, 100 and 1000 microM) dissolved in ethanol (0.3%). Gene expressions for StAR, P450scc, aromatase-alpha (P450aromA) and cyclin-B were detected using validated real-time PCR with specific primer pairs. Cellular localization of the StAR protein and P450scc were performed using the immunohistochemical technique with antisera prepared against synthetic peptide for both proteins. Steroid hormones (estradiol-17beta: E2 and testosterone: T) levels were estimated using enzyme immunoassay. Our data showed significant concentration-specific increase (at day 1 and 5) and decrease (at day 10 and 20) of the StAR mRNA expression after exposure to MT. P450scc expression showed a MT concentration-specific decrease during the exposure periods and cyclin-B mRNA expression was decreased in MT concentration-dependent manner at days 10 and 20 (reaching almost total inhibition after exposure to 1000 microM MT). MT exposure produced variable effects on the P450aromA mRNA expression that can be described as concentration-specific increase (day 1) and decrease (days 5 and 10). Cellular localization of the StAR protein and P450scc demonstrated their expression mainly in ovarian follicular cells. MT produced an apparent concentration-and time-dependent increase of E2 and T levels. Thus, the present study reveals some novel effects of pharmaceutical endocrine disruptor on the development of previtellogenic oocytes in cod. The impaired steroidogenesis and hormonal imbalance reported in the present study may have potential consequences for the vitellogenic process and overt fecundity in teleosts.