Metabolism and fatty acid profile in fat and lean rainbow trout lines fed with vegetable oil: effect of carbohydrates

Dietary ARA, DHA, and Carbohydrate Ratios Affect the Immune Status of Gilthead Sea Bream Juveniles upon Bacterial Challenge

This study aims to assess the effects of different dietary n-6/n-3 long-chain polyunsaturated fatty acid ratios and CHO content in the immune response of gilthead seabream. For that purpose, gilthead sea bream juveniles (initial body weight = 47.5 g) were fed for 84 days with four isoproteic (47% crude protein) and isolipidic (18% crude lipids) diets with high (20%) or low (5%) level of gelatinized starch (HS or LS diets, respectively) and included approximately 2.4% ARA or DHA. At the end of the trial, the DHA-enriched groups presented increased red blood cell (RBC) count, hemoglobin, plasmatic nitric oxide (NO) content, and antiprotease and alternative complement activities. The ARA groups had increased thrombocyte count, and plasmatic bactericidal activity against Vibrio anguillarum was lower in the fish fed the ARA/LS diet. After the feeding trial, the fish were challenged with an intraperitoneal injection (i.p.) of killed Photobacterium damselae subsp. piscicida (Phdp) and sampled at 4 and 24 h after the challenge. At 4 h after i.p., the ARA groups presented increased plasma total immunoglobulins (Ig) and bactericidal activity against V. anguillarum. In addition, the fish fed the ARA/LS diet presented lower white blood cell (WBC) and alternative complement activity. At 24 h after i.p., the ARA groups presented increased RBC, WBC, and thrombocyte numbers, total IG, plasma peroxidase activity, and casp3 expression in the distal intestine. The HS groups presented increased plasma NO content and bactericidal activity against Phdp and decreased protease, antiprotease activity, and bactericidal activity against V. anguillarum. In conclusion, high dietary DHA levels seemed to improve the immune status of unchallenged gilthead sea bream juveniles, while high dietary ARA levels improved the fish immune response to a bacterial challenge. The energy provided by dietary starch seems to be important to promote a fast response by the fish immune system after a challenge.

Establishing brown trout primary hepatocyte spheroids as a new alternative experimental model-Testing the effects of 5α-dihydrotestosterone on lipid pathways

Three-dimensional (3D) fish liver cultures mimic the in vivo cellular microenvironment, which is ideal for ecotoxicological research. Despite that, the application of these cultures to evaluate toxic effects in fish is scarce. A 3D model of brown trout (Salmo trutta f. fario) primary hepatocyte spheroids was optimized in this study by using DMEM/F-12 with 15 mM of HEPES, 10 mL/L of an antibiotic and antimycotic solution and FBS 10% (v/v), at 18 °C with ∼100 rpm. The selection of optimal conditions was based on a multiparametric characterization of the spheroids, including biometry, viability, microanatomy and immunohistochemistry. Biometric and morphologic stabilization of spheroids was reached within 12-16 days of culture. To our knowledge, this study is the first to culture and characterize viable spheroids from brown trout primary hepatocytes for over 30 days. Further, the 3D model was tested to explore the androgenic influences on lipidic target genes after 96 h exposures to control, solvent control, 10 and 100 µM of 5α-dihydrotestosterone (DHT), a non-aromatizable androgen. Spheroids exposed to 100 µM of DHT had decreased sphericity. DHT at 100 µM also significantly down-regulated Acox1-3I, PPARγ and fatty acid synthesis targets (i.e., ACC), and significantly up-regulated Fabp1. Acsl1 was significantly up-regulated after exposure to both 10 and 100 µM of DHT. The results support that DHT modulates distinct lipidic pathways in brown trout and show that this 3D model is a new valuable tool for physiological and toxicological mechanistic studies.

Time-Restricted Feeding Could Not Reduce Rainbow Trout Lipid Deposition Induced by Artificial Night Light

Artificial night light (ALAN) could lead to circadian rhythm disorders and disrupt normal lipid metabolism, while time-restricted feeding (TRF) could maintain metabolic homeostasis. In mammals, TRF has been demonstrated to have extraordinary effects on the metabolic regulation caused by circadian rhythm disorders, but studies in lower vertebrates such as fish are still scarce. In this study, the impacts of ALAN on the body composition and lipid metabolism of juvenile rainbow trout were investigated by continuous light (LL) exposure as well as whether TRF could alleviate the negative effects of LL. The results showed that LL upregulated the expression of lipid synthesis (fas and srebp-1c) genes and suppressed the expression of lipid lipolysis (pparβ, cpt-1a, and lpl) genes in the liver, finally promoting lipid accumulation in juvenile rainbow trout. However, LL downregulated the expression of genes (Δ6-fad, Δ9-fad, elovl2, and elovl5) related to long-chain polyunsaturated fatty acid (LC-PUFA) synthesis, resulting in a significant decrease in the proportion of LC-PUFA in the dorsal muscle. In serum, LL led to a decrease in glucose (Glu) levels and an increase in triglyceride (TG) and high-density lipoprotein cholesterol (H-DLC) levels. On the other hand, TRF (mid-dark stage feeding (D)) and mid-light stage feeding (L)) upregulated the expression of both the lipid synthesis (srebp-1c and pparγ), lipolysis (pparα, pparβ, and cpt-1a), and lipid transport (cd36/fat and fatp-1) genes, finally increasing the whole-body lipid, liver protein, and lipid content. Meanwhile, TRF (D and L groups) increased the proportion of polyunsaturated fatty acid (PUFA) and LC-PUFA in serum. In contrast, random feeding (R group) increased the serum Glu levels and decreased TG, total cholesterol (T-CHO), and H-DLC levels, suggesting stress and poor nutritional status. In conclusion, ALAN led to lipid accumulation and a significant decrease in muscle LC-PUFA proportion, and TRF failed to rescue these negative effects.

Fatty Acid Incorporation in the Muscle, Oxidative Markers, Lipid Peroxidation and PPAR-α and SREBP-2 Expression of Zebrafish Fed Linseed Oil and Clove Leaf Essential Oil

The objective of this study is to assess, in zebrafish, the effects of combining linseed oil (LO) and clove leaf essential oil (CLEO) on the incorporation of fatty acids in the muscle, oxidative markers, lipid peroxidation and expression of the PPAR-α (Peroxisome Proliferator-Activated Receptor-α) and the SREBP-2 (Sterol Regulatory Element Binding Protein-2) genes. Six diets were prepared, containing combinations of LO (3, 6 and 9%) and CLEO (0.5 and 1%): 3% LO + 0.5% CLEO; 3% LO + 1% CLEO; 6% LO + 0.5% CLEO; 6% LO + 1% CLEO; 9% LO + 0.5% CLEO; 9% LO + 1% CLEO. Results showed increase in the incorporation of n-3 fatty acids in the muscle concomitantly with the addition of LO and CLEO. The activities of superoxide dismutase and catalase were reduced and the glutathione content had increased. Lipid peroxidation was lower in the treatment with 1% CLEO, regardless of LO content. The expression of the PPAR-α and the SREBP-2 genes was higher in animals fed 9% LO + 0.5% CLEO. Therefore, for a greater incorporation and protection against the oxidative damages of n-3 fatty acids, a combined use of 9% LO with 0.5% CLEO is recommended for zebrafish.

Concurrent changes in thermal tolerance thresholds and cellular heat stress response reveals novel molecular signatures and markers of high temperature acclimation in rainbow trout

The objective of this study was to better understand the molecular mechanisms which regulate acclimatory responses and thermal safety margins of rainbow trout (Oncorhynchus mykiss) at temperatures above physiological optimum. For this, we investigated the time course of changes in critical thermal tolerance thresholds and associated hepatic and renal transcript abundance of molecular markers related to cellular stress response, during high temperature acclimation. The experimental fish were initially acclimated to 17 °C and later exposed to a gradually raised elevated temperature regime (22 °C) for a period of 30 days. CT_max, CT_min and mRNA expression of candidate markers were examined before the thermal challenge (T₀) and over the time-course (days) of high temperature exposure (T₁, T₃, T₇, T₁₅ and T₃₀). With respect to organismal response, CT_max was significantly elevated at T₃, but the degree of gain in heat tolerance was not persistent. Contrarily, we observed a gradual loss in cold tolerance with highest CT_min estimate at T₃₀. Based on the time-course of mRNA expression, the studied markers could be categorized into those which were persistently elevated (hsp70a, hsp70b, hspa5, hsp90a, hsp90b, stip1 and serpinh1 in kidney and hsp90b in liver); those which concurred with changes in CT_min (hspbp1, hsp90b, stip1, gr1, hif1a, hyou1, tnfa and tlr5 in kidney); and those which concurred with changes in CT_max (hsp90a, serpinh1, tlr5 and lmo2 in liver). Apparently, transcriptional changes in kidney and liver reflected CT_min and CT_max trend, respectively. Expression profile of stip1 and tlr5 suggest that they are potential novel markers which could reflect thermal limits in rainbow trout. Hepatic metabolic markers were either initially elevated (alt, glud, g6pase1) or down-regulated at different time-points (ast2, gls1, fas, cpt1b, mtor), linked to gluconeogenesis and metabolic depression, respectively. Whereas, growth-axis markers showed no significant differences. Overall, this time-course analysis has revealed potential associations in organismal and tissue-specific cellular response to high temperature acclimation in a thermally sensitive coldwater ectotherm.

Genetic architecture and genomic selection of fatty acid composition predicted by Raman spectroscopy in rainbow trout

Background

In response to major challenges regarding the supply and sustainability of marine ingredients in aquafeeds, the aquaculture industry has made a large-scale shift toward plant-based substitutions for fish oil and fish meal. But, this also led to lower levels of healthful n−3 long-chain polyunsaturated fatty acids (PUFAs)—especially eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids—in flesh. One potential solution is to select fish with better abilities to retain or synthesise PUFAs, to increase the efficiency of aquaculture and promote the production of healthier fish products. To this end, we aimed i) to estimate the genetic variability in fatty acid (FA) composition in visceral fat quantified by Raman spectroscopy, with respect to both individual FAs and groups under a feeding regime with limited n-3 PUFAs; ii) to study the genetic and phenotypic correlations between FAs and processing yields- and fat-related traits; iii) to detect QTLs associated with FA composition and identify candidate genes; and iv) to assess the efficiency of genomic selection compared to pedigree-based BLUP selection.

Results

Proportions of the various FAs in fish were indirectly estimated using Raman scattering spectroscopy. Fish were genotyped using the 57 K SNP Axiom™ Trout Genotyping Array. Following quality control, the final analysis contained 29,652 SNPs from 1382 fish. Heritability estimates for traits ranged from 0.03 ± 0.03 (n-3 PUFAs) to 0.24 ± 0.05 (n-6 PUFAs), confirming the potential for genomic selection. n-3 PUFAs are positively correlated to a decrease in fat deposition in the fillet and in the viscera but negatively correlated to body weight. This highlights the potential interest to combine selection on FA and against fat deposition to improve nutritional merit of aquaculture products. Several QTLs were identified for FA composition, containing multiple candidate genes with indirect links to FA metabolism. In particular, one region on Omy1 was associated with n-6 PUFAs, monounsaturated FAs, linoleic acid, and EPA, while a region on Omy7 had effects on n-6 PUFAs, EPA, and linoleic acid. When we compared the effectiveness of breeding programmes based on genomic selection (using a reference population of 1000 individuals related to selection candidates) or on pedigree-based selection, we found that the former yielded increases in selection accuracy of 12 to 120% depending on the FA trait.

Conclusion

This study reveals the polygenic genetic architecture for FA composition in rainbow trout and confirms that genomic selection has potential to improve EPA and DHA proportions in aquaculture species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08062-7.

Digesta and Plasma Metabolomics of Rainbow Trout Strains with Varied Tolerance of Plant-Based Diets Highlights Potential for Non-Lethal Assessments of Enteritis Development

The replacement of fishmeal in aquafeeds is essential to the sustainability of aquaculture. Besides the procurement of alternative protein sources, fish can also be selected for better performance on plant-based alternative diets. Rainbow trout (Oncorhynchus mykiss) is one such species in which the strain ARS-Sel has been selected for higher growth and enhanced utilization when fed soy-based diets. The aim of this study was to compare fish growth and plasma and digesta metabolomes between ARS-Sel and two commercial strains (CS-1 and CS-2), when fed plant-protein (PM) and fishmeal-based (FM) diets, and to correlate them with the onset of enteritis. An NMR-metabolomics approach was taken to assess plasma and digesta metabolite profiles. Diet and strain showed significant effects on fish growth, with the ARS-Sel fish receiving the PM diet reaching the highest final weight at sampling. Multivariate analysis revealed differences between plasma and digesta metabolite profiles of ARS-Sel and CS (CS-1 considered together with CS-2) PM-fed groups in the early stages of enteritis development, which was confirmed by intestinal histology. As reported in previous studies, the ARS-Sel strain performed better than the commercial strains when fed the PM diet. Our findings also suggest that metabolomic profiles of plasma and digesta, samples of which can be obtained through non-lethal methods, offer valuable insight in monitoring the occurrence of enteritis in carnivorous aquaculture species due to plant-based diets.

Interplay between dietary lipids and cadmium exposure in rainbow trout liver: Influence on fatty acid metabolism, metal accumulation and stress response

The present study aimed at investigating interactive effects between dietary lipids and both short- and long-term exposures to a low, environmentally realistic, cadmium (Cd) concentration. Juvenile rainbow trout were fed four isolipidic diets (31.7 g/kg) enriched in either linoleic acid (LA, 18:2n-6), alpha-linolenic acid (ALA, 18:3n-3), eicosapentaenoic acid (EPA, 20:5n-3) or docosahexaenoic acid (DHA, 22:6n-3). From the 4th week of this 10-week experiment, the lipid level of the diet was increased (120.0 g/kg) and half of the fish fed each diet were aqueously exposed to Cd (0.3 μg/L) while the other half were not exposed to Cd (control). Fish were sampled and their liver was harvested for fatty acid profile, hepatic Cd and calcium concentrations, total glutathione level and gene expression assessment, either (i) after 4 weeks of feeding and 24 h of Cd contamination (day 29) (short-term Cd exposure) or (ii) after 10 weeks of feeding and 6 weeks of Cd contamination (day 70) (long-term Cd exposure). We found that both dietary lipids and Cd exposure influenced fatty acid homeostasis and metabolism. The hepatic fatty acid profile mostly reflected that of the diet (e.g. n-3/n-6 ratio) with some differences, including selective retention of specific long chain polyunsaturated fatty acids (LC-PUFAs) like DHA and active biotransformation of dietary LA and ALA into LC-PUFAs. Cd effects on hepatic fatty acid profiles were influenced by the duration of the exposure and the nutritional status of the fish. The effects of diet and Cd exposure on the fatty acid profiles were only sparsely explained by variation of the expression pattern of genes involved in fatty acid metabolism. The biological responses to Cd were also influenced by dietary lipids. Fish fed the ALA-enriched diet seemed to be the least affected by the Cd exposure, as they showed a higher detoxifying ability against Cd with an early upregulation of protective metallothionein a (MTa) and apoptosis regulator BCL2-Like1 (BCLx) genes, an increased long-term phospholipid synthesis and turnover and fatty acid bioconversion efficiency, as well as a lower long-term accumulation of Cd in their liver. In contrast, fish fed the EPA-enriched diet seemed to be the most sensitive to a long-term Cd exposure, with an impaired growth performance and a decreased antioxidant capacity (lower glutathione level). Our results highlight that low, environmentally realistic aqueous concentrations of Cd can affect biological response in fish and that these effects are influenced by the dietary fatty acid composition.

Effects of different dietary carbohydrate-to-lipid ratios on growth, plasma biochemical indexes, digestive, and immune enzymes activities of sub-adult orange-spotted grouper Epinephelus coioides

A 10-week feeding trial was conducted to investigate the effects of dietary carbohydrate-to-lipid (CHO:L) ratios on glycogen content, hematological indices, liver, and intestinal enzyme activity of sub-adult grouper Epinephelus coioides. Five iso-nitrogenous (496.0 g kg^-1 protein) and iso-energetic (21.6 KJ g^-1 gross energy) diets with varying CHO: L ratios of 0.65 (D1), 1.31 (D2), 2.33 (D3), 4.24 (D4), and 8.51 (D5), respectively, were fed to triplicate groups of 20 fish (average 275.1 ± 1.86 g). Results showed that the weight gain rate (WGR), specific growth rate (SGR), and protein efficiency ratio (PER) of sub-adult grouper increased and then stable when dietary CHO:L ratios reach D4 (CHO:L = 4.24). The trend of feed conversion ratio (FCR) was opposite to PER. Along with the dietary CHO:L ratios, the liver and muscle glycogen level increased gradually. Plasma triglycerides (TG) and glucose (GLU) were all maximized at D5 (CHO:L = 8.51) group, cholesterol (CHOL) at D4 (CHO:L = 4.24) group. Digestive enzyme activities were significantly affected by dietary CHO:L ratios. Liver hexokinase (HK), alkaline phosphatase (AKP), and glucose-6-phosphate dehydrogenase (G6PDH) activity increased significantly as CHO:L ratios increased. Liver lysozyme (LYZ) and superoxide dismutase (SOD) activity of sub-adult grouper fed the D4 diet was significantly higher than that of the D2 (CHO:L = 1.31) diet. The trend of acid phosphatase (ACP) is opposite to AKP. The regression model analysis showed that the most suitable dietary CHO:L ratio to reach the highest SGR is 6.06.

Effects of Dietary Carbohydrate to Lipid Ratios on Growth Performance, Muscle Fatty Acid Composition, and Intermediary Metabolism in Juvenile Black Seabream (Acanthopagrus schlegelii)

An 8-week feeding trial was conducted to evaluate the effects of dietary carbohydrate to lipid (CHO:L) ratios on growth performance, muscle fatty acid composition, and intermediary metabolism in juvenile black seabream (Acanthopagrus schlegelii). Five isonitrogenous and isoenergetic diets (48.0% crude protein and 18.0 MJ kg-1 gross energy) were formulated to contain different CHO:L ratios ranging from 0.33 to 3.75. Triplicate groups of 20 fish averaging 0.51 ± 0.01 g were fed with experimental diets twice daily to apparent satiation. The results indicated that final body weight (FBW), percentage weight gain (PWG), specific growth rate (SGR), and protein efficiency ratio (PER) were significantly influenced by the dietary CHO:L ratios (p < 0.05). The highest FBW, PWG, and SGR were observed in fish fed the diet with a CHO:L ratio of 1.36 (p < 0.05). A two-slope broken-line regression analysis based on PWG indicated that the optimal dietary CHO:L is 1.08. Lipid content in the whole body decreased, and glycogen concentration in the liver increased with the increase of dietary CHO:L ratios from 0.33 to 3.75 (p < 0.05). Moreover, there was a positive correlation between muscle fatty acid composition and dietary fatty acid composition. The relative expression levels of genes involved in glucose metabolism, such as gk, pepck, and glut2 were upregulated by increasing the dietary CHO:L ratio. Also, the mRNA expression level of genes related to lipid synthesis, such as fas and accα were significantly upregulated with dietary CHO:L ratios increasing from 0.33 to 3.75. The highest expression of genes involved in fatty acid β-oxidation, such as cpt1 and acox1, were observed in fish fed the 1.36 CHO:L ratio diet. The gene expression of Δ6 fatty acyl desaturase (fads2) in the liver significantly increased with increase of dietary CHO:L ratios from 0.33 to 3.75. Fish fed the diet with CHO:L ratios of 2.26 and 3.75 had lower expression levels of elovl5 than those fed the other diets. These results demonstrate that dietary optimal CHO:L ratios could improve PWG and SGR but also influence expression of genes involved in glucose and lipid metabolism. Based on the overall results, the optimal dietary CHO:L ratio is 1.08 for black seabream.

Diets supplemented with Saccharina latissima influence the expression of genes related to lipid metabolism and oxidative stress modulating rainbow trout (Oncorhynchus mykiss) fillet composition

This study aimed to evaluate the impact of diets including increasing amounts (1, 2 and 4%) of an iodine-rich macroalgae, Saccharina latissima, on gene expression and fillet composition of commercial-sized rainbow trout. Liver and muscle expression of genes related to growth, iodine, oxidative stress, and lipid metabolism, and the fillet content of fatty acids, cholesterol, and vitamin D3 were assessed. The highest kelp inclusion led to lower final body weight and HSI, without significant differences in mRNA transcription of genes involved in growth (ghr1, ghr2 and igf1) or iodine metabolism (dio1, thra, and thrb). A significant downregulation of an oxidative stress marker, gpx1b2, was observed in fish fed 2% S. latissima, which might suggest the need for less endogenous antioxidants. Dietary inclusion of kelp impacted lipid metabolism, with a downregulation of fatty acid synthase, accompanied by a general decrease of fatty acids in fillet. The present study demonstrated that supplementation of diets with 1 or 2% S. latissima can be achieved without detrimental effects on rainbow trout final weight. Evidence suggest a lipid-lowering effect of diets that did not compromise fillet EPA and DHA concentrations, being 3.7 times above the recommended levels for human consumption.

Impact of dietary starch on extrahepatic tissue lipid metabolism in farmed European (Dicentrarchus labrax) and Asian seabass (Lates calcarifer)

In aquaculture, there is high interest in substituting marine-derived with vegetable-based ingredients as energy source. Farmed carnivorous fish under high carbohydrate diets tend to increase adiposity but it remains unclear if this happens by increased lipid retention/accumulation, promotion of lipogenic pathways, or both. In order to determine the response of extrahepatic tissue to dietary starch, European (Dicentrarchus labrax) and Asian (Lates calcarifer) seabass were fed a control (low starch; LS) or experimental (high starch; HS) diet, for at least 21 days and then transferred for 6 days to saltwater enriched with deuterated water ²H₂O. Incorporation of ²H-labelling follows well-defined metabolic steps, and analysis of triacylglycerols (TAG) ²H-enrichment by ²HNMR allowed evaluation of de novo lipogenesis (DNL) in muscle and visceral adipose tissue (VAT). Fractional synthetic rates for TAG-bound fatty acids and glycerol were quantified separately providing a detailed lipogenic profile. The FA profile differed substantially between muscle and VAT in both species, but their lipogenic fluxes revealed even greater differences. In European seabass, HS promoted DNL of TAG-bound FA, in muscle and VAT. High ²H-enrichment also found in muscle TAG-bound glycerol was indicative of its role on lipid cycling. In Asian seabass, HS had no effect on muscle FA composition and lipogenic flux, with no ²H-enriched TAG being detected. VAT on the other hand revealed a strong enhancement of DNL in HS-fed fish along with high TAG-bound glycerol cycling. This study consolidated the use of ²H₂O as tracer for fish lipid metabolism in different tissues, under different dietary conditions and suitable to use in different fish models.

Exploring the interactions between polyunsaturated fatty acids and cadmium in rainbow trout liver cells: a genetic and proteomic study

Polyunsaturated fatty acids (PUFAs) have key biological roles in fish cells. We recently showed that the phospholipid composition of rainbow trout liver cells (RTL-W1 cell line) modulates their tolerance to an acute cadmium (Cd) challenge. Here, we investigated (i) the extent to which PUFAs and Cd impact fatty acid homeostasis and metabolism in these cells and (ii) possible mechanisms by which specific PUFAs may confer cytoprotection against Cd. First, RTL-W1 cells were cultivated for one week in growth media spiked with 50 μmol L^-1 of either alpha-linolenic acid (ALA, 18:3n-3), eicosapentaenoic acid (EPA, 20:5n-3), linoleic acid (LA, 18:2n-6) or arachidonic acid (AA, 20:4n-6) in order to modulate their fatty acid profile. Then, the cells were challenged with Cd (0, 50 or 100 μmol L^-1) for 24 h prior to assaying viability, fatty acid profile, intracellular Cd content, proteomic landscape and expression levels of genes involved in fatty acid metabolism, synthesis of PUFA-derived signalling molecules and stress response. We observed that the fatty acid supply and, to a lesser extent, the exposure to Cd influenced cellular fatty acid homeostasis and metabolism. The cellular fatty acid composition of fish liver cells modulated their tolerance to an acute Cd challenge. Enrichments in ALA, EPA, and, to a lesser extent, AA conferred cytoprotection while enrichment in LA had no impact on cell viability. The present study ruled out the possibility that cytoprotection reflects a decreased Cd burden. Our results rather suggest that the PUFA-derived cytoprotection against Cd occurs through a reduction of the oxidative stress induced by Cd and a differential induction of the eicosanoid cascade, with a possible role of peroxiredoxin and glutaredoxin (antioxidant enzymes) as well as cytosolic phospholipase A2 (enzyme initiating the eicosanoid cascade).

Regulation by Dietary Carbohydrates of Intermediary Metabolism in Liver and Muscle of Two Isogenic Lines of Rainbow Trout

Rainbow trout (Oncorhynchus mykiss) is recognized as a typical "glucose-intolerant" fish, and the limits of dietary carbohydrate utilization have been investigated for many years. In this study, the objective was to test the molecular effects of dietary carbohydrates on intermediary metabolism in two major metabolic tissues, liver and muscle. Another objective was also to study if the response to carbohydrate intake depended on the genetic background. We fed two isogenic lines of rainbow trout (named A22h and N38h) with high carbohydrate diet (carbohydrate, 22.9%) or low carbohydrate diet (carbohydrate, 3.6%) for 12 weeks. Carbohydrates were associated with higher feed utilization owned by the well-known protein-sparing effect, with better fish growth performance. However, atypical regulation of glycolysis and gluconeogenesis in liver and absence of hk and glut4 induction in muscle, were also observed. Regarding the effects of carbohydrates on other metabolism, we observed an increased, at a molecular level, of hepatic cholesterol biosynthesis, fatty acid oxidation and mitochondrial energy metabolism. Genetic variability (revealed by the differences between the two isogenic lines) was observed for some metabolic genes especially for those involved in the EPA and DHA biosynthetic capacity. Finally, our study demonstrates that dietary carbohydrate not only affect glucose metabolism but also strongly impact the lipid and energy metabolism in liver and muscle of trout.

Transcriptional effects of phospholipid fatty acid profile on rainbow trout liver cells exposed to methylmercury

Lipids, and their constitutive fatty acids, are key nutrients for fish health as they provide energy, maintain cell structure, are precursors of signalling molecules and act as nuclear receptor ligands. These specific roles may be of crucial importance in a context of exposure to pollutants. We recently showed that the fatty acid profile of rainbow trout liver cell phospholipids modulates sensitivity to an acute methylmercury challenge. In order to investigate mechanisms of effects, we herein tested whether specific polyunsaturated fatty acids (PUFAs) may protect cells from methylmercury through decreasing intracellular mercury accumulation and/or enhancing cellular defences (e.g. via modulation of gene expression patterns). We also investigated the inverse relationship and assessed the impact of methylmercury on cellular fatty acid metabolism. To do so, the fatty acid composition of rainbow trout liver cell phospholipids was first modified by incubating them in a medium enriched in a specific PUFA from either the n-3 family (alpha-linolenic acid, ALA; eicosapentaenoic acid, EPA) or the n-6 family (linoleic acid, LA; arachidonic acid, AA). Cells were then exposed to methylmercury (0.15 or 0.50 μM) for 24 h and sampled thereafter for assessing phospholipid fatty acid profile, intracellular total mercury burden, and expression pattern of genes involved in fatty acid metabolism, synthesis of PUFA-derived signalling molecules and stress response. We observed that cells incorporated the given PUFA and some biotransformation products in their phospholipids. Methylmercury had few impacts on this cellular phospholipid composition. None of the PUFA enrichments affected the cellular mercury burden, suggesting that the previously observed cytoprotection conferred by ALA and EPA was not linked to a global decrease in cellular accumulation of mercury. Fatty acid enrichments and methylmercury exposure both modulated gene expression patterns. Genes involved in the synthesis of PUFA-derived signalling molecules, in stress response and the orphan cytochrome P450 20A1 were identified as possible sites of interaction between fatty acids and methylmercury in rainbow trout liver cells.

Social status affects lipid metabolism in rainbow trout, Oncorhynchus mykiss

Juvenile rainbow trout ( Oncorhynchus mykiss) confined in pairs form social hierarchies in which socially subordinate fish display characteristic traits, including reduced growth rates and altered glucose metabolism. These effects are, in part, mediated by chronically elevated cortisol levels and/or reduced feeding. To determine the effects of social status on lipid metabolism, trout were held in pairs for 4 days, following which organismal and liver-specific indexes of lipid metabolism were measured. At the organismal level, circulating triglycerides were elevated in dominant trout, whereas subordinate trout exhibited elevated concentrations of circulating free fatty acids (FFAs) and lowered plasma total cholesterol levels. At the molecular level, increased expression of lipogenic genes in dominant trout and cpt1a in subordinate trout was identified, suggesting a contribution of increased de novo lipogenesis to circulating triglycerides in dominant trout and reliance on circulating FFAs for β-oxidation in the liver of subordinates. Given the emerging importance of microRNAs (miRNA) in the regulation of hepatic lipid metabolism, candidate miRNAs were profiled, revealing increased expression of the lipogenic miRNA-33 in dominant fish. Because the Akt-TOR-S6-signaling pathway is an important upstream regulator of hepatic lipid metabolism, its signaling activity was quantified. However, the only difference detected among groups was a strong increase in S6 phosphorylation in subordinate trout. In general, the changes observed in lipid metabolism of subordinates were not mimicked by either cortisol treatment or fasting alone, indicating the existence of specific, emergent effects of subordinate social status itself on this fuel.

Cloning and expression characterization of peroxisome proliferator-activated receptors (PPARs) with their agonists, dietary lipids, and ambient salinity in rabbitfish Siganus canaliculatus

Rabbitfish Siganus canaliculatus is the first marine teleost reported to have the ability of biosynthesizing C_20-22 long-chain polyunsaturated fatty acids (LC-PUFA) from C₁₈ precursors, and thus provides a model for studying the regulatory mechanisms of LC-PUFA biosynthesis in teleosts. To investigate the possible roles of peroxisome proliferator-activated receptors (PPARs), critical transcription factors involved in the regulation of lipid metabolism, in the regulation of LC-PUFA biosynthesis in rabbitfish, the PPAR genes were cloned and their expression characterization with PPAR agonists, dietary lipid resource, and ambient salinity were examined. Three cDNA sequences respectively encoding 477, 516 and 519 amino acids of PPARα, PPARβ, and PPARγ isoforms were obtained. PPARα exhibited a wide tissue expression with its highest levels in the heart and brain; PPARβ was predominantly expressed in the gills, while PPARγ was highly expressed in the intestine and gills. In rabbitfish primary hepatocytes, both the PPAR agonists 2-bromopalmitate (2-Bro) and fenofibrate (FF) increased the expression of PPARγ, SREBP1c and Elovl5, whereas FF depressed the expression of Δ6/Δ5 Fad. Moreover, a higher hepatic PPARβ expression was observed in fish fed diets with vegetable oils (VO) than that with fish oil (FO), in the former the expression of PPARα, PPARβ, and PPARγ were increased at the low ambient salinity (10ppt), where an increasing expression of Δ5/Δ6 Fad, Δ4 Fad and Elovl5 genes was previously reported. These results suggest that PPARs might be involved in the upregulation of LC-PUFA biosynthesis with dietary VO and low ambient salinity in rabbitfish.

Nutrient Sensing Systems in Fish: Impact on Food Intake Regulation and Energy Homeostasis

Evidence obtained in recent years in a few species, especially rainbow trout, supports the presence in fish of nutrient sensing mechanisms. Glucosensing capacity is present in central (hypothalamus and hindbrain) and peripheral [liver, Brockmann bodies (BB, main accumulation of pancreatic endocrine cells in several fish species), and intestine] locations whereas fatty acid sensors seem to be present in hypothalamus, liver and BB. Glucose and fatty acid sensing capacities relate to food intake regulation and metabolism in fish. Hypothalamus is as a signaling integratory center in a way that detection of increased levels of nutrients result in food intake inhibition through changes in the expression of anorexigenic and orexigenic neuropeptides. Moreover, central nutrient sensing modulates functions in the periphery since they elicit changes in hepatic metabolism as well as in hormone secretion to counter-regulate changes in nutrient levels detected in the CNS. At peripheral level, the direct nutrient detection in liver has a crucial role in homeostatic control of glucose and fatty acid whereas in BB and intestine nutrient sensing is probably involved in regulation of hormone secretion from endocrine cells.

Dietary selenium disrupts hepatic triglyceride stores and transcriptional networks associated with growth and Notch signaling in juvenile rainbow trout

Dietary Se has been shown to adversely affect adult fish by altering growth rates and metabolism. To determine the underlying mechanisms associated with these observations, we measured biochemical and transcriptomic endpoints in rainbow trout following dietary Se exposures. Treatment groups of juvenile rainbow trout were fed either control Lumbriculus variegatus worms or worms cultured on selenized yeast. Selenized yeast was cultured at four nominal doses of 5, 10, 20 or 40mg/kg Se dry weight (measured dose in the worms of 7.1, 10.7, 19.5, and 31.8mg/kgSedw respectively) and fish were fed for 60days. At 60 d, hepatic triglycerides, glycogen, total glutathione, 8-isoprostane and the transcriptome response in the liver (n=8/group) were measured. Fish fed the nominal dose of 20 and 40mg/kg Se dry weight had lower body weight and a shorter length, as well as lower triglyceride in the liver compared to controls. Evidence was lacking for an oxidative stress response and there was no change in total glutathione, 8-isoprostane levels, nor relative mRNA levels for glutathione peroxidase isoforms among groups. Microarray analysis revealed that molecular networks for long-chain fatty acid transport, lipid transport, and low density lipid oxidation were increased in the liver of fish fed 40mg/kg, and this is hypothesized to be associated with the lower triglyceride levels in these fish. In addition, up-regulated gene networks in the liver of 40mg/kg Se treated fish included epidermal growth factor receptor signaling, growth hormone receptor, and insulin growth factor receptor 1 signaling pathways. These molecular changes are hypothesized to be compensatory and related to impaired growth. A gene network related to Notch signaling, which is involved in cell-cell communication and gene transcription regulation, was also increased in the liver following dietary treatments with both 20 and 40mg/kg Se. Transcriptomic data support the hypothesis that dietary Se increases the expression of networks for growth-related signaling cascades in addition to those related to fatty acid synthesis and metabolism. We propose that the disruption of metabolites related to triglyceride processing and storage, as well as gene networks for epidermal growth factor and Notch signaling in the liver, represent key molecular initiating events for adverse outcomes related to growth and Se toxicity in fish.

Replacement of Marine Fish Oil with de novo Omega-3 Oils from Transgenic Camelina sativa in Feeds for Gilthead Sea Bream (Sparus aurata L.)

Omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFA) are essential components of the diet of all vertebrates. The major dietary source of n-3 LC-PUFA for humans has been fish and seafood but, paradoxically, farmed fish are also reliant on marine fisheries for fish meal and fish oil (FO), traditionally major ingredients of aquafeeds. Currently, the only sustainable alternatives to FO are vegetable oils, which are rich in C18 PUFA, but devoid of the eicosapentaenoic (EPA) and docosahexaenoic acids (DHA) abundant in FO. Two new n-3 LC-PUFA sources obtained from genetically modified (GM) Camelina sativa containing either EPA alone (ECO) or EPA and DHA (DCO) were compared to FO and wild-type camelina oil (WCO) in juvenile sea bream. Neither ECO nor DCO had any detrimental effects on fish performance, although final weight of ECO-fed fish (117 g) was slightly lower than that of FO- and DCO-fed fish (130 and 127 g, respectively). Inclusion of the GM-derived oils enhanced the n-3 LC-PUFA content in fish tissues compared to WCO, although limited biosynthesis was observed indicating accumulation of dietary fatty acids. The expression of genes involved in several lipid metabolic processes, as well as fish health and immune response, in both liver and anterior intestine were altered in fish fed the GM-derived oils. This showed a similar pattern to that observed in WCO-fed fish reflecting the hybrid fatty acid profile of the new oils. Overall the data indicated that the GM-derived oils could be suitable alternatives to dietary FO in sea bream.

Transcriptome Analyses Reveal Lipid Metabolic Process in Liver Related to the Difference of Carcass Fat Content in Rainbow Trout (Oncorhynchus mykiss)

Excessive accumulation of carcass fat in farm animals, including fish, has a significant impact on meat quality and on the cost of feeding. Similar to farmed animals and humans, the liver can be considered one of the most important organs involved in lipid metabolism in rainbow trout (Oncorhynchus mykiss). RNA-seq based whole transcriptome sequencing was performed to liver tissue of rainbow trout with high and low carcass fat content in this study. In total 1,694 differentially expressed transcripts were identified, including many genes involved in lipid metabolism, such as L-FABP, adiponectin, PPAR-α, PPAR-β, and IGFBP1a. Evidence presented in this study indicated that lipid metabolic process in liver may be related to the difference of carcass fat content. The relevance of PPAR-α and PPAR-β as molecular markers for fat storage in liver should be worthy of further investigation.

Regulation of glucose and lipid metabolism by dietary carbohydrate levels and lipid sources in gilthead sea bream juveniles

The long-term effects on growth performance, body composition, plasma metabolites, liver and intestine glucose and lipid metabolism were assessed in gilthead sea bream juveniles fed diets without carbohydrates (CH–) or carbohydrate-enriched (20 % gelatinised starch, CH+) combined with two lipid sources (fish oil; or vegetable oil (VO)). No differences in growth performance among treatments were observed. Carbohydrate intake was associated with increased hepatic transcripts of glucokinase but not of 6-phosphofructokinase. Expression of phosphoenolpyruvate carboxykinase was down-regulated by carbohydrate intake, whereas, unexpectedly, glucose 6-phosphatase was up-regulated. Lipogenic enzyme activities (glucose-6-phosphate dehydrogenase, malic enzyme, fatty acid synthase) and ∆6 fatty acyl desaturase (FADS2) transcripts were increased in liver of fish fed CH+ diets, supporting an enhanced potential for lipogenesis and long-chain PUFA (LC-PUFA) biosynthesis. Despite the lower hepatic cholesterol content in CH+ groups, no influence on the expression of genes related to cholesterol efflux (ATP-binding cassette G5) and biosynthesis (lanosterol 14α-demethylase, cytochrome P450 51 cytochrome P450 51 (CYP51A1); 7-dehydrocholesterol reductase) was recorded at the hepatic level. At the intestinal level, however, induction of CYP51A1 transcripts by carbohydrate intake was recorded. Dietary VO led to decreased plasma phospholipid and cholesterol concentrations but not on the transcripts of proteins involved in phospholipid biosynthesis (glycerol-3-phosphate acyltransferase) and cholesterol metabolism at intestinal and hepatic levels. Hepatic and muscular fatty acid profiles reflected that of diets, despite the up-regulation ofFADS2transcripts. Overall, this study demonstrated that dietary carbohydrates mainly affected carbohydrate metabolism, lipogenesis and LC-PUFA biosynthesis, whereas effects of dietary lipid source were mostly related with tissue fatty acid composition, plasma phospholipid and cholesterol concentrations, and LC-PUFA biosynthesis regulation. Interactions between dietary macronutrients induced modifications in tissue lipid and glycogen content.

Recognition of purified beta 1,3/1,6 glucan and molecular signalling in the intestine of Atlantic salmon

Atlantic salmon was orally intubated with a highly purified β-glucan product (MacroGard(®)) to study the recognition of the molecule by the receptor genes, the regulation of the downstream signalling genes and global proteins, and the micromorphological changes in the intestine. The β-glucan receptor genes of Atlantic salmon, sclra, sclrb, sclrc and cr3, seem to recognize the molecule, and initiate the downstream ITAM-motif signalling, as evident from the significantly high mRNA levels of ksyk, mapkin2, il1b and mip2a levels. Among the altered proteins, the Apoa4 (involved in carbohydrate and lipid metabolism); Tagln, Actb (uptake of β-glucan); Psma2 (associated with substrate recognition); and Ckt (energy metabolism-related) were the overexpressed ones. The underexpressed proteins included the Uk114, Rpl9, Ctsb and Lgal that are connected to proliferation, LPS-stimulation, Il1b and lactose recognition, respectively. Furthermore, the mRNA levels of igt and the number of immune cells in the distal intestine were found to increase upon β-glucan uptake by the fish. This study provides some clues on the mechanisms by which the β-glucan evokes response in Atlantic salmon, particularly at the intestinal level.

Dietary carbohydrate and lipid source affect cholesterol metabolism of European sea bass (Dicentrarchus labrax) juveniles

Plant feedstuffs (PF) are rich in carbohydrates, which may interact with lipid metabolism. Thus, when considering dietary replacement of fishery by-products with PF, knowledge is needed on how dietary lipid source (LS) and carbohydrates affect lipid metabolism and other metabolic pathways. For that purpose, a 73-d growth trial was performed with European sea bass juveniles (IBW 74 g) fed four diets differing in LS (fish oil (FO) or a blend of vegetable oils (VO)) and carbohydrate content (0 % (CH-) or 20 % (CH+) gelatinised starch). At the end of the trial no differences among diets were observed on growth and feed utilisation. Protein efficiency ratio was, however, higher in the CH+ groups. Muscle and liver fatty acid profiles reflected the dietary LS. Dietary carbohydrate promoted higher plasma cholesterol and phospholipids (PL), whole-body and hepatic (mainly 16 : 0) lipids and increased muscular and hepatic glycogen. Except for PL, which were higher in the FO groups, no major alterations between FO and VO groups were observed on plasma metabolites (glucose, TAG, cholesterol, PL), liver and muscle glycogen, and lipid and cholesterol contents. Activities of glucose-6-phosphate dehydrogenase and malic enzyme - lipogenesis-related enzymes - increased with carbohydrate intake. Hepatic expression of genes involved in cholesterol metabolism was up-regulated with carbohydrate (HMGCR and CYP3A27) and VO (HMGCR and CYP51A1) intake. No dietary regulation of long-chain PUFA biosynthesis at the transcriptional level was observed. Overall, very few interactions between dietary carbohydrates and LS were observed. However, important insights on the direct relation between dietary carbohydrate and the cholesterol biosynthetic pathway in European sea bass were demonstrated.

Feeding rainbow trout with a lipid-enriched diet: effects on fatty acid sensing, regulation of food intake, and cellular signaling pathways

Using rainbow trout fed with low fat (LF) or high-fat (HF) diets we aimed to determine if the response of food intake, mRNA abundance of hypothalamic neuropeptides involved in the metabolic regulation of food intake, and fatty acid (FA) sensing systems in hypothalamus and liver is similar to that previously observed when levels of specific FA were raised by injection. Moreover, we also aimed to determine if the phosphorylation state of intracellular energy sensor 5′-AMP-activated protein kinase (AMPK), and proteins involved in cellular signalling such as protein kinase B (Akt) and target of rapamycin (mTOR) display changes that could be related to FA-sensing and the control of food intake. The increased levels of FA in hypothalamus and liver of rainbow trout fed the HF diet only partially activated FA-sensing systems and did not elicit changes in food intake suggesting that FA-sensing response in fish to increased levels of FA is more dependent on the presence of specific FA such as oleate or octanoate rather than to the global increase in FA. We also obtained, for the first time in fish, evidence for the presence and function of energy sensors like AMPK and proteins involved in cellular signaling like mTOR and Akt in hypothalamus. These proteins in hypothalamus and liver were generally activated in fish fed the HF vs LF diet suggesting the activation of the cellular signaling pathways in response to the increased availability of FA.

Comparison of glucose and lipid metabolic gene expressions between fat and lean lines of rainbow trout after a glucose load

Two experimental rainbow trout lines developed through divergent selection for low (Lean ‘L’ line) or high (Fat ‘F’ line) muscle fat content were used as models to study the genetic determinism of fat depots. Previous nutritional studies suggested that the F line had a better capability to use glucose than the L line during feeding trials. Based on that, we put forward the hypothesis that F line has a greater metabolic ability to clear a glucose load effectively, compared to L line. In order to test this hypothesis, 250 mg/kg glucose was intraperitoneally injected to the two rainbow trout lines fasted for 48 h. Hyperglycemia was observed after glucose treatment in both lines without affecting the phosphorylation of AMPK (cellular energy sensor) and Akt-TOR (insulin signaling) components. Liver glucokinase and glucose-6-phosphate dehydrogenase expression levels were increased by glucose, whereas mRNA levels of β-oxidation enzymes (CPT1a, CPT1b, HOAD and ACO) were down-regulated in the white skeletal muscle of both lines. Regarding the genotype effect, concordant with normoglycemia at 12 h after glucose treatment, higher muscle glycogen was found in F line compared to L line which exhibited hyperglycemia. Moreover, mRNA levels of hepatic glycolytic enzymes (GK, 6PFK and PK), gluconeogenic enzyme PEPCK and muscle fatty acid oxidation enzymes (CPT1a, CPT1b and HOAD) were concurrently higher in the F line. Overall, these findings suggest that F line may have a better ability to maintain glucose homeostasis than L line.