Modulations of lipid metabolism and development of the Atlantic salmon (Salmo salar) fry in response to egg-to-fry rearing conditions.

In stocking program, the use of artificial incubation conditions in hatcheries from the fertilisation of eggs to the release of unfed fry could reduce their ability to adapt to the natural environment. This study evaluates the effects of three factors on the fitness and physiology of salmon fry at their emergence, the origin of water (river vs drilling), the type of support in the incubator (support matrix vs plastic sheets) and the type of incubators (Californian vs vertical trays), and compares them to a semi-natural incubation method in river. Key biological functions including nutritional and immune status were compared among experimental conditions using biometric parameters, lipid composition and gene expression analyses. Our findings demonstrated that fry incubated in vertical trays supplied with river water had no significant difference in growth and lipid composition compared to those in semi-natural incubators. Besides, fry incubated on a substrate matrix in Californian trays exhibited phenotypic characteristics closest to those incubated in river. This support matrix improved fish growth, lipid consumption and distribution compared to fry on plastic sheets. Moreover, the large amounts of several PUFAs in these fry could allow a better membrane fluidity ensuring a better adaptation to temperature variation under cold conditions. In addition, drilling water improved the survival rate compared to river water due to lower numbers of fine particles, known to be responsible for the clogging of eggs. To conclude, using a substrate combined with drilling water in artificial incubators could increase fry fitness and its adaption to wild life.

A n-3 PUFA depletion applied to rainbow trout fry (Oncorhynchus mykiss) does not modulate its subsequent lipid bioconversion capacity.

Nutritional strategies are currently developed to produce farmed fish rich in n-3 long-chain PUFA (LC-PUFA) whilst replacing fish oil by plant-derived oils in aquafeeds. The optimisation of such strategies requires a thorough understanding of fish lipid metabolism and its nutritional modulation. The present study evaluated the fatty acid bioconversion capacity of rainbow trout (Oncorhynchus mykiss) fry previously depleted in n-3 PUFA through a 60-d pre-experimental feeding period with a sunflower oil-based diet (SO) followed by a 36-d experimental period during which fish were fed either a linseed oil-based diet (LO) (this treatment being called SO/LO) or a fish oil-based diet (FO) (this treatment being called SO/FO). These treatments were compared with fish continuously fed on SO, LO or FO for 96 d. At the end of the 36-d experimental period, SO/LO and SO/FO fish recovered >80 % of the n-3 LC-PUFA reported for LO and FO fish, respectively. Fish fed on LO showed high apparent in vivo elongation and desaturation activities along the n-3 biosynthesis pathway. However, at the end of the experimental period, no impact of the fish n-3 PUFA depletion was observed on apparent in vivo elongation and desaturation activities of SO/LO fish as compared with LO fish. In contrast, the fish n-3 PUFA depletion negatively modulated the n-6 PUFA bioconversion capacity of fish in terms of reduced apparent in vivo elongation and desaturation activities. The effects were similar after 10 or 36 d of the experimental period, indicating the absence of short-term effects.

Effects of the total replacement of fish-based diet with plant-based diet on the hepatic transcriptome of two European sea bass (Dicentrarchus labrax) half-sibfamilies showing different growth rates with the plant-based diet.

BACKGROUND: Efforts towards utilisation of diets without fish meal (FM) or fish oil (FO) in finfish aquaculture have been being made for more than two decades. Metabolic responses to substitution of fishery products have been shown to impact growth performance and immune system of fish as well as their subsequent nutritional value, particularly in marine fish species, which exhibit low capacity for biosynthesis of long-chain poly-unsaturated fatty acids (LC-PUFA). The main objective of the present study was to analyse the effects of a plant-based diet on the hepatic transcriptome of European sea bass (Dicentrarchus labrax). RESULTS: We report the first results obtained using a transcriptomic approach on the liver of two half-sibfamilies of the European sea bass that exhibit similar growth rates when fed a fish-based diet (FD), but significantly different growth rates when fed an all-plant diet (VD). Overall gene expression was analysed using oligo DNA microarrays (GPL9663). Statistical analysis identified 582 unique annotated genes differentially expressed between groups of fish fed the two diets, 199 genes regulated by genetic factors, and 72 genes that exhibited diet-family interactions. The expression of several genes involved in the LC-PUFA and cholesterol biosynthetic pathways was found to be up-regulated in fish fed VD, suggesting a stimulation of the lipogenic pathways. No significant diet-family interaction for the regulation of LC-PUFA biosynthesis pathways could be detected by microarray analysis. This result was in agreement with LC-PUFA profiles, which were found to be similar in the flesh of the two half-sibfamilies. In addition, the combination of our transcriptomic data with an analysis of plasmatic immune parameters revealed a stimulation of complement activity associated with an immunodeficiency in the fish fed VD, and different inflammatory status between the two half-sibfamilies. Biological processes related to protein catabolism, amino acid transaminations, RNA splicing and blood coagulation were also found to be regulated by diet, while the expression of genes involved in protein and ATP synthesis differed between the half-sibfamilies. CONCLUSIONS: Overall, the combined gene expression, compositional and biochemical studies demonstrated a large panel of metabolic and physiological effects induced by total substitution of both FM and FO in the diets of European sea bass and revealed physiological characteristics associated with the two half-sibfamilies.

Molecular cloning and 3D model of a fatty-acid elongase in a carnivorous freshwater teleost, the European perch (Perca fluviatilis).

The European perch (Perca fluviatilis) is a carnivorous freshwater fish able to metabolise polyunsaturated fatty acids (PUFA) into highly unsaturated fatty acids (HUFA). This makes it a potential candidate for sustainable aquaculture development. In this study, special attention is given to the fatty-acid elongase (ELOVL) family, one of the two enzymatic systems implied in the HUFA biosynthesis. Structural information on European perch enzyme converting PUFA into HUFA is obtained by both molecular cloning and in silico characterization of an ELOVL5-like elongase from P. fluviatilis (pfELOVL). The full-length cDNA sequence consists of a 885-base pair Open Reading Frame coding for a 294-amino acid protein. Phylogenetic analysis and sequence alignment with fish elongases predict the pfELOVL clusters within the ELOVL5 sub-group. The amino-acid sequence displays the typical ELOVL features: several transmembrane α helices (TMH), an endoplasmic reticulum (ER) retention signal, and four "conserved boxes" involved in the catalytic site. In addition, the topology analysis predicts a 7-TMH structure addressed in the ER membrane. A 3D model of the protein embedded in an ER-like membrane environment is also provided using de novo modelling and molecular dynamics. From docking studies, two putative enzyme-substrate-binding modes, including H bonds and CH-π interactions, emphasize the role of specific residues in the "conserved boxes".

Tropical Vegetable Oils Do Not Alter Growth Performance in African Catfish through a High n-6 Polyunsaturated Fatty Acids Biosynthesis Capacity.

The main objective of this study was to determine the best vegetable oils (VO) for nutrition of African catfish by assessing the effects of a complete replacement of fish oil (FO) by different VO sources on its growth performance, fatty acid composition, and elongase-desaturase gene expression levels. Fish (16.2 g of initial body weight) were fed with five experimental isonitrogenous, isolipidic, and isoenergetic diets in which FO was totally replaced by cottonseed oil (CO), palm oil (PO), desert date oil (DO), or Shea butter (SB). Complete replacement of FO with VO did not affect growth performance except for low values in fish fed SB diet. Muscle n-3 LC-polyunsaturated fatty acids (PUFA) were significantly reduced in fish fed VO-based diets when compared with FO fed fish. However, the muscle arachidinic acid (ARA) levels in phospholipid class were 1.4 to 1.6 times higher in fish fed CO and DO diets than FO fed fish despite the lower ARA suppliers from these VO-based diets, suggesting endogenous LC-PUFA biosynthesis from PUFA precursors in fish fed these VO. The fads2 and elovl5 gene expression levels in liver of fish fed DO were also higher compared to FO controls. Therefore, all the results support the hypothesis that African catfish has higher biosynthesis capacity to convert vegetable n-6 PUFA precursors like linoleic acid (LNA, 18:2n-6) into n-6 LC-PUFA of the ARA type, compared to the conversion of vegetable α-linolenic acid (ALA, 18:3n-3) into n-3 LC-PUFA of the eicosapentanoic acid (EPA) or docosahexanoic acid (DHA) type. The results also indicate that DO can be recommended as the best alternative to FO replacement in African catfish nutrition.

Temperature Increase Negatively Affects the Fatty Acid Bioconversion Capacity of Rainbow Trout (Oncorhynchus mykiss) Fed a Linseed Oil-Based Diet.

Aquaculture is meant to provide fish rich in omega-3 long chain polyunsaturated fatty acids (n-3 LC-PUFA). This objective must be reached despite (1) the necessity to replace the finite and limited fish oil in feed production and (2) the increased temperature of the supply water induced by the global warming. The objective of the present paper was to determine to what extent increased water temperature influences the fatty acid bioconversion capacity of rainbow trout (Oncorhynchus mykiss) fed a plant-derived diet. Fish were fed two diets formulated with fish oil (FO) or linseed oil (LO) as only added lipid source at the optimal water temperature of 15°C or at the increased water temperature of 19°C for 60 days. We observed that a temperature increase close to the upper limit of the species temperature tolerance range negatively affected the feed efficiency of rainbow trout fed LO despite a higher feed intake. The negative impact of increased water temperature on fatty acid bioconversion capacity appeared also to be quite clear considering the reduced expression of fatty acid desaturase 2 in liver and intestine and the reduced Δ6 desaturase enzymatic activity in intestinal microsomes. The present results also highlighted a negative impact of increased temperature on the apparent in vivo enzymatic activity of Δ5 and Δ6 desaturases of fish fed LO. Interestingly, this last parameter appeared less affected than those mentioned above. This study highlights that the increased temperature that rainbow trout may face due to global warming could reduce their fatty acid bioconversion capacity. The unavoidable replacement of finite fish oil by more sustainable, readily available and economically viable alternative lipid sources in aquaculture feeds should take this undeniable environmental issue on aquaculture productivity into account.

Characteristics of fads2 gene expression and putative promoter in European sea bass (Dicentrarchus labrax): comparison with salmonid species and analysis of CpG methylation.

Marine fish species exhibit low capacity to biosynthesise highly unsaturated fatty acid (HUFA) in comparison to strict freshwater and anadromous species. It is admitted that the Delta(6) desaturase (FADS2) is a key enzyme in the HUFA biosynthetic pathway. We investigated by quantitative PCR the relative amounts of FADS2 mRNA in European sea bass (Dicentrarchus labrax) in comparison with a salmonid species, the rainbow trout (Oncorhynchus mykiss L.). The analysis of the expression data was performed regarding the difference of the characteristics of a critical fragment of the fads2 gene promoter between sea bass and Atlantic salmon. The lower level of fads2 gene expression observed in sea bass suggested that fads2 gene putative promoter, which exhibited an E-box like Sterol Regulatory Element (SRE) site but lacked a Sp1 site, is less active in this marine species. The cytosine methylation of CpG sites in the putative promoter region including E-box like SRE and NF-Y binding sites of sea bass fads2 gene was also investigated following a nutritional conditioning of larvae. However, no significant difference of CpG methylation could be found for any of the 28 CpGs analysed between larvae fed diet with high or low HUFA contents. In conclusion, the present data revealed lower constitutive expression of the fads2 gene possibly related to different characteristics of gene promoter in sea bass in comparison with salmonid species, and indicated that long-term conditioning of fads2 gene expression did not influence the methylation of the gene promoter at potential SRE binding site.

Cloning, tissue expression analysis, and functional characterization of two Δ6-desaturase variants of sea bass (Dicentrarchus labrax L.).

Fish are the main source of the n-3 highly unsaturated fatty acids, which are crucial for human health. Their synthesis from C(18) precursors is mediated by desaturases and elongases, but the activity of these enzymes has not been conclusively established in marine fish species. This study reports the cloning, tissue expression, and functional characterization of a sea bass (Dicentrarchus labrax L.) Δ6-desaturase and one of its splicing variants. Two cDNAs with open reading frames of 1,346 and 1,354 bp were cloned and named D6D and D6D-V, respectively. Both deduced protein sequences (445 and 387 amino acids, respectively) contained two transmembrane regions and the N-terminal cytochrome b(5) domain with the HPGG motif characteristic of microsomal desaturases. D6D presents three histidine-rich regions, whereas in D6D-V, an insertion of eight nucleotides in the boundaries of exons 10 and 11 modified the third histidine-rich domain and led to insertion of a premature STOP codon, resulting in a shorter predicted protein. Quantitative real-time polymerase chain reaction assay of gene expression showed that D6D was highly expressed in the brain and intestine, and to a lesser extent, in muscle and liver; meanwhile, D6D-V was expressed in all tissues tested, but at level at least 200-fold lower than D6D. Functional analysis in yeast showed that sea bass D6D encodes a fully functional Δ6-desaturase with no residual Δ5-desaturase activity. This desaturase does not exhibit a clear preference for n-3 versus n-6 C(18) substrates. Interestingly, D6D-V is a nonfunctional protein, suggesting that the C-terminal end is indispensable for protein activity.