Long-chain polyunsaturated fatty acid synthesis in fish: Comparative analysis of Atlantic salmon (Salmo salar L.) and Atlantic cod (Gadus morhua L.) Δ6 fatty acyl desaturase gene promoters

Fads2b Plays a Dominant Role in ∆6/∆5 Desaturation Activities Compared with Fads2a in Common Carp (Cyprinus carpio)

Highly unsaturated fatty acids (HUFAs) are essential for mammalian health, development and growth. However, most mammals, including humans, are incapable of synthesizing n-6 and n-3 HUFAs. Fish can convert C18 unsaturated fatty acids into n-6 and n-3 HUFAs via fatty acid desaturase (Fads), in which Fads2 is a key enzyme in HUFA biosynthesis. The allo-tetraploid common carp theoretically encode two duplicated fads2 genes. The expression patterns and desaturase functions of these two homologous genes are still unknown. In this study, the full length of the fads2a and fads2b were identified in common carp (Cyprinus carpio). Expression analyses indicate that both genes were mainly expressed in the liver and the expression of fads2b is higher than fads2a at different developmental stages in carp embryos. Heterogenous expression and 3D docking analyses suggested that Fads2b demonstrated stronger ∆6 and ∆5 desaturase activities than Fads2a. The core promotor regions of fads2a and fads2b were characterized and found to have different potential transcriptional binding sites. These results revealed the same desaturase functions, but different activities of two homologues of fasd2 genes in common carp. The data showed that fads2b played a more important role in HUFA synthesis through both expression and functional analyses.

Molecular Regulation of Biosynthesis of Long Chain Polyunsaturated Fatty Acids in Atlantic Salmon

Salmon is a rich source of health-promoting omega-3 long chain polyunsaturated fatty acids (n-3 LC-PUFA), such as eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3). The LC-PUFA biosynthetic pathway in Atlantic salmon is one of the most studied compared to other teleosts. This has largely been due to the massive replacement of LC-PUFA-rich ingredients in aquafeeds with terrestrial plant oils devoid of these essential fatty acids (EFA) which ultimately pushed dietary content towards the minimal requirement of EFA. The practice would also reduce tissue content of n-3 LC-PUFA compromising the nutritional value of salmon to the human consumer. These necessitated detailed studies of endogenous biosynthetic capability as a contributor to these EFA. This review seeks to provide a comprehensive and concise overview of the current knowledge about the molecular genetics of PUFA biosynthesis in Atlantic salmon, highlighting the enzymology and nutritional regulation as well as transcriptional control networks. Furthermore, we discuss the impact of genome duplication on the complexity of salmon LC-PUFA pathway and highlight probable implications on endogenous biosynthetic capabilities. Finally, we have also compiled and made available a large RNAseq dataset from 316 salmon liver samples together with an R-script visualization resource to aid in explorative and hypothesis-driven research into salmon lipid metabolism.

Insulin can up-regulate LC-PUFA biosynthesis with the involvement of Srebp-1c and stimulatory protein 1 (Sp1) in marine teleost Siganus canaliculatus

The rabbitfish Siganus canaliculatus is the first marine teleost found to have the biosynthetic ability of long-chain polyunsaturated fatty acids (LC-PUFA) from C₁₈ precursors catalyzed by fatty acyl desaturases (Δ6/Δ5 Fads, Δ4 Fads) and elongases of very long chain fatty acids (Elovls). Previously, we predicted the existence of insulin (INS) response elements (IREs) including nuclear factor Y (NF-Y) and sterol regulatory element (SRE) in the core promoter region of rabbitfish Δ6/Δ5 fads and Δ4 fads. To clarify the potential regulatory effect and mechanism of INS in LC-PUFA biosynthesis, INS responding region was identified at -456 bp to + 51 bp of Δ6/Δ5 fads core promoter, but not in Δ4 fads promoter. Moreover, a unique stimulatory protein 1 (Sp1) element was predicted in the INS responding region of Δ6/Δ5 fads. Subsequently, SRE, NF-Y and Sp1 elements were proved as IREs in Δ6/Δ5 fads promoter. The up-regulation of INS on gene expression of Srebp-1c, Sp1, Δ6/Δ5 fads and elovl5 as well as the LC-PUFA biosynthesis was further demonstrated in S. canaliculatus hepatocyte line (SCHL) cells, but no influence was detected on Δ4 fads. Besides, inhibitors of transcription factors Srebp-1c (Fatostatin, PF-429242) and Sp1 (Mithramycin) could inhibit the gene expression of Srebp-1c, Δ6/Δ5 fads and elovl5, and abolish the up-regulation of INS on these genes' expression and LC-PUFA biosynthesis. These results indicated that INS could up-regulate LC-PUFA biosynthesis with the involvement of Srebp-1c and Sp1 in rabbitfish S. canaliculatus, which is the first report in teleost.

Regulation of Δ6Fads2 Gene Involved in LC-PUFA Biosynthesis Subjected to Fatty Acid in Large Yellow Croaker (Larimichthys crocea) and Rainbow Trout (Oncorhynchus mykiss)

Δ6 fatty acyl desaturase (Δ6Fads2) is regarded as the first rate-limiting desaturase that catalyzes the biosynthesis of long-chain polyunsaturated fatty acids (LC-PUFA) from 18-carbon fatty acid in vertebrates, but the underlying regulatory mechanism of fads2 has not been comprehensively understood. This study aimed to investigate the regulation role of fads2 subjected to fatty acid in large yellow croaker and rainbow trout. In vivo, large yellow croaker and rainbow trout were fed a fish oil (FO) diet, a soybean oil (SO) diet or a linseed oil (LO) diet for 10 weeks. The results show that LO and SO can significantly increase fads2 expression (p < 0.05). In vitro experiments were conducted in HEK293T cells or primary hepatocytes to determine the transcriptional regulation of fads2. The results show that CCAAT/enhancer-binding protein α (C/EBPα) can up-regulate fads2 expression. GATA binding protein 3 (GATA3) can up-regulate fads2 expression in rainbow trout but showed opposite effect in large yellow croaker. Furthermore, C/EBPα protein levels were significantly increased by LO and SO (p < 0.05), gata3 expression was increased in rainbow trout by LO but decreased in large yellow croaker by LO and SO. In conclusion, we revealed that FO replaced by LO and SO increased fads2 expression through a C/EBPα and GATA3 dependent mechanism in large yellow croaker and rainbow trout. This study might provide critical insights into the regulatory mechanisms of fads2 expression and LC-PUFA biosynthesis.

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.

Seafood Intake as a Method of Non-Communicable Diseases (NCD) Prevention in Adults

Seafood (fish in particular) is one of the main food groups in nutrition models with proven health benefits. Seafood has long been considered a very valuable dietary component, mainly due to presence of n-3 polyunsaturated fatty acids (n-3 PUFA) but it is also an important source of protein (including collagen), anserine, taurine, iodine, selenium, vitamin A, vitamin K, vitamin D, tocopherols, B vitamins and astaxanthin. Considering the beneficial effects of these ingredients on blood pressure, lipid profile and the inflammatory process, seafood should be an essential component of the diet. Non-communicable diseases (NCD) such as cardiovascular diseases, cancer, diabetes and mental disorder, chronic respiratory diseases are common diseases associated with advanced age. Promotion of a healthy lifestyle (including proper nutritional behavior) and prevention of diseases are the most effective and efficient ways to decrease premature mortality from NCD and to maintain mental health and well-being. This review article shows the potential preventive and therapeutic effects of seafood with an emphasis on fish. Our narrative review presents the results of systematic reviews and meta-analysis.

Modulation of hepatic miRNA expression in Atlantic salmon (Salmo salar) by family background and dietary fatty acid composition

This study finds significant differences in hepatic fatty acid composition between four groups of Atlantic salmon (Salmo salar) consisting of offspring from families selected for high and low capacities to express the delta 6 desaturase isomer b and fed diets with 10% or 75% fish oil. The results demonstrated that hepatic lipid metabolism was affected by experimental conditions (diet/family). The fatty acid composition in the four groups mirrored the differences in dietary composition, but it was also associated with the family groups. Small RNA sequencing followed by RT-qPCR identified 12 differentially expressed microRNAs (DE miRNAs), with expression associated with family groups (miR-146 family members, miR-200b, miR-214, miR-221, miR-125, miR-135, miR-137, miR_nov_1), diets (miR-203, miR-462) or both conditions. All the conserved DE miRNAs have been reported as associated with lipid metabolism in other vertebrates. In silico predictions revealed 37 lipid metabolism pathway genes, including desaturases, transcription factors and key enzymes in the synthesis pathways as putative targets (e.g., srebp-1 and 2, Δ6fad_b and c, hmdh, elovl4 and 5b, cdc42). RT-qPCR analysis of selected target genes showed expression changes that were associated with diet and with family groups (d5fad, d6fad_a, srebp-1). There was a reciprocal difference in the abundance of ssa-miR-203a-3p and srebp-1 in one group comparison, whereas other predicted targets did not reveal any evidence of being negatively regulated by degradation. More experimental studies are needed to validate and fully understand the predicted interactions and how the DE miRNAs may participate in the regulation of hepatic lipid metabolism.

Regulation of long-chain polyunsaturated fatty acid biosynthesis in teleost fish

Omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFA, C_20-24), including eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3), are involved in numerous biological processes and have a range of health benefits. Fish have long been considered as the main source of n-3 LC-PUFA in human diets. However, the capacity for endogenous biosynthesis of LC-PUFA from C₁₈ PUFA varies in fish species based on the presence, expression and activity of key enzymes including fatty acyl desaturases (Fads) and elongation of very long-chain fatty acids (Elovl) proteins. In this article, we review progress on the identified Fads and Elovl, as well as the regulatory mechanisms of LC-PUFA biosynthesis both at transcriptional and post-transcriptional levels in teleosts. The most comprehensive advances have been obtained in rabbitfish Siganus canaliculatus, a marine teleost demonstrated to have the entire pathway for LC-PUFA biosynthesis, including the roles of transcription factors hepatocyte nuclear factor 4α (Hnf4α), liver X receptor alpha (Lxrα), sterol regulatory element-binding protein 1 (Srebp-1), peroxisome proliferator-activated receptor gamma (Pparγ) and stimulatory protein 1 (Sp1), as well as post-transcriptional regulation by individual microRNA (miRNA) or clusters. This research has, for the first time, demonstrated the involvement of Hnf4α, Pparγ and miRNA in the regulation of LC-PUFA biosynthesis in vertebrates. The present review provides readers with a relatively comprehensive overview of the progress made into understanding LC-PUFA biosynthetic systems in teleosts, and some insights into improving endogenous LC-PUFA biosynthesis capacity aimed at reducing the dependence of aquafeeds on fish oil while maintaining or increasing flesh LC-PUFA content and the nutritional quality of farmed fish.

Transcriptional regulation mechanism of sterol regulatory element binding proteins on Δ6 fatty acyl desaturase in razor clam Sinonovacula constricta

The razor clam, Sinonovacula constricta, contains high levels of long-chain PUFA (LC-PUFA), which are critical for human health. In addition, S. constricta is the first marine mollusc demonstrated to possess Δ6 fatty acyl desaturase (Fad) and complete LC-PUFA biosynthetic ability, providing a good representative to investigate the molecular mechanism of sterol regulatory element binding proteins (SREBP) in regulating Δ6 Fad for LC-PUFA biosynthesis in marine molluscs. Herein, S. constricta SREBP and Δ6 Fad promoter were cloned and characterised. Subsequently, dual luciferase and electrophoretic mobility shift assays were conducted to explore the SREBP binding elements in the core regulatory region of S. constricta Δ6 Fad promoter. Results showed that S. constricta SREBP had a very conservative basic helix-loop-helix-leucine zipper motif, while S. constricta Δ6 Fad promoter exhibited very poor identity with teleost Fads2 promoters, indicating their differentiation during evolution. A 454 bp region harbouring a core sequence in S. constricta Δ6 Fad promoter was predicted to be essential for the transcriptional activation by SREBP. This was the first report on the regulatory mechanism of LC-PUFA biosynthesis in marine molluscs, which would facilitate optimising the LC-PUFA biosynthetic pathway of bivalves in further studies.

The requirements for sterol regulatory element-binding protein (Srebp) and stimulatory protein 1 (Sp1)-binding elements in the transcriptional activation of two freshwater fish Channa striata and Danio rerio elovl5 elongase

Fish are a major source of beneficial n-3 LC-PUFA in human diet, and there is considerable interest to elucidate the mechanism and regulatory aspects of LC-PUFA biosynthesis in farmed species. Long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis involves the activities of two groups of enzymes, the fatty acyl desaturase (Fads) and elongase of very long-chain fatty acid (Elovl). The promoters of elovl5 elongase, which catalyses the rate-limiting reaction of elongating polyunsaturated fatty acid (PUFA), have been previously described and characterized from several marine and diadromous teleost species. We report here the cloning and characterization of elovl5 promoter from two freshwater fish species, the carnivorous snakehead fish (Channa striata) and zebrafish. Results show the presence of sterol-responsive elements (SRE) in the core regulatory region of both promoters, suggesting the importance of sterol regulatory element-binding protein (Srebp) in the regulation of elovl5 for both species. Mutagenesis luciferase and electrophoretic mobility shift assays further validate the role of SRE for basal transcriptional activation. In addition, several Sp1-binding sites located in close proximity with SRE were present in the snakehead promoter, with one having a potential synergy with SRE in the regulation of elovl5 expression. The core zebrafish elovl5 promoter fragment also directed in vivo expression in the yolk syncytial layer of developing zebrafish embryos.

The Relationship between the Expression of Fatty Acyl Desaturase 2 (fads2) Gene in Peripheral Blood Cells (PBCs) and Liver in Gilthead Seabream, Sparus aurata Broodstock Fed a Low n-3 LC-PUFA Diet

The principle aim of this study is to elucidate the relationship between the fatty acid desaturase 2 gene (fads2) expression pattern in peripheral blood cells (PBCs) and liver of gilthead seabream (GSB), Sparus aurata broodstock in order to determine the possible use of fads2 expression as a potential biomarker for the selection of broodstock. This selection could be utilized for breeding programs aiming to improve reproduction, health, and nutritional status. Passive Integrated Transponder (PIT)-tagged GSB broodstock (Male-1.22 ± 0.20 kg; 44.8 ± 2 cm and female-2.36 ± 0.64 kg; 55.1 cm) were fed a diet containing low levels of fish meal and fish oil (EPA 2.5; DHA 1.7 and n-3 LC-PUFA 4.6% TFA) for one month. After the feeding period, fads2 expression in PBCs and liver of both male and female broodstock were highly significantly correlated (r = 0.89; p < 0.001). Additionally, in male broodstock, liver fads2 expression was significantly correlated (p < 0.05) to liver contents in 16:0 (r = 0.95; p = 0.04) and total saturates (r = 0.97; p = 0.03) as well as to 20:3n–6/20:2n–6 (r = 0.98; p = 0.02) a Fads2 product/precursor ratio. Overall, we found a positive and significant correlation between fads2 expression levels in the PBCs and liver of GSB broodstock. PBCs fads2 expression levels indicate a strong potential for utilization as a non-invasive method to select animals having increased fatty acid bioconversion capability, better able to deal with diets free of fish meal and fish oil.

Functional characterisation of fatty acyl desaturase, Fads2, and elongase, Elovl5, in the Boddart's goggle-eyed goby Boleophthalmus boddarti (Gobiidae) suggests an incapacity for long-chain polyunsaturated fatty acid biosynthesis

The biosynthesis of long-chain polyunsaturated fatty acids (LC-PUFA), a process to convert C18 polyunsaturated fatty acids into eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) or arachidonic acid (ARA), requires the concerted activities of two enzymes, the fatty acyl desaturase (Fads) and elongase (Elovl). This study highlights the cloning, functional characterisation and tissue expression pattern of a Fads and an Elovl from the Boddart's goggle-eyed goby (Boleophthalmus boddarti), a mudskipper species widely distributed in the Indo-Pacific region. Phylogenetic analysis revealed that the cloned fads and elovl are clustered with other teleost orthologs, respectively. The investigation of the genome of several mudskipper species, namely Boleophthalmus pectinirostris, Periophthalmus schlosseri and Periophthalmus magnuspinnatus, revealed a single Fads2 and two elongases, Elovl5 and Elovl4 for each respective species. A heterologous yeast assay indicated that the B. boddarti Fads2 possessed low desaturation activity on C18 PUFA and no desaturation on C20 and C22 PUFA substrates. In comparison, the Elovl5 showed a wide range of substrate specificity, with a capacity to elongate C18, C20 and C22 PUFA substrates. An amino acid residue that affects the capacity to elongate C22:5n-3 was identified in the B. boddarti Elovl5. Both genes are highly expressed in brain tissue. Among all tissues, DHA is highly concentrated in neuron-rich tissues, whereas EPA is highly deposited in gills. Taken together, the results showed that due to the inability to perform desaturation steps, B. boddarti is unable to biosynthesise LC-PUFA, relying on dietary intake to acquire these nutrients.

Plasma biochemistry, gene expression and liver histomorphology in common carp (Cyprinus carpio) fed with different dietary fat sources

Demand for omega-3 long chain polyunsaturated fatty acids has become global challenge for aquaculture and different components have been used to increase nutritional value of fillets. The aim of this study was to evaluate influences of feeds on zootechnical parameters, biochemical plasma parameters, expression of lipid-dependent genes, hepatocyte histomorphologies, and fatty acid profiles in common carp fillets. We compared a control diet (CTRL), mimicking a commercial feed formulation for common carp, with three diets containing blends of vegetable oils and a DHA-rich alga (Schizochytrium sp.) included at 3.125% (CB1) or 1.563% (CB2), and 2.1% salmon oil (CB3). The study revealed no differences in final body weight of fish fed CB1-3 diets in comparison with significantly lower CTRL. Concentrations of all biochemical parameters in plasma increased gradually in fish fed CB1-3 diets when compared to CTRL diet, with exception of triacylglycerol levels. Expression of hepatic fas, elovl-5a and pparα genes increased significantly in fish fed CB1 and CB2. Additionally, eicosapentaenoic (EPA) and docosahexaenoic acid (DHA) accumulation in muscle tissue was directly proportional to the amounts supplied in the diets. Our study revealed that carp fillet profiles can be manipulated for DHA and EPA-contents using enriched diets, depending on the source of fat.

Sp1 is Involved in Vertebrate LC-PUFA Biosynthesis by Upregulating the Expression of Liver Desaturase and Elongase Genes

The rabbitfish Siganus canaliculatus was the first marine teleost demonstrated to have the ability for the biosynthesis of long-chain (≥C20) polyunsaturated fatty acids (LC-PUFA) from C18 PUFA precursors, and all the catalytic enzymes including two fatty acyl desaturase 2 (Δ4 Fads2 and Δ6/Δ5 Fads2) and two elongases (Elovl4 and Elovl5) have been identified, providing a good model for studying the regulatory mechanisms of LC-PUFA biosynthesis in fish. Stimulatory protein 1 (Sp1) has been speculated to be a vital transcription factor in determining the promoter activity of Fads-like genes in fish, however its regulatory effects on gene expression and LC-PUFA biosynthesis have not been demonstrated. Bioinformatic analysis predicted potential Sp1 binding sites in the promoters of the rabbitfish Δ6/Δ5 fads2 and elovl5, but not in Δ4 fads2 promoter. Here we cloned full-length cDNA of the rabbitfish sp1 gene, which encoded a putative protein of 701 amino acids, and was expressed in all tissues studied with highest levels in gill and eyes. The dual luciferase reporter assay in HepG2 line cells demonstrated the importance of the Sp1 binding site for the promoter activities of both Δ6/Δ5 fads2 and elovl5. Moreover, the electrophoretic mobility shift assay confirmed the direct interaction of Sp1 with the two promoters. Insertion of the Sp1 binding site of Δ6/Δ5 fads2 promoter into the corresponding region of the Δ4 fads2 promoter significantly increased activity of the latter. In the Siganus canaliculatus hepatocyte line (SCHL) cells, mRNA levels of Δ6/Δ5 fads2 and elovl5 were positively correlated with the expression of sp1 when sp1 was overexpressed or knocked-down by RNAi or antagonist (mithramycin) treatment. Moreover, overexpression of sp1 also led to a higher conversion of 18:2n−6 to 18:3n−6, 18:2n−6 to 20:2n−6, and 18:3n−3 to 20:3n−3, which related to the functions of Δ6/Δ5 Fads2 and Elovl5, respectively. These results indicated that Sp1 is involved in the transcriptional regulation of LC-PUFA biosynthesis by directly targeting Δ6/Δ5 fads2 and elovl5 in rabbitfish, which is the first report of Sp1 involvement in the regulation of LC-PUFA biosynthesis in vertebrates.

MicroRNAs Involved in the Regulation of LC-PUFA Biosynthesis in Teleosts: miR-33 Enhances LC-PUFA Biosynthesis in Siganus canaliculatus by Targeting insig1 which in Turn Upregulates srebp1

Post-transcriptional regulatory mechanisms play important roles in the regulation of LC-PUFA biosynthesis. Our previous study revealed that miR-33 could increase the expression of fatty acyl desaturases (fads2) in the rabbitfish Siganus canaliculatus, but the specific mechanism is unknown. Here, we confirmed that miR-33 could target the 3'UTR of insulin-induced gene 1 (insig1), resulting in downregulation of its protein level in the rabbitfish hepatocyte line (SCHL). In vitro overexpression of miR-33 inhibited the mRNA level of insig1 and increased the mRNA levels of Δ6Δ5 fads2 and elovl5, as well as srebp1. In SCHL cells, proteolytic activation of sterol-regulatory-element-binding protein-1 (Srebp1) was blocked by Insig1, with overexpression of insig1 decreasing mature Srebp1 level, while inhibition of insig1 led to the opposite effect. Srebp1 could enhance the promoter activity of Δ6Δ5 fads2 and elovl5, whose expression levels decreased with knockdown of srebp1 in SCHL. Overexpression of miR-33 also resulted in a higher conversion of 18:3n-3 to 18:4n-3 and 20:5n-3 to 22:5n-3, linked to desaturation and elongation via Δ6Δ5 Fads2 and Elovl5, respectively. The results suggested that the mechanism by which miR-33 regulates LC-PUFA biosynthesis in rabbitfish is through enhancing the expression of srebp1 by targeting insig1. The findings here provide more insight to the mechanism of miRNAs involvement in the regulation of LC-PUFA biosynthesis in teleosts.

CRISPR/Cas9-mediated ablation of elovl2 in Atlantic salmon (Salmo salar L.) inhibits elongation of polyunsaturated fatty acids and induces Srebp-1 and target genes

Atlantic salmon can synthesize polyunsaturated fatty acids (PUFAs), such as eicosapentaenoic acid (20:5n-3), arachidonic acid (20:4n-6) and docosahexaenoic acid (22:6n-3) via activities of very long chain fatty acyl elongases (Elovls) and fatty acyl desaturases (Fads), albeit to a limited degree. Understanding molecular mechanisms of PUFA biosynthesis and regulation is a pre-requisite for sustainable use of vegetable oils in aquafeeds as current sources of fish oils are unable to meet increasing demands for omega-3 PUFAs. By generating CRISPR-mediated elovl2 partial knockout (KO), we have shown that elovl2 is crucial for multi-tissue synthesis of 22:6n-3 in vivo and that endogenously synthesized PUFAs are important for transcriptional regulation of lipogenic genes in Atlantic salmon. The elovl2-KOs showed reduced levels of 22:6n-3 and accumulation of 20:5n-3 and docosapentaenoic acid (22:5n-3) in the liver, brain and white muscle, suggesting inhibition of elongation. Additionally, elovl2-KO salmon showed accumulation of 20:4n-6 in brain and white muscle. The impaired synthesis of 22:6n-3 induced hepatic expression of sterol regulatory element binding protein-1 (srebp-1), fatty acid synthase-b, Δ6fad-a, Δ5fad and elovl5. Our study demonstrates key roles of elovl2 at two penultimate steps of PUFA synthesis in vivo and suggests Srebp-1 as a main regulator of endogenous PUFA synthesis in Atlantic salmon.

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.

Performance, feed utilization, and hepatic metabolic response of weaned juvenile Atlantic bluefin tuna (Thunnus thynnus L.): effects of dietary lipid level and source

Two trials were performed using extruded diets as on-growing feeds for weaned Atlantic bluefin tuna (Thunnus thynnus; ABT) to establish adequate dietary levels of both lipid and omega-3 long-chain polyunsaturated fatty acids (LC-PUFAs), and impacts on lipid metabolism via liver gene expression. In trial A, ABT were fed with either a commercial feed (Magokoro®; MGK) as a reference diet or two experimental feeds differing in lipid levels (15 or 20%) using krill oil (KO) as the single lipid source in order to estimate suitable lipid content. Fish fed MGK displayed the highest growth, followed by 15KO, and therefore a dietary lipid content of 15% was considered preferable to 20% at this stage. In trial B, fish were fed MGK, 15KO, or a feed containing 15% lipid with a blend of KO and rapeseed oil (RO) (1:1, v/v; 15KORO). Fish fed 15KO and 15KORO showed no difference in weight gain, specific growth rate, and fork length. Increasing dietary lipid level or including vegetable oil, RO, in the feeds did not increase liver lipid content. Liver fatty acid compositions largely reflected dietary profiles confirming very limited endogenous LC-PUFA biosynthesis. Liver of ABT fed 15KO and 20KO displayed the highest contents of docosahexaenoic acid (DHA). The hepatic expression of genes encoding enzymes and transcription factors involved in lipid and fatty acid metabolism, as well as genes encoding antioxidant enzymes, showed that many of these genes were regulated by dietary lipid and LC-PUFA content. Results suggested that ABT juveniles can be on-grown on inert dry feeds that support good fish growth and the accumulation of DHA.

Pparγ Is Involved in the Transcriptional Regulation of Liver LC-PUFA Biosynthesis by Targeting the Δ6Δ5 Fatty Acyl Desaturase Gene in the Marine Teleost Siganus canaliculatus

As the first marine teleost demonstrated to have the ability of long-chain polyunsaturated fatty acids (LC-PUFA) biosynthesis from C₁₈ PUFA precursors, the rabbitfish Siganus canaliculatus provides us a unique model for clarifying the regulatory mechanisms of LC-PUFA biosynthesis in teleosts aiming at the replacement of dietary fish oil (rich in LC-PUFA) with vegetable oils (rich in C₁₈ PUFA precursors but devoid of LC-PUFA). In the study of transcription regulation of gene encoding the Δ6Δ5 fatty acyl desaturase (Δ6Δ5 Fads), a rate-limiting enzyme catalyzing the first step of LC-PUFA biosynthesis in rabbitfish, a binding site for the transcription factor (TF), peroxisome proliferator-activated receptor γ (Pparγ), was predicted in Δ6Δ5 fads2 promoter by bioinformatics analysis, and thus the present study focused on the regulatory roles of Pparγ on Δ6Δ5 fads2. First, the activity of the Δ6Δ5 fads2 promoter was proved to be downregulated by pparγ overexpression and upregulated by treatment of Pparγ antagonist (GW9662), respectively, in HEK 293T cells with the dual luciferase reporter assay. Pparγ was further confirmed to interact with the promoter by electrophoretic mobility shift assay. Moreover, in S. canaliculatus hepatocyte line (SCHL) cells, GW9662 decreased the expression of pparγ together with increase of Δ6Δ5 fads2 mRNA. Besides, Δ6Δ5 fads2 expression was increased by pparγ RNAi knockdown and reduced by its mRNA overexpression. Furthermore, knockdown of pparγ induced a high conversion of 18:3n-3 to 18:4n-3 and 18:2n-6 to 18:3n-6, while pparγ mRNA overexpression led to a lower conversion of that, and finally a significant decrease of 20:4n-6(ARA), 20:5n-3(EPA), and 22:6n-3(DHA) production. The results indicate that Pparγ is involved in the transcriptional regulation of liver LC-PUFA biosynthesis by targeting Δ6Δ5 fads2 in rabbitfish, which is the first report of Pparγ involvement in the regulation of LC-PUFA biosynthesis in teleosts.

Long-chain polyunsaturated fatty acid biosynthesis and its response to cadmium exposure in silver pomfret

Despite a close interaction between cadmium (Cd) and long-chain polyunsaturated fatty acid (LC-PUFA) metabolism, the influence of Cd exposure on the endogenous synthesis of LC-PUFA has received little attention. In the present study, we hypothesized that Cd exposure would affect the synthesis of LC-PUFA in the marine fish silver pomfret (Pampus argenteus). Therefore, the molecular basis of LC-PUFA biosynthesis and regulation was investigated as the first step to understanding the mechanisms underpinning the effects of Cd exposure. Thereafter, transcriptional regulation of the genes that participate in LC-PUFA biosynthesis and regulation by Cd exposure were also explored. Our results showed that fatty acyl desaturase 2 (Fads2) and elongases of very long-chain fatty acids 5 (Elovl5), two key enzymes involved in LC-PUFA biosynthesis, enabled silver pomfret to biosynthesize 20:3n-6 and 20:4n-3 from 18:2n-6 and 18:3n-3. The results also raise the possibility that silver pomfret may have the ability to produce docosahexaenoic acid (DHA, 22:6n-3) from endogenous eicosapentaenoic acid (EPA, 20:5n-3). The expression of silver pomfret fads2 and elovl5 was transcriptionally regulated by the peroxisome proliferator activated receptor α (Pparα). The expression of fads2, elovl5 and pparα in the brain was significantly increased in response to Cd exposure. In addition, Cd exposure significantly reduced the DHA concentration and significantly increased the malondialdehyde concentration in the brain of silver pomfret. Cd exposure likely increases brain-specific DHA synthesis from EPA by transcriptionally activating fads2 and elovl5 via Pparα in silver pomfret. This regulation may be a coping mechanism for the reduction of DHA caused by Cd-oxidative stress in the brains of silver pomfret.

Characteristics of the fads2 gene promoter in marine teleost Epinephelus coioides and role of Sp1-binding site in determining promoter activity

Δ6 fatty acyl desaturase (Fads2) is a rate-limiting enzyme in long-chain polyunsaturated fatty acids (LC-PUFA) biosynthesis. Comparative analysis of gene promoters of Fads2 between salmonids and carnivorous marine fish suggested that the lack of binding site for stimulatory protein 1 (Sp1) was responsible for the low expression of fads2 gene of carnivorous marine species. To confirm this speculation, the fads2 candidate promoter (2646 bp) was cloned from carnivorous marine teleost Epinephelus coioides, and 330 bp core regulatory region was identified. Several binding sites for transcriptional factors such as nuclear factor 1, nuclear factor Y, sterol regulatory element and hepatocyte nuclear factor 4γ were identified, while that for Sp1 was shown to be absent in the promoter by both bioinformatic analysis and site-directed mutation. Moreover, after the Sp1-binding site from the fads2 promoter of herbivorous Siganus canaliculatus, the first marine teleost demonstrated to have LC-PUFA biosynthetic ability, was inserted into the corresponding region of E. coioides fads2 promoter, activity was significantly increased. The results provided direct data for the importance of the Sp1-binding site in determining fads2 promoter activity, and indicated that its lack may be a reason for low expression of fads2 and poor LC-PUFA biosynthetic ability in E. coioides.

Transcriptional activation of zebrafish fads2 promoter and its transient transgene expression in yolk syncytial layer of zebrafish embryos

The front-end desaturases (Fads) are rate-limiting enzymes responsible for production of long-chain polyunsaturated fatty acids (LC-PUFA). The full spectrum of the transcriptional regulation of fads is still incomplete, as cloning of fads promoter is limited to a few species. Here, we described the cloning and characterisation of the zebrafish fads2 promoter. Using 5'-deletion and mutation analysis on this promoter, we identified a specific region containing the sterol regulatory element (SRE) which is responsible for the activation of the fads2 promoter. In tandem, two conserved CCAAT boxes were also present adjacent to the SRE and mutation of either of these binding sites attenuates the transcriptional activation of the fads2 promoter. An in vivo analysis employing GFP reporter gene in transiently transfected zebrafish embryos showed that this 1754 bp upstream region of the fads2 gene specifically directs GFP expression in the yolk syncytial layer (YSL) region. This indicates a role for LC-PUFA in the transport of yolk lipids through this tissue layer. In conclusion, besides identifying novel core elements for transcriptional activation in zebrafish fads2 promoter, we also reveal a potential role for fads2 or LC-PUFA in YSL during development.

An oil containing EPA and DHA from transgenic Camelina sativa to replace marine fish oil in feeds for Atlantic salmon (Salmo salar L.): Effects on intestinal transcriptome, histology, tissue fatty acid profiles and plasma biochemistry

New de novo sources of omega 3 (n-3) long chain polyunsaturated fatty acids (LC-PUFA) are required as alternatives to fish oil in aquafeeds in order to maintain adequate levels of the beneficial fatty acids, eicosapentaenoic and docosahexaenoic (EPA and DHA, respectively). The present study investigated the use of an EPA+DHA oil derived from transgenic Camelina sativa in Atlantic salmon (Salmo salar) feeds containing low levels of fishmeal (35%) and fish oil (10%), reflecting current commercial formulations, to determine the impacts on tissue fatty acid profile, intestinal transcriptome, and health of farmed salmon. Post-smolt Atlantic salmon were fed for 12-weeks with one of three experimental diets containing either a blend of fish oil/rapeseed oil (FO), wild-type camelina oil (WCO) or transgenic camelina oil (DCO) as added lipid source. The DCO diet did not affect any of the fish performance or health parameters studied. Analyses of the mid and hindgut transcriptomes showed only mild effects on metabolism. Flesh of fish fed the DCO diet accumulated almost double the amount of n-3 LC-PUFA than fish fed the FO or WCO diets, indicating that these oils from transgenic oilseeds offer the opportunity to increase the n-3 LC-PUFA in farmed fish to levels comparable to those found a decade ago.

Regulation of FADS2 transcription by SREBP-1 and PPAR-α influences LC-PUFA biosynthesis in fish

The present study was conducted to explore the mechanisms leading to differences among fishes in the ability to biosynthesize long-chain polyunsaturated fatty acids (LC-PUFAs). Replacement of fish oil with vegetable oil caused varied degrees of increase in 18-carbon fatty acid content and decrease in n-3 LC-PUFA content in the muscle and liver of rainbow trout (Oncorhynchus mykiss), Japanese seabass (Lateolabrax japonicus) and large yellow croaker (Larimichthys crocea), suggesting that these fishes have differing abilities to biosynthesize LC-PUFAs. Fish oil replacement also led to significantly up-regulated expression of FADS2 and SREBP-1 but different responses of the two PPAR-α homologues in the livers of these three fishes. An in vitro experiment indicated that the basic transcription activity of the FADS2 promoter was significantly higher in rainbow trout than in Japanese seabass or large yellow croaker, which was consistent with their LC-PUFA biosynthetic abilities. In addition, SREBP-1 and PPAR-α up-regulated FADS2 promoter activity. These regulatory effects varied considerably between SREBP-1 and PPAR-α, as well as among the three fishes. Taken together, the differences in regulatory activities of the two transcription factors targeting FADS2 may be responsible for the different LC-PUFA biosynthetic abilities in these three fishes that have adapted to different ambient salinity.

Addition of different tuna meal levels to pizza dough

Abstract The aim of this study was to develop pizza dough with different levels of tuna meal (Tunnus spp.). In order to produce tuna meal, tuna torsos without fins were used, cooked for 1 hour, pressed, milled and dehydrated for 24 hours at 60 °C. Pizza dough was produced without (0%) or with the addition of 5, 10, 15 and 20% of tuna meal, calculated based on the quantity of wheat flour. The tuna meal and different pizza pastries were analyzed for moisture content, crude protein, total lipids, ash, carbohydrates, caloric value and fatty acid profiles. Microbiological and sensory analyses were also carried out on the pizza pastries. The increasing addition of tuna meal resulted in gains in the crude protein (10.89 to 18.94%), total lipid (4.63 to 5.89%) and ash (2.54 to 3.54%) contents of the pizza pastries, not influencing the moisture content or caloric value. The inclusion of tuna meal linearly increased the quantity of n-3 series fatty acids in the pizza pastry, from 1.56 to 5.93 g/kg with the addition of 20% tuna meal. The ratio between the polyunsaturated and saturated fatty acids in the tuna meal and pizza pastries varied from 1.21 to 1.85. The microbiological analyses showed that the pizza pastries were produced under proper hygiene conditions. It was also observed that the addition of 5 to 20% of tuna meal to the pizza pastry did not significantly (p>0.05) alter the parameters of aroma, flavor, overall impression and purchase intention. It was therefore concluded that the addition of 5 to 20% tuna meal is effective in improving the nutritional value and fatty acid profile of pizza pastry.

Regulation of the Omega-3 Fatty Acid Biosynthetic Pathway in Atlantic Salmon Hepatocytes

Limited availability of the n-3 fatty acids EPA and DHA have led to an interest in better understanding of the n-3 biosynthetic pathway and its regulation. The biosynthesis of alpha-linolenic acid to EPA and DHA involves several complex reaction steps including desaturation-, elongation- and peroxisomal beta-oxidation enzymes. The aims of the present experiments were to gain more knowledge on how this biosynthesis is regulated over time by different doses and fatty acid combinations. Hepatocytes isolated from salmon were incubated with various levels and combinations of oleic acid, EPA and DHA. Oleic acid led to a higher expression of the Δ6 fatty acid desaturase (fad) genes Δ6fad_a, Δ6fad_b, Δ6fad_c and the elongase genes elovl2 compared with cells cultured in medium enriched with DHA. Further, the study showed rhythmic variations in expression over time. Levels were reached where a further increase in specific fatty acids given to the cells not stimulated the conversion further. The gene expression of Δ6fad_a_and Δ6fad_b responded similar to fatty acid treatment, suggesting a co-regulation of these genes, whereas Δ5fad and Δ6fad_c showed a different regulation pattern. EPA and DHA induced different gene expression patterns, especially of Δ6fad_a. Addition of radiolabelled alpha-linolenic acid to the hepatocytes confirmed a higher degree of elongation and desaturation in cells treated with oleic acid compared to cells treated with DHA. This study suggests a complex regulation of the conversion process of n-3 fatty acids. Several factors, such as that the various gene copies are differently regulated, the gene expression show rhythmic variations and gene expression only affected to a certain level, determines when you get the maximum conversion of the beneficial n-3 fatty acids.

Minor lipid metabolic perturbations in the liver of Atlantic salmon (Salmo salar L.) caused by suboptimal dietary content of nutrients from fish oil

The present study was conducted to evaluate the effects on Atlantic salmon hepatic lipid metabolism when fed diets with increasing substitution of fish oil (FO) with a vegetable oil (VO) blend. Four diets with VOs replacing 100, 90, 79 and 65 % of the FO were fed for 5 months. The levels of eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3) in the experimental diets ranged from 1.3 to 7.4 % of fatty acids (FAs), while cholesterol levels ranged from 0.6 to 1.2 g kg(-1). In hepatocytes added [1-(14)C] α-linolenic acid (ALA, 18:3n-3), more ALA was desaturated and elongated to EPA and DHA in cells from fish fed 100 % VO, while in fish fed 65 % VO, ALA was elongated to eicosatrienoic acid (ETE; 20:3n-3), indicating reduced Δ6 desaturation activity. Despite increased desaturation activity and activation of the transcription factor Sp1 in fish fed 100 % VO, liver phospholipids contained less EPA and DHA compared with the 65 % VO group. The cholesterol levels in the liver of the 100 % VO group exceeded the levels in fish fed the 65 % VO diet, showing an inverse relationship between cholesterol intake and liver cholesterol content. For the phytosterols, levels in liver were generally low. The area as a proxy of volume of lipid droplets was significantly higher in salmon fed 100 % VO compared with salmon fed 65 % VO. In conclusion, the current study suggests that suboptimal dietary levels of cholesterol in combination with low levels of EPA and DHA (1.3 % of FAs) can result in minor metabolic perturbations in the liver of Atlantic salmon.

Hepatocyte Nuclear Factor 4α (HNF4α) Is a Transcription Factor of Vertebrate Fatty Acyl Desaturase Gene as Identified in Marine Teleost Siganus canaliculatus

Rabbitfish Siganus canaliculatus was the first marine teleost demonstrated to have the capability of biosynthesizing long-chain polyunsaturated fatty acids (LC-PUFA) from C₁₈ precursors, and to possess a Δ4 fatty acyl desaturase (Δ4 Fad) which was the first report in vertebrates, and is a good model for studying the regulatory mechanisms of LC-PUFA biosynthesis in teleosts. In order to understand regulatory mechanisms of transcription of Δ4 Fad, the gene promoter was cloned and characterized in the present study. An upstream sequence of 1859 bp from the initiation codon ATG was cloned as the promoter candidate. On the basis of bioinformatic analysis, several binding sites of transcription factors (TF) including GATA binding protein 2 (GATA-2), CCAAT enhancer binding protein (C/EBP), nuclear factor 1 (NF-1), nuclear factor Y (NF-Y), hepatocyte nuclear factor 4α (HNF4α) and sterol regulatory element (SRE), were identified in the promoter by site-directed mutation and functional assays. HNF4α and NF-1 were confirmed to interact with the core promoter of Δ4 Fad by gel shift assay and mass spectrometry. Moreover, over-expression of HNF4α increased promoter activity in HEK 293T cells and mRNA level of Δ4 Fad in rabbitfish primary hepatocytes, respectively. The results indicated that HNF4α is a TF of rabbitfish Δ4 Fad. To our knowledge, this is the first report on promoter structure of a Δ4 Fad, and also the first demonstration of HNF4α as a TF of vertebrate Fad gene involved in transcription regulation of LC-PUFA biosynthesis.

Molecular Cloning, Functional Characterization and Nutritional Regulation of the Putative Elongase Elovl5 in the Orange-Spotted Grouper (Epinephelus coioides)

The enzymes involved in the biosynthesis of long-chain polyunsaturated fatty acids (LC-PUFAs) are widely studied in fish species, as fish are the main source of n-3 LC-PUFAs for human beings. In the present study, a putative gene for elovl5, which encodes a key enzyme involved in LC-PUFA synthesis, was cloned and functionally characterized, and its transcription in response to dietary n-3 LC-PUFA exposure was investigated. Moreover, cell transfection and luciferase assays were used to explore the mechanism underlying the regulation of elovl5. The full-length cDNA of elovl5 was 1242 bp (excluding the polyA tail), including an 885 bp coding region encoding a 295 amino acid protein that possesses all of the characteristic features of elovl proteins. Functional characterization of heterologously expressed grouper Elovl5 indicated that it effectively elongates both C18 (18:2n-6, 18:3n-3, 18:3n-6 and 18:4n-3) and C20 (20:4n-6 and C20:5n-3) PUFAs, but not the C22 substrates. The expression of elovl5 was significantly affected by dietary n-3 LC-PUFA exposure: a high n-3 LC-PUFA level repressed the expression of elovl5 by slightly down-regulating the expression of sterol regulatory element-binding protein (SREBP)-1 and liver X receptor (LXR) α, which are major regulators of hepatic lipid metabolism. Promoter studies showed that grouper elovl5 reporter activity was induced by over-expression of LXRα but not SREBP-1. This finding suggests that elovl5 is a direct target of LXRα, which is involved in the biosynthesis of PUFAs via transcriptional regulation of elovl5. These findings may contribute to a further understanding of the mechanism underlying the regulation of LC-PUFA biosynthesis in marine fish species.

Assessment of a land-locked Atlantic salmon (Salmo salar L.) population as a potential genetic resource with a focus on long-chain polyunsaturated fatty acid biosynthesis

The natural food for Atlantic salmon (Salmo salar) in freshwater has relatively lower levels of omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFA) than found in prey for post-smolt salmon in seawater. Land-locked salmon such as the Gullspång population feed exclusively on freshwater type lipids during its entire life cycle, a successful adaptation derived from divergent evolution. Studying land-locked populations may provide insights into the molecular and genetic control mechanisms that determine and regulate n-3 LC-PUFA biosynthesis and retention in Atlantic salmon. A two factorial study was performed comparing land-locked and farmed salmon parr fed diets formulated with fish or rapeseed oil for 8 weeks. The land-locked parr had higher capacity to synthesise n-3 LC-PUFA as indicated by higher expression and activity of desaturase and elongase enzymes. The data suggested that the land-locked salmon had reduced sensitivity to dietary fatty acid composition and that dietary docosahexaenoic acid (DHA) did not appear to suppress expression of LC-PUFA biosynthetic genes or activity of the biosynthesis pathway, probably an evolutionary adaptation to a natural diet lower in DHA. Increased biosynthetic activity did not translate to enhanced n-3 LC-PUFA contents in the flesh and diet was the only factor affecting this parameter. Additionally, high lipogenic and glycolytic potentials were found in land-locked salmon, together with decreased lipolysis which in turn could indicate increased use of carbohydrates as an energy source and a sparing of lipid.

Effects of dietary linseed oil on innate immune system of Eurasian perch and disease resistance after exposure to Aeromonas salmonicida achromogen

This study was designated to investigate the effects of dietary fish oil (FO diet) replacement by linseed oil (LO diet) on regulation of immune response and disease resistance in Eurasian perch (Perca fluviatilis). A control diet containing fish oil (FO = cod liver oil) and characterized by high levels of n-3 high LC-PUFA (6% EPA, 7.5% of total fatty acids (FAs)) was compared to linseed oil diet (LO diet) composed of low LC-PUFA contents (1% EPA, 2.3% DHA of total FAs) but high C18 fatty acids levels. The experiment was conducted in quadruplicate groups of 80 fish each. After 10 weeks of feeding, the innate immune status was evaluated in various organs (liver, spleen, and head-kidney) (feeding condition). Two days later, a bacterial challenge was performed on fish from 2 rearing conditions: fish infected with Aeromonas salmonicida (bacteria condition) and fish injected with sterile medium but maintained in the same flow system that fish challenged with bacteria (sentinel condition). Three days after injection of bacteria, a significant decrease of lymphocyte, thrombocyte and basophil populations was observed while neutrophils were not affected. In addition, plasma lysozyme activity and reactive oxygen species production in kidney significantly increased in fish challenged with A. salmonicida while the plasma alternative complement pathway activity was not affected. Increase of plasma lysozyme activity as well as reactive oxygen species production in spleen and kidney of sentinel fish suggest that these immune defenses can also be activated, but at lower bacteria concentration than infected fish. No differences in leucocyte populations, plasma lysozyme and alternative complement pathway activities were observed between dietary treatments. Similarly, expression of genes related to eicosanoid synthesis in liver were not affected by the dietary oil source but were strongly stimulated in fish challenged with A. salmonicida. These findings demonstrated that the use of linseed oil does not deplete the innate immune system of Eurasian perch juveniles.

Dietary Linseed Oil Reduces Growth While Differentially Impacting LC-PUFA Synthesis and Accretion into Tissues in Eurasian Perch (Perca fluviatilis)

The aim of this study was to evaluate the impact of replacing dietary fish oil (FO) with linseed oil (LO) on growth, fatty acid composition and regulation of lipid metabolism in Eurasian perch (Perca fluviatilis) juveniles. Fish (17.5 g initial body weight) were fed isoproteic and isoenergetic diets containing 116 g/kg of lipid for 10 weeks. Fish fed the LO diet displayed lower growth rates and lower levels of DHA in the liver and muscle than fish fed the FO diet, while mortality was not affected by dietary treatment. However, DHA content recorded in the liver and muscle of fish fed the LO diet remained relatively high, despite a weight gain of 134 % and a reduced dietary level of long-chain polyunsaturated fatty acids (LC-PUFA), suggesting endogenous LC-PUFA biosynthesis. This was supported by the higher amounts of pathway intermediates, including 18:4n-3, 20:3n-3, 20:4n-3, 18:3n-6 and 20:3n-6, recorded in the liver of fish fed the LO diet in comparison with those fed the FO diet. However, fads2 and elovl5 gene expression and FADS2 enzyme activity were comparable between the two groups. Similarly, the expression of genes involved in eicosanoid synthesis was not modulated by dietary LO. Thus, the present study demonstrated that in fish fed LO for 10 weeks, growth was reduced but DHA levels in tissues were largely maintained compared to fish fed FO, suggesting a physiologically relevant rate of endogenous LC-PUFA biosynthesis capacity.

Cloning and characterization of SREBP-1 and PPAR-α in Japanese seabass Lateolabrax japonicus, and their gene expressions in response to different dietary fatty acid profiles

In the present study, putative cDNA of sterol regulatory element-binding protein 1 (SREBP-1) and peroxisome proliferator-activated receptor α (PPAR-α), key regulators of lipid homoeostasis, were cloned and characterized from liver of Japanese seabass (Lateolabrax japonicus), and their expression in response to diets enriched with fish oil (FO) or fatty acids such as palmitic acid (PA), stearic acid (SA), oleic acid (OA), α-linolenic acid (ALA), and n-3 long-chain polyunsaturated fatty acid (n-3 LC-PUFA), was investigated following feeding. The SREBP-1 of Japanese seabass appeared to be equivalent to SREBP-1a of mammals in terms of sequence feature and tissue expression pattern. The stimulation of the mRNA expression level of SREBP-1 in liver of Japanese seabass by dietary fatty acids significantly ranked as follows: PA, OA>SA, ALA, and n-3 LC-PUFA>FO. A new PPAR-α subtype in Japanese seabass, PPAR-α2, was cloned in this study, which is not on the same branch with Japanese seabass PPAR-α1 and mammalian PPAR-α in the phylogenetic tree. Liver gene expression of PPAR-α1 of Japanese seabass was inhibited by diets enriched with ALA or FO compared to diets enriched with PA or OA, while the gene expression of PPAR-α2 of Japanese seabass was up-regulated by diets enriched with ALA or n-3 LC-PUFA compared to diets enriched with OA or FO. This was the first evidence for the great divergence in response to dietary fatty acids between PPAR-α1 and PPAR-α2 of fish, which indicated probable functional distribution between PPAR-α isotypes of fish.

Influence of dietary docosahexaenoic acid in combination with other long-chain polyunsaturated fatty acids on expression of biosynthesis genes and phospholipid fatty acid compositions in tissues of post-smolt Atlantic salmon (Salmo salar)

To investigate interactions of dietary LC-PUFA, a dose-response study with a range of docosahexaenoic acid (DHA; 22:6n-3) levels (1 g kg(-1), 5 g kg(-1), 10 g kg(-1), 15 g kg(-1) and 20 g kg(-1)) was performed with post-smolts (111 ± 2.6g; mean ± S.D.) over a nine-week feeding period. Additional diets included 10 g kg(-1) DHA in combination with 10 g kg(-1) of either eicosapentaenoic acid (EPA; 20:5n-3) or arachidonic acid (ARA; 20:4n-6), and a diet containing 5 g kg(-1) each of DHA and EPA. The liver, brain, head kidney and gill were collected at the conclusion of the trial, and lipid and fatty acid compositions were determined as well as expression of genes of LC-PUFA biosynthesis. Total lipid content and class composition were largely unaffected by changes in dietary LC-PUFA. However, phospholipid (PL) fatty acid compositions generally reflected that of the diet, although the response varied between tissues. The liver most strongly reflected diet, followed by the head kidney. In both tissues increasing dietary DHA led to significantly increased DHA in PL and inclusion of EPA or ARA led to higher levels of these fatty acids. The brain showed the most conserved composition and gene expression profile, with increased dietary LC-PUFA resulting in only minor changes in PL fatty acids. Dietary LC-PUFA significantly affected the expression of Δ6 and Δ5 desaturases, Elovl 2, 4 and 5, and SREBPs although this varied between tissues with greatest effects observed in the liver followed by the head kidney, similar to PL fatty acid compositions.

Regulation of tissue LC-PUFA contents, Δ6 fatty acyl desaturase (FADS2) gene expression and the methylation of the putative FADS2 gene promoter by different dietary fatty acid profiles in Japanese seabass (Lateolabrax japonicus)

The present study was conducted to evaluate the influences of different dietary fatty acid profiles on the tissue content and biosynthesis of LC-PUFA in a euryhaline species Japanese seabass reared in seawater. Six diets were prepared, each with a characteristic fatty acid: Diet PA: Palmitic acid (C16:0); Diet SA: Stearic acid (C18:0); Diet OA: Oleic acid (C18:1n-9); Diet LNA: α-linolenic acid (C18:3n-3); Diet N-3 LC-PUFA: n-3 LC-PUFA (DHA+EPA); Diet FO: the fish oil control. A 10-week feeding trial was conducted using juvenile fish (29.53 ± 0.86 g). The results showed that Japanese seabass had limited capacity to synthesize LC-PUFA and fish fed PA, SA, OA and LNA showed significantly lower tissue n-3 LC-PUFA contents compared to fish fed N-3 LC-PUFA and FO. The putative gene promoter and full-length cDNA of FADS2 was cloned and characterized. The protein sequence was confirmed to be homologous to FADS2s of marine teleosts and possessed all the characteristic features of microsomal fatty acid desaturases. The FADS2 transcript levels in liver of fish fed N-3 LC-PUFA and FO were significantly lower than those in fish fed other diets except LNA while Diet PA significantly up-regulated the FADS2 gene expression compared to Diet LNA, N-3 LC-PUFA and FO. Inversely, fish fed N-3 LC-PUFA and FO showed significantly higher promoter methylation rates of FADS2 gene compared to fish fed the LC-PUFA deficient diets. These results suggested that Japanese seabass had low LC-PUFA synthesis capacity and LC-PUFA deficient diets caused significantly reduced tissue n-3 LC-PUFA contents. The liver gene expression of FADS2 was up-regulated in groups enriched in C16:0, C18:0 and C18:1n-9 respectively but not in the group enriched in C18:3n-3 compared to groups with high n-3 LC-PUFA contents. The FADS2 gene expression regulated by dietary fatty acids was significantly negatively correlated with the methylation rate of putative FADS2 gene promoter.

Conservation of lipid metabolic gene transcriptional regulatory networks in fish and mammals

Lipid content and composition in aquafeeds have changed rapidly as a result of the recent drive to replace ecologically limited marine ingredients, fishmeal and fish oil (FO). Terrestrial plant products are the most economic and sustainable alternative; however, plant meals and oils are devoid of physiologically important cholesterol and long-chain polyunsaturated fatty acids (LC-PUFA), eicosapentaenoic (EPA), docosahexaenoic (DHA) and arachidonic (ARA) acids. Although replacement of dietary FO with vegetable oil (VO) has little effect on growth in Atlantic salmon (Salmo salar), several studies have shown major effects on the activity and expression of genes involved in lipid homeostasis. In vertebrates, sterols and LC-PUFA play crucial roles in lipid metabolism by direct interaction with lipid-sensing transcription factors (TFs) and consequent regulation of target genes. The primary aim of the present study was to elucidate the role of key TFs in the transcriptional regulation of lipid metabolism in fish by transfection and overexpression of TFs. The results show that the expression of genes of LC-PUFA biosynthesis (elovl and fads2) and cholesterol metabolism (abca1) are regulated by Lxr and Srebp TFs in salmon, indicating highly conserved regulatory mechanism across vertebrates. In addition, srebp1 and srebp2 mRNA respond to replacement of dietary FO with VO. Thus, Atlantic salmon adjust lipid metabolism in response to dietary lipid composition through the transcriptional regulation of gene expression. It may be possible to further increase efficient and effective use of sustainable alternatives to marine products in aquaculture by considering these important molecular interactions when formulating diets.

Cloning and tissue distribution of a fatty acyl Δ6-desaturase-like gene and effects of dietary lipid levels on its expression in the hepatopancreas of Chinese mitten crab (Eriocheir sinensis)

Fatty acyl Δ6-desaturase is the rate-limiting enzyme in the biosynthetic pathway of highly unsaturated fatty acids (HUFAs) in vertebrates. In this report, a fatty acyl Δ6-desaturase-like cDNA was cloned from the hepatopancreas of Eriocheir sinensis (Chinese mitten crab) and characterized by performing rapid-amplification of cDNA ends. The 2278-bp long full-length cDNA encodes a polypeptide with 442 amino acids. Gene expression analysis via real-time quantitative polymerase chain reaction revealed that the fatty acyl Δ6-desaturase-like transcripts are widely distributed in various tissues, with high expression levels in the hepatopancreas and cranial ganglia. This study focuses on the nutritional regulation of genes involved in the HUFA biosynthetic pathway in Chinese mitten crab. A feeding trial was performed whereby crablets were fed for 238 d with four different diets: control diet without oil lipids (added with 3% basic lipid of the fundamental diets); fish oil diet (FO; added with 3% of the fundamental diets); soybean oil diet (SO; added with 3% of the fundamental diets); and FO/SO diet (1:1; added with 3% of the fundamental diets). The hepatopancreas of crabs sampled at 168 d and 238 d to determine the effects on fatty acyl Δ6-desaturase-like mRNA expression. The results show that the expression of fatty acyl Δ6-desaturase-like is higher in the hepatopancreas of crabs fed with SO diet than those fed with FO diet. Furthermore, gene expression increased by 2.45-fold in the hepatopancreas of crabs fed with SO after 238 d than those fed after 168 d but remained steady for those fed with FO after 238 d.

Nervonic Acid (24:1n-9) is a Dominant Unsaturated Fatty Acid in the Intestinal Brush Border of Atlantic Cod

Atlantic cod is a coldwater teleost of commercial importance. The intestinal epithelium is a large organ in vertebrates serving an important role in nutrient selection and uptake as well as an immunological barrier. Here, we perform lipid and fatty acid analysis of the plasma membrane from the cod intestinal enterocytes after separation of the brush border membrane and the basolateral membrane fractions. Our results show that both membrane fractions contain an unusually high amount of cholesterol and glycolipids but low levels of glycerophospholipids compared with other reported studies on fish. Sphingomyelin was the dominant lipid in the brush border fraction and was also prominent in the basolateral fraction where phosphatidylcholine was the dominant glycerophospholipid. Furthermore, our results show a distinct difference in fatty acids content, where monounsaturated fatty acids (MUFA) were more abundant than polyunsaturated fatty acid (PUFA). Nervonic acid (24:1n-9) was a prominent fatty acid in the BBM at ~50% of the total MUFA. We hypothesize that the high cholesterol content and the presence of this rare fatty acid may serve to maintain membrane fluidity in the cold environment.

Transcriptomic analyses of intestinal gene expression of juvenile Atlantic cod (Gadus morhua) fed diets with Camelina oil as replacement for fish oil

For aquaculture of marine species to continue to expand, dietary fish oil (FO) must be replaced with more sustainable vegetable oil (VO) alternatives. Most VO are rich in n-6 polyunsaturated fatty acids (PUFA) and few are rich in n-3 PUFA but Camelina oil (CO) is unique in that, besides high 18:3n-3 and n-3/n-6 PUFA ratio, it also contains substantial long-chain monoenes, commonly found in FO. Cod (initial mass ~1.4 g) were fed for 12 weeks diets in which FO was replaced with CO. Growth performance, feed efficiency and biometric indices were not affected but lipid levels in liver and intestine tended to increase and those of flesh, decrease, with increasing dietary CO although only significantly for intestine. Reflecting diet, tissue n-3 long-chain PUFA levels decreased whereas 18:3n-3 and 18:2n-6 increased with inclusion of dietary CO. Dietary replacement of FO by CO did not induce major metabolic changes in intestine, but affected genes with potential to alter cellular proliferation and death as well as change structural properties of intestinal muscle. Although the biological effects of these changes are unclear, given the important role of intestine in nutrient absorption and health, further attention should be given to this organ in future.

Genotype-specific responses in Atlantic salmon (Salmo salar) subject to dietary fish oil replacement by vegetable oil: a liver transcriptomic analysis

Background

Expansion of aquaculture is seriously limited by reductions in fish oil (FO) supply for aquafeeds. Terrestrial alternatives such as vegetable oils (VO) have been investigated and recently a strategy combining genetic selection with changes in diet formulations has been proposed to meet growing demands for aquaculture products. This study investigates the influence of genotype on transcriptomic responses to sustainable feeds in Atlantic salmon.

Results

A microarray analysis was performed to investigate the liver transcriptome of two family groups selected according to their estimated breeding values (EBVs) for flesh lipid content, 'Lean' or 'Fat', fed diets containing either FO or a VO blend. Diet principally affected metabolism genes, mainly of lipid and carbohydrate, followed by immune response genes. Genotype had a much lower impact on metabolism-related genes and affected mostly signalling pathways. Replacement of dietary FO by VO caused an up-regulation of long-chain polyunsaturated fatty acid biosynthesis, but there was a clear genotype effect as fatty acyl elongase (elovl2) was only up-regulated and desaturases (Δ5 fad and Δ6 fad) showed a higher magnitude of response in Lean fish, which was reflected in liver fatty acid composition. Fatty acid synthase (FAS) was also up-regulated by VO and the effect was independent of genotype. Genetic background of the fish clearly affected regulation of lipid metabolism, as PPARα and PPARβ were down-regulated by the VO diet only in Lean fish, while in Fat salmon SREBP-1 expression was up-regulated by VO. In addition, all three genes had a lower expression in the Lean family group than in the Fat, when fed VO. Differences in muscle adiposity between family groups may have been caused by higher levels of hepatic fatty acid and glycerophospholipid synthesis in the Fat fish, as indicated by the expression of FAS, 1-acyl-sn-glycerol-3-phosphate acyltransferase and lipid phosphate phosphohydrolase 2.

Conclusions

This study has identified metabolic pathways and key regulators that may respond differently to alternative plant-based feeds depending on genotype. Further studies are required but data suggest that it will be possible to identify families better adapted to alternative diet formulations that might be appropriate for future genetic selection programmes.

Molecular cloning and ontogenic mRNA expression of fatty acid desaturase in the carnivorous striped snakehead fish (Channa striata)

There is very little information on the capacity of freshwater carnivorous fish to biosynthesize highly unsaturated fatty acids (HUFA). The striped snakehead fish (Channa striata) is a carnivorous species cultured inland of several Southeast Asian countries due to its pharmaceutical properties in wound healing enhancement. We described here the full-length cDNA cloning of a striped snakehead fatty acid desaturase (fads), which is responsible for desaturation of unsaturated fatty acids in the HUFA biosynthesis. Bioinformatics analysis reveals a protein coding region with length of 445 amino acids containing all characteristic features of desaturase enzyme, including a cytochrome b5-domain with the heme-binding motif, two transmembrane domains and three histidine-rich regions. The striped snakehead fads amino acid sequence shares high similarity with known fads of other teleosts. The mRNA expression of striped snakehead fads also showed an ontogenic-related increase in expression in 0-20 days after hatch larva. Using ISH, we localized the presence of fads in larva brain, liver and intestinal tissues.