Functional diversification of teleost Fads2 fatty acyl desaturases occurs independently of the trophic level

Short-term dietary changes are reflected in the cerebral content of adult ring-billed gulls

Omega-3 long-chain polyunsaturated fatty acids (n3-LCPUFAs) are produced primarily in aquatic ecosystems and are considered essential nutrients for predators given their structural role in vertebrates' cerebral tissues. Alarmingly, with urbanization, many aquatic animals now rely on anthropogenic foods lacking n3-LCPUFAs. In this study undertaken in Newfoundland (Canada), we tested whether recent or longer term diet explains the cerebral fatty acid composition of ring-billed gulls (Larus delawarensis), a seabird that now thrives in cities. During the breeding season, cerebral levels of n3-LCPUFAs were significantly higher for gulls nesting in a natural habitat and foraging on marine food (mean ± s.d.: 32 ± 1% of total identified fatty acids) than for urban nesters exploiting rubbish (27 ± 1%). Stable isotope analysis of blood and feathers showed that urban and natural nesters shared similar diets in autumn and winter, suggesting that the difference in cerebral n3-LCPUFAs during the breeding season was owing to concomitant and transient differences in diet. We also experimentally manipulated gulls' diets throughout incubation by supplementing them with fish oil rich in n3-LCPUFAs, a caloric control lacking n3-LCPUFAs, or nothing, and found evidence that fish oil increased urban nesters' cerebral n3-LCPUFAs. These complementary analyses provide evidence that the brain of this seabird remains plastic during adulthood and responds to short-term dietary changes.

Dietary LC-PUFA and environmental salinity modulate the fatty acid biosynthesis capacity of the euryhaline teleost thicklip grey mullet (Chelon labrosus)

The capacity to biosynthesise long-chain (≥C20) polyunsaturated fatty acids (LC-PUFA) depends upon the complement and function of key enzymes commonly known as fatty acyl desaturases and elongases. The presence of a Δ5/Δ6 desaturase enabling the biosynthesis of docosahexaenoic acid (22:6n-3, DHA) through the "Sprecher pathway" has been reported in Chelon labrosus. Research in other teleosts have demonstrated that LC-PUFA biosynthesis can be modulated by diet and ambient salinity. The present study aimed to assess the combined effects of partial dietary replacement of fish oil (FO) by vegetable oil (VO) and reduced ambient salinity (35 ppt vs 20 ppt) on the fatty acid composition of muscle, enterocytes and hepatocytes of C. labrosus juveniles. Moreover, the enzymatic activity over radiolabelled [1-14C] 18:3n-3 (α-linolenic acid, ALA) and [1-14C] 20:5n-3 (eicosapentaenoic acid, EPA) to biosynthesise n-3 LC-PUFA in hepatocytes and enterocytes, and the gene regulation of the C. labrosus fatty acid desaturase-2 (fads2) and elongation of very long chain fatty acids protein 5 (elovl5) in liver and intestine was also investigated. Recovery of radiolabelled products including stearidonic acid (18:4n-3, SDA), 20:5n-3, tetracosahexaenoic acid (24:6n-3, THA) and 22:6n-3 in all treatments except FO35-fish, provided compelling evidence that a complete pathway enabling the biosynthesis of EPA and DHA from ALA is present and active in C. labrosus. Low salinity conditions upregulated fads2 in hepatocytes and elovl5 in both cell types, regardless of dietary composition. Interestingly, FO20-fish showed the highest amount of n-3 LC-PUFA in muscle, while no differences in VO-fish reared at both salinities were found. These results demonstrate a compensatory capacity of C. labrosus to biosynthesise n-3 LC-PUFA under reduced dietary supply, and emphasise the potential of low salinity conditions to stimulate this pathway in euryhaline fish.

Detailed whole-body nutrient analysis identifies differences in feeding ecology between related fish species: The case of Orestias native Andean killifish in Lake Titicaca

Body nutrient profiles in ecological studies allow for relating the nutritional status of consumers and their effects on the movement and retention of elements in ecosystems, as well as reflecting feeding conditions and habitat quality. This study compared the detailed whole-body nutrient composition (macronutrients, minerals, fatty acids and amino acids) of two omnivorous natives Orestias killifish from Lake Titicaca (Orestias agassizii and Orestias luteus, Valenciennes), the largest lake in the Andes, as an indirect tool to understand differences in their feeding ecology. Although both species are usually described as omnivorous fish, both have amphipods (Hyalella spp) as their main food source. Our results showed that both killifish had a comparable macronutrient composition, and the mineral concentrations of Mg, P and Ca (reflecting bony structures) differed between them. Many of the saturated fatty acids were significantly lower in O. luteus, and O. agassizii had higher concentrations of cis-vaccenic acid (18:1n11 (cis)), supporting the idea of a higher algal contribution to the diet of this fish. The lower histidine and higher taurine concentrations in O. agassizii compared with O. luteus (independent of body size) may reflect its ubiquitous behaviour and plasticity. This study shows how whole-body nutrient analysis can identify differences in feeding ecology and feeding behaviour between related species.

Genome-wide comparative identification and analysis of membrane-FADS-like superfamily genes in freshwater economic fishes

Membrane fatty acid desaturase (FADS)-like superfamily proteins (FADSs) are essential for the synthesis of unsaturated fatty acids (UFAs). Recently, studies on FADS in fishes have mostly focused on marine species, and a comprehensive analysis of the FADS superfamily, including the FADS, stearoyl-CoA desaturase (SCD), and sphingolipid delta 4-desaturase (DEGS) families, in freshwater economic fishes is urgently required. To this end, we conducted a thorough analysis of the number, gene/protein structure, chromosomal location, gene linkage map, phylogeny, and expression of the FADS superfamily. We identified 156 FADSs genes in the genome of 27 representative species. Notably, FADS1 and SCD5 were lost in most freshwater fish and other teleosts. All FADSs proteins contain 4 transmembrane helices and 2-3 amphipathic α-helices. FADSs in the same family are often linked on the same chromosome; moreover, FADS and SCD or DEGS are frequently collocated on the same chromosome. In addition, FADS, SCD, and DEGS family proteins share similar evolutionary patterns. Interestingly, FADS6, as a member of the FADS family, exhibits a similar gene structure and chromosome location to that of SCD family members, which may be the transitional form of FADS and SCD. This study shed light on the type, structure, and phylogenetic relationship of FADSs in freshwater fishes, offering a new perspective into the functional mechanism analysis of FADSs.

Metabolic and molecular evidence for long-chain PUFA biosynthesis capacity in the grass carp Ctenopharyngodon idella

There is a growing interest to understand the capacity of farmed fish species to biosynthesise the physiologically important long-chain (≥C₂₀) n-3 and n-6 polyunsaturated fatty acids (LC-PUFAs), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and arachidonic acid (ARA), from their C₁₈ PUFA precursors available in the diet. In fish, the LC-PUFA biosynthesis pathways involve sequential desaturation and elongation reactions from α-linolenic acid (ALA) and linoleic acid (LA), catalysed by fatty acyl desaturases (Fads) and elongation of very long-chain fatty acids (Elovl) proteins. Our current understanding of the grass carp (Ctenopharyngodon idella) LC-PUFA biosynthetic capacity is limited despite representing the most farmed finfish produced worldwide. To address this knowledge gap, this study first aimed at characterising molecularly and functionally three genes (fads2, elovl5 and elovl2) with putative roles in LC-PUFA biosynthesis. Using an in vitro yeast-based system, we found that grass carp Fads2 possesses ∆8 and ∆5 desaturase activities, with ∆6 ability to desaturase not only the C₁₈ PUFA precursors (ALA and LA) but also 24:5n-3 to 24:6n-3, a key intermediate to obtain DHA through the "Sprecher pathway". Additionally, the Elovl5 showed capacity to elongate C₁₈ and C₂₀ PUFA substrates, whereas Elovl2 was more active over C₂₀ and C₂₂. Collectively, the molecular cloning and functional characterisation of fads2, elovl5 and elovl2 demonstrated that the grass carp has all the enzymatic activities required to obtain ARA, EPA and DHA from LA and ALA. Importantly, the hepatocytes incubated with radiolabelled fatty acids confirmed the yeast-based results and demonstrated that these enzymes are functionally active.

A complete inventory of long-chain polyunsaturated fatty acid biosynthesis pathway enzymes in the miniaturized cyprinid Paedocypris micromegethes

The capacity for long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis activity in a species depends on the enzymatic activities of fatty acyl desaturase (Fads) and elongation of very long-chain fatty acid (Elovl). The miniaturized fish Paedocypris micromegethes is a developmentally truncated cyprinid living in highly acidic water conditions in tropical peat swamps. The capacity for LC-PUFA biosynthesis in this species, which has a reduced genome size, is unknown. A high-quality de novo transcriptome assembly enabled the identification of a putative Fads2 and four Elovl. The Fads2 was verified as a P. micromegethes Fads2 ortholog with in vitro Δ5 and Δ6 activities. The Elovl sequences were established as an Elovl5, Elovl2, and two Elovl4 paralogs, namely Elovl4a and Elovl4b. These Elovl enzymes, mainly Elovl5 and Elovl2, fulfill the necessary C18, C20, and C22 PUFA elongation steps for LC-PUFA biosynthesis. Collectively, these results validate the presence of a complete repertoire of LC-PUFA biosynthesis enzymes in a peat swamp miniatured freshwater fish.

Effect of Dietary Tryptophan on Growth, Intestinal Microbiota, and Intestinal Gene Expression in an Improved Triploid Crucian Carp

Tryptophan (Trp) has received increasing attention in the maintenance of intestinal function. In this study, improved triploid crucian carp (ITCC) fed diets containing 6.35 g kg⁻¹ Trp had higher average daily gain (ADG) and improved villus height (VH) and crypt depth (CD) in the intestine compared to the control group. To elucidate the potential mechanisms, we used RNA sequencing (RNA-seq) to investigate changes in the intestinal transcriptome and 16S rRNA gene sequencing to measure the intestinal microbiota in response to 6.35 g kg⁻¹ Trp feeding in ITCC. Dietary Trp altered intestinal gene expression involved in nutrient transport and metabolism. Differentially expressed transcripts (DETs) were highly enriched in key pathways containing protein digestion and absorption and the AMPK signaling pathway. 16S rRNA sequencing showed that 6.35 g kg⁻¹ Trp significantly increased the abundance of the genus Cetobacterium, and the Firmicutes/Bacteroidetes ratio at the phylum level (P < 0.05). In addition, bacterial richness indices (Simpson index) significantly increased (P < 0.05) community evenness in response to 6.35 g kg⁻¹ Trp. In conclusion, appropriate dietary Trp improves the growth performance, and influences the intestinal flora of ITCC. This study might be helpful to guide the supply of dietary exogenous Trp in ITCC breeding.

Lack of ∆5 Desaturase Activity Impairs EPA and DHA Synthesis in Fish Cells from Red Sea Bream and Japanese Flounder

Long-chain (≥ C₂₀) polyunsaturated fatty acids (LC-PUFA), such as eicosapentaenoic acid (20:5n-3, EPA) and docosahexaenoic acid (22:6n-3, DHA), are necessary for human health and are obtained from marine fish-derived oils. Marine fish are LC-PUFA-rich animals; however, many of them require LC-PUFA for growth. Therefore, it is suggested that they do not have sufficient ability to biosynthesize LC-PUFA. To evaluate in vivo LC-PUFA synthetic activity in fish cells, fish-derived cell lines from red sea bream (Pagrus major, PMS and PMF), Japanese flounder (Paralichthys olivaceus, HINAE), and zebrafish (Danio rerio, BRF41) were incubated with n-3 fatty acids labeled by radioisotopes or stable isotopes, and then, n-3 PUFA were analyzed by thin-layer chromatography or liquid chromatography-mass spectrometry. Labeled EPA and DHA were biosynthesized from labeled α-linolenic acid (18:3n-3) in BRF41, whereas they were not detected in PMS, PMF, or HINAE cells. We next cloned the fatty acid desaturase 2 (Fads2) cDNAs from PMF cells and zebrafish, expressed in budding yeasts, and then analyzed the substrate specificities of enzymes. As a result, we found that Fads2 from PMF cells was a ∆6/∆8 desaturase. Collectively, our study indicates that cell lines from red sea bream and Japanese flounder were not able to synthesize EPA or DHA by themselves, possibly due to the lack of ∆5 desaturase activity. Furthermore, this study provides a sensitive and reproducible non-radioactive method for evaluating LC-PUFA synthesis in fish cells using a stable isotope and liquid chromatography-mass spectrometry.

Expression of long-chain polyunsaturated fatty acids biosynthesis genes during the early life-cycle stages of the tropical gar Atractosteus tropicus

Long-chain (≥C₂₀) polyunsaturated fatty acids (LC-PUFA), including eicosapentaenoic acid (EPA, 20:5n-3), arachidonic acid (ARA, 20:4n-6) and docosahexaenoic acid (DHA, 22:6n-3), are essential in multiple physiological processes, especially during early development of vertebrates. LC-PUFA biosynthesis is achieved by two key families of enzymes, fatty acyl desaturases (Fads) and elongation of very long-chain fatty acid (Elovl). The present study determined the expression patterns of genes encoding desaturases (fads1 and fads2) and elongases (elovl2 and elovl5) involved in the LC-PUFA biosynthesis during early life-stages of the tropical gar Atractosteus tropicus. We further analyzed the fatty acid profiles during early development of A. tropicus to evaluate the impact of Fads and Elovl enzymatic activities. Specific oligonucleotides were designed from A. tropicus transcriptome to perform qPCR (quantitative polymerase chain reaction) on embryonic and larval stages, along with several organs (intestine, white muscle, brain, liver, heart, mesenteric adipose, kidney, gill, swim bladder, stomach, and spleen) collected from juvenile specimens. Fatty acid content of feeds and embryonic and larval stages were analyzed. Results show that fads1, fads2, elovl2 and elovl5 expression was detected from embryonic stages with expression peaks from day 15 post hatching, which could be related to transcriptional and dietary factors. Moreover, fads1, fads2 and elovl2 showed a higher expression in intestine, while elovl5 showed a higher expression in liver, suggesting that the tropical gar activates its LC-PUFA biosynthetic machinery to produce ARA, EPA and DHA to satisfy physiological demands at crucial developmental milestones during early development.

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.

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.

Comparative and functional analysis of desaturase FADS1 (∆5) and FADS2 (∆6) orthologues of marine organisms

Fatty acid desaturases are key enzymes involved in unsaturated fatty acid biosynthesis, which insert double bonds at specific positions of fatty acids, playing a pivotal role in unsaturated fatty acid synthesis required for membrane lipid fluidity. The ∆5 and ∆6 desaturases are responsible for producing long chain-polyunsaturated fatty acids (LC-PUFA) through their precursors α-linolenic acid and linoleic acid in organisms lacking or with very low ability to synthesize LC-PUFA by themselves. Extensive studies of fatty acid desaturases are available in model organisms, such as humans and mouse; however, the diversity of these genes in the marine biodiversity is less known. This study performed an exhaustive analysis to identify the ∆5 and ∆6 desaturases in the available marine genomes in databases, as well as transcriptomes and EST databases, and their coding sequences were compared to the well-characterized ∆5 and ∆6 desaturases from humans. The FADS1 and FADS2 genetic structures are well conserved among all the organisms analyzed. A common amino acid pattern was identified to discriminate between ∆5 and ∆6 desaturases. The analysis of the conserved motif involved in catalysis showed that 20% of the desaturases, ∆5 and ∆6, have lost motifs required for catalysis. Additionally, bifunctional ∆5/∆6 desaturases were able to be identified by amino acid sequence patterns found in previously described enzymes. A revision of the expression profiles and functional activity on sequences in databases and scientific literature provided information regarding the function of these marine organism enzymes.