Molecular and Functional Characterization of Elovl4 Genes in Sparus aurata and Solea senegalensis Pointing to a Critical Role in Very Long-Chain (>C24) Fatty Acid Synthesis during Early Neural Development of Fish

Fish oil replacement with different vegetable oils in Onychostoma macrolepis: Effects on fatty acid metabolism based on whole-body fatty acid balance method and genes expression

To evaluate the fatty acid (FA) metabolism status and possibility as a DHA source of farmed Onychostoma macrolepis, a total of 168 fish (2.03 ± 0.23 g) were fed four diets supplemented with fish oil (FO), linseed oil (LO), soybean oil (SO), and a mixture of LO and SO oil (MO), respectively, for 70 days. Body FA compositions were modified reflecting dietary FAs. Comparing liver and intestine fatty acids with fish fed four diets, the content of ARA in fish fed SO was significantly higher than others (P < 0.05), but showed no difference in muscle. The tissue FA profile showed that the FO-fed group successfully deposited DHA, while the LO-fed group converted ALA to DHA effectively, as well as the liver and intestine EPA was notably highest in the FO group, whereas no difference between the FO and LO group in the muscle. The FA results showed that the DHA contents in the muscle of Onychostoma macrolepis are at a medium-high level compared with several other fish species with the highest aquaculture yield. Correspondingly, in the fish fed diet with LO, SO, and MO, the genes of most FA biosynthesis, transportation, and transcriptional regulation factors were increased in the liver and muscle, but no significant difference was observed in the gene expression of Elovl4b, FATP1, and FABP10 in the muscle. In addition, the enzyme activity involved in PUFA metabolism was higher in fish fed vegetable oil-based diets, corroborating the results of the gene expression. Increased in vivo elongase and desaturase (Δ5, Δ6, and Δ9) activities were recorded in fish fed fish oil-devoid diets, which resulted in the appearance of products associated with elongase and desaturase activities in fish. Besides, as the specific n-3 PUFA synthesis substrate, the dietary supplementation of ALA not only retains most of the nutrition value but also ensures the muscular texture, such as fiber diameter and density. It is concluded that farmed O. macrolepis owns strong n-3 LC-PUFA biosynthetic capacity and high DHA contents so it can be a good DHA source for the population.

Determination of dietary essential fatty acids in a deep-sea fish, the splendid alfonsino Beryx splendens: functional characterization of enzymes involved in long-chain polyunsaturated fatty acid biosynthesis

The splendid alfonsino Beryx splendens is a commercially important deep-sea fish in East Asian countries. Because the wild stock of this species has been declining, there is an urgent need to develop aquaculture systems. In the present study, we investigated the long-chain polyunsaturated fatty acid (LC-PUFA) requirements of B. splendens, which are known as essential dietary components in many carnivorous marine fish species. The fatty acid profiles of the muscles, liver, and stomach contents of B. splendens suggested that it acquires substantial levels of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from its natural diet. The functional characterization of a fatty acid desaturase (Fads2) and three elongases (Elovl5, Elovl4a, and Elovl4b) from B. splendens confirmed their enzymatic capabilities in LC-PUFA biosynthesis. Fads2 showed Δ6 and Δ8 bifunctional desaturase activities. Elovl5 showed preferential elongase activities toward C₁₈ and C₂₀ PUFA substrates, whereas Elovl4a and Elovl4b showed activities toward various C_18-22 substrates. Given that Fads2 showed no Δ5 desaturase activity and no other fads-like sequence was found in the B. splendens genome, EPA and arachidonic acid cannot be synthesized from C₁₈ precursors; hence, they can be categorized as dietary essential fatty acids in B. splendens. EPA can be converted into DHA in B. splendens via the so-called Sprecher pathway. However, given that fads2 is only expressed in the brain, it is unlikely that the capacity of B. splendens to biosynthesize DHA from EPA can fulfill its physiological requirements. These results will be useful to researchers developing B. splendens aquaculture methods.

Fatty acid-binding protein genes in gilthead seabream: molecular cloning and nutritional regulation under low water temperatures

The molecular characteristics and tissue disruption of 10 fatty acid-binding protein (fabp) genes in gilthead seabream (Sparus aurata) were investigated, and their expression levels were found in the fish fed diets with different vegetable oil (VO) sources, which may explore the potential function of fabp genes in S. aurata. For this purpose, the open reading frames of fabp genes involved in the transport and ß-oxidation of fatty acids (FA) were molecularly cloned and characterized. S. aurata was then exposed to a two-staged feeding trial (the grow-out period following a wash-out period) at low water temperatures. In the grow-out period, the fish were fed diets containing 50% and 100% ratios of various VOs for 60 days, and in the wash-out period, the fish were fed a diet containing 100% fish oil (FO) for 30 days. It has been determined that (a) S. aurata and vertebrate fabp/FABP genes are orthologues; (b) spatio-temporal differences in tissue-specific patterns of fabp genes differ importantly; for instance, the difference between the highest and lowest values reaches 13 × 10⁵ -fold in the fabp10a; and (c) VO-based diets upregulated fabp transcript levels in the liver and muscle with some exceptions, such as liver fabp11a and muscle fabp7a. Gene expressions of only the hepatic fabp7b and fabp10a genes were diminished at the end of the wash-out period. In this study, the authors provide further evidence that dietary FAs affect fabp mRNA expressions in S. aurata. This might be useful in the nutritional control of fabp genes to maintain lipid homeostasis in marine fish fed VO-based diets at low water temperatures.

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.

Very long chain fatty acids

Very long chain fatty acids (VLCFAs) are important components of various lipid classes in most organisms, from bacteria to higher plants and mammals, including humans. VLCFAs, or very long chain polyunsaturated fatty acids (VLCPUFAs), can be defined as fatty acids with 23 or more carbon atoms in the molecule. The main emphasis in this review is on the analysis of these acids, including obtaining standards from natural sources or their synthesis. Furthermore, the occurrence and analysis of these compounds in both lower (bacteria, invertebrates) and higher organisms (flowering plants or mammals) are discussed in detail. Attention is paid to their biosynthesis, especially the elongation of very long chain fatty acids protein (ELOVL4). This review deals with papers describing these very interesting compounds, whose chemical, biochemical and biological properties have not been fully explored.

Antagonistic effects and mechanisms of carbendazim and chlorpyrifos on the neurobehavior of larval zebrafish

Residues from multiple pesticides are frequently detected on vegetables, which may produce combined toxicity not predicted by individual toxicity data. As these combined effects present additional dangers to food safety, we have compared individual to combined effects for a variety of pesticides. Carbendazim and chlorpyrifos are the two most commonly detected pesticides in vegetables, and previous studies reported that combined exposure results in synergistic developmental toxicity to zebrafish embryos. In this study, individual and combined effects on zebrafish motor activity were examined following individual and combined exposure to assess nervous system toxicity. Further, transcriptomics methods were used to identify potential molecular mechanisms for individual and combined toxicity. Carbendazim alone induced a disorganized swim pattern characterized by increased angular velocity, turn angle, meander, and acceleration during light-dark transition, while chlorpyrifos alone reduced average swim speed and light-dark acceleration. Combined treatment significantly reduced average swim velocity and total distance traveled. Combination indices indicated strong antagonism between compounds for average speed and light-dark acceleration. Transcriptomics (RNA-seq) showed that carbendazim significantly altered the expression of genes involved in antigen processing and presentation, apoptosis, autophagy, and metabolism, including ctslb, cyp7a1, hsp70l, and ugt1a1. Alternatively, chlorpyrifos significantly altered genes involved in various nervous system-related pathways, including glutamatergic, GABAergic, dopaminergic, and calcium signaling. Protein-protein interaction (PPI) network analysis suggested that chlorpyrifos significantly downregulated genes related to light transduction, resulting in decreased sensitivity to light-dark transitions, while antagonism mainly reflected divergent effects on phototransduction and retinol metabolism. Carbendazim had no significant effects on vision-related genes such as gnat1 and gngt1, while chlorpyrifos downregulated expression, an effect reversed by the combination. Comprehensive toxicity analyses must include joint effects of co-applied pesticides for enhanced food safety.

Influence of Dietary Lipids and Environmental Salinity on the n-3 Long-Chain Polyunsaturated Fatty Acids Biosynthesis Capacity of the Marine Teleost Solea senegalensis

Fish vary in their ability to biosynthesise long-chain polyunsaturated fatty acids (LC-PUFA) depending upon the complement and function of key enzymes commonly known as fatty acyl desaturases and elongases. It has been reported in Solea senegalensis the existence of a Δ4 desaturase, enabling the biosynthesis of docosahexaenoic acid (DHA) from eicosapentaenoic acid (EPA), which can be modulated by the diet. The present study aims to evaluate the combined effects of the partial replacement of fish oil (FO) with vegetable oils and reduced environmental salinity in the fatty acid composition of relevant body compartments (muscle, hepatocytes and enterocytes), the enzymatic activity over α-linolenic acid (ALA) to form n-3 LC-PUFA through the incubation of isolated hepatocytes and enterocytes with [1-¹⁴C] 18:3 n-3, and the regulation of the S. senegalensis fads2 and elovl5 in the liver and intestine. The presence of radiolabelled products, including 18:4n-3, 20:4n-3 and EPA, provided compelling evidence that a complete pathway enabling the biosynthesis of EPA from ALA, establishing S. senegalensis, has at least one Fads2 with ∆6 activity. Dietary composition prevailed over salinity in regulating the expression of fads2, while salinity did so over dietary composition for elovl5. FO replacement enhanced the proportion of DHA in S. senegalensis muscle and the combination with 20 ppt salinity increased the amount of n-3 LC-PUFA in hepatocytes.

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.

Identification of new, very long-chain polyunsaturated fatty acids in fish by gas chromatography coupled to quadrupole/time-of-flight mass spectrometry with atmospheric pressure chemical ionization

The characterization of very long-chain (>C24) polyunsaturated fatty acids (VLC-PUFAs), which are essential in the vision, neural function, and reproduction of vertebrates, is challenging because of the lack of reference standards and their very low concentrations in certain lipid classes. In this research, we have developed a new methodology for VLC-PUFA identification based on gas chromatography coupled to quadrupole/time-of-flight mass spectrometry with an atmospheric pressure chemical ionization source (GC-APCI-QTOF MS). The mass accuracy attainable with the innovative QTOF instrument, together with the soft ionization of the APCI source, provides valuable information on the intact molecule, traditionally lost with electron ionization sources due to the extensive fragmentation suffered. We have identified, for the first time, VLC-PUFAs with chains up to 44 carbons in eyes, brain, and gonads of gilthead sea bream, a commercially important fish in the Mediterranean. The added value of ion mobility-mass spectrometry (IMS), recently developed in combination with GC-QTOF MS, and the contribution of the collisional cross section (CCS) parameter in the characterization of novel VLC-PUFAs (for which reference standards are not available) have been also evaluated. The methodology developed has allowed assessing qualitative differences between farmed and wild fish, and opens new perspectives in a still scarcely known field of research.