Dynamics of PPARs, fatty acid metabolism genes and lipid classes in eggs and early larvae of a teleost

The Peroxisome Proliferator-Activated Receptors of Ray-Finned Fish: Unique Structures, Elusive Functions

The Actinopterygian and specifically the Teleostean peroxisome proliferator-activated receptors (PPARs) present an impressive variability and complexity in their structures, both at the gene and protein levels. These structural differences may also reflect functional divergence from their mammalian homologs, or even between fish species. This review, taking advantage of the data generated from the whole-genome sequencing of several fish species, highlights the differences in the primary structure of the receptors, while discussing results from the literature pertaining to the functions of fish PPARs and their activation by natural and synthetic compounds.

Getting fat and stressed: Effects of dietary intake of titanium dioxide nanoparticles in the liver of turbot Scophthalmus maximus

The extensive use of nanomaterials, including titanium dioxide nanoparticles (TiO2 NPs), raises concerns about their persistence in ecosystems. Protecting aquatic ecosystems and ensuring healthy and safe aquaculture products requires the assessment of the potential impacts of NPs on organisms. Here, we study the effects of a sublethal concentration of citrate-coated TiO2 NPs of two different primary sizes over time in flatfish turbot, Scophthalmus maximus (Linnaeus, 1758). Bioaccumulation, histology and gene expression were assessed in the liver to address morphophysiological responses to citrate-coated TiO2 NPs. Our analyses demonstrated a variable abundance of lipid droplets (LDs) in hepatocytes dependent on TiO2 NPs size, an increase in turbot exposed to smaller TiO2 NPs and a depletion with larger TiO2 NPs. The expression patterns of genes related to oxidative and immune responses and lipid metabolism (nrf2, nfκb1, and cpt1a) were dependent on the presence of TiO2 NPs and time of exposure supporting the variance in hepatic LDs distribution over time with the different NPs. The citrate coating is proposed as the likely catalyst for such effects. Thus, our findings highlight the need to scrutinize the risks associated with exposure to NPs with distinct properties, such as primary size, coatings, and crystalline forms, in aquatic organisms.

Early Molecular Immune Responses of Turbot (Scophthalmus maximus L.) Following Infection with Aeromonas salmonicida subsp. salmonicida

Turbot aquaculture production is an important economic activity in several countries around the world; nonetheless, the incidence of diseases, such furunculosis, caused by the etiological agent A. salmonicida subsp. salmonicida, is responsible for important losses to this industry worldwide. Given this perspective, this study aimed to evaluate early immune responses in turbot (S. maximus L.) following infection with A. salmonicida subsp. salmonicida. For this, 72 fish were individually weighed and randomly distributed into 6 tanks in a circulating seawater system. For the bacterial challenge, half of the individuals (3 tanks with 36 individuals) were infected using a peritoneal injection with the bacterial suspension, while the other half of individuals were injected with PBS and kept as a control group. Several factors linked to the innate immune response were studied, including not only haematological (white blood cells, red blood cells, haematocrit, haemoglobin, mean corpuscular volume, mean cell haemoglobin, mean corpuscular haemoglobin concentration, neutrophils, monocytes, lymphocytes, thrombocytes) and oxidative stress parameters, but also the analyses of the expression of 13 key immune-related genes (tnf-α, il-1β, il-8, pparα-1, acox1, tgf-β1, nf-kB p65, srebp-1, il-10, c3, cpt1a, pcna, il-22). No significant differences were recorded in blood or innate humoral parameters (lysozyme, anti-protease, peroxidase) at the selected sampling points. There was neither any evidence of significant changes in the activity levels of the oxidative stress indicators (catalase, glutathione S-transferase, lipid peroxidation, superoxide dismutase). In contrast, pro-inflammatory (tnf-α, il-1β), anti-inflammatory (il-10), and innate immune-related genes (c3) were up-regulated, while another gene linked with the lipid metabolism (acox1) was down-regulated. The results showed new insights about early responses of turbot following infection with A. salmonicida subsp. salmonicida.

Genome-wide identification and expression of the peroxisome proliferator-activated receptor gene family in the Tibetan highland fish Gymnocypris przewalskii

Peroxisome proliferator-activated receptor (PPAR) plays an important role in the regulation of lipid metabolism and has been widely identified in diverse species. Gymnocypris przewalskii is a native fish of the Qinghai Tibetan Plateau that survives in a chronically cold environment. In the current study, we conducted genome-wide identification of PPAR genes, revealing the existence of seven PPARs in the G. przewalskii genome. Collinearity was observed between two copies of PPARαb and PPARγ in G. przewalskii, suggesting that the additional copy might be gained through whole genome duplication. Both phylogenetic and multiple sequence alignment analyses indicated that PPARs in G. przewalskii were conserved with teleosts. The cold treatment (10 °C and 4 °C) led to the developmental delay of G. przewalskii embryos. Continuous expression of PPARs was observed during the embryonic development of G. przewalskii under normal and cold conditions, with significantly different transcriptional patterns. These results indicated that PPARs participated in the embryonic development of G. przewalskii, and were involved in the cold response during development. The current study proposed a potential role of PPARs in the cold response in the embryonic development of G. przewalskii, which shed light on understanding cold adaptation in Tibetan highland fish.

Characterizing toxicity pathways of fluoxetine to predict adverse outcomes in adult fathead minnows (Pimephales promelas)

Current ecotoxicity testing programs are impeded as they predominantly rely on slow and expensive animal tests measuring adverse outcomes. Therefore, new approach methodologies (NAMs) increasingly involve short-term mechanistic assays that employ molecular endpoints to predict adverse outcomes of regulatory relevance. This study aimed to elucidate the application of NAMs in adult fathead minnows using fluoxetine (FLX) as a model compound. Fish were exposed to three FLX concentrations (measured: 2.42, 10.7, and 56.7 μgL^-1) and a control. After 96 h, molecular toxicity signatures were characterized using proteomics and transcriptomics analyses in livers and brains of a sub-set of fish. The remaining fish were sampled at 21 days and assessed for liver histopathology and morphometric measurements. Fecundity was monitored throughout the study. In the livers, 56.7 μgL^-1 FLX caused enrichment of PPAR signaling in the proteome and fatty acid-related pathways in the transcriptome, potential upstream responses that led to lipid-type vacuolation of hepatocytes, observed via histopathology. Upregulated genes in the brain suggested alterations in serotonin-related signaling processes and reproductive behaviour, which may explain the observed significant decrease in fecundity. While the relationships between molecular responses and adverse outcomes remain complex, this research provided important insights into the mechanistic toxicity of FLX.

Brains in Metamorphosis: Temporal Transcriptome Dynamics in Hatchery-Reared Flatfishes

Metamorphosis is a captivating process of change during which the morphology of the larva is completely reshaped to face the new challenges of adult life. In the case of fish, this process initiated in the brain has traditionally been considered to be a critical rearing point and despite the pioneering molecular work carried out in other flatfishes, the underlying molecular basis is still relatively poorly characterized. Turbot brain transcriptome of three developmental stages (pre-metamorphic, climax of metamorphosis and post-metamorphic) were analyzed to study the gene expression dynamics throughout the metamorphic process. A total of 1570 genes were differentially expressed in the three developmental stages and we found a specific pattern of gene expression at each stage. Unexpectedly, at the climax stage of metamorphosis, we found highly expressed genes related to the immune response, while the biological pathway enrichment analysis in pre-metamorphic and post-metamorphic were related to cell differentiation and oxygen carrier activity, respectively. In addition, our results confirm the importance of thyroid stimulating hormone, increasing its expression during metamorphosis. Based on our findings, we assume that immune system activation during the climax of metamorphosis stage could be related to processes of larval tissue inflammation, resorption and replacement, as occurs in other vertebrates.

Incorporation of dietary lipids and fatty acids into red drum Sciaenops ocellatus eggs

Embryonic and early larval development and metabolism are fueled entirely by maternally derived nutritional resources (yolk and oil) before the onset of exogenous feeding. Composition of these maternally derived nutrients depends partly on maternal diet. Diet-egg relationships for fatty acids are well described for some species, but little is known about lipid transfer to eggs. To examine the effects of maternal diet on the egg composition, we fed adult red drum Sciaenops ocellatus six different diets, and measured lipid class and fatty acid composition of eggs they produced. Egg lipid class profiles remained relatively stable with only subtle differences in the concentrations of several lipid classes. Neutral lipid classes (wax ester/steryl ester (WE/SE), triglyceride (TG), sterol) varied more than polar lipid classes, with egg TG content being directly related to TG content of maternal diets. Dietary variations rapidly affected fatty acid composition of all major lipid classes in eggs (TG, WE/SE, phosphatidylcholine), with greater effects on neutral lipids than on the polar lipid. Results suggest a degree of maternal control over the provisioning of lipids as structural components (phospholipids) and energy substrates (neutral lipids), which may ensure proper development of larvae. But, egg fatty acid composition within lipid classes is more variable, and this may have consequences for larval survival and performance. This study also suggests that the pathways of maternal-offspring nutrient transfer are likely different for neutral and polar lipids.

Sex steroid dynamics and mRNA transcript profiles of growth- and development-related genes during embryogenesis following induced follicular maturation in European eel

Hormones and mRNA transcripts of maternal origin deposited in the egg may affect early embryonic development in oviparous species. These hormones include steroids, such as estradiol-17β (E2), testosterone (T), 11-ketotestosterone (11-kt), 17α,20ß-dihydroxy-4-pregnen-3-one (DHP), and cortisol, which also play an important role in fish reproduction. In European eel, Anguilla anguilla, which does not reproduce naturally in captivity, vitellogenesis in female broodstock is commonly induced by administration of salmon or carp pituitary extract (PE) as an exogenous source of gonadotropins, while follicular maturation is stimulated by a priming dose of PE followed by provision of DHP as a maturation inducing hormone. In this regard, the main purpose of the present study was to evaluate effects of induced follicular maturation on reproductive success in European eel, focusing on maternal transfer and dynamics of steroids and mRNA transcripts of growth- and development-related genes throughout embryogenesis. The results showed that maternal blood plasma concentrations of E2, T and DHP were reflected in the unfertilized eggs. Moreover, a negative relationship between concentrations of E2 and DHP in eggs and embryos and quality parameters measured as fertilization success, cleavage abnormalities, embryonic survival, and hatch success was found. Concomitant mRNA transcript abundance analysis including genes involved in stress response (hsp70, hsp90), somatotropic axis (gh, igf1, igf2a, igf2b), lipid (cpt1a, cpt1b, pigf5) and thyroid metabolism (dio1, dio2, dio3, thrαb, thrβa, thrβb) varied among unfertilized egg batches. For the majority of genes, mRNA abundance increased during the maternal-to-zygotic transition in connection to activation of the transcription of the embryos own genome. mRNA abundance of dio1, cpt1a and cpt1b throughout embryogenesis was related to embryonic developmental competence. Notably, mRNA abundance of dio3 was positively associated with E2 concentrations, while the mRNA abundance of thrαb was negatively related to T concentrations in the unfertilized eggs, which may suggest an interaction between the thyroid and steroid hormone systems. Altogether, maternal plasma concentrations of E2 and DHP were reflected in the eggs, with high concentrations of these steroids in the eggs being negatively associated with embryonic developmental competence. Additionally, high transcript levels of two of the investigated genes (dio1, cpt1b) were positively associated with embryonic developmental competence. This study reveals maternal transfer of steroids and mRNA transcripts to the eggs, which may be significant contributors to the variability in embryonic survival observed in European eel captive reproduction.

Deciphering influences of testosterone and dihydrotestosterone on lipid metabolism genes using brown trout primary hepatocytes

Despite of physiological and toxicological relevance, the potential of androgens to influence fish lipid metabolism remains poorly explored. Here, brown trout primary hepatocytes were exposed to six concentrations (1 nM to 100 μM) of dihydrotestosterone (DHT) and testosterone (T), to assess changes in the mRNA levels of genes covering diverse lipid metabolic pathways. Acsl1, essential for fatty acid activation, was up-regulated by T and DHT, whereas the lipogenic enzymes FAS and ACC were up-regulated by the highest (100 μM) concentration of T and DHT, respectively. ApoA1, the major component of high-density lipoprotein (HDL), was down-regulated by both androgens. PPARγ, linked to adipogenesis and peroxisomal β-oxidation, was down-regulated by T and DHT, while Acox1-3I, rate-limiting in peroxisomal β-oxidation, was down-regulated by T. Fabp1, StAR and LPL were not altered. Our findings suggest that androgens may impact on lipid transport, adipogenesis and fatty acid β-oxidation and promote lipogenesis in fish liver.

Resveratrol Induces Expression of Metabolic and Antioxidant Machinery and Protects Tilapia under Cold Stress

Exposures to low ambient temperature require ectothermic fish to not only adjust their metabolic machinery but also to mount protective responses against oxidative stress. In this study, we tested whether diets supplemented with resveratrol (RSV), a naturally occurring polyphenol known to stimulate metabolic and protective responses in various animals, would be beneficial to tilapia (Oreochromis mossambicus) under hypothermic challenge. Feeding tilapia with RSV-supplemented diet promoted liver expression of sirtuins and their known targets, including metabolic/antioxidative enzymes. After exposure to 15 °C cold conditions for three days, the oxygen–nitrogen (O:N) ratio was decreased in the control-diet-fed tilapia but not in their RSV-fed counterparts. Moreover, at 27 °C, RSV-fed tilapia showed significantly higher prolonged swim speed compared with controls. RSV feeding produced no significant effect on upper and bottom layer preference between the control- and RSV-treated tilapia at either 27 °C or 15 °C. Together, these findings suggest that RSV stimulates beneficial metabolic/antioxidative adjustments in teleosts and may serve as a valuable feed supplement for tropical fish exposed to cold stress during winter.

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.

Hepatic transcriptome of the euryhaline teleost Japanese seabass (Lateolabrax japonicus) fed diets characterized by α-linolenic acid or linoleic acid

To investigate the different effects of dietary α-linolenic acid (ALA) and linoleic acid (LA) on the euryhaline fish Japanese seabass, a feeding trial followed by hepatic transcriptome assay was conducted. Two experimental diets containing 10% LA-rich sunflower seed oil (diet LA) or 10% ALA-rich perilla oil (diet ALA) were used in the feeding trial. LA and ALA in diets were characteristically incorporated into fish tissues while no significant difference was observed in growth performance and body proximate composition between groups LA and ALA. Compared to LA, ALA up-regulated transcription of 49 unigenes and down-regulated those of 311 unigenes. Quantitative RT-PCR studies on eight lipid metabolism-related genes and seven randomly selected genes were conducted to validate the transcriptomic results. Lipid metabolism-related genes ApoA1, ApoA4, ApoE, FABP1, FABP3, FABP4, FATP6, and DGAT1, as well as ribosomal proteins L9e, L13e, and S4e, were transcriptionally down-regulated by ALA. The differentially expressed genes (DEGs) were primarily enriched in Gene Ontology terms such as Lipid transport, Protein metabolic process, and Ribosome biogenesis, as well as in KEGG pathways such as Complement and coagulation cascades and Ribosome. The Protein-Protein Interaction (PPI) network based on the peptide biosynthesis-related DEGs showed that ribosomal proteins such as SAe, L4e, S4e, L15e, L9e, and L13Ae had high betweenness centrality in the dietary regulation of peptide biosynthetic processes. In conclusion, under the present experimental conditions, a high level of dietary α-linolenic acid tended to suppress lipid transport and protein biosynthetic processes in the liver of Japanese seabass at the gene expression level.

Description of amino acid and fatty acid content during initial development of Lophiosilurus alexandri (Siluriformes: Pseudopimelodidae), a carnivorous freshwater catfish

ABSTRACT Samples of eggs, newly hatched larvae (NHL), and larvae at the end of the lecithotrophic period (eight days after hatching) (LPL) of Lophiosilurus alexandri were collected to determine the amino acid and fatty acid profiles. Crude protein did not change throughout initial development and the concentration of lipids was highest in NHL. The content of the indispensable amino acids (IAA) isoleucine, leucine, and valine decreased in LPL, while in eggs and NHL they remained high and similar in value. The dispensable amino acids (DAA), such as aspartic acid, tyrosine, and glycine, increased in LPL, while alanine decreased. The percentage of neutral lipids increased in LPL. The saturated fatty acid content decreased during ontogeny, while monounsaturated fatty acids decreased only in LPL. The polyunsaturated fatty acid content was highest in LP. Polar fatty acids were found in higher percentages in eggs and NHL, but lower in LPL. Saturated fatty acid content decreased during ontogenetic development, while that of monounsaturated fatty acids decreased only in LPL. Polyunsaturated fatty acid content was highest in LPL. Protein content was maintained during ontogenetic development, while amino acid classes experienced changes. Lophiosilurus alexandri preferentially uses saturated and monosaturated fatty acids as an energy source during its early development.

The Mitotic and Metabolic Effects of Phosphatidic Acid in the Primary Muscle Cells of Turbot (Scophthalmus maximus)

Searching for nutraceuticals and understanding the underlying mechanism that promote fish growth is at high demand for aquaculture industry. In this study, the modulatory effects of soy phosphatidic acids (PA) on cell proliferation, nutrient sensing, and metabolic pathways were systematically examined in primary muscle cells of turbot (Scophthalmus maximus). PA was found to stimulate cell proliferation and promote G1/S phase transition through activation of target of rapamycin signaling pathway. The expression of myogenic regulatory factors, including myoD and follistatin, was upregulated, while that of myogenin and myostatin was downregulated by PA. Furthermore, PA increased intracellular free amino acid levels and enhanced protein synthesis, lipogenesis, and glycolysis, while suppressed amino acid degradation and lipolysis. PA also was found to increased cellular energy production through stimulated tricarboxylic acid cycle and oxidative phosphorylation. Our results identified PA as a potential nutraceutical that stimulates muscle cell proliferation and anabolism in fish.

Nutrient sensing and metabolic changes after methionine deprivation in primary muscle cells of turbot (Scophthalmus maximus L.)

The low methionine content in plant-based diets is a major limiting factor for feed utilization by animals. However, the molecular consequences triggered by methionine deficiency have not been well characterized, especially in fish species, whose metabolism is unique in many aspects and important for aquaculture industry. In the present study, the primary muscle cells of turbot (Scophthalmus maximus L.) were isolated and treated with or without methionine for 12 h in culture. The responses of nutrient sensing pathways, the proteomic profiling of metabolic processes, and the expressions of key metabolic molecules were systematically examined. Methionine deprivation (MD) suppressed target of rapamycin (TOR) signaling, activated AMP-activated protein kinase (AMPK) and amino acid response (AAR) pathways. Reduced cellular protein synthesis and increased protein degradation by MD led to increased intracellular free amino acid levels and degradations. MD also reduced glycolysis and lipogenesis while stimulated lipolysis, thus resulted in decreased intracellular lipid pool. MD significantly enhanced energy expenditure through stimulated tricarboxylic acid (TCA) cycle and oxidative phosphorylation. Collectively, our results identified a comprehensive set of transcriptional, proteomic, and signaling responses generated by MD and provided the molecular insight into the integration of cell homeostasis and metabolic controls in fish species.

Lipid metabolism-related gene expression pattern of Atlantic bluefin tuna (Thunnus thynnus L.) larvae fed on live prey

The present study is the first to evaluate lipid metabolism in first-feeding Atlantic bluefin tuna (ABT; Thunnus thynnus L.) larvae fed different live prey including enriched rotifers Brachionus plicatilis and Acartia sp. copepod nauplii from 2 days after hatch. Understanding the molecular basis of lipid metabolism and regulation in ABT will provide insights to optimize diet formulations for this high-value species new to aquaculture. To this end, we investigated the effect of dietary lipid on whole larvae lipid class and fatty acid compositions and the expression of key genes involved in lipid metabolism in first feeding ABT larvae fed different live prey. Additionally, the expression of lipid metabolism genes in tissues of adult broodstock ABT was evaluated. Growth and survival data indicated that copepods were the best live prey for first feeding ABT and that differences in growth performance and lipid metabolism observed between larvae from different year classes could be a consequence of broodstock nutrition. In addition, expression patterns of lipid metabolic genes observed in ABT larvae in the trials could reflect differences in lipid class and fatty acid compositions of the live prey. The lipid nutritional requirements, including essential fatty acid requirements of larval ABT during the early feeding stages, are unknown, and the present study represents a first step in addressing these highly relevant issues. However, further studies are required to determine nutritional requirements and understand lipid metabolism during development of ABT larvae and to apply the knowledge to the commercial culture of this iconic species.

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.

Chronic rapamycin treatment on the nutrient utilization and metabolism of juvenile turbot (Psetta maxima)

High dietary protein inclusion is necessary in fish feeds and also represents a major cost in the aquaculture industry, which demands improved dietary conversion into body proteins in fish. In mammals, the target of rapamycin (TOR) is a key nutritionally responsive molecule governing postprandial anabolism. However, its physiological significance in teleosts has not been fully examined. In the present study, we examined the nutritional physiology of turbot after chronic rapamycin inhibition. Our results showed that a 6-week inhibition of TOR using dietary rapamycin inclusion (30 mg/kg diet) reduced growth performance and feed utilization. The rapamycin treatment inhibited TOR signaling and reduced expression of key enzymes in glycolysis, lipogenesis, cholesterol biosynthesis, while increasing the expression of enzymes involved in gluconeogenesis. Furthermore, rapamycin treatment increased intestinal goblet cell number in turbot, while the expressions of Notch and Hes1 were down regulated. It was possible that stimulated goblet cell differentiation by rapamycin was mediated through Notch-Hes1 pathway. Therefore, our results demonstrate the important role of TOR signaling in fish nutritional physiology.

Postprandial nutrient-sensing and metabolic responses after partial dietary fishmeal replacement by soyabean meal in turbot (Scophthalmus maximus L.)

In this study, we chose a carnivorous fish, turbot (Scophthalmus maximus L.), to examine its nutrient-sensing and metabolic responses after ingestion of diets with fishmeal (FM), or 45% of FM replaced by soyabean meal (34·6% dry diet) balanced with or without essential amino acids (EAA) to match the amino acid profile of FM diet for 30 d. After a 1-month feeding trial, fish growth, feed efficiency and nutrient retention were markedly reduced by soyabean meal-incorporated (SMI) diets. Compared with the FM diet, SMI led to a reduction of postprandial influx of free amino acids, hypoactivated target of rapamycin signalling and a hyperactivated amino acid response pathway after refeeding, a status associated with reduced protein synthesis, impaired postprandial glycolysis and lipogenesis. These differential effects were not ameliorated by matching an EAA profile of soyabean meal to that of the FM diet through dietary amino acid supplementation. Therefore, this study demonstrated that the FM diet and SMI diets led to distinct nutrient-sensing responses, which in turn modulated metabolism and determined the utilisation efficiency of diets. Our results provide a new molecular explanation for the role of nutrient sensing in the inferior performance of aquafeeds in which FM is replaced by soyabean meal.

A comparative analysis of the response of the hepatic transcriptome to dietary docosahexaenoic acid in Atlantic salmon (Salmo salar) post-smolts

Background

The present study aimed to explore the impact of dietary docosahexaenoic acid (DHA) on aspects of the metabolism of Atlantic salmon (Salmo salar). The effects of diets containing increasing levels of DHA (1 g kg⁻¹, 3 g kg⁻¹, 6 g kg⁻¹, 10 g kg⁻¹ and 13 g kg⁻¹) on the liver transcriptome of post-smolt salmon was examined to elucidate patterns of gene expression and responses of specific metabolic pathways. Total RNA was isolated from the liver of individual fish and analyzed using a custom gene expression 44K feature Atlantic salmon oligo-microarray.

Results

The expression of up to 911 unique annotated genes was significantly affected by dietary DHA inclusion relative to a low DHA reference diet. Analysis of a total of 797 unique genes were found with a significant linear correlation between expression level and dietary DHA. Gene-Set Enrichment Analysis (GSEA) identified a range of pathways that were significantly affected by dietary DHA content.

Conclusions

Pathways that showed a significant response to dietary DHA level included those for long-chain polyunsaturated fatty acid biosynthesis, fatty acid elongation, steroid biosynthesis, glycan biosynthesis, protein export and protein processing in the endoplasmic reticulum. These findings suggest that in addition to clear roles in influencing lipid metabolic pathways, DHA might also have key functional roles in other pathways distinct from lipid metabolism.

Effects of the PPARα agonist WY-14,643 on plasma lipids, enzymatic activities and mRNA expression of lipid metabolism genes in a marine flatfish, Scophthalmus maximus

Fibrates and other lipid regulator drugs are widespread in the aquatic environment including estuaries and coastal zones, but little is known on their chronic effects on non-target organisms as marine fish. In the present study, turbot juveniles were exposed to the PPARα model agonist WY-14,643 for 21 days by repeated injections at the concentrations of 5mg/kg (lo-WY) and 50mg/kg (hi-WY), and samples taken after 7 and 21 days. Enzyme activity and mRNA expression of palmitoyl-CoA oxidase and catalase in the liver were analyzed as first response, which validated the experiment by demonstrating interactions with the peroxisomal fatty acid oxidation and oxidative stress pathways in the hi-WY treatment. In order to get mechanistic insights, alterations of plasma lipids (free cholesterol, FC; HDL associated cholesterol, C-HDL; triglycerides, TG; non-esterified fatty acids, NEFA) and hepatic mRNA expression of 17 genes involved in fatty acid and lipid metabolism were studied. The exposure to hi-WY reduced the quantity of plasma FC, C-HDL, and NEFA. Microsomal triglyceride transfer protein and apolipoprotein E mRNA expression were higher in hi-WY, and indicated an increased formation of VLDL particles and energy mobilization from liver. It is speculated that energy depletion by PPARα agonists may contribute to a higher susceptibility to environmental stressors.

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.

Daily rhythms in expression of genes of hepatic lipid metabolism in Atlantic salmon (Salmo salar L.)

In mammals, several genes involved in liver lipid and cholesterol homeostasis are rhythmically expressed with expression shown to be regulated by clock genes via Rev-erb 1α. In order to elucidate clock gene regulation of genes involved in lipid metabolism in Atlantic salmon (Salmo salar L.), the orphan nuclear receptor Rev-erb 1α was cloned and 24 h expression of clock genes, transcription factors and genes involved in cholesterol and lipid metabolism determined in liver of parr acclimated to a long-day photoperiod, which was previously shown to elicit rhythmic clock gene expression in the brain. Of the 31 genes analysed, significant daily expression was demonstrated in the clock gene Bmal1, transcription factor genes Srebp1, Lxr, Pparα and Pparγ, and several lipid metabolism genes Hmgcr, Ipi, ApoCII and El. The possible regulatory mechanisms and pathways, and the functional significance of these patterns of expression were discussed. Importantly and in contrast to mammals, Per1, Per2, Fas, Srebp2, Cyp71α and Rev-erb 1α did not display significant daily rhythmicity in salmon. The present study is the first report characterising 24 h profiles of gene expression in liver of Atlantic salmon. However, more importantly, the predominant role of lipids in the nutrition and metabolism of fish, and of feed efficiency in determining farming economics, means that daily rhythmicity in the regulation of lipid metabolism will be an area of considerable interest for future research in commercially important species.

Normalization strategies for gene expression studies by real-time PCR in a marine fish species, Scophthalmus maximus

Thorough evaluation of normalization approaches is a fundamental aspect in real-time quantitative RT-PCR experiments to avoid artificial introduced intergroup variations. In our study, we tested three normalization strategies in an experimental data set derived from a toxicological exposure of Scophthalmus maximus to the peroxisome proliferator-activated receptor alpha (PPARα) agonist WY-14643. Juvenile turbots were exposed by repeated injections to 5 mg or 50 mg WY-14643/kg, and liver samples were taken at day 1, 7 and 21. Specifically, the mRNA expression of peroxiredoxin 5 (prdx5) was normalized to the cDNA content, to the mRNA expression of single reference genes (b-actin, b-act; elongation factor 1 α, ef1a; glyceraldehyde-3-phosphate dehydrogenase, gapdh; ribosomal protein L8, rpl8; tata-box binding protein, tbp; tubulin beta 2C chain, tubb2c; ubiquitin-conjugating enzyme E2L 3, ub2l3) or to a combination of multiple reference genes using geNorm, BestKeeper or NormFinder algorithms. Four single reference genes (ef1a, rpl8, tubb2c, tbp) did not show any significant differences between the treatment groups over time, while significant intergroup variations were observed for cDNA content, gapdh, b-act and ub2l3. The normalization of prdx5 to the valid (not altered) single reference genes led to significant up-regulated (prdx5/rpl8), not-regulated (prdx5/ef1a; prdx5/tbp) or down-regulated (prdx5/tubb2c) mRNA expression pattern. The multiple reference gene approaches resulted in different rankings and combinations of the most stable expressed reference genes (geNorm: ef1a>rpl8>b-act; BestKeeper: ub2l3>gapdh>ef1a; NormFinder: b-act>ef1a). However, the normalization with the three multiple reference gene procedures demonstrated consistent expression pattern with a significant up-regulation of prdx5 in response to the higher concentration after 21 days. Concluding, even if not yet established as "gold" standard for expression profiling in environmental toxicology or physiology using freshwater or marine fish models, the multiple reference gene approach is recommended, since it eliminates any biased results, which represented the major flaw of single reference genes.