Cloning, functional characterization and nutritional regulation of Δ6 fatty acyl desaturase in the herbivorous euryhaline teleost Scatophagus argus

Effects of environmental salinity on the immune response of the coastal fish Scatophagus argus during bacterial infection

The coastal aquaculture is characterized with environmental salinity fluctuation, and the effects of salinity stress on the immunity of cultured fish are needed to be further explored. Scatophagus argus is an important species in the wild fisheries and aquaculture industry, it would be of great value to reveal the impact of salinity change on the immune response in this species. Understanding the effects of salinity stress on immune response can provide valuable insights into salinity management in the aquacultural process. The head kidney, which is an organ unique for teleost fish, functions not only as a central immune organ but also as a crucial role in the stress response during which the secretion of immunoregulatory molecules i.e. cytokines is facilitated. In the present study, Individuals of S. argus acclimated to 3 different salinities [0‰ (FW), 10‰ (BW), and 25‰ (SW)] were injected intraperitoneally with A. hydrophila, and then monitored throughout one week. The effects of environmental salinity on the immune response in S. argus stimulated by A. hydrophila infection were investigated. mRNA expression profiles of cytokine genes IL-1β, IL-6, IL-10 and TNF-α in different salinity groups was quite different. mRNA expression of cytokine genes in BW group and SW group rose more quickly and significantly higher than FW group (p < 0.05) at early stages (6-24 hpi) after bacterial injection, and before 96 hpi, the highest value of cytokine expression at each time point was recorded in SW group. Immune parameters such as lysozyme level, complement C3 activity and IgM content in BW and FW groups were lower than SW group at each time point from 24 to 144 hpi after bacterial injection. In addition, leukocyte profiles in the head kidney and blood were also investigated. Although hypoosmotic acclimation could temporarily stimulate monocyte and neutrophil proliferation, it was observed that the number of monocytes, neutrophils and lymphocytes of the head kidney and blood in SW group increased more quickly than BW and FW groups after bacterial infection. Our results indicate that hypoosmotic stress due to the decrease of environmental salinity has suppressive immunoregulatory effects on the immune response of S. argus.

Haematological and metabolic profiles associated with age and sex in giant kokopu (Galaxias argenteus) (Gmelin 1789) broodstock

This study characterized selected peripheral blood (PB) haematological parameters, liver, serum and muscle metabolic features in 3- and 5-year-old male and female giant kokopu (Galaxias argenteus) broodstock reared indoor at 16°C. Sex and age did not affect PB total cell count and haematocrit values. Nonetheless, higher erythrocytes in 5-year-old fish, elevated thrombocyte and lymphocyte counts in 3-year-old fish indicate age-specific cellular regulation. Higher thrombocyte counts in female fish suggest sex-specific regulation. At a metabolic level, liver abundance for long chain saturated fatty acids (FAs) was higher in males, whereas females had elevated levels of polyunsaturated FAs. Essential and non-essential amino acids (AAs) in liver and serum were also elevated in females compared to males. These findings suggest differential allocation of FAs and AAs to reflect requirements for gonadal, development and provisioning. Similarly, age significantly resulted in higher liver and serum abundances of some non-essential AAs in 3-year-olds compared to 5-year-old fish, suggesting higher metabolism in younger fish. Overall, results enhance our understanding of sex- and age-based differences in fish haematology, muscle, liver, and serum metabolite profiles in healthy G. argenteus. Future studies should carefully consider potential age- and sex-specific differences in metabolic responses.

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.

Elovl4a participates in LC-PUFA biosynthesis and is regulated by PPARαβ in golden pompano Trachinotus ovatus (Linnaeus 1758)

The elongases of very long-chain fatty acids (Elovls) are responsible for the rate-limiting elongation process in long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis. The transcription factor, PPARα, regulates lipid metabolism in mammals; however, the detailed mechanism whereby PPARαb regulates Elovls remains largely unknown in fish. In the present study, we report the full length cDNA sequence of Trachinotus ovatus Elovl4a (ToElovl4a), which encodes a 320 amino acid polypeptide that possesses five putative membrane-spanning domains, a conserved HXXHH histidine motif and an ER retrieval signal. Phylogenetic analysis revealed that the deduced protein of ToElovl4a is highly conserved with the Oreochromis niloticus corresponding homologue. Moreover, functional characterization by heterologous expression in yeast indicated that ToElovl4a can elongate C18 up to C20 polyunsaturated fatty acids. A nutritional study showed that the protein expressions of ToElovl4a in the brain and liver were not significantly affected among the different treatments. The region from PGL3-basic-Elovl4a-5 (−148 bp to +258 bp) is defined as the core promoter via a progressive deletion mutation of ToElovl4a. The results from promoter activity assays suggest that ToElovl4a transcription is positively regulated by PPARαb. Mutation analyses indicated that the M2 binding site of PPARαb is functionally important for protein binding, and transcriptional activity of the ToElovl4a promoter significantly decreased after targeted mutation. Furthermore, PPARαb RNA interference reduced ToPPARαb and ToElovl4a expression at the protein levels in a time-dependent manner. In summary, PPARαb may promote the biosynthesis of LC-PUFA by regulating ToElovl4a expression in fish.

The elongation of very long-chain fatty acid 6 gene product catalyses elongation of n-13 : 0 and n-15 : 0 odd-chain SFA in human cells

Normal odd-chain SFA (OCSFA), particularly tridecanoic acid (n-13 : 0), pentadecanoic acid (n-15 : 0) and heptadecanoic acid (n-17 : 0), are normal components of dairy products, beef and seafood. The ratio of n-15 : 0:n-17 : 0 in ruminant foods (dairy products and beef) is 2:1, while in seafood and human tissues it is 1:2, and their appearance in plasma is often used as a marker for ruminant fat intake. Human elongases encoded by elongation of very long-chain fatty acid (ELOVL)1, ELOVL3, ELOVL6 and ELOVL7 catalyse biosynthesis of the dominant even-chain SFA; however, there are no reports of elongase function on OCSFA. ELOVL transfected MCF7 cells were treated with n-13 : 0, n-15 : 0 or n-17 : 0 (80 µm) and products analysed. ELOVL6 catalysed elongation of n-13 : 0→n-15 : 0 and n-15 : 0→n-17 : 0; and ELOVL7 had modest activity toward n-15 : 0 (n-15 : 0→n-17 : 0). No elongation activity was detected for n-17 : 0→n-19 : 0. Our data expand ELOVL specificity to OCSFA, providing the first molecular evidence demonstrating ELOVL6 as the major elongase acting on OCSFA n-13 : 0 and n-15 : 0 fatty acids. Studies of food intake relying on OCSFA as a biomarker should consider endogenous human metabolism when relying on OCSFA ratios to indicate specific food intake.

Identification of Fatty Acid Desaturase 6 in Golden Pompano Trachinotus Ovatus (Linnaeus 1758) and Its Regulation by the PPARαb Transcription Factor

Fatty acid desaturases are rate-limiting enzymes in long-chain polyunsaturated fatty acid biosynthesis. The transcription factor peroxisome proliferator-activated receptor alpha b (PPARαb) regulates lipid metabolism in mammals, however, the mechanism whereby PPARαb regulates fatty acid desaturases is largely unknown in fish. In this study, we report the full length cDNA sequence of Trachinotus ovatus fatty acid desaturase, which encodes a 380 amino acid polypeptide, possessing three characteristic histidine domains. Phylogenetic and gene exon/intron structure analyses showed typical phylogeny: the T. ovatus fatty acid desaturase contained a highly conserved exon/intron architecture. Moreover, functional characterization by heterologous expression in yeast indicated that T. ovatus desaturase was a fatty acid desaturase, with Δ4/Δ5/Δ8 Fad activity. Promoter activity assays indicated that ToFads6 desaturase transcription was positively regulated by PPARαb. Similarly, PPARαb RNA interference decreased ToPPARαb and ToFads6 expression at the mRNA and protein levels in a time-dependent manner. Mutation analyses showed that the M2 binding site of PPARαb was functionally important for protein binding, and transcriptional activity of the ToFads6 promoter was significantly decreased after targeted mutation of M2. Electrophoretic mobile shift assays confirmed that PPARαb interacted with the binding site of the ToFads6 promoter region, to regulate ToFads6 transcription. In summary, PPARαb played a vital role in ToFads6 regulation and may promote the biosynthesis of long-chain polyunsaturated fatty acids by regulating ToFads6 expression.

Biosynthesis of Polyunsaturated Fatty Acids in the Razor Clam Sinonovacula constricta: Characterization of Δ5 and Δ6 Fatty Acid Desaturases

To investigate the endogenous long-chain polyunsaturated fatty acid (LC-PUFA) biosynthetic ability in Sinonovacula constricta, fatty acid desaturases (Fads) of this bivalve, namely, Scfad5a, Scfad5b, and Scfad6, were cloned and characterized in the current study. Meanwhile, the tissue distributions of S. constricta Fads and fatty acids (FAs) were examined. Heterologous expression in yeasts confirmed that Scfad5a and Scfad5b were both Δ5 Fads, while Scfad6 was a Δ6 Fad. However, compared with Fads in other organisms, the desaturation activities of S. constricta Fads were relatively low (especially for Scfad6), indicating an adaptation to living conditions. S. constricta Fads were expressed in all tissues examined, and particularly high expressions were found in intestine and gonad. Moreover, FAs were differently distributed among tissues, which might be correlated with their corresponding physiological roles. Taken together, the results provided an insight into LC-PUFA biosynthesis in S. constricta. Notably, Scfad6 was the first functionally characterized Δ6 Fad in marine molluscs to date.

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.

Dietary fish oil replacement by linseed oil: Effect on growth, nutrient utilization, tissue fatty acid composition and desaturase gene expression in silver barb (Puntius gonionotus) fingerlings

Silver barb (Puntius gonionotus) is considered a promising medium carp species for freshwater aquaculture in Asia. This study in silver barb was carried out to evaluate the effects of total or partial substitution of dietary fish oil (FO) with linseed oil (LO) on growth, nutrient utilization, whole-body composition, muscle and liver fatty acid composition. Fish (12.1±0.4g of initial body weight) were fed for 60days with five experimental iso-proteinous, iso-lipidic and iso-caloric diets in which FO (control diet) was replaced by 33.3%, 50%, 66.7% and 100% LO. Final weight, weight gain, percent weight gain, SGR decreased linearly (p<0.001) with increasing LO levels in the diets. Dietary LO substitution levels did not significantly (p>0.05) affect the feed conversion ratio (FCR), protein efficiency ratio (PER) and whole body proximate composition. Furthermore, enhanced level of LO increased α-linolenic acid (ALA; 18:3n3) and linoleic acid (LA; 18:2n6) and decreased eicosapentaenoic acid (EPA; 20:5n3) and docosahexaenoic acid (DHA; 22:6n3) in muscle and liver. To understand the molecular mechanism of long chain-polyunsaturated fatty acid (LC-PUFA) biosynthesis, we cloned and characterized the fatty acyl Δ6 desaturase (Δ6 fad) cDNA and investigated its expression in various organs/tissues following replacement of FO with LO in the diet. The full-length Δ6 fad cDNA was 2056bp encoding 444 amino acids and was widely expressed in various organs/tissues. Replacement of FO with LO increased the expression of Δ6 fad mRNA in liver, muscle and intestine but no significant difference was found in the brain.

Isolation and Functional Characterisation of a fads2 in Rainbow Trout (Oncorhynchus mykiss) with Δ5 Desaturase Activity

Rainbow trout, Oncorhynchus mykiss, are intensively cultured globally. Understanding their requirement for long-chain polyunsaturated fatty acids (LC-PUFA) and the biochemistry of the enzymes and biosynthetic pathways required for fatty acid synthesis is important and highly relevant in current aquaculture. Most gnathostome vertebrates have two fatty acid desaturase (fads) genes with known functions in LC-PUFA biosynthesis and termed fads1 and fads2. However, teleost fish have exclusively fads2 genes. In rainbow trout, a fads2 cDNA had been previously cloned and found to encode an enzyme with Δ6 desaturase activity. In the present study, a second fads2 cDNA was cloned from the liver of rainbow trout and termed fads2b. The full-length mRNA contained 1578 nucleotides with an open reading frame of 1365 nucleotides that encoded a 454 amino acid protein with a predicted molecular weight of 52.48 kDa. The predicted Fads2b protein had the characteristic traits of the microsomal Fads family, including an N-terminal cytochrome b5 domain containing the heme-binding motif (HPPG), histidine boxes (HDXGH, HFQHH and QIEHH) and three transmembrane regions. The fads2b was expressed predominantly in the brain, liver, intestine and pyloric caeca. Expression of the fasd2b in yeast generated a protein that was found to specifically convert eicosatetraenoic acid (20:4n-3) to eicosapentaenoic acid (20:5n-3), and therefore functioned as a Δ5 desaturase. Therefore, rainbow trout have two fads2 genes that encode proteins with Δ5 and Δ6 desaturase activities, respectively, which enable this species to perform all the desaturation steps required for the biosynthesis of LC-PUFA from C₁₈ precursors.

Dietary lipid levels impact lipoprotein lipase, hormone-sensitive lipase, and fatty acid synthetase gene expression in three tissues of adult GIFT strain of Nile tilapia, Oreochromis niloticus

The objective of this study was to assess the effects of dietary lipids on growth performance, body composition, serum parameters, and expression of genes involved in lipid metabolism in adult genetically improved farmed tilapia (GIFT strain) of Nile tilapia, Oreochromis niloticus. We randomly assigned adult male Nile tilapia (average initial body weight = 220.00 ± 9.54 g) into six groups consisting of four replicates (20 fish per replicate). Fish in each group were hand-fed a semi-purified diets containing different lipid levels [3.3 (the control group), 28.4, 51.4, 75.4, 101.9, and 124.1 g kg(-1)] for 8 weeks. The results indicated that there was no obvious effect in feeding rate among all groups (P > 0.05). The highest weight gain, specific growth rate, and protein efficiency ratio in 75.4 g kg(-1) diet group were increased by 23.31, 16.17, and 22.02 % than that of fish in the control group (P < 0.05). Protein retention ratio was highest in 51.4 g kg(-1) diet group. The results revealed that the optimum dietary lipid level for maximum growth performance is 76.6-87.9 g kg(-1). Increasing dietary lipid levels contributed to increased tissue and whole body lipid levels. Saturated and monounsaturated fatty acids (MUFAs) decreased, and polyunsaturated fatty acids increased with increasing dietary lipid levels. With the exception of MUFAs, the fatty acid profiles of liver and muscle were similar. Dietary lipid levels were negatively correlated with low-density lipoprotein- cholesterol content and positively with triacylglycerol and glucose contents. In the lipid-fed groups, there was a significant down-regulation of fatty acid synthase (FAS) mRNA in liver, muscle, and visceral adipose tissues. There was a rapid up-regulation of lipoprotein lipase (LPL) mRNA in muscle and liver with increasing dietary lipid levels. In visceral adipose tissue, LPL mRNA was significantly down-regulated in the lipid-fed groups. Dietary lipids increased hormone-sensitive lipase (HSL) mRNA expression levels in the three tissues. These results strongly suggested that moderate dietary lipid levels were beneficial for adult tilapia growth performance and feed efficiency. However, excessive dietary lipid levels contributed to lipid deposition. Additionally, excessive dietary lipids may induce a competition between lipolysis and lipogenesis. FAS did not have tissue-specific regulation; however, the regulation of dietary lipids on LPL expression is tissue specific. FAS was a negative feedback regulator on fat deposition, and HSL was an indicator of fat content in tilapia.

Diversification of substrate specificities in teleostei Fads2: characterization of Δ4 and Δ6Δ5 desaturases of Chirostoma estor

Currently existing data show that the capability for long-chain PUFA (LC-PUFA) biosynthesis in teleost fish is more diverse than in other vertebrates. Such diversity has been primarily linked to the subfunctionalization that teleostei fatty acyl desaturase (Fads)2 desaturases have undergone during evolution. We previously showed that Chirostoma estor, one of the few representatives of freshwater atherinopsids, had the ability for LC-PUFA biosynthesis from C18 PUFA precursors, in agreement with this species having unusually high contents of DHA. The particular ancestry and pattern of LC-PUFA biosynthesis activity of C. estor make this species an excellent model for study to gain further insight into LC-PUFA biosynthetic abilities among teleosts. The present study aimed to characterize cDNA sequences encoding fatty acyl elongases and desaturases, key genes involved in the LC-PUFA biosynthesis. Results show that C. estor expresses an elongase of very long-chain FA (Elovl)5 elongase and two Fads2 desaturases displaying Δ4 and Δ6/Δ5 specificities, thus allowing us to conclude that these three genes cover all the enzymatic abilities required for LC-PUFA biosynthesis from C18 PUFA. In addition, the specificities of the C. estor Fads2 enabled us to propose potential evolutionary patterns and mechanisms for subfunctionalization of Fads2 among fish lineages.