Distinct developmental expression of two elongase family members in zebrafish

Excess feeding increases adipogenesis but lowers leptin transcript abundance in zebrafish larvae

Although fish exposed to municipal wastewater effluents (MWWE) show higher lipid accumulation, whether this is due to adipogenesis is unclear. The objective here was to identify molecular markers of adipogenesis in zebrafish (Danio rerio) larvae for use as high throughput screening tools for environmental contaminants, including obesogens in MWWE. Zebrafish larvae were fed a commercial diet at a maintenance level (5 % body mass) or in excess (25 or 50 % body mass) from day 6 to 30 days post-fertilization (dpf) to stimulate adipogenesis. We monitored fat accumulation and markers of lipid metabolism, including peroxisome proliferator-activated receptor γ (ppar γ), fatty acid synthase (fas), ELOVL fatty acid elongase 2 (elovl2), diacylglycerol O-acyltransferase 2 (dgat2), leptin (lepa and lepb), leptin receptor (lepr), and lipoprotein lipase (lpl). Excess feeding led to a higher growth rate, protein content and an increase in igf1 transcript abundance. Also, these larvae had higher triglyceride levels and accumulated lipids droplets in the abdominal cavity and viscera. The molecular markers of adipogenesis, including fas, elovl2, and dgat2, were upregulated, while the transcript abundance of lpl, a lipolytic gene, was transiently lower due to excess feeding. The increased adiposity seen at 30 dpf due to excess feeding coincided with a lower lep but not lepr transcript abundance in zebrafish. Our results demonstrate that excess feeding alters the developmental programming of key genes involved in lipid homeostasis, leading to excess lipid accumulation in zebrafish larvae. Overall, fas, elovl2, lpl, and dgat2, but not lep or ppar γ, have the potential to be biomarkers of adipogenesis in zebrafish larvae.

Therapeutic Role of ELOVL in Neurological Diseases

Fatty acids play an important role in controlling the energy balance of mammals. De novo lipogenesis also generates a significant amount of lipids that are endogenously produced in addition to their ingestion. Fatty acid elongation beyond 16 carbons (palmitic acid), which can lead to the production of very long chain fatty acids (VLCFA), can be caused by the rate-limiting condensation process. Seven elongases, ELOVL1-7, have been identified in mammals and each has a unique substrate specificity. Researchers have recently developed a keen interest in the elongation of very long chain fatty acids protein 1 (ELOVL1) enzyme as a potential treatment for a variety of diseases. A number of neurological disorders directly or indirectly related to ELOVL1 involve the elongation of monounsaturated (C20:1 and C22:1) and saturated (C18:0-C26:0) acyl-CoAs. VLCFAs and ELOVL1 have a direct impact on the neurological disease. Other neurological symptoms such as ichthyotic keratoderma, spasticity, and hypomyelination have also been linked to the major enzyme (ELOVL1). Recently, ELOVL1 has also been heavily used to treat a number of diseases. The current review focuses on in-depth unique insights regarding the role of ELOVL1 as a therapeutic target and associated neurological disorders.

Lipid-related metabolism during zebrafish embryogenesis under unbalanced copper homeostasis

Copper (Cu) is an essential trace element, playing an important role in lipid metabolism, and its transporters ATP7A and ATP7B, as Cu-transporting P-type ATPases, are involved in maintaining the Cu homeostasis in cells. Numerous studies in mammals have shown that Cu homeostasis and lipid metabolism are closely related, but studies on the link between the effects of excess Cu, ATP7A, and ATP7B on lipid metabolism during vertebrate embryogenesis are scarce. In this study, zebrafish disease models with Cu overload and ATP7A and ATP7B inactivation, respectively, were used to study the lipid metabolism-related differentially expressed genes (DEGs) which were enriched in the models. The dynamic and spatiotemporal expressions of the DEGs in WTs, atp7a^-/-, and atp7b^-/- mutants with or without Cu stress were unveiled in this study and they mostly distributed in brain at 24 hpf then in liver and intestine at 96 hpf, suggesting their potential roles in lipid and glycogen metabolism to apply energy for normal development in zebrafish. Meanwhile, the correlation analysis for the DEGs among the three groups unveiled that most of the DEGs were involved in the glyceride metabolism pathway. This is the first report to establish the relationship between atp7a and atp7b with Cu-stimulated intestinal and liver lipid metabolism during fish embryogenesis, and this study will provide a theoretical basis for fish embryonic development and lipid metabolism disorders under unbalanced copper homeostasis.

Effects of different pharmaceutical residues on embryos of fish species native to Central Europe

Environmental concentrations of pharmacologically active substances are increasing dramatically throughout the world, to the point where they are now considered a serious threat to the aquatic environment. This high occurrence of pharmaceutical residues in the aquatic environment is due to an increase in i) the prescription and consumption of drugs, and ii) their subsequent discharge into wastewater and its imperfect purification in wastewater treatment plants. Recent surveys have clearly shown that such substances can have serious negative effects on non-target organisms. In the present study, we tested the effects of several commonly used pharmaceuticals, such as antidepressants, analgesics and antibiotics, on the embryonic stages of different fishes. Specifically, we applied concentration ranges of tramadol, enrofloxacin and nortriptylined on a common toxicological model organism, the zebrafish (Danio rerio), and other species native to Central European freshwaters, i.e. common carp (Cyprinus carpio), catfish (Silurus glanis) and tench (Tinca tinca). Our results show that, though malformation and negative impacts on hatching and mortality were only observed at the highest test concentrations, gene expression indicated that even low environmentally relevant concentrations (0.1 μg/L) can cause significant changes in early development of embryo.

Evolution and Functional Characteristics of the Novel elovl8 That Play Pivotal Roles in Fatty Acid Biosynthesis

Elongation of very long-chain fatty acid (Elovl) proteins are key enzymes that catalyze the rate-limiting step in the fatty acid elongation pathway. The most recently discovered member of the Elovl family, Elovl8, has been proposed to be a fish-specific elongase with two gene paralogs described in teleosts. However, the biological functions of Elovl8 are still to be elucidated. In this study, we showed that in contrast to previous findings, elovl8 is not unique to teleosts, but displays a rather unique and ample phylogenetic distribution. For functional determination, we generated elovl8a (elovl8a^−/−) and elovl8b (elovl8b^−/−) zebrafish using CRISPR/Cas9 technology. Fatty acid composition in vivo and zebrafish liver cell experiments suggest that the substrate preference of Elovl8 overlapped with other existing Elovl enzymes. Zebrafish Elovl8a could elongate the polyunsaturated fatty acids (PUFAs) C18:2n-6 and C18:3n-3 to C20:2n-6 and C20:3n-3, respectively. Along with PUFA, zebrafish Elovl8b also showed the capacity to elongate C18:0 and C20:1. Gene expression quantification suggests that Elovl8a and Elovl8b may play a potentially important role in fatty acid biosynthesis. Overall, our results provide novel insights into the function of Elovl8a and Elovl8b, representing additional fatty acid elongases not previously described in chordates.

Perinatal Dietary Polyunsaturated Fatty Acids in Brain Development, Role in Neurodevelopmental Disorders

n-3 and n-6 polyunsaturated fatty acids (PUFAs) are essential fatty acids that are provided by dietary intake. Growing evidence suggests that n-3 and n-6 PUFAs are paramount for brain functions. They constitute crucial elements of cellular membranes, especially in the brain. They are the precursors of several metabolites with different effects on inflammation and neuron outgrowth. Overall, long-chain PUFAs accumulate in the offspring brain during the embryonic and post-natal periods. In this review, we discuss how they accumulate in the developing brain, considering the maternal dietary supply, the polymorphisms of genes involved in their metabolism, and the differences linked to gender. We also report the mechanisms linking their bioavailability in the developing brain, their transfer from the mother to the embryo through the placenta, and their role in brain development. In addition, data on the potential role of altered bioavailability of long-chain n-3 PUFAs in the etiologies of neurodevelopmental diseases, such as autism, attention deficit and hyperactivity disorder, and schizophrenia, are reviewed.

Elovl2 Is Required for Robust Visual Function in Zebrafish

Omega-3 and omega-6 polyunsaturated fatty acids (PUFAs) play critical roles in membrane stability and cell signaling within the retina. ELOVL2 (Elongation of Very Long Chain Fatty Acids-Like 2), an elongase involved in the synthesis of long chain polyunsaturated fatty acids (LC-PUFAs), has recently been implicated in regulating aging in the mammalian retina. In this work, we characterize the expression and function of elovl2 in the retina development in embryonic zebrafish. Whole mount in situ hybridization shows elovl2 is expressed in the Muller glia in embryonic and adult zebrafish. Lipidomics analysis of elovl2 crispants whole embryos at day 2 and eyes at day 7 demonstrated significant changes in lipids composition, especially on the level of lipids containing docosahexaenoic acid (DHA). Histological analysis of zebrafish lacking elovl2 revealed increased retinal thickness compared to controls at day 7 without gross disruptions of the retinal architecture. Finally, elovl2 crispants showed differences in the visual motor reflex light off (VMR-OFF) at day 7 compared to controls. In sum, inactivation of elovl2 in zebrafish embryos caused changes in lipid composition and in visual behavior, further confirming the important role of LC-PUFAs in healthy vision.

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

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

Molecular cloning and 3D model of a fatty-acid elongase in a carnivorous freshwater teleost, the European perch (Perca fluviatilis)

The European perch (Perca fluviatilis) is a carnivorous freshwater fish able to metabolise polyunsaturated fatty acids (PUFA) into highly unsaturated fatty acids (HUFA). This makes it a potential candidate for sustainable aquaculture development. In this study, special attention is given to the fatty-acid elongase (ELOVL) family, one of the two enzymatic systems implied in the HUFA biosynthesis. Structural information on European perch enzyme converting PUFA into HUFA is obtained by both molecular cloning and in silico characterization of an ELOVL5-like elongase from P. fluviatilis (pfELOVL). The full-length cDNA sequence consists of a 885-base pair Open Reading Frame coding for a 294-amino acid protein. Phylogenetic analysis and sequence alignment with fish elongases predict the pfELOVL clusters within the ELOVL5 sub-group. The amino-acid sequence displays the typical ELOVL features: several transmembrane α helices (TMH), an endoplasmic reticulum (ER) retention signal, and four "conserved boxes" involved in the catalytic site. In addition, the topology analysis predicts a 7-TMH structure addressed in the ER membrane. A 3D model of the protein embedded in an ER-like membrane environment is also provided using de novo modelling and molecular dynamics. From docking studies, two putative enzyme-substrate-binding modes, including H bonds and CH-π interactions, emphasize the role of specific residues in the "conserved boxes".

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

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

Vitamin E inadequacy in humans: causes and consequences

It is estimated that >90% of Americans do not consume sufficient dietary vitamin E, as α-tocopherol, to meet estimated average requirements. What are the adverse consequences of inadequate dietary α-tocopherol intakes? This review discusses health aspects where inadequate vitamin E status is detrimental and additional vitamin E has reversed the symptoms. In general, plasma α-tocopherol concentrations <12 μmol/L are associated with increased infection, anemia, stunting of growth, and poor outcomes during pregnancy for both the infant and the mother. When low dietary amounts of α-tocopherol are consumed, tissue α-tocopherol needs exceed amounts available, leading to increased damage to target tissues. Seemingly, adequacy of human vitamin E status cannot be assessed from circulating α-tocopherol concentrations, but inadequacy can be determined from “low” values. Circulating α-tocopherol concentrations are very difficult to interpret because, as a person ages, plasma lipid concentrations also increase and these elevations in lipids increase the plasma carriers for α-tocopherol, leading to higher circulating α-tocopherol concentrations. However, abnormal lipoprotein metabolism does not necessarily increase α-tocopherol delivery to tissues. Additional biomarkers of inadequate vitamin E status are needed. Urinary excretion of the vitamin E metabolite α-carboxy-ethyl-hydroxychromanol may fulfill this biomarker role, but it has not been widely studied with regard to vitamin E status in humans or with regard to health benefits. This review evaluated the information available on the adverse consequences of inadequate α-tocopherol status and provides suggestions for avenues for research.

Interactions between α-tocopherol, polyunsaturated fatty acids, and lipoxygenases during embryogenesis

α-Tocopherol is a lipid-soluble antioxidant that is specifically required for reproduction and embryogenesis. However, since its discovery, α-tocopherol's specific biologic functions, other than as an antioxidant, and the mechanism(s) mediating its requirement for embryogenesis remain unknown. As an antioxidant, α-tocopherol protects polyunsaturated fatty acids (PUFAs) from lipid peroxidation. α-Tocopherol is probably required during embryonic development to protect PUFAs that are crucial to development, specifically arachidonic (ARA) and docosahexaenoic (DHA) acids. Additionally, ARA and DHA are metabolized to bioactive lipid mediators via lipoxygenase enzymes, and α-tocopherol may directly protect, or it may mediate the production and/or actions of, these lipid mediators. In this review, we discuss how α-tocopherol (1) prevents the nonspecific, radical-mediated peroxidation of PUFAs, (2) functions within a greater antioxidant network to modulate the production and/or function of lipid mediators derived from 12- and 12/15-lipoxygenases, and (3) modulates 5-lipoxygenase activity. The application and implication of such interactions are discussed in the context of α-tocopherol requirements during embryogenesis.

Dietary and ontogenic regulation of fatty acid desaturase and elongase expression in broiler chickens

Effects of diet and ontogeny on the expression of fatty acid desaturases and elongases were examined in broiler chickens. In Study 1, 120 day-old male chicks received one of six diets with LA:ALA ranging from 46:4 to 16:34, for 33 days. Total n-6 PUFA decreased, and n-3 PUFA increased in response to a decrease in the dietary LA:ALA. FADS1, FADS2, ELOVL2 and ELOVL5 mRNAs were highest (P<0.05) in birds fed lower LA:ALA diets. In Study 2, 60 day-old male chicks were fed a basal diet, and liver samples were collected on day of hatch, and on days 2, 7, 14, 21 and 35 post-hatch. Total n-6 and n-3 PUFA increased (P<0.01) from days 7 to days 21. FADS1, FADS2 and ELOVL2 mRNAs generally increased (P<0.01) with age. These findings provide evidence for the dietary and developmental regulation of PUFA metabolism in broiler chickens.

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

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