De novo lipogenesis in Atlantic salmon adipocytes

A fish intestinal in vitro model for investigation of lipid metabolism and steatosis

Choline is now recognized as an essential nutrient to ensure lipid transport in Atlantic salmon. Its deficiency leads to excessive lipid accumulation in the enterocytes, a condition known as steatosis. The knowledge of lipid metabolism and steatosis in fish remains limited, motivating the use of in vitro intestinal models to perform deeper explorations. This study aimed to create an in vitro steatosis model using RTdi-MI, a new cell line derived from the distal intestine of rainbow trout. Cells were exposed to varying oleic acid (OA) concentrations over different time points (24 h, 72 h, and 168 h). Results indicated that the increasing OA concentration enhanced intracellular lipid droplet formation. Quantitative lipid analysis confirmed OA accumulation, which intensified with prolonged exposure and increased OA dose. Moreover, all cells, including controls, exhibited fatty acid metabolic activity. Such outcome was confirmed by light and fluorescence microscopy. Additionally, RTdi-MI cells expressed genes involved in lipid metabolism and synthesis similar to in vivo conditions. Collectively, our findings demonstrate the ability of RTdi-MI cells to accumulate OA in intracellular lipid droplets and mirror in vivo steatosis conditions, offering a new tool for exploring fish intestinal lipid metabolism.

Advances and Challenges in Cell Biology for Cultured Meat

Cultured meat is an emerging biotechnology that aims to produce meat from animal cell culture, rather than from the raising and slaughtering of livestock, on environmental and animal welfare grounds. The detailed understanding and accurate manipulation of cell biology are critical to the design of cultured meat bioprocesses. Recent years have seen significant interest in this field, with numerous scientific and commercial breakthroughs. Nevertheless, these technologies remain at a nascent stage, and myriad challenges remain, spanning the entire bioprocess. From a cell biological perspective, these include the identification of suitable starting cell types, tuning of proliferation and differentiation conditions, and optimization of cell-biomaterial interactions to create nutritious, enticing foods. Here, we discuss the key advances and outstanding challenges in cultured meat, with a particular focus on cell biology, and argue that solving the remaining bottlenecks in a cost-effective, scalable fashion will require coordinated, concerted scientific efforts. Success will also require solutions to nonscientific challenges, including regulatory approval, consumer acceptance, and market feasibility. However, if these can be overcome, cultured meat technologies can revolutionize our approach to food.

Molecular programming of the hepatic lipid metabolism via a parental high carbohydrate and low protein diet in rainbow trout

It is now recognised that parental diets could alter their offspring metabolism, concept known as nutritional programming. For agronomic purposes, it has been previously proposed that programming could be employed as a strategy to prepare individual for future nutritional challenges. Concerning cultured fish that belong to high trophic level, plant-derived carbohydrates are a possible substitute for the traditional protein-rich fishmeal in broodstock diet, lowering thus the dietary protein-to-carbohydrate ratio (HC/LP nutrition). However, in mammals, numerous studies have previously demonstrated that parental HC/LP nutrition negatively affects their offspring in the long term. Therefore, the question of possible adaptation to plant-based diets, via parental nutrition, should be explored. First, the maternal HC/LP nutrition induced a global DNA hypomethylation in the liver of their offspring. Interestingly at the gene expression level, the effects brought by the maternal and paternal HC/LP nutrition cumulated in the liver, as indicated by the altered transcriptome. The paternal HC/LP nutrition significantly enhanced cholesterol synthesis at the transcriptomic level. Furthermore, hepatic genes involved in long-chain polyunsaturated fatty acids were significantly increased by the parental HC/LP nutrition, affecting thus both hepatic and muscle fatty acid profiles. Overall, the present study demonstrated that lipid metabolism could be modulated via a parental nutrition in rainbow trout, and that such modulations have consequences on their progeny phenotypes.

Reassessment of adipocyte technology for cellular agriculture of alternative fat

Alternative proteins, such as cultivated meat, have recently attracted significant attention as novel and sustainable food. Fat tissue/cell is an important component of meat that makes organoleptic and nutritional contributions. Although adipocyte biology is relatively well investigated, there is limited focus on the specific techniques and strategies to produce cultivated fat from agricultural animals. In the assumed standard workflow, stem/progenitor cell lines are derived from tissues of animals, cultured for expansion, and differentiated into mature adipocytes. Here, we compile information from literature related to cell isolation, growth, differentiation, and analysis from bovine, porcine, chicken, other livestock, and seafood species. A diverse range of tissue sources, cell isolation methods, cell types, growth media, differentiation cocktails, and analytical methods for measuring adipogenic levels were used across species. Based on our analysis, we identify opportunities and challenges in advancing new technology era toward producing "alternative fat" that is suitable for human consumption.

Gilthead seabream (Sparus aurata) in vitro adipogenesis and its endocrine regulation by leptin, ghrelin, and insulin

Leptin, ghrelin, and insulin influence lipid metabolism and thus can directly affect adipose tissue characteristics, modulating the organoleptic quality of aquaculture fish. The present study explored gilthead seabream (Sparus aurata) cultured preadipocytes development, and the regulation of adipogenesis by those three hormones. Preadipocytes presented a fibroblast-like phenotype during the proliferation phase that changed to round-shaped with an enlarged cytoplasm filled with lipid droplets after complete differentiation, confirming the characteristics of mature adipocytes. peroxisome proliferator-activated receptor-γ (pparγ) expression was higher at the beginning of the culture, while fatty acid synthase and 3-hydroxyacyl-CoA dehydrogenase gradually increased with cell maturation. The expression of lipoprotein lipase-like, lysosomal acid lipase (lipa), fatty acid translocase/cluster of differentiation-36 (cd36), and leptin receptor (lepr) were not affected during cell culture development; and undetectable expression levels were observed for leptin. Concerning regulation, leptin inhibited lipid accumulation significantly reducing pparγ and cd36 gene expression, both in early differentiating and mature adipocytes, while ghrelin decreased the expression of pparγ in the early differentiating phase but did not reduce intracellular lipid content significantly. Additional insulin past the onset of adipogenesis did not affect lipid accumulation either. In conclusion, at present culture conditions leptin has an anti-adipogenic function in differentiating preadipocytes of gilthead seabream and continues exerting this role in mature adipocytes, while ghrelin and insulin do not seem to influence adipogenesis progression. A better understanding of leptin, ghrelin, and insulin impact on the adipogenic process could help in the prevention of fat accumulation, improving aquaculture fish production and quality.

Interaction between ω6 and ω3 fatty acids of different chain lengths regulates Atlantic salmon hepatic gene expression and muscle fatty acid profiles

Atlantic salmon smolts (approx. 20-months old) were fed experimental diets with different combinations of omega-6:omega-3 fatty acids (FAs) (high-ω6, high-ω3, or balanced) and eicosapentaenoic acid plus docosahexaenoic acid (EPA + DHA) levels (0.3, 1.0 or 1.4%) for 12 weeks. Muscle FA (% total FA) reflected dietary C₁₈-polyunsaturated FA; however, muscle EPA per cent and content (mg g^-1) were not different in salmon fed high-ω3 or balanced diets. Muscle DHA per cent was similar among treatments, while DHA content increased in fish fed 1.4% EPA + DHA, compared with those fed 0.3-1.0% EPA + DHA combined with high-ω6 FA. Muscle 20:3ω6 (DGLA) content was highest in those fed high-ω6 with 0.3% EPA + DHA. Quantitative polymerase chain reaction analyses on liver RNA showed that the monounsaturated FA synthesis-related gene, scdb, was upregulated in fish fed 1.0% EPA + DHA with high-ω6 compared to those fed 0.3% EPA + DHA. In high-ω3-fed salmon, liver elovl2 transcript levels were higher with 0.3% EPA + DHA than with 1.0% EPA + DHA. In high-ω6-fed fish, elovl2 did not vary with EPA + DHA levels, but it was positively correlated with muscle ARA, 22:4ω3 and DGLA. These results suggest dietary 18:3ω3 elongation contributed to maintaining muscle EPA + DHA levels despite a two- to threefold change in dietary proportions, while 18:2ω6 with 0.3% EPA + DHA increased muscle DGLA more than arachidonic acid (ARA). Positive correlations between hepatic elovl2 and fabp10a with muscle ω6:ω3 and EPA + DHA + ARA, respectively, were confirmed by reanalysing data from a previous salmon trial with lower variations in dietary EPA + DHA and ω6:ω3 ratios. This article is part of the theme issue 'The next horizons for lipids as 'trophic biomarkers': evidence and significance of consumer modification of dietary fatty acids'.

DHA Modulates Immune Response and Mitochondrial Function of Atlantic Salmon Adipocytes after LPS Treatment

Adipocytes play a central role in overall energy homeostasis and are important contributors to the immune system. Fatty acids (FAs) act as signaling molecules capable to modulate adipocyte metabolism and functions. To identify the effects of two commonly used FAs in Atlantic salmon diets, primary adipocytes were cultured in the presence of oleic (OA) or docosahexaenoic (DHA) acid. DHA decreased adipocyte lipid droplet number and area compared to OA. The increase in lipid load in OA treated adipocytes was paralleled by an increase in iNOS activity and mitochondrial SOD2-GFP activity, which was probably directed to counteract increase in oxidative stress. Under lipopolysaccharide (LPS)-induced inflammation, DHA had a greater anti-inflammatory effect than OA, as evidenced by the higher SOD2 activity and the transcriptional regulation of antioxidant enzymes and pro- and anti-inflammatory markers. In addition, DHA maintained a healthy mitochondrial structure under induced inflammation while OA led to elongated mitochondria with a thin thread like structures in adipocytes exposed to LPS. Overall, DHA possess anti-inflammatory properties and protects Atlantic salmon against oxidative stress and limits lipid deposition. Furthermore, DHA plays a key role in protecting mitochondria shape and function.

Exploring the Impact of a Low-Protein High-Carbohydrate Diet in Mature Broodstock of a Glucose-Intolerant Teleost, the Rainbow Trout

Sustainable aquaculture production requires a greater reduction in the use of marine-derived ingredients, and one of the most promising solutions today is the augmentation in the proportion of digestible carbohydrates in aquafeed. This challenge is particularly difficult for high trophic level teleost fish as they are considered to be glucose-intolerant (growth delay and persistent postprandial hyperglycemia observed in juveniles fed a diet containing more than 20% of carbohydrates). It was previously suggested that broodstock could potentially use carbohydrates more efficiently than juveniles, probably due to important metabolic changes that occur during gametogenesis. To investigate this hypothesis, 2-year old male and female rainbow trout (Oncorhynchus mykiss) were either fed a diet containing no carbohydrates (NC) or a 35%-carbohydrate diet (HC) for an entire reproductive cycle. Zootechnical parameters as well as the activities of enzymes involved in carbohydrate metabolism were measured in livers and gonads. Fish were then reproduced to investigate the effects of such a diet on reproductive performance. Broodstock consumed the HC diet, and in contrast to what is commonly observed in juveniles, they were able to grow normally and they did not display postprandial hyperglycemia. The modulation of their hepatic metabolism, with an augmentation of the glycogenesis, the pentose phosphate pathway and a possible better regulation of gluconeogenesis, may explain their improved ability to use dietary carbohydrates. Although the HC diet did induce precocious maturation, the reproductive performance of fish was not affected, confirming that broodstock are able to reproduce when fed a low-protein high-carbohydrate diet. In conclusion, this exploratory work has shown that broodstock are able to use a diet containing digestible carbohydrates as high as 35% and can then grow and reproduce normally over an entire reproductive cycle for females and at least at the beginning of the cycle for males. These results are highly promising and suggest that dietary carbohydrates can at least partially replace proteins in broodstock aquafeed.

Impact of dietary starch on extrahepatic tissue lipid metabolism in farmed European (Dicentrarchus labrax) and Asian seabass (Lates calcarifer)

In aquaculture, there is high interest in substituting marine-derived with vegetable-based ingredients as energy source. Farmed carnivorous fish under high carbohydrate diets tend to increase adiposity but it remains unclear if this happens by increased lipid retention/accumulation, promotion of lipogenic pathways, or both. In order to determine the response of extrahepatic tissue to dietary starch, European (Dicentrarchus labrax) and Asian (Lates calcarifer) seabass were fed a control (low starch; LS) or experimental (high starch; HS) diet, for at least 21 days and then transferred for 6 days to saltwater enriched with deuterated water ²H₂O. Incorporation of ²H-labelling follows well-defined metabolic steps, and analysis of triacylglycerols (TAG) ²H-enrichment by ²HNMR allowed evaluation of de novo lipogenesis (DNL) in muscle and visceral adipose tissue (VAT). Fractional synthetic rates for TAG-bound fatty acids and glycerol were quantified separately providing a detailed lipogenic profile. The FA profile differed substantially between muscle and VAT in both species, but their lipogenic fluxes revealed even greater differences. In European seabass, HS promoted DNL of TAG-bound FA, in muscle and VAT. High ²H-enrichment also found in muscle TAG-bound glycerol was indicative of its role on lipid cycling. In Asian seabass, HS had no effect on muscle FA composition and lipogenic flux, with no ²H-enriched TAG being detected. VAT on the other hand revealed a strong enhancement of DNL in HS-fed fish along with high TAG-bound glycerol cycling. This study consolidated the use of ²H₂O as tracer for fish lipid metabolism in different tissues, under different dietary conditions and suitable to use in different fish models.

Effects of glucose administration on glucose and lipid metabolism in two strains of gibel carp (Carassius gibelio)

We compared the glucose clearance ability of gibel carp CAS III (A strain) with gibel carp Dongting (DT strain). A previous study suggested that these two strains responded to insulin differently. As insulin plays an important role in glucose utilization, we hypothesized that the ability to eliminate excess glucose after a glucose load would differ between A strain and DT strain. To test this hypothesis, fasted specimens of both strains of gibel carp were injected with glucose. As expected, glucose induced hyperglycemia in both A strain and DT strain. In both strains, mRNA levels of the glycolytic enzyme 6-phosphofructokinase (6PFK) increased in the white skeletal muscle 8 h post-injection, while expression levels of glucose-6-phosphatase (G6Pase), fructose 1,6-bisphosphatase (FBPase), and phosphoenolpyruvate carboxykinase (PEPCK) decreased in the liver 8 h post-injection. In the DT strain, both GLUT4 expression and muscular glycolytic processes increased, as reflected by elevated hexokinase 2 (HK2) and pyruvate kinase (PK) mRNA expression levels. The DT strain also returned to basal glycemia more quickly than the A strain (within 6 h versus more than 12 h). The glycogen concentration in the liver of the DT strain was higher than that of the A strain, indicating that the DT strain was better able to store glucose as glycogen than the A strain. Overall, the DT strain was better able to clear excess blood glucose after the glucose tolerance test than the A strain.

Adipogenesis in fish

White adipose tissue (AT) is the main lipid storage depot in vertebrates. Initially considered to be a simple lipid store, AT has recently been recognized as playing a role as an endocrine organ that is implicated in processes such as energy homeostasis and as a rich source of stem cells. Interest in adipogenesis has increased not only because of the prevalence of obesity, metabolic syndrome and type 2 diabetes in humans, but also in aquaculture because of the excessive fat deposition experienced in some cultured fish species, which may compromise both their welfare and their final product quality. Adipocyte development is well conserved among vertebrates, and this conservation has facilitated the rapid characterization of several adipogenesis models in fish. This Review presents the main findings of adipogenesis research based in primary cultures of the preadipocytes of farmed fish species. Zebrafish has emerged as an excellent model for studying the early stages of adipocyte fish development in vivo. Nevertheless, larger fish species are more suitable for the isolation of preadipocytes from visceral AT and for studies in which preadipocytes are differentiated in vitro to form mature adipocytes. Differentiated adipocytes contain lipid droplets and express adipocyte marker genes such as those encoding the peroxisome proliferator activated receptor γ (pparγ), CCAAT-enhancer-binding protein α (c/ebpα), lipoprotein lipase (lpl), fatty acid synthase (fas), fatty acid binding protein 11 (fabp11), fatty acid transporter protein1 (fatp1), adiponectin and leptin. Differentiated adipocytes also have elevated glycerol 3-phosphate (G3P) dehydrogenase (GPDH) activity. To better understand fish adipocyte development and regulation, different adipokines, fatty acids, growth factors and PPAR agonists have been studied, providing relevant insights into which factors affect these processes and counterbalance AT dysregulation.

Tributyltin and triphenyltin exposure promotes in vitro adipogenic differentiation but alters the adipocyte phenotype in rainbow trout

Numerous environmental pollutants have been identified as potential obesogenic compounds affecting endocrine signaling and lipid homeostasis. Among them, well-known organotins such as tributyltin (TBT) and triphenyltin (TPT), can be found in significant concentrations in aquatic environments. The aim of the present study was to investigate in vitro the effects of TBT and TPT on the development and lipid metabolism of rainbow trout (Onchorynchus mykiss) primary cultured adipocytes. Results showed that TBT and TPT induced lipid accumulation and slightly enhanced peroxisome proliferator-activated receptor gamma (PPARγ) and CCAAT enhancer binding protein alpha (C/EBPα) protein expression when compared to a control, both in the presence or absence of lipid mixture. However, the effects were higher when combined with lipid, and in the absence of it, the organotins did not cause complete mature adipocyte morphology. Regarding gene expression analyses, exposure to TBT and TPT caused an increase in fatty acid synthase (fasn) mRNA levels confirming the pro-adipogenic properties of these compounds. In addition, when added together with lipid, TBT and TPT significantly increased cebpa, tumor necrosis factor alpha (tnfa) and ATP-binding cassette transporter 1 (abca1) mRNA levels suggesting a synergistic effect. Overall, our data highlighted that TBT and TPT activate adipocyte differentiation in rainbow trout supporting an obesogenic role for these compounds, although by themselves they are not able to induce complete adipocyte development and maturation suggesting that these adipocytes might not be properly functional.

Caffeic acid and hydroxytyrosol have anti-obesogenic properties in zebrafish and rainbow trout models

Some natural products, known sources of bioactive compounds with a wide range of properties, may have therapeutic values in human health and diseases, as well as agronomic applications. The effect of three compounds of plant origin with well-known dietary antioxidant properties, astaxanthin (ATX), caffeic acid (CA) and hydroxytyrosol (HT), on zebrafish (Danio rerio) larval adiposity and rainbow trout (Onchorynchus mykiss) adipocytes was assessed. The zebrafish obesogenic test (ZOT) demonstrated the anti-obesogenic activity of CA and HT. These compounds were able to counteract the obesogenic effect produced by the peroxisome proliferator-activated receptor gamma (PPARγ) agonist, rosiglitazone (RGZ). CA and HT suppressed RGZ-increased PPARγ protein expression and lipid accumulation in primary-cultured rainbow trout adipocytes. HT also significantly reduced plasma triacylglycerol concentrations, as well as mRNA levels of the fasn adipogenic gene in the adipose tissue of HT-injected rainbow trout. In conclusion, in vitro and in vivo approaches demonstrated the anti-obesogenic potential of CA and HT on teleost fish models that may be relevant for studying their molecular mode of action. Further studies are required to evaluate the effect of these bioactive components as food supplements for modulating adiposity in farmed fish.

Different regulation of insulin on glucose and lipid metabolism in 2 strains of gibel carp

To test the hypothesis that response to insulin by regulating glucose and lipid metabolism in gibel carp A strain may be different from that in DT strain, bovine insulin was injected into both strains of gibel carp after previous fasting for 48h. The results showed that insulin induced hypoglycemia at 3h in 2 strains, and that this was coupled with increased expression of glucose transporters (GLUT2 in the liver and GLUT1, GLUT4 in the muscle) and glycolytic enzyme (HK2 in the muscle) in both strains. Insulin induced increased glycolysis (GK) and fatty acid oxidation (ACO3 in the liver and CPT1a, ACO3 in the muscle) in the DT strain. Conversely, very strong lipogenic capacity, as indicated by higher mRNA levels of transcription factor of fatty acid anabolism (SREBP1) and lipogenic enzymes (ACC, ACLY, and FAS) and decrease lipolytic capacity as indicated by lower mRNA levels of fatty acid oxidation enzymes in the liver (ACO3) and muscle (CPT1a and ACO3) detected in the A strain after insulin injection. Higher plasma insulin levels and decreased plasma free fatty acid levels were detected at 8h post insulin injection in A strain induced hypoglycemia. However, plasma glucose levels returned to baseline and no effect on fatty acid levels in the DT strain was observed in response to insulin treatment at the same point in time. These insulin-strain interactions demonstrated that insulin induced different changes in glucose and lipid metabolism in these 2 strains as expected.

In vivo imaging and quantification of regional adiposity in zebrafish

Adipose tissues (ATs) are lipid-rich structures that supply and sequester energy-dense lipid in response to the energy status of an organism. As such, ATs provide an organism energetic insurance during periods of adverse physiological burden. ATs are deposited in diverse anatomical locations, and excessive accumulation of particular regional ATs modulates disease risk. Therefore, a model system that facilitates the visualization and quantification of regional adiposity holds significant biomedical promise. The zebrafish (Danio rerio) has emerged as a new model system for AT research in which the entire complement of regional ATs can be imaged and quantified in live individuals. Here we present detailed methods for labeling adipocytes in live zebrafish using fluorescent lipophilic dyes, and for identifying and quantifying regional zebrafish ATs.

Adipogenic Gene Expression in Gilthead Sea Bream Mesenchymal Stem Cells from Different Origin

During the last decades, adipogenesis has become an emerging field of study in aquaculture due to the relevance of the adipose tissue in many physiological processes and its connection with the endocrine system. In this sense, recent studies have translated into the establishment of preadipocyte culture models from several fish species, sometimes lacking information on the mRNA levels of adipogenic genes. Thus, the aim of this study was to determine the gene expression profile of gilthead sea bream (Sparus aurata) primary cultured mesenchymal stem cells (MSCs) from different origin (adipose tissue and vertebra bone) during adipogenesis. Both cell types differentiated into adipocyte-like cells, accumulating lipids inside their cytoplasm. Adipocyte differentiation of MSCs from adipose tissue resulted in downregulation of several adipocyte-related genes (such as lpl, hsl, pparα, pparγ and gapdh2) at day 4, gapdh1 at day 8, and fas and pparβ at day 12. In contrast, differences in lxrα mRNA expression were not observed, while g6pdh levels increased during adipocyte maturation. Gapdh and Pparγ protein levels were also detected in preadipocyte cultures; however, only the former increased its expression during adipogenesis. Moreover, differentiation of bone-derived cells into adipocytes also resulted in the downregulation of several adipocyte gene markers, such as fas and g6pdh at day 10 and hsl, pparβ, and lxrα at day 15. On the other hand, the osteogenic genes fib1a, mgp, and op remained stable, but an increase in runx2 expression at day 20 was observed. In summary, the present study demonstrates that gilthead sea bream MSCs, from both adipose tissue and bone, differentiate into adipocyte-like cells, although revealed some kind of species- and cell lineage-specific regulation with regards to gene expression. Present data also provide novel insights into some of the potential key genes controlling adipogenesis in gilthead sea bream that can help to better understand the regulation of lipid storage in fish.