Contribution of dietary starch to hepatic and systemic carbohydrate fluxes in European seabass (Dicentrarchus labrax L.)

A mechanism for red coloration in vertebrates

Red coloration is a salient feature of the natural world. Many vertebrates produce red color by converting dietary yellow carotenoids into red ketocarotenoids via an unknown mechanism. Here, we show that two enzymes, cytochrome P450 2J19 (CYP2J19) and 3-hydroxybutyrate dehydrogenase 1-like (BDH1L), are sufficient to catalyze this conversion. In birds, both enzymes are expressed at the sites of ketocarotenoid biosynthesis (feather follicles and red cone photoreceptors), and genetic evidence implicates these enzymes in yellow/red color variation in feathers. In fish, the homologs of CYP2J19 and BDH1L are required for ketocarotenoid production, and we show that these enzymes are sufficient to produce ketocarotenoids in cell culture and when ectopically expressed in fish skin. Finally, we demonstrate that the red-cone-enriched tetratricopeptide repeat protein 39B (TTC39B) enhances ketocarotenoid production when co-expressed with CYP2J19 and BDH1L. The discovery of this mechanism of ketocarotenoid biosynthesis has major implications for understanding the evolution of color diversity in vertebrates.

Hepatic Glucose Metabolism and Its Disorders in Fish

Carbohydrate, which is the most abundant nutrient in plant-sourced feedstuffs, is an economically indispensable component in commercial compound feeds for fish. This nutrient can enhance the physical quality of diets and allow for pellet expansion during extrusion. There is compelling evidence that an excess dietary intake of starch causes hepatic disorders, thereby further reducing the overall food consumption and growth performance of fish species. Among the severe metabolic disturbances are glycogenic hepatopathy (hepatomegaly caused by the excessive accumulation of glycogen in hepatocytes) and hepatic steatosis (the accumulation of large vacuoles of triacylglycerols in hepatocytes). The development of those disorders is mainly due to the limited ability of fish to oxidize glucose and control blood glucose concentration. The prolonged elevations of blood glucose increase glucose intake by the liver, and excess glucose is stored either as glycogen through glycogenesis in hepatocytes or as triglycerides via lipogenesis in tissues, depending on the species. In some fish species (e.g., largemouth bass), the liver has a low ability to regulate glycolysis, gluconeogenesis, and glycogen breakdown in response to high starch intake. For most species of fish, the liver size increases with lipid or glycogen accumulation when they have a high starch intake. It is a challenge to develop the same set of diagnostic criteria for all fish species as their physiology or metabolic patterns differ. Although glycogenic hepatopathy appears to be a common disease in carnivorous fish, it has been under-recognized in many studies. As a result, understanding these diseases and their pathogeneses in different fish species is crucial for manufacturing cost-effective pellet diets to promote the health, growth, survival, and feed efficiency of fish in future.

Limitations to Starch Utilization in Barramundi (Lates calcarifer) as Revealed by NMR-Based Metabolomics

Practical diets for commercial barramundi production rarely contain greater than 10% starch, used mainly as a binding agent during extrusion. Alternative ingredients such as digestible starch have shown some capacity to spare dietary protein catabolism to generate glucose. In the present study, a carnivorous fish species, the Asian seabass (Lates calcarifer) was subjected to two diets with the same digestible energy: Protein (P) – with high protein content (no digestible starch); and Starch (S) – with high digestible (pregelatinized) starch content. The effects of a high starch content diet on hepatic glycogen synthesis as well as the muscle and liver metabolome were studied using a complementary approach of ¹H and ²H NMR. The hepatosomatic index was lower for fish fed high starch content diet while the concentration of hepatic glycogen was similar between groups. However, increased glycogen synthesis via the direct pathway was observed in the fish fed high starch content diet which is indicative of increased carbohydrate utilization. Multivariate analysis also showed differences between groups in the metabolome of both tissues. Univariate analysis revealed more variations in liver than in muscle of fish fed high starch content diet. Variations in metabolome were generally in agreement with the increase in the glycogen synthesis through direct pathway, however, this metabolic shift seemed to be insufficient to keep the growth rate as ensured by the diet with high protein content. Although liver glycogen does not make up a substantial quantity of total stored dietary energy in carnivorous fish, it is a key regulatory intermediate in dietary energy utilization.

Dietary starch promotes hepatic lipogenesis in barramundi (Lates calcarifer)

Barramundi (Lates calcarifer) are a highly valued aquaculture species, and, as obligate carnivores, they have a demonstrated preference for dietary protein over lipid or starch to fuel energetic growth demands. In order to investigate how carnivorous fish regulate nutritional cues, we examined the metabolic effects of feeding two isoenergetic diets that contained different proportions of digestible protein or starch energy. Fish fed a high proportion of dietary starch energy had a higher proportion of liver SFA, but showed no change in plasma glucose levels, and few changes in the expression of genes regulating key hepatic metabolic pathways. Decreased activation of the mammalian target of rapamycin growth signalling cascade was consistent with decreased growth performance values. The fractional synthetic rate (lipogenesis), measured by TAG 2H-enrichment using 2H NMR, was significantly higher in barramundi fed with the starch diet compared with the protein diet (0·6 (se 0·1) v. 0·4 (se 0·1) % per d, respectively). Hepatic TAG-bound glycerol synthetic rates were much higher than other closely related fish such as sea bass, but were not significantly different (starch, 2·8 (se 0·3) v. protein, 3·4 (se 0·3) % per d), highlighting the role of glycerol as a metabolic intermediary and high TAG-FA cycling in barramundi. Overall, dietary starch significantly increased hepatic TAG through increased lipogenesis. Compared with other fish, barramundi possess a unique mechanism to metabolise dietary carbohydrates and this knowledge may define ways to improve performance of advanced formulated feeds.

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.

Administration of chitosan-tripolyphosphate-DNA nanoparticles to knockdown glutamate dehydrogenase expression impairs transdeamination and gluconeogenesis in the liver

Glutamate dehydrogenase (GDH) plays a major role in amino acid catabolism. To increase the current knowledge of GDH function, we analysed the effect of GDH silencing on liver intermediary metabolism from gilthead sea bream (Sparus aurata). Sequencing of GDH cDNA from S. aurata revealed high homology with its vertebrate orthologues and allowed us to design short hairpin RNAs (shRNAs) to knockdown GDH expression. Following validation of shRNA-dependent downregulation of S. aurata GDH in vitro, chitosan-tripolyphosphate (TPP) nanoparticles complexed with a plasmid encoding a selected shRNA (pCpG-sh2GDH) were produced to address the effect of GDH silencing on S. aurata liver metabolism. Seventy-two hours following intraperitoneal administration of chitosan-TPP-pCpG-sh2GDH, GDH mRNA levels and immunodetectable protein decreased in the liver, leading to reduced GDH activity in both oxidative and reductive reactions to about 53-55 % of control values. GDH silencing decreased glutamate, glutamine and aspartate aminotransferase activity, while increased 2-oxoglutarate content, 2-oxoglutarate dehydrogenase activity and 6-phosphofructo-1-kinase/fructose-1,6-bisphosphatase activity ratio. Our findings show for the first time that GDH silencing reduces transdeamination and gluconeogenesis in the liver, hindering the use of amino acids as gluconeogenic substrates and enabling protein sparing and metabolisation of dietary carbohydrates, which would reduce environmental impact and production costs of aquaculture.

Response to dietary carbohydrates in European seabass (Dicentrarchus labrax) muscle tissue as revealed by NMR-based metabolomics

INTRODUCTION

Feed optimization is a key step to the environmental and economic sustainability of aquaculture, especially for carnivorous species. Plant-derived ingredients can contribute to reduce costs and nitrogenous effluents while sparing wild fish stocks. However, the metabolic use of carbohydrates from vegetable sources by carnivorous fish is still not completely understood.

OBJECTIVES

We aimed to study the effects of diets with carbohydrates of different digestibilities, gelatinized starch (DS) and raw starch (RS), in the muscle metabolome of European seabass (Dicentrarchus labrax).

METHODS

We followed an NMR-metabolomics approach, using two sample preparation procedures, the intact muscle (HRMAS) and the aqueous muscle extracts (¹H NMR), to compare the variations in muscle metabolome between the two diets.

RESULTS

In muscle, multivariate analysis revealed similar metabolome shifts for DS and RS diets, when compared with the control diet. HRMAS of intact muscle, which included both hydrophobic and hydrophilic metabolites, showed increased lipid in DS-fed fish by univariate analysis. Regardless of the nature of the starch, increased glycine and phenylalanine, and decreased proline were observed when compared to the Ctr diet. Combined univariate analysis of intact muscle and aqueous extracts indicated specific diet related changes in lipid and amino acid metabolism, consistent with increased dietary carbohydrate supplementation.

CONCLUSIONS

Due to differential sample processing, outputs differ in detail but provide complementary information. After tracing nutritional alterations by profiling fillet components, DS seems to be the most promising alternative to fishmeal-based diets in aquaculture. This approach should be reproducible for other farmed fish species and provide valuable information on nutritional and organoleptic properties of the final product.

Role of upstream stimulatory factor 2 in glutamate dehydrogenase gene transcription

Glutamate dehydrogenase (Gdh) plays a central role in ammonia detoxification by catalysing reversible oxidative deamination of l-glutamate into α-ketoglutarate using NAD⁺ or NADP⁺ as cofactor. To gain insight into transcriptional regulation of glud, the gene that codes for Gdh, we isolated and characterised the 5' flanking region of glud from gilthead sea bream (Sparus aurata). In addition, tissue distribution, the effect of starvation as well as short- and long-term refeeding on Gdh mRNA levels in the liver of S. aurata were also addressed. 5'-Deletion analysis of glud promoter in transiently transfected HepG2 cells, electrophoretic mobility shift assays, chromatin immunoprecipitation (ChIP) and site-directed mutagenesis allowed us to identify upstream stimulatory factor 2 (Usf2) as a novel factor involved in the transcriptional regulation of glud Analysis of tissue distribution of Gdh and Usf2 mRNA levels by reverse transcriptase-coupled quantitative real-time PCR (RT-qPCR) showed that Gdh is mainly expressed in the liver of S. aurata, while Usf2 displayed ubiquitous distribution. RT-qPCR and ChIP assays revealed that long-term starvation down-regulated the hepatic expression of Gdh and Usf2 to similar levels and reduced Usf2 binding to glud promoter, while refeeding resulted in a slow but gradual restoration of both Gdh and Usf2 mRNA abundance. Herein, we demonstrate that Usf2 transactivates S. aurata glud by binding to an E-box located in the proximal region of glud promoter. In addition, our findings provide evidence for a new regulatory mechanism involving Usf2 as a key factor in the nutritional regulation of glud transcription in the fish liver.

Disposition of a Glucose Load into Hepatic Glycogen by Direct and Indirect Pathways in Juvenile Seabass and Seabream

In carnivorous fish, conversion of a glucose load to hepatic glycogen is widely used to assess their metabolic flexibility towards carbohydrate utilization, but the activities of direct and indirect pathways in this setting are unclear. We assessed the conversion of an intraperitoneal glucose load (2 g.kg-1) enriched with [U-13C6]glucose to hepatic glycogen in juvenile seabass and seabream. 13C-NMR analysis of glycogen was used to determine the contribution of the load to glycogen synthesis via direct and indirect pathways at 48-hr post-injection. For seabass, [U-13C6]glucose was accompanied by deuterated water and 2H-NMR analysis of glycogen 2H-enrichment, allowing endogenous substrate contributions to be assessed as well. For fasted seabass and seabream, 47 ± 5% and 64 ± 10% of glycogen was synthesized from the load, respectively. Direct and indirect pathways contributed equally (25 ± 3% direct, 21 ± 1% indirect for seabass; 35 ± 7% direct, 29 ± 4% indirect for seabream). In fasted seabass, integration of 2H- and 13C-NMR analysis indicated that endogenous glycerol and anaplerotic substrates contributed an additional 7 ± 2% and 7 ± 1%, respectively. In fed seabass, glucose load contributions were residual and endogenous contributions were negligible. Concluding, direct and indirect pathways contributed equally and substantially to fasting hepatic glycogen repletion from a glucose load in juvenile seabream and seabass.

Effects of dietary carbohydrate on hepatic de novo lipogenesis in European seabass (Dicentrarchus labrax L.)

Farmed seabass have higher adiposity than their wild counterparts and this is often attributed to carbohydrate (CHO) feeding. Whether this reflects a reduction in fat oxidation, increased de novo lipogenesis (DNL), or both, is not known. To study the effects of high CHO diets on hepatic TG biosynthesis, hepatic TG deuterium ((2)H) enrichment was determined following 6 days in (2)H-enriched tank water for fish fed with a no-CHO control diet (CTRL), and diets with digestible starch (DS) and raw starch (RS). Hepatic fractional synthetic rates (FSRs, percent per day(-1)) were calculated for hepatic TG-glyceryl and FA moieties through (2)H NMR analysis. Glyceryl FSRs exceeded FA FSRs in all cases, indicating active cycling. DS fish did not show increased lipogenic potential compared to CTRL. RS fish had lower glyceryl FSRs compared with the other diets and negligible levels of FA FSRs despite similar hepatic TG levels to CTRL. DS-fed fish showed higher activity for enzymes that can provide NADPH for lipogenesis, relative to CTRL in the case of glucose-6-phosphate dehydrogenase (G6PDH) and relative to RS for both G6PDH and 6-phosphogluconate dehydrogenase. This approach indicated that elevated hepatic adiposity from DS feeding was not attributable to increased DNL.

Hepatic fatty acid biosynthesis is more responsive to protein than carbohydrate in rainbow trout during acute stimulations

The link between dietary carbohydrate/protein and de novo lipogenesis (DNL) remains debatable in carnivorous fish. We aimed to evaluate and compare the response of hepatic lipogenic gene expression to dietary carbohydrate intake/glucose and dietary protein intake/amino acids (AAs) during acute stimulations using both in vivo and in vitro approaches. For the in vivo trial, three different diets and a controlled-feeding method were employed to supply fixed amount of dietary protein or carbohydrate in a single meal; for the in vitro trial, primary hepatocytes were stimulated with a low or high level of glucose (3 mM or 20 mM) and a low or high level of AAs (one-fold or four-fold concentrated AAs). In vitro data showed that a high level of AAs upregulated the expression of enzymes involved in DNL [fatty acid synthase (FAS) and ATP citrate lyase (ACLY)], lipid bioconversion [elongation of very long chain fatty acids like-5 (Elovl5), Elovl2, Δ6 fatty acyl desaturase (D6D) and stearoyl-CoA desaturase-1 (SCD1)], NADPH production [glucose-6-phosphate dehydrogenase (G6PDH) and malic enzyme (ME)], and transcriptional factor sterol regulatory element binding protein 1-like, while a high level of glucose only elevated the expression of ME. Data in trout liver also showed that high dietary protein intake induced higher lipogenic gene expression (FAS, ACLY, and Elovl2) regardless of dietary carbohydrate intake, while high carbohydrate intake markedly suppressed the expression of acetyl-CoA carboxylase (ACC) and Elovl5. Overall, we conclude that, unlike rodents or humans, hepatic fatty acid biosynthetic gene expression in rainbow trout is more responsive to dietary protein intake/AAs than dietary carbohydrate intake/glucose during acute stimulations. This discrepancy probably represents one important physiological and metabolic difference between carnivores and omnivores.