Hypoxia tolerance in fish depends on catabolic preference between lipids and carbohydrates

Hypoxia is a common environmental stress factor in aquatic organisms,which varies among fish species.However,the mechanisms underlying the ability of fish species to tolerate hypoxia are not well known.Here,we showed that hypoxia response in different fish species was affected by lipid catabolism and preference for lipid or carbohydrate energy sources.Activation of biochemical lipid catabolism through peroxisome proliferator-activated receptor alpha(Pparα)or increasing mitochondrial fat oxidation in tilapia decreased tolerance to acute hypoxia by increasing oxygen consumption and oxidative damage and reducing carbohydrate catabolism as an energy source.Conversely,lipid catabolism inhibition by suppressing entry of lipids into mitochondria in tilapia or individually knocking out three key genes of lipid catabolism in zebrafish increased tolerance to acute hypoxia by decreasing oxygen consumption and oxidative damage and promoting carbohydrate catabolism.However,anaerobic glycolysis suppression eliminated lipid catabolism inhibition-promoted hypoxia tolerance in adipose triglyceride lipase(atgl)mutant zebrafish.Using 14 fish species with different trophic levels and taxonomic status,the fish preferentially using lipids for energy were more intolerant to acute hypoxia than those preferentially using carbohydrates.Our study shows that hypoxia tolerance in fish depends on catabolic preference for lipids or carbohydrates,which can be modified by regulating lipid catabolism.

Uridine alleviates high-carbohydrate diet-induced metabolic syndromes by activating sirt1/AMPK signaling pathway and promoting glycogen synthesis in Nile tilapia(Oreochromis niloticus)

Carbohydrates have a protein sparing effect,but long-term feeding of a high-carbohydrate diet(HCD)leads to metabolic disorders due to the limited utilization efficiency of carbohydrates in fish.How to mitigate the negative effects induced by HCD is crucial for the rapid development of aquaculture.Uridine is a pyrimidine nucleoside that plays a vital role in regulating lipid and glucose metabolism,but whether uridine can alleviate metabolic syndromes induced by HCD remains unknown.In this study,a total of480 Nile tilapia(Oreochromis niloticus)(average initial weight 5.02±0.03 g)were fed with 4 diets,including a control diet(CON),HCD,HCD+500 mg/kg uridine(HCUL)and HCD+5,000 mg/kg uridine(HCUH),for 8 weeks.The results showed that addition of uridine decreased hepatic lipid,serum glucose,triglyceride and cholesterol(P<0.05).Further analysis indicated that higher concentration of uridine activated the sirtuin1(sirt1)/adenosine 5-monophosphate-activated protein kinase(AMPK)signaling pathway to increase lipid catabolism and glycolysis while decreasing lipogenesis(P<0.05).Besides,uridine increased the activity of glycogen synthesis-related enzymes(P<0.05).This study suggested that uridine could alleviate HCD-induced metabolic syndrome by activating the sirt1/AMPK signaling pathway and promoting glycogen synthesis.This finding reveals the function of uridine in fish metabolism and facilitates the development of new additives in aquatic feeds.

Morphology, mitochondrial development and adipogenic-related genes expression during adipocytes differentiation in grass carp(Ctenopharyngodon idellus)

To investigate the differentiation mechanism of grass carp preadipocytes, a primary adipocytes culture system was established. Confluent preadipocytes were induced to differentiation, and the morphology and gene expression were evaluated at different stages. It was shown that preadipocytes were gradually filled with droplets and the cellular lipid content increased during the differentiation. Ultrastructure observation indicated that the number of mitochondria increased with adipocytes differentiation.Consistently, the mitochondrial protein content was elevated in the differentiating adipocytes. q RT-PCR showed that the expression level of lipogenesis-related genes such as peroxisome proliferator activator receptor c(PPAR c),lipoprotein lipase(LPL), fatty acid synthase(FAS) and stearoyl-Co A desaturase(SCD) increased during adipocytes differentiation. The mitochondrial relevant gene also elevated when adipocyte differentiation, such as PPAR coactivator-1(PGC-1 a), PGC-1 b and nuclear respiratory factor(NRF-1). However, the expression of carnitine palmitoyltransferase I a(CPT-1 a) gene decreased at the initial stage, but increased at the last stage of cell differentiation. These results indicated that the differentiation process of grass carp preadipocytes is similar to that of landanimals, but the molecular mechanisms are not exactly the same. The findings revealed in this study provides new information to the study of fish adipocyte differentiation.

Partial replacement of soybean meal by yellow mealworm(Tenebrio molitor)meal influences the flesh quality of Nile tilapia(Oreochromis niloticus)

This study investigated the effects of yellow mealworm meal(YM)replacing soybean meal(SBM)at different proportions(0%,15%,30%and 45%,referred as YM0,YM15,YM30 and YM45,respectively)on the flesh quality of Nile tilapia.A total of 360 fish(70.0±0.12 g)were randomly divided into 4 groups(3 tanks per group).Fish were fed the experimental diet twice daily for 10 wk.The results showed that muscle protein content significantly decreased in YM30 and YM45,while the lipid content significantly decreased in YM45(P<0.05).The essential amino acids and flavor amino acids of the muscle were not affected by the YM substitution,while saturated fatty acid content decreased in YM30 and YM45 compared with YM0(P<0.05).Fillets in YM45 had higher hardness,gumminess,and a higher proportion of thin myofibers(≤100μm,P<0.05)than those in other groups.Further analysis revealed that apoptosis and atrophy related genes were up-regulated,while the muscle antioxidant capacity decreased significantly in YM45(P<0.05),which may be related to the high acid value in YM45 diet.Our findings indicated that YM could replace up to 30%SBM without substantially altering the flesh quality.When the replacement ratio increased to 45%,the flesh quality would change.Special attention should be paid to avoid feed rancidity which may affect the flesh quality of fish.

Double-edged effect of sodium citrate in Nile tilapia(Oreochromis niloticus):Promoting lipid and protein deposition vs.causing hyperglycemia and insulin resistance

Citrate is an essential substrate for energy metabolism that plays critical roles in regulating glucose and lipid metabolic homeostasis.However,the action of citrate in regulating nutrient metabolism in fish remains poorly understood.Here,we investigated the effects of dietary sodium citrate on growth performance and systematic energy metabolism in juvenile Nile tilapia(Oreochromis niloticus).A total of 270Nile tilapia(2.81±0.01 g)were randomly divided into three groups(3 replicates per group,30 fish per replicate)and fed with control diet(35%protein and 6%lipid),2%and 4%sodium citrate diets,respectively,for 8 weeks.The results showed that sodium citrate exhibited no effect on growth performance(P>0.05).The whole-body crude protein,serum triglyceride and hepatic glycogen contents were significantly increased in the 4%sodium citrate group(P<0.05),but not in the 2%sodium citrate group(P>0.05).The 4%sodium citrate treatment significantly increased the serum glucose and insulin levels at the end of feeding trial and also in the glucose tolerance test(P<0.05).The 4%sodium citrate significantly enhanced the hepatic phosphofructokinase activity and inhibited the expression of pyruvate dehydrogenase kinase isozyme 2 and phosphor-pyruvate dehydrogenase E1 component subunit alpha proteins(P<0.05).Additionally,the 4%sodium citrate significantly increased hepatic triglyceride and acetyl-Co A levels,while the expressions of carnitine palmitoyl transferase 1a protein were significantly down-regulated by the 4%sodium citrate(P<0.05).Besides,the 4%sodium citrate induced crude protein deposition in muscle by activating m TOR signaling and inhibiting AMPK signaling(P<0.05).Furthermore,the 4%sodium citrate significantly suppressed serum aspartate aminotransferase and alanine aminotransferase activities,along with the lowered expression of pro-inflammatory genes,such as nfκb,tnfa and il8(P<0.05).Although the 4%sodium citrate significantly increased phosphor-nuclear factor-k B p65 protein expression(P<0.05),no significant tissue damage or inflammation occurred.Taken together,dietary supplementation of sodium citrate could exhibit a double-edged effect in Nile tilapia,with the positive aspect in promoting nutrient deposition and the negative aspect in causing hyperglycemia and insulin resistance.

High cholesterol intake remodels cholesterol turnover and energy homeostasis in Nile tilapia(Oreochromis niloticus)

The roles of dietary cholesterol in fish physiology are currently contradictory.The issue reflects the limited studies on the metabolic consequences of cholesterol intake in fish.The present study investigated the metabolic responses to high cholesterol intake in Nile tilapia(Oreochromis niloticus),which were fed with four cholesterol-contained diets(0.8,1.6,2.4 and 3.2%)and a control diet for eight weeks.All fish-fed cholesterol diets showed increased body weight,but accumulated cholesterol(the peak level was in the 1.6%cholesterol group).Then,we selected 1.6%cholesterol and control diets for further analysis.The high cholesterol diet impaired liver function and reduced mitochondria number in fish.Furthermore,high cholesterol intake triggered protective adaptation via(1)inhibiting endogenous cholesterol synthesis,(2)elevating the expression of genes related to cholesterol esterification and efflux,and(3)promoting chenodeoxycholic acid synthesis and efflux.Accordingly,high cholesterol intake reshaped the fish gut microbiome by increasing the abundance of Lactobacillus spp.and Mycobacterium spp.,both of which are involved in cholesterol and/or bile acids catabolism.Moreover,high cholesterol intake inhibited lipid catabolic activities through mitochondrialβ-oxidation,and lysosome-mediated lipophagy,and depressed insulin signaling sensitivity.Protein catabolism was elevated as a compulsory response to maintain energy homeostasis.Therefore,although high cholesterol intake promoted growth,it led to metabolic disorders in fish.For the first time,this study provides evidence for the systemic metabolic response to high cholesterol intake in fish.This knowledge contributes to an understanding of the metabolic syndromes caused by high cholesterol intake or deposition in fish.

Lipophagy is essential for lipid metabolism in fish

Lipophagy is a form of autophagy in mammals.In this cellular process,lipid droplets(LDs)are degraded through the lysosomal degradative pathway[1].During lipophagy,LDs sequestrated in autophagosomes are delivered to lysosomes,which fuse to form autophagosomes and then develop into autophagolysosomes,leading to LDs degradation of[1].A number of studies have demonstrated that lipophagy play important roles in lipid

Inhibition of pyruvate dehydrogenase kinase improves carbohydrate utilization in Nile tilapia by regulating PDK2/4-PDHE1αaxis and insulin sensitivity

Pyruvate dehydrogenase kinases(PDKs)-pyruvate dehydrogenase E1αsubunit(PDHE1α)axis plays an important role in regulating glucose metabolism in mammals.However,the regulatory function of PDKsPDHE1α axis in the glucose metabolism of fish is not well known.This study determined whether PDKs inhibition could enhance PDHE1αactivity,and improve glucose catabolism in fish.Nile tilapia fingerlings(1.90±0.11 g)were randomly divided into 4 treatments in triplicate(30 fish each)and fed control diet without dichloroacetate(DCA)(38% protein,7% lipid and 45% corn starch)and the control diet supplemented with DCA,which inhibits PDKs through binding the allosteric sites,at 3.75(DCA3.75),7.50(DCA7.50)and 11.25 g/kg(DCA11.25),for 6 wk.The results showed that DCA3.75,DCA7.50 and DCA11.25significantly increased weight gain,carcass ratio and protein efficiency ratio(P<0.05)and reduced feed efficiency(P<0.05)of Nile tilapia.To investigate the effects of DCA on growth performance of Nile tilapia,we selected the lowest dose DCA3.75 for subsequent analysis.Nile tilapia fed on DCA3.75significantly reduced the mesenteric fat index,serum and liver triglyceride concentration and total lipid content in whole fish,and down-regulated the expressions of genes related to lipogenesis(P<0.05)compared to the control.The DCA3.75 treatment significantly improved glucose oxidative catabolism and glycogen synthesis in the liver,but significantly reduced the conversion of glucose to lipid(P<0.05).Furthermore,the DCA3.75 treatment significantly decreased the PDK2/4 gene and protein expressions(P<0.05),accordingly stimulated PDHE1αactivity by decreasing the phosphorylated PDHE1αprotein level.In addition,DCA3.75 treatment significantly increased the phosphorylated levels of key proteins involved in insulin signaling pathway and glycogen synthase kinase 3β(P<0.05).Taken together,the present study demonstrates that PDK2/4 inhibition by using DCA promotes glucose utilization in Nile tilapia by activating PDHE1αand improving insulin sensitivity.Our study helps to understand the regulatory mechanism of glucose metabolism for improving dietary carbohydrate utilization in farmed fish.