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.

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.

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 ele- vated in the differentiating adipocytes, qRT-PCR showed that the expression level of lipogenesis-related genes such as peroxisome proliferator activator receptor 7 （PPAR 7）, lipoprotein lipase （LPL）, fatty acid synthase （FAS） and stearoyl-CoA desaturase （SCD） increased during adipocytes differentiation. The mitochondrial relevant gene also elevated when adipocyte differentiation, such as PPAR coactivator-1 （PGC-1 α）, PGC-1β and nuclear respiratory factor （NRF-1）. However, the expression of carnitine palmitoyltransferase 1α （CPT-1 α） gene decreased at the initial stage, but increased at the last stage of cell differ- entiation. These results indicated that the differentiation process of grass carp preadipocytes is similar to that of land animals, 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.