Lipophagy is essential for lipid metabolism in fish

Network-based integration of omics, physiological and environmental data in real-world Elbe estuarine Zander

Coastal and estuarine environments are under endogenic and exogenic pressures jeopardizing survival and diversity of inhabiting biota. Information of possible synergistic effects of multiple (a)biotic stressors and holobiont interaction are largely missing in estuaries like the Elbe but are of importance to estimate unforeseen effects on animals' physiology. Here, we seek to leverage host-transcriptional RNA-seq and gill mucus microbial 16S rRNA metabarcoding data coupled with physiological and abiotic measurements in a network analysis approach to decipher the impact of multiple stressors on the health of juvenile Sander lucioperca along one of the largest European estuaries. We find mesohaline areas characterized by gill tissue specific transcriptional responses matching osmosensing and tissue remodeling. Liver transcriptomes instead emphasized that zander from highly turbid areas were undergoing starvation which was supported by compromised body condition. Potential pathogenic bacteria, including Shewanella, Acinetobacter, Aeromonas and Chryseobacterium, dominated the gill microbiome along the freshwater transition and oxygen minimum zone. Their occurrence coincided with a strong adaptive and innate transcriptional immune response in host gill and enhanced energy demand in liver tissue supporting their potential pathogenicity. Taken together, we show physiological responses of a fish species and its microbiome to abiotic factors whose impact is expected to increase with consequences of climate change. We further present a method for the close-meshed detection of the main stressors and bacterial species with disease potential in a highly productive ecosystem.

Selenium reduces hepatopancreas lipid accumulation of grass carp (Ctenopharyngodon idella) fed high-fat diet via lipophagy activation

It has been reported that selenium (Se) can reduce hepatopancreas lipid accumulation induced by high-fat diet. However, its mechanism is still unknown. This study aims to investigate the specific mechanisms by which Se alleviates high-fat diet-induced lipid accumulation. Grass carp were fed control diet (4.8% lipid, Con), high-fat diet (8.8% lipid, HFD) or HFD supplemented with 0.3 mg/kg nano-Se (HSe0.3) for 10 weeks. Growth performance, Se deposition, lipid accumulation, hepatic ultrastructure, and gene and protein expression levels associated with autophagy were examined. Furthermore, oleic acid (OA) was used to incubate the grass carp hepatocytes (L8824) for 24 h, and then the L8824 were incubated with sodium selenite in presence or absence of an autophagy inhibitor for 24 h. L8824 was analyzed for triglyceride concentration, immunofluorescence, and gene and protein expression levels associated with autophagy. We found that dietary nano-Se improved the growth of fish fed HFD and also decreased hepatosomatic index and intraperitoneal fat ratio of fish fed HFD (P < 0.05). HFD significantly increased hepatopancreas lipid accumulation and decreased autophagic activity (P < 0.05). Treatment of grass carp fed HFD with nano-Se decreased lipid accumulation and restored hepatic autophagy (P < 0.05). In vitro, Se (100 μM sodium selenite) obviously activated autophagy in L8824 incubated with OA, and consequently reduced the lipid accumulation induced by OA (P < 0.05). Furthermore, using pharmacological inhibition (chloroquine) of the autophagy greatly diminished the beneficial effects of Se on alleviating OA-induced lipid accumulation and increased the co-localization of lipid droplets with autophagosome (P < 0.05), which indicated that Se increased autophagic flux. In conclusion, these results suggest that Se alleviates HFD-induced hepatopancreas lipid accumulation by activating lipophagy.

Vitamin D modulates disordered lipid metabolism in zebrafish (Danio rerio) liver caused by exposure to polystyrene nanoplastics

In this study, zebrafish (Danio rerio) were exposed to polystyrene nanoplastics (PS-NPs, 80 nm) at 0, 15, or 150 μg/L for 21 days and supplied with a low or high vitamin D (VD) diet (280 or 2800 IU/kg, respectively, indicated by - or +) to determine whether and how VD can regulate lipid metabolism disorder induced by PS-NPs. Six groups were created according to the PS-NP concentration and VD diet status: 0-, 0+, 15-, 15+,150-, and 150 +. Transmission electron microscopy showed that PS-NPs accumulated in the livers of zebrafish, which led to large numbers of vacuoles and lipid droplets in liver cell matrices; this accumulation was most prominent in the 150- group, wherein the number of lipid droplets increased significantly by 136.36%. However, the number of lipid droplets decreased significantly by 76.92% in the 150+ group compared with the 150- group. An examination of additional biochemical indicators showed that the high VD diet partially reversed the increases in the triglyceride and total cholesterol contents induced by PS-NPs (e.g., triglycerides decreased by 58.52% in the 150+ group, and total cholesterol decreased by 44.64% in the 15+ group), and regulated lipid metabolism disorder mainly by inhibiting lipid biosynthesis. Untargeted lipidomics analysis showed that exposure to PS-NPs was associated mainly with changes in the lipid molecular content related to cell membrane function and lipid biosynthesis and that the high VD diet reduced the content of lipid molecules related to lipid biosynthesis, effectively alleviating cell membrane damage and lipid accumulation. These findings highlight the potential of VD to alleviate lipid metabolism disorder caused by PS-NP exposure, thereby providing new insights into how the toxic effects of NPs on aquatic organisms could be reduced.

ATGL-mediated lipophagy balances cholesterol-induced inflammation in pathogen infected Apostichopus japonicus coelomocytes

Cholesterol metabolism can be dynamically altered in response to pathogen infection that ensure proper macrophage inflammatory function in mammals. However, it is unclear whether the dynamic between cholesterol accumulation and breakdown could induce or suppress inflammation in aquatic animal. Here, we aimed to investigate the cholesterol metabolic response to LPS stimulation in coelomocytes of Apostichopus japonicus, and to elucidate the mechanism of lipophagy in regulating cholesterol-related inflammation. LPS stimulation significantly increased intracellular cholesterol levels at early time point (12 h), and the increase in cholesterol levels is associated with AjIL-17 upregulation. Excessive cholesterol in coelomocytes of A. japonicus was rapidly converted to cholesteryl esters (CEs) and stored in lipid droplets (LDs) after 12 h of LPS stimulation and prolonged for 18 h. Then, increased colocalization of LDs with lysosomes was observed at late time point of LPS treatment (24 h), accompanied by elevated expression of AjLC3 and decreased expression of Ajp62. At the same time, the expression of AjABCA1 rapidly increased, suggesting lipophagy induction. Moreover, we demonstrated that AjATGL is required for induction of lipophagy. Inducing lipophagy by AjATGL overexpression attenuated cholesterol-induced AjIL-17 expression. Overall, our study provides evidence that cholesterol metabolic response occurs upon LPS stimulation, which is actively involved in regulating the inflammatory response of coelomocytes. AjATGL-mediated lipophagy is responsible for cholesterol hydrolysis, thereby balancing cholesterol-induced inflammation in the coelomocytes of A. japonicus.

Hif1α/Dhrs3a Pathway Participates in Lipid Droplet Accumulation via Retinol and Ppar-γ in Fish Hepatocytes

Excessive hepatic lipid accumulation is a common phenomenon in cultured fish; however, its underlying mechanisms are poorly understood. Lipid droplet (LD)-related proteins play vital roles in LD accumulation. Herein, using a zebrafish liver cell line (ZFL), we show that LD accumulation is accompanied by differential expression of seven LD-annotated genes, among which the expression of dehydrogenase/reductase (SDR family) member 3 a/b (dhrs3a/b) increased synchronously. RNAi-mediated knockdown of dhrs3a delayed LD accumulation and downregulated the mRNA expression of peroxisome proliferator-activated receptor gamma (pparg) in cells incubated with fatty acids. Notably, Dhrs3 catalyzed retinene to retinol, the content of which increased in LD-enriched cells. The addition of exogenous retinyl acetate maintained LD accumulation only in cells incubated in a lipid-rich medium. Correspondingly, exogenous retinyl acetate significantly increased pparg mRNA expression levels and altered the lipidome of the cells by increasing the phosphatidylcholine and triacylglycerol contents and decreasing the cardiolipin, phosphatidylinositol, and phosphatidylserine contents. Administration of LW6, an hypoxia-inducible factor 1α (HIF1α) inhibitor, reduced the size and number of LDs in ZFL cells and attenuated hif1αa, hif1αb, dhrs3a, and pparg mRNA expression levels. We propose that the Hif-1α/Dhrs3a pathway participates in LD accumulation in hepatocytes, which induces retinol formation and the Ppar-γ pathway.

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 mitochondrial β-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.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-022-00158-7.

Prostaglandin 2α Promotes Autophagy and Mitochondrial Energy Production in Fish Hepatocytes

Fatty liver, characterized by excessive lipid droplet (LD) accumulation in hepatocytes, is a common physiological condition in humans and aquaculture species. Lipid mobilization is an important strategy for modulating the number and size of cellular LDs. Cyclooxygenase (COX)-mediated arachidonic acid derivatives are known to improve lipid catabolism in fish; however, the specific derivatives remain unknown. In the present study, we showed that serum starvation induced LD degradation via autophagy, lipolysis, and mitochondrial energy production in zebrafish hepatocytes, accompanied by activation of the COX pathway. The cellular concentration of PGF2α, but not other prostaglandins, was significantly increased. Administration of a COX inhibitor or interference with PGF2α synthase abolished serum deprivation-induced LD suppression, LD–lysosome colocalization, and expression of autophagic genes. Additionally, exogenous PGF2α suppressed the accumulation of LDs, promoted the accumulation of lysosomes with LD and the autophagy marker protein LC3A/B, and augmented the expression of autophagic genes. Moreover, PGF2α enhanced mitochondrial accumulation and ATP production, and increased the transcript levels of β-oxidation- and mitochondrial respiratory chain-related genes. Collectively, these findings demonstrate that the COX pathway is implicated in lipid degradation induced by energy deprivation, and that PGF2α is a key molecule triggering autophagy, lipolysis, and mitochondrial development in zebrafish hepatocytes.

Protective Effects of Bile Acids Against Hepatic Lipid Accumulation in Hybrid Grouper Fed a High-Lipid Diet

Bile acids (BAs) usually display growth-promoting and lipid-lowering properties when supplemented to the diet. The effects of a high-lipid diet (HD) and BAs supplementation on growth performance and lipid deposition of hybrid grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂) was evaluated in this study. Compared to the control diet (CD), the HD did not significantly affect the fish growth performance, but it promoted lipid deposition, as revealed by a significantly higher crude lipid content of the whole body, muscle, and liver. Among the HD supplemented with taurocholic acid (BD) groups, and compared to the HD, fish fed dietary supplementation of BAs at 900 mg kg⁻¹ exhibited the best growth performance and lowest hepatic lipid deposition. In most BD groups, the content of total cholesterol, low-density lipoprotein cholesterol, and triglycerides in serum, as well as the content of total cholesterol in the liver, were decreased, whereas the content of high-density lipoprotein cholesterol in serum was increased. In addition, the most strongly influenced pathways between the control, HD, and B3D groups were fatty acid biosynthesis, insulin signaling pathway, and AMPK signaling pathway. The improvement of lipid metabolism induced by the supplementation of BAs may be attributed to decreased expression of lipogenesis genes and proteins (enzymes), and increased lipolysis. In conclusion, dietary supplementation of BAs at 900 mg kg⁻¹ promoted growth performance and reduced lipid accumulation, whereas BAs supplementation improved the hepatic lipid metabolism by enhancing hepatic lipolysis, inhibiting lipogenesis, and regulating associated transcriptional factors in hybrid grouper.

Protective effects of taurocholic acid on the excessive hepatic lipid accumulation via regulation of bile acids metabolism in grouper

Dietary bile acids (BAs) supplementation can notably ameliorate the fatty liver disease caused by high dietary lipids, but the mechanism behind this is poorly understood. The present study was aimed...

Selective autophagy: the rise of the zebrafish model

Selective autophagy is a specific elimination of certain intracellular substrates by autophagic pathways. The most studied macroautophagy pathway involves tagging and recognition of a specific cargo by the autophagic membrane (phagophore) followed by the complete sequestration of targeted cargo from the cytosol by the double-membrane vesicle, autophagosome. Until recently, the knowledge about selective macroautophagy was minimal, but now there is a panoply of links elucidating how phagophores engulf their substrates selectively. The studies of selective autophagy processes have further stressed the importance of using the in vivo models to validate new in vitro findings and discover the physiologically relevant mechanisms. However, dissecting how the selective autophagy occurs yet remains difficult in living organisms, because most of the organelles are relatively inaccessible to observation and experimental manipulation in mammals. In recent years, zebrafish (Danio rerio) is widely recognized as an excellent model for studying autophagic processes in vivo because of its optical accessibility, genetic manipulability and translational potential. Several selective autophagy pathways, such as mitophagy, xenophagy, lipophagy and aggrephagy, have been investigated using zebrafish and still need to be studied further, while other selective autophagy pathways, such as pexophagy or reticulophagy, could also benefit from the use of the zebrafish model. In this review, we shed light on how zebrafish contributed to our understanding of these selective autophagy processes by providing the in vivo platform to study them at the organismal level and highlighted the versatility of zebrafish model in the selective autophagy field.Abbreviations: AD: Alzheimer disease; ALS: amyotrophic lateral sclerosis; Atg: autophagy-related; CMA: chaperone-mediated autophagy; CQ: chloroquine; HsAMBRA1: human AMBRA1; KD: knockdown; KO: knockout; LD: lipid droplet; MMA: methylmalonic acidemia; PD: Parkinson disease; Tg: transgenic.

Oleic and palmitic acids induce hepatic angiopoietin-like 4 expression predominantly via PPAR-γ in Larimichthys crocea

Angiopoietin-like 4 (ANGPTL4) is a potent regulator of TAG metabolism, but knowledge of the mechanisms underlying ANGPTL4 transcription in response to fatty acids is still limited in teleost. In the current study, we explored the molecular characterisation of ANGPTL4 and regulatory mechanisms of ANGPTL4 in response to fatty acids in large yellow croaker (Larimichthys crocea). Here, croaker angptl4 contained a 1416 bp open reading frame encoding a protein of 471 amino acids with highly conserved 12-amino acid consensus motif. Angptl4 was widely expressed in croaker, with the highest expression in the liver. In vitro, oleic and palmitic acids (OA and PA) treatments strongly increased angptl4 mRNA expression in croaker hepatocytes. Moreover, angptl4 expression was positively regulated by PPAR family (PPAR-α, β and γ), and expression of PPARγ was also significantly increased in response to OA and PA. Moreover, inhibition of PPARγ abrogated OA- or PA-induced angptl4 mRNA expression. Beyond that, PA might increase angptl4 expression partly via the insulin signalling. Overall, the expression of ANGPTL4 is strongly upregulated by OA and PA via PPARγ in the liver of croaker, which contributes to improve the understanding of the regulatory mechanisms of ANGPTL4 in fish.

Lipolysis and lipophagy play individual and interactive roles in regulating triacylglycerol and cholesterol homeostasis and mitochondrial form in zebrafish

Neutral lipases-mediated lipolysis and acid lipases-moderated lipophagy are two main processes for degradation of lipid droplets (LDs). However, the individual and interactive roles of these metabolic pathways are not well known across vertebrates. This study explored the roles of lipolysis and lipophagy from the aspect of neutral and acid lipases in zebrafish. We established zebrafish strains deficient in either adipose triglyceride lipase (atgl^-/-; AKO fish) or lysosomal acid lipase (lal^-/-; LKO fish) respectively, and then inhibited lipolysis in the LKO fish and lipophagy in the AKO fish by feeding diets supplemented with the corresponding inhibitors Atglistatin and 3-Methyladenine, respectively. Both the AKO and LKO fish showed reduced growth, swimming activity, and oxygen consumption. The AKO fish did not show phenotypes in adipose tissue, but mainly accumulated triacylglycerol (TAG) in liver, also, they had large LDs in the hepatocytes, and did not stimulate lipophagy as a compensation response but maintained basal lipophagy. The LKO fish reduced total lipid accumulation in the body but had high cholesterol content in liver; also, they accumulated small LDs in the hepatocytes, and showed increased lipolysis, especially Atgl expression, as a compensatory mechanism. Simultaneous inhibition of lipolysis and lipophagy in zebrafish resulted in severe liver damage, with the potential to trigger mitophagy. Overall, our study illustrates that lipolysis and lipophagy perform individual and interactive roles in maintaining homeostasis of TAG and cholesterol metabolism. Furthermore, the interactive roles of lipolysis and lipophagy may be essential in regulating the functions and form of mitochondria.

The reduction of lipid-sourced energy production caused by ATGL inhibition cannot be compensated by activation of HSL, autophagy, and utilization of other nutrients in fish

The adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL)-mediated lipolysis play important roles in lipid catabolism. ATGL is considered the central rate-limiting enzyme in the mobilization of fatty acids in mammals. Currently, severe fat accumulation has been commonly detected in farmed fish globally. However, the ATGL-mediated lipolysis and the potential synergy among ATGL, HSL, and autophagy, which is another way for lipid breakdown, have not been intensively understood in fish. In the present study, we added Atglistatin as an ATGL-specific inhibitor into the zebrafish diet and fed to the fish for 5 weeks. The results showed that the Atglistatin-treated fish exhibited severe fat deposition, reduced oxygen consumption, and fatty acid β-oxidation, accompanied with increased oxidative stress and inflammation. Furthermore, the Atglistatin-treated fish elevated total and phosphorylation protein expressions of HSL. However, the free fatty acids and lipase activities in organs were still systemically reduced in the Atglistatin-treated fish, and the autophagy marker LC3 was also decreased in the liver. On the other hand, glycogenolysis was stimulated but blood glucose was higher in the Atglistatin-treated fish. The transcriptomic analysis also provided the hint that the protein turnover efficiency in Atglistatin-treated fish was likely to be accelerated, but the protein content in whole fish was not affected. Taken together, ATGL plays crucial roles in energy homeostasis such that its inhibition causes loss of lipid-sourced energy production, which cannot be compensated by activation of HSL, autophagy, and utilization of other nutrients.

Energy response and fatty acid metabolism in Onychostoma macrolepis exposed to low-temperature stress

Temperature is a key environmental factor, and understanding how its fluctuations affect physiological and metabolic processes is critical for fish. The present study characterizes the energy response and fatty acid metabolism in Onychostoma macrolepis exposed to low temperature (10 °C). The results demonstrated that cold stress remarkably disrupted the energy homeostasis of O. macrolepis, then the AMP-activated protein kinase (AMPK) could strategically mobilize carbohydrates and lipids. In particular, when the O. macrolepis were faced with cold stress, the lipolysis was stimulated along with the enhanced fatty acid β-oxidation for energy, while the fatty acid synthesis was supressed in the early stage. Additionally, the fatty acid composition analysis suggested that saturated fatty acid (SFA) might accumulate while monounsaturated fatty acid (MUFA) and polyunsaturated fatty acid (PUFA) in storage lipids (mainly containing non-polar lipid, NPL) could be utilized to supply energy during cold acclimation. Altogether, this study may provide some meritorious for understanding the cold-tolerant mechanism of fish in the viewpoint of energy balance combined with fatty acid metabolism, and thus to contribute to this species rearing in fish farms in the future.

Impaired peroxisomal fat oxidation induces hepatic lipid accumulation and oxidative damage in Nile tilapia

Many metabolic diseases in fish are often associated with lowered peroxisomal fatty acid (FA) β-oxidation. However, the physiological role of peroxisomal FA oxidation in lipid metabolism in fish still remains unclear. In the present study, a specific peroxisomal FA β-oxidation inhibitor, 10,12-tricosadiynoic acid (TDYA), was used to investigate the effects of impaired peroxisomal β-oxidation on growth performance, health status, and lipid metabolism in Nile tilapia. The results showed that the dietary TDYA treatment did not affect weight gain, but significantly decreased peroxisomal β-oxidation in the liver, and increased body fat accumulation. The fish with impaired peroxisomal β-oxidation exhibited higher contents of serum lipid and peroxidation products, and alanine aminotransferase activity, and significantly lowered hepatic activities of superoxide dismutase and catalase. The inhibited peroxisomal β-oxidation did not enhance mitochondrial β-oxidation activity, but compensatorily upregulated FA β-oxidation-related gene expression, and downregulated the gene expressions in lipolysis and lipogenesis. Taken together, TDYA treatment markedly induced lipid accumulation and hepatic oxidative damage via systemically depressing lipid catabolism and antioxidant capacity. Our findings reveal the pivotal roles of peroxisomal β-oxidation in maintaining health and lipid homeostasis in fish, and could be helpful in understanding metabolic diseases in fish.

Identification and analysis of lipid droplet-related proteome in the adipose tissue of grass carp (Ctenopharyngodon idella) under fed and starved conditions

Fat accumulation in the mesenteric adipose tissue is a serious problem in grass carp (Ctenopharyngodon idella) culture. Lipid droplet-related proteins (LDRPs) are involved in the formation, degradation, and biological functions of lipid droplets. In this study, we aimed to provide reference proteomics data to study lipid droplet regulation in fish. We isolated LDRPs from the mesenteric adipose tissue of grass carp (1-year-old) after normal feeding and 7 days of starvation, and identified and analysed them using isobaric tags for relative and absolute quantitation (iTRAQ) technology. Short-term starvation had no significant effect on the body weight, condition factor, visceral index, hepatopancreas index, intraperitoneal fat index, adipose tissue triglyceride content, and adipocyte size of grass carp. Nine hundred and fifty proteins were identified and annotated using the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases; they are involved in a variety of metabolic and signalling pathways, including amino acid, lipid, and carbohydrate metabolism, and the PI3K-Akt signalling pathway. There were 296 differentially expressed proteins (DEPs), with 143 up-regulated and 153 down-regulated proteins. Three proteins involved in triglyceride and fatty acid syntheses and two proteins involved in autophagy were up-regulated, and six proteins involved in lipid catabolism were down-regulated. These results indicate that under short-term starvation, lipid droplets in the adipose tissue of grass carp may maintain their shape by promoting fat production and inhibiting lipolysis, and autophagy may be one of the main strategies for coping with short-term energy deprivation.

Feeding Faba Beans (Vicia faba L.) Reduces Myocyte Metabolic Activity in Grass Carp (Ctenopharyngodon idellus)

In this study, we aimed to explore the effects of faba bean (Vicia faba L.) on the energy metabolism of grass carp (Ctenopharyngodon idellus). A total of 180 fish (∼2900 g) were randomly assigned to six tanks (2.5 × 2.5 × 1.2 m; 30 individuals per tank) and fed either faba bean (Vicia faba L.) or a commercial diet for 120 days (3% body weight, twice per day). The results showed that faba bean-fed grass carp (FBFG) had significantly lower growth and higher fat accumulation in the mesenteric adipose tissue and hepatopancreas than commercial diet-fed grass carp (CDFG). Compared with CDFG, FBFG exhibited no significant difference in proximate composition of the muscle; however, an obvious decrease in muscle fiber size and significantly higher hardness, chewiness, and gumminess were observed. Transcriptome results showed that a total of 197 genes were differentially regulated in the dorsal muscle. Down-regulated genes included four genes annotated with myocyte development and 12 transcripts annotated with components of myofibrils. In addition, the FBFG group exhibited significantly lower expression of genes associated with oxygen transport, the mitochondrial respiratory chain, and creatine metabolism, suggesting reduced energy availability in the muscle of the FBFG. Moreover, using western-blotting and enzyme assays, we found decreased protein levels in the mitochondrial electron transport respiratory chain and creatine metabolism activities, as well as increased expression of autophagy marker protein levels, in the muscle of FBFG. Overall, our results suggest that an abnormal energy distribution may exist in grass carps after feeding with faba bean, which is reflected by a mass of fat deposition in the adipose tissue and hepatopancreas and subdued metabolic activity in the muscle.

Effects of feed restriction on growth performance, lipid mobilization, and gene expression in rose spotted snapper (Lutjanus guttatus)

The effects of feed deprivation were evaluated for 1 week and 2 weeks in Lutjanus guttatus juveniles. A significant reduction in body mass was observed in both feed deprivation schemes, as well as in hepatosomatic, viscerosomatic and mesenteric fat indexes. The composition of fasted fish was characterized by a decrease in lipid content; the liver displayed an intense reduction of lipid reserves in both fasted groups, and increased expression of the lysosomal acid lipase. 1 week after re-feeding, both experimental groups showed an increase in specific growth rate, feed intake and feed conversion ratio. A recovery in hepatic lipid reserves was observed, and the expression of the lysosomal acid lipase decreased, although lipid content in both groups was still significantly lower than in control groups. Hepatic expression of the growth hormone receptor decreased after fasting, and remained low even after the fish were fed, whereas the expression of insulin-like growth factor 1 and 2 increased after fasting and decreased in both groups when fish were fed again. Altogether, these results showed a partial compensatory growth response in L. guttatus juveniles after fasting, with enhanced growth rates and improved feed efficiency. Fish used stored lipid reserves as the main energy source, and the expression of growth-related and lipid mobilization marker genes in the liver showed similar patterns in both fasting schemes. Based on the results, we suggest as much as 1-week fasting intervals during grow-out programmes to reduce visceral fat and increase growth rate in this species.

Effects of dietary monocalcium phosphate supplementation on the anti-oxidative capacity, anti-bacteria function in immune organs of obscure puffer (Takifugu obscurus) after infection with Aeromonas hydrophila

The purpose of the present study was to explore the impaired anti-bacteria ability in immune organs and immune systems of obscure puffer induced by chronic dietary phosphorus (P) deficiency. Fish were fed diets supplemented with 6 g/kg P (P6) and 0 g/kg P (P0) respectively for 15 weeks, and lower final body weight, feed intake, weight gain, whole body P content and bone P content were observed in fish fed P0 diet (P < 0.05). Then the fish were continued to feed for 3 weeks and intraperitoneal injection with PBS (P6+PBS) and Aeromonas hydrophila (A.hydrophila) (P6 + A.hydrophila and P0 + A.hydrophila), and sampled at 3, 6, 12 and 24 h. The results showed that dietary P deficiency lowered survival rate, total hemocyte count, whereas enhanced ROS production and apoptosis rate of obscure puffer compared to the 6 g/kg P supplemented group after infection. Moreover, compared to the P sufficient group, puffer fish fed P deficient diet decreased the expressions of antioxidant genes catalase (cat) and glutathione reductase (gr), immune-related genes toll-like receptor 2 (tlr-2) and anti-inflammatory factors transforming growth factor β1 (tgf-β1) and interleukin 11 (il-11) while increased pro-inflammatory cytokines tumor necrosis factor α (tnf-α), interleukin 1β (il-1β) and interleukin 8 (il-8) in head kidney post-infection. In addition, dietary P deficiency decreased the hepatic gene expressions of anti-apoptotic factor B-cell lymphoma 2 (bcl-2) and bax-inhibitor 1 (bi-1), accompanied by increasing the mRNA expressions of pro-apoptotic factor caspase 3, caspase 8 and caspase 9 compared to the P sufficient group after A.hydrophila infection. In conclusion, dietary P deficiency impaired the anti-bacteria function of the immune system as well as immune organs by increasing oxidative stress and aggravating the inflammatory response and apoptosis in obscure puffer under the A.hydrophila challenge.

Inhibited Lipophagy Suppresses Lipid Metabolism in Zebrafish Liver Cells

Lipophagy degrades lipid droplets (LDs) through the lysosomal degradative pathway, thus plays important roles in regulating lipid metabolism in mammals. However, information on the existence and functions of lipophagy in fish lipid metabolism is still limited. In the present study, we confirmed the existence of lipophagy by observing the structures of LDs sequestered in autophagic vacuoles in the zebrafish liver cell line (ZFL) via electronic microscopy. Moreover, starved cells increased the mRNA expression of the microtubule-associated protein 1A/1B light chain 3 beta (LC3), which is a marker protein for autophagy and protein conversion from LC3-I to LC3-II. Inhibiting autophagy with chloroquine increased significantly the LDs content and decreased fatty acid β-oxidation and esterification activities in the ZFL cells cultured in the fed state. Furthermore, inhibiting autophagy function downregulated the mRNA expression of the genes and their proteins related to lipid metabolism. Altogether, the present study verified the existence of lipophagy and its essential regulatory roles in lipid metabolism in fish cells.

The Autophagic Flux Inhibitor Bafilomycine A1 Affects the Expression of Intermediary Metabolism-Related Genes in Trout Hepatocytes

Autophagy is an evolutionarily conserved process of cellular self-eating which emerged these last years as a major adaptive metabolic response to various stresses such as fasting, hypoxia, or environmental pollutants. However, surprisingly very few data is currently available on its role in fish species which are directly exposed to frequent environmental perturbations. Here, we report that the treatment of fasted trout hepatocytes with the autophagy inhibitor Bafilomycine A1 lowered the mRNA levels of many of the gluconeogenesis-related genes and increased those of genes involved in intracellular lipid stores. Concurrently, intracellular free amino acid levels dropped and the expression of the main genes involved in the endoplasmic reticulum (ER) stress exhibited a sharp increase in autophagy inhibited cells. Together these results highlight the strong complexity of the crosstalk between ER, autophagy and metabolism and support the importance of considering this function in future studies on metabolic adaptation of fish to environmental stresses.