Response of AMP-activated protein kinase and lactate metabolism of largemouth bass (Micropterus salmoides) under acute hypoxic stress

Long-term hypoxia-induced physiological response in turbot Scophthalmus maximus L

Hypoxia affects fish's survival, growth, and physiological metabolism processes. In this study, turbot plasma glucose and cortisol contents, hepatic glycolysis (hexokinase [HK], phosphofructokinase [PFK], pyruvate kinase [PK]) and lipolysis (fatty acid synthetase [FAS], lipoprotein lipase [LPL]) enzyme activities, anti-oxidant enzyme (superoxide dismutase [SOD], catalase [CAT], glutathione peroxidase [GSH-Px]) activities, malondialdehyde (MDA), lactate and glycogen contents, gill histological parameters (lamellar length [SLL], width [SLW], interlamellar distance [ID]), respiratory frequency (RF), the proportion of the secondary lamellae available for gas exchange (PAGE), and hifs (hif-1α, hif-2α, hif-3α) expression were determined during long-term hypoxia and reoxygenation. Results showed that long-term hypoxia (3.34 ± 0.17 mg L-1) significantly elevated plasma cortisol and glucose contents; increased hepatic HK, PK, PFK, FAS, and LPL activity; decreased hepatic glycogen, lactate contents, and lipid drop numbers; and caused changes of hepatocyte (vacuolation, pyknotic, and lytic nucleus) after treatment for 4 weeks. Hepatic SOD, CAT, GSH-Px activity, and MDA contents; lamellar perimeter, SLL, ID, RF, and PAGE; and hepatic hif-1α, hif-2α, and hif-3α manifested similar results. Meanwhile, hif-1α is significantly higher than hif-2α, and hif-3α. Interestingly, females and males demonstrated no sex dimorphism significantly different from the above parameters (except hepatic FAS, LPL activity, and lipid drop number) under hypoxia. The above parameters recovered to normal levels after reoxygenation treatment for 4 weeks. Thus, long-term hypoxia promotes turbot hepatic glycogenolysis and lipolysis, induces oxidative damage and stimulates hepatic antioxidant capacity, and alters gill morphology to satisfy insufficient energy demand and alleviate potential damage, while hif-1α plays critical roles in the above physiological process.

Gamma-Aminobutyric Acid (GABA) Avoids Deterioration of Transport Water Quality, Regulates Plasma Biochemical Indices, Energy Metabolism, and Antioxidant Capacity of Tawny Puffer (Takifugui flavidus) under Transport Stress

Live fish transportation is crucial for managing aquaculture but can pose health risks to fish due to stressors encountered during transportation. Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter that plays a crucial role in the central nervous system and is considered to exhibit anti-stress effects. This study aims to investigate the effects of GABA on the transport water quality, plasma biochemical indices, energy metabolism, and antioxidant capacity of tawny puffer (Takifugu flavidus) under transport stress. Tawny puffer were pretreated by immersing in aquariums containing GABA (final concentrations at 0, 5, 50, and 150 mg/L) seawater for 3 days; then, simulated transport was conducted using oxygen-filled polyethylene bags containing the same concentration of GABA seawater as the pretreatment period. Water samples, plasma, and liver were collected after 0, 6, and 12 h of transport. The results revealed that with the prolongation of transportation time, the control group's water quality deteriorated, stress-related plasma biochemical indices increased, glycolytic substrate contents decreased, glycolytic enzyme activities and product contents increased, and aerobic metabolic enzyme activities exhibited initial increases followed by declines, ATPase activities decreased, antioxidant enzyme activities decreased, and the lipid peroxidation marker contents increased. It is noteworthy that GABA treatment could avoid water quality deterioration during transportation, inhibit an elevation in stress-related biochemical indicators, regulate energy metabolism, and reduce oxidative damage in tawny puffer, especially at 50 and 150 mg/L concentrations. In summary, GABA treatment can effectively alleviate the transport stress of tawny puffer.

Salicornia ramosissima Biomass as a Partial Replacement of Wheat Meal in Diets for Juvenile European Seabass (Dicentrarchus labrax)

The green tips of Salicornia ramosissima are used for human consumption, while, in a production scenario, the rest of the plant is considered a residue. This study evaluated the potential of incorporating salicornia by-products in diets for juvenile European seabass, partially replacing wheat meal, aspiring to contribute to their valorization. A standard diet and three experimental diets including salicornia in 2.5%, 5% and 10% inclusion levels were tested in triplicate. After 62 days of feeding, no significant differences between treatments were observed in fish growth performances, feeding efficiency and economic conversation ratio. Nutrient digestibility of the experimental diets was unaffected by the inclusion of salicornia when compared to a standard diet. Additionally, salicornia had significant modulatory effects on the fish muscle biochemical profiles, namely by significantly decreasing lactic acid and increasing succinic acid levels, which can potentially signal health-promoting effects for the fish. Increases in DHA levels in fish fed a diet containing 10% salicornia were also shown. Therefore, the results suggest that salicornia by-products are a viable alternative to partially replace wheat meal in diets for juvenile European seabass, contributing to the valorization of a residue and the implementation of a circular economy paradigm in halophyte farming and aquaculture.

Effects of acute hypoxia and reoxygenation on histological structure, antioxidant response, and apoptosis in razor clam Sinonovacula constricta

Hypoxia is one of the major environmental problems limiting the healthy development of intensive aquaculture. Marine benthic shellfish are encountering heightened problems related to hypoxic stress as a result of ongoing human activities and aquaculture operations. Razor clam Sinonovacula constricta, a commercially valuable shellfish, has not yet been reported in studies on physiological changes caused by hypoxia and reoxygenation. To understand the negative effects of hypoxia and reoxygenation on the clams, we set up two low-oxygen concentration groups (DO 2.0 mg/L and DO 0.5 mg/L) and assessed multiple aspects of oxidative damage to their hepatopancreas and gills. After the hypoxic stress, the two tissues of the razor clam suffered varying degrees of damage, including cell degeneration and disruption of mitochondrial cristae. After reoxygenation, the 2.0 mg/L group recovered substantially, but the clams in the 0.5 mg/L group still unrecovered. The activities of antioxidant enzymes (MDA, T-AOC, SOD, GPX, and CAT) in clams were considerably altered by acute hypoxia and reoxygenation. Briefly, there was a growing and then declining trend in MDA, T-AOC, and SOD activities in the hepatopancreas, whereas GPX and CAT activities showed the converse trend. In the hepatopancreas and gills, the level of anti-apoptotic gene Bcl-2 transcripts gradually decreased with the duration of hypoxia and increased following reoxygenation. However, changes in the transcript level of the pro-apoptotic gene Bax were in contrast to that of Bcl-2. The TUNEL assay revealed that hypoxia caused apoptosis. Furthermore, at DO 0.5 mg/L, the degree of apoptosis was more significant than at DO 2.0 mg/L, and hepatopancreatic apoptosis was more severe than gill apoptosis. Collectively, our findings imply that hypoxia induces oxidative stress, histological damage, and apoptosis in razor clams in a concentration-dependent and tissue-specific manner. These consequences serve as a reminder that prolonged recovery periods may be required for razor clams to fully recover from oxidative damage resulting from hypoxia-reoxygenation episodes.

Effects of acute hypoxia on nutrient metabolism and physiological function in turbot, Scophthalmus maximus

Acute hypoxia is a common stress in aquaculture, and causes energy deficiency, oxidative damage and death in fish. Many studies have confirmed that acute hypoxia activated hif1α expression, anaerobic glycolysis and antioxidant system in fish, but the effects of acute hypoxia on lipid and protein metabolism, organelle damage, and the functions of hif2α and hif3α in economic fishes have not been well evaluated. In the present study, turbot was exposed to acute hypoxia (2.0 ± 0.5 mg/L) for 6 h, 12 h, and 24 h, respectively. Then, the contents of hemoglobin (HB), metabolite, gene expressions of hifα isoforms, energy homeostasis, endoplasmic reticulum (ER) stress, and apoptosis were measured. The results suggested that turbot is intolerant to acute hypoxia and the asphyxiation point is about 1.5 mg/L. Acute hypoxia induced perk-mediated ER stress, and increased lipid peroxidation and liver injury in turbot. The blood HB level and liver vegfab expression were increased under hypoxia, which enhances oxygen transport. At hypoxia stress, hif3α, anaerobic glycolysis-related genes expression, and lactate content were increased in the liver, and glycogen was broken down to ensure ATP supply. Meanwhile, hif2α, lipid synthesis-related genes expression, and TG content were increased in the liver, but the lipid catabolism and protein synthesis were suppressed during hypoxia, which reduced the oxygen consumption and ROS generation. Our results systematically illustrate the metabolic and physiological changes under acute hypoxia in turbot, and provide important guidance to improve hypoxia tolerance in fish.

Integrated Transcriptomics and Metabolomics Reveal Changes in Cell Homeostasis and Energy Metabolism in Trachinotus ovatus in Response to Acute Hypoxic Stress

Trachinotus ovatus is an economically important mariculture fish, and hypoxia has become a critical threat to this hypoxia-sensitive species. However, the molecular adaptation mechanism of T. ovatus liver to hypoxia remains unclear. In this study, we investigated the effects of acute hypoxic stress (1.5 ± 0.1 mg·L-1 for 6 h) and re-oxygenation (5.8 ± 0.3 mg·L-1 for 12 h) in T. ovatus liver at both the transcriptomic and metabolic levels to elucidate hypoxia adaptation mechanism. Integrated transcriptomics and metabolomics analyses identified 36 genes and seven metabolites as key molecules that were highly related to signal transduction, cell growth and death, carbohydrate metabolism, amino acid metabolism, and lipid metabolism, and all played key roles in hypoxia adaptation. Of these, the hub genes FOS and JUN were pivotal hypoxia adaptation biomarkers for regulating cell growth and death. During hypoxia, up-regulation of GADD45B and CDKN1A genes induced cell cycle arrest. Enhancing intrinsic and extrinsic pathways in combination with glutathione metabolism triggered apoptosis; meanwhile, anti-apoptosis mechanism was activated after hypoxia. Expression of genes related to glycolysis, gluconeogenesis, amino acid metabolism, fat mobilization, and fatty acid biosynthesis were up-regulated after acute hypoxic stress, promoting energy supply. After re-oxygenation for 12 h, continuous apoptosis favored cellular function and tissue repair. Shifting from anaerobic metabolism (glycolysis) during hypoxia to aerobic metabolism (fatty acid β-oxidation and TCA cycle) after re-oxygenation was an important energy metabolism adaptation mechanism. Hypoxia 6 h was a critical period for metabolism alteration and cellular homeostasis, and re-oxygenation intervention should be implemented in a timely way. This study thoroughly examined the molecular response mechanism of T. ovatus under acute hypoxic stress, which contributes to the molecular breeding of hypoxia-tolerant cultivars.

The Impact of Niclosamide Exposure on the Activity of Antioxidant Enzymes and the Expression of Glucose and Lipid Metabolism Genes in Black Carp (Mylopharyngodon piceus)

Niclosamide (NIC, 2',5-dichloro-4'-nitrosalicylanilide) is a salicylanilide molluscicide, and the extensive utilization and environmental pollution associated with NIC engender a potential hazard to both human health and the wellbeing of aquatic organisms. However, the mechanism of the chronic toxicity of NIC at environmentally relevant concentrations in terms of oxidative stress, metabolic disorder, and barrier functions in black carp (Mylopharyngodon piceus) is unknown. Therefore, healthy juvenile black carp (M. piceus) (average weight: 38.2 ± 2.5 g) were exposed to NIC at an environmentally realistic concentration (0, 10, and 50 μg/L) for 28 days. The findings of this study indicate that exposure to NIC resulted in reductions in weight gain, decreased activity of antioxidant enzymes, and increased expression of the Nrf2 gene. Furthermore, the liver demonstrated a greater accumulation of NIC than that in the gut and gills, as determined with a chemical analysis. Additionally, NIC exposure led to a significant reduction in ATP content and the activity of Na+/K+-ATPase and Ca2+/Mg2+-ATPase in the gut. Meanwhile, exposure to NIC resulted in a decrease in the liver glucose (Glu) level, gut cholesterol (CHO), and glycogen (Gln) and triglyceride (TG) content in all examined tissues. Conversely, it led to an increase in tissue lactic acid (LA) and acetyl-CoA levels, as well as LDH activity. Furthermore, NIC exposure at environmentally relevant concentrations demonstrated an upregulation in the expression of genes associated with glycolysis, such as PK and GK, while concurrently downregulating the gluconeogenesis gene G6Pase. Additionally, NIC exhibited an upregulation in the expression of genes related to β-oxidation, such as CPT1 and ACOX, while downregulating genes involved in triglyceride synthesis, including SREBP1, GPAT, FAS, and ACC1. Moreover, NIC facilitated fatty acid transportation through the overexpression of FATP and Fat/cd36. These results suggest that chronic exposure to NIC is associated with oxidative stress, compromised barrier function, and metabolic disorder. Moreover, these results underscore the significance of assessing the potential consequences of NIC for black carp and aquatic environments for aquaculture.

Alteration of antioxidant status, glucose metabolism, and hypoxia signal pathway in Eirocheir sinensis after acute hypoxic stress and reoxygenation

Dissolved oxygen (DO) is crucial for the survival of Chinese mitten crab (Eirocheir sinensis); low DO levels adversely affect the health of these crabs. In this study, we evaluated the underlying response mechanism of E. sinensis to acute hypoxic stress by analyzing antioxidant parameters, glycolytic indicators, and hypoxia signaling factors. The crabs were exposed to hypoxia for 0, 3, 6, 12, and 24 h and reoxygenated for 1, 3, 6, 12, and 24 h. The hepatopancreas, muscle, gill, and hemolymph were sampled at different exposure times to detect the biochemical parameters and gene expression. The results showed that the activity of catalase, antioxidants, and malondialdehyde in tissues significantly increased under acute hypoxia and gradually decreased during the reoxygenation phase. Under acute hypoxic stress, glycolysis indices, including hexokinase (HK), phosphofructokinase, pyruvate kinase (PK), pyruvic acid (PA), lactate dehydrogenase (LDH), lactic acid (LA), succinate dehydrogenase (SDH), glucose, and glycogen in the hepatopancreas, hemolymph, and gills increased to varying degrees but recovered to the control levels after reoxygenation. Gene expression data showed that hypoxia signaling pathway-related genes, including hypoxia-inducible factor-1α/β (HIF1α/β), prolyl hydroxylase (PHD), factor inhibiting hypoxia-inducible factor (FIH), and glycolysis-related factors (HK and PK) were upregulated, showing that the HIF signaling pathway was activated under hypoxic conditions. In conclusion, acute hypoxic exposure activated the antioxidant defense system, glycolysis, and HIF pathway to respond to adverse conditions. These data contribute to elucidating the defense and adaptive mechanisms of crustaceans to acute hypoxic stress and reoxygenation.

Transcriptome analysis revealed the mechanism of Luciobarbus capito (L. capito) adapting high salinity: Antioxidant capacity, heat shock proteins, immunity

Salinity has a significant influence on the physiology of freshwater aquatic organisms. However, there are few studies on the hematology and immunology of freshwater fish under high salinity. In the current study, we aimed to analyze the adaptive effect of salt stress on L. capito spleen immune function and hematology using transcriptomic analysis. We replicated a L. capito acute salinity stress model, and collected blood and spleens from freshwater and saltwater fish. It was found that salinity affected significantly the numbers of leukocytes, lymphocytes, neutrophils, and red blood cells, as well as the content of haemoglobin. Salt treatment resulted in a significant increase in the expression of HSP70, HSP90, CAT, SOD, and GPX1 genes in L. capito spleens. Transcriptomic analysis revealed a total of 546 differentially expressed genes (DEGs) in spleens, including 224 up-regulated DEGs and 322 down-regulated DEGs. In addition, GO enrichment analysis revealed immune system process, multicellular organismal process, and biological regulation of genes with the most differences in biological processes. KEGG enrichment analysis showed that the regulation of lipolysis in adipocyte, FoxO signaling pathway, Hematopoietic cell lineage signaling pathway, and HIF-1 signaling pathway were significantly enriched. L. capito adapted oxidative to high salinity through FoxO signaling pathway and immune to high salinity through Hematopoietic cell lineage signaling pathway. At the same time, we selected 10 DEGs for qRT-PCR detection, and the results showed that the qRT-PCR results were consistent with our RNA-Seq results, indicating that transcriptome sequencing was accurate and reliable. In conclusion, our results demonstrated that the improvement of antioxidant capacity, heat shock protein and immunity are involved in the molecular mechanism of L. capito adapting to high salinity. Our findings provided a rationale for further study on high salinity adaptation and related enrichment pathways.

The protective effect of resveratrol on largemouth bass (Micropterus salmoides) during out-of-season spawning

In aquaculture production, out-of-season spawning is beneficial to solve the seasonal shortage of fry that are normally produced once annually by species such as largemouth bass (Micropterus salmoides), thereby implementing year-round fry production. Maintaining low temperature over a period of several months can delay largemouth bass ovarian development, but it can cause severe stress to their reproductive function, leading to decreased fertility during out-of-season spawning. Feeding with antioxidants is one of the most effective methods to alleviate the negative effects of low temperature stress. Therefore, the purpose of this study is to: (a) evaluate the changes in oocyte morphology, antioxidant capacity, reproductive hormone-related index, cell apoptosis and autophagy during the out-of-season spawning of largemouth bass, and (b) to investigate the protective effect of the antioxidant resveratrol on this fish during out-of-season spawning from May through August. The study was divided into two groups (three replicates per group, 2000 fish per replicate): control group (Control) (exposure to water temperature of 12-17 °C) and resveratrol supplementation group (Res) (exposure to water temperature of 12-17 °C and fed with 200 mg/kg resveratrol). The results show that: (1) The serum hormones LH and E2 increased first and then remained unchanged, and the ovarian section showed that the ovary remained in stage IV. (2) In the process of off-season reproduction, a large number of follicles experienced follicular atresia, accompanied by endoplasmic reticulum expansion, nuclear chromatin condensation and mitochondrial swelling, which was relieved after feeding resveratrol. (3) Resveratrol decreased the ovarian ROS content and improved the activities of CAT and other antioxidant enzymes in the ovary and liver to some extent. (4) Resveratrol reduced the level of pro-apoptotic (Bax, Caspase3, Caspase8, Caspase9) and autophagy-related components (LC3-B, Beclin-1) while increasing the transcription level of anti-apoptotic (Bcl-2) factors. These findings suggest that resveratrol alleviates some adverse effects of largemouth bass during out-of-season spawning to some extent and provide a model for efficient and high-quality out-of-season spawning.

Resveratrol attenuated oxidative stress and inflammatory and mitochondrial dysfunction induced by acute ammonia exposure in gibel carp (Carassius gibelio)

Ammonia is recognized as an environmental stressor for fish. As resveratrol (RES) has anti-inflammatory and antioxidant properties, we hypothesized that RES could attenuate the response to ammonia exposure in gibel carp. Therefore, gibel carp were fed a diet containing RES for eight weeks, followed by acute ammonia stimulation. Stress induced by acute ammonia exposure could be ameliorated by RES, manifested by down-regulated plasma glucose, and up-regulated C3 and IgM levels. Furthermore, decreased AST and LDH; enhanced T-AOC, SOD, and GPx in the liver; and reduced damage to gill and liver tissues indicated that RES attenuated oxidative and tissue damage induced by ammonia exposure. Moreover, RES activated the Nrf2/HO-1 pathway and up-regulated the expression of several antioxidant genes. RES enhanced anti-inflammatory activity as reflected by activation of the NF-κB pathway, down-regulated the expression of pro-inflammatory cytokines (nfκb, tnf-α, and il-1β), and up-regulated the expression of anti-inflammatory cytokines (il-4 and il-10). In terms of mitochondrial function, RES up-regulated protein levels of p-AMPK, SIRT1, and PGC-1α; inhibited mitochondrial fission; promoted mitochondrial fusion and biogenesis-related gene expression. Overall, the results suggest that RES mediated the Nrf2/HO-1, NF-κB, and AMPK/SIRT1/PGC-1α pathways to attenuate oxidative stress, inflammation, and mitochondrial dysfunction induced by ammonia in gibel carp.

Acute deltamethrin exposure induces oxidative stress, triggers endoplasmic reticulum stress, and impairs hypoxic resistance of crucian carp

Deltamethrin (Del) has been widely used for effectively controlling ectoparasites of crucian carp and was also strictly prohibited in a hypoxic environment. A previous study indicated that Del exposure causes gill injury in Carassius auratus, which is associated with oxidative stress and endoplasmic reticulum stress (ER stress), but the precise mechanism is not well understood. Here, crucian carp were exposed to Del (0.61, 1.22, 2.44, 4.88 μg/L) for 24 h and then subjected to acute hypoxia challenge (1.0 mg/L) for 24 h. The results revealed that acute exposure to Del notably increased MDA content but markedly decreased CAT activities. Moreover, the T-AOC and SOD activities first increased and then decreased in the 4.88 μg/L Del group. Likewise, the mRNA levels of Nrf2 signaling and its target genes (ho-1, mt, sod, cat, and gpx1) were significantly downregulated in the high concentration exposure groups, while the mRNA levels of keap1 showed the opposite change trend. Meanwhile, Del exposure evoked the PERK-ATF4-CHOP and IRE1 signaling pathways and triggered ER stress in a dose-dependent manner in crucian carp. Importantly, we found that Del exposure significantly decreased the survival rate of crucian carp after hypoxia challenge by reducing oxygen uptake, modifying energy metabolism, and promoting lactate accumulation. Additionally, Del exposure aggravated gill damage and apoptosis under hypoxic stress, which was confirmed by histological assays. Collectively, we inferred that acute exposure to deltamethrin induces oxidative stress and ER stress and impairs hypoxic resistance of crucian carp.

Transcriptome and 16S rRNA Analyses Reveal That Hypoxic Stress Affects the Antioxidant Capacity of Largemouth Bass (Micropterus salmoides), Resulting in Intestinal Tissue Damage and Structural Changes in Microflora

Dissolved oxygen (DO) is a key factor affecting the health of aquatic organisms in an intensive aquaculture environment. In this study, largemouth bass (Micropterus salmoides) were subjected to acute hypoxic stress for 96 h (DO: 1.00 mg/L) followed by recovery under sufficient DO conditions (DO: 7.50 mg/L) for 96 h. Serum biochemical indices, intestinal histomorphology, the transcriptome, and intestinal microbiota were compared between hypoxia-treated fish and those in a control group. The results showed that hypoxia caused oxidative stress, exfoliation of the intestinal villus epithelium and villus rupture, and increased cell apoptosis. Transcriptome analyses revealed that antioxidant-, inflammation-, and apoptosis-related pathways were activated, and that the MAPK signaling pathway played an important role under hypoxic stress. In addition, 16S rRNA sequencing analyses revealed that hypoxic stress significantly decreased bacterial richness and identified the dominant phyla (Proteobacteria, Firmicutes) and genera (Mycoplasma, unclassified Enterobacterales, Cetobacterium) involved in the intestinal inflammatory response of largemouth bass. Pearson's correlation analyses showed that differentially expressed genes in the MAPK signaling pathway were significantly correlated with some microflora. The results of this study will help to develop strategies to reduce damage caused by hypoxic stress in aquacultured fish.

Acute hypoxia promotes the liver angiogenesis of largemouth bass (Micropterus salmoides) by HIF - Dependent pathway

A 24-h hypoxia exposure experiment was conducted to determine how hypoxia exposure induce liver angiogenesis in largemouth bass. Nitrogen (N₂) was pumped into water to exclude dissolved oxygen into 1.2 ± 0.2 mg/L, and liver tissues were sampled during hypoxia exposure of 0 h, 4 h, 8 h, 12 h, 24 h and re-oxygenation for 12 h. Firstly, the results showed that hypoxia exposure promoted the angiogenesis occurrence by immunohistochemical analysis of vascular endothelial growth factor receptor 2 (VEGFR2). Secondly, the concentration of vasodilation factor increased and it's activity was elevated during 8 h exposure, such as nitric oxide (NO) and nitric oxide synthase (NOS) (p < 0.05). Thirdly, hypoxia exposure promoted angiogenesis through up-regulation the expression of matrix metalloproteinase 2 (MMP-2), jagged, protein kinase B (AKT), phosphoinositide-3-kinase (PI3K), mitogen-activated protein kinase (MAPK) at 4 h; contrarily, the expression of inhibiting angiogenesis genes presented up-regulated at 8 h (p < 0.05), such as matrix metalloproteinase inhibitor-2 (TIMP-2), matrix metalloproteinase inhibitor-3 (TIMP-3). Finally, the genes and proteins that regulate angiogenesis presented obvious chronological order. Parts of them promoted the budding and extension of blood vessels were up-regulated during 4 h-8 h (p < 0.05), such as vascular endothelial growth factor a (VEGFA), VEGFR2, monocarboxylic acid transporter 1 (MCT1), CD147, prolyl hydroxylase (PHD), nuclear factor kappa-B (NF-κB); other part of them promoted blood vessel maturation were highly expressed during 12 h-24 h (p < 0.05), such as angiogenin-1 (Ang-1) and angiogenin-2 (Ang-2). In short, acute hypoxia can promote the liver angiogenesis of largemouth bass by HIF - dependent pathway.

Blood biomarkers as diagnostic tools: An overview of climate-driven stress responses in fish

Global climate change due to anthropogenic activities affects the dynamics of aquatic communities by altering the adaptive capacities of their inhabitants. Analysis of blood provides valuable insights in the form of a comprehensive representation of the physiological and functional status of fish under various environmental and treatment conditions. This review synthesizes currently available information about blood biomarkers used in climate change induced stress responses in fish. Alterations in informative blood-based indicators are used to monitor the physiological fitness of individual fishes or entire populations. Specific characteristics of fish blood, such as serum and plasma metabolites, cell composition, cellular abnormalities, cellular and antioxidant enzymes necessitate adapted protocols, as well as careful attention to experimental designs and meticulous interpretation of patterns of data. Moreover, the sampling technique, transportation, type of culture system, acclimation procedure, and water quality must all be considered for valid interpretation of hemato-biochemical parameters. Besides, blood collection, handling, and storage time of blood samples can all have significant impacts on the results of a hematological analysis, so it is optimal to perform hemato-biochemical evaluations immediately after blood collection because long-term storage can alter the results of the analyses, at least in part as a result of storage-related degenerative changes that may occur. However, the scarcity of high-throughput sophisticated approaches makes fish blood examination studies promising for climate-driven stress responses in fish.

Metabolic response provides insights into the mechanism of adaption to hypoxia in largemouth bass (Micropterus salmoides) under intermittent hypoxic conditions

In metabolism, molecular oxygen is a necessary substrate. Oxygen imbalances are linked to a variety of circumstances in the organism's homeostasis. Recently, the positive effects of hypoxia treatment in improving exercise ability and hypoxia tolerance have become a research focus. We explored the effects of intermittent hypoxia exposure (IHE, for one hour or three hours per day) on the hypoxia tolerance of largemouth bass in this study. The results showed that (1) IHE significantly reduced the LOEcrit (the critical O₂ tension for loss of equilibrium) value of largemouth bass, indicating that its hypoxia tolerance was enhanced. (2) The level of oxidative stress in the liver decreased in the HH3 group (exposed to a hypoxic condition for 3 h per day) compared to HH1 group (exposed to a hypoxic condition for 1 h per day). (3) IHE reduced the content of lactic acid and enhanced the process of gluconeogenesis in the liver. (4) Importantly, lipid mobilization and fatty acid oxidation in the liver of largemouth bass were significantly enhanced during IHE. In short, the results of this study indicate that IHE can improve hypoxia tolerance by regulating the energy metabolism of largemouth bass.

Effects of Acute Hypoxic Stress on Physiological and Hepatic Metabolic Responses of Triploid Rainbow Trout (Oncorhynchus mykiss)

This experiment simulated the hypoxic environment caused by actual production operations in fish farming (i.e., catching, gathering, transferring, and weighting) to study the effects of acute hypoxic conditions on the physiological and metabolic responses of triploid rainbow trout (O. mykiss). Two groups of fish weighting 590 g were sampled in the normoxia group (dissolved oxygen above 7 mg/L) and hypoxia group (dissolved oxygen ranged from 2 to 5 mg/L for 10 min). The results showed that 1) regarding stress response, hypoxia increased plasma levels of cortisol, heat shock protein 70 (HSP-70), lysozyme, alanine aminotransferase (ALT), aspartate aminotransferase (AST) and creatine phosphokinase (CPK); induced the expression of hepatic genes encoding nuclear factor erythroid 2 related factor 2 (Nrf2), interferon γ (IFN-γ) and interleukin-1β (IL-1β). 2) Regarding metabolism response, hypoxia increased plasma levels of globulin (GLOB), glucose (GLU), triglyceride (TG) and lactate dehydrogenase (LDH); upregulated the hepatic gene expression of phosphoenolpyruvate carboxykinase, (PEPCK), pyruvate dehydrogenase kinase (PDK1), acetyl-CoA carboxylase (ACC) and acetyl-CoA oxidase (ACO); downregulated the hepatic gene expression of carnitine palmitoyl transferase 1 (CPT1); and unchanged the expression of hepatic genes in glycolysis and autophagy. 3) In response to hypoxia-inducible factors (HIFs), the hepatic HIF-2α gene was activated in the hypoxia group, but HIF-1α gene expression remained unchanged. Thus, during acute hypoxic stress, triploid rainbow trout were in a defensive state, with an enhanced immune response and altered antioxidant status. Additionally, the hepatic mitochondrial oxidation of glucose- and lipid-derived carbon in trout was suppressed, and hepatic gluconeogenesis and lipid synthesis were activated, which might be regulated by the HIF-2α pathway.

Vitamin C Attenuates Oxidative Stress, Inflammation, and Apoptosis Induced by Acute Hypoxia through the Nrf2/Keap1 Signaling Pathway in Gibel Carp (Carassius gibelio)

Previous studies have found that vitamin C (VC) has protective effects in fish. However, the efficacy of VC on hypoxia-induced liver injury in fish remains unknown. Therefore, to investigate the protective mechanism of VC on liver injury after acute hypoxic stimulation in fish, gibel carp were fed a diet containing VC for eight weeks, then were subjected to acute hypoxia stimulation. The specific growth rate of fish was increased by the supplementation of VC. Plasma stress markers (glucose, lactic acid, and cortisol) were decreased by the VC supplementation. Moreover, the levels of the inflammatory cytokines (tnf-α, il-2, il-6, and il-12) were increased by enhancing the Nrf2/Keap1 signaling pathway. Upregulation of the antioxidant enzymes activity (CAT, SOD, and GPx); T-AOC; and anti-inflammatory factors (il-4 and tgf-β) highlighted the antioxidant and anti-inflammatory activities of VC. The results showed that VC reduced the apoptotic index of the fish hypothalamus. The expression of GRP78 protein in the liver and endoplasmic reticulum stress and apoptosis induced by hypoxia were inhibited by VC. Taken together, the results indicate that VC can attenuate oxidative damage, inflammation, and acute hypoxia induced apoptosis in gibel carp via the Nrf2/Keap1 signaling pathway. The results identify a new defense strategy of gibel carp in response to hypoxic conditions.

The transcriptomic responses of blunt snout bream (Megalobrama amblycephala) to acute hypoxia stress alone, and in combination with bortezomib

BACKGROUND

Blunt snout bream (Megalobrama amblycephala) is sensitive to hypoxia. A new blunt snout bream strain, "Pujiang No.2", was developed to overcome this shortcoming. As a proteasome inhibitor, bortezomib (PS-341) has been shown to affect the adaptation of cells to a hypoxic environment. In the present study, bortezomib was used to explore the hypoxia adaptation mechanism of "Pujiang No.2". We examined how acute hypoxia alone (hypoxia-treated, HN: 1.0 mg·L^- 1), and in combination with bortezomib (hypoxia-bortezomib-treated, HB: Use 1 mg bortezomib for 1 kg fish), impacted the hepatic ultrastructure and transcriptome expression compared to control fish (normoxia-treated, NN).

RESULTS

Hypoxia tolerance was significantly decreased in the bortezomib-treated group (LOE_crit, loss of equilibrium, 1.11 mg·L^- 1 and 1.32 mg·L^- 1) compared to the control group (LOE_crit, 0.73 mg·L^- 1 and 0.85 mg·L^- 1). The HB group had more severe liver injury than the HN group. Specifically, the activities of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) in the HB group (52.16 U/gprot, 32 U/gprot) were significantly (p < 0.01) higher than those in the HN group (32.85 U/gprot, 21. 68 U/gprot). In addition, more severe liver damage such as vacuoles, nuclear atrophy, and nuclear lysis were observed in the HB group. RNA-seq was performed on livers from the HN, HB and NN groups. KEGG pathway analysis disclosed that many DEGs (differently expressed genes) were enriched in the HIF-1, FOXO, MAPK, PI3K-Akt and AMPK signaling pathway and their downstream.

CONCLUSION

We explored the adaptation mechanism of "Pujiang No.2" to hypoxia stress by using bortezomib, and combined with transcriptome analysis, accurately captured the genes related to hypoxia tolerance advantage.

High carbohydrate diet induced endoplasmic reticulum stress and oxidative stress, promoted inflammation and apoptosis, impaired intestinal barrier of juvenile largemouth bass (Micropterus salmoides)

This study assessed the effects of feed carbohydrate content on intestinal physical barrier and immunity in juvenile largemouth bass (Micropterus salmoides). Triplicate groups of juvenile fish (4.1 ± 0.2 g) were fed low (LCD, 7%), medium (MCD, 12%) and high (HCD, 17%) carbohydrate diets for eight weeks. Gut histology revealed the slight infiltration of inflammatory cells and moderate loss of mucous membrane layer in HCD group. Expression of ZO1, occluding, and claudin7 genes and epidermal growth factor receptor (EGFR) gene were significantly decreased in HCD group indicating the impairment of tight junction and epithelial cell regeneration. The results showed the significant (P < 0.05) reduction of antioxidant capacity in HCD group compared to LCD. Furthermore, expression of intestinal ERS-related genes such as IRE1, Eif2α, GRP78, CHOPα and CHOPβ in HCD group was significantly higher than the LCD group. In addition, HCD induced the up-regulated expression of inflammatory (IL-8, IL-1β, TNFα and COX2) and apoptosis (TRAF2, bax, casepase3, caspase8 and casepase9) related genes in fish intestine. The data generated in this study clearly demonstrated that HCD induced ERS and oxidative stress, which promoted intestinal inflammation and apoptosis in juvenile largemouth bass.

Lactate ions induce synaptic plasticity to enhance output from the central respiratory network

Lactate ion sensing has emerged as a process that regulates ventilation during metabolic challenges. Most work has focused on peripheral sensing of lactate for the control of breathing. However, lactate also rises in the central nervous system (CNS) during disturbances to blood gas homeostasis and exercise. Using an amphibian model, we recently showed that lactate ions, independently of pH and pyruvate metabolism, act directly in the brainstem to increase respiratory-related motor outflow. This response had a long washout time and corresponded with potentiated excitatory synaptic strength of respiratory motoneurons. Thus, we tested the hypothesis that lactate ions enhance respiratory output using cellular mechanisms associated with long-term synaptic plasticity within motoneurons. In this study, we confirm that 2 mM sodium lactate, but not sodium pyruvate, increases respiratory motor output in brainstem-spinal cord preparations, persisting for 2 h upon the removal of lactate. Lactate also led to prolonged increases in the amplitude of AMPA-glutamate receptor (AMPAR) currents in individual motoneurons from brainstem slices. Both motor facilitation and AMPAR potentiation by lactate required classic effectors of synaptic plasticity, L-type Ca²⁺ channels and NMDA receptors, as part of the transduction process but did not correspond with increased expression of immediate-early genes often associated with activity-dependent neuronal plasticity. Altogether these results show that lactate ions enhance respiratory motor output by inducing conserved mechanisms of synaptic plasticity and suggest a new mechanism that may contribute to coupling ventilation to metabolic demands in vertebrates. KEY POINTS: Lactate ions, independently of pH and metabolism, induce long-term increases in respiratory-related motor outflow in American bullfrogs. Lactate triggers a persistent increase in strength of AMPA-glutamatergic synapses onto respiratory motor neurons. Long-term plasticity of motor output and synaptic strength by lactate involves L-type Ca²⁺ channels and NMDA-receptors as part of the transduction process. Enhanced AMPA receptor function in response to lactate in the intact network is causal for motor plasticity. In sum, well-conserved synaptic plasticity mechanisms couple the brainstem lactate ion concentration to respiratory motor drive in vertebrates.

Distinct metabolic shifts occur during the transition between normoxia and hypoxia in the hybrid and its maternal abalone

Due to anthropogenic activities that have increased global climate change and nutrient discharges, severe hypoxic events have frequently occurred in coastal waters in recent years. Relying on coastal waters, the aquaculture area has suffered ecological and economic losses caused by hypoxia, especially in summer. In this study, to investigate the stress resistance of the Pacific abalone Haliotis discus hannai (DD) and the hybrid H. discus hannai ♀ × H. fulgens ♂ (DF), a combination of physiological, biochemical, and metabolomic methods were used to compare the metabolic responses of these two abalones to acute hypoxia (~0.5 mg O₂/L, 12 h) and reoxygenation (~6.6 mg O₂/L, 10-20 h). Hemolymph characteristics and aerobic/anaerobic respiratory capacity changed significantly under hypoxia or reoxygenation conditions, and they were regulated in different trends in two abalones. The contents of hepatopancreas glycogen in two abalones reached the trough after 10 h recovery, implying that short-term hypoxia leads to a long-lasting (several hours) imprint on the energy storage of abalone. In response to dissolved oxygen fluctuation, metabolic profiles of two abalones changed in distinct ways both in the hypoxia group or the reoxygenation group. The conversion of carbohydrate metabolism and amino acid metabolism indicated that hypoxia prompts abalone to change the way of energy metabolism, which may also reflect the difference in the energy utilization of DD and DF abalones. In addition, 3 metabolites (L-glutamate, 2-hydroxy-butanoic acid, and 2-methyl-3-hydroxybutyric acid) as potential biomarkers for hypoxia and reoxygenation response in abalone were determined by operating characteristic analysis (ROC). Overall, this study provides information towards understanding the damage caused by frequent hypoxic events and implies the metabolic shifts that occur under hypoxia and reoxygenation conditions in DD and DF abalones.

Chronic chlorpyrifos exposure induces oxidative stress, apoptosis and immune dysfunction in largemouth bass (Micropterus salmoides)

This study was undertaken to (a) evaluate the destructive effects of chronic exposure to low-dose of chlorpyrifos (CPF) on antioxidant system and immune function in largemouth bass (Micropterus salmoides), and (b) to examine whether dietary supplementation of curcumin can mitigate the adverse effects induced by CPF contamination. The experiment consisted of three groups (with three replicates, 30 fish per replicate) which lasted for 60 days: A control group (without CPF exposure or CU application), CP group (exposed to 0.004 mg/L of CPF), and CU group (exposed to 0.004 mg/L of CPF and fed a diet containing 100 mg curcumin per kg feed). The results showed that CPF contamination leads to reduced weight gain, severe histopathological lesions, decreased activity of antioxidant enzymes and down-regulated expression of antioxidant-related genes. Moreover, CPF upregulated the expression of pro-inflammatory genes such as TNF-α, IL-8, IL-15, downregulated anti-inflammatory genes TGF-β1, IL-10, and promoted apoptosis through overexpression of Caspase-3, Caspase-8, caspase-9 and Bax. In addition, curcumin supplementation showed significant improvement in oxidative stress, apoptosis and immune dysfunction, but the improved effect gradually weakened during the exposure last. Gas chromatography-mass spectrometry (GC-MS) analysis for accumulation of CPF in muscle supported the changes of general physiological structure, excessive apoptotic responses, abnormal antioxidant and immune system functions and posed potential human health risks to children based on target hazard quotient. These results suggested that chronic exposure to CPF can cause oxidative stress, apoptosis and immune dysfunction, and that curcumin have the potential to reduce pesticides residues in fish. This also highlights the importance of monitoring pesticides residues in aquatic products and aquaculture aquatic environments.

The biological activity of the organic UV filter ethylhexyl methoxycinnamate in rainbow trout (Oncorhynchus mykiss)

UV filters are able to enter the aquatic environment and negatively affect non-target organisms. The aim of this study was to evaluate subchronic exposure to ethylhexyl methoxycinnamate (EHMC) in rainbow trout. The tested EHMC concentrations of 6.9 (low), 96.0 (medium) and 395.6 μg/kg (high) were used. The lowest concentration was based on environmentally relevant concentrations. The higher concentrations were chosen as a multiple of the lowest one to determine the dose-response relationship. EHMC was incorporated into feed pellets. The experiment was conducted for six weeks in a semi-static system. Haematological, biochemical and oxidative stress indices were determined at the end of the experiment and supplemented by histological examination. Significant changes were proven at medium and high concentrations of EHMC. Specifically, increases of glucose, lactate and decrease of albumin and total protein in plasma indicate a stress ethology. Moreover, a decrease of plasma cholesterol, triacylglycerols and ammonia were observed even in the experimental group exposed to the lowest concentration of EHMC, perceived as an environmentally relevant concentration. The ferric reducing ability of plasma was decreased in all tested concentrations. Exposure to the highest concentration of EHMC resulted in a decrease in leukocyte counts. Increased activity of glutathione peroxidase in liver was recorded for the medium and the highest concentration of EHMC. The level of the thiobarbituric acid reactive substances in kidney was elevated for the highest concentration. Decrease of the activity of glutathione-S-transferase in gills for medium concentration of EHMC was registered. Histopathological examination revealed massive destruction of hepatic parenchyma at the highest concentration of EHMC. All these results support the finding of a stress load on the fish organism. In summary, although subchronic exposure to EHMC had no effect on behaviour, mortality or feed intake, this exposure resulted in the alteration of saccharide, lipid and protein metabolism and weakened antioxidant capacity.

Integration of Nutrient Sensing in Fish Hypothalamus

The knowledge regarding hypothalamic integration of metabolic and endocrine signaling resulting in regulation of food intake is scarce in fish. Available studies pointed to a network in which the activation of the nutrient-sensing (glucose, fatty acid, and amino acid) systems would result in AMP-activated protein kinase (AMPK) inhibition and activation of protein kinase B (Akt) and mechanistic target of rapamycin (mTOR). Changes in these signaling pathways would control phosphorylation of transcription factors cAMP response-element binding protein (CREB), forkhead box01 (FoxO1), and brain homeobox transcription factor (BSX) leading to food intake inhibition through changes in the expression of neuropeptide Y (NPY), agouti-related peptide (AgRP), pro-opio melanocortin (POMC), and cocaine and amphetamine-related transcript (CART). The present mini-review summarizes information on the topic and identifies gaps for future research.

Multi-omics analysis reveals the glycolipid metabolism response mechanism in the liver of genetically improved farmed Tilapia (GIFT, Oreochromis niloticus) under hypoxia stress

BACKGROUND

Dissolved oxygen (DO) in the water is a vital abiotic factor in aquatic animal farming. A hypoxic environment affects the growth, metabolism, and immune system of fish. Glycolipid metabolism is a vital energy pathway under acute hypoxic stress, and it plays a significant role in the adaptation of fish to stressful environments. In this study, we used multi-omics integrative analyses to explore the mechanisms of hypoxia adaptation in Genetically Improved Farmed Tilapia (GIFT, Oreochromis niloticus).

RESULTS

The 96 h median lethal hypoxia (96 h-LH50) for GIFT was determined by linear interpolation. We established control (DO: 5.00 mg/L) groups (CG) and hypoxic stress (96 h-LH50: 0.55 mg/L) groups (HG) and extracted liver tissues for high-throughput transcriptome and metabolome sequencing. A total of 581 differentially expressed (DE) genes and 93 DE metabolites were detected between the CG and the HG. Combined analyses of the transcriptome and metabolome revealed that glycolysis/gluconeogenesis and the insulin signaling pathway were down-regulated, the pentose phosphate pathway was activated, and the biosynthesis of unsaturated fatty acids and fatty acid metabolism were up-regulated in GIFT under hypoxia stress.

CONCLUSIONS

The results show that lipid metabolism became the primary pathway in GIFT under acute hypoxia stress. Our findings reveal the changes in metabolites and gene expression that occur under hypoxia stress, and shed light on the regulatory pathways that function under such conditions. Ultimately, this information will be useful to devise strategies to decrease the damage caused by hypoxia stress in farmed fish.

MicroRNA regulation in hypoxic environments: differential expression of microRNAs in the liver of largemouth bass (Micropterus salmoides)

Environmental changes in intensive aquaculture commonly lead to hypoxic stress for cultured largemouth bass (Micropterus salmoides). To better to understand the hypoxic stress response mechanisms, the miRNA expression profiles of the livers of largemouth bass exposed for 24 h to three different dissolved oxygen levels (7.0 ± 0.2 mg/L as control, 3.0 ± 0.2 mg/L and 1.2 ± 0.2 mg/L) were compared. In this study, a total of 266 known miRNAs were identified, 84 of which were differentially expressed compared with the control group. Thirteen of the differentially expressed miRNAs (miR-15b-5p, miR-30a-3p, miR-133a-3p, miR-19d-5p, miR-1288-3p, miR456, miR-96-5p, miR-23a-3p, miR-23b-5p, miR-214, miR-24, miR-20a-3p, and miR-2188-5p) were significantly enriched in VEGF signaling pathway, MAPK signaling pathway, and phosphatidylinositol signaling system. These miRNAs were significantly downregulated during stress, especially after a 4-h exposure to hypoxia. In contrast, their target genes (vegfa, pla2g4a, raf1a, pik3c2a, clam2a, inpp1, pi4k2b, mtmr14, ip6k, itpkca, map3k7, and Jun) were significant upregulated after 4 h of hypoxic stress. Moreover, two potential hypoxia-tolerance signal transduction pathways (MAPK signaling pathway and phosphatidylinositol signaling system) were revealed, both of which may play important roles in responding to acute hypoxic stress. We see that miRNAs played an important role in regulating gene expression related to physiological responses to hypoxia. Potential functional network regulated by miRNAs under hypoixic stress in the liver of largemouth bass (Micropterus salmoides). Blue boxes indicated that the expression of miRNA or target genes were down-regulated. Red boxes indicated that the expression of miRNA or target genes wasere up-regulated.

Effects of Sudden Drop in Salinity on Osmotic Pressure Regulation and Antioxidant Defense Mechanism of Scapharca subcrenata

Salinity is an important ecological factor that impacts the growth and survival of aquatic organisms. The salinity of seawater in coastal and estuarine areas is often subject to dynamic changes because of seasonal rainfall and continental runoff. Thus, the current study investigated the effects of sudden changes in salinity on the survival rate and osmotic pressure regulation mechanisms of bottom-sowing seedlings of the economically important ark shell, Scapharca subcrenata. By simulating the sudden changes that occur in seawater salinity after rainstorms, the results showed that the osmotic pressure of the hemolymph and Na⁺, K⁺, Ca²⁺, and Cl^– concentrations first decreased and then increased. When the salinity decreased from 30 to 14‰, hemoglobin, soluble total protein, taurine, and total free amino acid gradually increased; maximum levels of hemoglobin, soluble total protein, and taurine occurred once the salinity increased to 22‰ at 96 h. After 96 h, the total free amino acid content increased until 144 h. The reactive oxygen species (ROS) content and total antioxidant capacity (T-AOC) peaked at 96 h, whereas the expression levels of Mn-superoxide dismutase (MnSOD) and catalase (CAT) increased earlier, indicating that, with continuous ROS generation, antioxidant defense mechanisms were activated to avoid oxidative damage. Expression levels of cathepsin C (CTSC), cathepsin D (CTSD), heat shock protein 20 (HSP20), and heat shock protein 70 (HSP70) were significantly higher than in the control group at 48 h (salinity level 14‰); the expression levels of HSP20, heat shock protein 90 (HSP90), MnSOD, and glutathione peroxidase (GP_x) remained high, indicating that they were still required for osmotic pressure regulation to maintain the dynamic balance between the generation and removal of ROS as the salinity level increased. These results not only add to our basic understanding of the aquatic ecology of S. subcrenata, but also provide a theoretical ground for improving the survival rate of bottom-sowing, propagation, and release of S. subcrenata seedlings.

Combined exposure to hypoxia and ammonia aggravated biological effects on glucose metabolism, oxidative stress, inflammation and apoptosis in largemouth bass (Micropterus salmoides)

Hypoxia and ammonia are unavoidable environmental factors in aquaculture, and have been shown cause various adverse effects in fish. In the present study, a two-factor crossover experiment was carried out to evaluate the combined effect of hypoxia and ammonia on oxidative stress and glucose metabolism endpoints in largemouth bass. The fish were divided into four experimental groups: hypoxia and ammonia group, hypoxia group, ammonia group, and control group. The results showed that hypoxia and ammonia exposures both induced antioxidant response and oxidative stress (superoxide dismutase [SOD] and catalase [CAT] activities increased first then decreased, and malondialdehyde accumulated) and anaerobic glycolysis (increase of blood glucose, decrease of liver glycogen, accumulation of lactate, and increased lactate dehydrogenase activity). In addition, hypoxia and ammonia upregulated antioxidant enzyme genes (Cu/ZnSOD, CAT, and GPx), apoptosis genes (caspase 3, caspase 8, and caspase 9), as well as inflammatory genes (interleukin [IL]-1β and IL-8) and downregulated an anti-inflammatory gene (IL-10), suggesting that apoptosis and inflammation may be related to oxidative stress. The increased expression of GLUT1, LDH, and MCT4 were induced by hypoxia and ammonia, suggesting that anaerobic glycolysis was increased. Furthermore, fish suffering from hypoxia or ammonia exposure showed some changes in gill tissues histology, and the most severe lesions of gill tissues appeared in simultaneous exposure. Overall, both hypoxia and ammonia affected homeostasis, and simultaneous exposure led to more deleterious effects on largemouth bass than exposure to the individual stressors.

Effects of low salinity on gill and liver glycogen metabolism of great blue-spotted mudskippers (Boleophthalmus pectinirostris)

This study investigated the effects of low salinity exposure on glycogen and its metabolism biomarkers, glycogen synthase (GS) and glycogen phosphorylase (GP), representing glycogen synthesis and catabolism, respectively, in the gills and liver of great blue-spotted mudskippers (Boleophthalmus pectinirostris). The fish were accumulated at 10‰ salinity seawater for 1 week, then 270 healthy great blue-spotted mudskippers with similar size were randomly transferred to 10‰ (control group) or 3‰ (low salinity group) seawater for 72-hour stress experiment. Fish significantly elevated their blood glucose levels 12 h after low salinity challenge. At the end of experiments, a decrease in liver glycogen contents was observed in both the control and low salinity groups, the latter showing a pronounced decrease, while the gill glycogen contents were not changed for either group. The mRNA abundance and enzyme activity of GS and GP were both elevated in gill tissues, showing a rising glycogen synthesis and catabolism, probably resulting in the unchanging gill glycogen content. While in liver tissues, the mRNA abundance and enzyme activity were decreased for GS and increased for GP, showing a net increase for breaking down glycogen in liver, probably for supplying a sufficient glucose level for gills and other tissues/organs involved in the response to salinity changes.

High-carbohydrate diet promotes the adaptation to acute hypoxia in zebrafish

Oxygen deprivation (hypoxia) is a common challenge in water environment, which causes lack of energy and oxidative damage in organisms. Many studies have indicated a number of physiological and metabolic changes under hypoxia, but the effects of dietary nutrients on hypoxia tolerance have not been well evaluated. In the present 7-week feeding trial, we fed zebrafish with low-protein diet (LP), high-protein diet (HP), low-fat diet (LF), high-fat diet (HF), low-carbohydrate diet (LC), and high-carbohydrate diet (HC), respectively. Afterward, the resistance to acute hypoxia challenge, growth, body composition, activities of metabolic enzymes, and expressions of energy homeostasis-related genes and six hifαs genes were measured. The results indicated that only the HC diet could significantly improve the resistance to hypoxia challenge. Moreover, the HC diet feeding caused higher glycogen deposition in the liver and muscle, and these glycogens were significantly reduced after 6-h acute hypoxia challenge. Meanwhile, the lactate content in the liver and blood was increased in the HC groups. At hypoxia status, the relative mRNA expressions of the genes related to glycolysis, ATP production, insulin signaling pathway, and hif-3a (hif1al) were all significantly increased in the muscle of the HC diet-fed fish. This study revealed that high-carbohydrate diet could improve the resistance to hypoxia by activating glycolysis and hif/insulin signaling pathway in zebrafish, mainly in the muscle, to efficiently supply energy. Therefore, our results highlight the importance of dietary carbohydrate in resisting hypoxia in fish.

Interactive effect of thermal and hypoxia on largemouth bass (Micropterus salmoides) gill and liver: Aggravation of oxidative stress, inhibition of immunity and promotion of cell apoptosis

High temperatures and low oxygen in aquatic environments, such as intensive aquaculture or in natural watersheds, inevitably cause stress in fish. Fish are exposed to high temperatures during the summer, which exacerbates hypoxia. Hypoxia (1.2 ± 0.2 mg/L) under 20 °C (20 HG) and 26 °C (26 HG) was simulated to induce stress in largemouth bass (Micropterus salmoides). Related enzymes and genes involved in antioxidant, immune, and apoptotic responses were selected to explore the interactive effects of temperature and hypoxia on largemouth bass. The results showed that malondialdehyde (MDA) levels in plasma, gill, and liver increased in the 26 HG (p < 0.05). Liver superoxide dismutase (SOD) activity increased in the 26 HG. Peak SOD (SOD1, SOD2, SOD3a, and SOD3b), CAT, and GSH-Px mRNA levels in the gill and liver were observed at 12-24 h of stress. The levels of gill and liver total antioxidant capacity, catalase (CAT), glutathione peroxidase (GSH-Px) activities and other enzyme activities and genes in the 26 HG were higher than those in the 20 HG (p < 0.05). The gill and liver acid phosphatase and alkaline phosphatase activities increased with time in the 26 HG (p < 0.05), while gill and liver lysozyme activities in the 26 HG were lower than those in the 20 HG (p < 0.05). Tumor necrosis factor-α mRNA level was upregulated in the gill and downregulated in the liver at 24 h in the 26 HG. Interleukin (IL)-1β and IL-8 mRNA levels were upregulated in the gill and liver in the 26 HG at 24 h, whereas IL-15 mRNA level was downregulated in the 26 HG at 12 h. Transforming growth factor-β1 mRNA level was upregulated in the gill in the 20 HG at 24 h, but downregulated in gill and liver in the 26 HG at 24 h. Similarly, IL-10, Hepcidin-1, and Hepcidin-2 showed lower expression levels in the 26 HG. Gill and liver caspase-3 activities were higher in the 26 HG (p < 0.05), and gill caspase-3 activity was higher than that in the liver. The mRNA levels of proapoptotic genes (caspase-3, caspase-8, and caspase-9) were higher in the 26 HG. The present study demonstrates the interactive effects of temperature and hypoxia on stress in largemouth bass gill and liver. These results will be helpful to understand the mechanisms of stress induced by temperature and hypoxia in fish and provide a theoretical basis for aquaculture management.

MicroRNA-124 regulates lactate transportation in the muscle of largemouth bass (micropterus salmoides) under hypoxia by targeting MCT1

Carbohydrate metabolism switches from aerobic to anaerobic (glycolysis) to supply energy in response to acute hypoxic stress. Acute hypoxic stress with dissolved oxygen (DO) levels of 1.2 ± 0.1 mg/L for 24 h and 12 h re-oxygenation was used to investigate the response of the anaerobic glycolytic pathway in Micropterus salmoides muscle. The results showed that the glucose concentration was significantly lower in muscle, while the lactic acid and pyruvic acid concentrations tended to increase during hypoxic stress. No significant difference was observed in muscle glycogen, and ATP content fluctuated significantly. The activities of gluconeogenesis-related enzymes were slightly elevated, such as phosphoenolpyruvate carboxykinase (PEPCK). The activities of the glycolytic enzymes increased after the induction of hypoxia, such as hexokinase (HK), pyruvate kinase (PK), and lactate dehydrogenase (LDH). Curiously, phosphofructokinase (PFK) activity was significantly down-regulated within 4 h during hypoxia, although these effects were transient, and most indices returned to control levels after 12 h of re-oxygenation. Upregulated hif-1α, ampkα, hk, glut1, and ldh mRNA expression suggested that carbohydrate metabolism was reprogrammed under hypoxia. Lactate transport was regulated by miR-124-5p according to quantitative polymerase chain reaction and dual luciferase reporter assays. Our findings provide new insight into the molecular regulatory mechanism of hypoxia in Micropterus salmoides muscle.