Amino acids are major energy substrates for tissues of hybrid striped bass and zebrafish

Characteristics of the Digestive Tract of Dogs and Cats

As for other mammals, the digestive system of dogs (facultative carnivores) and cats (obligate carnivores) includes the mouth, teeth, tongue, pharynx, esophagus, stomach, small intestine, large intestine, and accessory digestive organs (salivary glands, pancreas, liver, and gallbladder). These carnivores have a relatively shorter digestive tract but longer canine teeth, a tighter digitation of molars, and a greater stomach volume than omnivorous mammals such as humans and pigs. Both dogs and cats have no detectable or a very low activity of salivary α-amylase but dogs, unlike cats, possess a relatively high activity of pancreatic α-amylase. Thus, cats select low-starch foods but dogs can consume high-starch diets. In contrast to many mammals, the vitamin B12 (cobalamin)-binding intrinsic factor for the digestion and absorption of vitamin B12 is produced in: (a) dogs primarily by pancreatic ductal cells and to a lesser extent the gastric mucosa; and (b) cats exclusively by the pancreatic tissue. Amino acids (glutamate, glutamine, and aspartate) are the main metabolic fuels in enterocytes of the foregut. The primary function of the small intestine is to digest and absorb dietary nutrients, and its secondary function is to regulate the entry of dietary nutrients into the blood circulation, separate the external from the internal milieu, and perform immune surveillance. The major function of the large intestine is to ferment undigested food (particularly fiber and protein) and to absorb water, short-chain fatty acids (serving as major metabolic fuels for epithelial cells of the large intestine), as well as vitamins. The fermentation products, water, sloughed cells, digestive secretions, and microbes form feces and then pass into the rectum for excretion via the anal canal. The microflora influences colonic absorption and cell metabolism, as well as feces quality. The digestive tract is essential for the health, survival, growth, and development of dogs and cats.

Integrated Amino Acids and Transcriptome Analysis Reveals Arginine Transporter SLC7A2 Is a Novel Regulator of Myogenic Differentiation

Skeletal muscle differentiation is a precisely coordinated process. While many of the molecular details of myogenesis have been investigated extensively, the dynamic changes and functions of amino acids and related transporters remain unknown. In this study, we conducted a comprehensive analysis of amino acid levels during different time points of C2C12 myoblast differentiation using high-performance liquid chromatography (HPLC). Our findings revealed that the levels of most amino acids exhibited an initial increase at the onset of differentiation, reaching their peak typically on the fourth or sixth day, followed by a decline on the eighth day. Particularly, arginine and branched-chain amino acids showed a prominent increase during this period. Furthermore, we used RNA-seq analysis to show that the gene encoding the arginine transporter, Slc7a2, is significantly upregulated during differentiation. Knockdown of Slc7a2 gene expression resulted in a significant decrease in myoblast proliferation and led to a reduction in the expression levels of crucial myogenic regulatory factors, hindering the process of myoblast differentiation, fusion, and subsequent myotube formation. Lastly, we assessed the expression level of Slc7a2 during aging in humans and mice and found an upregulation of Slc7a2 expression during the aging process. These findings collectively suggest that the arginine transporter SLC7A2 plays a critical role in facilitating skeletal muscle differentiation and may hold potential as a therapeutic target for sarcopenia.

Molecular mechanisms of physiological change under acute total dissolved gas supersaturation stress in yellow catfish (Pelteobagrus fulvidraco)

During the dam discharging period, the strong aeration of high-speed water leads to the supersaturation of total dissolved gas (TDG) in the downstream water, which causes gas bubble disease (GBD) in fish and threatens their survival. TDG supersaturation has now become an ecological and environmental issue of global concern; however, the molecular mechanism underlying the physiological effect of TDG supersaturation on fish is poorly known. Here, we comprehensively investigated the effect of TDG supersaturation on Pelteobagrus fulvidraco at the histopathological, biochemical, transcriptomic, and metabolomic levels. After exposure to 116% TDG, P. fulvidraco exhibited classic GBD symptoms and pathological changes in gills. The level of superoxide dismutase was highly significantly decreased. Transcriptomic results revealed that heat shock proteins (HSPs) and a large number of genes involved in immunity were increased by TDG stress. A key environmental sensor PI3K/Akt/mTOR pathway was significantly stimulated for defence against stress. Integrated transcriptomic and metabolomic analyses revealed that key metabolites and genes were upregulated in the triacylglycerol synthesis pathway and that amino acid levels decreased, which might be associated with TDG supersaturation stress. The present study demonstrated that TDG supersaturation could cause severe physiological damage in fish. HSP genes, immune functions, and energy metabolic pathways were enhanced to counteract the adverse effects.

The Effects of Lipoic Acid on Yolk Nutrient Utilization, Energy Metabolism, and Redox Balance over Time in Artemia sp

Lipoic acid (LA) is a mitochondrial coenzyme that, depending on the concentration and exposure time, can behave as an antioxidant or pro-oxidant agent and has a proven ability to modulate metabolism by promoting lipid and glucose oxidation for energy production. To assess the effects of LA on energy metabolism and redox balance over time, Artemia sp. nauplii was used as an animal model. The administered concentrations of the antioxidant were 0.05, 0.1, 0.5, 1.0, 5.0, and 10.0 µM. Therefore, possible differences in protein, triglyceride, glucose, and lactate concentrations in the artemia samples and total ammoniacal nitrogen (TAN) in the culture water were evaluated. We also measured the effects of LA on in vivo activity of the electron transport system (ETS), antioxidant capacity, and production of reactive oxygen species (ROS) at 6, 12, 18, and 24 h post-hatching. There was a decrease in glucose concentration in the LA-treated animals, and a decrease in ammonia production was observed in the 0.5 µM LA treatment. ETS activity was positively regulated by the addition of LA, with the most significant effects at concentrations of 5.0 and 10.0 µM at 12 and 24 h. For ETS activity, treatments with LA presented the highest values at 24 h, a period when ROS production decreased significantly, for the treatment with 10.0 µM. LA showed positive regulation of energy metabolism together with a decrease in ROS and TAN excretion.

Determining the Potential Roles of Branched-Chain Amino Acids in the Regulation of Muscle Growth in Common Carp (Cyprinus carpio) Based on Transcriptome and MicroRNA Sequencing

Branched-chain amino acids (BCAAs) can be critically involved in skeletal muscle growth and body energy homeostasis. Skeletal muscle growth is a complex process; some muscle-specific microRNAs (miRNAs) are involved in the regulation of muscle thickening and muscle mass. Additionally, the regulatory network between miRNA and messenger RNA (mRNA) in the modulation of the role of BCAAs on skeletal muscle growth in fish has not been studied. In this study, common carp was starved for 14 days, followed by a 14-day gavage therapy with BCAAs, to investigate some of the miRNAs and genes that contribute to the regulation of normal growth and maintenance of skeletal muscle in response to short-term BCAA starvation stress. Subsequently, the transcriptome and small RNAome sequencing of carp skeletal muscle were performed. A total of 43,414 known and 1,112 novel genes were identified, in addition to 142 known and 654 novel miRNAs targeting 22,008 and 33,824 targets, respectively. Based on their expression profiles, 2,146 differentially expressed genes (DEGs) and 84 differentially expressed miRNA (DEMs) were evaluated. Kyoto Encyclopedia of Genes and Genome pathways, including the proteasome, phagosome, autophagy in animals, proteasome activator complex, and ubiquitin-dependent protein catabolic process, were enriched for these DEGs and DEMs. Our findings revealed the role of atg5, map1lc3c, ctsl, cdc53, psma6, psme2, myl9, and mylk in skeletal muscle growth, protein synthesis, and catabolic metabolism. Furthermore, miR-135c, miR-192, miR-194, and miR-203a may play key roles in maintaining the normal activities of the organism by regulating genes related to muscle growth, protein synthesis, and catabolism. This study on transcriptome and miRNA reveals the potential molecular mechanisms underlying the regulation of muscle protein deposition and provides new insights into genetic engineering techniques to improve common carp muscle development.

Effect of Early Peptide Diets on Zebrafish Skeletal Development

Incorporation of dietary peptides has been correlated with decreased presence of skeletal abnormalities in marine larvae. In an attempt to clarify the effect of smaller protein fractions on fish larval and post-larval skeleton, we designed three isoenergetic diets with partial substitution of their protein content with 0% (C), 6% (P6) and 12% (P12) shrimp di- and tripeptides. Experimental diets were tested in zebrafish under two regimes, with inclusion (ADF-Artemia and dry feed) or lack (DF-dry feed only) of live food. Results at the end of metamorphosis highlight the beneficial effect of P12 on growth, survival and early skeletal quality when dry diets are provided from first feeding (DF). Exclusive feeding with P12 also increased the musculoskeletal resistance of the post-larval skeleton against the swimming challenge test (SCT). On the contrary, Artemia inclusion (ADF) overruled any peptide effect in total fish performance. Given the unknown species' larval nutrient requirements, a 12% dietary peptide incorporation is proposed for successful rearing without live food. A potential nutritional control of the larval and post-larval skeletal development even in aquaculture species is suggested. Limitations of the current molecular analysis are discussed to enable the future identification of the peptide-driven regulatory pathways.

Dietary glycine supplementation improves the growth performance of 110- to 240-g (phase II) hybrid striped bass (Morone saxatilis ♀× Morone chrysops ♂) fed soybean meal-based diets

We recently reported that supplementing glycine to soybean meal (SBM)-based diets is necessary for optimum growth of 5- to 40-g (phase I) hybrid striped bass (HSB). The present study tested the hypothesis that supplementing glycine to SBM-based diets may enhance the growth of 110- to 240-g (phase II) HSB. HSB (the initial body weight of approximately 110 g) were fed an SBM (58%)-based diet supplemented with 0%, 1%, or 2% of glycine, with l-alanine serving as the isonitrogenous control. There were four tanks per dietary group, with four fish per tank. The fish were fed their respective diets to apparent satiation twice daily. The feed intake and body weight of fish were recorded daily and every 2 wk, respectively. At the end of the 56-d feeding trial, plasma and tissue samples were collected to determine amino acid concentrations and histological alterations, and tissues were used to measure the oxidation of l-glutamate, l-glutamine, l-aspartate, and glycine. Results showed that dietary supplementation with 1% and 2% glycine dose-dependently increased (P < 0.05) the concentration of glycine in the plasma of HSB by 48% and 99%, respectively. Compared with the 0%-glycine group, dietary supplementation with 1% glycine did not affect (P > 0.05) the feed intake of HSB but increased (P < 0.05) their final body weight, weight gain, and gain:feed ratio during the whole period by 13%, 29%, and 21%, respectively. Compared with the 1% glycine group, dietary supplementation with 2% glycine increased (P < 0.05) the feed intake, final body weight, and weight gain of HSB by 13%, 7%, and 14%, respectively. Compared with the 0%-glycine group, fish fed with the 1%-glycine and 2%-glycine diets had a greater (P < 0.05) villus height in the proximal intestine, when compared with the 0%-glycine group. Collectively, these results indicated that SBM-based diets did not provide sufficient glycine for phase II HSB (110 to 240 g) and that dietary glycine supplementation is essential for their optimum growth and intestinal structure.

Dietary glycine supplementation enhances the growth performance of hybrid striped bass (Morone saxatilis ♀× Morone chrysops ♂) fed soybean meal-based diets

This study was conducted to test the hypothesis that supplementing 1% and 2% glycine to soybean meal (SBM)-based diets can improve the growth performance of juvenile hybrid striped bass (HSB). The basal diets contained 15% fishmeal and 58% SBM (DM basis). Alanine was used as the isonitrogenous control in different diets. All diets contained 44% crude protein and 10% lipids (DM basis). There were four tanks (15 fish per tank) per dietary group, with the mean of the initial body weight (BW) of fish being 5.3 g. Fish were fed to apparent satiation twice daily, and their BW was recorded every 2 wk. The trial lasted for 8 wk. Results indicated that the BW, weight gain, protein efficiency ratio, and retention of dietary lipids in fish were enhanced (P < 0.05) by dietary supplementation with 1% or 2% glycine. In addition, dietary supplementation with glycine did not affect (P > 0.05) the feed intake of fish but increased (P < 0.05) the retention of dietary nitrogen, most amino acids, and phosphorus in the body, compared to the 0% glycine group. Dietary supplementation with 1% and 2% glycine dose-dependently augmented (P < 0.05) the villus height of the proximal intestine and reduced the submucosal thickness of the gut, while preventing submucosal and lamina propria hemorrhages. Compared with the 0% glycine group, dietary supplementation with 1% or 2% glycine decreased (P < 0.05) the proportion of skeletal-muscle fibers with diameters of 40 to 60 µm but increased (P < 0.05) the proportion of skeletal-muscle fibers with diameters of 80 to 100 µm and > 100 µm. Collectively, these findings indicate that glycine in SBM-based diets is inadequate for maximum growth of juvenile HSB and that dietary supplementation with 1% or 2% glycine is required to improve their weight gain and feed efficiency. Glycine is a conditionally essential amino acid for this fish.

Metabolomic responses in livers of female and male zebrafish (Danio rerio) following prolonged exposure to environmental levels of zinc oxide nanoparticles

Zinc oxide nanoparticles (ZnONPs) are widespread pollutants that are present in diverse environmental samples. Here, we determined metabolomic and bioenergetic responses in the liver of female and male zebrafish exposed to a prolonged environmentally relevant concentration of ZnONPs. Metabolome analysis revealed that exposure to 500 μg/L ZnONPs reduced the abundance of metabolites in the tricarboxylic acid (TCA) cycle by modulating the activities of rate-limiting enzymes α-ketoglutarate dehydrogenase and isocitrate dehydrogenase. Moreover, oxidative phosphorylation (OXPHOS) was negatively impacted in the liver based upon decreased activities of mitochondrial Complex I and V in both female and male livers. Our results revealed that bioenergetic responses were not attributed to dissolved Zn²⁺ and were not sex-specific. However, the metabolic responses in liver following exposure to ZnONPs did show sex-specific responses. Females exposed to ZnONPs compensated for the energetic stress via increasing fatty acids and amino acids metabolism, while males compensated to ZnONPs exposure by adjusting amino acids metabolism, based upon transcript profiles. This study demonstrates that zebrafish adjust the transcription of metabolic enzymes in the liver to compensate for metabolic disruption following ZnONPs exposure. Taken together, this study contributes to a comprehensive understanding of risks related to ZnONPs exposure in relation to metabolic activity in the liver. Environmental implication Zinc oxide nanoparticles (ZnONPs) are widely used in industry and are subsequently released into environments. However, biological responses between female and male following ZnONPs exposure has never been compared. Our data revealed for the first time that female and male zebrafish showed comparable bioenergetic responses, but different metabolic responses to ZnONPs at an environmentally relevant dose. Females compensated for the energetic stress via increasing fatty acids and amino acids metabolism, while males compensated to ZnONPs exposure by adjusting amino acids metabolism in livers. This study reveals that sex may be an important variable to consider in risk assessments of nanoparticles released into environments.

Different Types of Non-Starch Polysaccharides Alter the Growth, Intestinal Flora and Serum Metabolite Profile of Grass Carp, Ctenopharyngodon idella

Dietary non-starch polysaccharides (NSPs) broadly influence fish intestinal flora and physiological metabolism, but limited information is available on grass carp (Ctenopharyngodon idella). This study investigated the effects of different types of NSPs on the growth, nutrient metabolism status, gut microbiota, and serum metabolome of grass carp. Fish were fed with diets containing 4.4% insoluble NSPs (INSP), 9.24% soluble NSPs (SNSP), 13.64% NSPs (4.4% INSP + 9.24% SNSP, NSP) and non NSPs (FM), respectively, for 9 weeks. Results showed that dietary SNSP decreased protein efficiency ratio and serum protein content, but increased feed coefficient ratio, feed intake, plasma blood urea nitrogen content, and plasma aspartate aminotransferase activity (AST); conversely, dietary INSP decreased plasma AST activity. Dietary INSP and SNSP increased serum free cholesterol content. Dietary NSPs altered the abundance of dominant bacteria and serum metabolite profiles. The differential metabolites between groups were significantly enriched in amino acid synthesis and metabolic pathways. In conclusion, dietary INSP exhibited a growth-promoting effect compared to SNSP. Dietary INSP is beneficial for improving nutrient metabolism and intestinal health. Moreover, dietary NSPs may regulate the physiological metabolism and feeding behavior of grass carp by altering amino acid synthesis and metabolism.

Intestinal metabolomics of juvenile lenok (Brachymystax lenok) in response to heat stress

Changes in the metabolic profile within the intestine of lenok (Brachymystax lenok) when challenged to acute and lethal heat stress (HS) are studied using no-target HPLC-MS/MS metabonomic analysis. A total of 51 differentially expressed metabolites (VIP > 1, P < 0.05) were identified in response to HS, and 34 occurred in the positive ion mode and 17 in negative ion mode, respectively. After heat stress, changes in metabolites related to glycolysis (i.e., alpha-D-glucose, stachyose, and L-lactate) were identified. The metabolites (acetyl carnitine, palmitoylcarnitine, carnitine, and erucic acid) related to fatty acid β-oxidation accumulated significantly, and many amino acids (L-tryptophan, D-proline, L-leucine, L-phenylalanine, L-aspartate, L-tyrosine, L-methionine, L-histidine, and L-glutamine) were significantly decreased in HS-treated lenok. The mitochondrial β-oxidation pathway might be inhibited, while severe heat stress might activate the anaerobic glycolysis and catabolism of amino acid for energy expenditure. Oxidative damage in HS-treated lenok was indicated by the decreased glycerophospholipid metabolites (i.e., glycerophosphocholine, 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphoethanolamine, 1-palmitoyl-sn-glycero-3-phosphocholine, 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine, and 1, 2-dioleoyl-sn-glycero-3-phosphatidylcholine) and the increased oxylipin production (12-HETE and 9R, 10S-EpOME). The minor oxidative pathways (omega-oxidation and peroxisomal beta-oxidation) were likely to be induced in HS-treated lenok.

LC-qTOF-MS analysis of fish immune organs reveals the distribution of amino acids in response to metabolic adaptation of the survival phenotype in grouper against Vibrio infection

Epinephelus fuscoguttatus is economically crucial to various Southeast Asia countries where they are reared in fish farms to meet the demand for supply. However, a systemic infectious disease known as vibriosis has steadily and extensively affected the fish farming industry. The disease is caused by Vibrio spp., which are pathogenic gram-negative bacteria. This study focused on understanding the host's metabolic adaptation against Vibrio vulnificus infection, which features a survival phenotype, by profiling the metabolites in grouper fingerlings that survived the experimental infection. Mapping of the pathways is crucial to explain the roles of metabolites in fish immunity. A solvent extraction method was used on the grouper's immune organs (gills, liver and spleen) prior to Liquid Chromatography-Quadrupole Time-of-Flight Mass Spectrometry (LC-qTOF-MS) analysis. The metabolites identified in fingerlings that survived experimental infections were mostly amino acids (primary metabolites). Glutamine (0.44%), alanine (0.68%), phenylalanine (2.63%) and tyrosine (2.60%) were highly abundant in survived-infected gills. Aspartic acid (13.57%) and leucine (4.01%) were highly abundant in the livers of the survived-infected fish and lysine was highly abundant in both gills (2.94%) and liver (3.64%) of the survived-infected fish. Subsequent bioinformatics analysis revealed the involvement of the identified functional amino acids in various immune-related pathways. The current findings facilitate the comprehension of the metabolic adaptation of grouper fingerlings that exhibited a survival phenotype against Vibrio infection.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03269-1.

Proteomic analysis of hepatic fibrosis induced by a high starch diet in largemouth bass (Micropterus salmoides)

Largemouth bass is sensitive to the dietary starch level and excess starch can induce metabolic liver diseases (MLD). Hepatic fibrosis is a typical pathological phenotype of MLD in largemouth bass, but the molecular basis underlying is largely unclear. This study fed fish with a low or high starch diet for 4 weeks. Liver tissues with or without fibrotic symptoms were recognized through histopathological and molecular markers analysis of hepatic fibrosis, following TMT Quantitative proteomics and conducted Parallel Reaction Monitoring (PRM) analyses. 2455 differentially expressed proteins with 1618 up-regulated and 837 down-regulated were identified in this study. In GO terms, up-regulated proteins were correlated with cytoskeleton organization, supramolecular fiber, cytoskeleton protein binding, and actin-binding, while down-regulated proteins were involved in mainly metabolism-related processes, and molecular binding activity. Down-regulated proteins were enriched in 63 KEGG pathways and concentrated in metabolism-related pathways, especially glucose, lipid, and amino acid metabolism. 70 KEGG pathways of up-regulated proteins mainly included immunity and inflammation-related pathways. The expression trends of 11 differentially expressed proteins were consistent with proteome results by PRM analysis. In conclusion, the development of hepatic fibrosis induced by high starch may be related to multi-signaling pathways, metabolism processes, and targets, which provides important data for further study on revealing the molecular mechanism of hepatic fibrosis.

Nitrate exposure reprograms hepatic amino acid and nutrient sensing pathways prior to exercise: A metabolomic and transcriptomic investigation in zebrafish (Danio rerio)

Scope: Nitrate supplementation is a popular ergogenic aid that improves exercise performance by reducing oxygen consumption during exercise. We investigated the effect of nitrate exposure and exercise on metabolic pathways in zebrafish liver.

Materials and methods: Fish were exposed to sodium nitrate (606.9 mg/L), or control water, for 21 days and analyzed at intervals during an exercise test. We utilized untargeted liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis and measured gene expression of 24 genes central to energy metabolism and redox signaling.

Results: We observed a greater abundance of metabolites involved in endogenous nitric oxide (NO) metabolism and amino acid metabolism in nitrate-treated liver at rest, compared to rested controls. In the absence of exercise, nitrate treatment upregulated expression of genes central to nutrient sensing (pgc1a), protein synthesis (mtor) and purine metabolism (pnp5a and ampd1) and downregulated expression of genes involved in mitochondrial fat oxidation (acaca and cpt2).

Conclusion: Our data support a role for sub-chronic nitrate treatment in the improvement of exercise performance, in part, by improving NO bioavailability, sparing arginine, and modulating hepatic gluconeogenesis and glycolytic capacity in the liver.

Ovine conceptus homogenates metabolize fructose for metabolic support during the peri-implantation period of pregnancy

Roles of fructose in elongating ovine conceptuses are poorly understood, despite it being the major hexose sugar in fetal fluids and plasma throughout gestation. Therefore, we determined if elongating ovine conceptuses utilize fructose via metabolic pathways for survival and development. Immunohistochemical analyses revealed that trophectoderm and extra-embryonic endoderm express ketohexokinase and aldolase B during the peri-implantation period of pregnancy for conversion of fructose into fructose-1-phosphate for entry into glycolysis and related metabolic pathways. Conceptus homogenates were cultured with 14C-labeled glucose and/or fructose under oxygenated and hypoxic conditions to assess contributions of glucose and fructose to the pentose cycle (PC), tricarboxylic acid cycle, glycoproteins, and lipid synthesis. Results indicated that both glucose and fructose contributed carbons to each of these pathways, except for lipid synthesis, and metabolized to pyruvate and lactate, with lactate being the primary product of glycolysis under oxygenated and hypoxic conditions. We also found that: 1) conceptuses preferentially oxidized glucose over fructose (P < 0.05); 2) incorporation of fructose and glucose at 4 mM each into the PC by Day 17 conceptus homogenates was similar in the presence or absence of glucose, but incorporation of glucose into the PC was enhanced by the presence of fructose (P < 0.05); 3) incorporation of fructose into the PC in the absence of glucose was greater under oxygenated conditions (P < 0.01); and 4) incorporation of glucose into the PC under oxygenated conditions was greater in the presence of fructose (P = 0.05). These results indicate that fructose is an important metabolic substrate for ovine conceptuses.

Effects of heat shock on energy metabolism and antioxidant defence in a tropical fish species Psalidodon bifasciatus

Predictions about global warming have raised interest in assessing whether ectothermic organisms will be able to adapt to these changes. Understanding the physiological mechanisms and metabolic adjustment capacity of fish subjected to heat stress can provide subsidies that may contribute to decision-making in relation to ecosystems and organisms subjected to global climate change. This study investigated the antioxidant defence system and energy metabolism of carbohydrate and protein responses in the gill, liver and kidney tissues of Psalidodon bifasciatus (Garavello & Sampaio 2010), a Brazilian freshwater fish used in aquaculture and in biological studies, following exposure to heat shock at 31°C for 2, 6, 12, 24 and 48 h. The fish presented signs of stress in all tissues tested, as evidenced by increased lipid peroxidation concentration at 2 h and phosphofructokinase, hexokinase and malate dehydrogenase activity at 48 h in the gills; increased glutathione-S-transferase activity at 12 h, citrate synthase activity at 24 h and concentration of reduced glutathione (GSH) concentration at 12 and 48 h in the liver; and through increased activity of superoxide dismutase at 48 h, glutathione reductase at 24 h, glucose-6-phosphate dehydrogenase at 48 h and concentration of GSH at 24 h in the kidney. In the kidneys, changes in the antioxidant system were more prominent, whereas in the gills, there were greater changes in the carbohydrate metabolism. These results indicated the importance of glycolysis and aerobic metabolism in the gills, aerobic metabolism in the liver and pentose-phosphate pathway in the kidneys during homeostasis. The biomarker response was tissue specific, with the greatest number of biomarkers altered in the gills, followed by those in the kidneys and liver.

Oxidation of amino acids, glucose, and fatty acids as metabolic fuels in enterocytes of developing pigs

Enterocytes of young pigs are known to use glutamine, glutamate, and glucose as major metabolic fuels. However, little is known about the roles of aspartate, alanine, and fatty acids as energy sources for these cells. Therefore, this study simultaneously determined the oxidation of the amino acids and glucose as well as short- and long-chain fatty acids in enterocytes of developing pigs. Jejunal enterocytes were isolated from 0-, 7-, 14- and 21-day-old piglets, and incubated at 37 °C for 30 min in Krebs-Henseleit bicarbonate buffer (pH 7.4) containing 5 mM D-glucose and one of the following: D-[U-¹⁴C]glucose, 0.5-5 mM L-[U-¹⁴C]glutamate, 0.5-5 mM L-[U-¹⁴C]glutamine, 0.5-5 mM L-[U-¹⁴C]aspartate, 0.5-5 mM L-[U-¹⁴C]alanine, 0.5-2 mM L-[U-¹⁴C]palmitate, 0.5-5 mM [U-¹⁴C]propionate, and 0.5-5 mM [1-¹⁴C]butyrate. At the end of the incubation, ¹⁴CO₂ produced from each ¹⁴C-labeled substrate was collected. Rates of oxidation of each substrate by enterocytes from all age groups of piglets increased (P < 0.05) gradually with increasing its extracellular concentrations. The rates of oxidation of glutamate, glutamine, aspartate, and glucose by enterocytes from 0- to 21-day-old pigs and of alanine from newborn pigs were much greater (P < 0.05) than those for the same concentrations of palmitate, propionate, and butyrate. Compared with 0-day-old pigs, the rates of oxidation of glutamate, aspartate, glutamine, alanine, and glucose by enterocytes from 21-day-old pigs decreased (P < 0.05) markedly, without changes in palmitate oxidation. Oxidation of alanine, propionate, butyrate and palmitate by enterocytes of pigs was limited during their postnatal growth. At each postnatal age, the oxidation of glutamate, glutamine, aspartate, and glucose produced much more ATP than alanine, propionate, butyrate and palmitate. The degradation of glutamate was initiated primarily by glutamate-pyruvate and glutamate-oxaloacetate transaminases. Our results indicated that amino acids (glutamate plus glutamine plus aspartate) are the major metabolic fuels in enterocytes of 0- to 21-day-old pigs.

Oxidation of amino acids, glucose, and fatty acids as metabolic fuels in enterocytes of post-hatching developing chickens

This study determined the oxidation of amino acids, glucose and fatty acid in enterocytes of developing chickens. Jejunal enterocytes were isolated from 0-, 7-, 21-, and 42-d-old broiler chickens, and incubated at 40°C for 30 min in Krebs-Henseleit bicarbonate buffer (pH 7.4) containing 5 mM D-glucose and one of the following: 0.5-5 mM L-[U-14C]glutamate, 0.5-5 mM L-[U-14C]glutamine, 0.5-5 mM L-[U-14C]aspartate, 0.5-5 mM L-[U-14C]alanine, 0.5-2 mM [U-14C]palmitate, D-[U-14C]glucose, 0.5-5 mM [U-14C]propionate, and 0.5-5 mM [1-14C]butyrate. 14CO2 produced from each 14C-labeled substrate was collected for determination of radioactivity. Among all the substrates studied, glutamate had the greatest rate of oxidation in enterocytes from 0- to 42-d-old chickens. Glutamate transaminases, rather than glutamate dehydrogenase, may be primarily responsible for initiating glutamate degradation. Rates of amino acid and fatty acid oxidation by cells increased (P < 0.05) with increasing their extracellular concentrations from 0.5 to 5 mM. Rates of glutamate and glucose oxidation in enterocytes decreased (P < 0.05) with increasing age, and rates of glutamine, aspartate, propionate, and butyrate oxidation were lower (P < 0.05) in 42-d-old chickens than in 0-d-old chickens. By contrast, oxidation of palmitate at 2 mM increased (P < 0.05) by 118% in cells from 42-d-old chickens, compared with 0-d-old chickens. Compared with glutamate, oxidation of glutamine, aspartate, alanine, propionate, butyrate, and palmitate was limited in cells from all age groups of chickens. Collectively, these results indicate that glutamate is the major metabolic fuel in enterocytes of 0- to 42-d-old chickens.

Hepatic Glucose Metabolism and Its Disorders in Fish

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

Protein-Sourced Feedstuffs for Aquatic Animals in Nutrition Research and Aquaculture

Aquatic animals have particularly high requirements for dietary amino acids (AAs) for health, survival, growth, development, and reproduction. These nutrients are usually provided from ingested proteins and may also be derived from supplemental crystalline AA. AAs are the building blocks of protein (a major component of tissue growth) and, therefore, are the determinants of the growth performance and feed efficiency of farmed fish. Because protein is generally the most expensive ingredient in aqua feeds, much attention has been directed to ensure that dietary protein feedstuff is of high quality and cost-effective for feeding fish, crustaceans, and other aquatic animals worldwide. Due to the rapid development of aquaculture worldwide and a limited source of fishmeal (the traditionally sole or primary source of AAs for aquatic animals), alternative protein sources must be identified to feed aquatic animals. Plant-sourced feedstuffs for aquatic animals include soybean meal, extruded soybean meal, fermented soybean meal, soybean protein concentrates, soybean protein isolates, leaf meal, hydrolyzed plant protein, wheat, wheat hydrolyzed protein, canola meal, cottonseed meal, peanut meal, sunflower meal, peas, rice, dried brewers grains, and dried distillers grains. Animal-sourced feedstuffs include fishmeal, fish paste, bone meal, meat and bone meal, poultry by-product meal, chicken by-product meal, chicken visceral digest, spray-dried poultry plasma, spray-dried egg product, hydrolyzed feather meal, intestine-mucosa product, peptones, blood meal (bovine or poultry), whey powder with high protein content, cheese powder, and insect meal. Microbial sources of protein feedstuffs include yeast protein and single-cell microbial protein (e.g., algae); they have more balanced AA profiles than most plant proteins for animal feeding. Animal-sourced ingredients can be used as a single source of dietary protein or in complementary combinations with plant and microbial sources of proteins. All protein feedstuffs must adequately provide functional AAs for aquatic animals.

Comprehensive Interrogation of Metabolic and Bioenergetic Responses of Early-Staged Zebrafish (Danio rerio) to a Commercial Copper Hydroxide Nanopesticide

The use of copper hydroxide nanopesticide can pose exposure risks to aquatic organisms. In this study, the toxicity of a copper hydroxide nanopesticide, compared to conventional copper sulfate at environmentally relevant doses, was evaluated using metabolomics and bioenergetic assays in embryonic zebrafish. At a copper concentration of 100 μg/L, the nanopesticide caused higher mortality and deformity compared to copper ions alone; despite higher copper accumulation, increased metallothionein and elevated ATP-binding cassette (ABC) transporter activity in zebrafish exposed to copper ions were observed. Both nanopesticide and copper ions reduced the abundance of metabolites of glycolysis and induced energetic stress in zebrafish. The nanopesticide also increased concentrations of several organic acids involved in the tricarboxylic acid (TCA) cycle and elevated the activity of isocitrate dehydrogenase and α-ketoglutarate dehydrogenase, suggesting enhanced TCA cycle activity. Nanopesticide exposure depleted both glutamate and glutamine parallel to the upregulation of the TCA cycle. In addition, zebrafish exposed to the nanopesticide appeared to shift metabolism toward amino acid catabolism and lipid accumulation based upon altered expression profiles of glutaminase, glutamate dehydrogenase, fatty acid synthase, and acetyl-CoA carboxylase. Lastly, the ability of the ions to increase oxidative phosphorylation to alleviate energetic stress was reduced in the case of the nanopesticide. We hypothesize that, unlike copper ions alone, the nanopesticide induces higher toxicity to zebrafish because of increased protein catabolism. This study provides a comprehensive understanding of the risks of copper hydroxide nanopesticide exposure in relation to metabolic activity and mitochondrial function.

Growth Hormone Overexpression Induces Hyperphagia and Intestinal Morphophysiological Adaptations to Improve Nutrient Uptake in Zebrafish

The excess of circulating growth hormone (GH) in most transgenic animals implies mandatory growth resulting in higher metabolic demand. Considering that the intestine is the main organ responsible for the digestion, absorption, and direction of dietary nutrients to other tissues, this study aimed to investigate the mechanisms by which gh overexpression modulates the intestine to support higher growth. For this purpose, we designed an 8-weeks feeding trial to evaluate growth parameters, feed intake, and intestinal morphometric indices in the adult gh-transgenic zebrafish (Danio rerio) model. To access the sensitivity of the intestine to the excess of circulating GH, the messenger RNA (mRNA) expression of intestine GH receptors (GHRs) (ghra and ghrb) was analyzed. In addition, the expression of insulin-like growth factor 1a (igf1a) and genes encoding for di and tripeptide transporters (pept1a and pept1b) were assessed. Gh-transgenic zebrafish had better growth performance and higher feed intake compared to non-transgenic sibling controls. Chronic excess of GH upregulates the expression of its cognate receptor (ghrb) and the main growth factor related to trophic effects in the intestine (igf1a). Moreover, transgenic zebrafish showed an increased intestinal absorptive area and higher expression of crucial genes related to the absorption of products from meal protein degradation. These results reinforce the ability of GH to modulate intestinal morphology and the mechanisms of assimilation of nutrients to sustain the energy demand for the continuous growth induced by the excess of circulating GH.

Investigation of dependences of the morphological composition of body and amino acid composition of trout meat proteins (Oncorhynchus mykiss) on levels of the energy value of feeds

The article studies the effect of using complete compound feeds with different energy levels on the morphological composition of the body and the amino acid composition of trout meat proteins. The experiment aimed to establish the influence of different levels of energy nutrition of commercial rainbow trout on the morphological composition of their body and the amino acid composition of meat proteins. For this purpose, five experimental groups were formed using the analog method. The study lasted 210 days and was divided into two periods: comparative (10 days) and main (200 days). During the comparative period, the study fish consumed compound feed of the control group. During the main period, the energy level in experimental compound feeds for different experimental trout groups ranged from 16 to 20 mJ per 1 kg. It was found that with an increase in the mass of two-year-old trout, the mass of muscle tissue and the yield of edible parts probably increases. Feeding fish with an increased amount of metabolic energy (20 mJ.kg-1) resulted in a significant change in the weight of internal organs, including the heart, liver, and kidneys. Increasing the metabolic energy in the compound feeds of fish from 18 mJ.kg-1 to 19 – 20 mJ.kg-1 leads to a significant increase in bowel mass by 13.3 – 5.0%. An increase in the level of metabolic energy in rainbow trout diets from 18 mJ.kg-1 to 19 – 20 mJ.kg-1 contributed to a likely increase in the methionine content in meat. A similar pattern was observed for the tryptophan content. It was found that the content of most essential amino acids in the protein of rainbow trout meat exceeds the corresponding values in the "ideal" protein, except for the content of isoleucine and leucine, which refers them to limiting amino acids.

Nitrate-induced improvements in exercise performance are coincident with exuberant changes in metabolic genes and the metabolome in zebrafish (Danio rerio) skeletal muscle

Dietary nitrate supplementation improves exercise performance by reducing the oxygen cost of exercise and enhancing skeletal muscle function. However, the mechanisms underlying these effects are not well understood. The purpose of this study was to assess changes in skeletal muscle energy metabolism associated with exercise performance in a zebrafish model. Fish were exposed to sodium nitrate (60.7 mg/L, 303.5 mg/L, 606.9 mg/L), or control water, for 21 days and analyzed at intervals (5, 10, 20, 30, 40 cm/s) during a 2-h strenuous exercise test. We measured oxygen consumption during an exercise test and assessed muscle nitrate concentrations, gene expression, and the muscle metabolome before, during, and after exercise. Nitrate exposure reduced the oxygen cost of exercise and increased muscle nitrate concentrations at rest, which were reduced with increasing exercise duration. In skeletal muscle, nitrate treatment upregulated expression of genes central to nutrient sensing (mtor), redox signaling (nrf2a), and muscle differentiation (sox6). In rested muscle, nitrate treatment increased phosphocreatine (P = 0.002), creatine (P = 0.0005), ATP (P = 0.0008), ADP (P = 0.002), and AMP (P = 0.004) compared with rested-control muscle. Following the highest swimming speed, concentration of phosphocreatine (P = 8.0 × 10^-5), creatine (P = 6.0 × 10^-7), ATP (P = 2.0 × 10^-6), ADP (P = 0.0002), and AMP (P = 0.004) decreased compared with rested nitrate muscle. Our data suggest nitrate exposure in zebrafish lowers the oxygen cost of exercise by changing the metabolic programming of muscle prior to exercise and increasing availability of energy-rich metabolites required for exercise.NEW & NOTEWORTHY We show that skeletal muscle nitrate concentration is higher with supplementation at rest and was lower in groups with increasing exercise duration in a zebrafish model. The higher availability of nitrate at rest is associated with upregulation of key nutrient-sensing genes and greater availability of energy-producing metabolites (i.e., ATP, phosphocreatine, glycolytic intermediates). Overall, nitrate supplementation may lower oxygen cost of exercise through improved fuel availability resulting from metabolic programming of muscle prior to exercise.

Limits of temperature adaptation and thermopreferendum

Background

Managing the limits of temperature adaptation is relevant both in medicine and in biotechnology. There are numerous scattered publications on the identification of the temperature limits of existence for various organisms and using different methods. Dmitry Petrovich Kharakoz gave a general explanation for many of these experimental results. The hypothesis implied that each cycle of synaptic exocytosis includes reversible phase transitions of lipids of the presynaptic membrane due to the entry and subsequent removal of calcium ions from the synaptic terminal. The correspondence of the times of phase transitions has previously been experimentally shown on isolated lipids in vitro. In order to test the hypothesis of D.P. Kharakoz in vivo, we investigated the influence of the temperature of long-term acclimatization on the temperature of heat and cold shock, as well as on the kinetics of temperature adaptation in zebrafish. Testing the hypothesis included a comparison of our experimental results with the results of other authors obtained on various models from invertebrates to humans.

Results

The viability polygon for Danio rerio was determined by the minimum temperature of cold shock (about 6 °C), maximum temperature of heat shock (about 43 °C), and thermopreferendum temperature (about 27 °C). The ratio of the temperature range of cold shock to the temperature range of heat shock was about 1.3. These parameters obtained for Danio rerio describe with good accuracy those for the planarian Girardia tigrina, the ground squirrel Sermophilus undulatus, and for Homo sapiens.

Conclusions

The experimental values of the temperatures of cold shock and heat shock and the temperature of the thermal preferendum correspond to the temperatures of phase transitions of the lipid-protein composition of the synaptic membrane between the liquid and solid states. The viability range for zebrafish coincides with the temperature range, over which enzymes function effectively and also coincides with the viability polygons for the vast majority of organisms. The boundaries of the viability polygon are characteristic biological constants. The viability polygon of a particular organism is determined not only by the genome, but also by the physicochemical properties of lipids that make up the membrane structures of synaptic endings. The limits of temperature adaptation of any biological species are determined by the temperature range of the functioning of its nervous system.

Nutrition and Functions of Amino Acids in Fish

Aquaculture is increasingly important for providing humans with high-quality animal protein to improve growth, development and health. Farm-raised fish and shellfish now exceed captured fisheries for foods. More than 70% of the production cost is dependent on the supply of compound feeds. A public debate or concern over aquaculture is its environmental sustainability as many fish species have high requirements for dietary protein and fishmeal. Protein or amino acids (AAs), which are the major component of tissue growth, are generally the most expensive nutrients in animal production and, therefore, are crucial for aquatic feed development. There is compelling evidence that an adequate supply of both traditionally classified nutritionally essential amino acids (EAAs) and non-essential amino acids (NEAAs) in diets improve the growth, development and production performance of aquatic animals (e.g., larval metamorphosis). The processes for the utilization of dietary AAs or protein utilization by animals include digestion, absorption and metabolism. The digestibility and bioavailability of AAs should be carefully evaluated because feed production processes and AA degradation in the gut affect the amounts of dietary AAs that enter the blood circulation. Absorbed AAs are utilized for the syntheses of protein, peptides, AAs, and other metabolites (including nucleotides); biological oxidation and ATP production; gluconeogenesis and lipogenesis; and the regulation of acid-base balance, anti-oxidative reactions, and immune responses. Fish producers usually focus on the content or digestibility of dietary crude protein without considering the supply of AAs in the diet. In experiments involving dietary supplementation with AAs, inappropriate AAs (e.g., glycine and glutamate) are often used as the isonitrogenous control. At present, limited knowledge is available about either the cell- and tissue-specific metabolism of AAs or the effects of feed processing methods on the digestion and utilization of AAs in different fish species. These issues should be addressed to develop environment-friendly aquafeeds and reduce feed costs to sustain the global aquaculture.

Amino Acids in the Nutrition, Metabolism, and Health of Domestic Cats

Domestic cats (carnivores) require high amounts of dietary amino acids (AAs) for normal growth, development, and reproduction. Amino acids had been traditionally categorised as nutritionally essential (EAAs) or nonessential (NEAAs), depending on whether they are synthesized de novo in the body. This review will focus on AA nutrition and metabolism in cats. Like other mammals, cats do not synthesize the carbon skeletons of twelve proteinogenic AAs: Arg, Cys, His, Ile, Leu, Lys, Met, Phe, Thr, Trp, Tyr, and Val. Like other feline carnivores but unlike many mammals, cats do not synthesize citrulline and have a very limited ability to produce taurine from Cys. Except for Leu and Lys that are strictly ketogenic AAs, most EAAs are both glucogenic and ketogenic AAs. All the EAAs (including taurine) must be provided in diets for cats. These animals are sensitive to dietary deficiencies of Arg and taurine, which rapidly result in life-threatening hyperammonemia and retinal damage, respectively. Although the National Research Council (NCR, Nutrient requirements of dogs and cats. National Academies Press, Washington, DC, 2006) does not recommend dietary requirements of cats for NEAAs, much attention should be directed to this critical issue of nutrition. Cats can synthesize de novo eight proteinogenic AAs: Ala, Asn, Asp, Gln, Glu, Gly, Pro, and Ser, as well as some nonproteinogenic AAs, such as γ-aminobutyrate, ornithine, and β-alanine with important physiological functions. Some of these AAs (e.g., Gln, Glu, Pro, and Gly) are crucial for intestinal integrity and health. Except for Gln, AAs in the arterial blood of cats may not be available to the mucosa of the small intestine. Plant-source foodstuffs lack taurine and generally contain inadequate Met and Cys and, therefore, should not be fed to cats in any age group. Besides meat, animal-source foodstuffs (including ruminant meat & bone meal, poultry by-product meal, porcine mucosal protein, and chicken visceral digest) are good sources of proteinogenic AAs and taurine for cats. Meeting dietary requirements for both EAAs and NEAAs in proper amounts and balances is crucial for improving the health, wellbeing, longevity, and reproduction of cats.

Putative imbalanced amino acid metabolism in rainbow trout long term fed a plant-based diet as revealed by 1H-NMR metabolomics

The long-term effect of a plant (P)-based diet was assessed by proton nuclear magnetic resonance (¹H-NMR) metabolomics in rainbow trout fed a marine fish meal (FM)-fish oil (FO) diet (M), a P-based diet and a control commercial-like diet (C) starting with the first feeding. Growth performances were not heavily altered by long-term feeding on the P-based diet. An ¹H-NMR metabolomic analysis of the feed revealed significantly different soluble chemical compound profiles between the diets. A set of soluble chemical compounds was found to be specific either to the P-based diet or to the M diet. Pterin, a biomarker of plant feedstuffs, was identified both in the P-based diet and in the plasma of fish fed the P-based diet. ¹H-NMR metabolomic analysis on fish plasma and liver and muscle tissues at 6 and 48 h post feeding revealed significantly different profiles between the P-based diet and the M diet, while the C diet showed intermediate results. A higher amino acid content was found in the plasma of fish fed the P-based diet compared with the M diet after 48 h, suggesting either a delayed delivery of the amino acids or a lower amino acid utilisation in the P-based diet. This was associated with an accumulation of essential amino acids and the depletion of glutamine in the muscle, together with an accumulation of choline in the liver. Combined with an anticipated absorption of methionine and lysine supplemented in free form, the present results suggest an imbalanced essential amino acid supply for protein metabolism in the muscle and for specific functions of the liver.

Nutrition and Functions of Amino Acids in Aquatic Crustaceans

Crustaceans (e.g., shrimp and crabs) are a good source of protein-rich foods for human consumption. They are the second largest aquaculture species worldwide. Understanding the digestion of dietary protein, as well as the absorption, metabolism and functions of amino acids (AAs) and small peptides is essential to produce cost-effective and sustainable aquafeeds. Hepatopancreas (the midgut gland) is the main site for the digestion of dietary protein as well as the absorption of small peptides and AAs into the hemolymph. Besides serving as the building blocks of protein, AAs (particularly aspartate, glutamate, glutamine and alanine) are the primary metabolic fuels for the gut and extra-hepatopancreas tissues (e.g., kidneys and skeletal muscle) of crustaceans. In addition, AAs are precursors for the syntheses of glucose, lipids, H₂S, and low-molecular-weight molecules (e.g., nitric oxide, glutathione, polyamines, histamine, and hormones) with enormous biological importance, such as physical barrier, immunological and antioxidant defenses. Therefore, both nutritionally essential and nonessential AAs are needed in diets to improve the growth, development, molt rate, survival, and reproduction of crustaceans. There are technical difficulties and challenges in the use of crystalline AAs for research and practical production due to the loss of free AAs during feed processing, the leaching of in-feed free AAs to the surrounding water environment, and asynchronous absorption with peptide-bounded AAs. At present, much knowledge about AA metabolism and functions in crustaceans is based on studies of mammals and fish species. Basic research in this area is necessary to lay a solid foundation for improving the balances and bioavailability of AAs in the diets for optimum growth, health and wellbeing of crustaceans, while preventing and treating their metabolic diseases. This review highlights recent advances in AA nutrition and metabolism in aquatic crustacean species at their different life stages. The new knowledge is expected to guide the development of the next generation of their improved diets.

Oxidation of Energy Substrates in Tissues of Fish: Metabolic Significance and Implications for Gene Expression and Carcinogenesis

Fish are useful animal models for studying effects of nutrients and environmental factors on gene expression (including epigenetics), toxicology, and carcinogenesis. To optimize the response of the animals to substances of interest (including toxins and carcinogens), water pollution, or climate changes, it is imperative to understand their fundamental biochemical processes. One of these processes concerns energy metabolism for growth, development, and survival. We have recently shown that tissues of hybrid striped bass (HSB), zebrafish, and largemouth bass (LMB) use amino acids (AAs; such as glutamate, glutamine, aspartate, alanine, and leucine) as major energy sources. AAs contribute to about 80% of ATP production in the liver, proximal intestine, kidney, and skeletal muscle tissue of the fish. Thus, as for mammals (including humans), AAs are the primary metabolic fuels in the proximal intestine of fish. In contrast, glucose and fatty acids are only minor metabolic fuels in the fish. Fish tissues have high activities of glutamate dehydrogenase, glutamate-oxaloacetate transaminase, and glutamate-pyruvate transaminase, as well as high rates of glutamate uptake. In contrast, the activities of hexokinase, pyruvate dehydrogenase, and carnitine palmitoyltransferase 1 in all the tissues are relatively low. Furthermore, unlike mammals, the skeletal muscle (the largest tissue) of HSB and LMB has a limited uptake of long-chain fatty acids and barely oxidizes fatty acids. Our findings explain differences in the metabolic patterns of AAs, glucose, and lipids among various tissues in fish. These new findings have important implications for understanding metabolic significance of the tissue-specific oxidation of AAs (particularly glutamate and glutamine) in gene expression (including epigenetics), nutrition, and health, as well as carcinogenesis in fish, mammals (including humans), and other animals.

Amino Acid Nutrition for Optimum Growth, Development, Reproduction, and Health of Zoo Animals

Proteins are large polymers of amino acids (AAs) linked via peptide bonds, and major components for the growth and development of tissues in zoo animals (including mammals, birds, and fish). The proteinogenic AAs are alanine, arginine, aspartate, asparagine, cysteine, glutamate, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. Except for glycine, they are all present in the L-isoform. Some carnivores may also need taurine (a nonproteinogenic AA) in their diet. Adequate dietary intakes of AAs are necessary for the growth, development, reproduction, health and longevity of zoo animals. Extensive research has established dietary nutrient requirements for humans, domestic livestock and companion animals. However, this is not true for many exotic or endangered species found in zoos due to the obstacles that accompany working with these species. Information on diets and nutrient profiles of free-ranging animals is needed. Even with adequate dietary intake of crude protein, dietary AAs may still be unbalanced, which can lead to nutrition-related diseases and disorders commonly observed in captive zoo species, such as dilated cardiomyopathy, urolithiasis, gut dysbiosis, and hormonal imbalances. There are differences in AA metabolism among carnivores, herbivores and omnivores. It is imperative to consider these idiosyncrasies when formulating diets based on established nutritional requirements of domestic species. With optimal health, populations of zoo animals will have a vastly greater chance of thriving in captivity. For endangered species especially, maintaining stable captive populations is crucial for conservation. Thus, adequate provision of AAs in diets plays a crucial role in the management, sustainability and expansion of healthy zoo animals.

Oxidation of energy substrates in tissues of largemouth bass (Micropterus salmoides)

This study tested the hypothesis that amino acids are oxidized at higher rates than glucose and palmitate for ATP production in tissues of largemouth bass (LMB, a carnivorous fish). Slices (10 to 50 mg) of liver, proximal intestine, kidney, and skeletal muscle isolated from LMB were incubated at 26 °C for 2 h in oxygenated Krebs-Henseleit bicarbonate buffer (pH 7.4, with 5 mM D-glucose) containing either D-[U-¹⁴C]glucose, 2 mM L-alanine plus L-[U-¹⁴C]alanine, 2 mM L-aspartate plus L-[U-¹⁴C]aspartate, 2 mM L-glutamate plus L-[U-¹⁴C]glutamate, 2 mM L-glutamine plus L-[U-¹⁴C]glutamine, 2 mM L-leucine plus L-[U-¹⁴C]leucine, or 2 mM palmitate plus [U-¹⁴C]palmitate. In parallel experiments, tissues were incubated with a [U-¹⁴C]-labeled tracer and a mixture of unlabeled substrates [alanine, aspartate, glutamate, glutamine, leucine, and palmitate (2 mM each) plus 5 mM glucose]. ¹⁴CO₂ was collected to calculate the rates of substrate oxidation. In separate experiments, O₂ consumption by each tissue was measured in the presence of individual or a mixture of substrates. The activities of key metabolic enzymes were also measured. Results indicated that the liver and skeletal muscle had a limited ability to oxidize glucose and palmitate to CO₂ for ATP production in the presence of individual or a mixture of substrates due to low activities of carnitine palmitoyltransferase-I, hexokinase and pyruvate dehydrogenase. In the presence of individual substrates, each amino acid was actively oxidized by all the tissues. In the presence of a mixture of substrates, glutamine and glutamate were the major metabolic fuels in the proximal intestine and kidney, as glutamine for the liver and aspartate for skeletal muscle. All the tissues had high activities of glutaminase, glutamate dehydrogenase, and transaminases. At the same extracellular concentration of amino acids (2 mM) in a mixture of energy substrates, glutamine was the major metabolic fuel for the liver of the LMB, glutamine and glutamate for the proximal intestine and kidneys, and aspartate for the skeletal muscle. Glutamine plus glutamate plus aspartate generated 60-70% of ATP in LMB tissues.

Effects of dietary starch and lipid levels on the protein retention and growth of largemouth bass (Micropterus salmoides)

Protein accretion in some fish species is affected by dietary lipids, starch and their interactions, but this aspect of nutrition is largely unknown in largemouth bass (LMB). Therefore, we designed six experimental diets with three starch levels (5%, 10%, and 15%; dry matter  basis) and two lipid levels (10% and 12.5%; dry matter basis) to evaluate the effects of dietary starch and lipid levels on the protein retention, growth, feed utilization, and liver histology of LMB. There were three tanks (18 fish per tank, ~ 4.85 g per fish) per dietary treatment group and the trial lasted for 8 weeks. Fish were fed to apparent satiation twice daily. Results indicated that increasing the dietary starch level from 5 to 15% reduced (P < 0.05) absolute feed intake (AFI; - 9.0%, - 15% and - 14% on days 14-28, 28-42, and 42-56, respectively) and weight gains (- 4.4% and - 6.5% on days 42 and 56, respectively) of LMB. Increasing the dietary lipid level from 10 to 12.5% reduced (P < 0.05) AFI (- 9.7%, - 11.7% and - 11.9% on days 14-28, 28-42; and 42-56, respectively), weight gains (- 4.2%, - 5.9% and - 6.9% on days 28, 42 and 56, respectively), and survival rate (by a 5.6% unit) of LMB. The retention of dietary protein and some amino acids in the body was affected by dietary starch or lipid levels and their interactions. The viscerosomatic index (VSI), hepatosomatic index (HSI), and intraperitoneal fat ratio (IPFR) increased with increasing the dietary starch level from 5 to 15%. Compared with 10% lipids, 12.5% lipids in diets increased IPFR but had no effect on VSI or HSI. The concentrations of glucose in serum increased with increasing the dietary starch level from 5 to 15% at 4 to 24 h after feeding, with the effect of dietary lipids being time-dependent. Compared with a 5%-starch diet, fish fed a diet with 10%- or 15%-starch exhibited an enlarged and pale liver with excessive glycogen. Based on these findings, we recommend dietary lipid and starch levels to be 10% and < 10%, respectively, for juvenile LMB to maximize the retention of dietary protein in their bodies.

Effect of dietary L-glutamate levels on growth, digestive and absorptive capability, and intestinal physical barrier function in Jian carp (Cyprinus carpio var. Jian)

The present study explored effects of L-glutamate (Glu) levels on growth, digestive and absorptive capability, and intestinal physical barrier functions of Jian carp (Cyprinus carpio). A total of 600 Jian carp (126.40 ± 0.21 g) were randomly distributed into 5 groups with 3 replicates each, fed diets containing graded levels of Glu (53.4 [control], 57.2, 60.6, 68.4, and 83.4 g/kg) for 63 d. Results showed compared with control diet, feed intake and percent weight gain (PWG) in fish fed 83.4 g of Glu/kg diet were increased and feed conversion ratio in fish fed 68.4 g of Glu/kg diet was decreased (P < 0.05). Similarly, body crude protein and lipid contents in fish fed 68.4 g of Glu/kg diet were higher (P < 0.05). The activities of trypsin and chymotrypsin in the hepatopancreas and intestine, and amylase, alkaline phosphatase (AKP), Na⁺, K⁺-ATPase (NKA), and creatine kinase (CK) in intestine were higher in fish fed 68.4 g of Glu/kg diet (P < 0.05). Dietary Glu (57.2 to 83.4 g/kg diet) decreased malondialdehyde (MDA) and protein carbonyl (PCO) contents in the intestine (P < 0.05). The activities of catalase (CAT), glutathione peroxidase (GPx), and glutathione S-transferase (GST) in the hepatopancreas and intestine were higher in fish fed 60.6 and 68.4 g of Glu/kg diets (P < 0.05). Intestinal the glutathione reductase (GR) activity and glutathione (GSH) content in fish fed 60.6, 68.4, and 83.4 g of Glu/kg diet were increased (P < 0.05). The GPx1a, GST, and nuclear factor erythroid 2-related factor 2 (Nrf2) mRNA expressions in the intestine were up-regulated in fish fed 60.6 and 68.4 g of Glu/kg diet (P < 0.05). The zonula occludens protein-1 (ZO-1), occludin1, and claudin3 mRNA expressions were also up-regulated in fish fed 83.4 g of Glu/kg diet (P < 0.05). Fish fed 68.4 g of Glu/kg diet had higher levels of claudin 2, claudin7, and protein kinase C (PKC) mRNA (P < 0.05). These results indicated that Glu improved fish growth, digestive and absorptive ability, and intestinal physical barrier functions. Based on the quadratic regression analysis of PWG, and MDA of the hepatopancreas and intestine, the optimal dietary Glu levels were estimated to be 81.97, 71.06, and 71.36 g/kg diet, respectively.

Nutrition and metabolism of glutamate and glutamine in fish

Glutamate (Glu) and glutamine (Gln) comprise a large proportion of total amino acids (AAs) in fish in the free and protein-bound forms. Both Glu and Gln are synthesized de novo from other α-amino acids and ammonia. Although these two AAs had long been considered as nutritionally non-essential AAs for an aquatic animal, they must be included adequately in its diet to support optimal health (particularly intestinal health) and maximal growth. In research on fish nutrition, Glu has been used frequently as an isonitrogenous control on the basis of the assumption that this AA has no nutritional or physiological function. In addition, purified diets used for feeding fish generally lack glutamine. As functional AAs, Glu and Gln are major metabolic fuels for tissues of fish (including the intestine, liver, kidneys, and skeletal muscle), and play important roles not only in protein synthesis but also in glutathione synthesis and anti-oxidative reactions. The universality of Glu and Gln as abundant intracellular AAs depends on their enormous versatility in metabolism. Dietary supplementation with Glu and Gln to farmed fish can improve their growth performance, intestinal development, innate and adaptive immune responses, skeletal muscle development and fillet quality, ammonia removal, and the endocrine status. Glu (mainly as monosodium glutamate), glutamine, or AminoGut (a mixture of Glu and Gln) is a promising feed additive to reduce the use of fishmeal, while gaining the profitability of global aquaculture production. Thus, the concept of dietary requirements of fish for Glu and Gln is a paradigm shift in the nutrition of aquatic animals (including fish).

The impact of intracellular aminopeptidase on C2C12 myoblast proliferation and differentiation

The ubiquitin-proteasome pathway is essential for skeletal muscle growth and development. Proteasomes generate oligopeptides in the cytoplasm, and these peptides are considered to be rapidly degraded to amino acids by several intracellular aminopeptidases. However, the role of intracellular aminopeptidases in muscle growth remains unknown. In this study, therefore, we investigated the role of intracellular aminopeptidases in C2C12 myoblast proliferation and differentiation. Inhibition of intracellular aminopeptidases by Bestatin methyl ester (Bes-ME) decreased leucine and alanine aminopeptidase activity, and impaired proliferation and differentiation of C2C12 myoblasts. Furthermore, we observed that the inhibition of intracellular aminopeptidases reduced intracellular levels of amino acid and ATP level, and suppressed the phosphorylation of the mTOR pathway. These results suggested that intracellular aminopeptidases affect C2C12 myoblast proliferation and differentiation via mTOR pathway; however, further studies are required to clarify the role of aminopeptidase in skeletal muscle.

Metabolism of Amino Acids in the Brain and Their Roles in Regulating Food Intake

Amino acids (AAs) and their metabolites play an important role in neurological health and function. They are not only the building blocks of protein but are also neurotransmitters. In the brain, glutamate and aspartate are the major excitatory neurotransmitters, whereas γ-aminobutyrate (GABA, a metabolite of glutamate) and glycine are the major inhibitory neurotransmitters. Nitric oxide (NO, a metabolite of arginine), H₂S (a metabolite of cysteine), serotonin (a metabolite of tryptophan) and histamine (a metabolite of histidine), as well as dopamine and norepinephrine (metabolites of tyrosine) are neurotransmitters to modulate synaptic plasticity, neuronal activity, learning, motor control, motivational behavior, emotion, and executive function. Concentrations of glutamine (a precursor of glutamate and aspartate), branched-chain AAs (precursors of glutamate, glutamine and aspartate), L-serine (a precursor of glycine and D-serine), methionine and phenylalanine in plasma are capable of affecting neurotransmission through the syntheses of glutamate, aspartate, and glycine, as well as the competitive transport of tryptophan and tyrosine across from the blood-brain barrier. Adequate consumption of AAs is crucial to maintain their concentrations and the production of neurotransmitters in the central nervous system. Thus, the content and balance of AAs in diets have a profound impact on food intake by animals. Knowledge of AA transport and metabolism in the brain is beneficial for improving the health and well-being of humans and animals.

Hepatotoxicity evaluation of Euphorbia kansui on zebrafish larvae in vivo

BACKGROUND

Euphorbia kansui is effective in treating various diseases, such as ascites and edema, but its liver toxicity is a major obstacle in its wide use in the clinic. However, further investigations have suggested that Euphorbia kansui can cause liver injury.

HYPOTHESIS

The study aims to investigate the effect of Euphorbia kansui exposure on zebrafish, and explain the underlying toxicity mechanisms from a comprehensive perspective.

STUDY DESIGN

The 4dpf zebrafish larvae were exposed to Euphorbia kansui at a sub-lethal concentration.

METHODS

We evaluated the effect of Euphorbia kansui on the ultrastructure and function of the liver, apoptosis of liver cells by PCR and western blot, and metabolic profile by GC-MS based on sub-lethal concentrations.

RESULTS

Our results suggested Euphorbia kansui could lead to liver injury and significant alteration of the metabolomics of the zebrafish larvae in sub-lethal concentration conditions. It could also induce alterations in liver microstructure, hepatic function, gene expression and protein associated with the apoptosis process, as well as endogenous metabolism. KEGG pathway analysis identified some biological processes on the basis of different metabolisms and their associated processes especially for amino acid metabolism.

CONCLUSION

The results bring us closer to an in-depth understanding of the toxic effects of Euphorbia kansui on zebrafish liver, which will be significantly helpful in effectively guiding safer clinical application of this herb in the clinic. Furthermore, our results also showed the zebrafish model is reliable for evaluation of Euphorbia kansui extract hepatotoxicity and as a methodological reference for the evaluation of Traditional Chinese Medicine with underlying liver toxicity.

Hirsutella sinensis Treatment Shows Protective Effects on Renal Injury and Metabolic Modulation in db/db Mice

Hirsutella sinensis (HS) is the anamorph of the traditional Chinese medicine Cordyceps sinensis. Although the renal protective effect of HS has been reported, its effect on diabetic nephropathy (DN) remains unclear. In this study, db/db mice were used as the DN model, and the renal protective effect was evaluated after oral administration of HS for 6 and 12 weeks. Plasma, urine, and kidney samples were collected, and biochemical indicator measurements, pathological analysis, and metabolomics studies were performed. Biochemical assays showed that HS reduced the levels of fasting blood glucose (FBG), urinary albumin/creatinine ratio (ACR), and N-acetyl-beta-D-glucosaminidase (NAG) and increased the creatinine clearance (Ccr). HS alleviated glomerular and tubular glycogen accumulation and fibrosis and normalized the disordered ultrastructure of the glomerular filtration barrier. Metabolomics analysis of metabolites in the plasma, urine, and kidney indicated that HS modulated the perturbed glycolipid metabolism and amino acid turnover. HS reduced the elevated levels of metabolites involved in energy metabolism (TCA cycle, glycolysis, and pentose phosphate pathway) and nucleotide metabolism (pyrimidine metabolism and purine metabolism) in the kidneys of db/db mice. These results suggest that HS can protect against renal injury and that its efficacy involved metabolic modulation of the disturbed metabolome in db/db mice.

Regulation of protein synthesis in porcine mammary epithelial cells by L-valine

This study was conducted to determine the catabolism of L-valine in porcine mammary epithelial cells (PMECs) and its role in stimulating protein synthesis in these cells. PMECs were incubated with 0.05-, 0.10-, 0.25-, 0.5-, and 1.0-mM L-valine at 37 ^oC for 2 h. Cell viability and expressions of α-lactalbumin and β-casein were measured after culture with L-valine for 3 days. L-[1-¹⁴C]valine was used to study valine catabolism, whereas [³H]phenylalanine was employed as a tracer to determine protein synthesis and degradation in PMECs. The abundances of proteins involved in the mTOR signaling pathway and the mRNA levels for the related key genes were determined using the western blot and RT-PCR techniques, respectively. Cell numbers and the synthesis of proteins (including α-lactalbumin and β-casein) were greater (P < 0.05) in the presence of 0.5-mM L-valine, compared with 0.05- or 0.1-mM L-valine. L-Valine at 0.5 mM also enhanced (P < 0.05) the production of α-lactalbumin by PMECs, in comparison with 0.25 mM L-valine. Increasing the extracellular concentration of L-valine from 0.05 to 0.5 mM stimulated protein synthesis in a concentration-dependent manner without affecting proteolysis. Although L-valine was actively transaminated in PMECs, its α-ketoacid product (α-ketoisovalerate) at 0.05-0.2 mM did not affect protein synthesis or degradation in the cells. Thus, the effect of L-valine on protein synthesis was independent of its metabolism to yield α-ketoisovalerate. At the molecular level, 0.5-mM L-valine increased (P < 0.05) the mRNA levels for Ras, ERK1/2, and p70S6K, and the abundances of mTOR, p-4EBP1, total 4EBP1, p-ERK1/2, and total ERK1/2 proteins. These findings establish the critical role of L-valine in enhancing PMEC growth and milk protein synthesis possibly by regulating the mTOR and Ras/ERK signaling pathways. Further studies are warranted to understand how L-valine regulates gene expression and mTOR activation in PMECs.

Domestication and Temperature Modulate Gene Expression Signatures and Growth in the Australasian Snapper Chrysophrys auratus

Identifying genes and pathways involved in domestication is critical to understand how species change in response to human-induced selection pressures, such as increased temperatures. Given the profound influence of temperature on fish metabolism and organismal performance, a comparison of how temperature affects wild and domestic strains of snapper is an important question to address. We experimentally manipulated temperature conditions for F₁-hatchery and wild Australasian snapper (Chrysophrys auratus) for 18 days to mimic seasonal extremes and measured differences in growth, white muscle RNA transcription and hematological parameters. Over 2.2 Gb paired-end reads were assembled de novo for a total set of 33,017 transcripts (N50 = 2,804). We found pronounced growth and gene expression differences between wild and domesticated individuals related to global developmental and immune pathways. Temperature-modulated growth responses were linked to major pathways affecting metabolism, cell regulation and signaling. This study is the first step toward gaining an understanding of the changes occurring in the early stages of domestication, and the mechanisms underlying thermal adaptation and associated growth in poikilothermic vertebrates. Our study further provides the first transcriptome resources for studying biological questions in this non-model fish species.

Amino acid-specific δ15N trophic enrichment factors in fish fed with formulated diets varying in protein quantity and quality

Compound-specific isotope analysis (CSIA) of amino acids (AAs) in consumer tissues is a developing technique with wide-ranging applications for identifying nitrogen (N) sources and estimating animal trophic level. Controlled experiments are essential for determining which dietary conditions influence variability in N stable isotopes (δ15N) trophic enrichment factors in bulk tissue (TEFbulk) and AAs (TEFAA). To date, however, studies have not independently evaluated the effect of protein quantity and quality (digestibility) on TEFs, complicating the application of AA-δ15N values for estimating trophic levels. We conducted a 98-d feeding experiment using five formulated isoenergetic feeds prepared with a high-quality protein source to evaluate the effect of protein quantity and quality on TEFs of liver and muscle tissues of juvenile Pacific yellowtail (Seriola lalandi), a carnivorous fish species. We decreased protein digestibility using well-established protocols that do not change AA profiles. Growth rates were higher in diets with higher protein content, and isotopic equilibrium was reached for both fish tissues and all treatments. Protein quantity and quality influenced isotope discrimination depending on tissue type and AA. In liver tissue, bulk TEFs showed a limited but significant relationship with protein quality, but did not differ with protein quantity or quality in muscle. None of the pre-established source AAs (Lys, Met, Phe, and Gly) TEFs varied significantly with protein quantity or quality in liver tissue. However, in muscle tissue, TEFPhe increased significantly with protein content and decreased in response to reduced digestibility, indicating it may not serve as proxy for baseline isotopic values used to calculate trophic level. Among trophic AAs, TEFLeu decreased significantly with increasing protein quantity in liver tissue, while both Leu and Ile TEFs decreased with lower protein digestibility in muscle tissue. Our results indicate that CSIA-AA in liver tissue provides more robust source and trophic AA-δ15N values than in muscle.

Species-specific low plasma glucose in fish is associated with relatively high tissue glucose content and is inversely correlated with cardiac glycogen content

The relationship between plasma glucose concentration and intracellular glucose (liver, heart, brain, gill, gonad, intestine, kidney, spleen, white muscle) was determined in fish species with a range in plasma glucose (Atlantic cod, 5.06 mM; cunner, 3.8 mM; rainbow trout, 3.7 mM; lumpfish, 0.9 mM; short-horned sculpin, 0.6 mM; and winter flounder, 0.6 mM). The ratio of intracellular glucose/plasma glucose was always higher than one in liver for all species consistent with a diffusion gradient from the tissue to the plasma. In all other tissues in Atlantic cod, cunner, and rainbow trout the diffusion gradient was from the plasma to the intracellular space. In short-horned sculpin, the mean ratio in heart and white muscle exceeded one and in winter flounder the ratio was significantly greater than one at 5.97 and 2.92 for heart and muscle, respectively. The presence of an active glucose 6-phosphatase in white muscle could account for elevated amounts of free glucose. The white muscle of all species displayed phosphoenolpyruvate carboxykinase and in winter flounder the activity was as high in white muscle as in liver suggesting that gluconeogenesis may be associated with a relatively high-muscle glucose content. The glycogen content was highest in liver followed by heart with lower amounts in all other tissues. There was an inverse correlation between heart glycogen content and plasma glucose concentration when all species were included. It is contended that in species with low plasma glucose, heart glycogen is accumulated at a slow rate under normoxia, to be called upon under hypoxic conditions when the gradient for inward diffusion is unfavourable for high rates of glucose metabolism.