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

Astaxanthin attenuates glucose-induced liver injury in largemouth bass: role of p38MAPK and PI3K/Akt signaling pathways

BACKGROUND

Astaxanthin (ASX) has been documented to exert beneficial influence on various processes in fish. Largemouth bass (Micropterus salmoides) serves as a common model for studying glucose-induced liver disease, making it imperative to investigate the regulatory mechanisms underlying its liver health.

METHODS

Largemouth bass were fed with a control diet (CON), a high carbohydrate diet (HC), or a HC diet supplemented astaxanthin (HCA) for 8-weeks, followed by the glucose tolerance test (GTT). Primary hepatocytes were treated with low glucose and high glucose combined with different concentrations of astaxanthin for 48 h. The histopathology, enzymology, transcriptomics, molecular biology and cell biology were combined to investigate the mechanism of liver injury.

RESULTS

This study provides evidence for the protective effects of ASX against growth performance reduction and hepatic liver injure in largemouth bass fed HC diet. In GTT, HCA diet exhibited an improvement in glucose tolerance following glucose loading. Although HCA diet did not restore the expression of insulin resistance-related genes in livers at different time during the GTT, the addition of ASX in the long-term HC diet did improve the insulin resistance pathway by regulating the PTP1B/PI3K/Akt signaling pathway. Hepatic transcriptome analyses showed that ASX plays an essential role in the modulation of glucose homeostasis in response to treated with HC diet. In in vitro study, ASX treatment resulted in an exaltation in cell viability and a reduction in the rate of cell apoptosis and reactive oxygen species (ROS). Additionally, astaxanthin was observed to improve apoptosis induced by high-glucose via p38MAPK/bcl-2/caspase-3 signaling pathway.

CONCLUSIONS

Astaxanthin exhibited a protective effect against apoptosis by regulating p38MAPK/bcl-2/caspase-3 pathway, and ameliorated insulin resistance by activating the PTP1B/PI3K/Akt pathway. This study elucidated the mechanism of astaxanthin in the liver injury of largemouth bass from a new perspective and provided a new target for the treatment of insulin resistance.

An In-Depth Insight into the Profile, Mechanisms, Functions, and Transfer of Essential Amino Acids from Mulberry Leaves to Silkworm Bombyx mori L. Pupae and Fish

The silkworm Bombyx mori, the second most varied group of insects, is a fascinating insect that belongs to the Lepidoptera species. We aimed to deepen our knowledge about the composition and significance of amino acids (AA) from the sericulture chain to fish. AAs are the most prevalent molecules throughout the growth process of silkworms. We described AAs classification, occurrence, metabolism, and functions. Online datasets revealed that the essential AAs (EAA) level in fish meal and silkworm pupae (SWP) is comparable. SWP have a high content of methionine and lysine, which are the principal limiting AAs in fish diets, indicating that SWP have nutritional potential to be added to fish diets. Additionally, an overview of the data analyzed displays that SWP have a higher protein efficiency ratio than fish meal, the classical protein-rich source (>1.19 times), and compared to soybean meal, the second-most preferred source of protein in aquaculture (>2.08 times), indicating that SWP can be considered effective for animal feeding. In this study, we provide an overview of the current knowledge concerning AAs, paying special emphasis to EAAs and explaining, to some extent, certain mechanisms and functions of these compounds, from mulberry leaves to larvae-pupae and fish diets.

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.

Characteristics of Nutrition and Metabolism in Dogs and Cats

Domestic dogs and cats have evolved differentially in some aspects of nutrition, metabolism, chemical sensing, and feeding behavior. The dogs have adapted to omnivorous diets containing taurine-abundant meat and starch-rich plant ingredients. By contrast, domestic cats must consume animal-sourced foods for survival, growth, and development. Both dogs and cats synthesize vitamin C and many amino acids (AAs, such as alanine, asparagine, aspartate, glutamate, glutamine, glycine, proline, and serine), but have a limited ability to form de novo arginine and vitamin D3. Compared with dogs, cats have greater endogenous nitrogen losses and higher dietary requirements for AAs (particularly arginine, taurine, and tyrosine), B-complex vitamins (niacin, thiamin, folate, and biotin), and choline; exhibit greater rates of gluconeogenesis; are less sensitive to AA imbalances and antagonism; are more capable of concentrating urine through renal reabsorption of water; and cannot tolerate high levels of dietary starch due to limited pancreatic α-amylase activity. In addition, dogs can form sufficient taurine from cysteine (for most breeds); arachidonic acid from linoleic acid; eicosapentaenoic acid and docosahexaenoic acid from α-linolenic acid; all-trans-retinol from β-carotene; and niacin from tryptophan. These synthetic pathways, however, are either absent or limited in all cats due to (a) no or low activities of key enzymes (including pyrroline-5-carboxylate synthase, cysteine dioxygenase, ∆6-desaturase, β-carotene dioxygenase, and quinolinate phosphoribosyltransferase) and (b) diversion of intermediates to other metabolic pathways. Dogs can thrive on one large meal daily, select high-fat over low-fat diets, and consume sweet substances. By contrast, cats eat more frequently during light and dark periods, select high-protein over low-protein diets, refuse dry food, enjoy a consistent diet, and cannot taste sweetness. This knowledge guides the feeding and care of dogs and cats, as well as the manufacturing of their foods. As abundant sources of essential nutrients, animal-derived foodstuffs play important roles in optimizing the growth, development, and health of the companion animals.

Different Starch Sources Affect the Growth Performance and Hepatic Health Status of Largemouth Bass (Micropterus salmoides) in a High-Temperature Environment

The experiment was designed to investigate the effects of different starch types on the growth performance and liver health status of largemouth bass in a high-temperature environment (33-35 °C). In this study, we designed five diets using corn starch (CS), tapioca starch (TS), sweet potato starch (SPS), potato starch (PS), and wheat starch (WS) as the starch sources (10%). We selected 225 healthy and uniformly sized largemouth bass (199.6 ± 0.43 g) and conducted the feeding experiment for 45 days. The results showed that the WS group had the highest WGR, SGR, and SR and the lowest FCR. Among the five groups, the WS group had the highest CAT activity, SOD activity, and GSH content, while the SPS group had the highest MDA content. Furthermore, oil red O staining of liver samples showed that the TS group had the largest positive region, indicating high lipid accumulation. Lastly, the gene expression results revealed that compared with the WS group, the CS, TS, and SPS groups showed suppressed expression of nrf2, keap1, cat, sod, gpx, il-8, and il-10. Therefore, our results demonstrated the effect of different starch sources on largemouth bass growth performance and hepatic health in a high-temperature environment.

Dietary lysophospholipids improves growth performance and hepatic lipid metabolism of largemouth bass (Micropterussalmoides)

This study was conducted to evaluate the influence of dietary lysophospholipids combined with 1% dietary fish oil reduction on the growth performance and hepatic lipid metabolism of largemouth bass (Micropterus salmoides). Five isonitrogenous feeds were prepared with lysophospholipids at 0% (fish oil group, FO), 0.05% (L-0.05), 0.1% (L-0.1), 0.15% (L-0.15) and 0.2% (L-0.2), respectively. The dietary lipid was 11% in the FO diet and 10% in the other diets. Largemouth bass were fed for 68 d (initial body weight = 6.04 ± 0.01 g) with 4 replicates per group and 30 fish per replicate. The results showed that the fish fed diet containing 0.1% lysophospholipids had higher digestive enzyme activity and obtained better growth performance compared to the fish fed FO diet (P < 0.05). The feed conversion rate in the L-0.1 group was significantly lower than that in the other groups. Serum total protein and triglyceride contents in L-0.1 group were significantly higher than those in other groups (P < 0.05) and the contents of total cholesterol and low-density lipoprotein cholesterol in L-0.1 group were significantly lower than those in FO group (P < 0.05). The activity and genes expression of hepatic glucolipid metabolizing enzymes in L-0.15 group were significantly increased compared to those in FO group (P < 0.05). Reducing 1% fish oil along with 0.1% lysophospholipids added to the feed could improve the digestion and absorption of nutrients, enhance the activity of liver glycolipid metabolizing enzymes, and thus effectively promote the growth of largemouth bass.

Amino acid nutrition and metabolism in domestic cats and dogs

Domestic cats and dogs are carnivores that have evolved differentially in the nutrition and metabolism of amino acids. This article highlights both proteinogenic and nonproteinogenic amino acids. Dogs inadequately synthesize citrulline (the precursor of arginine) from glutamine, glutamate, and proline in the small intestine. Although most breeds of dogs have potential for adequately converting cysteine into taurine in the liver, a small proportion (1.3%-2.5%) of the Newfoundland dogs fed commercially available balanced diets exhibit a deficiency of taurine possibly due to gene mutations. Certain breeds of dogs (e.g., golden retrievers) are more prone to taurine deficiency possibly due to lower hepatic activities of cysteine dioxygenase and cysteine sulfinate decarboxylase. De novo synthesis of arginine and taurine is very limited in cats. Thus, concentrations of both taurine and arginine in feline milk are the greatest among domestic mammals. Compared with dogs, cats have greater endogenous nitrogen losses and higher dietary requirements for many amino acids (e.g., arginine, taurine, cysteine, and tyrosine), and are less sensitive to amino acid imbalances and antagonisms. Throughout adulthood, cats and dogs may lose 34% and 21% of their lean body mass, respectively. Adequate intakes of high-quality protein (i.e., 32% and 40% animal protein in diets of aging dogs and cats, respectively; dry matter basis) are recommended to alleviate aging-associated reductions in the mass and function of skeletal muscles and bones. Pet-food grade animal-sourced foodstuffs are excellent sources of both proteinogenic amino acids and taurine for cats and dogs, and can help to optimize their growth, development, and health.

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.

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.

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.

Dietary Glutamine Inclusion Regulates Immune and Antioxidant System, as Well as Programmed Cell Death in Fish to Protect against Flavobacterium columnare Infection

The susceptibility of animals to pathogenic infection is significantly affected by nutritional status. The present study took yellow catfish (Pelteobagrus fulvidraco) as a model to test the hypothesis that the protective roles of glutamine during bacterial infection are largely related to its regulation on the immune and antioxidant system, apoptosis and autophagy. Dietary glutamine supplementation significantly improved fish growth performance and feed utilization. After a challenge with Flavobacterium columnare, glutamine supplementation promoted il-8 and il-1β expression via NF-κB signaling in the head kidney and spleen, but inhibited the over-inflammation in the gut and gills. Additionally, dietary glutamine inclusion also enhanced the systematic antioxidant capacity. Histological analysis showed the protective role of glutamine in gill structures. Further study indicated that glutamine alleviated apoptosis during bacterial infection, along with the reduced protein levels of caspase-3 and the reduced expression of apoptosis-related genes. Moreover, glutamine also showed an inhibitory role in autophagy which was due to the increased activation of the mTOR signaling pathway. Thus, our study for the first time illustrated the regulatory roles of glutamine in the fish immune and antioxidant system, and reported its inhibitory effects on fish apoptosis and autophagy during bacterial infection.

Nutrition and Metabolism: Foundations for Animal Growth, Development, Reproduction, and Health

Consumption of high-quality animal protein plays an important role in improving human nutrition, growth, development, and health. With an exponential growth of the global population, demands for animal-sourced protein are expected to increase by 60% between 2021 and 2050. In addition to the production of food protein and fiber (wool), animals are useful models for biomedical research to prevent and treat human diseases and serve as bioreactors to produce therapeutic proteins. For a high efficiency to transform low-quality feedstuffs and forages into high-quality protein and highly bioavailable essential minerals in diets of humans, farm animals have dietary requirements for energy, amino acids, lipids, carbohydrates, minerals, vitamins, and water in their life cycles. All nutrients interact with each other to influence the growth, development, and health of mammals, birds, fish, and crustaceans, and adequate nutrition is crucial for preventing and treating their metabolic disorders (including metabolic diseases) and infectious diseases. At the organ level, the small intestine is not only the terminal site for nutrient digestion and absorption, but also intimately interacts with a diverse community of intestinal antigens and bacteria to influence gut and whole-body health. Understanding the species and metabolism of intestinal microbes, as well as their interactions with the intestinal immune systems and the host intestinal epithelium can help to mitigate antimicrobial resistance and develop prebiotic and probiotic alternatives to in-feed antibiotics in animal production. As abundant sources of amino acids, bioactive peptides, energy, and highly bioavailable minerals and vitamins, animal by-product feedstuffs are effective for improving the growth, development, health, feed efficiency, and survival of livestock and poultry, as well as companion and aquatic animals. The new knowledge covered in this and related volumes of Adv Exp Med Biol is essential to ensure sufficient provision of animal protein for humans, while helping reduce greenhouse gas emissions, minimize the urinary and fecal excretion of nitrogenous and other wastes to the environment, and sustain animal agriculture (including aquaculture).

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.

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.

Use of alternative protein sources for fishmeal replacement in the diet of largemouth bass (Micropterus salmoides). Part II: effects of supplementation with methionine or taurine on growth, feed utilization, and health

Fishmeal has long been a staple protein feedstuff for fish, but its global shortage and high price have prompted its replacement with alternative sustainable sources. In this experiment involving largemouth bass (a carnivorous fish), a new mixture of feedstuffs (45% poultry byproduct meal, 30% soybean meal, 15% blood meal, and 10% krill shrimp meal) was added to low (14.5%) fishmeal diets along with 0.0%, 0.5% taurine, 0.5% methionine, or 0.5% taurine plus 0.5% methionine (dry matter basis). The positive control diet [65.3% fishmeal (46% crude protein on dry matter basis)] and all low-fishmeal diets contained 40% true protein and 10% lipids. There were 3 tanks per treatment group (20 fish/tank). Fish with the mean initial body weight of 16.6 g were fed to satiety twice daily. Compared with the unsupplemented low-fishmeal group, supplementing either 0.5% methionine or 0.5% methionine plus 0.5% taurine to the low-fishmeal diet improved (P < 0.05) the growth, feed utilization, retention of dietary protein and lipids, and health of largemouth bass, reduced (P < 0.05) the occurrence of black skin syndrome from ~ 40 to ~ 10%. Histological sections of tissues from the fish with black skin syndrome showed retina degeneration, liver damage, and enteritis in the intestine. Compared with methionine supplementation, supplementing 0.5% taurine alone to the low-fishmeal diet did not affect the growth or feed efficiency of fish and had less beneficial effects (P < 0.05) on ameliorating the black skin syndrome. These results indicated that: (a) the basal low-fishmeal diet was inadequate in methionine or taurine; and (b) dietary supplementation with methionine was an effective method to improve the growth performance, feed efficiency, and health of largemouth bass. Further studies are warranted to understand the pathogenesis of the black skin syndrome in largemouth bass.

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.

Use of alternative protein sources for fishmeal replacement in the diet of largemouth bass (Micropterus salmoides). Part I: effects of poultry by-product meal and soybean meal on growth, feed utilization, and health

Five isonitrogenous and isocaloric diets [containing 54, 30, 15, 10, and 5% fishmeal crude-protein (CP), dry matter (DM) basis] were prepared by replacing fishmeal with poultry by-product meal plus soybean meal to feed juvenile largemouth bass (LMB, with an initial mean body weight of 4.9 g) for 8 weeks. All diets contained 54% CP and 13% lipids. There were four tanks of fish per treatment group (15 fish/tank). The fish were fed twice daily with the same feed intake (g/fish) in all the dietary groups. Results indicated that the inclusion of 15% fishmeal protein in the diet is sufficient for LMB growth. However, some of the fish that were fed diets containing ≤ 15% fishmeal CP had black skin syndrome (characterized by skin darkening and retinal degeneration, as well as intestinal and liver atrophies and structural abnormalities). The concentrations of taurine, methionine, threonine and histidine in serum were reduced (P < 0.05) in fish fed the diets containing 5, 10 and 15% fishmeal CP, compared with the 30 and 54% fishmeal CP diets. Interestingly, the concentrations of tyrosine and tryptophan in serum were higher in fish fed diets with ≤ 15% fishmeal CP than those in the 54% fishmeal CP group. These results indicated that 15% fishmeal CP in the diet containing poultry by-product meal and soybean meal was sufficient for the maximum growth and feed efficiency in LMB but inadequate for their intestinal, skin, eye, and liver health. A reduction in dietary methionine and taurine content and the possible presence of antinutritional factors in the fishmeal replacements diets containing high inclusion levels of soybean meal may contribute to black skin syndrome in LMB. We recommend that the diets of juvenile LMB contain 30% fishmeal CP (DM basis).

Effects of dietary protein and lipid levels on the growth performance, feed utilization, and liver histology of largemouth bass (Micropterus salmoides)

The reported requirements of largemouth bass (LMB, which is native to North America) for dietary protein and lipids varied substantially among previous studies, and this fish fed current formulated diets exhibit poor growth performance and pale liver syndrome. Because amino acids and lipids are known to affect hepatic metabolism and function in mammals, it is imperative to understand the impacts of these dietary macronutrients on the growth and liver morphology of LMB. In this study, we designed six isocaloric diets to determine the effects of different dietary crude protein (CP; 40%, 45%, and 50%; dry matter basis) and lipid levels (7.5% and 10%; dry matter basis) on fat and glycogen deposits, as well as hepatosis in LMB. There were four tanks (12 fish per tank, an average initial weight of 18.4 g/fish) per dietary treatment group and the trial lasted for 8 weeks. Fish were fed to apparent satiation three times daily. Results indicated that LMB fed the 45% or 50% CP diet grew faster (P < 0.05), had less (P < 0.05) glycogen in the liver and smaller (P < 0.05) hepatocyte sizes than fish fed the 40% CP diet, but there was no difference in weight gain or feed efficiency between the 45% and 50% CP diets. The hepatic lipid content did not differ between LMB fed the 40% and 45% CP diets, and the values for these two groups were 29% lower (P < 0.05) than those for LMB fed the 50% CP diet. Compared with the 40% CP group, LMB fed the 45% or 50% CP diet had 8-12% lower content of total minerals, phosphorus, and calcium in the body. Increasing the dietary lipid level from 7.5 to 10% enhanced the weight gains (+ 15%) and feed efficiency (+ 22%), as well as the retention of dietary protein (+ 18%), energy (+ 25%), and phosphorus (+ 7.6%) in the body. No fatty liver occurred in any group of LMB (with hepatic lipid concentrations being < 2%, wet weight basis). Based on these growth, metabolic and histologic data, we recommend dietary CP and lipids levels to be 45% and 10%, respectively, for juvenile LMB.

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.