Glutamate promotes nucleotide synthesis in the gut and improves availability of soybean meal feed in rainbow trout

Comparison of the gut microbiota and untargeted gut tissue metabolome of Chinese mitten crabs (Eriocheir sinensis) with different shell colors

Introduction

The Chinese mitten crab (Eriocheir sinensis) is a highly valued freshwater crustacean in China. While the natural shell color of E. sinensis is greenish brown (GH), we found a variety with a brownish-orange shell color (RH). Although RH is more expensive, it exhibits a lower molting frequency and growth rate compared with GH, which significantly reduces its yield and hinders large-scale farming. The growth and development of animals are closely related to their gut microbiota and gut tissue metabolic profiles.

Methods

In this study, we compared the gut microbiome communities and metabolic profiles of juvenile RH and GH crabs using 16S rRNA gene sequencing and liquid chromatography-mass spectrometry (LC-MS), respectively.

Results

Our findings indicated that the intestinal microbial composition and metabolic characteristics of E. sinensis differed significantly between RH and GH. At the operational taxonomic unit (OTU) level, the α-diversity of the gut microbiota did not differ significantly between RH and GH, while the β-diversity of the RH gut microbiota was higher than that of the GH gut microbiota. At the species level, the richness of unclassified_c_Alphaproteobacteria was significantly higher in the GH group, while the RH group had a significantly higher richness of three low-abundance species, Flavobacteria bacterium BAL38, Paraburkholderia ferrariae, and uncultured_bacterium_g__Legionella. In the current study, 598 gut tissue metabolites were identified, and 159 metabolites were significantly different between GH and RH. The metabolite profile of RH was characteristic of a low level of most amino acids and lipid metabolites and a high level of several pigments compared with that of GH. These metabolites were enriched in 102 KEGG pathways. Four pathways, including (1) Central carbon metabolism in cancer, (2) protein digestion and absorption, (3) alanine, aspartate and glutamate metabolism, and (4) aminoacyl-tRNA biosynthesis, were significantly enriched. The correlation analysis between metabolites and microbiotas indicated that most key differential metabolites were positively correlated with the abundance of Shewanella_sp_MR-7.

Discussion

This research provided a greater understanding of the physiological conditions of E. sinensis varieties with different shell colors by comparing the gut microbiota and gut tissue metabolome.

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.

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.

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).

Effect of monosodium glutamate on growth performance and blood biochemical parameters of rainbow trout (Oncorhynchus mykiss W.)

Aim:

This study aimed to evaluate the effect of dietary monosodium glutamate (MSG) on growth performance and blood biochemical parameters of rainbow trout (Oncorhynchus mykiss W.).

Materials and Methods:

A total of 200 trouts were allotted in five experimental groups (n=40) that for 60 days received feed supplemented with MSG at the following levels: 0% (K) or 0.5% (E) or 1% (F) or 3% (G) or 5% (H).

Results:

The average initial weight of trouts was 116.68±1.51 g (K), 116.58±1.33 g (E), 116.43±1.39 g (F), 117.40±1.47 g (G), and 115.95±1.88 g (H). At the end of the trial, control fish (K) had the lowest live weight (242.90±3.87 g) compared to Groups E (252.70±5.69 g), F (250.93±4.82 g), G (248.25±4.98 g), and H (247.95±4.74 g). Feed conversion ratio (FCR) of control group (K) was higher (1.11±0.02) versus FCR values established in Groups E (1.03±0.01), F (1.04±0.02), G (1.07±0.03), and H (1.06±0.02). Albumin, aspartate aminotransferase, gamma-glutamyl transpeptidase, alkaline phosphatase, phosphorus, magnesium, and triglyceride exhibited statistically significant differences versus controls.

Conclusion:

The results from the study showed that dietary supplementation of rainbow trouts with MSG increased live weight and reduced FCR. The optimum level of MSG recommended for addition to feed was 0.5%.

Bromodeoxyuridine DNA labelling reveals host and parasite proliferation in a fish-myxozoan model

Enteromyxum leei is a myxozoan parasite responsible for enteritis in gilthead sea bream (Sparus aurata). The parasite proliferates in the paracellular space of the intestinal epithelium and induces an inflammatory reaction. To assess intestinal cell turnover and parasite proliferation, fish were infected with the parasite by anal intubation; after 17 and 64 days, the cell proliferative marker bromodeoxyuridine (BrdU) was administered; and after 24 hr, tissue samples were taken for immunohistochemical detection. Parasite exposure induced increased epithelial and immune cell proliferation in all intestinal segments at all time points, even before parasite establishment. This increased turnover was triggered early after intubation and mainly at a local level, as shown by an increased proliferating cell nuclear antigen (pcna) gene expression only at the posterior intestine after 17 days (not found in lymphohaematopoietic organs). Incorporation of BrdU in parasite secondary and tertiary daughter cells indicated that parasite endogeny is not by schizogonial division, which uses de novo synthesis pathway of pyrimidines. Altogether, BrdU immunolabelling and pcna gene expression showed the rapid proliferative response of the fish intestines upon a myxozoan infection and how this response is effectively triggered even before the parasite reaches or establishes in the site.

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

Fish generally have much higher requirements for dietary protein than mammals, and this long-standing puzzle remains unsolved. The present study was conducted with zebrafish (omnivores) and hybrid striped bass (HSB, carnivores) to test the hypothesis that AAs are oxidized at a higher rate than carbohydrates (e.g., glucose) and fatty acids (e.g., palmitate) to provide ATP for their tissues. Liver, proximal intestine, kidney, and skeletal muscle isolated from zebrafish and HSB were incubated at 28.5 °C (zebrafish) or 26 °C (HSB) for 2 h in oxygenated Krebs-Henseleit bicarbonate buffer (pH 7.4, with 5 mM D-glucose) containing 2 mM L-[U-¹⁴C]glutamine, L-[U-¹⁴C]glutamate, L-[U-¹⁴C]leucine, or L-[U-¹⁴C]palmitate, or a trace amount of D-[U-¹⁴C]glucose. In parallel experiments, tissues were incubated with a tracer and  a mixture of unlabeled substrates [glutamine, glutamate, leucine, and palmitate (2 mM each) plus 5 mM D-glucose]. ¹⁴CO₂ was collected to calculate the rates of substrate oxidation. In the presence of glucose or a mixture of substrates, the rates of oxidation of glutamate and ATP production from this AA by the proximal intestine, liver, and kidney of HSB   were much higher than those for glucose and palmitate. This was also true for glutamate in the skeletal muscle and glutamine in the liver of both species, glutamine in the HSB kidney, and leucine in the zebrafish muscle, in the presence of a mixture of substrates. We conclude that glutamate plus glutamine plus leucine contribute to ~80% of ATP production in the liver, proximal intestine, kidney, and skeletal muscle of zebrafish and HSB. Our findings provide the first direct evidence that the major tissues of fish use AAs (mainly glutamate and glutamine) as primary energy sources instead of carbohydrates or lipids.