Glutamine accumulation and up-regulation of glutamine synthetase activity in the swamp eel, Monopterus albus (Zuiew), exposed to brackish water

Effects of CRISPR/Cas9 targeting of the myo-inositol biosynthesis pathway on hyper-osmotic tolerance of tilapia cells

Myo-inositol is an important compatible osmolyte in vertebrates. This osmolyte is produced by the myo-inositol biosynthesis (MIB) pathway composed of myo-inositol phosphate synthase and inositol monophosphatase. These enzymes are among the highest upregulated proteins in tissues and cell cultures from teleost fish exposed to hyperosmotic conditions indicating high importance of this pathway for tolerating this type of stress. CRISPR/Cas9 gene editing of tilapia cells produced knockout lines of MIB enzymes and control genes. Metabolic activity decreased significantly for MIB KO lines in hyperosmotic media. Trends of faster growth of the MIB knockout lines in isosmotic media and faster decline of MIB knockout lines in hyperosmotic media were also observed. These results indicate a decline in metabolic fitness but only moderate effects on cell survival when tilapia cells with disrupted MIB genes are exposed to hyperosmolality. Therefore MIB genes are required for full osmotolerance of tilapia cells.

The role of the balance between energy production and ammonia detoxification mediated by key amino acids in divergent hypersaline adaptation among crassostrea oysters

Global ocean salinity is changing under rapid climate change and intensified anthropogenic activity. Increased differences in salinity threaten marine biodiversity, organismal survival, and evolution, particularly sessile invertebrates dwelling in highly fluctuating intertidal and estuarine environments. Comparing the responses of closely related species to salinity changes can provide insights into the adaptive mechanisms underlying inter- and intraspecific divergence in salinity tolerance, but are poorly understood in marine bivalves. We collected wild individuals of four Crassostrea species, in addition to two populations of the same species from their native habitats and determined the dynamics of hydrolyzed amino acids (HAAs) and transcriptional responses to hypersaline stress. In response to hypersaline stress, species/populations inhabiting natural high-salinity sea environments showed higher survival and less decline in HAAs than that of congeners inhabiting low-salinity estuaries. Thus, native environmental salinity shapes oyster tolerance. Notably, a strong negative correlation between the decline in HAAs and survival indicated that the HAAs pool could predict tolerance to hypersaline challenge. Four HAAs, including glutamine (Glu), aspartic acid (Asp), alanine (Ala) and glycine (Gly), were identified as key amino acids that contributed substantially to the emergency response to hypersaline stress. High-salinity-adapted oyster species only induced substantial decreases in Glu and Asp, whereas low-salinity-adapted congeners further incresaed Ala and Gly metabolism under hypersaline stress. The dynamics of the content and gene expression responsible for key amino acids pathways revealed the importance of maintaining the balance between energy production and ammonia detoxification in divergent hypersaline responses among oyster species/populations. High constructive or plastic expression of evolutionarily expanded gene copies in high-salinity-adapted species may contribute to their greater hypersaline tolerance. Our findings reveal the adaptive mechanism of key amino acids in salinity adaptation in marine bivalves and provide new avenues for the prediction of adaptive potential and aquaculture with high-salinity tolerant germplasms.

Free amino acids in response to salinity changes in fishes: relationships to osmoregulation

Free amino acids (FAAs) are believed to play important roles in osmoregulation and buffer capacity in some aquatic animals, such as fishes. However, the potential roles of FAAs have not been systematically summarized and characterized until now. In the present study, the meta-analysis was conducted to investigate the relationships between FAAs and environmental salinities. Twenty published documents were included, accounting for 106 study cases. The effect sizes of total free amino acids (TFAAs), total essential amino acids (TEAAs), and total non-essential amino acids (TNEAAs) to salinity increase were calculated and determined by the restricted maximum likelihood (REML) method. It clearly showed that the elevated salinities significantly induced the contents of TFAAs, TEAAs, and TNEAAs at the ratio of 36%, 27%, and 29%, respectively. Faced to the salinity changes, the contents of FAAs in fishes under freshwater and seawater varied significantly, while the individuals under brackish water displayed relatively constant contents of FAAs. When salinity elevated, the contents of 17 amino acids in muscles significantly increased, suggesting the important roles of FAA metabolism in osmoregulation in fishes. The results also indicated that the effect sizes of TFAAs were positively related to the rates of salinity increases, and exhibited a significant quadratic linear relationship with temperatures. Additionally, the contents of FAAs also showed positive correlation with osmotic pressure, concentrations of plasma Na+, Cl-, and urea, implying their potential roles of FAAs in osmoregulation in fishes. These findings suggested that elevated salinities greatly induced the contents of FAAs in fishes, making a great contribution to maintaining the homeostasis of fishes in response to environmental salinity changes.

The immune function of heme oxygenase-1 from grass carp (Ctenopharyngodon idellus) in response to bacterial infection

Heme oxygenase (HO)-1, a rate-limiting enzyme in heme catabolism, results in the formation of equivalent amounts of biliverdin (BV), carbon monoxide (CO) and ferrous iron (Fe²⁺). Previous studies have revealed that HO-1 plays an important role in immune responses. However, the mechanism underlying the immune responses against bacterial infection of teleost HO-1 remains enigmatic. To decipher the mechanisms, we have cloned and characterized the HO-1 gene of grass carp (designated as GcHO-1) in this research. The results showed that the open reading frame (ORF) of GcHO-1 was 819 bp, which encoded a putative protein of 272 amino acids. The deduced amino acid sequence phylogenetically shared the highest identity with other teleosts, and contained two domains of heme-oxygenase and a single-pass transmembrane domain. The mRNA expressions of GcHO-1 in healthy grass carp have widely existed in examined tissues in the following order of spleen > head-kidney > middle head-kidney > intestines > liver > gills > heart > muscle > brain. Besides, the mRNA and protein transcription of GcHO-1 were both significantly up-regulated in the liver and head-kidney tissues after Staphylococcus aureus and Aeromonas hydrophila infection. In addition, overexpression of GcHO-1 in kidney cell line (CIK) cells of grass carp could reduce the expression of inflammatory cytokines (IL-1β, IL-8, TNFα, CCL1 and IL-6). Herein, we demonstrate that GcHO-1 plays an anti-inflammatory role in innate immunity. Our results shed new light on the mechanisms underlying the antibacterial immunity of teleost.

An osmolality/salinity-responsive enhancer 1 (OSRE1) in intron 1 promotes salinity induction of tilapia glutamine synthetase

Euryhaline tilapia (Oreochromis mossambicus) are fish that tolerate a wide salinity range from fresh water to > 3× seawater. Even though the physiological effector mechanisms of osmoregulation that maintain plasma homeostasis in fresh water and seawater fish are well known, the corresponding molecular mechanisms that control switching between hyper- (fresh water) and hypo-osmoregulation (seawater) remain mostly elusive. In this study we show that hyperosmotic induction of glutamine synthetase represents a prominent part of this switch. Proteomics analysis of the O. mossambicus OmB cell line revealed that glutamine synthetase is transcriptionally regulated by hyperosmolality. Therefore, the 5' regulatory sequence of O. mossambicus glutamine synthetase was investigated. Using an enhancer trapping assay, we discovered a novel osmosensitive mechanism by which intron 1 positively mediates glutamine synthetase transcription. Intron 1 includes a single, functional copy of an osmoresponsive element, osmolality/salinity-responsive enhancer 1 (OSRE1). Unlike for conventional enhancers, the hyperosmotic induction of glutamine synthetase by intron 1 is position dependent. But irrespective of intron 1 position, OSRE1 deletion from intron 1 abolishes hyperosmotic enhancer activity. These findings indicate that proper intron 1 positioning and the presence of an OSRE1 in intron 1 are required for precise enhancement of hyperosmotic glutamine synthetase expression.

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).

Development of a Gill Assay Library for Ecological Proteomics of Threespine Sticklebacks (Gasterosteus aculeatus)

A data-independent acquisition (DIA) assay library for quantitative analyses of proteome dynamics has been developed for gills of threespine sticklebacks (Gasterosteus aculeatus). A raw spectral library was generated by data-dependent acquisition (DDA) and annotation of tryptic peptides to MSMS spectra and protein database identifiers. The assay library was constructed from the raw spectral library by removal of low-quality, ambiguous, and low-signal peptides. Only unique proteins represented by at least two peptides are included in the assay library, which consists of 1506 proteins, 5074 peptides, 5104 precursors, and 25,322 transitions. This assay library was used with DIA data to identify biochemical differences in gill proteomes of four populations representing different eco- and morpho-types of threespine sticklebacks. The assay library revealed unique and reproducible proteome signatures. Warm-adapted, low-plated, brackish-water fish from Laguna de la Bocana del Rosario (Mexico) show elevated HSP47, extracellular matrix, and innate immunity proteins whereas several immunoglobulins, interferon-induced proteins, ubiquitins, proteolytic enzymes, and nucleic acid remodeling proteins are reduced. Fully-plated, brackish-water fish from Westchester Lagoon (Alaska) display elevated ion regulation, GTPase signaling, and contractile cytoskeleton proteins, altered abundances of many ribosomal, calcium signaling and immunity proteins, and depleted transcriptional regulators and metabolic enzymes. Low-plated freshwater fish from Lake Solano (California) have elevated inflammasomes and proteolytic proteins whereas several iron containing and ion regulatory proteins are reduced. Gills of fully-plated, marine fish from Bodega Harbor (California) have elevated oxidative metabolism enzymes and reduced transglutaminase 2, collagens, and clathrin heavy chains. These distinct proteome signatures represent targets for testing ecological and evolutionary influences on molecular mechanisms of gill function in threespine sticklebacks. Furthermore, the gill assay library represents a model for other tissues and paves the way for accurate and reproducible network analyses of environmental context-dependent proteome dynamics in complex organisms.

Tolerance of hyperammonemia in brain of Heteropneustes fossilis is supported by glutamate-glutamine cycle

This report presents analysis of molecular switches associated with tolerance to hyperammonemia in Heteropneustes fossilis because it tolerates about 100-fold more ammonia than mammals. Brains of Heteropneustes fossilis exposed to 100mM ammonium chloride were dissected after Zero hour as control, 16h and 20h exposure. The status of neuron and glia were analysed by Golgi staining, Luxol Fast Blue, and Nissl's staining. The expression patterns of genes associated to homeostasis of neuron and glia, management of oxidative stress and inflammation, ammonia metabolism and brain derived neurotrophic factor were analysed through reverse-transcriptase-polymerase chain reaction. After 20h of hyperammonemia glia were more degenerated than neurons. The expression of mRNA of lactate dehydrogenase (Ldh), super oxide dismutase (Sod), catalase (Catalase), arginase-I (Arg I), inducible nitric oxide (iNos), glutaminase (GA), and brain derived neurotrophic factor (Bdnf) was up-regulated than the control. The levels of mRNA of Arg II, glutamate dehydrogenase (Gdh), glutamine synthetase (GS), glial fibrillary acidic protein (Gfap), proliferating cell nuclear antigen (Pcna) and S100β were down-regulated than control due to hyperammonemia. It appears first observation on impact of hyperammonemia on the status of neurons, myelination and glial cells in brain of Heteropneustes fossilis by Golgi staining, Nissl's and Luxol Fast Blue staining. The distribution of ammonia and glutamate metabolising enzymes in brain supports multi-centric mechanism (s) of regulation. The expression of Arg I and Arg II gets inversely regulated and glutamate-glutamine cycle also operates in Heteropneustes fossilis against hyperammonemia in brain.

Inhibition of human glutamine synthetase by L-methionine-S,R-sulfoximine-relevance to the treatment of neurological diseases

At high concentrations, the glutamine synthetase inhibitor L-methionine-S,R-sulfoximine (MSO) is a convulsant, especially in dogs. Nevertheless, sub-convulsive doses of MSO are neuroprotective in rodent models of hyperammonemia, acute liver disease, and amyotrophic lateral sclerosis and suggest MSO may be clinically useful. Previous work has also shown that much lower doses of MSO are required to produce convulsions in dogs than in primates. Evidence from the mid-20th century suggests that humans are also less sensitive. In the present work, the inhibition of recombinant human glutamine synthetase by MSO is shown to be biphasic-an initial reversible competitive inhibition (K i 1.19 mM) is followed by rapid irreversible inactivation. This K i value for the human enzyme accounts, in part, for relative insensitivity of primates to MSO and suggests that this inhibitor could be used to safely inhibit glutamine synthetase activity in humans.

Seawater acclimation and inositol monophosphatase isoform expression in the European eel (Anguilla anguilla) and Nile tilapia (Orechromis niloticus)

Inositol monophosphatase (IMPA) is responsible for the synthesis of inositol, a polyol that can function as an intracellular osmolyte helping re-establish cell volume when exposed to hypertonic environments. Some epithelial tissues in euryhaline teleosts such as the eel and tilapia encounter considerable hyperosmotic challenge when fish move from freshwater (FW) to seawater (SW) environments; however, the roles played by organic osmolytes, such as inositol, have yet to be determined. Syntenic analysis has indicated that, as a result of whole genome- and tandem-duplication events, up to six IMPA isoforms can exist within teleost genomes. Four isoforms are homologs of the mammalian IMPA1 gene, and two isoforms are homologs of the mammalian IMPA2 gene. Although the tissue-dependent isoform expression profiles of the teleost isoforms appear to be species-specific, it was primarily mRNA for the IMPA1.1 isoform that was upregulated in epithelial tissues after fish were transferred to SW (up to 16-fold in eel and 90-fold in tilapia). Although up-regulation of IMPA1.1 expression was evident in many tissues in the eel, more substantial increases in IMPA1.1 expression were found in tilapia tissues, where SW acclimation resulted in up to 2,000-fold increases in protein expression, 16-fold increases in enzyme activity and 15-fold increases in tissue inositol contents. Immunohistochemical studies indicated that the tissue and cellular distribution of IMPA1.1 protein differed slightly between eels and tilapia; however, in both species the basal epithelial cell layers within the skin and fin, and the branchial epithelium and interstitial cells within the kidney, exhibited high levels of IMPA1.1 protein expression.

Glutamine synthetase activity and the expression of three glul paralogues in zebrafish during transport

The enzyme glutamine synthetase (GS; glutamate-ammonia ligase, EC 6.3.1.2) plays an important role in the nitrogen metabolism of fish. In this study the GS activity and the corresponding genes were examined to understand how they are regulated in zebrafish in response to hyperammonemic stress during a 72 h simulated transport. Whole body ammonia levels, the activity of the enzyme GS and the mRNA expression of the splice variants of three paralogues of glul, glutamine synthetase gene (glula, glulb and glulc) were examined in brain, liver and kidney of zebrafish. Whole body ammonia reached significantly higher levels by 48 h, while brain showed higher levels as early as 24 h, compared to the values at the start of the transport. The GS activities in brain, liver and kidney were significantly higher at the end of 72 h transport than those at the start. However, only the expression of mRNA of glulb-002 and glulb-003 were significantly upregulated during the simulated transport. In silico analysis of the putative promoter regions of glul paralogues revealed glucocorticoid receptor binding sites. However, glucocorticoid response elements of glulb were not different. The up-regulation of GS enzyme activity and hitherto unreported mRNA expression of glul paralogues during zebrafish transport indicate a physiological response of fish to ammonia.

Gene Cloning and mRNA Expression of Glutamate Dehydrogenase in the Liver, Brain, and Intestine of the Swamp Eel, Monopterus albus (Zuiew), Exposed to Freshwater, Terrestrial Conditions, Environmental Ammonia, or Salinity Stress

The swamp eel, Monopterus albus, is an obligatory air-breathing teleost which can undergo long period of emersion, has high environmental and tissue ammonia tolerance, and can survive in brackish water. We obtained a cDNA sequence of glutamate dehydrogenase (gdh), which consisted of a 133-bp 5′ UTR, a complete coding sequence region spanning 1629 bp and a 3′ UTR of approximately 717 bp, from the liver, intestine, and brain of M. albus. The translated Gdh amino acid sequence had 542 residues, and it formed a monophyletic clade with Bostrychus sinensis Gdh1a, Tetraodon nigroviridis Gdh1a, Chaenocephalus aceratus Gdh1a, Salmo salar Gdh1a1 and Gdh1a2, and O. mykiss Gdh1a. One day of exposure to terrestrial conditions or 75 mmol l⁻¹ NH₄Cl, but not to water at salinity 20, resulted in a significant increase in mRNA expression of gdh1a and Gdh amination activity in the liver of M. albus. However, exposure to brackish water, but not to terrestrial conditions or 75 mmol l⁻¹ NH₄Cl, led to a significant increase in the mRNA expression of gdh1a and Gdh amination activity in the intestine. By contrast, all the three experimental conditions had no significant effects on the mRNA expression of gdh1a in the brain of M. albus, despite a significant decrease in the Gdh amination activity in the brain of fish exposed to 75 mmol l⁻¹ NH₄Cl for 6 days. Our results indicate for the first time that the mRNA expression of gdh1a was differentially up-regulated in the liver and intestine of M. albus in response to ammonia toxicity and salinity stress, respectively. The increases in mRNA expression of gdh1a and Gdh amination activity would probably lead to an increase in glutamate production in support of increased glutamine synthesis for the purpose of ammonia detoxification or cell volume regulation under these two different environmental conditions.

Ammonia production, excretion, toxicity, and defense in fish: a review

Many fishes are ammonotelic but some species can detoxify ammonia to glutamine or urea. Certain fish species can accumulate high levels of ammonia in the brain or defense against ammonia toxicity by enhancing the effectiveness of ammonia excretion through active NH4+transport, manipulation of ambient pH, or reduction in ammonia permeability through the branchial and cutaneous epithelia. Recent reports on ammonia toxicity in mammalian brain reveal the importance of permeation of ammonia through the blood-brain barrier and passages of ammonia and water through transporters in the plasmalemma of brain cells. Additionally, brain ammonia toxicity could be related to the passage of glutamine through the mitochondrial membranes into the mitochondrial matrix. On the other hand, recent reports on ammonia excretion in fish confirm the involvement of Rhesus glycoproteins in the branchial and cutaneous epithelia. Therefore, this review focuses on both the earlier literature and the up-to-date information on the problems and mechanisms concerning the permeation of ammonia, as NH(3), NH4+ or proton-neutral nitrogenous compounds, across mitochondrial membranes, the blood-brain barrier, the plasmalemma of neurons, and the branchial and cutaneous epithelia of fish. It also addresses how certain fishes with high ammonia tolerance defend against ammonia toxicity through the regulation of the permeation of ammonia and related nitrogenous compounds through various types of membranes. It is hoped that this review would revive the interests in investigations on the passage of ammonia through the mitochondrial membranes and the blood-brain barrier of ammonotelic fishes and fishes with high brain ammonia tolerance, respectively.

The freshwater Amazonian stingray, Potamotrygon motoro, up-regulates glutamine synthetase activity and protein abundance, and accumulates glutamine when exposed to brackish (15‰) water

This study aimed to examine whether the stenohaline freshwater stingray, Potamotrygon motoro, which lacks a functional ornithine—urea cycle, would up-regulate glutamine synthetase (GS) activity and protein abundance, and accumulate glutamine during a progressive transfer from freshwater to brackish (15‰) water with daily feeding. Our results revealed that, similar to other freshwater teleosts, P. motoro performed hyperosmotic regulation, with very low urea concentrations in plasma and tissues, in freshwater. In 15‰ water, it was non-ureotelic and non-ureoosmotic, acting mainly as an osmoconformer with its plasma osmolality, [Na+] and [Cl−] comparable to those of the external medium. There were significant increases in the content of several free amino acids (FAAs), including glutamate, glutamine and glycine, in muscle and liver, but not in plasma, indicating that FAAs could contribute in part to cell volume regulation. Furthermore, exposure of P. motoro to 15‰ water led to up-regulation of GS activity and protein abundance in both liver and muscle. Thus, our results indicate for the first time that, despite the inability to synthesize urea and the lack of functional carbamoyl phosphate synthetase III (CPS III) which uses glutamine as a substrate, P. motoro retained the capacity to up-regulate the activity and protein expression of GS in response to salinity stress. Potamotrygon motoro was not nitrogen (N) limited when exposed to 15‰ water with feeding, and there were no significant changes in the amination and deamination activities of hepatic glutamate dehydrogenase. In contrast, P. motoro became N limited when exposed to 10‰ water with fasting and could not survive well in 15‰ water without food.