Ontogeny of glutamine synthetase in rat small intestine

Free Amino Acids in Human Milk: A Potential Role for Glutamine and Glutamate in the Protection Against Neonatal Allergies and Infections

Breastfeeding is indicated to support neonatal immune development and to protect against neonatal infections and allergies. Human milk composition is widely studied in relation to these unique abilities, which has led to the identification of various immunomodulating components in human milk, including various bioactive proteins. In addition to proteins, human milk contains free amino acids (FAAs), which have not been well-studied. Of those, the FAAs glutamate and glutamine are by far the most abundant. Levels of these FAAs in human milk sharply increase during the first months of lactation, in contrast to most other FAAs. These unique dynamics are globally consistent, suggesting that their levels in human milk are tightly regulated throughout lactation and, consequently, that they might have specific roles in the developing neonate. Interestingly, free glutamine and glutamate are reported to exhibit immunomodulating capacities, indicating that these FAAs could contribute to neonatal immune development and to the unique protective effects of breastfeeding. This review describes the current understanding of the FAA composition in human milk. Moreover, it provides an overview of the effects of free glutamine and glutamate on immune parameters relevant for allergic sensitization and infections in early life. The data reviewed provide rationale to study the role of free glutamine and glutamate in human milk in the protection against neonatal allergies and infections.

Ontogeny of methionine utilization and splanchnic uptake in critically ill children

To determine the rates of methionine splanchnic uptake and utilization in critically ill pediatric patients we used two kinetic models: the plasma methionine enrichment and the "intracellular" homocysteine enrichment. Twenty four patients, eight infants, eight children, and eight adolescents, were studied. They received simultaneous, primed, constant, intravenous infusions of l-[(2)H(3)]methylmethionine and enteral l-[1-(13)C]methionine. The ratio of [(13)C]homocysteine to [(13)C]methionine enrichment was 1.0 ± 0.15, 0.80 ± 0.20, and 0.66 ± 0.10, respectively, for the infants, children, and adolescents, and it was different between the infants and adolescents (P < 0.01). Methionine splanchnic uptake was 63, 45, and 36%, respectively, in the infants, children, and adolescents, and it was higher (P < 0.01) in the infants compared with the adolescents. The infants utilized 73% of methionine flux for nonoxidative disposal, while 27% was used for transulfuration (P < 0.001). Conversely, in the adolescents, 40% was utilized for nonoxidative disposal, while 60% was used for transulfuration. There is ontogeny on the rates of methionine splanchnic uptake and on the fate of methionine utilization in critically ill children, with greater methionine utilization for synthesis of proteins and methionine-derived compounds (P < 0.01) and decreased transulfuration rates in the infants (P < 0.01), while the opposite was observed in the adolescents. The plasma model underestimated methionine kinetics in children and adolescents but not in the infants, suggesting lesser dilution and greater compartmentation of methionine metabolism in the infant population. All patients were in negative methionine balance, indicating that the current enteral nutritional support is inadequate in these patients.

Does the ornithine-alpha-ketoglutarate ratio influence ornithine alpha-ketoglutarate metabolism in healthy rats?

Ornithine alpha-ketoglutarate (OKG) is a salt composed of 2 molecules of ornithine (ORN) and one molecule of alpha-ketoglutarate (alphaKG). OKG has been used successfully via oral, enteral, and parenteral routes to improve protein status in patients with chronic and acute protein depletion, but its mechanism of action, which is probably multifactorial, is still unclear. A specific metabolic interaction between alphaKG and ORN has been shown to be a key factor in the effects of OKG, but the impact of the ORN/alphaKG ratio (2 molecules of ORN for 1 molecule of alphaKG) has never been discussed. To clarify this point, young (3 weeks old) male Wistar rats in the postabsorptive state received 5 g/kg of either OKG or a mono-ornithine alphaKG (MOKG) salt (ORN/alphaKG ratio = 1:1) in amounts that were either isonitrogenous or isomolar to OKG, or a saline solution (controls) and were killed 1 hour later. In a second experiment, a kinetic study was performed in which rats were killed 1, 2, 3, or 6 hours after OKG, MOKG, or saline administration. Amino acid contents were analyzed in the plasma, liver, jejunal and ileal mucosae, and the extensor digitorum longus (EDL) muscle. The major metabolites detected after intake of OKG or MOKG (ie, ORN, proline [PRO], and glutamate; OKG and MOKG vs control, P < .05) together with the absence of increased arginine and citrulline levels suggested that ORN was mainly metabolized by the ORN aminotransferase pathway, leading to glutamate and PRO production with accumulation persisting at 6 hours postadministration. This study provides new and important data on the influence of the ORN/alphaKG ratio on OKG metabolism: MOKG-treated rats presented less intestinal ORN than OKG-treated rats (MOKG vs OKG, P < .05), suggesting that ORN/alphaKG ratio influences the rate of ORN availability and metabolism. In addition, the metabolic interaction between ORN and alphaKG (ie, in the presence of alphaKG, ORN metabolism is partially diverted toward PRO production), which is characteristic of OKG metabolism, still takes place even if the salt contains only 1 molecule of ORN instead of two.

Ornithine alpha-ketoglutarate metabolism in the healthy rat in the postabsorptive state

To gain further insight into the ability of ornithine alpha-ketoglutarate (OKG) to generate key metabolites, the aim of this work was to study the short-term metabolism, that is, 1 hour after administration, of OKG in plasma and tissues. Particular attention was paid to keto acids (alpha-ketoglutarate and branched-chain keto acids). Young (3 weeks old) male Wistar rats in the postabsorptive state received either 1.5 g/kg of monohydrated OKG (OKG group, n = 8) diluted in distilled water or an equivalent volume of saline solution at 0.9% (control group, n = 8) by gavage and were killed 1 hour later. Plasma, liver, jejunal and ileal mucosa, and the extensor digitorum longus muscle were removed to analyze amino and keto acid contents. Major metabolites detected after OKG ingestion (ornithine [ORN], alpha-ketoglutarate, proline and glutamate; OKG vs control, P < .05) and the absence of increased arginine (and even a decrease in jejunum and muscle) and citrulline levels suggested that ORN was mainly metabolized by the ORN aminotransferase pathway. In addition, significantly decreased plasma branched-chain keto acids and increased hepatic branched-chain amino acids (OKG vs control, P < .05) were observed upon OKG ingestion. Finally, glutamine accumulation restricted to the intestine, as evidenced in this short-term study, suggests that the effects of OKG on glutamine pools in other tissues in various pathological states after several days of treatment, as observed in previous studies, may be related to a long-term induction of glutamine synthetase.

Substitutes for glutamine in proliferation of rat intestinal epithelial cells

OBJECTIVES

Glutamine (Gln) is important for intestinal epithelial proliferation. The purpose of this study was to determine whether glutamate (Glu), a mixture of nucleotide monophosphates, arginine, or glucosamine could support proliferation of rat intestinal crypt cells (IEC-6) in the absence of Gln.

METHODS

Glu with added ammonia acetate, glucosamine, arginine, and nucleotide monophosphates were tested at concentrations that were isonitrogenous with respect to Gln. To determine whether de novo synthesis of Gln was affected by these nutrients, a duplicate set of treatment groups was also tested with 1.0 mM/L of methionine sulfoximine, an inhibitor of Gln synthetase.

RESULTS

Gln + methionine sulfoximine-treated cells showed suboptimal proliferation below 0.6 mM/L but normal proliferation between 0.6 and 4.0 mM/L of Gln. In the absence of exogenous Gln, isonitrogenous concentrations of Glu, glucosamine, arginine, or nucleotide monophosphates yielded similar proliferation as Gln. Cells treated with Glu, glucosamine, arginine, or nucleotide monophosphate mixture showed a decrease in proliferation compared with cells treated with Gln across all treatment doses (P < 0.03).

CONCLUSIONS

The importance of these results is that, in the presence of active Gln synthetase, these nutrients can maintain intestinal epithelial proliferation similar to that observed with Gln.

Effects of protein deprivation on growth and small intestine morphology are not improved by glutamine or glutamate in gastrostomy-fed rat pups

OBJECTIVES

Critically ill neonates often have their enteral intake severely limited shortly after birth. Whether glutamine (Gln) or glutamate (Glu) can preserve intestinal structure and function in the neonate undergoing limited enteral feeding is not clear. We hypothesize that Gln and Glu can similarly preserve intestinal structure in the developing small intestine of infant rats fed a low protein diet.

METHODS

Using a gastrostomy-fed "pup-in-a-cup" rat model, the effects of Gln and Glu on the developing rat small intestine were examined. Four groups of 6- to 7-day-old pups were fed rat milk substitute (RMS) via gastrostomy tube. One group was provided 100% and three were provided 25% of the protein normally received from their mothers. Two of the groups fed 25% protein received additional Gln or Glu for 6 days.

RESULTS

Pups receiving the 100% protein RMS were larger than pups receiving the 25% protein RMS with or without Gln/Glu supplementation (P < 0.001). Average villus height (P < 0.01) and area (P < 0.01) were greater in pups receiving 100% protein RMS than in pups given 25% protein RMS formula. There was no significant difference among the groups in mucosal maltase or alkaline phosphatase activities. Tight junction protein claudin-1 was significantly higher in the group fed 100% protein RMS diet, while occludin did not differ among the 4 groups. Neither Gln nor Glu increased claudin-1 or occludin in rats fed 25% protein.

CONCLUSIONS

These results suggest that neither Gln nor Glu supplementation can substitute effectively for whole protein in the developing rat small intestine for the outcomes that were evaluated.

Glutamine supplementation and deprivation: effect on artificially reared rat small intestinal morphology

The mechanisms of how glutamine benefits critically ill patients have not been established. The purpose of this study was to determine the effects of dietary and endogenously produced glutamine on small intestinal morphology using light and transmission electron microscopy in artificially reared rat pups. It was hypothesized that deprivation of dietary glutamine leads to intestinal disease that is exacerbated by inhibition of glutamine synthetase by methionine sulfoximine (MS). Rat pups were placed into five different test groups: The first was a reference group that was reared by their mother. The other four groups were reared artificially and received a 10% Travasol amino acid solution at 5 g/kg per day, which does not contain glutamine, added to a mixture containing carbohydrates, lipids, and vitamins. This dose was chosen because it represents an approximation of the amount of glutamine these rats would be receiving in a normal rat diet (approximately 40 g/kg per day total protein, 10 to 15% of which is glutamine + glutamate). The glutamine was manipulated by adding glutamine (Q) or MS or both. The four groups were as follows: MS-Q-, MS-Q+, MS+Q-, and MS+Q+. Light microscopy revealed the greatest blunting of villus height in the ileum of rats from the MS+Q- group when compared with the MS-Q+ group (123 +/- 48.9 micro m versus 207 +/- 36 microm, p < 0.05). The other two groups exhibited intermediate villus heights, but all were shorter than the villi from the mother-reared animals. The number of villi per unit length of bowel was also lowest in the animals that were treated with MS and not provided with dietary glutamine. Transmission electron microscopy demonstrated breakdown of the epithelial junctions in the glutamine-deprived and glutamine synthetase-inhibited intestines. Glutamine-deprived animals also displayed sloughing of microvilli, decreased actin cores, and degeneration of the terminal web. In summary, these studies support the hypothesis that glutamine is involved with maintenance of intestinal epithelial integrity.

Glutamine and the bowel

Since the pioneering work of Windmueller and Spaeth, the importance of glutamine to the support of intestinal mucosal metabolic function has become generally accepted. Nevertheless, the mechanisms underlying this role still remain obscure. This paper explores a number of questions: 1) Is glutamine essential for intestinal function? 2) To what extent does this relate to its intermediary metabolism? 3) What is the importance of glutamine as a biosynthetic precursor? 4) Is glutamine supplementation of the nutrient mixture presented to patients of any metabolic or clinical benefit? As a result of this exploratory exercise, the following general conclusions were reached: 1) Much suggestive biochemical and physiologic evidence exists that implies that glutamine, especially systemic glutamine, supports the function of the intestinal mucosal system. 2) Despite the extensive metabolism of this amino acid by the intestinal tissues, most evidence suggests that if glutamine does play a physiologic role in the bowel, it is not compellingly related to its intermediary metabolism. 3) There is, on the other hand, evidence that the mucosal cells not only utilize extracellular glutamine but synthesize the amino acid. Given that inhibition of glutamine synthesis inhibits both proliferation and differentiation of mucosal cell cultures, this suggests some more subtle regulatory role. This notion is supported by the demonstration that glutamine will activate a number of genes associated with cell cycle progression in the mucosa. 4) Despite the accumulated evidence, the mechanisms underlying glutamine's function and the question whether glutamine supplementation uniformly benefits mucosal health remain equivocal at best.

Glutamine synthetase: a key enzyme for intestinal epithelial differentiation?

BACKGROUND

We have previously shown that glutamine synthetase protein and mRNA are concentrated in the crypt region of the rat small intestine and that the activity of this enzyme is highest around the time of weaning. This anatomical location and time of peak activity are sites and periods of active enterocyte differentiation. This led to our current hypothesis that glutamine synthetase is important in the differentiation of enterocytes.

METHODS

To test our hypothesis, we treated Caco-2 cells with physiologic (0.6 mM) glutamine concentrations in cell culture medium. The experimental group was treated with methionine sulfoximine, an irreversible glutamine synthetase inhibitor, and the control group with phosphate buffered saline. Three standard and well-defined markers of intestinal differentiation-sucrase-isomaltase activity, microvillus formation, and electrical impedance in transwell plates-were compared between the two groups.

RESULTS

The methionine-sulfoximine-inhibited group was found to have lower sucrase-isomaltase activity, a lower density of microvilli, and lower electrical impedance values over time compared with the control group.

CONCLUSION

The experimental group was found to be less differentiated by all three markers of differentiation. Therefore, glutamine synthetase is important for Caco-2 cell differentiation.

Inhibition of glutamine synthetase decreases proliferation of cultured rat intestinal epithelial cells

The importance of glutamine synthetase (GS) for cell proliferation was examined in rat intestinal crypt cells (IEC-6) by inhibiting its activity with 10 mmol/L methionine sulfoximine (MS) at varying extracellular glutamine (Q) concentrations. In uninhibited cultures, cell number, protein, and DNA accumulation and synthesis showed a dependence on extracellular Q over a concentration range of 0.06 to 1.06 mmol/L, with apparent half-maximal responses of 0.46 mmol/L extracellular Q. In contrast, proliferation of GS-inhibited cultures required >/=1.06 mmol/L extracellular Q, with an apparent half-maximal response of 2 mmol/L. MS inhibited GS activity >97% in extracts of washed cells and appeared to be specific because its effects on proliferation were overcome by 4.06 mmol/L Q and were reversible. The increased dependence of IEC-6 cells on extracellular Q when GS was inhibited suggests that Q derived from GS (GS-Q) contributes importantly to cell proliferation at physiologic levels of extracellular Q (0.6 mmol/L). The unexpectedly high concentration of extracellular Q required to rescue maximal proliferation during GS-inhibition, relative to a reported Km for Q-transport into the cell, indicates that intracellular Q derived from the extracellular medium (exo-Q) is inefficiently utilized. In a previous study, we found that GS-protein and mRNA are concentrated in the proliferative crypt region of the small intestine in vivo, and predicted that GS activity is important for crypt cell proliferation. Here, we show that enzyme activity is important for cell proliferation at physiologic concentrations of Q in this cell culture model. Finally, we speculate that exo-Q and GS-Q are utilized differently in the cell.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.

Characterization of glutamine synthetase transcript, protein, and enzyme activity in the human placenta

This study characterizes the molecular mechanisms necessary for glutamine synthesis in the human placenta. RNA hybridization and protein immunoblotting were used to verify the presence of glutamine synthetase (GS) transcripts and protein, respectively. Additionally, the presence of GS was determined by immunohistochemistry. RNA hybridization demonstrated the presence of 1.8- and 2.8-kB transcripts and protein immunoblotting yielded a single 49-kDa band, characteristics of GS transcripts and protein, respectively. The mean (+/- s.d.) specific activity of placental GS, expressed as mumol gamma-glutamyl hydroxamic acid/mg protein/h was 1.80 +/- 0.59, which is comparable to other organs which are net glutamine producers. Immunohistochemical analysis indicated the presence of GS within the cytotrophoblast and mesenchyme layers of placental villi, but not in the syncytiotrophoblast. Although these results suggest that the human placenta is capable of synthesizing glutamine, the fate of glutamine produced by this organ remains speculative.