Localization of rat small intestine glutamine synthetase using immunofluorescence and in situ hybridization

Hepatic autophagy is differentially regulated in periportal and pericentral zones - a general mechanism relevant for other tissues?

Background

Liver zonation, the fact that metabolic pathways are spatially separated along the liver sinusoids, is fundamental for proper functioning of this organ. For example, glutamine synthesis from glutamate and ammonia is localized pericentrally in only 7% of the hepatocytes concentrically arranged around the central veins. Recently, we found that FOXO transcription factors lead to upregulation of glutamine synthetase expression inducing autophagy via increasing glutamine production. Since in liver this mechanism can only be functioning in the pericentral zone it remains unclear how autophagy might be regulated in the rest of liver parenchyma.

Presentation of the hypothesis

We hypothesize that the regulation of autophagy by glutamine in liver is zonated. In the periportal zone, autophagy is inhibited by low intracellular glutamine but high essential amino acids, while in the pericentral zone it is stimulated by high intracellular glutamine. This zonation may be controlled by the Wnt and Hedgehog signalling pathways through reciprocal influence on the expression of amino acid transporters and metabolic enzymes in the different zones of the parenchyma.

Testing the hypothesis

The hypothesis can be tested in transgenic mice with conditional hepatocyte-specific modulation of Wnt and Hedgehog signalling. Isolated periportal and pericentral hepatocyte populations allow for determining the different activities of autophagy and its regulating mechanisms in different zones of the parenchyma.

Implications of the hypothesis

Zonation of the regulation of autophagy may allow adapting the extent of the proteolytic breakdown of proteins and organelles to different physiological needs in different zones of liver parenchyma. In this manner metabolic functions can be supported in one zone, for example maintenance of blood glucose levels during starvation which is a periportal issue, while simultaneously preventing cytotoxic events in the opposite zone. Likewise, lipid metabolism can be differentially influenced by uncoupling periportal lipophagy from pericentral breakdown of peroxisomes. Further implications concern the shaping of morphogen gradients along the sinusoidal axis by autophagy, and the different contribution of autophagy to the development of various different liver pathologies. The proposed dependence of the dual glutamine-dependent regulatory mechanisms of autophagy on inverse gradients of Wnt and hedgehog signalling may be relevant for other tissues in which GS is heterogeneously expressed.

Molecular cloning and characterization of glutamine synthetase, a tegumental protein from Schistosoma japonicum

Glutamine synthetase catalyzes the synthesis of glutamine, providing nitrogen for the production of purines, pyrimidines, amino acids, and other compounds required in many pivotal cellular events. Herein, a full-length cDNA encoding Schistosoma japonicum glutamine synthetase (SjGS) was isolated from 21-day schistosomes. The entire open reading frame of SjGS contains a 1,095-bp coding region corresponding to 364 amino acids with a calculated molecular weight of 40.7 kDa. NCBIP blast shows that the putative amino acid of SjGS contains a classic β-grasp domain and a catalytic domain of glutamine synthetase. The relative mRNA expression of SjGS was evaluated in 7-, 13-, 21-, 28-, 35-, and 42-day worms of S. japonicum in the final host and higher expression at day 21, and 42 worms were observed. This protein was also detected in worm extracts using Western blot. Immunofluorescence studies indicated that the SjGS protein was mainly distributed on tegument and parenchyma in 28-day adult worms. The recombinant glutamine synthetase with a molecular weight of 45 kDa was expressed in Escherichia coli and purified in its active form. The enzyme activity of the recombinant protein was 3.30 ± 0.67 U.μg-1. The enzyme activity was highly stable over a wide range of pH (6-9) and temperature (25-40 °C) under physiological conditions. The transcription of SjGS was upregulated in praziquantel-treated worms at 2-, 4-, and 24-h posttreatment compared with the untreated control. As a first step towards the clarification of the role of glutamine synthetase in schistosome species, we have cloned and characterized cDNAs encoding SjGS in S. japonicum, and the data presented suggest that SjGS is an important molecule in the development of the schistosome.

Metabolism and functions of L-glutamate in the epithelial cells of the small and large intestines

l-Glutamate is one of the most abundant amino acids in alimentary proteins, but its concentration in blood is among the lowest. This is largely because l-glutamate is extensively oxidized in small intestine epithelial cells during its transcellular journey from the lumen to the bloodstream and after its uptake from the bloodstream. This oxidative capacity coincides with a high energy demand of the epithelium, which is in rapid renewal and responsible for the nutrient absorption process. l-Glutamate is a precursor for glutathione and N-acetylglutamate in enterocytes. Glutathione is involved in the enterocyte redox state and in the detoxication process. N-acetylglutamate is an activator of carbamoylphosphate synthetase 1, which is implicated in l-citrulline production by enterocytes. Furthermore, l-glutamate is a precursor in enterocytes for several other amino acids, including l-alanine, l-aspartate, l-ornithine, and l-proline. Thus, l-glutamate can serve both locally inside enterocytes and through the production of other amino acids in an interorgan metabolic perspective. Intestinal epithelial cell capacity to oxidize l-glutamine and l-glutamate is already high in piglets at birth and during the suckling period. In colonocytes, l-glutamate also serves as a fuel but is provided from the bloodstream. Alimentary and endogenous proteins that escape digestion enter the large intestine and are broken down by colonic bacterial flora, which then release l-glutamate into the lumen. l-Glutamate can then serve in the colon lumen as a precursor for butyrate and acetate in bacteria. l-Glutamate, in addition to fiber and digestion-resistant starch, can thus serve as a luminally derived fuel precursor for colonocytes.

Glutamine synthetase gene expression during the regeneration of the annelid Enchytraeus japonensis

Enchytraeus japonensis is a highly regenerative oligochaete annelid that can regenerate a complete individual from a small body fragment in 4–5 days. In our previous study, we performed complementary deoxyribonucleic acid subtraction cloning to isolate genes that are upregulated during E. japonensis regeneration and identified glutamine synthetase (gs) as one of the most abundantly expressed genes during this process. In the present study, we show that the full-length sequence of E. japonensis glutamine synthetase (EjGS), which is the first reported annelid glutamine synthetase, is highly similar to other known class II glutamine synthetases. EjGS shows a 61–71% overall amino acid sequence identity with its counterparts in various other animal species, including Drosophila and mouse. We performed detailed expression analysis by in situ hybridization and reveal that strong gs expression occurs in the blastemal regions of regenerating E. japonensis soon after amputation. gs expression was detectable at the cell layer covering the wound and was found to persist in the epidermal cells during the formation and elongation of the blastema. Furthermore, in the elongated blastema, gs expression was detectable also in the presumptive regions of the brain, ventral nerve cord, and stomodeum. In the fully formed intact head, gs expression was also evident in the prostomium, brain, the anterior end of the ventral nerve cord, the epithelium of buccal and pharyngeal cavities, the pharyngeal pad, and in the esophageal appendages. In intact E. japonensis tails, gs expression was found in the growth zone in actively growing worms but not in full-grown individuals. In the nonblastemal regions of regenerating fragments and in intact worms, gs expression was also detected in the nephridia, chloragocytes, gut epithelium, epidermis, spermatids, and oocytes. These results suggest that EjGS may play roles in regeneration, nerve function, cell proliferation, nitrogenous waste excretion, macromolecule synthesis, and gametogenesis.

Cellular concentrations of glutamine synthetase in murine organs

Glutamine synthetase (GS) is the only enzyme that can synthesize glutamine, but it also functions to detoxify glutamate and ammonia. Organs with high cellular concentrations of GS appear to function primarily to remove glutamate or ammonia, whereas those with a low cellular concentration appear to primarily produce glutamine. To validate this apparent dichotomy and to clarify its regulation, we determined the GS concentrations in 18 organs of the mouse. There was a >100-fold difference in GS mRNA, protein, and enzyme-activity levels among organs, whereas there was only a 20-fold difference in the GS protein:mRNA ratio, suggesting extensive transcriptional and posttranscriptional regulation. In contrast, only small differences in the GS enzyme activity : protein ratio were found, indicating that posttranslational regulation is of minor importance. The cellular concentration of GS was determined by relating the relative differences in cellular GS concentration, detected using image analysis of immunohistochemically stained tissue sections, to the biochemical data. There was a >1000-fold difference in cellular concentrations of GS between GS-positive cells in different organs, and cellular concentrations were up to 20x higher in subpopulations of cells within organs than in whole organs. GS activity was highest in pericentral hepatocytes (approximately 485 micromol.g(-1).min-(1), followed in descending order by epithelial cells in the epididymal head, Leydig cells in the testicular interstitium, epithelial cells of the uterine tube, acid-producing parietal cells in the stomach, epithelial cells of the S3 segment of the proximal convoluted tubule of the kidney, astrocytes of the central nervous tissue, and adipose tissue. GS activity in muscle amounted to only 0.4 micromol.g(-1).min(-1). Our findings confirmed the postulated dichotomy between cellular concentration and GS function.

Alanyl-glutamine hastens morphologic recovery from 5-fluorouracil-induced mucositis in mice

OBJECTIVE

In this study, we postulated the beneficial role of oral alanyl-glutamine, a more stable glutamine derivative to decrease 5-fluorouracil (5-FU)-induced mucositis in mice.

METHODS

We measured different morphologic parameters to assess structural changes over time in the small bowel, including crypt depth, villus height, villus area, mitotic and apoptotic indices at the crypt level using terminal deoxyuridine triphosphate nick end labeling, and hematoxylin-eosin staining of ileal tissue. In addition, we analyzed the effect of different alanyl-glutamine concentrations on animal weight curves after 5-FU treatment.

RESULTS

Neither glutamine nor alanyl- glutamine prevented the 5-FU intestinal structural damage or apoptosis in crypt enterocytes at 24 h after 5-FU challenge. However, we found that alanyl-glutamine, but not glutamine, speeds intestinal recovery when compared with 5-FU-treated controls (P < 0.05), predominantly by enhancing mitotic activity and crypt length.

CONCLUSION

Our findings provide important data to support clinical studies of oral alanyl-glutamine in 5-FU-induced mucositis.

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.

Glutamine synthetase in tilapia gastrointestinal tract: zonation, cDNA and induction by cortisol

Glutamine synthetase, an enzyme generally associated with ammonia detoxication in the vertebrate brain and with hepatic nitrogen turnover in mammals, shows substantial activities in the gastrointestinal tract of teleostean fishes. Enzyme activity is highest in the central area of the stomach and reveals a distinct distribution pattern in stomach and along the intestine of tilapia (Oreochromis niloticus), rainbow trout (Oncorhynchus mykiss) and copper rockfish (Sebastes caurinus). In all three species, intestinal activity peaks in the distal region of the intestine. The brain contains the highest titre of the enzyme (46 U g(-1) in tilapia brain versus 15 U g(-1) in tilapia stomach), but because of the relative mass of the stomach, the largest glutamine synthetase pool in tilapia body appears to be localized in the stomach. Activities in white and red muscle are very modest at 0.1% of the brain. Independent of distribution, peak activities of glutamine synthetase in selected areas of tilapia stomach and intestine are significantly (two- to fourfold) increased after a 5-day treatment with an intraperitoneal cortisol deposit. Cortisol also increases glutamine synthetase activity in tilapia liver, white and red muscle, while activities in brain remain unaffected. We cloned and sequenced the predominant transcript of tilapia stomach glutamine synthetase (about 1.9 kb), encoding a 371-amino acid peptide. The open reading frame shows considerable identity with glutamine synthetase in toadfish (92% at peptide level, 87% at nucleotide level), but possesses a longer 3'-untranslated region than the toadfish. The tilapia glutamine synthetase mRNA contains a remnant of a putative mitochondrial leader sequence, but without a conserved second site for initiation of translation. We also find evidence for additional transcripts of glutamine synthetase in tilapia, suggesting multiple genes. Finally, we present evidence for similar abundance of glutamine synthetase transcripts in all regions of rockfish intestine. The physiological significance of the presence of glutamine synthetase in teleostean intestine is discussed.

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.

Starvation alters the activity and mRNA level of glutaminase and glutamine synthetase in the rat intestine

The metabolism of glutamine, the main respiratory fuel of enterocytes, is governed by the activity of glutaminase and glutamine synthetase. Because starvation induces intestinal atrophy, it might alter the rate of intestinal glutamine utilization. This study examined the effect of starvation on the activity, level of mRNA, and distribution of mRNA of glutaminase and glutamine synthetase in the rat intestine. Rats were randomized into groups and were either: (1) fed for 2 days with rat food ad libitum or (2) starved for 2 days. Standardized segments of jejunum and ileum were removed for the estimation of enzyme activity, level of mRNA, and in situ hybridization analysis. The jejunum of the fed rats had a greater activity of both enzymes per centimeter of intestine (P < 0.01), a greater glutaminase specific activity (1.97 +/- 0.45 vs. 1.09 +/- 0.34 micromol/hr/mg protein, P < 0.01), and a lower level of glutaminase and glutamine synthetase mRNA. The ileum of the fed rats had a greater activity of glutamine synthetase per centimeter of intestine (162.9 +/- 50.6 vs. 91.0 +/- 23.1 nmol/hr/cm bowel, P < 0.01), a lower level of glutaminase mRNA, and a greater level of glutamine synthetase mRNA. In situ hybridization analysis showed that starvation does not alter the distribution of glutaminase and glutamine synthetase mRNA in the intestinal mucosa. This study confirms that starvation decreases the total intestinal activity per centimeter of both glutaminase and glutamine synthetase. More importantly, the results indicate that the intestine adapts to starvation by accumulating glutaminase mRNA. This process prepares the intestine for a restoration of intake.

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.

Ontogeny of glutamine synthetase in rat small intestine

Glutamine synthetase (GS) is a key enzyme involved in the endogenous biosynthesis of glutamine, an amino acid known to be essential for small intestinal metabolism and function. This study describes the ontogeny of rat small intestinal GS from fetal life through adulthood with enzyme activities, protein immunoblotting, and steady state levels of GS mRNA by RNA gel blots and dot blots. Enzyme activities progressively increased from 21 d of fetal life to 32 d postnatally, then decreased in adulthood. The amount of GS immunoreactive protein in the small intestine increased from fetal life to 10-day-old infants and persisted into adulthood. GS mRNA, as quantified by dot blots was highest at 19 d postnatally. The ontogenic changes in rat small intestinal GS appear to correspond temporally with rapid growth and weaning. The steady increase in GS enzyme activity up to 32 d of age with a subsequent drop in adulthood is not paralleled by an increase in GS mRNA or protein. These findings suggest an apparent complex regulation of the enzyme activity at a transcriptional or translational levels.