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

The Interplay between Neurotransmitters and Calcium Dynamics in Retinal Synapses during Development, Health, and Disease

The intricate functionality of the vertebrate retina relies on the interplay between neurotransmitter activity and calcium (Ca2+) dynamics, offering important insights into developmental processes, physiological functioning, and disease progression. Neurotransmitters orchestrate cellular processes to shape the behavior of the retina under diverse circumstances. Despite research to elucidate the roles of individual neurotransmitters in the visual system, there remains a gap in our understanding of the holistic integration of their interplay with Ca2+ dynamics in the broader context of neuronal development, health, and disease. To address this gap, the present review explores the mechanisms used by the neurotransmitters glutamate, gamma-aminobutyric acid (GABA), glycine, dopamine, and acetylcholine (ACh) and their interplay with Ca2+ dynamics. This conceptual outline is intended to inform and guide future research, underpinning novel therapeutic avenues for retinal-associated disorders.

Ammonia transport is independent of PNH3 gradients across the gastrointestinal epithelia of the rainbow trout: A role for the stomach

Although the gastrointestinal tract (GIT) is an important site for nitrogen metabolism in teleosts, the mechanisms of ammonia absorption and transport remain to be elucidated. Both protein catabolism in the lumen and the metabolism of the GIT tissues produce ammonia which, in part, enters the portal blood through the anterior region of the GIT. The present study examined the possible roles of different GIT sections of rainbow trout (Oncorhynchus mykiss) in transporting ammonia in its unionized gas form-NH₃ -by changing the PNH₃ gradient across GIT epithelia using in vitro gut sac preparations. We also surveyed messenger RNA expression patterns of three of the identified Rh proteins (Rhbg, Rhcg1, and  Rhcg2) as potential NH₃ transporters and NKCC as a potential ammonium ion (NH₄ ⁺ ) transporter along the GIT of rainbow trout. We found that ammonia absorption is not dependent on the PNH₃ gradient despite expression of Rhbg and Rhcg2 in the intestinal tissues, and Rhcg2 in the stomach. We detected no expression of Rhbg in the stomach and no expression of Rhcg1 in any GIT tissues. There was also a lack of correlation between ammonia transport and [NH₄ ⁺ ] gradient despite NKCC expression in all GIT tissues. Regardless of PNH₃ gradients, the stomach showed the greatest absorption and net tissue consumption of ammonia. Overall, our findings suggest nitrogen metabolism zonation of GIT, with stomach serving as an important site for the absorption, handling and transport of ammonia that is independent of the PNH₃ gradient.

Hepatic glutamine synthetase augmentation enhances ammonia detoxification

The urea cycle and glutamine synthetase (GS) are the two main pathways for waste nitrogen removal and their deficiency results in hyperammonemia. Here, we investigated the efficacy of liver-specific GS overexpression for therapy of hyperammonemia. To achieve hepatic GS overexpression, we generated a helper-dependent adenoviral (HDAd) vector expressing the murine GS under the control of a liver-specific expression cassette (HDAd-GS). Compared to mice injected with a control vector expressing an unrelated reporter gene (HDAd-alpha-fetoprotein), wild-type mice with increased hepatic GS showed reduced blood ammonia levels and a concomitant increase of blood glutamine after intraperitoneal injections of ammonium chloride, whereas blood urea was unaffected. Moreover, injection of HDAd-GS reduced blood ammonia levels at baseline and protected against acute hyperammonemia following ammonia challenge in a mouse model with conditional hepatic deficiency of carbamoyl phosphate synthetase 1 (Cps1), the initial and rate-limiting step of ureagenesis. In summary, we found that upregulation of hepatic GS reduced hyperammonemia in wild-type and Cps1-deficient mice, thus confirming a key role of GS in ammonia detoxification. These results suggest that hepatic GS augmentation therapy has potential for treatment of both primary and secondary forms of hyperammonemia.

[¹³N]Ammonia positron emission tomographic/computed tomographic imaging targeting glutamine synthetase expression in prostate cancer

The purpose of this study was to investigate the expression of glutamine synthetase (GS) in prostate cancer (PCa) and the utility of [¹³N]ammonia positron emission tomography/computed tomography (PET/CT) in the imaging of PCa. The uptake ratio of [¹³N]ammonia and the expression of GS in PC3 and DU145 cells was measured. Thirty-four patients with suspected PCa underwent [¹³N]ammonia PET/CT imaging, and immunohistochemistry staining of GS was performed. The uptake of [¹³N]ammonia in PC3 and DU145 cells elevated along with the decrease in glutamine in medium. The expression of GS messenger ribonucleic acid and protein also increased when glutamine was deprived. In biopsy samples, the GS expression scores were significantly higher in PCa tissue than in benign tissues (p < .001), and there was a positive correlation between the maximum GS expression scores and Gleason scores (Spearman r  =  .52). In 34 patients, [¹³N]ammonia uptake in PCa segments was significantly higher than that in benign segments (p ≤ .01), and there was a weak correlation between GS expression scores and the uptake of [¹³N]ammonia (Spearman r  =  .47). The expression of GS in PCa cells upregulated along with the deprivation of glutamine. GS is the main reason for the uptake of [¹³N]ammonia, and [¹³N]ammonia is a useful tracer for PCa imaging.

Biodistribution and estimation of radiation-absorbed doses in humans for 13N-ammonia PET

OBJECTIVE

To evaluate the biodistribution of radiation-absorbed doses of (13)N-ammonia in healthy people.

METHODS

Five healthy human subjects underwent whole-body PET and CT scans after injection of 555-740 MBq of (13)N-ammonia. Five serial dynamic emission scans in each healthy volunteer were acquired. Regions of interest were drawn on the CT image and transferred to the corresponding transverse PET slice. Estimates of the radiation-absorbed doses were calculated using the medical internal radiation dosimetry method.

RESULTS

The highest concentrations of (13)N-ammonia were found in the heart and liver, followed by pancreas, brain, spleen and stomach. The highest absorbed organ doses were to the heart wall (7.14E-03 ± 3.63E-03 mGy/MBq) and kidneys (6.02E-03 ± 3.53E-03 mGy/MBq). The effective dose (ED) was 6.58E-03 ± 1.23E-03 mSv/MBq.

CONCLUSIONS

With these new estimates for (13)N-ammonia dosimetry, the results for Chinese people were not appreciably different from those of the previous study performed with old devices. As one of the most important myocardial perfusion PET tracers, the whole-body (13)N-ammonia PET appears to be safe for humans, yielding a relatively modest radiation burden that would allow multiple PET studies on the same subject per year.

The combination of 13N-ammonia and 18F-FDG in predicting primary central nervous system lymphomas in immunocompetent patients

OBJECTIVE

Accurate identification of primary central nervous system lymphoma (PCNSL) and its differentiation from other brain tumors remain difficult but are essential for treatment. In this study, we investigated whether (13)N-ammonia combined with (18)F-FDG could distinguish PCNSL from solid gliomas effectively.

METHODS

Ten consecutive patients with final diagnosis of PCNSL (5 female and 5 male patients; mean [SD] age, 59.10 [12.47] years; range, 43-74 years) and another fifteen consecutive patients with solid glioma lesions (5 female and 10 male patients; mean [SD] age, 46.73 [19.61] years; range, 14-72 years) were included in this study. PET/CT imaging was performed for all of them with both (18)F-FDG and (13)N-ammonia as tracers. Tumor-to-gray matter (T/G) ratios were calculated for the evaluation of tumor uptake. Both Student t test and discriminant analysis were recruited to assess the differential efficacy of these 2 tracers.

RESULTS

The T/G ratios of (18)F-FDG in PCNSL lesions were higher than in solid gliomas (3.26 [1.18] vs 1.56 [0.41], P < 0.001), whereas the T/G ratios of (13)N-ammonia in PCNSL lesions were lower than in solid gliomas significantly (1.38 [0.20] vs 2.11 [0.69], P < 0.001). All the lesions of PCNSL displayed higher T/G ratios of (18)F-FDG than (13)N-ammonia, whereas 14 (77.8%) of 18 glioma lesions showed contrary results. Tumor classification by means of canonical discriminant analysis yielded an overall accuracy of 96.9%, and only one glioma lesion was misclassified into the PCNSL group.

CONCLUSIONS

PCNSLs and solid gliomas have different metabolic profiles on N-ammonia and F-FDG imaging. The combination of these 2 tracers can distinguish these 2 clinical entities effectively and make an accurate prediction of PCNSL.

Regulation of astrocyte glutamine synthetase in epilepsy

Astrocytes play a crucial role in regulating and maintaining the extracellular chemical milieu of the central nervous system under physiological conditions. Moreover, proliferation of phenotypically altered astrocytes (a.k.a. reactive astrogliosis) has been associated with many neurologic and psychiatric disorders, including mesial temporal lobe epilepsy (MTLE). Glutamine synthetase (GS), which is found in astrocytes, is the only enzyme known to date that is capable of converting glutamate and ammonia to glutamine in the mammalian brain. This reaction is important, because a continuous supply of glutamine is necessary for the synthesis of glutamate and GABA in neurons. The known stoichiometry of glutamate transport across the astrocyte plasma membrane also suggests that rapid metabolism of intracellular glutamate via GS is a prerequisite for efficient glutamate clearance from the extracellular space. Several studies have indicated that the activity of GS in astrocytes is diminished in several brain disorders, including MTLE. It has been hypothesized that the loss of GS activity in MTLE leads to increased extracellular glutamate concentrations and epileptic seizures. Understanding the mechanisms by which GS is regulated may lead to novel therapeutic approaches to MTLE, which is frequently refractory to antiepileptic drugs. This review discusses several known mechanisms by which GS expression and function are influenced, from transcriptional control to enzyme modification.

Abnormalities in glutamate metabolism and excitotoxicity in the retinal diseases

In the physiological condition, glutamate acts as an excitatory neurotransmitter in the retina. However, excessive glutamate can be toxic to retinal neurons by overstimulation of the glutamate receptors. Glutamate excess is primarily attributed to perturbation in the homeostasis of the glutamate metabolism. Major pathway of glutamate metabolism consists of glutamate uptake by glutamate transporters followed by enzymatic conversion of glutamate to nontoxic glutamine by glutamine synthetase. Glutamate metabolism requires energy supply, and the energy loss inhibits the functions of both glutamate transporters and glutamine synthetase. In this review, we describe the present knowledge concerning the retinal glutamate metabolism under the physiological and pathological conditions.

Tissue proteomics by one-dimensional gel electrophoresis combined with label-free protein quantification

Label-free methods streamline quantitative proteomics of tissues by alleviating the need for metabolic labeling of proteins with stable isotopes. Here we detail and implement solutions to common problems in label-free data processing geared toward tissue proteomics by one-dimensional gel electrophoresis followed by liquid chromatography tandem mass spectrometry (geLC MS/MS). Our quantification pipeline showed high levels of performance in terms of duplicate reproducibility, linear dynamic range, and number of proteins identified and quantified. When applied to the liver of an adenomatous polyposis coli (APC) knockout mouse, we demonstrated an 8-fold increase in the number of statistically significant changing proteins compared to alternative approaches, including many more previously unidentified hydrophobic proteins. Better proteome coverage and quantification accuracy revealed molecular details of the perturbed energy metabolism.

Glutamine synthetase deficiency in murine astrocytes results in neonatal death

Glutamine synthetase (GS) is a key enzyme in the "glutamine-glutamate cycle" between astrocytes and neurons, but its function in vivo was thus far tested only pharmacologically. Crossing GS(fl/lacZ) or GS(fl/fl) mice with hGFAP-Cre mice resulted in prenatal excision of the GS(fl) allele in astrocytes. "GS-KO/A" mice were born without malformations, did not suffer from seizures, had a suckling reflex, and did drink immediately after birth, but then gradually failed to feed and died on postnatal day 3. Artificial feeding relieved hypoglycemia and prolonged life, identifying starvation as the immediate cause of death. Neuronal morphology and brain energy levels did not differ from controls. Within control brains, amino acid concentrations varied in a coordinate way by postnatal day 2, implying an integrated metabolic network had developed. GS deficiency caused a 14-fold decline in cortical glutamine and a sevenfold decline in cortical alanine concentration, but the rising glutamate levels were unaffected and glycine was twofold increased. Only these amino acids were uncoupled from the metabolic network. Cortical ammonia levels increased only 1.6-fold, probably reflecting reduced glutaminolysis in neurons and detoxification of ammonia to glycine. These findings identify the dramatic decrease in (cortical) glutamine concentration as the primary cause of brain dysfunction in GS-KO/A mice. The temporal dissociation between GS(fl) elimination and death, and the reciprocal changes in the cortical concentration of glutamine and alanine in GS-deficient and control neonates indicate that the phenotype of GS deficiency in the brain emerges coincidentally with the neonatal activation of the glutamine-glutamate and the associated alanine-lactate cycles.

Glutamine synthetase in muscle is required for glutamine production during fasting and extrahepatic ammonia detoxification

The main endogenous source of glutamine is de novo synthesis in striated muscle via the enzyme glutamine synthetase (GS). The mice in which GS is selectively but completely eliminated from striated muscle with the Cre-loxP strategy (GS-KO/M mice) are, nevertheless, healthy and fertile. Compared with controls, the circulating concentration and net production of glutamine across the hindquarter were not different in fed GS-KO/M mice. Only a approximately 3-fold higher escape of ammonia revealed the absence of GS in muscle. However, after 20 h of fasting, GS-KO/M mice were not able to mount the approximately 4-fold increase in glutamine production across the hindquarter that was observed in control mice. Instead, muscle ammonia production was approximately 5-fold higher than in control mice. The fasting-induced metabolic changes were transient and had returned to fed levels at 36 h of fasting. Glucose consumption and lactate and ketone-body production were similar in GS-KO/M and control mice. Challenging GS-KO/M and control mice with intravenous ammonia in stepwise increments revealed that normal muscle can detoxify approximately 2.5 micromol ammonia/g muscle.h in a muscle GS-dependent manner, with simultaneous accumulation of urea, whereas GS-KO/M mice responded with accumulation of glutamine and other amino acids but not urea. These findings demonstrate that GS in muscle is dispensable in fed mice but plays a key role in mounting the adaptive response to fasting by transiently facilitating the production of glutamine. Furthermore, muscle GS contributes to ammonia detoxification and urea synthesis. These functions are apparently not vital as long as other organs function normally.

Developmental changes in the concentrations of glutamine and other amino acids in plasma and skeletal muscle of the Standardbred foal

Glutamine is concentrated within skeletal muscle, where it has been proposed to play a regulatory role in maintaining protein homeostasis. The work presented here addressed the hypothesis that glutamine would be the most abundant free alpha-AA in plasma and skeletal muscle in the foal during the first year of life. Glycine, however, was the most abundant free alpha-AA in plasma at birth and between 3 and 12 mo of age. The concentration of glutamine, the second most abundant AA at birth, increased through the first 7 d (P < 0.05) and then returned to values similar to those at birth. This resulted in glutamine being the most abundant free alpha-AA in plasma from 1 d through 1 mo of age. The most abundant free alpha-AA in skeletal muscle at birth was glutamine, but the concentration fell by more than 50% by d 15 and continued to decrease, reaching about one-third of the original values by 1 yr of age (P < 0.05). Glutamine synthetase was barely detectable in skeletal muscle at birth, but the abundance increased rapidly within 15 d of birth. The concentration of glycine, the second most abundant alpha AA in muscle at birth, decreased by about 40% by d 15 (P < 0.05) and then stabilized at this value throughout the year. In contrast, glutamate, alanine, and serine concentrations, the third, fourth, and fifth most abundant free alpha-AA in muscle at birth, respectively, increased to new stable concentrations between 3 and 6 mo of age (P < 0.05). This resulted in alanine being the most abundant free alpha-AA in skeletal muscle at 12 mo of age, followed by glutamate, glutamine, and glycine. The decrease in intramuscular glutamine content, particularly during the first 2 wk after birth, is not compatible with a regulatory role for glutamine in muscle protein synthesis because it occurred at the time of maximum growth in these animals. The findings that, at certain times of development, glutamine was not the most abundant free alpha-AA in the foal is novel and signifies that intramuscular glutamine may have functions specific to muscle type and mammalian species.

Deprivation of protein or amino acid induces C/EBPbeta synthesis and binding to amino acid response elements, but its action is not an absolute requirement for enhanced transcription

A nutrient stress signalling pathway is triggered in response to protein or amino acid deprivation, namely the AAR (amino acid response), and previous studies have shown that C/EBPbeta (CCAAT/enhancer-binding protein beta) expression is up-regulated following activation of the AAR. DNA-binding studies, both in vitro and in vivo, have revealed increased C/EBPbeta association with AARE (AAR element) sequences in AAR target genes, but its role is still unresolved. The present results show that in HepG2 human hepatoma cells, the total amount of C/EBPbeta protein, both the activating [LAP* and LAP (liver-enriched activating protein)] and inhibitory [LIP (liver-enriched inhibitory)] isoforms, was increased in histidine-deprived cells. Immunoblotting of subcellular fractions and immunostaining revealed that most of the C/EBPbeta was located in the nucleus. Consistent with these observations, amino acid limitation caused an increase in C/EBPbeta DNA-binding activity in nuclear extracts and chromatin immunoprecipitation revealed an increase in C/EBPbeta binding to the AARE region in vivo, but at a time when transcription from the target gene was declining. A constant fraction of the basal and increased C/EBPbeta protein was phosphorylated on Thr(235) and the phospho-C/EBPbeta did bind to an AARE. Induction of AARE-enhanced transcription was slightly greater in C/EBPbeta-deficient MEFs (mouse embryonic fibroblasts) or C/EBPbeta siRNA (small interfering RNA)-treated HepG2 cells compared with the corresponding control cells. Transient expression of LAP*, LAP or LIP in C/EBPbeta-deficient fibroblasts caused suppression of increased transcription from an AARE-driven reporter gene. Collectively, the results demonstrate that C/EBPbeta is not required for transcriptional activation by the AAR pathway but, when present, acts in concert with ATF3 (activating transcription factor 3) to suppress transcription during the latter stages of the response.

Glutamine synthetase is essential in early mouse embryogenesis

Glutamine synthetase (GS) is expressed in a tissue-specific and developmentally controlled manner, and functions to remove ammonia or glutamate. Furthermore, it is the only enzyme that can synthesize glutamine de novo. Since congenital deficiency of GS has not been reported, we investigated its role in early development. Because GS is expressed in embryonic stem (ES) cells, we generated a null mutant by replacing one GS allele in-frame with a beta-galactosidase-neomycine fusion gene. GS(+/LacZ) mice have no phenotype, but GS(LacZ/LacZ) mice die at ED3.5, demonstrating GS is essential in early embryogenesis. Although cells from ED2.5 GS(LacZ/LacZ) embryos and GS(GFP/LacZ) ES cells survive in vitro in glutamine-containing medium, these GS-deficient cells show a reduced fitness in chimera analysis and fail to survive in tetraploid-complementation assays. The survival of heavily (>90%) chimeric mice up to at least ED16.5 indicates that GS deficiency does not entail cell-autonomous effects and that, after implantation, GS activity is not essential until at least the fetal period. We hypothesize that GS-deficient embryos die when they move from the uterine tube to the harsher uterine environment, where the embryo has to catabolize amino acids to generate energy and, hence, has to detoxify ammonia, which requires GS activity.

Glutamine directly downregulates glutamine synthetase protein levels in mouse C2C12 skeletal muscle myotubes

This study examined the regulation of glutamine synthetase protein levels, in response to changes in external glutamine concentration, in mouse C2C12 skeletal muscle cells. Glutamine, at concentrations as low as 0.25 mmol/L, downregulated endogenous and exogenous (plasmid encoded) glutamine synthetase with maximal effect at 2 mmol/L. Glutamine appears to act by changing the stability of the glutamine synthetase protein, and the effect was partially blocked by the proteasome inhibitor MG132. The addition of the glutamine structural analog and glutaminase inhibitor, 6-diazo-5-oxo-L-norleucine, in the presence or absence of glutamine, also resulted in low glutamine synthetase protein levels. Otherwise, the effect was specific for glutamine, and the only compounds able to mimic the effect of glutamine were amino acids, glutamate, alanine, and ornithine, which can be converted to glutamine. Other amino acids, analogs, and products of glutamine metabolism were without effect. Methionine sulfoximine, an inhibitor of glutamine synthetase, stabilized the protein and prevented the glutamine effect. Thus, in mouse C2C12 skeletal muscle cells, glutamine synthetase protein expression is regulated by glutamine through changes in the rate of degradation of the protein. The effect is specific to glutamine, which acts directly without requiring prior metabolism.

Glutamine, insulin and glucocorticoids regulate glutamine synthetase expression in C2C12 myotubes, Hep G2 hepatoma cells and 3T3 L1 adipocytes

The cell-specific regulation of glutamine synthetase expression was studied in three cell lines. In C2C12 myotubes, glucocorticoids increased the abundance of both glutamine synthetase protein and mRNA. Culture in the absence of glutamine also resulted in very high glutamine synthetase protein abundance but mRNA levels were unchanged. Glucocorticoids also increased the abundance of glutamine synthetase mRNA in Hep G2 hepatoma cells but this was not reflected in changes in protein abundance. Culture of Hep G2 cells without glutamine resulted in very high levels of protein, again with no change in mRNA abundance. Insulin was without effect in both C2C12 and Hep G2 cells. In 3T3 L1 adipocytes glucocorticoids increased the abundance of both glutamine synthetase mRNA and protein, insulin added alone had no effect but in the presence of glucocorticoids resulted in lower mRNA levels than seen with glucocorticoids alone, although protein levels remained high under such conditions. In contrast to the other cell lines glutamine synthetase protein levels were relatively unchanged by culture in the absence of glutamine. The results support the hypothesis that in myocytes, and hepatomas, but not in adipocytes, glutamine acts to moderate glutamine synthetase induction by glucocorticoids.

Hepatocellular expression of glutamine synthetase: an indicator of morphogen actions as master regulators of zonation in adult liver

Glutamine synthetase (GS) has long been known to be expressed exclusively in pericentral hepatocytes most proximal to the central veins of liver lobuli. This enzyme as well as its peculiar distribution complementary to the periportal compartment for ureogenesis plays an important role in nitrogen metabolism, particularly in homeostasis of blood levels of ammonium ions and glutamine. Despite this fact and intensive studies in vivo and in vitro, many aspects of the regulation of its activity on the protein and on the genetic level remained enigmatic. Recent experimental advances using transgenic mice and new analytic tools have revealed the fundamental role of morphogens such as wingless-type MMTV integration site family member signals (Wnt), beta-catenin, and adenomatous polyposis coli in the regulation of this particular enzyme. In addition, novel information concerning the structure of transcription factor binding sites within regulatory regions of the GS gene and their interactions with signalling pathways could be collected. In this review we focus on all aspects of the regulation of GS in the liver and demonstrate how the new findings have changed our view of the determinants of liver zonation. What appeared as a simple response of hepatocytes to blood-derived factors and local cellular interactions must now be perceived as a fundamental mechanism of adult tissue patterning by morphogens that were considered mainly as regulators of developmental processes. Though GS may be the most obvious indicator of morphogen action among many other targets, elucidation of the complex regulation of the expression of the GS gene could pave the road for a better understanding of the mechanisms involved in patterning of liver parenchyma. Based on current knowledge we propose a new concept of how morphogens, hormones and other factors may act in concert, in order to restrict gene expression to small subpopulations of one differentiated cell type, the hepatocyte, in different anatomical locations. Although many details of this regulatory network are still missing, and an era of exciting new discoveries is still about to come, it can already be envisioned that similar mechanisms may well be active in other organs contributing to the fine-tuning of organ-specific functions.

Alterations in glutamine synthetase activity in rat skeletal muscle are associated with advanced age

OBJECTIVE

Glutamine synthetase (GS), a key enzyme in the production of glutamine, is preserved in skeletal muscle during early aging (<24 mo). Because the effects of advanced age on GS are unknown, we investigated the effect of advanced age (>24 mo) on GS activity in skeletal muscle. We hypothesized that advanced age would enhance muscle GS activity.

METHODS

Muscle GS activities were assessed in adult (8 mo), mature adult (15 mo), aged (20-22 mo), advanced age (25-27 mo), or very advanced age (29-32 mo) female Wistar rats. Male Wistar (6-27 mo) were used to investigate the effect of gender on this activity.

RESULTS

Glutamine synthetase activity remained low and unaltered in rats from 8 to 22 mo of age, as previously demonstrated. In contrast, GS activity was high ( approximately 75% of individual values were higher than the low value mean) in 25-mo to 27-mo-old rats. In very-old-aged rats (29-32 mo), approximately 55% of GS activity data points exhibited low values. Changes in GS protein content paralleled those in GS activities. In male rats, GS activity was also high ( approximately 80% of individual values were higher than the mean value of 6-mo to 19-mo-old rats) at the upper limit of life expectancy (27 mo).

CONCLUSION

There is enhanced GS activity in old female and male rats suggesting a greater need for glutamine. In some very old rats, low GS activity may be associated with longevity or reflect a limitation in glutamine production due to extremely advanced age per se.

The 3′-UTR of the glutamine-synthetase gene interacts specifically with upstream regulatory elements, contains mRNA-instability elements and is involved in glutamine sensing

Glutamine synthetase (GS) is expressed at various levels in a wide range of tissues, suggesting that a complex network of modules regulates its expression. We explored the interactions between the upstream enhancer, regulatory regions in the first intron, and the 3'-untranslated region and immediate downstream genomic sequences of the GS gene (the GS "tail"), and compared the results with those obtained previously in conjunction with the bovine growth hormone (bGH) tail. The statistical analysis of these interactions revealed that the GS tail was required for full enhancer activity of the combination of the upstream enhancer and either the middle or the 3'-intron element. The GS tail also prevented a productive interaction between the upstream enhancer and the 5'-intron element, whereas the bGH tail did not, suggesting that the 5'-intron element is a regulatory element that needs to be silenced for full GS expression. Using the CMV promoter/enhancer and transfection experiments, we established that the 2.8 kb GS mRNA polyadenylation signal is approximately 10-fold more efficient than the 1.4 kb mRNA signal. Because the steady-state levels of both mRNAs are similar, the intervening conserved elements destabilize the long mRNA. Indeed, one but not all constructs containing these elements had a shorter half life in FTO-2B cells. A construct containing only 300 bases before and 100 bases after the 2.8 kb mRNA polyadenylation site sufficed for maximal expression. A stretch of 21 adenines inside this fragment conferred, in conjunction with the upstream enhancer and the 3'-part of the first intron, sensitivity of GS expression to ambient glutamine.

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.

Analysis of reproductive toxicity and classification of glufosinate-ammonium

CONCLUSION REGARDING CLASSIFICATION OF GLUFOSINATE-AMMONIUM: Science Partners' Evaluation Group (Evaluation Group) has conducted an independent analysis of the herbicide glufosinate-ammonium (GA) relative to its potential to cause reproductive toxicity in humans. Further, the Evaluation Group has evaluated the implementation of Annex 6 of Commission Directive 2001/59/EC (28th ATP of Council Directive 67/548/EEC) and Council Directive 91/414/EEC, with respect to classification of chemicals posing potential reproductive hazards. After consideration of all information available to us relevant to the potential of glufosinate-ammonium (GA) to cause reproductive toxicity, the Science Partners Evaluation Group concludes that no classification of GA is justified. The following form the basis of this conclusion. There are no human data to suggest that GA causes reproductive toxicity in women or in their conceptus. The issue concerning possible reproductive hazard to humans is raised solely on the basis of positive animal test results that show GA to cause preimplantation or implantation losses in rats. SPECIFICALLY: a. Daily treatment with GA had no detectable effect on the earliest stages of the reproductive sequence including gametogenesis, ovulation, mating and conception; b. Treatment with GA interfered with rat gestation before and at the stage when the conceptus implants into the uterus. This effect occurred at doses of 360 ppm in the feed (corresponding to daily doses of 27.8 mg/kg bw) and above; and c. After implantation, no further effect of GA on prenatal and post-natal development was recognized. Previous concerns that GA might be toxic to embryonic stages after implantation were not supported by the data. Abortions and stillbirth seen were associated with, and regarded as secondary to, maternal toxicity. There was no evidence suggesting the induction of malformations in the offspring. The mechanism underlying this adverse effect in experimental laboratory animals is identified-inhibition of glutamine synthetase. Glutamine is essential to the viability of the embryo. The embryo is dependent on a maternal source of the amino acid. For embryo lethality to occur, a significant reduction of maternal glutamine is required. Such reduction in maternal glutamine depends on a significant inhibition of glutamine synthetase by GA. This can only occur when the mother is exposed to very high levels of GA. SPECIFICALLY: a. The reproductive toxicity of GA is confined to very short, early stages of reproduction, during which the conceptus is dependent on maternal glutamine; and b. In order for the effect to occur, significant reduction in maternal blood glutamine level is required, which in turn depends on a significant inhibition of glutamine synthetase, induced by high levels of GA in the maternal system. There is no evidence for accumulation of GA in the mammalian organism beyond a factor of two and no evidence for its metabolic toxification. To raise a concern in humans, women would have to be exposed to GA during the very limited time frame of preimplantation or implantation and the exposure would have to be to the exceedingly high levels necessary to alter the maternal metabolism and, correspondingly, result in glutamine levels in maternal tissue and blood plasma being drastically reduced. There is no basis to suggest that such exposures would occur under conditions of normal handling and use. SPECIFICALLY: a. Under conditions of normal handling and use, operators would never be exposed to GA levels that could potentially inhibit glutamine synthetase to the extent that this inhibition could impair preimplantation or implantation. b. All acceptable exposure measurements and predictive calculations confirm this conclusion, and in fact demonstrate that reasonably foreseeable exposure of workers would be to levels significantly below the AOEL. c. The evidence is also clear that there is no reproductive toxicity hazard to workers upon reentry tosprayed fields, bystanders, consumers or toddlers. The safety margin compared to the NOAEL in animal studies is sufficiently large to assure protection of the health of workers using GA as well as bystanders, consumers, and toddlers. Pursuant to Annex 6 of Commission Directive 2001/59/EC (28th ATP of Council Directive 67/548/EEC), to justify a classification of category 2 there must be sufficient evidence to produce a strong presumption that human exposure to the substance may result in impaired fertility in humans. It is the conclusion of the Science Partners Evaluation Group that there is no reasonable evidence to suggest a strong presumption of impairment. To the contrary, there is clear evidence demonstrating a strong presumption that exposure to GA would not cause the adverse effect demonstrated in rats. Pursuant to Annex 6 of Commission Directive 2001/59/EC (28th ATP of Council Directive 67/548/EEC), to justify a classification of category 3, there must be sufficient evidence to provide a strong suspicion of impaired fertility in humans. There is no basis to conclude that the animal data demonstrating impaired preimplantation or implantation has any relevance to humans in that the effect found in rats only occurs at levels which would never be experienced by workers under conditions of normal handling and use or by bystanders, consumers, or toddlers.

Nutritional control of gene expression: how mammalian cells respond to amino acid limitation

The amino acid response (AAR) pathway in mammalian cells is designed to detect and respond to amino acid deficiency. Limiting any essential amino acid initiates this signaling cascade, which leads to increased translation of a "master regulator," activating transcription factor (ATF) 4, and ultimately, to regulation of many steps along the pathway of DNA to RNA to protein. These regulated events include chromatin remodeling, RNA splicing, nuclear RNA export, mRNA stabilization, and translational control. Proteins that are increased in their expression as targets of the AAR pathway include membrane transporters, transcription factors from the basic region/leucine zipper (bZIP) superfamily, growth factors, and metabolic enzymes. Significant progress has been achieved in understanding the molecular mechanisms by which amino acids control the synthesis and turnover of mRNA and protein. Beyond gaining additional knowledge of these important regulatory pathways, further characterization of how these processes contribute to the pathology of various disease states represents an interesting aspect of future research in molecular nutrition.

The RL-ET-14 cell line mediates expression of glutamine synthetase through the upstream enhancer/promoter region

BACKGROUND/AIMS

The expression of glutamine synthetase (GS) in the mammalian liver is confined to the hepatocytes surrounding the central vein and can be induced in cultures of periportal hepatocytes by co-cultivation with the rat-liver epithelial cell line RL-ET-14. We exploited these observations to identify the regulatory regions of the GS gene and the responsible signal-transduction pathway that mediates this effect.

METHODS

Fetal hepatocytes of wild-type or GS-transgenic mice were co-cultured with RL-ET-14 cells to induce GS expression. Small-interfering RNA was employed to silence beta-catenin expression in the fetal hepatocytes prior to co-culture.

RESULTS

Co-cultivation of RL-ET-14 cells with fetal mouse hepatocytes induced GS expression 4.2-fold. The expression of another pericentral enzyme, ornithine aminotransferase and a periportal enzyme, carbamoylphosphate synthetase, were not affected. Co-culture of RL-ET-14 cells with transgenic fetal mouse hepatocytes demonstrated that GS expression was induced via its upstream enhancer located at -2.5 kb and that the signal mediator required a functional beta-catenin pathway.

CONCLUSIONS

The 'RL-ET-14' factor specifically induces GS expression, working via its upstream enhancer in a beta-catenin-dependent fashion.

Glucocorticoids induce glutamine synthetase expression in human osteoblastic cells: a novel observation in bone

Glucocorticoids have marked effects on bone metabolism, and continued exposure of skeletal tissue to excessive amounts of these steroids results in osteoporosis. Therefore, in the present proteomic study, we characterized the potential effects of glucocorticoids on protein expression in human osteoblastic cells. Using two-dimensional gel electrophoresis and mass spectrometry, we identified an increased expression of glutamine synthetase (GS) in dexamethasone (Dex)-treated human MG-63 osteosarcoma cells. GS is an enzyme catalyzing the conversion of glutamate and ammonia to glutamine. Intracellular and extracellular glutamate levels may be important in cell signalling mediated by glutamate transporters and receptors which have recently been found in bone cells. The induction of GS protein by Dex was accompanied by an increase in mRNA level and enzyme activity. Dex induction of GS was also mediated by glucocorticoid receptors (GRs) because it was blocked by the GR antagonist RU-38486. In addition, Dex induction of GS expression was partially blocked by cyclohexamide indicating that it at least partly required new protein synthesis. GS induction by Dex was not associated with apoptosis as determined by Bax/Bcl-2 ratio and DNA staining. In addition to MG-63 cells, Dex induction of GS was also observed in human G-292 osteosarcoma cells as well as conditionally immortalized human preosteoblastic (HOB-03-C5) and mature osteoblastic (HOB-03-CE6) cells. However, in two other human osteosarcoma cell lines, SaOS-2 and U2-OS, GS expression was not affected by Dex. This observation may be explained by the lower levels of GR protein in these cells. In summary, this is the first report of the regulation of GS expression by glucocorticoids in bone cells. The role of GS in bone cell metabolism and glucocorticoid action on the skeleton is not yet known, but as a modulator of intracellular glutamate and glutamine levels, it may have an important role in these processes.

N-terminal extension of canine glutamine synthetase created by splicing alters its enzymatic property

It was found that an extra exon exists in the first intron of glutamine synthetase gene, generated by means of alternative splicing. Inclusion of this exon decreased the translation of glutamine synthetase (GS) in human, dog, and mouse. When translated in vitro with the canine GS transcript containing the exon, we obtained two different species of GS enzymes. Besides the known 45-kDa protein, the extended form of GS was identified with additional 40 amino acids on its N-terminal end. An upstream ATG in the extra exon served as a translation initiator for the long form of GS. When the long transcript was translated in vivo in animal cells, only the long GS was expressed. On the other hand, the long GS is less predominant relative to the short one in canine tissues including brain and liver. Subcellular fractionation of canine brain revealed that the long GS is present in all cellular compartments as is the short one, which is consistent with fluorescence microscopy data obtained with green fluorescent protein fused to GS. The short (SGS) and long (LGS) forms of canine GS were purified in Escherichia coli and shown to have similar Km values for l-glutamate and hydroxylamine. However, the Km values for ATP were slightly altered, 1.3 and 1.9 mm for the short and long GSs, respectively. The Kis for l-methionine-S-sulfoximine (MSOX), a highly potent ATP-dependent inactivator of GS, were considerably different such that the values are 0.067 and 0.124 mm for the short and long forms, respectively. When the intrinsic fluorescences of tryptophans were monitored upon bindings of chloride and metal ions without any effect on the oligomeric state, the pattern of quenching in LGS was significantly different from that of SGS. Taken together, the N-terminal extension in the long isoform of GS induces a conformational change of core enzyme, leading to a change in affinity to its substrates as well as in the effector-induced conformational alterations.

A splice variant acquiring an extra transcript leader region decreases the translation of glutamine synthetase gene

The expression of glutamine synthetase (GS), catalysing the ATP-dependent conversion of glutamate and ammonia into glutamine, is transcriptionally and post-transcriptionally regulated. The genomic structure of dog GS shown in the present study is basically similar to that of other mammals in that it is composed of seven exons and six introns. Using 5'-cRACE (where cRACE stands for circular rapid amplification of cDNA ends) and reverse transcriptase-PCR, we identified an additional exon (120 bp) in the first intron, designated in the present study as exon 1'. By means of alternative splicing, the GS gene produces an altered form of GS transcript with 5'-untranslated region (UTR) containing the exon 1'. This alternative transcript is abundantly expressed in brain, whereas it is found at lower levels in other tissues. In the human and mouse GS genes, extra exons are also found at the corresponding site of the intron 1 but with different sizes. An exon-trapping experiment for the GS gene in COS-7, Madin-Darby canine kidney and SK-N-SH cells revealed that the pattern of alternative splicing is variable in different cell types. The propensity of forming a secondary structure is predicted to be considerably higher in the presence of extra 5'-UTR, suggesting the possibility of a translational effect. To test this, we performed a reporter assay for fusions with different 5'-UTRs, demonstrating that the long form with extra 5'-UTR was translated 20- and 10-fold less than the short one in SK-N-SH and Neuro-2A cells respectively. Similarly, translations of human and mouse transcripts with extra 5'-UTRs were less efficient, showing 6-8-fold reductions in SK-N-SH cells. Furthermore, when we mutated an ATG sequence contained in the exon 1', the suppression of translation was partially relieved, suggesting that the negative regulation by an extra 5'-UTR is, to some extent, due to an abortive translation from the upstream ATG.

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.

Upstream and intronic regulatory sequences interact in the activation of the glutamine synthetase promoter

Glutamine synthetase (GS) is expressed at high levels in subsets of cells in some tissues and at low levels in all cells of other tissues, suggesting that the GS gene is surrounded by multiple regulatory elements. We searched for such elements in the 2.5-kb upstream region and in the 2.6-kb first intron of the GS gene, using FTO-2B hepatoma and C2/7 muscle cells as representatives of both cell types and transient transfection assays as our tools. In addition to the entire upstream region and entire intron, an upstream enhancer module at -2.5 kb, and 5', middle and 3' modules of the first intron were tested. The main effects of the respective modules and their combinatorial interactions were quantified using the analysis of variance (anova) technique. The upstream enhancer was strongly stimulatory, the middle intron module strongly inhibitory, and the 3'-intron module weakly stimulatory in both hepatoma and muscle cells. The 5'-intron module was strongly stimulatory in muscle cells only. The major new finding was that in both cell types, the upstream enhancer and 5'-intron module needed to be present simultaneously to fully realize their transactivational potencies. This interaction was responsible for a pronounced inhibitory effect of the 5'-intron module in the absence of the upstream enhancer in hepatoma cells, and for a strong synergistic effect of these two modules, when present simultaneously in muscle cells. The main difference between hepatoma and muscle cells therefore appeared to reside in tissue-specific differences in activity of the respective regulatory elements due to interactions rather than in the existence of tissue-specific regulatory elements.

Hepatic glutamine metabolism

Expression of high activities of both glutamine synthetase and glutaminase allows the liver to play a major role in the regulation of glutamine homeostasis. The liver shows net glutamine output in metabolic acidosis, in prolonged starvation and animals bearing tumors, net glutamine uptake in the postabsorptive state, on consuming high protein diets, and in uncontrolled diabetes or sepsis. Liver glutamine synthetase is expressed only in a small population of perivenous cells that allows it to salvage any ammonia not incorporated into urea in periportal cells. Hepatic glutaminase is a unique isozyme found only in periportal liver parenchymal cells where it provides glutamate and ammonia for the urea cycle. Control of hepatic glutamine metabolism occurs almost exclusively through changes in the activity of glutaminase, with no change in glutamine synthetase flux.

Do sex steroids regulate glutamine synthesis with age?

Glutamine synthetase, a key enzyme in the production of glutamine, is known to be induced by glucocorticoids and preserved in skeletal muscle during aging, but the effect of other steroids, such as sex steroids (progesterone, estradiol), is unknown in vivo. The aim of this study was to determine whether progesterone or estradiol plays a role in the regulation of glutamine synthetase (GS) with aging. The effects of glucocorticoids and sex steroids on muscle GS activity and mRNA expression were measured in adult (6-8 mo; n = 7 in each group) and aged (26 mo; n = 10 in each group) female Wistar rats after adrenalectomy (ADX), ovariectomy (OV), or both (ADXOV) and were compared with those in sham-operated (Sham) control rats. In tibialis anterior muscle, ADX noticeably decreased both GS activity and expression irrespective of age (50-60%; P < 0.05), whereas OV had no effect at either age. Progesterone and estradiol replacement had no effect on the recovery of muscle GS response in either ADX or OV rats, regardless of age. In contrast, heart GS activity was decreased by ADX in aged animals only. These results suggest that the reproductive endocrine status of female rats does not affect muscle GS activity either in muscle or in heart, in young or aged animals, and that the heart GS response to steroids may be differently regulated in aged rats.

CCAAT/enhancer-binding protein-beta is a mediator of the nutrient-sensing response pathway that activates the human asparagine synthetase gene

Transcription from the human asparagine synthetase (AS) gene is increased in response to either amino acid (amino acid response) or glucose (unfolded protein response) deprivation. These two independent pathways converge on the same set of genomic cis-elements within the AS promoter, which are referred to as nutrient-sensing response element (NSRE)-1 and -2, both of which are absolutely necessary for gene activation. The NSRE-1 sequence was used to identify the corresponding transcription factor by yeast one-hybrid screening. Based on those results, electrophoretic mobility shift assays for individual CCAAT/enhancer-binding protein-beta (C/EBP) family members were performed to test for supershifting of complexes by specific antibodies. The results indicated that of all the family members, C/EBPbeta bound to the NSRE-1 sequence to the greatest extent and that the absolute amount of this complex was increased when extracts from amino acid- or glucose-deprived cells were tested. Using electrophoretic mobility shift assays, mutation of the NSRE-1 sequence completely prevented formation of the C/EBPbeta-containing complexes. In contrast, mutation of the NSRE-2 sequence did not block C/EBPbeta binding. Overexpression in HepG2 hepatoma cells of the activating isoform of C/EBPbeta increased AS promoter-driven transcription, whereas the inhibitory dominant-negative isoform of C/EBPbeta blocked enhanced transcription following amino acid or glucose deprivation. Collectively, the results provide both in vitro and in vivo evidence for a role of C/EBPbeta in the transcriptional activation of the AS gene in response to nutrient deprivation.

Multiple adaptive mechanisms affect asparagine synthetase substrate availability in asparaginase-resistant MOLT-4 human leukaemia cells

Childhood acute lymphoblastic leukaemia is treated by combination chemotherapy with a number of drugs, almost always including the enzyme L-asparaginase (ASNase). Although the initial remission rate is quite high, relapse and associated drug resistance remain a problem. In vitro studies have demonstrated an adaptive increase in asparagine synthetase (AS) expression in ASNase-resistant cells, which is believed to permit ASNase-resistant human leukaemia cells to survive in vivo. The present results, obtained with ASNase-sensitive and -resistant human MOLT-4 leukaemia cell lines, illustrate that several other adaptive processes occur to provide sufficient amounts of the AS substrates, aspartate and glutamine, required to support this increased enzymic activity. In both cell populations, aspartate is derived almost exclusively from intracellular sources, whereas the necessary glutamine arises from both intracellular and extracellular sources. Transport of glutamine into ASNase-resistant cells is significantly enhanced compared with the parental cells, whereas amino acid efflux (e.g. asparagine) is reduced. Most of the adaptive change for the amino acid transporters, Systems A, ASC and L, is rapidly (12 h) reversed following ASNase removal. The enzymic activity of glutamine synthetase is also enhanced in ASNase-resistant cells by a post-transcriptional mechanism. The results demonstrate that there are several sites of metabolic adaptation in ASNase-treated leukaemia cells that serve to promote the replenishment of both glutamine and asparagine.

Hypoxia regulates glutamate metabolism and membrane transport in rat PC12 cells

We investigated the effect of hypoxia on glutamate metabolism and uptake in rat pheochromocytoma (PC12) cells. Various key enzymes relevant to glutamate production, metabolism and transport were coordinately regulated by hypoxia. PC12 cells express two glutamate-metabolizing enzymes, glutamine synthetase (GS) and glutamate decarboxylase (GAD), as well as the glutamate-producing enzyme, phosphate-activated glutaminase (PAG). Exposure to hypoxia (1% O(2)) for 6 h or longer increased expression of GS mRNA and protein and enhanced GS enzymatic activity. In contrast, hypoxia caused a significant decrease in expression of PAG mRNA and protein, and also decreased PAG activity. In addition, hypoxia led to an increase in GAD65 and GAD67 protein levels and GAD enzymatic activity. PC12 cells express three Na(+)-dependent glutamate transporters; EAAC1, GLT-1 and GLAST. Hypoxia increased EAAC1 and GLT-1 protein levels, but had no effect on GLAST. Chronic hypoxia significantly enhanced the Na(+)-dependent component of glutamate transport. Furthermore, chronic hypoxia decreased cellular content of glutamate, but increased that of glutamine. Taken together, the hypoxia-induced changes in enzymes related to glutamate metabolism and transport are consistent with a decrease in the extracellular concentration of glutamate. This may have a role in protecting PC12 cells from the cytotoxic effects of glutamate during chronic hypoxia.

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.

Dietary glutamine suppresses endogenous glutamine turnover in the rat

Plasma glutamine turnover was determined using 1-14C-labeled glutamine in rats that consumed crystalline amino acid diets containing the equivalent of 16% protein with 25% of the amino acids as glutamine or a control diet containing no glutamine (or glutamate) for 10 days. Glutamine turnover in glutamine-fed animals was 66% of the rate in the control group. Glutamine feeding caused 20% higher levels of arterial plasma glutamine. Arterial-portal differences across the portal-drained viscera showed net glutamine uptake in control animals but no net uptake or release in the glutamine-fed group. Skeletal muscle glutamine synthetase activity was similar in both groups. The results indicate that long-term consumption of relatively large amounts of dietary glutamine decreases the turnover of plasma glutamine and thus reduces the need for endogenous glutamine synthesis.

Glutamine synthetase expression in perinatal spiny mouse liver

The pronounced increase in the protein/mRNA ratio of ammonia-metabolising enzymes in rat liver in the last prenatal week represents a clear example of a post-transcriptional level of control of gene expression. Both the underlying mechanism, namely an increase in translational efficiency of the mRNA and/or enhanced stability of the protein, and its importance for perinatal adaptation are unknown. We investigated this process in spiny mouse liver, because the comparison of rat and spiny mouse can discriminate adaptively from developmentally regulated processes in the perinatal period. We focused on glutamine synthetase (GS) because of the conveniently small size of its mRNA. Prenatally, GS enzyme activity slowly accumulated to approximately 1.3 U x g-1 liver at birth and postnatally more rapidly to 5.5 U x g-1 at 2 weeks. Both phases of enzyme accumulation obeyed exponential functions. Western-blot analysis showed that changes in GS activity reflected changes in GS protein content. GS mRNA content of the liver was 45 fmol x g-1 at 2 weeks before birth and slowly declined to approximately 25 fmol x g-1 at 2 weeks after birth. The GS protein/mRNA ratio increased 2.5-fold prenatally and sixfold postnatally. Analysis of prenatal and postnatal polysome profiles revealed no evidence of GS mRNA-containing ribonucleoprotein particles. Instead, GS mRNAs were (fully) occupied by 12 ribosomes, indicating regulation at the level of elongation. The kinetics of GS protein accumulation, in conjunction with GS mRNA content, are consistent with an approximately sixfold increase in its rate of synthesis at birth as the result of a corresponding stimulation of the rate of elongation.