Nicotinamide prevention in diabetes-induced alterations in the rat liver.

Objective. The study was performed to elucidate whether nicotinamide (NAm) can attenuate the diabetes-induced liver damage by correction of ammonia detoxifying function and disbalance of NAD-dependent processes in diabetic rats. Methods. After four weeks of streptozotocin-induced diabetes, Wistar male rats were treated for two weeks with or without NAm. Urea concentration, arginase, and glutamine synthetase activities, NAD+ levels, and NAD+/NADH ratio were measured in cytosolic liver extracts. Expression of parp-1 gene in the liver was estimated by quantitative polymerase chain reaction and PARP-1 cleavage evaluated by Western blotting. Results. Despite the blood plasma lipid peroxidation products in diabetic rats were increased by 60%, the activity of superoxide dismutase (SOD) was reduced. NAm attenuated the oxidative stress, but did not affect the enzyme activity in diabetic rats. In liver of the diabetic rats, urea concentration and arginase activity were significantly higher than in the controls. The glutamine synthetase activity was decreased. Decline in NAD+ level and cytosolic NAD+/NADH ratio in the liver of diabetic rats was observed. Western blot analysis demonstrated a significant up-regulation of PARP-1 expression accompanied by the enzyme cleavage in the diabetic rat liver. However, no correlation was seen between mRNA expression of parp-1 gene and PARP-1 protein in the liver of diabetic rats. NAm markedly attenuated PARP-1 cleavage induced by diabetes, but did not affect the parp-1 gene expression. Conclusions. NAm counteracts diabetes-induced impairments in the rat liver through improvement of its detoxifying function, partial restoration of oxidative stress, NAD+ level, normalization of redox state of free cytosolic NAD+/NADH-couples, and prevention of PARP-1 cleavage.

Effects of the ammonium stress on photosynthesis and ammonium assimilation in submerged leaves of Ottelia cordata - an endangered aquatic plant.

Although ammonium (NH4+-N) is an important nutrient for plants, increases in soil nitrogen (N) input and atmospheric deposition have made ammonium toxicity a serious ecological problem. In this study, we explored the effects of NH4+-N stress on the ultrastructure, photosynthesis, and NH4+-N assimilation of Ottelia cordata (Wallich) Dandy, an endangered heteroblastic plant native to China. Results showed that 15 and 50 mg L-1 NH4+-N damaged leaf ultrastructure and decreased the values of maximal quantum yield (Fv/Fm), maximal fluorescence (Fm), and relative electron transport rate (rETR) in the submerged leaves of O. cordata. Furthermore, when NH4+-N was ≥ 2 mg L-1, phosphoenolpyruvate carboxylase activity (PEPC) and soluble sugar and starch contents decreased significantly. The content of dissolved oxygen in the culture water also decreased significantly. The activity of the NH4+-N assimilation enzyme glutamine synthetase (GS) significantly increased when NH4+-N was ≥ 10 mg L-1 and NADH-glutamate synthase (NADH-GOGAT) and Fd-glutamate synthase (Fd-GOGAT) increased when NH4+-N was at 50 mg L-1. However, the activity of nicotinamide adenine dinucleotide-dependent glutamate dehydrogenase (NADH-GDH) and nicotinamide adenine dinucleotide phosphate-dependent glutamate dehydrogenase (NADPH-GDH) did not change, indicating that GS/GOGAT cycle may play an important role in NH4+-N assimilation in the submerged leaves of O. cordata. These results show that short-term exposure to a high concentration of NH4+-N is toxic to O. cordata.

NMDA receptor-mediated modulation on glutamine synthetase and glial glutamate transporter GLT-1 is involved in the antidepressant-like and neuroprotective effects of guanosine.

Guanosine has been reported to elicit antidepressant-like responses in rodents, but if these actions are associated with its ability to afford neuroprotection against glutamate-induced toxicity still needs to be fully understood. Therefore, this study investigated the antidepressant-like and neuroprotective effects elicited by guanosine in mice and evaluated the possible involvement of NMDA receptors, glutamine synthetase, and GLT-1 in these responses. We found that guanosine (0.05 mg/kg, but not 0.01 mg/kg, p. o.) was effective in producing an antidepressant-like effect and protecting hippocampal and prefrontocortical slices against glutamate-induced damage. Our results also unveiled that ketamine (1 mg/kg, but not 0.1 mg/kg, i. p, an NMDA receptor antagonist) effectively elicited antidepressant-like actions and protected hippocampal and prefrontocortical slices against glutamatergic toxicity. Furthermore, the combined administration of sub-effective doses of guanosine (0.01 mg/kg, p. o.) with ketamine (0.1 mg/kg, i. p.) promoted an antidepressant-like effect and augmented glutamine synthetase activity and GLT-1 immunocontent in the hippocampus, but not in the prefrontal cortex. Our results also showed that the combination of sub-effective doses of ketamine and guanosine, at the same protocol schedule that exhibited an antidepressant-like effect, effectively abolished glutamate-induced damage in hippocampal and prefrontocortical slices. Our in vitro results reinforce that guanosine, ketamine, or sub-effective concentrations of guanosine plus ketamine protect against glutamate exposure by modulating glutamine synthetase activity and GLT-1 levels. Finally, molecular docking analysis suggests that guanosine might interact with NMDA receptors at the ketamine or glycine/d-serine co-agonist binding sites. These findings provide support for the premise that guanosine has antidepressant-like effects and should be further investigated for depression management.

Insm1 promotes differentiation of retinal progenitor cells toward photoreceptor cells in the developing retina through up-regulation of SHH.

This article investigated the effect of Insm1 on RPC differentiation in mice and the underlying mechanism. The retinal tissues of mouse embryo at 12.5 days (E12.5) and postnatal 14 days (P14) were collected, following by the detection of Insm1 and corresponding markers by immunofluorescent staining. RPCs isolated from retinal tissues at P1 were cultured in culture medium for 7 days. The differentiation of photoreceptor and glial cells was assessed after RPCs transferred to the differentiation medium for 20 days. Next, the effect of Insm1 overexpression on the differentiation of RPCs toward rod photoreceptor and glial cells were assessed. Insm1 was highly expressed in RPCs of retinal tissues and decline in photoreceptor cells, while hardly expressed in glial cells. Based on the results of Pax-6 positive immunofluorescent staining and flow cytometry detection, RPCs were successfully isolated from retinal tissues. After the culture in differentiation medium, RPCs showed positive staining of Rhodopsin and glial fibrillary acidic protein (GFAP). Further results showed that overexpression of Insm1 significantly increased the percentage of Rhodopsin positive cells, and up-regulated Sonic Hedgehog (SHH), hairy and enhancer of split homolog-1(Hes1), S-opsin and Rhodopsin levels, while decreased the percentage of Glutamine synthetase positive cells, and reduced Glutamine synthetase and GFAP levels. Whereas, the effect of Insm1 overexpression on these protein levels were partly abolished by the knockdown of SHH or Hes1. We conclude that Insm1 promotes the differentiation of RPCs into photoreceptor cells in the developing retina through up-regulation of SHH.

Phosphonate analog of 2-oxoglutarate regulates glutamate-glutamine homeostasis and counteracts amyloid beta induced learning and memory deficits in rats.

BACKGROUND: Metabolic alteration is a mainstream concept underlying the cognitive decline in neurodegenerative disorders including Alzheimer's disease (AD). Mitochondrial enzyme α-ketoglutarate dehydrogenase complex (α-KGDHC) seems to play a dual-edged sword role in cytotoxic insult. Here, using succinyl phosphonate (SP), a specific α-KGDHC inhibitor, we aimed to examine its potential action on AD progression. METHODS: Male Wistar rats were assigned to two separate experiments. First, they were bilaterally microinjected into the dorsal CA1 area by amyloid-beta (Aß)25-35 for four consecutive days. Seven days after the last injection, they were trained to acquire Morris Water Maze (MWM) task for three successive days when they were treated with SP after each training session. In the second experiment, SP was administered 30 min after the first Aß microinjection and behavioral tests were performed one week after the last Aß administration. The activity of glutamate dehydrogenase (GDH), and glutamine synthetase (GS), as key enzymes involved in glutamate-glutamine homeostasis and histological assays were evaluated in the hippocampi. RESULTS: Our behavioral results indicated that post-training SP treatment enhanced task acquisition but did not change memory performance in Aß-treated rats. However, administration of SP at the time of Aß injection precludes the deteriorative effect of Aß and neuronal injury on both spatial learning and memory performances indicating its preventive action against Aß pathology at its early stages. Measurement of enzymes activity shows that α-KGDHC activity was reduced in the Aß treated group, and SP administration restored its activity; also, GDH and GS activities were increased and decreased respectively due to Aß, and SP reversed the action of Aß on these enzymes. CONCLUSIONS: This study proposes that SP possibly a promising therapeutic approach to improve memory impairment in AD, especially in the early phases of this disease.

Rutin prevents seizures in kainic acid-treated rats: evidence of glutamate levels, inflammation and neuronal loss modulation.

Rutin, a naturally derived flavonoid molecule with known neuroprotective properties, has been demonstrated to have anticonvulsive potential, but the mechanism of this effect is still unclear. The current study aimed to investigate the probable antiseizure mechanisms of rutin in rats using the kainic acid (KA) seizure model. Rutin (50 and 100 mg kg-1) and carbamazepine (100 mg kg-1) were administered daily by oral gavage for 7 days before KA (15 mg kg-1) intraperitoneal (i.p.) injection. Seizure behavior, neuronal cell death, glutamate concentration, excitatory amino acid transporters (EAATs), glutamine synthetase (GS), glutaminase, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits GluA1 and GluA2, N-methyl-D-aspartate (NMDA) receptor subunits GluN2A and GluN2B, activated astrocytes, and inflammatory and anti-inflammatory molecules in the hippocampus were evaluated. Supplementation with rutin attenuated seizure severity in KA-treated rats and reversed KA-induced neuronal loss and glutamate elevation in the hippocampus. Decreased glutaminase and GluN2B, and increased EAATs, GS, GluA1, GluA2 and GluN2A were observed with rutin administration. Rutin pretreatment also suppressed activated astrocytes, downregulated the protein levels of inflammatory molecules [interleukin-1ß (IL-1ß), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), high mobility group Box 1 (HMGB1), interleukin-1 receptor 1 (IL-1R1), and Toll-like receptor-4 (TLR-4)] and upregulated anti-inflammatory molecule interleukin-10 (IL-10) protein expression. Taken together, the results indicate that the preventive treatment of rats with rutin attenuated KA-induced seizures and neuronal loss by decreasing glutamatergic hyperactivity and suppressing the IL-1R1/TLR4-related neuroinflammatory cascade.

Phosphomimetic Dicer S1016E triggers a switch to glutamine metabolism in gemcitabine-resistant pancreatic cancer.

OBJECTIVE: Dicer is an enzyme that processes microRNAs (miRNAs) precursors into mature miRNAs, which have been implicated in various aspects of cancer progressions, such as clinical aggressiveness, prognosis, and survival outcomes. We previously showed that high expression of Dicer is associated with gemcitabine (GEM) resistance in pancreatic ductal adenocarcinoma (PDAC); thus, in this study, we aimed to focus on how Dicer is involved in GEM resistance in PDAC, including cancer prognosis, cell proliferation, and metabolic regulation. METHODS: We generated stable shRNA knockdown of Dicer in GEM-resistant PANC-1 (PANC-1 GR) cells and explored cell viability by MTT and clonogenicity assays. Metabolomic profiling was employed to investigate metabolic changes between parental cells, PANC-1, and PANC-1 GR cells, and further implied to compare their sensitivity to the glutaminase inhibitor, CB839, and GEM treatments. To identify putative phosphorylation site involves with Dicer and its effects on GEM resistance in PDAC cells, we further generated phosphomimetic or phosphomutant Dicer at S1016 site and examined the changes in drug sensitivity, metabolic alteration, and miRNA regulation. RESULTS: We observed that high Dicer levels in pancreatic ductal adenocarcinoma cells were positively correlated with advanced pancreatic cancer and acquired resistance to GEM. Metabolomic analysis indicated that PANC-1 GR cells rapidly utilised glutamine as their major fuel and increased levels of glutaminase (GLS): glutamine synthetase (GLUL) ratio which is related to high Dicer expression. In addition, we found that phosphomimetic Dicer S1016E but not phosphomutant Dicer S1016A facilitated miRNA maturation, causing an imbalance in GLS and GLUL and resulting in an increased response to GLS inhibitors. CONCLUSION: Our results suggest that phosphorylation of Dicer on site S1016 affects miRNA biogenesis and glutamine metabolism in GEM-resistant pancreatic cancer.

The glutamate/GABA system in the retina of male rats: effects of aging, neurodegeneration, and supplementation with melatonin and antioxidant SkQ1.

Glutamate and -aminobutyric acid (GABA) are the most abundant amino acids in the retina. An imbalance of the glutamate/GABA system is involved in the pathogenesis of various neurodegenerative disorders. Here we for the first time analyzed alterations of expression of glutamate- and GABA-synthesizing enzymes, transporters, and relevant receptors in the retina with age in Wistar rats and in senescence-accelerated OXYS rats who develop AMD-like retinopathy. We noted consistent age-dependent expression changes of GABAergic-system proteins (GAD67, GABA-T, and GAT1) in OXYS and Wistar rats: upregulation by age 3 months and downregulation at age 18 months. At a late stage of AMD-like retinopathy in OXYS rats (18 months), there was significant upregulation of glutaminase and downregulation of glutamine synthetase, possibly indicating an increasing level of glutamate in the retina. AMD-like-retinopathy development in the OXYS strain was accompanied by underexpression of glutamate transporter GLAST. Prolonged supplementation with both melatonin and SkQ1 (separately) suppressed the progression of the AMD-like pathology in OXYS rats without affecting the glutamate/GABA system but worsened the condition of the Wistar rat's retina during normal aging. We observed decreasing protein levels of glutamine synthetase, GLAST, and GABAAR1 and an increasing level of glutaminase in Wistar rats. In summary, both melatonin and mitochondrial antioxidant SkQ1 had different effect on the retinal glutamate / GABA in healthy Wistar and senescence-accelerated OXYS rats.

Hypoxia-triggered O-GlcNAcylation in the brain drives the glutamate-glutamine cycle and reduces sensitivity to sevoflurane in mice.

BACKGROUND: Hypersensitivity to general anaesthetics predicts adverse postoperative outcomes in patients. Hypoxia exerts extensive pathophysiological effects on the brain; however, whether hypoxia influences sevoflurane sensitivity and its underlying mechanisms remain poorly understood. METHODS: Mice were acclimated to hypoxia (oxygen 10% for 8 h day-1) for 28 days and anaesthetised with sevoflurane; the effective concentrations for 50% of the animals (EC50) showing loss of righting reflex (LORR) and loss of tail-pinch withdrawal response (LTWR) were determined. Positron emission tomography-computed tomography, O-glycoproteomics, seahorse analysis, carbon-13 tracing, site-specific mutagenesis, and electrophysiological techniques were performed to explore the underlying mechanisms. RESULTS: Compared with the control group, the hypoxia-acclimated mice required higher concentrations of sevoflurane to present LORR and LTWR (EC50LORR: 1.61 [0.03]% vs 1.46 [0.04]%, P<0.01; EC50LTWR: 2.46 [0.14]% vs 2.22 [0.06]%, P<0.01). Hypoxia-induced reduction in sevoflurane sensitivity was correlated with elevation of protein O-linked N-acetylglucosamine (O-GlcNAc) modification in brain, especially in the thalamus, and could be abolished by 6-diazo-5-oxo-l-norleucine, a glutamine fructose-6-phosphate amidotransferase inhibitor, and mimicked by thiamet-G, a selective O-GlcNAcase inhibitor. Mechanistically, O-GlcNAcylation drives de novo synthesis of glutamine from glucose in astrocytes and promotes the glutamate-glutamine cycle, partially via glycolytic flux and activation of glutamine synthetase. CONCLUSIONS: Intermittent hypoxia exposure decreased mouse sensitivity to sevoflurane anaesthesia through enhanced O-GlcNAc-dependent modulation of the glutamate-glutamine cycle in the brain.

DNA-Encoded Glutamine Synthetase Enzyme as Ammonia-Lowering Therapeutic for Hyperammonemia.

Hyperammonemia is a dangerous life-threatening metabolic complication characterized by markedly elevated ammonia levels that can lead to irreversible brain damage if not carefully monitored. Current pharmacological treatment strategies available for hyperammonemia patients are suboptimal and associated with major side effects. In this study, we focus on developing and evaluating the in vivo delivery of novel DNA-encoded glutamine synthetase (GS) enzymes for the treatment of hyperammonemia. Direct in vivo delivered DNA-encoded GS enzyme was evaluated in ammonium acetate-induced hyperammonemia and thioacetamide-induced acute liver injury (ALI) models in C57BL/6 mice. In ammonium acetate-induced hyperammonemia model, we achieved a 30.5% decrease in blood ammonia levels 15 min postadministration of ammonium acetate, with DNA-encoded GS-treated group. Significant increase in survival was observed in ALI model with the treated mice. A comparison of the secreted versus intracellular DNA-encoded GS enzyme demonstrated similar increases in survival in the ALI model, with 40% mortality in the secreted enzymes and 30% mortality in the intracellular enzymes, as compared with 90% mortality in the control group. Direct in vivo delivery of DNA-encoded GS demonstrated important ammonia-lowering potential. These results provide the initial steps toward development of delivered DNA as a potential new approach to ammonia-lowering therapeutics.

Glutamine synthetase protects the spinal cord against hypoxia-induced and GABA(A) receptor-activated axonal depressions.

BACKGROUND: We investigated the effects of exogenous GS on hypoxia- and GABA(A) receptor-induced axonal depression in neonatal rats. METHODS: To assess the effects of GS on spinal cord axons, CAPs were recorded. Hemicords were exposed to hypoxia by 30-minute superfusion with Ringer's solution saturated with 95% N(2) and 5% CO(2) followed by 60-minute exposure to 95% N(2) and 5% CO(2) gassing (N(2) gassing phase) and then 90 minutes of resuperfusion with oxygenated Ringer's solution (resuperfusion phase). Exogenous high GS (15 U) or low GS (1.5 U) was delivered during the N(2) gassing phase. The effects of GS on GABA(A) receptor-induced axonal depression were analyzed with oxygenated isolated dorsal columns. RESULTS: The high GS significantly reduced the decline in the CAP amplitudes during the N(2) gassing and resuperfusion phases (P = .0185) compared to the hypoxia control. The low GS treatment showed a trend toward recovery during the N(2) gassing and resuperfusion phases, but the effect was not significant (P = .3953). In isolated dorsal columns, GS significantly reduced the CAP amplitude depression induced by GABA(A) receptor agonist. CONCLUSIONS: Our findings suggest that GS had dose-dependent protective effects on the spinal cord against hypoxia-induced axonal depression. It may inhibit the depression of CAP amplitudes by blocking GABA(A) receptors.

Exercise restores immune cell function in energy-restricted rats.

OBJECTIVE: To evaluate the effect of chronic moderate-intensity exercise upon the alterations of immune system cell function induced by energy restriction. METHODS: Forty male Wistar rats were randomly assigned to the following groups: sedentary animals fed ad libitum (SF, N = 10) or submitted to energy restriction (SER, N = 10, receiving 50% of the mean amount of chow consumed by SF); and trained animals fed ad libitum (TF, N = 10) or submitted to energy restriction (TER, N = 10), who exercised on a treadmill (at 60-65%VO(2max) 5 d.wk(-1) for 10 wk(-1), after 30 d under the restriction protocol. The incorporation of [2-(14)C]-thymidine by lymphocytes obtained from the spleen and mesenteric lymph nodes, plasma glucose and glutamine concentration, and cytokine production by cells cultivated in the presence of glutamine were measured in all groups, 24 h after the last exercise session. Two-way ANOVA and Tukey's posttest were employed for the statistical analysis. RESULTS: Training induced an increase in the proliferative response and in the production of gamma-interferon and interleukin-1 (P < 0.05) in cells from the spleen and lymph nodes of SER, in which these parameters were diminished when compared with SF (P < 0.05). SER spleen and lymph node cells produced more TNF (26 and 42%, respectively) and IL-2 (49 and 42%, respectively) than SF. The Th1-like diversion of the immune response observed in SER persisted after training. Partial recovery of the decreased SER plasma glutamine concentration and muscle glutamine synthase mRNA was observed. CONCLUSIONS: Training induced the recovery of the proliferative capacity of lymphocytes from SER, probably due to the partial restoration of plasma glutamine levels, but did not interfere with the diversion towards a Th1-type immune response induced by food restriction.

Contribution of glutamine synthetase to ammonia-induced apoptosis in gastric mucosal cells.

BACKGROUND/AIMS: Glutamine synthetase is a key enzyme necessary for ammonia detoxification in the brain, but excessive activation of this enzyme can be cytotoxic to neural cells as a consequence of excessive consumption of ATP and glutamate. The stomach also expresses high levels of glutamine synthetase and this study aimed to investigate a possible pathophysiological role of glutamine synthetase in ammonia-induced gastric mucosal injury. METHODS: Normal rat gastric mucosal epithelial (RGM-1) cells were treated with ammonia, and a specific glutamine synthetase inhibitor (methionine sulfoximine) was used to assess the action of glutamine synthetase. RESULTS: Treatment with ammonia induced apoptotic cell death. Increased expression of p21 and Bax, decreased expression of Bcl-2, cytochrome C release from the mitochondria into the cytosol and subsequent activation of caspase-9 and -3 were identified in the cells treated with ammonia, although there was no apparent change in p53 expression. On the other hand, pretreatment with various concentrations of methionine sulfoximine reduced the glutamine synthetase activity in ammonia-treated RGM-1 cells, and prevented the induction of apoptosis and the reduction in intracellular ATP levels in a dose-dependent manner. CONCLUSIONS: Our results suggested that the energy exhaustion which resulted from an overload of ammonia to glutamine synthetase may have initiated the apoptotic signaling in gastric mucosal cells.

The role of adrenal hormones in the response of glutamine synthetase to fasting in adult and old rats.

BACKGROUND & AIMS: During fasting, skeletal muscle exports increased amounts of glutamine (Gln) while increasing the production of this amino acid by glutamine synthetase (GS) in order to maintain the intramuscular Gln pool. Glucocorticoid hormones are believed to be the principal mediators of GS induction during stress conditions. The aim of this study was to evaluate (1) the effect of fasting on GS activity and expression in skeletal muscle during aging and consequently, (2) the role of glucocorticoids in fasting-induced GS activity. METHODS: Male Wistar rats (6-, 22-month old) were fasted for 5 days and both the activity and expression of GS were measured in tibialis anterior muscle. To better demonstrate the role of glucocorticoids in the response of GS to fasting, we suppressed their action by RU38486 administration (a potent glucocorticoid antagonist) and their production by adrenalectomy in fed and fasted rats. RESULTS: An increase in fasting-induced GS activity was observed in skeletal muscles from both adult and aged rats. Adrenalectomy, but surprisingly not RU38486, suppressed the fasting-induced increase in GS activity and expression. CONCLUSION: The data clearly show that the GS responsiveness to fasting was not modified by aging in skeletal muscle.

Glutamine synthetase and glutamate dehydrogenase in the prefrontal cortex of patients with schizophrenia.

Basing primarily on the facts of altered levels of glutamate neurotransmitter, its receptors and transporters in schizophrenic brain, the "glutamatergic hypothesis" of schizophrenia has been broadened into the field of brain glutamate metabolism. Significantly changed levels of glutamine synthetase (GS) and glutamate dehydrogenase (GDH), the key enzymes involved in glutamine-glutamate cycling between neurons and glia, have been found in the prefrontal cortex (area 10) of patients with schizophrenia compared to controls (P<.01). The data were obtained by enzymatic activity determinations as well as immunoreactivity level evaluations for GS, glutamine synthetase-like protein (GSLP), and three GDH isoenzymes in brain extracts by immunoblotting using specific polyclonal and monoclonal antibodies. Inverse changes in amounts of proteins of GS and GSLP, as well as elevation in amounts of GDH isoenzymes have been observed in schizophrenia. The presented results provide evidence for the impairment of glutamate metabolism and, in turn, abnormalities in functioning of the glutamate-glutamine cycle in the frontal cortex of patients with schizophrenia.

Inhibition of glutamine synthetase activity prevents memory consolidation.

Methionine sulfoximine, a specific inhibitor of the exclusively glial enzyme glutamine synthetase, was shown, at a concentration of 3.5-4.5 mM, to prevent consolidation of memory for a passive avoidance task in day-old chicks. Provided the drug was administered 5-20 min before the learning task, significant retention loss was observed from the normal time of onset of the second of three postulated stages in the memory formation sequence but the drug had to be administered considerably earlier. The amnestic effect of methionine sulfoximine was successfully counteracted by L-glutamine (10 mM) and monosodium glutamate (4 mM), and also by a cocktail of alpha-ketoglutarate (5 mM) and alanine (5 mM). This effect of methionine sulfoximine is attributed to its blockade of the production of glutamine via the glutamate-glutamine cycle, leading to a reduced capacity of neurons to replenish their transmitter glutamate.

Glutamine synthetase-induced enhancement of beta-amyloid peptide A beta (1-40) neurotoxicity accompanied by abrogation of fibril formation and A beta fragmentation.

beta-Amyloid peptide (A beta) is the main constituent in both senile plaques and diffuse deposits in Alzheimer's diseased brains. It was previously shown that synthetic A beta s were able to form free radical species in aqueous solution and cause both oxidative damage to cell proteins and inactivation of key metabolic enzymes. We also previously demonstrated that an interaction of A beta (1-40) with the oxidatively sensitive enzyme glutamine synthetase (GS) resulted in both inactivation of GS and an increase of A beta toxicity to hippocampal cell cultures. In the present study the enhancement of A beta toxicity during interaction with GS was found to be accompanied by abrogation of fibril formation and partial fragmentation of A beta (1-40). HPLC elution profiles demonstrated the production of several peptide fragments. Analysis of the amino acid sequence of the major fragments identified them as the first 15 and the last six amino acids of A beta (1-40). The fragmentation of A beta was inhibited by immunoprecipitation of GS.

Enhancement of beta-amyloid peptide A beta(1-40)-mediated neurotoxicity by glutamine synthetase.

The beta-amyloid peptide (A beta), a main constituent in both senile and diffuse plaques in Alzheimer's disease brains, was previously shown to be neurotoxic and to be able to interact with several macromolecular components of brain tissue. Previous investigations carried out in our laboratory demonstrated free radical species formation in aqueous solutions of A beta(1-40) and its C-end fragment, A beta(25-35). Toxic forms of A beta rapidly inactivate the oxidation-sensitive cytosolic enzyme glutamine synthetase (GS). In this regard, we suggested and subsequently demonstrated that A beta radicals can cause an oxidative damage of cell proteins and lipids resulting in disruption of membrane functions, enzyme inactivation, and cell death. Because GS can be a substrate for A beta-derived oxidizing species, the present study was conducted to determine if GS could protect against A beta neurotoxicity. In contrast to this initial hypothesis, we here report that GS significantly enhances the neurotoxic effects of A beta(1-40). The A beta-mediated inactivation of GS was found to be accompanied by the loss of immunoreactive GS and the significant increase of A beta(1-40) neurotoxicity.

Characterization of the glutamine synthetase amplifiable eukaryotic expression system applied to an integral membrane protein--the human thyrotropin receptor.

An amplifiable eukaryotic expression system, based upon glutamine synthetase, has been applied to the production of a complex integral membrane glycoprotein, the human receptor for the polypeptide hormone thyrotropin (TSH). Production of recombinant protein was achieved in chinese hamster ovary (CHO) cells at levels at least 10-fold higher than has been achieved in any other system. After amplification of the inserted gene, the gene copy number was found to be increased in most (but not all) subclones in the range of 3- to 50-fold; mRNA levels of the individual cell lines broadly followed their gene copy number. The level of protein production (measured both functionally and structurally, by radioligand binding and cytofluorimetry, respectively) also reflected these increases in DNA and RNA, but appeared to be limited to a maximum value which we conclude is the maximum that the cells can tolerate without impairing their viability. The receptor is efficiently coupled to adenylate cyclase (22-45 pM TSH producing a 50% response), although the coupling mechanism appeared to be saturated at higher receptor numbers. The high level of expression has allowed, for the first time, the detection of recombinant TSH receptor by immunochemical means. This expression system should prove very useful, not only in facilitating characterization of the TSH receptor, but also for the production of many other integral membrane proteins in their native form.

Effects of L-glutamine, glutaminase and glutamine synthetase on CAP threshold of cochlear nerve of guinea pig.

Negative direct current (-DC 300 microA) stimulation was applied to the round window of the guinea pig cochlea to exhaust the pre-synaptic intracellular reserves of the transmitter in hair cells, and then the scala tympani was perfused respectively with L-glutamine, glutamine synthetase and glutaminase. Experimental results showed that the negative DC electrical stimulation applied to the round window elevated the CAP threshold of the cochlear nerve in the basal turn of the cochlea, which recovered over a period of approximately 17-39 min. The perfusion of L-glutamine apparently elevated the CAP threshold. The recovery of the CAP threshold following electrical stimulation, however, was accelerated by the perfusion of 10 mmol/L L-glutamine. The time for recovery only took about 5-6 min. The perfusion of enzyme glutamine synthetase elevated the CAP threshold by 50 dB, while glutaminase had little effect. These results suggest that the effect of L-glutamine on the CAP threshold in the cochlea of the guinea pig appears to be that of a potent depolarizing agent which accelerates the recovery of the CAP threshold during the depletion of the transmitter, and L-glutamine may be the candidate for the afferent excitatory transmitter.