Newer aspects of glutamine/glutamate metabolism: the role of acute pH changes

Effects of higher dietary acid load: a narrative review with special emphasis in children

Metabolic effects of high diet acid load (DAL) have been studied for years in adults, although only recently in children. Contemporary diets, especially those of Western societies, owe their acidogenic effect to high animal-origin protein content and low contribution of base-forming elements, such as fruits and vegetables. This imbalance, where dietary acid precursors exceed the body's buffering capacity, results in an acid-retaining state known by terms such as "eubicarbonatemic metabolic acidosis," "low-grade metabolic acidosis," "subclinical acidosis," or "acid stress". Its consequences have been linked to chronic systemic inflammation, contributing to various noncommunicable diseases traditionally considered more common in adulthood, but now have been recognized to originate at much earlier ages. In children, effects of high DAL are not limited to growth impairment caused by alterations of bone and muscle metabolism, but also represent a risk factor for conditions such as obesity, insulin resistance, diabetes, hypertension, urolithiasis, and chronic kidney disease (CKD). The possibility that high DAL may be a cause of chronic acid-retaining states in children with growth impairment should alert pediatricians and pediatric nephrologists, since its causes have been attributed traditionally to inborn errors of metabolism and renal pathologies such as CKD and renal tubular acidosis. The interplay between DAL, overall diet quality, and its cascading effects on children's health necessitates comprehensive nutritional assessments and interventions. This narrative review explores the clinical relevance of diet-induced acid retention in children and highlights the potential for prevention through dietary modifications, particularly by increasing fruit and vegetable intake alongside appropriate protein consumption.

Hepatic and extrahepatic metabolic modulation in hbv-related decompensated cirrhosis and acute-on-chronic liver failure

Acute-on-chronic liver failure (ACLF) and decompensated cirrhosis (DC) are life-threatening syndromes that can develop at the end-stage of chronic hepatitis B virus (HBV) infection. Both ACLF and DC are complicated by hepatic and extrahepatic pathogeneses. To better understand the compartment-specific metabolic modulations related to their pathogenesis, HBV-DC, HBV-ACLF patients, and controls (30 each) were analyzed by metabolomics using portal (Port), hepatic vein (Hep), and peripheral (Peri) serum. Compartment ratios of metabolites (RatioHep/Port, RatioPeri/Hep, and RatioPort/Peri) were calculated. The liver tissues (10 per group) were analyzed using transcriptomics and metabolomics. An additional 75 patients with ACLF, 20 with DC, and 20 with liver cirrhosis (LC) were used to confirm oxlipid dysregulation. Both multi-omics datasets suggest suppressed energy, amino acid, and pyrimidine metabolism in the ACLF/DC liver. The serum metabolomic variations were contributed primarily by disease rather than sampling compartments, as both HBV-ACLF and HBV-DC patients demonstrated abnormal profiles of amino acids and peptides, indoles, purines, steroids, and benzimidazoles. In ACLF/DC patients, impaired hepatic metabolism resulted in a highly correlated hepatic and portal vein serum metabolome and release of inflammatory lipids and heme metabolites from the liver. HBV-ACLF showed higher RatioPeri/Hep of extrahepatic inflammatory oxlipids, while HBV-DC patients showed higher RatioPort/Peri of gut microbial metabolites. An inflammatory oxlipid outburst was confirmed in the early stages of HBV-ACLF. The inflammatory effects of the selected oxlipids were confirmed in monocytes. These findings support a synergy between liver-specific mechanisms and systemic inflammation in ACLF/DC development, and that pro-inflammatory oxlipids are metabolic signatures of early HBV-ACLF.

Exogenous putrescine plays a switch-like influence on the pH stress adaptability of biofilm-based activated sludge

Microbial community adaptability to pH stress plays a crucial role in biofilm formation. This study aims to investigate the regulatory mechanisms of exogenous putrescine on pH stress, as well as enhance understanding and application for the technical measures and molecular mechanisms of biofilm regulation. Findings demonstrated that exogenous putrescine acted as a switch-like distributor affecting microorganism pH stress, thus promoting biofilm formation under acid conditions while inhibiting it under alkaline conditions. As pH decreases, the protonation degree of putrescine increases, making putrescine more readily adsorbed. Protonated exogenous putrescine could increase cell membrane permeability, facilitating its entry into the cell. Subsequently, putrescine consumed intracellular H+ by enhancing the glutamate-based acid resistance strategy and the γ-aminobutyric acid metabolic pathway to reduce acid stress on cells. Furthermore, putrescine stimulated ATPase expression, allowing for better utilization of energy in H+ transmembrane transport and enhancing oxidative phosphorylation activity. However, putrescine protonation was limited under alkaline conditions, and the intracellular H+ consumption further exacerbated alkali stress and inhibits cellular metabolic activity. Exogenous putrescine promoted the proportion of fungi and acidophilic bacteria under acidic stress and alkaliphilic bacteria under alkali stress while having a limited impact on fungi in alkaline biofilms. Increasing Bdellovibrio under alkali conditions with putrescine further aggravated the biofilm decomposition. This research shed light on the unclear relationship between exogenous putrescine, environmental pH, and pH stress adaptability of biofilm. By judiciously employing putrescine, biofilm formation could be controlled to meet the needs of engineering applications with different characteristics.IMPORTANCEThe objective of this study is to unravel the regulatory mechanism by which exogenous putrescine influences biofilm pH stress adaptability and understand the role of environmental pH in this intricate process. Our findings revealed that exogenous putrescine functioned as a switch-like distributor affecting the pH stress adaptability of biofilm-based activated sludge, which promoted energy utilization for growth and reproduction processes under acidic conditions while limiting biofilm development to conserve energy under alkaline conditions. This study not only clarified the previously ambiguous relationship between exogenous putrescine, environmental pH, and biofilm pH stress adaptability but also offered fresh insights into enhancing biofilm stability within extreme environments. Through the modulation of energy utilization, exerting control over biofilm growth and achieving more effective engineering goals could be possible.

Gluconeogenesis in frogs during cooling and dehydration exposure: new insights into tissue plasticity of the gluconeogenic pathway dependent on abiotic factors

Anurans undergo significant physiological changes when exposed to environmental stressors such as low temperatures and humidity. Energy metabolism and substrate management play a crucial role in their survival success. Therefore, understanding the role of the gluconeogenic pathway and demonstrating its existence in amphibians is essential. In this study, we exposed the subtropical frog Boana pulchella to cooling (-2.5°C for 24 h) and dehydration conditions (40% of body water loss), followed by recovery (24 h), and assessed gluconeogenesis activity from alanine, lactate, glycerol and glutamine in the liver, muscle and kidney. We report for the first time that gluconeogenesis activity by 14C-alanine and 14C-lactate conversion to glucose occurs in the muscle tissue of frogs, and this tissue activity is influenced by environmental conditions. Against the control group, liver gluconeogenesis from 14C-lactate and 14C-glycerol was lower during cooling and recovery (P<0.01), and gluconeogenesis from 14C-glutamine in the kidneys was also lower during cooling (P<0.05). In dehydration exposure, gluconeogenesis from 14C-lactate in the liver was lower during recovery, and that from 14C-alanine in the muscle was lower during dehydration (P<0.05). Moreover, we observed that gluconeogenesis activity and substrate preference respond differently to cold and dehydration. These findings highlight tissue-specific plasticity dependent on the nature of the encountered stressor, offering valuable insights for future studies exploring this plasticity, elucidating the importance of the gluconeogenic pathway and characterizing it in anuran physiology.

Evolution of Glutamate Metabolism via GLUD2 Enhances Lactate-Dependent Synaptic Plasticity and Complex Cognition

Human evolution is characterized by rapid brain enlargement and the emergence of unique cognitive abilities. Besides its distinctive cytoarchitectural organization and extensive inter-neuronal connectivity, the human brain is also defined by high rates of synaptic, mainly glutamatergic, transmission, and energy utilization. While these adaptations' origins remain elusive, evolutionary changes occurred in synaptic glutamate metabolism in the common ancestor of humans and apes via the emergence of GLUD2, a gene encoding the human glutamate dehydrogenase 2 (hGDH2) isoenzyme. Driven by positive selection, hGDH2 became adapted to function upon intense excitatory firing, a process central to the long-term strengthening of synaptic connections. It also gained expression in brain astrocytes and cortical pyramidal neurons, including the CA1-CA3 hippocampal cells, neurons crucial to cognition. In mice transgenic for GLUD2, theta-burst-evoked long-term potentiation (LTP) is markedly enhanced in hippocampal CA3-CA1 synapses, with patch-clamp recordings from CA1 pyramidal neurons revealing increased sNMDA receptor currents. D-lactate blocked LTP enhancement, implying that glutamate metabolism via hGDH2 potentiates L-lactate-dependent glia-neuron interaction, a process essential to memory consolidation. The transgenic (Tg) mice exhibited increased dendritic spine density/synaptogenesis in the hippocampus and improved complex cognitive functions. Hence, enhancement of neuron-glia communication, via GLUD2 evolution, likely contributed to human cognitive advancement by potentiating synaptic plasticity and inter-neuronal connectivity.

Alteration of the Intestinal Permeability Are Reflected by Changes in the Urine Metabolome of Young Autistic Children: Preliminary Results

Several metabolomics-based studies have provided evidence that autistic subjects might share metabolic abnormalities with gut microbiota dysbiosis and alterations in gut mucosal permeability. Our aims were to explore the most relevant metabolic perturbations in a group of autistic children, compared with their healthy siblings, and to investigate whether the increased intestinal permeability may be mirrored by specific metabolic perturbations. We enrolled 13 autistic children and 14 unaffected siblings aged 2–12 years; the evaluation of the intestinal permeability was estimated by the lactulose:mannitol test. The urine metabolome was investigated by proton nuclear magnetic resonance (1H-NMR) spectroscopy. The lactulose:mannitol test unveiled two autistic children with altered intestinal permeability. Nine metabolites significantly discriminated the urine metabolome of autistic children from that of their unaffected siblings; however, in the autistic children with increased permeability, four additional metabolites—namely, fucose, phenylacetylglycine, nicotinurate, and 1-methyl-nicotinamide, strongly discriminated their urine metabolome from that of the remaining autistic children. Our preliminary data suggest the presence of a specific urine metabolic profile associated with the increase in intestinal permeability.

An acidic residue buried in the dimer interface of isocitrate dehydrogenase 1 (IDH1) helps regulate catalysis and pH sensitivity

Isocitrate dehydrogenase 1 (IDH1) catalyzes the reversible NADP+-dependent conversion of isocitrate to α-ketoglutarate (αKG) to provide critical cytosolic substrates and drive NADPH-dependent reactions like lipid biosynthesis and glutathione regeneration. In biochemical studies, the forward reaction is studied at neutral pH, while the reverse reaction is typically characterized in more acidic buffers. This led us to question whether IDH1 catalysis is pH-regulated, which would have functional implications under conditions that alter cellular pH, like apoptosis, hypoxia, cancer, and neurodegenerative diseases. Here, we show evidence of catalytic regulation of IDH1 by pH, identifying a trend of increasing kcat values for αKG production upon increasing pH in the buffers we tested. To understand the molecular determinants of IDH1 pH sensitivity, we used the pHinder algorithm to identify buried ionizable residues predicted to have shifted pKa values. Such residues can serve as pH sensors, with changes in protonation states leading to conformational changes that regulate catalysis. We identified an acidic residue buried at the IDH1 dimer interface, D273, with a predicted pKa value upshifted into the physiological range. D273 point mutations had decreased catalytic efficiency and, importantly, loss of pH-regulated catalysis. Based on these findings, we conclude that IDH1 activity is regulated, at least in part, by pH. We show this regulation is mediated by at least one buried acidic residue ∼12 Å from the IDH1 active site. By establishing mechanisms of regulation of this well-conserved enzyme, we highlight catalytic features that may be susceptible to pH changes caused by cell stress and disease.

Physiological Role of Glutamate Dehydrogenase in Cancer Cells

NH 4 + increased growth rates and final densities of several human metastatic cancer cells. To assess whether glutamate dehydrogenase (GDH) in cancer cells may catalyze the reverse reaction of NH 4 + fixation, its covalent regulation and kinetic parameters were determined under near-physiological conditions. Increased total protein and phosphorylation were attained in NH 4 + -supplemented metastatic cells, but total cell GDH activity was unchanged. Higher V _max values for the GDH reverse reaction vs. forward reaction in both isolated hepatoma (HepM) and liver mitochondria [rat liver mitochondria (RLM)] favored an NH 4 + -fixing role. GDH sigmoidal kinetics with NH 4 + , ADP, and leucine fitted to Hill equation showed n _H values of 2 to 3. However, the K _0.5 values for NH 4 + were over 20 mM, questioning the physiological relevance of the GDH reverse reaction, because intracellular NH 4 + in tumors is 1 to 5 mM. In contrast, data fitting to the Monod-Wyman-Changeux (MWC) model revealed lower K _m values for NH 4 + , of 6 to 12 mM. In silico analysis made with MWC equation, and using physiological concentrations of substrates and modulators, predicted GDH N-fixing activity in cancer cells. Therefore, together with its thermodynamic feasibility, GDH may reach rates for its reverse, NH 4 + -fixing reaction that are compatible with an anabolic role for supporting growth of cancer cells.

Changes in hypothermal stress-induced hepatic mitochondrial metabolic patterns between fresh water- and seawater-acclimated milkfish, Chanos chanos

Milkfish (Chanos chanos) is a tropical euryhaline species. It can acclimate to fresh water (FW) or seawater (SW) and be cultured in both. In winter, cold snaps cause huge losses in milkfish revenue. Compared to FW-acclimated individuals, SW-acclimated milkfish have better low-temperature tolerance. Under hypothermal stress, a stable energy supply is critical to maintain normal liver function. In this study, the levels of key mitochondrial enzymes (citrate synthase (CS) and cytochrome c oxidase (COX)) in milkfish livers were examined. The CS:COX activity ratio in FW milkfish significantly increased under hypothermal stress (18 °C) whereas ATP (the end product of aerobic metabolism) was downregulated. Therefore, the activities of the enzymes involved in mitochondrial amino acid biosynthesis (aspartate aminotransferase (AST) and glutamate dehydrogenase (GDH)) were evaluated to elucidate energy flow in milkfish livers under hypothermal stress. In FW milkfish, GDH activity was upregulated whereas AST activity was downregulated. Nevertheless, the levels of all the aforementioned enzymes did not significantly change in SW milkfish under hypothermal stress. In summary, we clarified the mechanism accounting for the fact that SW milkfish have superior low-temperature tolerance to FW milkfish and demonstrated that SW and FW milkfish have different and unique strategies for regulating energy flow.

Acid-Induced Downregulation of ASS1 Contributes to the Maintenance of Intracellular pH in Cancer

Downregulation of the urea cycle enzyme argininosuccinate synthase (ASS1) by either promoter methylation or by HIF1α is associated with increased metastasis and poor prognosis in multiple cancers. We have previously shown that in normoxic conditions, ASS1 downregulation facilitates cancer cell proliferation by increasing aspartate availability for pyrimidine synthesis by the enzyme complex CAD. Here we report that in hypoxia, ASS1 expression in cancerous cells is downregulated further by HIF1α-mediated induction of miR-224-5p, making the cells more invasive and dependent on upstream substrates of ASS1 for survival. ASS1 was downregulated under acidic conditions, and ASS1-depleted cancer cells maintained a higher intracellular pH (pHi), depended less on extracellular glutamine, and displayed higher glutathione levels. Depletion of substrates of urea cycle enzymes in ASS1-deficient cancers decreased cancer cell survival. Thus, ASS1 levels in cancer are differentially regulated in various environmental conditions to metabolically benefit cancer progression. Understanding these alterations may help uncover specific context-dependent cancer vulnerabilities that may be targeted for therapeutic purposes. SIGNIFICANCE: Cancer cells in an acidic or hypoxic environment downregulate the expression of the urea cycle enzyme ASS1, which provides them with a redox and pH advantage, resulting in better survival.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/79/3/518/F1.large.jpg.

The Effect of Electrospun Fiber Diameter on Astrocyte-Mediated Neurite Guidance and Protection

The topography of electrospun fiber scaffolds modifies astrocytes toward in vivo-like morphologies and behaviors. However, little is known about how electrospun fiber diameter influences astrocyte behavior. In this work, aligned fibers with two distinct nanoscale fiber diameters (808 and 386 nm) were prepared, and the astrocyte response was measured over time. Astrocytes on the large diameter fibers showed significantly increased elongation as early as 2 h after seeding and remained significantly more elongated for up to 4 days compared to those on small diameter fibers. Astrocytes extending along larger diameter fibers were better equipped to support long neurite outgrowth from dorsal root ganglia neurons, and neurite outgrowth along these astrocytes was less branched than outgrowth along astrocytes cultured on small diameter fibers. The differences in astrocyte shape observed on the small or large diameter fibers did not translate into differences in GLT-1, GFAP, or GLAST protein expression. Thus, different fiber diameters were unable to influence astrocyte protein expression uniquely. Nevertheless, astrocytes cultured in either small or large fibers significantly increased their expression of GLT-1 compared to astrocytes cultured on nonfiber (film) controls. Fibrous-induced increases in astrocyte GLT-1 expression protected astrocyte/neuron cocultures from toxicity generated by high extracellular glutamate. Alternatively, astrocytes/neurons cultured on films were less able to protect these cells from culture conditions consisting of high glutamate levels. Biomaterials, such as the fibrous materials presented here, may help stimulate astrocytes to increase GLT-1 expression and uptake more glutamate, since astrocytes are less likely to uptake glutamate in neurodegenerative pathologies or following central nervous system injury.

Aberrant interactions of peripheral measures and neurometabolites with lipids in complex regional pain syndrome using magnetic resonance spectroscopy: A pilot study

Background The aim of this study was to assess peripheral measures and central metabolites associated with lipids using magnetic resonance spectroscopy. Results Twelve patients with complex regional pain syndrome (CRPS) and 11 healthy controls participated. Using magnetic resonance spectroscopy, we measured the levels of lipid 13a (Lip13a) and lipid 09 (Lip09) relative to total creatine (tCr) levels in the right and left thalamus. We found negative correlations of Lip13a/tCr in the right thalamus with red blood cells or neutrophils, but a positive correlation between Lip13a/tCr and lymphocytes in the controls. We found negative correlations between Lip09/tCr and peripheral pH or platelets in the controls. There were positive correlations between Lip09a/tCr and myo-inositol/tCr, between Lip13a/tCr and N-acetylaspartate (NAA)/tCr, and between Lip09/tCr and NAA/tCr in healthy controls. On the other hand, there were positive correlations between Lip13a/tCr and Lip09/tCr and urine pH in CRPS patients. There were significant correlations between Lip13a/tCr or Lip09/tCr and different peripheral measures depending on the side of the thalamus (right or left) in CRPS patients. Conclusion This is the first report indicating that abnormal interactions of Lip13a and Lip09 in the thalamus with peripheral measures and central metabolites may mediate the complex pathophysiological mechanisms underlying CRPS.

Peripheral and Central Metabolites Affecting Depression, Anxiety, Suicidal Ideation, and Anger in Complex Regional Pain Syndrome Patients Using a Magnetic Resonance Spectroscopy: A Pilot Study

OBJECTIVE

This study investigated peripheral and central metabolites affecting depression, anxiety, suicidal ideation, and anger in complex regional pain syndrome (CRPS) patients.

METHODS

Metabolite levels were determined in the right and left thalamus and insula, in 12 CRPS patients using magnetic resonance spectroscopy (MRS).

RESULTS

There were positive correlations between valine (Val)/tNAA (N-acetylaspartate+N-acetylaspartylglutamate) and the anxiety, and a negative correlation between glutamine (Gln)/NAA and the depression. There were positive correlations between alanine (Ala)/Gln and the depression and suicidal ideation, between glutamate (Glu)/Gln and the depression and suicidal ideation, between N-acetylaspartylglutamate (NAAG)/Gln and the depression. There was a positive correlation between Ala/NAAG and the trait anger and a negative correlation between creatine (Cr)/N-acetylaspartate (NAA) and the trait anger. There was a negative correlation between Cr/Glx (Glu+Gln) and the trait anger. High hemoglobin and alkaline phosphatase were associated with low pain levels, but CO2 and chloride showed positive correlations with pain levels in CRPS patients. Peripheral glucose, CO2 and chloride were associated with depression, anxiety, anger and suicidal ideation.

CONCLUSION

The specific central and peripheral metabolites were associated with psychological disorders including depression, anxiety, suicidal ideation and anger in CRPS patients, showing pathological interactions between a painful body and mind.

Renal Tubular Acidosis: H+/Base and Ammonia Transport Abnormalities and Clinical Syndromes

Renal tubular acidosis (RTA) represents a group of diseases characterized by (1) a normal anion gap metabolic acidosis; (2) abnormalities in renal HCO3- absorption or new renal HCO3- generation; (3) changes in renal NH4+, Ca2+, K+, and H2O homeostasis; and (4) extrarenal manifestations that provide etiologic diagnostic clues. The focus of this review is to give a general overview of the pathogenesis of the various clinical syndromes causing RTA with a particular emphasis on type I (hypokalemic distal RTA) and type II (proximal) RTA while reviewing their pathogenesis from a physiological "bottom-up" approach. In addition, the factors involved in the generation of metabolic acidosis in both type I and II RTA are reviewed highlighting the importance of altered renal ammonia production/partitioning and new HCO3- generation. Our understanding of the underlying tubular transport and extrarenal abnormalities has significantly improved since the first recognition of RTA as a clinical entity because of significant advances in clinical acid-base chemistry, whole tubule and single-cell H+/base transport, and the molecular characterization of the various transporters and channels that are functionally affected in patients with RTA. Despite these advances, additional studies are needed to address the underlying mechanisms involved in hypokalemia, altered ammonia production/partitioning, hypercalciuria, nephrocalcinosis, cystic abnormalities, and CKD progression in these patients.

Hepatic glutamate transport and glutamine synthesis capacities are decreased in finished vs. growing beef steers, concomitant with increased GTRAP3-18 content

Hepatic glutamate uptake and conversion to glutamine is critical for whole-body N metabolism, but how this process is regulated during growth is poorly described. The hepatic glutamate uptake activities, protein content of system [Formula: see text] transporters (EAAC1, GLT-1) and regulatory proteins (GTRAP3-18, ARL6IP1), glutamine synthetase (GS) activity and content, and glutathione (GSH) content, were compared in liver tissue of weaned Angus steers randomly assigned (n = 8) to predominantly lean (growing) or predominantly lipid (finished) growth regimens. Steers were fed a cotton seed hull-based diet to achieve final body weights of 301 or 576 kg, respectively, at a constant rate of growth. Liver tissue was collected at slaughter and hepatic membranes fractionated. Total (75%), Na+-dependent (90%), system [Formula: see text]-dependent (abolished) glutamate uptake activity, and EAAC1 content (36%) in canalicular membrane-enriched vesicles decreased as steers developed from growing (n = 6) to finished (n = 4) stages, whereas Na+-independent uptake did not change. In basolateral membrane-enriched vesicles, total (60%), Na+-dependent (60%), and Na+-independent (56%) activities decreased, whereas neither system [Formula: see text]-dependent uptake nor protein content changed. EAAC1 protein content in liver homogenates (n = 8) decreased in finished vs. growing steers, whereas GTRAP3-18 and ARL6IP1 content increased and GLT-1 content did not change. Concomitantly, hepatic GS activity decreased (32%) as steers fattened, whereas GS and GSH contents did not differ. We conclude that hepatic glutamate uptake and GS synthesis capacities are reduced in livers of finished versus growing beef steers, and that hepatic system [Formula: see text] transporter activity/EAAC1 content is inversely proportional to GTRAP3-18 content.

Effect of styrene exposure on plasma parameters, molecular mechanisms and gene expression in rat model islet cells

Styrene is an aromatic hydrocarbon compound present in the environment and have primary exposure through plastic industry. The current study was designed to evaluate styrene-induced toxicity parameters in rat plasma fasting blood glucose (FBG) level, oral glucose tolerance, insulin secretion, oxidative stress, and inflammatory cytokines in cellular and molecular levels. Styrene was dissolved in corn oil and administered at different doses (250, 500, 1000, 1500, 2000mg/kg/day and control) to each rat, for 42days. In treated groups, styrene significantly increased fasting blood glucose, plasma insulin (p<0.001) and glucose tolerance. Glucose tolerance, insulin resistance and hyperglycemia were found to be the main consequences correlating gene expression of islet cells. Styrene caused a significant enhancement of oxidative stress markers (p<0.001) and inflammatory cytokines in a dose and concentration-dependent manner in plasma (p<0.001). Moreover, the activities of caspase-3 and -9 of the islet cells were significantly up-regulated by this compound at 1500 and 2000mg/kg/day styrene administrated groups (p<0.001). The relative fold change of GLUD1 was downregulated (p<0.05) and upregulated at 1500 and 2000mg/kg, respectively (p<0.01). The relative fold changes of GLUT2 were down regulated at 250 and 1000mg/kg and up regulated in 500, 1500 and 2000mg/kg doses of styrene (p<0.01). The expression level of GCK indicated a significant upregulation at 250mg/kg and downregulation of relative fold changes in the remaining doses of styrene, except for no change at 2000mg/kg of styrene for GCK. Targeting genes (GLUD1, GLUT2 and GCK) of the pancreatic islet cells in styrene exposed groups, disrupted gluconeogenesis, glycogenolysis pathways and insulin secretory functions. The present study illustrated that fasting blood glucose, insulin pathway, oxidative balance, inflammatory cytokines, cell viability and responsible genes of glucose metabolism are susceptible to styrene, which consequently lead to other abnormalities in various organs.

Perfused Gills Reveal Fundamental Principles of pH Regulation and Ammonia Homeostasis in the Cephalopod Octopus vulgaris

In contrast to terrestrial animals most aquatic species can be characterized by relatively higher blood [Formula: see text] concentrations despite its potential toxicity to the central nervous system. Although many aquatic species excrete [Formula: see text] via specialized epithelia little information is available regarding the mechanistic basis for NH₃/[Formula: see text] homeostasis in molluscs. Using perfused gills of Octopus vulgaris we studied acid-base regulation and ammonia excretion pathways in this cephalopod species. The octopus gill is capable of regulating ammonia (NH₃/[Formula: see text]) homeostasis by the accumulation of ammonia at low blood levels (<260 μM) and secretion at blood ammonia concentrations exceeding in vivo levels of 300 μM. [Formula: see text] transport is sensitive to the adenylyl cyclase inhibitor KH7 indicating that this process is mediated through cAMP-dependent pathways. The perfused octopus gill has substantial pH regulatory abilities during an acidosis, accompanied by an increased secretion of [Formula: see text]. Immunohistochemical and qPCR analyses revealed tissue specific expression and localization of Na⁺/K⁺-ATPase, V-type H⁺-ATPase, Na⁺/H⁺-exchanger 3, and Rhesus protein in the gill. Using the octopus gill as a molluscan model, our results highlight the coupling of acid-base regulation and nitrogen excretion, which may represent a conserved pH regulatory mechanism across many marine taxa.

Fetal Alcohol Exposure: The Common Toll

Alcohol has always been present in human life, and currently it is estimated that 50% of women of childbearing age consume alcohol. It has become increasingly clear over the last years that alcohol exposure during fetal development can have detrimental effects on various organ systems, and these effects are exerted by alcohol through multiple means, including effects on free radical formation, cellular apoptosis, as well as gene expression. Fetal alcohol exposure can lead to a spectrum of short term as well as long-term problems, with Fetal Alcohol Syndrome being on the more severe end of that spectrum. This syndrome is morbid, yet preventable, and is characterized by midfacial hypoplasia, thin upper lip, widely spaced small eyes, long smooth philtrum and inner epicanthal folds. Other findings include growth restriction as well as various neurodevelopmental abnormalities. This article is the first comprehensive review combining the molecular as well as the gross physiological and anatomical effects of alcohol exposure during pregnancy on various organ systems in the body. Our knowledge of these various mechanisms is crucial for our understanding of how alcohol exposure during fetal development can lead to its detrimental effects.

Multiple Forms of Glutamate Dehydrogenase in Animals: Structural Determinants and Physiological Implications

Glutamate dehydrogenase (GDH) of animal cells is usually considered to be a mitochondrial enzyme. However, this enzyme has recently been reported to be also present in nucleus, endoplasmic reticulum and lysosomes. These extramitochondrial localizations are associated with moonlighting functions of GDH, which include acting as a serine protease or an ATP-dependent tubulin-binding protein. Here, we review the published data on kinetics and localization of multiple forms of animal GDH taking into account the splice variants, post-translational modifications and GDH isoenzymes, found in humans and apes. The kinetic properties of human GLUD1 and GLUD2 isoenzymes are shown to be similar to those published for GDH1 and GDH2 from bovine brain. Increased functional diversity and specific regulation of GDH isoforms due to alternative splicing and post-translational modifications are also considered. In particular, these structural differences may affect the well-known regulation of GDH by nucleotides which is related to recent identification of thiamine derivatives as novel GDH modulators. The thiamine-dependent regulation of GDH is in good agreement with the fact that the non-coenzyme forms of thiamine, i.e., thiamine triphosphate and its adenylated form are generated in response to amino acid and carbon starvation.

Minireview on Glutamine Synthetase Deficiency, an Ultra-Rare Inborn Error of Amino Acid Biosynthesis

Glutamine synthetase (GS) is a cytosolic enzyme that produces glutamine, the most abundant free amino acid in the human body. Glutamine is a major substrate for various metabolic pathways, and is thus an important factor for the functioning of many organs; therefore, deficiency of glutamine due to a defect in GS is incompatible with normal life. Mutations in the human GLUL gene (encoding for GS) can cause an ultra-rare recessive inborn error of metabolism—congenital glutamine synthetase deficiency. This disease was reported until now in only three unrelated patients, all of whom suffered from neonatal onset severe epileptic encephalopathy. The hallmark of GS deficiency in these patients was decreased levels of glutamine in body fluids, associated with chronic hyperammonemia. This review aims at recapitulating the clinical history of the three known patients with congenital GS deficiency and summarizes the findings from studies done along with the work-up of these patients. It is the aim of this paper to convince the reader that (i) this disorder is possibly underdiagnosed, since decreased concentrations of metabolites do not receive the attention they deserve; and (ii) early detection of GS deficiency may help to improve the outcome of patients who could be treated early with metabolites that are lacking in this condition.

Widening Spectrum of Cellular and Subcellular Expression of Human GLUD1 and GLUD2 Glutamate Dehydrogenases Suggests Novel Functions

Mammalian glutamate dehydrogenase1 (GDH1) (E.C. 1.4.1.3) is a mitochondrial enzyme that catalyzes the reversible oxidative deamination of glutamate to α-ketoglutarate and ammonia while reducing NAD+ and/or NADP+ to NADH and/or NADPH. It links amino acid with carbohydrate metabolism, contributing to Krebs cycle anaplerosis, energy production, ammonia handling and redox homeostasis. Although GDH1 was one of the first major metabolic enzymes to be studied decades ago, its role in cell biology is still incompletely understood. There is however growing interest in a novel GDH2 isoenzyme that emerged via duplication in primates and underwent rapid evolutionary selection concomitant with prefrontal human cortex expansion. Also, the anaplerotic function of GDH1 and GDH2 is currently under sharp focus as this relates to the biology of glial tumors and other neoplasias. Here we used antibodies specific for human GDH1 (hGDH1) and human GDH2 (hGDH2) to study the expression of these isoenzymes in human tissues. Results revealed that both hGDH1 and hGDH2 are expressed in human brain, kidney, testis and steroidogenic organs. However, distinct hGDH1 and hGDH2 expression patterns emerged. Thus, while the Sertoli cells of human testis were strongly positive for hGDH2, they were negative for hGDH1. Conversely, hGDH1 showed very high levels of expression in human liver, but hepatocytes were virtually devoid of hGDH2. In human adrenals, both hGDHs were densely expressed in steroid-producing cells, with hGDH2 expression pattern matching that of the cholesterol side chain cleavage system involved in steroid synthesis. Similarly in human ovaries and placenta, both hGDH1 and hGDH2 were densely expressed in estrogen producing cells. In addition, hGDH1, being a housekeeping enzyme, was also expressed in cells that lack endocrine function. Regarding human brain, study of cortical sections using immunofluorescence (IF) with confocal microscopy revealed that hGDH1 and hGDH2 were both expressed in the cytoplasm of gray and white matter astrocytes within coarse structures resembling mitochondria. Additionally, hGDH1 localized to the nuclear membrane of a subpopulation of astrocytes and of the vast majority of oligodendrocytes and their precursors. Remarkably, hGDH2-specific staining was detected in human cortical neurons, with different expression patterns having emerged. One pattern, observed in large cortical neurons (some with pyramidal morphology), was a hGDH2-specific labeling of cytoplasmic structures resembling mitochondria. These were distributed either in the cell body-axon or on the cell surface in close proximity to astrocytic end-feet that encircle glutamatergic synapses. Another pattern was observed in small cortical neurons with round dense nuclei in which the hGDH2-specific staining was found in the nuclear membrane. A detailed description of these observations and their functional implications, suggesting that the GDH flux is used by different cells to serve some of their unique functions, is presented below.

What do we know about gliotransmitter release from astrocytes?

Astrocytes participate in information processing by actively modulating synaptic properties via gliotransmitter release. Various mechanisms of astrocytic release have been reported, including release from storage organelles via exocytosis and release from the cytosol via plasma membrane ion channels and pumps. It is still not fully clear which mechanisms operate under which conditions, but some of them, being Ca²⁺-regulated, may be physiologically relevant. The properties of Ca²⁺-dependent transmitter release via exocytosis or via ion channels are different and expected to produce different extracellular transmitter concentrations over time and to have distinct functional consequences. The molecular aspects of these two release pathways are still under active investigation. Here, we discuss the existing morphological and functional evidence in support of either of them. Transgenic mouse models, specific antagonists and localization studies have provided insight into regulated exocytosis, albeit not in a systematic fashion. Even more remains to be uncovered about the details of channel-mediated release. Better functional tools and improved ultrastructural approaches are needed in order fully to define specific modalities and effects of astrocytic gliotransmitter release pathways.

Glutamate receptors in the kidney

l-Glutamate (l-Glu) plays an essential role in the central nervous system (CNS) as an excitatory neurotransmitter, and exerts its effects by acting on a large number of ionotropic and metabotropic receptors. These receptors are also expressed in several peripheral tissues, including the kidney. This review summarizes the general properties of ionotropic and metabotropic l-Glu receptors, focusing on N-methyl-d-aspartate (NMDA) and Group 1 metabotropic glutamate receptors (mGluRs). NMDA receptors are expressed in the renal cortex and medulla, and appear to play a role in the regulation of renal blood flow, glomerular filtration, proximal tubule reabsorption and urine concentration within medullary collecting ducts. Sustained activation of NMDA receptors induces Ca(2+) influx and oxidative stress, which can lead to glomerulosclerosis, for example in hyperhomocysteinemia. Group 1 mGluRs are expressed in podocytes and probably in other cell types. Mice in which these receptors are knocked out gradually develop albuminuria and glomerulosclerosis. Several endogenous agonists of l-Glu receptors, which include sulfur-containing amino acids derived from l-homocysteine, and quinolinic acid (QA), as well as the co-agonists glycine and d-serine, are present in the circulation at concentrations capable of robustly activating ionotropic and metabotropic l-Glu receptors. These endogenous agonists may also be secreted from renal parenchymal cells, or from cells that have migrated into the kidney, by exocytosis or by transporters such as system x(-)(c), or by transporters involved in ammonia secretion. l-Glu receptors may be useful targets for drug therapy, and many selective orally-active compounds exist for investigation of these receptors as potential drug targets for various kidney diseases.

Nitrogen regulates AMPK to control TORC1 signaling

BACKGROUND

Cell growth and cell-cycle progression are tightly coordinated to enable cells to adjust their size (timing of division) to the demands of proliferation in varying nutritional environments. In fission yeast, nitrogen stress results in sustained proliferation at a reduced size.

RESULTS

Here, we show that cells can sense nitrogen stress to reduce target of rapamycin complex-1 (TORC1) activity. Nitrogen-stress-induced TORC1 inhibition differs from amino-acid-dependent control of TORC1 and requires the Ssp2 (AMPKα) kinase, the Tsc1/2 complex, and Rhb1 GTPase. Importantly, the β and γ regulatory subunits of AMPK are not required to control cell division in response to nitrogen stress, providing evidence for a nitrogen-sensing mechanism that is independent of changes in intracellular ATP/AMP levels. The CaMKK homolog Ssp1 is constitutively required for phosphorylation of the AMPKα(Ssp2) T loop. However, we find that a second homolog CaMKK(Ppk34) is specifically required to stimulate AMPKα(Ssp2) activation in response to nitrogen stress. Finally, ammonia also controls mTORC1 activity in human cells; mTORC1 is activated upon the addition of ammonium to glutamine-starved Hep3B cancer cells.

CONCLUSIONS

The alternative nitrogen source ammonia can simulate TORC1 activity to support growth and division under challenging nutrient settings, a situation often seen in cancer.

Gap junctions and hemichannels composed of connexins: potential therapeutic targets for neurodegenerative diseases

Microglia are macrophage-like resident immune cells that contribute to the maintenance of homeostasis in the central nervous system (CNS). Abnormal activation of microglia can cause damage in the CNS, and accumulation of activated microglia is a characteristic pathological observation in neurologic conditions such as trauma, stroke, inflammation, epilepsy, and neurodegenerative diseases. Activated microglia secrete high levels of glutamate, which damages CNS cells and has been implicated as a major cause of neurodegeneration in these conditions. Glutamate-receptor blockers and microglia inhibitors (e.g., minocycline) have been examined as therapeutic candidates for several neurodegenerative diseases; however, these compounds exerted little therapeutic benefit because they either perturbed physiological glutamate signals or suppressed the actions of protective microglia. The ideal therapeutic approach would hamper the deleterious roles of activated microglia without diminishing their protective effects. We recently found that abnormally activated microglia secrete glutamate via gap-junction hemichannels on the cell surface. Moreover, administration of gap-junction inhibitors significantly suppressed excessive microglial glutamate release and improved disease symptoms in animal models of neurologic conditions such as stroke, multiple sclerosis, amyotrophic lateral sclerosis, and Alzheimer's disease. Recent evidence also suggests that neuronal and glial communication via gap junctions amplifies neuroinflammation and neurodegeneration. Elucidation of the precise pathologic roles of gap junctions and hemichannels may lead to a novel therapeutic strategies that can slow and halt the progression of neurodegenerative diseases.

Effects of L-glutamine supplementation on maternal and fetal hemodynamics in gestating ewes exposed to alcohol

Not much is known about effects of gestational alcohol exposure on maternal and fetal cardiovascular adaptations. This study determined whether maternal binge alcohol exposure and L-glutamine supplementation could affect maternal-fetal hemodynamics and fetal regional brain blood flow during the brain growth spurt period. Pregnant sheep were randomly assigned to one of four groups: saline control, alcohol (1.75-2.5 g/kg body weight), glutamine (100 mg/kg body weight) or alcohol + glutamine. A chronic weekend binge drinking paradigm between gestational days (GD) 99 and 115 was utilized. Fetuses were surgically instrumented on GD 117 ± 1 and studied on GD 120 ± 1. Binge alcohol exposure caused maternal acidemia, hypercapnea, and hypoxemia. Fetuses were acidemic and hypercapnic, but not hypoxemic. Alcohol exposure increased fetal mean arterial pressure, whereas fetal heart rate was unaltered. Alcohol exposure resulted in ~40 % reduction in maternal uterine artery blood flow. Labeled microsphere analyses showed that alcohol induced >2-fold increases in fetal whole brain blood flow. The elevation in fetal brain blood flow was region-specific, particularly affecting the developing cerebellum, brain stem, and olfactory bulb. Maternal L-glutamine supplementation attenuated alcohol-induced maternal hypercapnea, fetal acidemia and increases in fetal brain blood flow. L-Glutamine supplementation did not affect uterine blood flow. Collectively, alcohol exposure alters maternal and fetal acid-base balance, decreases uterine blood flow, and alters fetal regional brain blood flow. Importantly, L-glutamine supplementation mitigates alcohol-induced acid-base imbalances and alterations in fetal regional brain blood flow. Further studies are warranted to elucidate mechanisms responsible for alcohol-induced programming of maternal uterine artery and fetal circulation adaptations in pregnancy.

Proteome analysis and conditional deletion of the EAAT2 glutamate transporter provide evidence against a role of EAAT2 in pancreatic insulin secretion in mice

Islet function is incompletely understood in part because key steps in glutamate handling remain undetermined. The glutamate (excitatory amino acid) transporter 2 (EAAT2; Slc1a2) has been hypothesized to (a) provide islet cells with glutamate, (b) protect islet cells against high extracellular glutamate concentrations, (c) mediate glutamate release, or (d) control the pH inside insulin secretory granules. Here we floxed the EAAT2 gene to produce the first conditional EAAT2 knock-out mice. Crossing with Nestin-cyclization recombinase (Cre) eliminated EAAT2 from the brain, resulting in epilepsy and premature death, confirming the importance of EAAT2 for brain function and validating the genetic construction. Crossing with insulin-Cre lines (RIP-Cre and IPF1-Cre) to obtain pancreas-selective deletion did not appear to affect survival, growth, glucose tolerance, or β-cell number. We found (using TaqMan RT-PCR, immunoblotting, immunocytochemistry, and proteome analysis) that the EAAT2 levels were too low to support any of the four hypothesized functions. The proteome analysis detected more than 7,000 islet proteins of which more than 100 were transporters. Although mitochondrial glutamate transporters and transporters for neutral amino acids were present at high levels, all other transporters with known ability to transport glutamate were strikingly absent. Glutamate-metabolizing enzymes were abundant. The level of glutamine synthetase was 2 orders of magnitude higher than that of glutaminase. Taken together this suggests that the uptake of glutamate by islets from the extracellular fluid is insignificant and that glutamate is intracellularly produced. Glutamine synthetase may be more important for islets than assumed previously.

Metabolic perturbations of kidney and spleen in murine cerebral malaria: (1)H NMR-based metabolomic study

A significant fraction of global population is under the threat of malaria. Majority of annual death is due to the more complicated form of the infection i.e. the cerebral form, also known as Cerebral Malaria (CM). Host parasite interaction is known to cause a cascade of events in various tissues like brain, liver, kidney, and spleen. We have employed (1)H NMR based metabolomics to understand the specific perturbations of various tissues in CM. In our previous paper we have delineated the differences between CM vis-a-vis non-cerebral malaria (NCM) mice in serum, liver and brain. In this paper we focus on their differences of metabolic profile in kidney and spleen as kidney dysfunction and splenomegaly are known to be associated to neurological outcome of the disease. Moreover we have also looked into how the biological compartments (kidney, spleen and serum) interact with each other. The various metabolites involved in such interactions and their correlational aspects across the compartments have been studied in CM, NCM and control mice. The idea was to find out the specific pathways that are altered in CM mice. Our results demonstrate that both the kidney as well as spleen metabolism are differentially perturbed in CM with respect to NCM. The results point out that glutamate levels are decreased in CM mice with respect to NCM mice both in case of spleen and kidney while creatine, myo-inositol and betaine levels are increased in kidney of CM mice with respect to NCM mice. From the analysis of Multiway Principal Component Analysis (MPCA) we see that lipid metabolism and TCA cycle is altered in kidney and spleen.

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.

Effect of alpha-ketoglutarate on growth and metabolism of cells cultured on three-dimensional cryogel matrix

Alpha- ketoglutarate (α-KG) is a well-known intermediate of Kreb's cycle and thus one of the important candidates in the role of cellular metabolism. The aim was to investigate the effect of α-KG on the growth and proliferation of different cells (fibroblasts and chondrocytes). Further change in the consumption of glucose and release of ammonia in the media, upon the addition of α-KG was also examined. NIH3T3 and chondrocytes were seeded on two different macroporous cryogel matrices synthesized by poly (hydroxyethylmethaacrylate)-gelatin (HG) and alginate-gelatin (AG). Higher proliferation was observed for both the cell lines in 2-D as well as 3-D where α-KG was added to the media compared to the controls. It was evident that the chondrocytes were metabolically more active in the case of α-KG containing samples as early extracellular matrix (ECM) accumulation was observed. In comparison to the 6-8 weeks duration required by the chondrocytes for ECM accumulation in normal in-vitro culture conditions, α-KG containing samples showed an earlier accumulation within 3 weeks. In order to further validate the results, scanning electron microscopic (SEM) analysis was performed showed high ECM deposition and cells embedded in the matrix. Homogenous distribution of cells on both the synthesized matrix was reported using 4'-6-diamidino-2-phenylindole (DAPI) and propidium iodide (PI) staining. The results clearly showed reduction in ammonia concentration in α-KG containing samples thus leading to decreased ammonia toxicity that builds up during long cell culture conditions thereby enhancing the proliferation and metabolic activity. Thus α-KG can be used potentially for long batch-cultures for the production of vaccines or antibody as well as can play an important role as a bioactive molecule for in vitro neo-cartilage generation.

Bioenergetic measurements in children with bipolar disorder: a pilot 31P magnetic resonance spectroscopy study

Background

Research exploring Bipolar Disorder (BD) phenotypes and mitochondrial dysfunction, particularly in younger subjects, has been insufficient to date. Previous studies have found abnormal cerebral pH levels in adults with BD, which may be directly linked to abnormal mitochondrial activity. To date no such studies have been reported in children with BD.

Methods

Phosphorus Magnetic Resonance Spectroscopy (³¹P MRS) was used to determine pH, phopshocreatine (PCr) and inorganic phosphate (Pi) levels in 8 subjects with BD and 8 healthy comparison subjects (HCS) ages 11 to 20 years old.

Results

There was no significant difference in pH between the patients and HCS. However, frontal pH values for patients with BD increased with age, contrary to studies of HCS and the pH values in the frontal lobe correlated negatively with the YMRS values. Global Pi was significantly lower in subjects with BD compared with HCS. There were no significant differences in PCr between the groups. Global PCr-to-Pi ratio (PCr/Pi) was significantly higher in subjects with BD compared with HCS.

Conclusions

The change in Pi levels for the patients with BD coupled with the no difference in PCr levels, suggest an altered mitochondrial phosphorylation. However, our findings require further investigation of the underlying mechanisms with the notion that a mitochondrial dysfunction may manifest itself differently in children than that in adults.

Limitations

Further investigations with larger patient populations are necessary to draw further conclusions.

Acute alcohol exposure, acidemia or glutamine administration impacts amino acid homeostasis in ovine maternal and fetal plasma

Fetal alcohol syndrome (FAS) is a significant problem in human reproductive medicine. Maternal alcohol administration alters maternal amino acid homeostasis and results in acidemia in both mother and fetus, causing fetal growth restriction. We hypothesized that administration of glutamine, which increases renal ammoniagenesis to regulate acid-base balance, may provide an intervention strategy. This hypothesis was tested using sheep as an animal model. On day 115 of gestation, ewes were anesthetized and aseptic surgery was performed to insert catheters into the fetal abdominal aorta as well as the maternal abdominal aorta and vena cava. On day 128 of gestation, ewes received intravenous administration of saline, alcohol [1.75 g/kg body weight (BW)/h], a bolus of 30 mg glutamine/kg BW, alcohol + a bolus of 30 mg glutamine/kg BW, a bolus of 100 mg glutamine/kg BW, alcohol + a bolus of 100 mg glutamine/kg BW, or received CO2 administration to induce acidemia independent of alcohol. Blood samples were obtained simultaneously from the mother and the fetus at times 0 and 60 min (the time of peak blood alcohol concentration) of the study. Administration of alcohol to pregnant ewes led to a reduction in concentrations of glutamine and related amino acids in plasma by 21-30%. An acute administration of glutamine to ewes, concurrent with alcohol administration, improved the profile of most amino acids (including citrulline and arginine) in maternal and fetal plasma. We suggest that glutamine may have a protective effect against alcohol-induced metabolic disorders and FAS in the ovine model.

Feed restriction and genetic selection on the expression and activity of metabolism regulatory enzymes in rabbits

This work aims at the identification of relevant intermediate metabolism enzymes contributing to improved meat production due to genetic selection. A wild rabbit (WR) breed and a highly meat selected breed (New Zealand (NZ) rabbit) were used. Food restriction was used as an experimental condition so as to enhance differences within the metabolic pathways under study. During a period of 30 days, NZ and WR experimental breeds were subjected to, respectively, 40% and 60% ad libitum food restriction leading to 17.7% and 21.1% initial weight. Hepatic glycolytic, lipidic and protein regulatory enzyme activity, transcriptional and metabolite levels were determined. Insulin-like growth factor (IGF-1), triiodothyronine, and cortisol were also evaluated. In the glycolytic pathways, the NZ control rabbits presented a higher phosphofructokinase and pyruvate kinase activity level when compared to the WR, while the latter group showed a higher expression of glycogen synthase, although with less glycogen content. In the nitrogen metabolism, our results showed a lower activity level of glutamate dehydrogenase in WR when subjected to food restriction. Within the lipid metabolism, results showed that although WR had a significantly higher mRNA hepatic lipase, non-esterified fatty acid levels were similar between the experimental groups. NZ rabbits presented a better glycemia control and greater energy substrate availability leading to enhanced productivities in which triiodothyronine and IGF-1 played a relevant role.

Decreased insulin secretion in islets from protein malnourished rats is associated with impaired glutamate dehydrogenase function: effect of leucine supplementation

We herein studied the role of glutamate dehydrogenase (GDH), in response to leucine (LEU) supplementation, upon insulin secretion of malnourished rats. Weaned male Wistar rats were fed normal-protein (17%) or low-protein diet (6%, LP) for 8 weeks. Half of the rats of each group were supplemented with LEU (1.5%) in the drinking water for the following 4 weeks. Gene and protein expressions, static insulin secretion, and cytoplasmic Ca(2+) oscillations were measured. Glutamate dehydrogenase messenger RNA was 58% lower in LP islets, and LEU supplementation augmented it in 28%. The LP islets secreted less insulin when exposed to 20 mmol/L LEU, 20 mmol/L LEU + 2 mmol/L glutamine (with or without 5 mmol/L aminooxyacetic acid, a branched chain aminotransferase inhibitor, or 20 μmol/L epigallocatechin gallate, a GDH inhibitor), 20 mmol/L α-ketoisocaproate, glutamine + 20 mmol/L β-2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (a GDH activator), and 22.2 mmol/L glucose. Leucine supplementation augmented insulin secretion to levels found in normal-protein islets in all the above conditions, an effect that was blunted when islets were incubated with epigallocatechin gallate. The glutamine + β-2-aminobicyclo[2.2.1]heptane-2-carboxylic acid-induced increased [Ca(2+)](i) and oscillations were higher than those for LP islets. Leucine supplementation normalized these parameters in LP islets. Impaired GDH function was associated with lower insulin release in LP islets, and LEU supplementation normalized insulin secretion via restoration of GDH function. In addition, GDH may contribute to insulin secretion through ameliorations of Ca(2+) handling in LP islets.

The role of glutamate dehydrogenase in mammalian ammonia metabolism

Glutamate dehydrogenase (GDH) catalyzes the reversible inter-conversion of glutamate to α-ketoglutarate and ammonia. High levels of GDH activity is found in mammalian liver, kidney, brain, and pancreas. In the liver, GDH reaction appears to be close-to-equilibrium, providing the appropriate ratio of ammonia and amino acids for urea synthesis in periportal hepatocytes. In addition, GDH produces glutamate for glutamine synthesis in a small rim of pericentral hepatocytes. Hence, hepatic GDH can be either a source for ammonia or an ammonia scavenger. In the kidney, GDH function produces ammonia from glutamate to control acidosis. In the human, the presence of two differentially regulated isoforms (hGDH1 and hGDH2) suggests a complex role for GDH in ammonia homeostasis. Whereas hGDH1 is sensitive to GTP inhibition, hGDH2 has dissociated its function from GTP control. Furthermore, hGDH2 shows a lower optimal pH than hGDH1. The hGDH2 enzyme is selectively expressed in human astrocytes and Sertoli cells, probably facilitating metabolic recycling processes essential for their supportive role. Here, we report that hGDH2 is also expressed in the epithelial cells lining the convoluted tubules of the renal cortex. As hGDH2 functions more efficiently under acidotic conditions without the operation of the GTP energy switch, its presence in the kidney may increase the efficacy of the organ to maintain acid base equilibrium.

Blockade of gap junction hemichannel suppresses disease progression in mouse models of amyotrophic lateral sclerosis and Alzheimer's disease

Background

Glutamate released by activated microglia induces excitotoxic neuronal death, which likely contributes to non-cell autonomous neuronal death in neurodegenerative diseases, including amyotrophic lateral sclerosis and Alzheimer's disease. Although both blockade of glutamate receptors and inhibition of microglial activation are the therapeutic candidates for these neurodegenerative diseases, glutamate receptor blockers also perturbed physiological and essential glutamate signals, and inhibitors of microglial activation suppressed both neurotoxic/neuroprotective roles of microglia and hardly affected disease progression. We previously demonstrated that activated microglia release a large amount of glutamate specifically through gap junction hemichannel. Hence, blockade of gap junction hemichannel may be potentially beneficial in treatment of neurodegenerative diseases.

Methods and Findings

In this study, we generated a novel blood-brain barrier permeable gap junction hemichannel blocker based on glycyrrhetinic acid. We found that pharmacologic blockade of gap junction hemichannel inhibited excessive glutamate release from activated microglia in vitro and in vivo without producing notable toxicity. Blocking gap junction hemichannel significantly suppressed neuronal loss of the spinal cord and extended survival in transgenic mice carrying human superoxide dismutase 1 with G93A or G37R mutation as an amyotrophic lateral sclerosis mouse model. Moreover, blockade of gap junction hemichannel also significantly improved memory impairments without altering amyloid β deposition in double transgenic mice expressing human amyloid precursor protein with K595N and M596L mutations and presenilin 1 with A264E mutation as an Alzheimer's disease mouse model.

Conclusions

Our results suggest that gap junction hemichannel blockers may represent a new therapeutic strategy to target neurotoxic microglia specifically and prevent microglia-mediated neuronal death in various neurodegenerative diseases.

Autophagy during cardiac stress: joys and frustrations of autophagy

The study of autophagy has been transformed by the cloning of most genes in the pathway and the introduction of GFP-LC3 as a reporter to allow visual assessment of autophagy. The field of cardiac biology is not alone in attempting to understand the implications of autophagy. The purpose of this review is to address some of the controversies and conundrums associated with the evolving studies of autophagy in the heart. Autophagy is a cellular process involving a complex orchestration of regulatory gene products as well as machinery for assembly, selective targeting, and degradation of autophagosomes and their contents. Our understanding of the role of autophagy in human disease is rapidly evolving as investigators examine the process in different tissues and different pathophysiological contexts. In the field of heart disease, autophagy has been examined in the settings of ischemia and reperfusion, preconditioning, cardiac hypertrophy, and heart failure. This review addresses the role of autophagy in cardioprotection, the balance of catabolism and anabolism, the concept of mitochondrial quality control, and the implications of impaired autophagic flux or frustrated autophagy.

Blockade of glutamate release from microglia attenuates experimental autoimmune encephalomyelitis in mice

Multiple sclerosis (MS) is a chronic inflammatory demyelinating and neurodegenerative disease of the central nervous system. Despite a variety of anti-inflammatory or immunomodulation drugs including interferon-beta are effective to reduce relapse risk, most patients have progressive neurological deterioration due to axonal degeneration. Accumulation of activated microglia is a pathological hallmark of active MS lesion. Microglia can act as not only antigen-presenting cells but also effector cells to damage other cells in the central nervous system. Especially, glutamate released by activated microglia induces excito-neurotoxicity and may contribute to neurodegeneration in MS. Gap junction is a major cell-to-cell channel and is composed of paired hemichannels on coupled cells. Recent studies showed that cells release various small molecules (including ions, ATP, and amino acids) from unpaired hemichannel of gap junction that is openly exposed to the extracellular space. We have previously revealed that activated microglia produce glutamate via glutaminase and release it through hemichannels of gap junctions. Thus, in this study, we examined whether the glutaminase inhibitor and the gap junction blocker relieved experimental autoimmune encephalomyelitis (EAE) that is an animal model of MS. Here we show that the gap junction blocker carbenoxolone (CBX) and the glutaminase inhibitor 6-diazo-5-oxo-L-norleucine (DON) decreased glutamate release from activated microglia and rescued neuronal death in a dose-dependent manner in vitro. In EAE mice, treatment with CBX or DON also attenuated EAE clinical symptoms. Thus, blockade of glutamate release from activated microglia with CBX or DON may be an effective therapeutic strategy against neurodegeneration in MS.

Chronic binge ethanol-mediated acidemia reduces availability of glutamine and related amino acids in maternal plasma of pregnant sheep

Heavy drinking during pregnancy can result in fetal alcohol syndrome (FAS), of which, fetal and postnatal growth retardation and central nervous system deficits are cardinal features. Although a number of mechanisms have been proposed, none fully account for these deficiencies. We have previously reported that maternal ethanol exposure (1.75 g/kg) results in transient acidemia in the mother and fetus. Alterations in pH are known to regulate glutamine homeostasis. Therefore, we hypothesized that chronic binge ethanol-mediated acidosis reduces glutamine concentrations in maternal plasma that result in decreases in the circulating levels of amino acids related to glutamine metabolism. Pregnant ewes were divided into three groups: ethanol (1.75 g/kg), saline control, and acidemia (inspired fractional carbon dioxide [CO(2)] was manipulated to mimic the maternal arterial pH pattern created by ethanol). The experiment was conducted on three consecutive days followed by four days without treatment beginning on gestational day (GD) 109, continuing to GD 132. Plasma samples were analyzed for nutrients and metabolites using HPLC and spectrophotometric methods. Maternal plasma concentrations of glutamate increased (58%), whereas glutamine, citrulline, and arginine decreased (between 14 and 53%) in response to an acute challenge after the chronic exposure in ethanol-treated ewes. No differences in these amino acid concentrations were noted between the ethanol and acidemic group subjects. Maternal plasma lactate levels increased by approximately 100% in response to ethanol, whereas glucose and urea levels did not change in any group. We conclude that maternal chronic binge ethanol consumption results in acidosis-mediated reductions in circulating levels of glutamine and related amino acids that could be responsible for neuronal deficits, altered fetal growth, development, and programming. We also speculate that the consequent increase in fetal glutamate during critical periods of brain development may contribute to the pathogenesis of FAS.

Dimethyl amiloride improves glucose homeostasis in mouse models of type 2 diabetes

Dimethyl amiloride (DMA) enhances insulin secretion in the pancreatic beta-cell. DMA also enhances time-dependent potentiation (TDP) and enables TDP to occur in situations where it is normally absent. As we have demonstrated before, these effects are mediated in part through inhibition of neuronal nitric oxide synthase (nNOS), resulting in increased availability of arginine. Thus both DMA and arginine have the potential to correct the secretory defect in diabetes by enabling or enhancing TDP. In the current study we have demonstrated the ability of these agents to improve blood glucose homeostasis in three mouse models of type 2 diabetes. The pattern of TDP under different conditions indicates that inhibition of NOS is not the only mechanism through which DMA exerts its positive effects. Thus we also have explored another possible mechanism through which DMA enables/enhances TDP, via the activation of mitochondrial alpha-ketoglutarate dehydrogenase.

Interpretation of plasma amino acids in the follow-up of patients: the impact of compartmentation

Results of plasma or urinary amino acids are used for suspicion, confirmation or exclusion of diagnosis, monitoring of treatment, prevention and prognosis in inborn errors of amino acid metabolism. The concentrations in plasma or whole blood do not necessarily reflect the relevant metabolite concentrations in organs such as the brain or in cell compartments; this is especially the case in disorders that are not solely expressed in liver and/or in those which also affect nonessential amino acids. Basic biochemical knowledge has added much to the understanding of zonation and compartmentation of expressed proteins and metabolites in organs, cells and cell organelles. In this paper, selected old and new biochemical findings in PKU, urea cycle disorders and nonketotic hyperglycinaemia are reviewed; the aim is to show that integrating the knowledge gained in the last decades on enzymes and transporters related to amino acid metabolism allows a more extensive interpretation of biochemical results obtained for diagnosis and follow-up of patients and may help to pose new questions and to avoid pitfalls. The analysis and interpretation of amino acid measurements in physiological fluids should not be restricted to a few amino acids but should encompass the whole quantitative profile and include other pathophysiological markers. This is important if the patient appears not to respond as expected to treatment and is needed when investigating new therapies. We suggest that amino acid imbalance in the relevant compartments caused by over-zealous or protocol-driven treatment that is not adjusted to the individual patient's needs may prolong catabolism and must be corrected.

Role of Brainstem Sodium/Proton Exchanger 3 for Breathing Control during Chronic Acid–Base Imbalance

RATIONALE

The sodium/proton exchanger (NHE) 3 is expressed in brainstem areas with prevalence for central chemosensitivity. Selective NHE3 inhibitors can evoke CO(2) mimetic responses both in vitro and in vivo, demonstrating the functional significance of this pH-regulating protein. Moreover, levels of NHE3 expression are inversely correlated to interindividual differences of baseline ventilation in conscious rabbits.

OBJECTIVES

We explored the influence of chronic acid-base disturbances on mRNA levels of brainstem NHE3 in relation to breathing control.

METHODS

Alveolar ventilation (Va), blood gases, systemic base excess (BE), and metabolic Vco(2) were determined in rabbits shortly after exposure to either CO(2)-enriched air for 3 days (n = 5) or to ammonium chloride with drinking water for 2 days (n = 6). Untreated animals served as controls (n = 24). NHE3 mRNA within the obex region was quantified by real-time reverse transcription-polymerase chain reaction.

MEASUREMENTS AND MAIN RESULTS

After chronic hypercapnia, we found a compensatory rise of BE (mean +/- SEM) to 5.3 +/- 0.5 mmol x L(-1) with slightly elevated Pa(CO(2)). Brainstem NHE3 mRNA as well as Va were not significantly different from control levels. In the NH(4)Cl group, arterial pH was approximately 0.09 units lower than control, and BE decreased to -6.5 +/- 1.6 mmol x L(-1) with slightly decreased Pa(CO(2)), but considerably reduced Va (by approximately 25%; P < 0.05) and Vco(2). Concomitantly, brainstem NHE3 mRNA had increased from control level of 1.45 +/- 0.19 to 3.64 +/- 0.37 fg cDNA/mug RNA; P < 0.01.

CONCLUSIONS

Expression of brainstem NHE3 is up-regulated by chronic metabolic acidosis but not by prolonged hypercapnia. It is proposed that elevated brainstem NHE3 expression contributes to limit maladaptive hyperventilation during metabolic acidosis.

Regulation of renal amino acid transporters during metabolic acidosis

The kidney plays a major role in acid-base homeostasis by adapting the excretion of acid equivalents to dietary intake and metabolism. Urinary acid excretion is mediated by the secretion of protons and titratable acids, particularly ammonia. NH(3) is synthesized in proximal tubule cells from glutamine taken up via specific amino acid transporters. We tested whether kidney amino acid transporters are regulated in mice in which metabolic acidosis was induced with NH(4)Cl. Blood gas and urine analysis confirmed metabolic acidosis. Real-time RT-PCR was performed to quantify the mRNAs of 16 amino acid transporters. The mRNA of phosphoenolpyruvate carboxykinase (PEPCK) was quantified as positive control for the regulation and that of GAPDH, as internal standard. In acidosis, the mRNA of kidney system N amino acid transporter SNAT3 (SLC38A3/SN1) showed a strong induction similar to that of PEPCK, whereas all other tested mRNAs encoding glutamine or glutamate transporters were unchanged or reduced in abundance. At the protein level, Western blotting and immunohistochemistry demonstrated an increased abundance of SNAT3 and reduced expression of the basolateral cationic amino acid/neutral amino acid exchanger subunit y(+)-LAT1 (SLC7A7). SNAT3 was localized to the basolateral membrane of the late proximal tubule S3 segment in control animals, whereas its expression was extended to the earlier S2 segment of the proximal tubule during acidosis. Our results suggest that the selective regulation of SNAT3 and y(+)LAT1 expression may serve a major role in the renal adaptation to acid secretion and thus for systemic acid-base balance.

Inborn error of amino acid synthesis: human glutamine synthetase deficiency

Glutamine synthetase (GS) is ubiquitously expressed in human tissues, being involved in ammonia detoxification and interorgan nitrogen flux. Inherited systemic deficiency of glutamine based on a defect of glutamine synthetase was recently described in two newborns with an early fatal course of disease. Glutamine was largely absent in their serum, urine and cerebrospinal fluid. Each of the patients had a homozygous mutation in the glutamine synthetase gene and enzymatic investigations confirmed that these mutations lead to a severely reduced glutamine synthetase activity. From the observation in the first patients with congenital glutamine synthetase deficiency, brain malformation can be expected as one of the leading signs. In addition, other organ systems are probably involved as observed in one of the index patients who suffered from severe enteropathy and necrolytic erythema of the skin. Deficiency of GS has to be added to the list of inherited metabolic disorders as a rare example of a defect in the biosynthesis of an amino acid.

Novel renal amino acid transporters

Reabsorption of amino acids, similar to that of glucose, is a major task of the proximal kidney tubule. Various amino acids are actively transported across the luminal brush border membrane into proximal tubule epithelial cells, most of which by cotransport. An important player is the newly identified cotransporter (symporter) B0AT1 (SLC6A19), which imports a broad range of neutral amino acids together with Na+ across the luminal membrane and which is defective in Hartnup disorder. In contrast, cationic amino acids and cystine are taken up in exchange for recycled neutral amino acids by the heterodimeric cystinuria transporter. The basolateral release of some neutral amino acids into the extracellular space is mediated by unidirectional efflux transporters, analogous to GLUT2, that have not yet been definitively identified. Additionally, cationic amino acids and some other neutral amino acids leave the cell basolaterally via heterodimeric obligatory exchangers.

Characteristics of glutamine metabolism in human precision-cut kidney slices: a 13C-NMR study

The metabolism of glutamine, a physiological substrate of the human kidney, plays a major role in systemic acid-base homoeostasis. Not only because of the limited availability of human renal tissue but also in part due to the lack of adequate cellular models, the mechanisms regulating the renal metabolism of this amino acid in humans have been poorly characterized. Therefore given the renewed interest in their use, human precision-cut renal cortical slices were incubated in Krebs-Henseleit medium (118 mM NaCl, 4.7 mM KCl, 1.18 mM KH2PO4, 1.18 mM MgSO4*7H2O, 24.9 mM NaHCO3 and 2.5 mM CaCl2*2H2O) with 2 mM unlabelled or 13C-labelled glutamine residues. After incubation, substrate utilization and product formation were measured by enzymatic and NMR spectroscopic methods. Glutamate accumulation tended to plateau but glutamine removal and ammonia, alanine and lactate production as well as flux through GLDH (glutamate dehydrogenase) increased to various extents with time for up to 4 h of incubation indicating the metabolic viability of the slices. Valproate, a stimulator of renal glutamine metabolism, markedly and in a dose-dependent fashion increased ammonia production. With [3-13C]glutamine as a substrate, and in the absence and presence of valproate, [13C]glutamate, [13C]alanine and [13C]lactate accounted for 81 and 96%, 34 and 63%, 30 and 46% of the glutamate, alanine and lactate accumulations measured enzymatically respectively. The slices also metabolized glutamine and retained their reactivity to valproate during incubations lasting for up to 48 h. These results demonstrate that, although endogenous metabolism substantially operates in the presence of glutamine, human precision-cut renal cortical slices are metabolically viable and strongly respond to the ammoniagenic effect of valproate. Thus, this experimental model is suitable for metabolic and pharmaco-toxicological studies.

Cellular distribution of glutamate transporters in the gastrointestinal tract of mice. An immunohistochemical and in situ hybridization approach

L-Glutamate transport by intestinal epithelial cells is an initial step of the entire glutamate metabolism pathway in the gut mucosa. The present study examined the cellular distribution of glutamate transporters in the digestive tract of adult mice using immunohistochemistry and in situ hybridization technique. Expression of EAAC1 mRNA was more intense in the ileum, where the epithelium in crypts and the basal half of intestinal villi showed high levels of transcripts, suggesting an essential role of EAAC1 in differentiating or premature epithelial cells. Electron-microscopically, EAAC1 immunoreactivity was predominantly localized in the striated border of enterocytes. Immunoreactivity for GLT-1 was found in the lateral membrane of epithelial cells at the bottom of gastric glands and at the intestinal crypts, and also in the lateral membrane of secretory cells at the duodenal gland. GLAST immunoreactivity was restricted to the fundic and pyloric glands, and was especially intense in the neck portion of both glands. However, in situ hybridization analysis failed to confirm the expression of GLT-1 and GLAST at the mRNA level, possibly due to limited sensitivity. The strong and specific luminal localization of EAAC1 in the intestinal epithelium suggests that EAAC1 is a predominant transporter of glutamate, at least in the lower part of the small intestine.

Anaplerotic input is sufficient to induce time-dependent potentiation of insulin release in rat pancreatic islets

Nutrients that induce biphasic insulin release, such as glucose and leucine, provide acetyl-CoA and anaplerotic input in the beta-cell. The first phase of release requires increased ATP production leading to increased intracellular Ca(2+) concentration ([Ca(2+)](i)). The second phase requires increased [Ca(2+)](i) and anaplerosis. There is strong evidence to indicate that the second phase is due to augmentation of Ca(2+)-stimulated release via the K(ATP) channel-independent pathway. To test whether the phenomenon of time-dependent potentiation (TDP) has similar properties to the ATP-sensitive K(+) channel-independent pathway, we monitored the ability of different agents that provide acetyl-CoA and anaplerotic input or both of these inputs to induce TDP. The results show that anaplerotic input is sufficient to induce TDP. Interestingly, among the agents tested, the nonsecretagogue glutamine, the nonhydrolyzable analog of leucine aminobicyclo[2.2.1]heptane-2-carboxylic acid, and succinic acid methyl ester all induced TDP, and all significantly increased alpha-ketoglutarate levels in the islets. In conclusion, anaplerosis that enhances the supply and utilization of alpha-ketoglutarate in the tricarboxylic acid cycle appears to play an essential role in the generation of TDP.