The complex mechanism of glutamate dehydrogenase in insulin secretion

Evolutionary Changes in Primate Glutamate Dehydrogenases 1 and 2 Influence the Protein Regulation by Ligands, Targeting and Posttranslational Modifications

There are two paralogs of glutamate dehydrogenase (GDH) in humans encoded by the GLUD1 and GLUD2 genes as a result of a recent retroposition during the evolution of primates. The two human GDHs possess significantly different regulation by allosteric ligands, which is not fully characterized at the structural level. Recent advances in identification of the GDH ligand binding sites provide a deeper perspective on the significance of the accumulated substitutions within the two GDH paralogs. In this review, we describe the evolution of GLUD1 and GLUD2 after the duplication event in primates using the accumulated sequencing and structural data. A new gibbon GLUD2 sequence questions the indispensability of ancestral R496S and G509A mutations for GLUD2 irresponsiveness to GTP, providing an alternative with potentially similar regulatory features. The data of both GLUD1 and GLUD2 evolution not only confirm substitutions enhancing GLUD2 mitochondrial targeting, but also reveal a conserved mutation in ape GLUD1 mitochondrial targeting sequence that likely reduces its transport to mitochondria. Moreover, the information of GDH interactors, posttranslational modification and subcellular localization are provided for better understanding of the GDH mutations. Medically significant point mutations causing deregulation of GDH are considered from the structural and regulatory point of view.

Quantitative Metabolomics and Lipoprotein Analysis of PDAC Patients Suggests Serum Marker Categories for Pancreatic Function, Pancreatectomy, Cancer Metabolism, and Systemic Disturbances

Pancreatic ductal adenocarcinoma (PDAC) is difficult to diagnose in the early stages and lacks reliable biomarkers. The scope of this project was to establish quantitative nuclear magnetic resonance (NMR) spectroscopy to comprehensively study blood serum alterations in PDAC patients. Serum samples from 34 PDAC patients obtained before and after pancreatectomy as well as 83 age- and sex-matched control samples from healthy donors were analyzed with in vitro diagnostics research (IVDr) proton NMR spectroscopy at 600 MHz. Uni- and multivariate statistics were applied to identify significant biofluid alterations. We identified 29 significantly changed metabolites and 98 lipoproteins when comparing serum from healthy controls with those of PDAC patients. The most prominent features were assigned to (i) markers of pancreatic function (e.g., glucose and blood triglycerides), (ii) markers related to surgery (e.g., ketone bodies and blood cholesterols), (iii) PDAC-associated markers (e.g., amino acids and creatine), and (iv) markers for systemic disturbances in PDAC (e.g., gut metabolites DMG, TMAO, DMSO2, and liver lipoproteins). Quantitative serum NMR spectroscopy is suited as a diagnostic tool to investigate PDAC. Remarkably, 2-hydroxybutyrate (2-HB) as a previously suggested marker for insulin resistance was found in extraordinarily high levels only after pancreatectomy, suggesting this metabolite is the strongest marker for pancreatic loss of function.

Association between protein undernutrition and diabetes: Molecular implications in the reduction of insulin secretion

Undernutrition is still a recurring nutritional problem in low and middle-income countries. It is directly associated with the social and economic sphere, but it can also negatively impact the health of the population. In this sense, it is believed that undernourished individuals may be more susceptible to the development of non-communicable diseases, such as diabetes mellitus, throughout life. This hypothesis was postulated and confirmed until today by several studies that demonstrate that experimental models submitted to protein undernutrition present alterations in glycemic homeostasis linked, in part, to the reduction of insulin secretion. Therefore, understanding the changes that lead to a reduction in the secretion of this hormone is essential to prevent the development of diabetes in undernourished individuals. This narrative review aims to describe the main molecular changes already characterized in pancreatic β cells that will contribute to the reduction of insulin secretion in protein undernutrition. So, it will provide new perspectives and targets for postulation and action of therapeutic strategies to improve glycemic homeostasis during this nutritional deficiency.

Structural Evolution of Primate Glutamate Dehydrogenase 2 as Revealed by In Silico Predictions and Experimentally Determined Structures

Glutamate dehydrogenase (GDH) interconverts glutamate to a-ketoglutarate and ammonia, interconnecting amino acid and carbohydrate metabolism. In humans, two functional GDH genes, GLUD1 and GLUD2, encode for hGDH1 and hGDH2, respectively. GLUD2 evolved from retrotransposition of the GLUD1 gene in the common ancestor of modern apes. These two isoenzymes are involved in the pathophysiology of human metabolic, neoplastic, and neurodegenerative disorders. The 3D structures of hGDH1 and hGDH2 have been experimentally determined; however, no information is available about the path of GDH2 structure changes during primate evolution. Here, we compare the structures predicted by the AlphaFold Colab method for the GDH2 enzyme of modern apes and their extinct primate ancestors. Also, we analyze the individual effect of amino acid substitutions emerging during primate evolution. Our most important finding is that the predicted structure of GDH2 in the common ancestor of apes was the steppingstone for the structural evolution of primate GDH2s. Two changes with a strong functional impact occurring at the first evolutionary step, Arg443Ser and Gly456Ala, had a destabilizing and stabilizing effect, respectively, making this step the most important one. Subsequently, GDH2 underwent additional modifications that fine-tuned its enzymatic properties to adapt to the functional needs of modern-day primate tissues.

Pancreatic β-cell glutaminase 2 maintains glucose homeostasis under the condition of hyperglycaemia

Glutaminase 2 (GLS2), a master regulator of glutaminolysis that is induced by p53 and converts glutamine to glutamate, is abundant in the liver but also exists in pancreatic β-cells. However, the roles of GLS2 in islets associated with glucose metabolism are unknown, presenting a critical issue. To investigate the roles of GLS2 in pancreatic β-cells in vivo, we generated β-cell-specific Gls2 conditional knockout mice (Gls2 CKO), examined their glucose homeostasis, and validated the findings using a human islet single-cell analysis database. GLS2 expression markedly increased along with p53 in β-cells from control (RIP-Cre) mice fed a high-fat diet. Furthermore, Gls2 CKO exhibited significant diabetes mellitus with gluconeogenesis and insulin resistance when fed a high-fat diet. Despite marked hyperglycaemia, impaired insulin secretion and paradoxical glucagon elevation were observed in high-fat diet-fed Gls2 CKO mice. GLS2 silencing in the pancreatic β-cell line MIN6 revealed downregulation of insulin secretion and intracellular ATP levels, which were closely related to glucose-stimulated insulin secretion. Additionally, analysis of single-cell RNA-sequencing data from human pancreatic islet cells also revealed that GLS2 expression was elevated in β-cells from diabetic donors compared to nondiabetic donors. Consistent with the results of Gls2 CKO, downregulated GLS2 expression in human pancreatic β-cells from diabetic donors was associated with significantly lower insulin gene expression as well as lower expression of members of the insulin secretion pathway, including ATPase and several molecules that signal to insulin secretory granules, in β-cells but higher glucagon gene expression in α-cells. Although the exact mechanism by which β-cell-specific GLS2 regulates insulin and glucagon requires further study, our data indicate that GLS2 in pancreatic β-cells maintains glucose homeostasis under the condition of hyperglycaemia.

Metabolic sensing and control in mitochondria

Mitochondria are membrane-enclosed organelles with endosymbiotic origins, harboring independent genomes and a unique biochemical reaction network. To perform their critical functions, mitochondria must maintain a distinct biochemical environment and coordinate with the cytosolic metabolic networks of the host cell. This coordination requires them to sense and control metabolites and respond to metabolic stresses. Indeed, mitochondria adopt feedback or feedforward control strategies to restrain metabolic toxicity, enable metabolic conservation, ensure stable levels of key metabolites, allow metabolic plasticity, and prevent futile cycles. A diverse panel of metabolic sensors mediates these regulatory circuits whose malfunctioning leads to inborn errors of metabolism with mild to severe clinical manifestations. In this review, we discuss the logic and molecular basis of metabolic sensing and control in mitochondria. The past research outlined recurring patterns in mitochondrial metabolic sensing and control and highlighted key knowledge gaps in this organelle that are potentially addressable with emerging technological breakthroughs.

NKCC transport mediates the insulinotropic effects of taurine and other small neutral amino acids

AIMS

Despite its high concentration in pancreatic islets of Langerhans and broad range of antihyperglycemic effects, the route facilitating the import of dietary taurine into pancreatic β-cell and mechanisms underlying its insulinotropic activity are unclear. We therefore studied the impact of taurine on beta-cell function, alongside that of other small neutral amino acids, L-alanine and L-proline.

MAIN METHODS

Pharmacological profiling of insulin secretion was conducted using clonal BRIN BD11 β-cells, the impact of taurine on the metabolic fate of glucose carbons was assessed using NMR and the findings were verified by real-time imaging of Ca²⁺ dynamics in the cytosol of primary mouse and human islet beta-cells.

KEY FINDINGS

In our hands, taurine, alanine and proline induced secretory responses that were dependent on the plasma membrane depolarisation, import of Ca²⁺, homeostasis of K⁺ and Na⁺ as well as on cell glycolytic and oxidative metabolism. Taurine shifted the balance between the oxidation and anaplerosis towards the latter, in BRIN BD11 beta-cells. Furthermore, the amino acid signalling was significantly attenuated by inhibition of Na⁺-K⁺-Cl^- symporter (NKCC).

SIGNIFICANCE

These data suggest that taurine, like L-alanine and L-proline, acutely induces glucose-dependent insulin-secretory responses by modulating electrogenic Na⁺ transport, with potential role of intracellular K⁺ and Cl^- in the signal transduction. The acute action delineated would be consistent with antidiabetic potential of dietary taurine supplementation.

Glutamate dehydrogenase hyperinsulinism: mechanisms, diagnosis, and treatment

Congenital hyperinsulinism (CHI) is a genetically heterogeneous disease, in which intractable, persistent hypoglycemia is induced by excessive insulin secretion and increased serum insulin concentration. To date,15 genes have been found to be associated with the pathogenesis of CHI. Glutamate dehydrogenase hyperinsulinism (GDH-HI) is the second most common type of CHI and is caused by mutations in the glutamate dehydrogenase 1 gene. The objective of this review is to summarize the genetic mechanisms, diagnosis and treatment progress of GDH-HI. Early diagnosis and treatment are extremely important to prevent long-term neurological complications in children with GDH-HI.

Multiple types of genomic variation contribute to adaptive traits in the mustelid subfamily Guloninae

Species of the mustelid subfamily Guloninae inhabit diverse habitats on multiple continents, and occupy a variety of ecological niches. They differ in feeding ecologies, reproductive strategies and morphological adaptations. To identify candidate loci associated with adaptations to their respective environments, we generated a de novo assembly of the tayra (Eira barbara), the earliest diverging species in the subfamily, and compared this with the genomes available for the wolverine (Gulo gulo) and the sable (Martes zibellina). Our comparative genomic analyses included searching for signs of positive selection, examining changes in gene family sizes, as well as searching for species-specific structural variants (SVs). Among candidate loci associated with phenotypic traits, we observed many related to diet, body condition and reproduction. For example, for the tayra, which has an atypical gulonine reproductive strategy of aseasonal breeding, we observe species-specific changes in many pregnancy-related genes. For the wolverine, a circumpolar hypercarnivore that must cope with seasonal food scarcity, we observed many changes in genes associated with diet and body condition. All types of genomic variation examined (single nucleotide polymorphisms, gene family expansions, structural variants) contributed substantially to the identification of candidate loci. This strongly argues for consideration of variation other than single nucleotide polymorphisms in comparative genomics studies aiming to identify loci of adaptive significance.

Metabolomics of Type 1 and Type 2 Diabetes: Insights into Risk Prediction and Mechanisms

PURPOSE OF REVIEW

Metabolomics enables rapid interrogation of widespread metabolic processes making it well suited for studying diabetes. Here, we review the current status of metabolomic investigation in diabetes, highlighting its applications for improving risk prediction and mechanistic understanding.

RECENT FINDINGS

Findings of metabolite associations with type 2 diabetes risk have confirmed experimental observations (e.g., branched-chain amino acids) and also pinpointed novel pathways of diabetes risk (e.g., dimethylguanidino valeric acid). In type 1 diabetes, abnormal metabolite patterns are observed prior to the development of autoantibodies and hyperglycemia. Diabetes complications display specific metabolite signatures that are distinct from the metabolic derangements of diabetes and differ across vascular beds. Lastly, metabolites respond acutely to pharmacologic treatment, providing opportunities to understand inter-individual treatment responses. Metabolomic studies have elucidated biological mechanisms underlying diabetes development, complications, and therapeutic response. While not yet ready for clinical translation, metabolomics is a powerful and promising precision medicine tool.

Amino acid transporters as modulators of glucose homeostasis

Amino acids modulate glucose homeostasis. Cytosolic levels of amino acids are regulated by amino acid transporters, modulating insulin release, protein synthesis, cell proliferation, cell fate, and metabolism. In β-cells, amino acid transporters modulate incretin-stimulated insulin release. In the liver, amino acid transporters provide glutamine and alanine for gluconeogenesis. Intestinal amino acid transporters facilitate the intake of amino acids causing protein restriction when inactive. Adipocyte development is regulated by amino acid transporters through activation of mechanistic target of rapamycin (mTORC1) and amino acid-related metabolites. The accumulation and metabolism of branched-chain amino acids (BCAAs) in muscle depends on transporters. The integration between amino acid metabolism and transport is critical for the maintenance and function of tissues and cells involved in glucose homeostasis.

Genetic pathogenesis, diagnosis, and treatment of short-chain 3-hydroxyacyl-coenzyme A dehydrogenase hyperinsulinism

Congenital hyperinsulinism (CHI), a major cause of persistent and recurrent hypoglycemia in infancy and childhood. Numerous pathogenic genes have been associated with 14 known genetic subtypes of CHI. Adenosine triphosphate-sensitive potassium channel hyperinsulinism (KATP-HI) is the most common and most severe subtype, accounting for 40–50% of CHI cases. Short-chain 3-hydroxyacyl-coenzyme A dehydrogenase hyperinsulinism (SCHAD-HI) is a rare subtype that accounts for less than 1% of all CHI cases that are caused by homozygous mutations in the hydroxyacyl-coenzyme A dehydrogenase (HADH) gene. This review provided a systematic description of the genetic pathogenesis and current progress in the diagnosis and treatment of SCHAD-HI to improve our understanding of this disease.

Glutamate Dehydrogenase as a Promising Target for Hyperinsulinism Hyperammonemia Syndrome Therapy

Hyperinsulinism-hyperammonemia syndrome (HHS) is a rare disease characterized by recurrent hypoglycemia and persistent elevation of plasma ammonia, and it can lead to severe epilepsy and permanent brain damage. It has been demonstrated that functional mutations of glutamate dehydrogenase (GDH), an enzyme in the mitochondrial matrix, are responsible for the HHS. Thus, GDH has become a promising target for the small molecule therapeutic intervention of HHS. Several medicinal chemistry studies are currently aimed at GDH, however, to date, none of the compounds reported has been entered clinical trials. This perspective summarizes the progress in the discovery and development of GDH inhibitors, including the pathogenesis of HHS, potential binding sites, screening methods, and research models. Future therapeutic perspectives are offered to provide a reference for discovering potent GDH modulators and encourage additional research that will provide more comprehensive guidance for drug development.

Effects of High-Fat Diet Induced Obesity and Fructooligosaccharide Supplementation on Cardiac Protein Expression

The mechanism by which high fat-diet induced obesity affects cardiac protein expression is unclear, and the extent to which this is modulated by prebiotic treatment is not known. These outcomes were assessed in rats initially fed a high-fat diet, then the top 40% weight gain group were randomly allocated to control (CON), high-fat (HF) and HF supplemented with fructooligosaccharide (32 g; HF-FOS) treatments for 12 weeks (n = 10/group). At sacrifice, left ventricles were either frozen or preserved in formalin. Serum was stored for glucose and insulin measurements. Protein spectra was obtained using an Orbitrap analyzer, processed with Sequest and fold changes assessed with Scaffold Q +. Treatment effects for body weights, glucose and insulin were assessed using one-way ANOVA, and the differential protein expression was assessed by a Mann-Whitney U test. The Database for Annotation, Visualization and Integrated Discovery and the Kyoto Encyclopedia of Genes and Genomes identified pathways containing overrepresented proteins. Hematoxylin and eosin sections were graded for hypertrophy and also quantified; differences were identified using Chi-square analyses and Mann-Whitney U tests. HF diet fed rats were significantly (p < 0.05) heavier than CON, and 23 proteins involved in mitochondrial function and lipid metabolism were differentially expressed between HF and CON. Between HF-FOS and HF, 117 proteins involved in contractility, lipid and carbohydrate metabolism were differentially expressed. HF cardiomyocytes were significantly (p < 0.05) more hypertrophic than CON. We conclude that high-fat feeding and FOS are associated with subcellular deviations in cardiac metabolism and contractility, which may influence myocardial function and alter the risk of cardiovascular disease.

Regulation of Body Size and Growth Control

The control of body and organ growth is essential for the development of adults with proper size and proportions, which is important for survival and reproduction. In animals, adult body size is determined by the rate and duration of juvenile growth, which are influenced by the environment. In nutrient-scarce environments in which more time is needed for growth, the juvenile growth period can be extended by delaying maturation, whereas juvenile development is rapidly completed in nutrient-rich conditions. This flexibility requires the integration of environmental cues with developmental signals that govern internal checkpoints to ensure that maturation does not begin until sufficient tissue growth has occurred to reach a proper adult size. The Target of Rapamycin (TOR) pathway is the primary cell-autonomous nutrient sensor, while circulating hormones such as steroids and insulin-like growth factors are the main systemic regulators of growth and maturation in animals. We discuss recent findings in Drosophila melanogaster showing that cell-autonomous environment and growth-sensing mechanisms, involving TOR and other growth-regulatory pathways, that converge on insulin and steroid relay centers are responsible for adjusting systemic growth, and development, in response to external and internal conditions. In addition to this, proper organ growth is also monitored and coordinated with whole-body growth and the timing of maturation through modulation of steroid signaling. This coordination involves interorgan communication mediated by Drosophila insulin-like peptide 8 in response to tissue growth status. Together, these multiple nutritional and developmental cues feed into neuroendocrine hubs controlling insulin and steroid signaling, serving as checkpoints at which developmental progression toward maturation can be delayed. This review focuses on these mechanisms by which external and internal conditions can modulate developmental growth and ensure proper adult body size, and highlights the conserved architecture of this system, which has made Drosophila a prime model for understanding the coordination of growth and maturation in animals.

Factors Influencing Mitochondrial Function as a Key Mediator of Glucose-Induced Insulin Release: Highlighting Nicotinamide Nucleotide Transhydrogenase

Pancreatic β-cells recognize blood glucose changes and release insulin that is a peptide hormone responsible for stable glycemia. Diabetes, a chronic disorder of insulin insufficiency, leads to disturbed glucose homeostasis and multi-organ problems. Glucose and insulin are key markers in the follow-up and control of this disease. Mitochondrial metabolism of pancreatic beta cells is a crucial part of glucose-stimulated cascade of insulin secretion. Effective factors on β-cells mitochondrial function in production of compounds such as tricarboxylic acid intermediates, glutamate, nicotinamide adenine dinucleotide phosphate, and reactive oxygen species can have great effects on the secretion of insulin under diabetes. This review enhances our knowledge of factors influencing mitochondrial function as a key mediator of glucose-induced insulin release that accordingly will be helpful to further our understanding of the mechanisms implicated in the progressive beta cell failure that results in diabetes.

The influence of dietary leucine above recommendations and fixed ratios to isoleucine and valine on muscle protein synthesis and degradation pathways in broilers

This study investigated the hypothesis that dietary supplementation of leucine (Leu) above actual recommendations activates protein synthesis and inhibits protein degradation pathways on the molecular level and supports higher muscle growth in broilers. Day-old male Cobb-500 broilers (n = 180) were allotted to 3 groups and phase-fed 3 different corn-wheat-soybean meal-based basal diets during periods 1 to 10, 11 to 21, and 22 to 35 D. The control group (L0) received the basal diet which met the broiler's requirements of nutrients and amino acids for maintenance and growth. Groups L1 and L2 received basal diets supplemented with Leu to exceed recommendations by 35 and 60%, respectively, and isoleucine (Ile) and valine (Val) were supplemented to keep Leu: Ile and Leu: Val ratios fixed. Samples of liver and breast muscle and pancreas were collected on days 10, 21, and 35. The gene expression and abundance of total and phosphorylated proteins involved in the mammalian target of rapamycin pathway of protein synthesis, in the ubiquitin-proteasome pathway and autophagy-lysosomal pathway of protein degradation, in the general control nonderepressible 2/eukaryotic translation initiation factor 2A pathway involved in the inhibition of protein synthesis, and in the myostatin-Smad2/3 pathway involved in myogenesis were evaluated in the muscle, as well as expression of genes involved in the growth hormone axis. Growth performance, feed intake, the feed conversion ratio, and carcass weights did not differ between the 3 groups (P > 0.05). Plasma concentrations of Leu, Ile, and Val and of their keto acids, and the activity of the branched-chain α-keto acid dehydrogenase in the pancreas increased dose dependently with increasing dietary Leu concentrations. In the breast muscle, relative mRNA abundances of genes and phosphorylation of selected proteins involved in all investigated pathways were largely uninfluenced by dietary Leu supplementation (P > 0.05). In summary, these data indicate that excess dietary Leu concentrations do not influence protein synthesis or degradation pathways, and subsequently do not increase muscle growth in broilers at fixed ratios to Ile and Val.

Glucose Homeostasis in Newborns: An Endocrinology Perspective

Physiologic adaptations in the postnatal period, along with gradual establishment of enteral feeding, help maintain plasma glucose concentrations in the neonatal period. The definition of normal plasma glucose in the neonatal period has been a subject of debate because of a lack of evidence linking a set plasma or blood glucose concentration to clinical symptoms or predictors of short- and long-term outcomes. However, there is consensus that maintaining plasma glucose in the normal range for age is important to prevent immediate and long-term neurodevelopmental consequences of hypoglycemia or hyperglycemia. The specific management strategy for abnormal glucose levels in neonates depends on the underlying etiology, and interventions could include nutritional changes, medications, hormone therapy, or even surgery. Here, we will review the physiological processes that help maintain plasma glucose in newborns and discuss the approach to a newborn with disordered glucose homeostasis, with an emphasis on the endocrine basis of abnormal glucose homeostasis.

Structures, functions, and mechanisms of filament forming enzymes: a renaissance of enzyme filamentation

Filament formation by non-cytoskeletal enzymes has been known for decades, yet only relatively recently has its wide-spread role in enzyme regulation and biology come to be appreciated. This comprehensive review summarizes what is known for each enzyme confirmed to form filamentous structures in vitro, and for the many that are known only to form large self-assemblies within cells. For some enzymes, studies describing both the in vitro filamentous structures and cellular self-assembly formation are also known and described. Special attention is paid to the detailed structures of each type of enzyme filament, as well as the roles the structures play in enzyme regulation and in biology. Where it is known or hypothesized, the advantages conferred by enzyme filamentation are reviewed. Finally, the similarities, differences, and comparison to the SgrAI endonuclease system are also highlighted.

Evaluation of Pancreatic and Extra Pancreatic Effects of Branched Amino Acids

Background and aims: Leucine, Isoleucine, and Valine collectively known as Branched-chain amino acids (BCAAs), can be closely associated with metabolic dysregulates and with insulin resistance. We aimed to explore the role of BCAAs as potential treatment option for diabetes.

Material and method: Bioassay the effect of BCAAs on MIN6 cell line on insulin secretion and pancreatic beta cells expansion, then were checked for inhibitory potential of pancreatic amylase, glucosidase and lipase as alternative approach for diabetes treatment.

Results: BCAAs significantly enhance insulin secretion parallel to L-alanine efficacy. Furthermore, BCAAs obtain a dose dependent β-cell proliferation similar to glucagon-like peptide-1. Moreover, these acids could restore the secretory function of MIN6 β-cell despite stressful gluco-lipo-toxicity; separately or combined. Moreover, BCAAs exerted a dose dependent dual inhibition of amylase, glucosidase and lipase.

Conclusions: Our current findings suggest that BCAAs supplementation may have a potential therapeutic effect against diabetes as insulin releasing agent and as specific inhibitors for both-amylase/α-amyloglucoside and lipase

Effects of leucine supplementation on muscle protein synthesis and degradation pathways in broilers at constant dietary concentrations of isoleucine and valine

The present study investigated the hypothesis that dietary concentrations of leucine (Leu) in excess of the breeder´s recommendations activates protein synthesis and decreases protein degradation in muscle of broilers. Day-old male Ross 308 broilers (n = 450) were phase-fed corn-soybean meal-based diets during starter (d 1-10), grower (d 11-22), and finisher (d 23-34) period. The basal diets fed to the control group (L0) met the broilers' requirements for nutrients and amino acids, and contained Leu, Leu:isoleucine (Ile) and Leu:valine (Val) ratios, close to those recommended by the breeder (Leu:Ile: 100:54, 100:52, 100:51; Leu:Val 100:64, 100:61, 100:58; in starter, grower and finisher diet, resp.). Basal diets were supplemented with Leu to exceed the breeder's recommendations by 35% (group L35) and 60% (group L60). Growth performance during 34 d, and carcass weights, and breast and thigh muscle weights on d 34 were similar among groups. Hepatic and muscle mRNA levels of genes involved in the somatotropic axis [growth hormone receptor, insulin-like growth factor (IGF)-1, IGF binding protein 2, IGF receptor] on d 34 were not influenced by Leu. In the breast muscle, relative mRNA abundances of genes involved in the mammalian target of rapamycin (mTOR) pathway of protein synthesis (mTOR, ribosomal p70 S6 kinase) and the ubiquitin-proteasome pathway of protein degradation (F-box only protein 32, Forkhead box protein O1, Muscle RING-finger protein-1) on d 34 were largely similar among groups. Likewise, relative phosphorylation and thus activation of mTOR and ribosomal protein S6 involved in the mTOR pathway, and of eukaryotic translation initiation factor 2A (eIF2a) involved in the general control nonderepressible 2 (GCN2)/eIF2a pathway of protein synthesis inhibition, were not influenced. These data indicate that dietary Leu concentrations exceeding the broiler´s requirements up to 60% neither influence protein synthesis nor degradation pathways nor muscle growth in growing broilers.

Amino acid transporters in the regulation of insulin secretion and signalling

Amino acids are increasingly recognised as modulators of nutrient disposal, including their role in regulating blood glucose through interactions with insulin signalling. More recently, cellular membrane transporters of amino acids have been shown to form a pivotal part of this regulation as they are primarily responsible for controlling cellular and circulating amino acid concentrations. The availability of amino acids regulated by transporters can amplify insulin secretion and modulate insulin signalling in various tissues. In addition, insulin itself can regulate the expression of numerous amino acid transporters. This review focuses on amino acid transporters linked to the regulation of insulin secretion and signalling with a focus on those of the small intestine, pancreatic β-islet cells and insulin-responsive tissues, liver and skeletal muscle. We summarise the role of the amino acid transporter B⁰AT1 (SLC6A19) and peptide transporter PEPT1 (SLC15A1) in the modulation of global insulin signalling via the liver-secreted hormone fibroblast growth factor 21 (FGF21). The role of vesicular vGLUT (SLC17) and mitochondrial SLC25 transporters in providing glutamate for the potentiation of insulin secretion is covered. We also survey the roles SNAT (SLC38) family and LAT1 (SLC7A5) amino acid transporters play in the regulation of and by insulin in numerous affective tissues. We hypothesise the small intestine amino acid transporter B⁰AT1 represents a crucial nexus between insulin, FGF21 and incretin hormone signalling pathways. The aim is to give an integrated overview of the important role amino acid transporters have been found to play in insulin-regulated nutrient signalling.

The pleiotropic effects of the glutamate dehydrogenase (GDH) pathway in Saccharomyces cerevisiae

Ammonium assimilation is linked to fundamental cellular processes that include the synthesis of non-essential amino acids like glutamate and glutamine. In Saccharomyces cerevisiae glutamate can be synthesized from α-ketoglutarate and ammonium through the action of NADP-dependent glutamate dehydrogenases Gdh1 and Gdh3. Gdh1 and Gdh3 are evolutionarily adapted isoforms and cover the anabolic role of the GDH-pathway. Here, we review the role and function of the GDH pathway in glutamate metabolism and we discuss the additional contributions of the pathway in chromatin regulation, nitrogen catabolite repression, ROS-mediated apoptosis, iron deficiency and sphingolipid-dependent actin cytoskeleton modulation in S.cerevisiae. The pleiotropic effects of GDH pathway in yeast biology highlight the importance of glutamate homeostasis in vital cellular processes and reveal new features for conserved enzymes that were primarily characterized for their metabolic capacity. These newly described features constitute insights that can be utilized for challenges regarding genetic engineering of glutamate homeostasis and maintenance of redox balances, biosynthesis of important metabolites and production of organic substrates. We also conclude that the discussed  pleiotropic features intersect with basic metabolism and set a new background for further glutamate-dependent applied research of biotechnological interest.

Chlorogenic acids inhibit glutamate dehydrogenase and decrease intracellular ATP levels in cultures of chick embryo retina cells

Chlorogenic acids (CGAs) are a group of phenolic compounds found in worldwide consumed beverages such as coffee and green tea. They are synthesized from an esterification reaction between cinnamic acids, including caffeic (CFA), ferulic and p-coumaric acids with quinic acid (QA), forming several mono- and di-esterified isomers. The most prevalent and studied compounds are 3-O-caffeoylquinic acid (3-CQA), 4-O-caffeoylquinic acid (4-CQA) and 5-O-caffeoylquinic acid (5-CQA), widely described as having antioxidant and cell protection effects. CGAs can also modulate glutamate release from microglia by a mechanism involving a decrease of reactive oxygen species (ROS). Increased energy metabolism is highly associated with enhancement of ROS production and cellular damage. Glutamate can also be used as an energy source by glutamate dehydrogenase (GDH) enzyme, providing α-ketoglutarate to the tricarboxylic acid (TCA) cycle for ATP synthesis. High GDH activity is associated with some disorders, such as schizophrenia and hyperinsulinemia/hyperammonemia syndrome. In line with this, our objective was to investigate the effect of CGAs on GDH activity. We show that CGAs and CFA inhibits GDH activity in dose-dependent manner, reaching complete inhibition at high concentration with IC50 of 52 μM for 3-CQA and 158.2 μM for CFA. Using live imaging confocal microscopy and microplate reader, we observed that 3-CQA and CFA can be transported into neuronal cells by an Na⁺-dependent mechanism. Moreover, neuronal cells treated with CGAs presented lower intracellular ATP levels. Overall, these data suggest that CGAs have therapeutic potential for treatment of disorders associated with high GDH activity.

Localization of Human Glutamate Dehydrogenases Provides Insights into Their Metabolic Role and Their Involvement in Disease Processes

Glutamate dehydrogenase (GDH) catalyzes the reversible deamination of L-glutamate to α-ketoglutarate and ammonia. In mammals, GDH contributes to important processes such as amino acid and carbohydrate metabolism, energy production, ammonia management, neurotransmitter recycling and insulin secretion. In humans, two isoforms of GDH are found, namely hGDH1 and hGDH2, with the former being ubiquitously expressed and the latter found mainly in brain, testis and kidney. These two iso-enzymes display highly divergent allosteric properties, especially concerning their basal activity, ADP activation and GTP inhibition. On the other hand, both enzymes are thought to predominantly localize in the mitochondrial matrix, even though alternative localizations have been proposed. To further study the subcellular localization of the two human iso-enzymes, we created HEK293 cell lines stably over-expressing hGDH1 and hGDH2. In these cell lines, immunofluorescence and enzymatic analyses verified the overexpression of both hGDH1 and hGDH2 iso-enzymes, whereas subcellular fractionation followed by immunoblotting showed their predominantly mitochondrial localization. Given that previous studies have only indirectly compared the subcellular localization of the two iso-enzymes, we co-expressed them tagged with different fluorescent dyes (green and red fluorescent protein for hGDH1 and hGDH2, respectively) and found them to co-localize. Despite the wealth of information related to the functional properties of hGDH1 and hGDH2 and the availability of the hGDH1 structure, there is still an ongoing debate concerning their metabolic role and their involvement in disease processes. Data on the localization of hGDHs, as the ones presented here, could contribute to better understanding of the function of these important human enzymes.

Glutamate dehydrogenase: role in regulating metabolism and insulin release in pancreatic β-cells

Regulation of insulin release and glucose homeostasis by pancreatic β-cells is dependent on the metabolism of glucose by glucokinase (GK) and the influence of that activity on oxidative phosphorylation. Genetic alterations that result in hyperactivity of mitochondrial glutamate dehydrogenase (GDH-1) can cause hypoglycemia-hyperammonemia following high protein meals, but the role of GDH-1 remains poorly understood. GDH-1 activity is strongly inhibited by GTP, to near zero in the absence of ADP, and cooperatively activated ( n = 2.3) by ADP. The dissociation constant for ADP is near 200 µM in vivo, but leucine and its nonmetabolized analog 2-amino-2-norbornane-carboxylic acid (BCH) can activate GDH-1 by increasing the affinity for ADP. Under physiological conditions, as [ADP] increases GDH-1 activity remains very low until ~35 µM (threshold) and then increases rapidly. A model for GDH-1 and its regulation has been combined with a previously published model for glucose sensing that coupled GK activity and oxidative phosphorylation. The combined model (GK-GDH-core) shows that GK activity determines the energy state ([ATP]/[ADP][Pi]) in β-cells for glucose concentrations > 5 mM ([ADP] < 35 µM). As glucose falls < 5 mM the [ADP]-dependent increase in GDH-1 activity prevents [ADP] from rising above ~70 µM. Thus, GDH-1 dynamically buffers β-cell energy metabolism during hypoglycemia, maintaining the energy state and the basal rate of insulin release. GDH-1 hyperactivity suppresses the normal increase in [ADP] in hypoglycemia. This leads to hypoglycemia following a high protein meal by increasing the basal rate of insulin release (β-cells) and decreasing glucagon release (α-cells). NEW & NOTEWORTHY A model of β-cell metabolism and regulation of insulin release is presented. The model integrates regulation of oxidative phosphorylation, glucokinase (GK), and glutamate dehydrogenase (GDH-1). GDH-1 is near equilibrium under physiological conditions, but the activity is inhibited by GTP. In hypoglycemia, however, GK activity is low and [ADP], a potent activator of GDH-1, increases. Reducing equivalents from GDH dynamically buffers the intramitochondrial [NADH]/[NAD⁺], and thereby the energy state, preventing hypoglycemia-induced substrate deprivation.

Palmitate-induced lipotoxicity alters acetylation of multiple proteins in clonal β cells and human pancreatic islets

Type 2 diabetes is characterized by progressive β cell dysfunction, with lipotoxicity playing a possible pathogenetic role. Palmitate is often used to examine the direct effects of lipotoxicity and it may cause mitochondrial alterations by activating protein acetylation. However, it is unknown whether palmitate influences protein acetylation in β cells. We investigated lysine acetylation in mitochondrial proteins from INS-1E β cells (INS-1E) and in proteins from human pancreatic islets (HPI) after 24 h palmitate exposure. First, we confirmed that palmitate damages β cells and demonstrated that chemical inhibition of deacetylation also impairs INS-1E function and survival. Then, by 2-D gel electrophoresis, Western Blot and Liquid Chromatography-Mass Spectrometry we evaluated the effects of palmitate on protein acetylation. In mitochondrial preparations from palmitate-treated INS-1E, 32 acetylated spots were detected, with 13 proteins resulting over-acetylated. In HPI, 136 acetylated proteins were found, of which 11 were over-acetylated upon culture with palmitate. Interestingly, three proteins, glutamate dehydrogenase, mitochondrial superoxide dismutase, and SREBP-1, were over-acetylated in both INS-1E and HPI. Therefore, prolonged exposure to palmitate induces changes in β cell protein lysine acetylation and this modification could play a role in causing β cell damage. Dysregulated acetylation may be a target to counteract palmitate-induced β cell lipotoxicity.

Mechanisms of the amplifying pathway of insulin secretion in the β cell

Pancreatic islet β cells secrete insulin in response to nutrient secretagogues, like glucose, dependent on calcium influx and nutrient metabolism. One of the most intriguing qualities of β cells is their ability to use metabolism to amplify the amount of secreted insulin independent of further alterations in intracellular calcium. Many years studying this amplifying process have shaped our current understanding of β cell stimulus-secretion coupling; yet, the exact mechanisms of amplification have been elusive. Recent studies utilizing metabolomics, computational modeling, and animal models have progressed our understanding of the metabolic amplifying pathway of insulin secretion from the β cell. New approaches will be discussed which offer in-roads to a more complete model of β cell function. The development of β cell therapeutics may be aided by such a model, facilitating the targeting of aspects of the metabolic amplifying pathway which are unique to the β cell.

The Pancreatic β-Cell: A Bioenergetic Perspective

The pancreatic β-cell secretes insulin in response to elevated plasma glucose. This review applies an external bioenergetic critique to the central processes of glucose-stimulated insulin secretion, including glycolytic and mitochondrial metabolism, the cytosolic adenine nucleotide pool, and its interaction with plasma membrane ion channels. The control mechanisms responsible for the unique responsiveness of the cell to glucose availability are discussed from bioenergetic and metabolic control standpoints. The concept of coupling factor facilitation of secretion is critiqued, and an attempt is made to unravel the bioenergetic basis of the oscillatory mechanisms controlling secretion. The need to consider the physiological constraints operating in the intact cell is emphasized throughout. The aim is to provide a coherent pathway through an extensive, complex, and sometimes bewildering literature, particularly for those unfamiliar with the field.

SIRT4 Is a Lysine Deacylase that Controls Leucine Metabolism and Insulin Secretion

Sirtuins are NAD⁺-dependent protein deacylases that regulate several aspects of metabolism and aging. In contrast to the other mammalian sirtuins, the primary enzymatic activity of mitochondrial sirtuin 4 (SIRT4) and its overall role in metabolic control have remained enigmatic. Using a combination of phylogenetics, structural biology, and enzymology, we show that SIRT4 removes three acyl moieties from lysine residues: methylglutaryl (MG)-, hydroxymethylglutaryl (HMG)-, and 3-methylglutaconyl (MGc)-lysine. The metabolites leading to these post-translational modifications are intermediates in leucine oxidation, and we show a primary role for SIRT4 in controlling this pathway in mice. Furthermore, we find that dysregulated leucine metabolism in SIRT4KO mice leads to elevated basal and stimulated insulin secretion, which progressively develops into glucose intolerance and insulin resistance. These findings identify a robust enzymatic activity for SIRT4, uncover a mechanism controlling branched-chain amino acid flux, and position SIRT4 as a crucial player maintaining insulin secretion and glucose homeostasis during aging.

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.

Glutamate dehydrogenase activator BCH stimulating reductive amination prevents high fat/high fructose diet-induced steatohepatitis and hyperglycemia in C57BL/6J mice

Individuals with non-alcoholic fatty liver disease (NAFLD) and type 2 diabetes (T2D) induced by high calorie western diet are characterized by enhanced lipogenesis and gluconeogenesis in the liver. Stimulation of reductive amination may shift tricarboxylic acid cycle metabolism for lipogenesis and gluconeogenesis toward glutamate synthesis with increase of NAD+/NADH ratio and thus, ameliorate high calorie diet-induced fatty liver and hyperglycemia. Stimulation of reductive amination through glutamate dehydrogenase (GDH) activator 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH) reduced both de novo lipogenesis and gluconeogenesis but increased the activities of sirtuins and AMP-activated kinase in primary hepatocytes. Long-term BCH treatment improved most metabolic alterations induced by high fat/high fructose (HF/HFr) diet in C57BL/6J mice. BCH prevented HF/HFr-induced fat accumulation and activation of stress/inflammation signals such as phospho-JNK, phospho-PERK, phospho-p38, and phospho-NFκB in liver tissues. Furthermore, BCH treatment reduced the expression levels of inflammatory cytokines such as TNF-α and IL-1β in HF/HFr-fed mouse liver. BCH also reduced liver collagen and plasma levels of alanine transaminase and aspartate transaminase. On the other hand, BCH significantly improved fasting hyperglycemia and glucose tolerance in HF/HFr-fed mice. In conclusion, stimulation of reductive amination through GDH activation can be used as a strategy to prevent high calorie western diet-induced NAFLD and T2D.

Exercise-induced alterations in pancreatic oxidative stress and mitochondrial function in type 2 diabetic Goto-Kakizaki rats

Progressive metabolic complications accompanied by oxidative stress are the hallmarks of type 2 diabetes. The precise molecular mechanisms of the disease complications, however, remain elusive. Exercise-induced nontherapeutic management of type 2 diabetes is the first line of choice to control hyperglycemia and diabetes associated complications. In this study, using 11-month-old type 2 Goto-Kakizaki (GK) rats, we have investigated the effects of exercise on mitochondrial metabolic and oxidative stress in the pancreas. Our results showed an increase in theNADPHoxidase enzyme activity and reactive oxygen species (ROS) production inGKrats, which was inhibited after exercise. Increased lipid peroxidation and protein carbonylation andSODactivity were also inhibited after exercise. Interestingly, glutathione (GSH) level was markedly high in the pancreas ofGKdiabetic rats even after exercise. However,GSH-peroxidase andGSH-reductase activities were significantly reduced. Exercise also induced energy metabolism as observed by increased hexokinase and glutamate dehydrogenase activities. A significant decrease in the activities of mitochondrial ComplexesII/IIIandIVwere observed in theGKrats. Exercise improved only ComplexIVactivity suggesting increased utilization of oxygen. We also observed increased activities of cytochrome P450s in the pancreas ofGKrats which was reduced significantly after exercise.SDS-PAGEresults have shown a decreased expression ofNF-κB, Glut-2, andPPAR-ϒ inGKrats which was markedly increased after exercise. These results suggest differential oxidative stress and antioxidant defense responses after exercise. Our results also suggest improved mitochondrial function and energy utilization in the pancreas of exercisingGKrats.

Requirement for the Mitochondrial Pyruvate Carrier in Mammalian Development Revealed by a Hypomorphic Allelic Series

Glucose and oxygen are two of the most important molecules transferred from mother to fetus during eutherian pregnancy, and the metabolic fates of these nutrients converge at the transport and metabolism of pyruvate in mitochondria. Pyruvate enters the mitochondrial matrix through the mitochondrial pyruvate carrier (MPC), a complex in the inner mitochondrial membrane that consists of two essential components, MPC1 and MPC2. Here, we define the requirement for mitochondrial pyruvate metabolism during development with a progressive allelic series of Mpc1 deficiency in mouse. Mpc1 deletion was homozygous lethal in midgestation, but Mpc1 hypomorphs and tissue-specific deletion of Mpc1 presented as early perinatal lethality. The allelic series demonstrated that graded suppression of MPC resulted in dose-dependent metabolic and transcriptional changes. Steady-state metabolomics analysis of brain and liver from Mpc1 hypomorphic embryos identified compensatory changes in amino acid and lipid metabolism. Flux assays in Mpc1-deficient embryonic fibroblasts also reflected these changes, including a dramatic increase in mitochondrial alanine utilization. The mitochondrial alanine transaminase GPT2 was found to be necessary and sufficient for increased alanine flux upon MPC inhibition. These data show that impaired mitochondrial pyruvate transport results in biosynthetic deficiencies that can be mitigated in part by alternative anaplerotic substrates in utero.

Exciting Times for Pancreatic Islets: Glutamate Signaling in Endocrine Cells

Glutamate represents a key excitatory neurotransmitter in the central nervous system, and also modulates the function and viability of endocrine cells in pancreatic islets. In insulin-secreting beta cells, glutamate acts as an intracellular messenger, and its transport into secretory granules promotes glucose- and incretin-stimulated insulin secretion. Mitochondrial degradation of glutamate also contributes to insulin release when glutamate dehydrogenase is allosterically activated. It also signals extracellularly via glutamate receptors (AMPA and NMDA receptors) to modulate glucagon, insulin and somatostatin secretion, and islet cell survival. Its degradation products, GABA and γ-hydroxybutyrate, are released and also influence islet cell behavior. Thus, islet glutamate receptors, such as the NMDA receptors, might serve as possible drug targets to develop new medications for adjunct treatment of diabetes.

Beta Cell Function and the Nutritional State: Dietary Factors that Influence Insulin Secretion

Approximately 366 million people worldwide have been diagnosed with type-2 diabetes (T2D). Chronic insulin resistance, decreased functional β-cell mass, and elevated blood glucose are defining characteristics of T2D. Great advances have been made in understanding the pathogenesis of T2D with respect to the effects of dietary macronutrient composition and energy intake on β-cell physiology and glucose homeostasis. It has been further established that obesity is a leading pathogenic factor for developing insulin resistance. However, insulin resistance may not progress to T2D unless β-cells are unable to secret an adequate amount of insulin to compensate for decreased insulin sensitivity. Therefore, pancreatic β-cell dysfunction plays an important role in the development of overt diabetes. This paper reviews recent research findings on the effects of several micronutrients (zinc, vitamin D, iron, vitamin A), leucine, and the phytochemical, genistein on pancreatic β-cell physiology with emphasis on their effects on insulin secretion, specifically in the context of T2D.

Interaction of ingested leucine with glycine on insulin and glucose concentrations

The majority of individual amino acids increase insulin and attenuate the plasma glucose response when ingested with glucose. Objective. To determine whether ingestion of two amino acids simultaneously, with glucose, would result in an additive effect. Leucine (Leu) and glycine (Gly) were chosen because they were two of the most potent glucose-lowering amino acids when given individually. Materials and Methods. Nine subjects received test items on four separate days. The first was a water control, then 25 g glucose, or Leu + Gly (1 mmol/kg fat-free mass each) ±25 g glucose, in random order. Glucose, insulin, and glucagon were measured frequently for 2.5 hours. Net areas were calculated. Results. The glucose area response decreased by 66%. The insulin area response increased by 24% after ingestion of Leu + Gly + glucose compared to ingestion of glucose alone. The decrease in glucose response was not additive; the increase in insulin response was far less than additive when compared to previously published individual amino acid results. The glucagon concentration remained unchanged. Conclusion. There is an interaction between Leu and Gly that results in a markedly attenuated glucose response. This occurred with a very modest increase in insulin response. Changes in glucagon response could not explain the results. The mechanism is unknown.

Glucose-stimulated single pancreatic islets sustain increased cytosolic ATP levels during initial Ca2+ influx and subsequent Ca2+ oscillations

In pancreatic islets, insulin secretion occurs via synchronous elevation of Ca(2+) levels throughout the islets during high glucose conditions. This Ca(2+) elevation has two phases: a quick increase, observed after the glucose stimulus, followed by prolonged oscillations. In these processes, the elevation of intracellular ATP levels generated from glucose is assumed to inhibit ATP-sensitive K(+) channels, leading to the depolarization of membranes, which in turn induces Ca(2+) elevation in the islets. However, little is known about the dynamics of intracellular ATP levels and their correlation with Ca(2+) levels in the islets in response to changing glucose levels. In this study, a genetically encoded fluorescent biosensor for ATP and a fluorescent Ca(2+) dye were employed to simultaneously monitor the dynamics of intracellular ATP and Ca(2+) levels, respectively, inside single isolated islets. We observed rapid increases in cytosolic and mitochondrial ATP levels after stimulation with glucose, as well as with methyl pyruvate or leucine/glutamine. High ATP levels were sustained as long as high glucose levels persisted. Inhibition of ATP production suppressed the initial Ca(2+) increase, suggesting that enhanced energy metabolism triggers the initial phase of Ca(2+) influx. On the other hand, cytosolic ATP levels did not fluctuate significantly with the Ca(2+) level in the subsequent oscillation phases. Importantly, Ca(2+) oscillations stopped immediately before ATP levels decreased significantly. These results might explain how food or glucose intake evokes insulin secretion and how the resulting decrease in plasma glucose levels leads to cessation of secretion.

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.

Ingestion of leucine + phenylalanine with glucose produces an additive effect on serum insulin but less than additive effect on plasma glucose

Most individual amino acids stimulate insulin secretion and attenuate the plasma glucose response when ingested with glucose. We determined whether ingestion of two amino acids simultaneously with glucose would result in an additive effect on the glucose area response compared with ingestion of amino acids individually. Leucine and phenylalanine were chosen because they were two of the most potent glucose-lowering amino acids when given individually. Eight healthy subjects were studied on four separate days. Test meals were given at 0800. The first meal was a water control. Subjects then received 25 g glucose or leucine + phenylalanine (1 mmol/kg fat free body mass each) ±25 g glucose in random order. Glucose, insulin and glucagon were measured frequently for 2.5 hours thereafter. Net areas under the curves were calculated using the mean fasting value as baseline. The insulin response to leucine + phenylalanine was additive. In contrast, the decrease in glucose response to leucine + phenylalanine + glucose was less than additive compared to the individual amino acids ingested with glucose. Interestingly, the insulin response to the combination was largely due to the leucine component, whereas the glucose response was largely due to the phenylalanine component. Glucose was unchanged when leucine or phenylalanine, alone or in combination, was ingested without glucose. This trial is registered with ClinicalTrials.gov NCT01471509.

Metabolic disturbances and defects in insulin secretion in rats with streptozotocin-nicotinamide-induced diabetes

Rats with diabetes induced by streptozotocin (STZ) and nicotinamide (NA) are often used in animal studies concerning various aspects of diabetes. In this experimental model, the severity of diabetes is different depending on doses of STZ and NA. Moreover, diabetic changes in rats with STZ-NA-induced diabetes are not fully characterized. In our present study, metabolic changes and insulin secretion were investigated in rats with diabetes induced by administration of 60 mg of STZ and 90 mg of NA per kg body weight. Four to six weeks after diabetes induction, insulin, glucagon and some metabolic parameters were determined to evaluate the severity of diabetes. Moreover, insulin secretory capacity of pancreatic islets isolated from control and diabetic rats was compared. It was demonstrated that administration of 60 mg of STZ and 90 mg of NA per kg body weight induced relatively mild diabetes, since insulin, glucagon and other analyzed parameters were only slightly affected in diabetic rats compared with control animals. In vitro studies revealed that insulin secretory response was preserved in pancreatic islets of diabetic rats, however, was lower than in islets of control animals. This effect was observed in the presence of different stimuli. Insulin secretion induced by 6.7 and 16.7 mmol/l glucose was moderately reduced in islets of diabetic rats compared with control islets. In the presence of leucine with glutamine, insulin secretion appeared to be also decreased in islets of rats with STZ-NA-induced diabetes. Insulinotropic action of 6.7 mmol/l glucose with forskolin was also deteriorated in diabetic islets. Moreover, it was demonstrated that at a non-stimulatory glucose, pharmacological depolarization of plasma membrane with a concomitant activation of protein kinase C evoked significant rise in insulin release in islets of control and diabetic rats. However, in diabetic islets, this effect was attenuated. These results indicate that impairment in insulin secretion in pancreatic islets of rats with mild diabetes induced by STZ and NA results from both metabolic and nonmetabolic disturbances in these islets.

Sirtuin 3 regulates mouse pancreatic beta cell function and is suppressed in pancreatic islets isolated from human type 2 diabetic patients

AIMS/HYPOTHESIS

Sirtuin (SIRT)3 is a mitochondrial protein deacetylase that regulates reactive oxygen species (ROS) production and exerts anti-inflammatory effects. As chronic inflammation and mitochondrial dysfunction are key factors mediating pancreatic beta cell impairment in type 2 diabetes, we investigated the role of SIRT3 in the maintenance of beta cell function and mass in type 2 diabetes.

METHODS

We analysed changes in SIRT3 expression in experimental models of type 2 diabetes and in human islets isolated from type 2 diabetic patients. We also determined the effects of SIRT3 knockdown on beta cell function and mass in INS1 cells.

RESULTS

SIRT3 expression was markedly decreased in islets isolated from type 2 diabetes patients, as well as in mouse islets or INS1 cells incubated with IL1β and TNFα. SIRT3 knockdown in INS1 cells resulted in lowered insulin secretion, increased beta cell apoptosis and reduced expression of key beta cell genes. SIRT3 knockdown also blocked the protective effects of nicotinamide mononucleotide on pro-inflammatory cytokines in beta cells. The deleterious effects of SIRT3 knockdown were mediated by increased levels of cellular ROS and IL1β.

CONCLUSIONS/INTERPRETATION

Decreased beta cell SIRT3 levels could be a key step in the onset of beta cell dysfunction, occurring via abnormal elevation of ROS levels and amplification of beta cell IL1β synthesis. Strategies to increase the activity or levels of SIRT3 could generate attractive therapies for type 2 diabetes.

Deregulation of glutamate dehydrogenase in human neurologic disorders

Mammalian glutamate dehydrogenase is an allosterically regulated enzyme that is central to glutamate metabolism. It contributes to important cellular processes, including Krebs cycle anaplerotic mechanisms, energy production, and ammonia homeostasis. In addition to this housekeeping hGDH1, humans have acquired through duplication an hGDH2 isoenzyme expressed in neural tissues with distinct regulatory properties. There is increasing evidence that deregulation of human GDHs leads to human disorders. Thus, in hGDH1, regulatory mutations that attenuate GTP inhibition can result in the hyperinsulinism/hyperammonemia syndrome, which is often associated with epileptic seizures, mental retardation, and generalized dystonia. Also, transgenic overexpression of GLUD1 in neurons has resulted in age-dependent degeneration of the CA1 behippocampal region, associated with upregulation of α-synuclein and other proteins linked to major human movement disorders. With regard to hGDH2, a rare T1492G variation in the GLUD2 gene, resulting in substitution of Ala for Ser445 in the regulatory domain of hGDH2, interacts significantly with Parkinson's disease (PD) onset. In two independent Greek and one North American PD cohorts, Ser445Ala hemizygous males, but not heterozygous females, developed PD 6-13 years earlier than subjects with other genotypes. The Ala445-hGDH2 variant displays increased catalytic activity that is amenable to inhibition by estrogens. Enhanced glutamate oxidation by Ala445-hGDH2 is thought to accelerate nigral cell degeneration in hemizygous males, and inhibition of the overactive variant by estrogens may protect heterozygous females. Hence, deregulation of hGDH1 and hGDH2 may play a role in degenerative processes, so these observations identify novel targets for therapeutic intervention in human disorders.

Do pancreatic β cells "taste" nutrients to secrete insulin?

Insulin secretion from pancreatic β cells is controlled by nutrients, hormones, and neurotransmitters. Unlike the latter, which work through classic receptors, glucose and most other nutrients do not interact with membrane receptors but must be metabolized by β cells to induce insulin secretion. Studies have revealed the presence of umami and sweet taste receptors and their downstream effectors in β cells. That the receptors are functional was established by the effects of fructose and artificial sweeteners, which induced signals similar to those produced in taste buds of the tongue. These signals mediated an increase in insulin secretion in the presence of glucose. However, the physiological implications of these pathways in insulin secretion are unclear because of the large differences between plasma concentrations of fructose or sweeteners and their effective concentrations in vitro.

Metabolomic analysis of pancreatic β-cell insulin release in response to glucose

Defining the key metabolic pathways that are important for fuel-regulated insulin secretion is critical to providing a complete picture of how nutrients regulate insulin secretion. We have performed a detailed metabolomics study of the clonal β-cell line 832/13 using a gas chromatography-mass spectrometer (GC-MS) to investigate potential coupling factors that link metabolic pathways to insulin secretion. Mid-polar and polar metabolites, extracted from the 832/13 β-cells, were derivatized and then run on a GC/MS to identify and quantify metabolite concentrations. Three hundred fifty-five out of 527 chromatographic peaks could be identified as metabolites by our metabolomic platform. These identified metabolites allowed us to perform a systematic analysis of key pathways involved in glucose-stimulated insulin secretion (GSIS). Of these metabolites, 41 were consistently identified as biomarker for GSIS by orthogonal partial least-squares (OPLS). Most of the identified metabolites are from common metabolic pathways including glycolytic, sorbitol-aldose reductase pathway, pentose phosphate pathway, and the TCA cycle suggesting these pathways play an important role in GSIS. Lipids and related products were also shown to contribute to the clustering of high glucose sample groups. Amino acids lysine, tyrosine, alanine and serine were upregulated by glucose whereas aspartic acid was downregulated by glucose suggesting these amino acids might play a key role in GSIS. In summary, a coordinated signaling cascade elicited by glucose metabolism in pancreatic β-cells is revealed by our metabolomics platform providing a new conceptual framework for future research and/or drug discovery.