No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Mitochondrial GTP Insensitivity Contributes to Hypoglycemia in Hyperinsulinemia Hyperammonemia by Inhibiting Glucagon Release
Abstract
Mitochondrial GTP (mtGTP)-insensitive mutations in glutamate dehydrogenase (GDH(H454Y)) result in fasting and amino acid-induced hypoglycemia in Hyperinsulinemia Hyperammonemia (HI/HA). Surprisingly, hypoglycemia may occur in this disorder despite appropriately suppressed insulin. To better understand the islet-specific contribution transgenic mice expressing the human activating mutation in beta-cells (H454Y mice) were characterized in vivo. As in the humans with HI/HA, H454Y mice had fasting hypoglycemia but plasma insulin concentrations were similar to the controls. Paradoxically, both glucose- and glutamine-stimulated insulin secretion were severely impaired in H454Y mice. Instead, lack of a glucagon response during hypoglycemic clamps identified impaired counter regulation. Moreover, both insulin and glucagon secretion were impaired in perifused islets. Acute pharmacologic inhibition of GDH restored both insulin and glucagon secretion and normalized glucose tolerance in vivo. These studies support the presence of a mtGTP-dependent signal generated via beta-cell GDH that inhibits alpha-cells. As such, in children with activating GDH mutations of HI/HA this insulin-independent glucagon suppression may contribute importantly to symptomatic hypoglycemia. The identification of a human mutation causing congenital hypoglucagonemic hypoglycemia highlights a central role of the mtGTP-GDH-glucagon axis in glucose homeostasis.
... Exons 6 and 7 encode the GTP allosteric binding domain in GDH, which is the main binding site for GTP. Mutations in these regions can disturb GTP binding and lead to decreased sensitivity of GDH to allosteric regulation by GTP, resulting in weakened inhibitory effect and enhanced GDH activity [12]. ...
... Their results showed that mitochondria guanosine triphosphate (mtGTP) in islet β cells was insensitive, inhibiting the secretion of glucagon by α cells and leading to glucagon deficiency. This study showed for the first time that GDH-induced hypoglycemia was associated with impaired function of α cells [12]. Impaired glucagon secretion is a contributing mechanism. ...
... The blood ammonia levels in children with GDH-HI are not as high as Urea cycle disorder (UCD) patients, and the blood ammonia level is stable. The blood ammonia levels in children with GDH-HI will not increase significantly due to protein intake and are not related to blood glucose levels as well [12]. ...
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.
... In this issue of Diabetes, Kibbey et al. (13) examine the effects of fasting and amino acids on glucose, insulin, and glucagon levels in mice with mitochondrial GTP (mtGTP)-insensitive mutations in GDH H454Y . This study convincingly demonstrates that the H454Y mice had fasting hypoglycemia despite the fact that their plasma insulin concentrations were similar to controls. ...
... In support of the utility of their model, the authors cited work (ref. 24 in [13]) supporting their notion that hypoglycemia could develop without increases in insulin secretion. Indeed, in that study, insulin secretion increased under protein tolerance test conditions. ...
... Indeed, in that study, insulin secretion increased under protein tolerance test conditions. As discussed above, GDH H454Y transgenic islets manifested decreased leucine-and glutamine-stimulated insulin secretion with glucose stimulation; this is discordant with the low insulin response following glutamine stimulation in the current study (13). A second caveat concerns studies supporting the concept that amino acids, especially branched-chain amino acids such as leucine, may enhance the mammalian target of rapamycin (mTOR) signaling pathway. ...
The hyperinsulinemia/hyperammonemia (HI/HA) syndrome—the secondmost common form of congenital hyperinsulinism—is a rare autosomal dominant disease manifested by hypoglycemic symptoms and elevated serum ammonia triggered by fasting or high-protein meals (1). In 1955, Cochrane et al. described a child and her father, both with hypoglycemia that was aggravated by consumption of a low-carbohydrate, high-protein diet (2). Subsequently, another group identified the gene GLUD1. This gene, located on chromosome 10q23.3, is composed of 13 exons and regulates mitochondrial enzyme glutamate dehydrogenase (GDH) (3). The GDH enzyme catalyzes glutamate metabolism and plays important roles in the regulation of amino acid−stimulated insulin secretion in β-cells, modulation of amino acid catabolism in hepatocytes, and ammoniagenesis in the brain (4). A total of 14 amino acid residues affected by GDH-activating mutations has been identified in patients with the HI/HA syndrome (5). GDH activity also is subject to complex regulation by GTP, ADP, and leucine (6). For example, the flux of glutamate into the tricarboxylic acid cycle for energy generation is modulated by the mitochondrial energy potential, which, in turn, is controlled by the ratio of GTP to ADP. When the energy potential is high, amino acid oxidation is not required, and GDH enzyme activity shuts down. When energy potential is low, GDH is activated to sustain energy generation through oxidation of amino acids (4). Interestingly, epigallocatechin gallate, a component of green tea, has been shown to be a potent allosteric inhibitor of GDH enzyme activity (7).
Insulin secretion is upregulated through increased cellular phosphate energy potential, which is manifested as an increase in the ATP/ADP ratio. Elevated ATP/ADP concentrations …
... Dominantly inherited GLUD1 mutations are associated with fasting and leucine (protein) induced postprandial HH, with elevated plasma ammonia (also known as HI/HA syndrome) concentration. Interestingly in a mutant GDH mouse model carrying the H454Y mutation, in addition to the loss of GTP inhibition on GDH activity, there was also inhibition of glucagon secretion (58). This inhibition of glucagon secretion may also contribute to the development symptomatic hypoglycemia in these patients (58). ...
... Interestingly in a mutant GDH mouse model carrying the H454Y mutation, in addition to the loss of GTP inhibition on GDH activity, there was also inhibition of glucagon secretion (58). This inhibition of glucagon secretion may also contribute to the development symptomatic hypoglycemia in these patients (58). ...
Pancreatic β-cells are finely tuned to secrete insulin so that plasma glucose levels are maintained within a narrow physiological range (3.5-5.5 mmol/L). Hyperinsulinaemic hypoglycaemia (HH) is the inappropriate seretion of insulin in the presence of low plasma glucose levels and leads to severe and persistent hypoglycaemia in neonates and children. Mutations in 12 different key genes (ABCC8, KCNJ11, GLUD1, GCK, HADH, SLC16A1, UCP2, HNF4A, HNF1A, HK1, PGM1 and PMM2) that are involved in the regulation of insulin secretion from pancreatic β-cells have been described to be responsible for the underlying molecular mechanisms leading to congenital HH. In HH due to the inhibitory effect of insulin on lipolysis and ketogenesis there is suppressed ketone body formation in the presence of hypoglycaemia thus leading to increased risk of hypoglycaemic brain injury. Therefore, a prompt diagnosis and immediate management of HH is essential to avoid hypoglycaemic brain injury and long-term neurological complications in children. Advances in molecular genetics, imaging techniques (18F-DOPA positron emission tomography/computed tomography scanning), medical therapy and surgical advances (laparoscopic and open pancreatectomy) have changed the management and improved the outcome of patients with HH. This review article provides an overview to the background, clinical presentation, diagnosis, molecular genetics and therapy in children with different forms of HH.
... It is thought that β-cells' mitochondrial GTP insensitivity impairs glucagon release from α-cells and leads to glucagon deficiency. Increased catabolism of glucagon activators or generation of glucagon inhibitors in the absence of mtGTP regulation could be involved, but the relevant second messenger is still unknown [15]. ...
... When hypoglycemic episodes are adequately recognized, they can be satisfactorily prevented by protein restriction and treatment with diazoxide [1,8]. Diazoxide acts as a K ATP channel opener that reduces insulin secretion by hyperpolarization of β-cells and in the same time stimulates glucagon secretion from α-cells, and thus reduces both protein-induced and fasting hypoglycemia [15]. HI/HA syndrome is almost always accompanied by hyperammonemia [4]; only in few patients with a GLUD1 mutation plasma ammonia concentrations were not elevated [6,7]. ...
Hyperinsulinism-hyperammonemia (HI/HA) syndrome is a rare autosomal dominant disease characterized by recurrent hypoglycemia and persistent mild elevation of plasma ammonia. HI/HA syndrome is one of the more common forms of congenital hyperinsulinism (CHI), caused by activating mutations within the GLUD1 gene that encodes the mitochondrial enzyme glutamate dehydrogenase (GDH). We report here on monozygotic twin girls presented with fasting-and protein-induced hypoglycemia and mild persistent hyperammonemia. Genetic analysis revealed that both girls were heterozygous for a novel missense mutation within exon 11 [c.1499A>T, p.(R443W)] of the GLUD1 gene. Despite early treatment with diazoxide and a low protein diet, they both developed non-hypoglycemic seizures in early childhood followed by cognitive impairment. In addition to their clinical course, a review of the literature on HI/HA syndrome is provided.
... Source data 1. Source data for excised patch clamp experiments in Figure 2. cataplerosis of PEP (Kibbey et al., 2014;Stark et al., 2009). We first examined whether restriction of mitochondrial PEP production in PCK2-βKO islets impacts the cytosolic ATP/ADP c ratio. ...
Pyruvate kinase (PK) and the phosphoenolpyruvate (PEP) cycle play key roles in nutrient-stimulated K ATP channel closure and insulin secretion. To identify the PK isoforms involved, we generated mice lacking β-cell PKm1, PKm2, and mitochondrial PEP carboxykinase (PCK2) that generates mitochondrial PEP. Glucose metabolism generates both glycolytic and mitochondrially-derived PEP, which triggers K ATP closure through local PKm1 and PKm2 signaling at the plasma membrane. Amino acids, which generate mitochondrial PEP without producing glycolytic fructose 1,6-bisphosphate to allosterically activate PKm2, signal through PKm1 to raise ATP/ADP, close K ATP channels, and stimulate insulin secretion. Raising cytosolic ATP/ADP with amino acids is insufficient to close K ATP channels in the absence of PK activity or PCK2, indicating that K ATP channels are primarily regulated by PEP that provides ATP via plasma membrane-associated PK, rather than mitochondrially-derived ATP. Following membrane depolarization, the PEP cycle is also involved in an 'off-switch' that facilitates K ATP channel reopening and Ca ²⁺ extrusion, as shown by PK activation experiments and β-cell PCK2 deletion, which prolongs Ca ²⁺ oscillations and increases insulin secretion. In conclusion, the differential response of PKm1 and PKm2 to the glycolytic and mitochondrial sources of PEP influences the β-cell nutrient response, and controls the oscillatory cycle regulating insulin secretion.
... Amino acids (AA) are obligate mitochondrial fuels that simultaneously feed oxidative and anaplerotic pathways. Independent of glycolysis, they generate PEP via glutamate dehydrogenase (GDH)-mediated anaplerosis that is followed by PCK2-mediated cataplerosis via the mitochondrial GTP cycle 8,21 . AA are a tool for separating mechanistic components of the secretion mechanism because at low glucose they raise ATP/ADP but only elicit a single wave of KATP channel closure, Ca 2+ , and insulin release. ...
As nutrient sensors for the organism, pancreatic β-cells use metabolism as a signaling pathway to elicit insulin secretion. Phosphoenolpyruvate (PEP), the substrate for pyruvate kinase (PK), may be a critical signaling intermediate based on its ability to locally control ATP-sensitive K ⁺ (K ATP ) channels on the plasma membrane. Using isoform-specific deletion, we show that constitutively-active PKm1 is sufficient for K ATP closure. Yet, it is the minor but allosterically-tunable PKm2 isoform that enables glucose-dependent regulation of the K ATP channel microcompartment. In contrast to glucose, PKm1 and PKm2 have non-overlapping responses to amino acids, which generate PEP via the mitochondrial PCK2 enzyme. β-cell deletion of PCK2 blocked amino acid regulation of K ATP and impacted Ca ²⁺ influx (via PKm1) and extrusion (via PKm2). Shifting the β-cell from PKm2 to PKm1 correspondingly increased secretory output. Together, these three knockout models indicate that the source of PEP and the isoforms of PK are key determinants of the β-cell nutrient response.
... Pancreata were excised from anesthetized rodents (rats and mice) and islets were isolated by collagenase P digestion followed by centrifugation with Histopaque 1100 solution for density separation of pure islets and then hand-picked as previously described (Kibbey et al., 2014). Islets were allowed to recover for 24-48 hours in RPMI with 10% FBS and antibiotics with either 5mM glucose (rats) or 11mM glucose (mice). ...
The mitochondrial GTP (mtGTP)-dependent phospho enol pyruvate (PEP) cycle is an anaplerotic-cataplerotic mitochondrial shuttle utilizing mitochondrial PEPCK (PCK2) and pyruvate kinase (PK). PEP cycling stimulates insulin secretion via OxPhos-independent lowering of ADP by PK. We assess in vivo whether islet PCK2 is necessary for glucose sensing and if speeding the PEP cycle via pharmacological PK activators amplifies insulin secretion. Pck2 -/- mice had severely impaired insulin secretion during islet perifusion, oral glucose tolerance tests and hyperglycemic clamps. Acute and chronic pharmacologic PK activator therapy improved islet insulin secretion from normal, high-fat diet (HFD) fed, or Zucker diabetic fatty (ZDF) rats, and glucolipotoxic or diabetic humans. A similar improvement in insulin secretion was observed in regular chow and HFD rats in vivo . Insulin secretion and cytosolic Ca ²⁺ during PK activation were dependent on PCK2. These data provide a preclinical rationale for strategies, such as PK activation, that target the PEP cycle to improve glucose homeostasis.
Highlights
Loss of mitochondrial phospho enol pyruvate (PEP) impairs insulin release in vivo .
Pyruvate kinase (PK) activators stimulate beta-cells in preclinical diabetes models.
PEP cycling in vivo depends on PK and mitochondrial PEPCK (PCK2) for insulin release.
Acute and 3-week oral PK activator amplifies insulin release during hyperglycemia.
ETOC Blurb
Abudukadier et al. show that small molecule pyruvate kinase activation in vivo and in vitro increases insulin secretion in rodent and human models of diabetes. The phospho enol pyruvate (PEP) cycling mechanism and its amplification are dependent on mitochondrial PEPCK (PCK2).
... Gain of function mutation results in excessive secretion of insulin by activating GDH via decreasing the sensitivity for GTP. Also, lack of a glucagon response was found in alpha cells in impaired counter-regulation of hyperinsulinism via mitochondrial GTP signal pathway generated by GDH in β cells, contributing to symptomatic hypoglycemia [5]. ...
... Activating "de novo" or dominant missense GLUD1 mutations decrease the enzyme sensitivity to the GTP allosteric inhibition and upregulate its activity in the presence of amino acid leucine, followed by increased insulin secretion (76). Interestingly, in a mutant GDH mouse model carrying the H454Y mutation, in addition to the loss of GTP inhibition on GDH activity, there is also an inhibition of glucagon secretion which contributes to the hypoglycaemic phenotype (79). ...
Congenital hyperinsulinism (CHI) is a heterogenous and complex disorder in which the unregulated insulin secretion from pancreatic beta-cells leads to hyperinsulinaemic hypoglycaemia. The severity of hypoglycaemia varies depending on the underlying molecular mechanism and genetic defects. The genetic and molecular causes of CHI include defects in pivotal pathways regulating the secretion of insulin from the beta-cell. Broadly these genetic defects leading to unregulated insulin secretion can be grouped into four main categories. The first group consists of defects in the pancreatic KATP channel genes (ABCC8 and KCNJ11). The second and third categories of conditions are enzymatic defects (such as GDH, GCK, HADH) and defects in transcription factors (for example HNF1α, HNF4α) leading to changes in nutrient flux into metabolic pathways which converge on insulin secretion. Lastly, a large number of genetic syndromes are now linked to hyperinsulinaemic hypoglycaemia. As the molecular and genetic basis of CHI has expanded over the last few years, this review aims to provide an up-to-date knowledge on the genetic causes of CHI.
... Since the ATP/ADP ratio regulates insulin secretion from β-cells, hGDH1 hyperactivity leads to excess insulin release that is inappropriate to the serum glucose levels [91]. It is also possible that these activating mutations in hGDH1 lead to impaired glucagon release from pancreatic α-cells and possibly increased glucagon-like peptide 1 secretion from L-cells [92,93]. The evidence above renders hGDHs an attractive target for diabetes treatment [94,95]. ...
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.
... Hypoglycemia in the context of appropriately suppressed insulin levels has been seen previously in HI/HA patients and in other forms of congenital HI. 7,12,13 In addition, recent studies in a transgenic mouse model with expression of p.H454Y GDH mutation in beta-cells suggest that inhibition of glucagon release may play a role in the mechanism of hypoglycemia associated with GDH activating mutation. 14 It is interesting that, despite evidence of more severe hypoglycemia, our patient's plasma ammonia concentrations were not higher than in patients with p.S445L heterozygous mutants or other GDH activating mutants in general. Recent work suggests that the elevated ammonia in HI/HA is caused by increased production of ammonia from GDH overactivity in the kidney, rather than impaired hepatic ureagenesis. ...
Activating mutations in the GLUD1 gene, which encodes glutamate dehydrogenase (GDH), result in the hyperinsulinism-hyperammonemia syndrome. GDH is an allosterically regulated enzyme responsible for amino acid-mediated insulin secretion via the oxidative deamination of glutamate to 2-oxoglutarate, leading to ATP production and insulin release. This study characterizes a novel combination of mutations in GLUD1 found in a neonate who presented on the first day of life with severe hypoglycemia, hyperammonemia, and seizures. Mutation analysis revealed a novel frameshift mutation (c.37delC) inherited from the asymptomatic mother that results in a truncated protein and a de novo activating mutation (p.S445L) close to the GTP binding site that has previously been reported. GTP inhibition of GDH enzyme activity in 293T cells expressing the p.S445L or wild-type GDH showed that the half-maximal inhibitory concentration (IC50) for GTP was approximately 800 times higher for p.S445L compared to wild type. GTP inhibition of GDH activity in lymphoblasts from the patient, from a heterozygote for the p.S445L mutation, and in wild-type lymphoblasts showed that the IC50 for GTP of the patient was approximately 200 times that of wild type and 7 times that of heterozygote. However, while the patient had a loss of GTP inhibition of GDH that was more severe than that of heterozygotes, the patient's clinical phenotype is similar to typical heterozygous mutations of GDH. This is the first time we have observed a functionally homozygous activating mutation of GDH in a human.
... GDH activity is highly controlled by allosteric regulation; mitochondrial GTP (mtGTP) inhibits GDH, whereas L-leucine activates GDH (22,23). mtGTP is an important mitochondrial energy sensor in both hepatocytes and pancreatic islet cells, reflecting the balance between tricarboxylic acid cycle and gluconeogenic fluxes implicated in glucose production, insulin secretion, and glucagon secretion (2,(24)(25)(26). In contrast, as an essential amino acid, L-leucine reflects high nutrient protein levels and can activate GDH even when mitochondrial energy is high. ...
Significance
The CDC projects that by the year 2050 one in three adults will be affected by diabetes in the United States alone. A key feature of type 2 diabetes is insulin resistance, which is associated with ectopic lipid deposition in tissues such as the liver. We have discovered a novel regulatory mechanism in the liver linking protein catabolism and ureagenesis to increased lipid oxidation. By increasing urea production, flux through the enzyme argininosuccinate synthetase is enhanced, leading to activation of AMP-activated protein kinase and increased hepatic fat oxidation. These findings may lead to the development of new drugs designed to reduce fat accumulation in the liver and reverse insulin resistance.
... Our study does not address a potential role of SCHAD in b-cells versus other cell types in the islet. In this regard, it is notable that insulin-independent glucagon suppression contributes to hypoglycemia in a mouse model of CHI caused by GDH mutation, suggesting that a signal generated via b-cell mitochondria and GDH specifically inhibits a-cells (18). As SCHAD inhibits GDH, impaired a-cell function may also be relevant for SCHAD deficiency, and of note, the first family described with the disease was regarded as glucagon deficient (19). ...
Congenital hyperinsulinism of infancy (CHI) can be caused by inactivating mutations in the gene encoding short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD), a ubiquitously expressed enzyme involved in fatty acid oxidation. The hypersecretion of insulin may be explained by a loss of interaction between SCHAD and glutamate dehydrogenase in the pancreatic beta-cells. However, in affected individuals there is also a general accumulation of metabolites specific for the enzymatic defect. It remains to be explored whether hypoglycemia in SCHAD-CHI can be uncoupled from the systemic effect on fatty acid oxidation. We therefore transplanted islets from global SCHAD knock-out (SCHADKO) mice into mice with streptozotocin-induced diabetes. Following transplantation, SCHADKO islet recipients exhibited significantly lower random and fasting blood glucose compared with mice transplanted with normal islets or non-diabetic, non-transplanted controls. Furthermore, intraperitoneal glucose tolerance was improved in animals receiving SCHADKO islets compared with those receiving normal islets. Graft beta-cell proliferation and apoptosis rates were similar in the two transplantation groups. We conclude that hypoglycemia in SCHAD-CHI is islet cell-autonomous.
... Glutamate anaplerosis requires a net loss of nitrogen either to ammonia or urea. Consequently, metabolic exchange is the most likely since at 9 mM glucose concentrations aspartate levels are reduced (Supplemental Table S5), transaminases do not generate free ammonia, and GDH is completely inhibited by mitochondrial GTP (Kibbey et al., 2014;Tanizawa et al., 2002). Since there was relatively little dilution in the distal reactions of the TCA cycle (Φ GS , Φ SM and Φ MO ; Figures 2F, 3C and Supplemental Figure S2) then glutamate is fully in exchange with αKG at a rate that is very fast relative to the TCA cycle rate. ...
Mass isotopomer multi-ordinate spectral analysis (MIMOSA) is a step-wise flux analysis platform to measure discrete glycolytic and mitochondrial metabolic rates. Importantly, direct citrate synthesis rates were obtained by deconvolving the mass spectra generated from [U-(13)C6]-D-glucose labeling for position-specific enrichments of mitochondrial acetyl-CoA, oxaloacetate, and citrate. Comprehensive steady-state and dynamic analyses of key metabolic rates (pyruvate dehydrogenase, β-oxidation, pyruvate carboxylase, isocitrate dehydrogenase, and PEP/pyruvate cycling) were calculated from the position-specific transfer of (13)C from sequential precursors to their products. Important limitations of previous techniques were identified. In INS-1 cells, citrate synthase rates correlated with both insulin secretion and oxygen consumption. Pyruvate carboxylase rates were substantially lower than previously reported but showed the highest fold change in response to glucose stimulation. In conclusion, MIMOSA measures key metabolic rates from the precursor/product position-specific transfer of (13)C-label between metabolites and has broad applicability to any glucose-oxidizing cell.
... The results are potentially important as to possible involvement of UCP2, but are controversial [38,39]. Using mice with mitochondrial GTP-insensitive mutations in glutamate dehydrogenase (GDH) in b cells, Kibbey et al. [40] (see also [41]) showed presence of a mitochondrial GTP-dependent signal generated via bcell GDH that suppresses the a cells. Overall, these in-vivo rodent studies suggest a key role for GDH in b-cell mitochondria that controls a-cell secretion. ...
Autoimmune destruction of the β cells is considered the key abnormality in type 1 diabetes mellitus and insulin replacement the primary therapeutic strategy. However, a lack of insulin is accompanied by disturbances in glucagon release, which is excessive postprandially, but insufficient during hypoglycaemia. In addition, replacing insulin alone appears insufficient for adequate glucose control. This review focuses on the growing body of evidence that glucagon abnormalities contribute significantly to the pathophysiology of diabetes and on recent efforts to target the glucagon axis as adjunctive therapy to insulin replacement.
This review discusses recent (since 2013) advances in abnormalities of glucagon regulation and their link to the pathophysiology of diabetes; new mechanisms of glucagon action and regulation; manipulation of glucagon in diabetes treatment; and analytical and systems biology tools to study glucagon regulation.
Recent efforts 'resurrected' glucagon as a key hormone in the pathophysiology of diabetes. New studies target its abnormal regulation and action that is key for improving diabetes treatment. The progress is promising, but major questions remain, including unravelling the mechanism of loss of glucagon counterregulation in type 1 diabetes mellitus and how best to manipulate glucagon to achieve more efficient and safer glycaemic control.
... Mitochondrial GTP drives K ATP channel independent, non-canonical insulin secretion through anapleurotic phosphoenolpyruvate cycling (Stark et al., 2009). In hypoglycemic hypoglucagonemic H454Y GDH transgenic mice, glucagon secretion is restored following pharmacologic GDH inhibition, which suggests that allosteric mtGTP-inhibition of GDH may also have paracrine effects on α-cells (Kibbey et al., 2014). These observations not only implicate both GDH and mtGTP in control of AASIS and hyperinsulinism, but also connect GDH and mtGTP to the maintenance of both α and β-cell function. ...
All forms of diabetes share the common etiology of insufficient pancreatic β-cell function to meet peripheral insulin demand. In pancreatic β-cells, mitochondria serve to integrate the metabolism of exogenous nutrients into energy output, which ultimately leads to insulin release. As such, mitochondrial dysfunction underlies β-cell failure and the development of diabetes. Mitochondrial regulation of β-cell function occurs through many diverse pathways, including metabolic coupling, generation of reactive oxygen species, maintenance of mitochondrial mass, and through interaction with other cellular organelles. In this chapter, we will focus on the importance of enzymatic regulators of mitochondrial fuel metabolism and control of mitochondrial mass to pancreatic β-cell function, describing how defects in these pathways ultimately lead to diabetes. Furthermore, we will examine the factors responsible for mitochondrial biogenesis and degradation and their roles in the balance of mitochondrial mass in β-cells. Clarifying the causes of β-cell mitochondrial dysfunction may inform new approaches to treat the underlying etiologies of diabetes.
Copyright © 2015 Elsevier Ltd. All rights reserved.
In this review, we focus on recent developments in our understanding of nutrient-induced insulin secretion that challenge a key aspect of the “canonical” model, in which an oxidative phosphorylation-driven rise in ATP production closes KATP channels. We discuss the importance of intrinsic β cell metabolic oscillations; the phasic alignment of relevant metabolic cycles, shuttles, and shunts; and how their temporal and compartmental relationships align with the triggering phase or the secretory phase of pulsatile insulin secretion. Metabolic signaling components are assigned regulatory, effectory, and/or homeostatic roles vis-à-vis their contribution to glucose sensing, signal transmission, and resetting the system. Taken together, these functions provide a framework for understanding how allostery, anaplerosis, and oxidative metabolism are integrated into the oscillatory behavior of the secretory pathway. By incorporating these temporal as well as newly discovered spatial aspects of β cell metabolism, we propose a much-refined MitoCat-MitoOx model of the signaling process for the field to evaluate.
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.
The mitochondrial GTP (mtGTP)-dependent phosphoenolpyruvate (PEP) cycle couples mitochondrial PEPCK (PCK2) to pyruvate kinase (PK) in the liver and pancreatic islets to regulate glucose homeostasis. Here, small molecule PK activators accelerated the PEP cycle to improve islet function, as well as metabolic homeostasis, in preclinical rodent models of diabetes. In contrast, treatment with a PK activator did not improve insulin secretion in pck2−/− mice. Unlike other clinical secretagogues, PK activation enhanced insulin secretion but also had higher insulin content and markers of differentiation. In addition to improving insulin secretion, acute PK activation short-circuited gluconeogenesis to reduce endogenous glucose production while accelerating red blood cell glucose turnover. Four-week delivery of a PK activator in vivo remodeled PK phosphorylation, reduced liver fat, and improved hepatic and peripheral insulin sensitivity in HFD-fed rats. These data provide a preclinical rationale for PK activation to accelerate the PEP cycle to improve metabolic homeostasis and insulin sensitivity.
Mechanisms coordinating pancreatic β cell metabolism with insulin secretion are essential for glucose homeostasis. One key mechanism of β cell nutrient sensing uses the mitochondrial GTP (mtGTP) cycle. In this cycle, mtGTP synthesized by succinyl-CoA synthetase (SCS) is hydrolyzed via mitochondrial PEPCK (PEPCK-M) to make phosphoenolpyruvate, a high-energy metabolite that integrates TCA cycling and anaplerosis with glucose-stimulated insulin secretion (GSIS). Several strategies, including xenotopic overexpression of yeast mitochondrial GTP/GDP exchanger (GGC1) and human ATP and GTP-specific SCS isoforms, demonstrated the importance of the mtGTP cycle. These studies confirmed that mtGTP triggers and amplifies normal GSIS and rescues defects in GSIS both in vitro and in vivo. Increased mtGTP synthesis enhanced calcium oscillations during GSIS. mtGTP also augmented mitochondrial mass, increased insulin granule number, and membrane proximity without triggering de-differentiation or metabolic fragility. These data highlight the importance of the mtGTP signal in nutrient sensing, insulin secretion, mitochondrial maintenance, and β cell health. : Jesinkey et al. report that mitochondrial GTP (mtGTP) is an integrative nutrient sentinel regulating β cell function. Signaling from mtGTP raises calcium independent of oxidative phosphorylation to promote insulin secretion. Without overworking the β cell, mtGTP cycling potentiates insulin secretion, nutrient sensing, and mitochondrial expansion alongside promoting health and increasing insulin reserves. Keywords: mitochondrial GTP, PEPCK-M, insulin secretion, oxidative phosphorylation, phosphoenolpyruvate, anaplerosis, succinyl-CoA synthetase, stable isotope, metabolic flux, MIMOSA
Context:
Congenital hyperinsulinism (HI) is the most common cause of hypoglycemia in children. The risk of permanent brain injury in infants with HI continues to be as high as 25-50% due to delays in diagnosis and inadequate treatment. Congenital HI has been described since the birth of the JCEM under various terms, including "idiopathic hypoglycemia of infancy," "leucine-sensitive hypoglycemia," or "nesidioblastosis."
Evidence acquisition:
In the past 20 years, it has become apparent that HI is caused by genetic defects in the pathways that regulate pancreatic β-cell insulin secretion.
Evidence synthesis:
There are now 11 genes associated with monogenic forms of HI (ABCC8, KCNJ11, GLUD1, GCK, HADH1, UCP2, MCT1, HNF4A, HNF1A, HK1, PGM1), as well as several syndromic genetic forms of HI (eg, Beckwith-Wiedemann, Kabuki, and Turner syndromes). HI is also the cause of hypoglycemia in transitional neonatal hypoglycemia and in persistent hypoglycemia in various groups of high-risk neonates (such as birth asphyxia, small for gestational age birthweight, infant of diabetic mother). Management of HI is one of the most difficult problems faced by pediatric endocrinologists and frequently requires difficult choices, such as near-total pancreatectomy and/or highly intensive care with continuous tube feedings. For 50 years, diazoxide, a KATP channel agonist, has been the primary drug for infants with HI; however, it is ineffective in most cases with mutations of ABCC8 or KCNJ11, which constitute the majority of infants with monogenic HI.
Conclusions:
Genetic mutation testing has become standard of care for infants with HI and has proven to be useful not only in projecting prognosis and family counseling, but also in diagnosing infants with surgically curable focal HI lesions. (18)F-fluoro-L-dihydroxyphenylalanine ((18)F-DOPA) PET scans have been found to be highly accurate for localizing such focal lesions preoperatively. New drugs under investigation provide hope for improving the outcomes of children with HI.
Synthesis of phosphoenolpyruvate (PEP) from oxaloacetate (OAA) is an absolute requirement for gluconeogenesis from mitochondrial substrates. Generally, this reaction has solely been attributed to the cytosolic isoform of PEPCK (PEPCK-C) though loss of the mitochondrial isoform (PEPCK-M) has never been assessed. Despite catalyzing the same reaction, to date the only significant role reported in mammals for the mitochondrial isoform is as a glucose sensor necessary for insulin secretion. We hypothesized that this nutrient-sensing mitochondrial GTP-dependent pathway contributes importantly to gluconeogenesis. PEPCK-M was acutely silenced in gluconeogenic tissues of rats using anti-sense oligonucleotides (ASOs) both in vivo and in isolated hepatocytes. Silencing PEPCK-M lowers plasma glucose, insulin, and triglycerides, reduces white adipose, depletes hepatic glycogen, but raises lactate. There is a switch of gluconeogenic substrate preference to glycerol that quantitatively accounts for a third of glucose production. In contrast to the severe mitochondrial deficiency characteristic of PEPCK-C knockout livers, hepatocytes from PEPCK-M deficient livers maintained normal oxidative function. Consistent with its predicted role, gluconeogenesis rates from hepatocytes lacking PEPCK-M are severely reduced for lactate, alanine, and glutamine, but not for pyruvate and glycerol. Thus, PEPCK-M has a direct role in fasted and fed glucose homeostasis and this mitochondrial GTP-dependent pathway should be reconsidered for its involvement in both normal and diabetic metabolism.
Context:
Hypoglycemia due to congenital hyperinsulinism (HI) is caused by mutations in 9 genes.
Objective:
Our objective was to correlate genotype with phenotype in 417 children with HI.
Methods:
Mutation analysis was carried out for the ATP-sensitive potassium (KATP) channel genes (ABCC8 and KCNJ11), GLUD1, and GCK with supplemental screening of rarer genes, HADH, UCP2, HNF4A, HNF1A, and SLC16A1.
Results:
Mutations were identified in 91% (272 of 298) of diazoxide-unresponsive probands (ABCC8, KCNJ11, and GCK), and in 47% (56 of 118) of diazoxide-responsive probands (ABCC8, KCNJ11, GLUD1, HADH, UCP2, HNF4A, and HNF1A). In diazoxide-unresponsive diffuse probands, 89% (109 of 122) carried KATP mutations; 2% (2 of 122) had GCK mutations. In mutation-positive diazoxide-responsive probands, 42% were GLUD1, 41% were dominant KATP mutations, and 16% were in rare genes (HADH, UCP2, HNF4A, and HNF1A). Of the 183 unique KATP mutations, 70% were novel at the time of identification. Focal HI accounted for 53% (149 of 282) of diazoxide-unresponsive probands; monoallelic recessive KATP mutations were detectable in 97% (145 of 149) of these cases (maternal transmission excluded in all cases tested). The presence of a monoallelic recessive KATP mutation predicted focal HI with 97% sensitivity and 90% specificity.
Conclusions:
Genotype to phenotype correlations were most successful in children with GLUD1, GCK, and recessive KATP mutations. Correlations were complicated by the high frequency of novel missense KATP mutations that were uncharacterized, because such defects might be either recessive or dominant and, if dominant, be either responsive or unresponsive to diazoxide. Accurate and timely prediction of phenotype based on genotype is critical to limit exposure to persistent hypoglycemia in infants and children with congenital HI.
Paracrine signaling between pancreatic islet β-cells and α-cells has been proposed to play a role in regulating glucagon responses
to elevated glucose and hypoglycemia. To examine this possibility in human islets, we used a metabolomic approach to trace
the responses of amino acids and other potential neurotransmitters to stimulation with [U-13C]glucose in both normal individuals and type 2 diabetics. Islets from type 2 diabetics uniformly showed decreased glucose
stimulation of insulin secretion and respiratory rate but demonstrated two different patterns of glucagon responses to glucose:
one group responded normally to suppression of glucagon by glucose, but the second group was non-responsive. The non-responsive
group showed evidence of suppressed islet GABA levels and of GABA shunt activity. In further studies with normal human islets,
we found that γ-hydroxybutyrate (GHB), a potent inhibitory neurotransmitter, is generated in β-cells by an extension of the
GABA shunt during glucose stimulation and interacts with α-cell GHB receptors, thus mediating the suppressive effect of glucose
on glucagon release. We also identified glycine, acting via α-cell glycine receptors, as the predominant amino acid stimulator
of glucagon release. The results suggest that glycine and GHB provide a counterbalancing receptor-based mechanism for controlling
α-cell secretory responses to metabolic fuels.
The role of uncoupling protein 2 (UCP2) in pancreatic β-cells is highly debated, partly because of the broad tissue distribution of UCP2 and thus limitations of whole-body UCP2 knockout mouse models. To investigate the function of UCP2 in the β-cell, β-cell-specific UCP2 knockout mice (UCP2BKO) were generated and characterized.
UCP2BKO mice were generated by crossing loxUCP2 mice with mice expressing rat insulin promoter-driven Cre recombinase. Several in vitro and in vivo parameters were measured, including respiration rate, mitochondrial membrane potential, islet ATP content, reactive oxygen species (ROS) levels, glucose-stimulated insulin secretion (GSIS), glucagon secretion, glucose and insulin tolerance, and plasma hormone levels.
UCP2BKO β-cells displayed mildly increased glucose-induced mitochondrial membrane hyperpolarization but unchanged rates of uncoupled respiration and islet ATP content. UCP2BKO islets had elevated intracellular ROS levels that associated with enhanced GSIS. Surprisingly, UCP2BKO mice were glucose-intolerant, showing greater α-cell area, higher islet glucagon content, and aberrant ROS-dependent glucagon secretion under high glucose conditions.
Using a novel β-cell-specific UCP2KO mouse model, we have shed light on UCP2 function in primary β-cells. UCP2 does not behave as a classical metabolic uncoupler in the β-cell, but has a more prominent role in the regulation of intracellular ROS levels that contribute to GSIS amplification. In addition, β-cell UCP2 contributes to the regulation of intraislet ROS signals that mediate changes in α-cell morphology and glucagon secretion.
Glutamate dehydrogenase (GDH) catalyzes the oxidative deamination of L-glutamate and, in animals, is extensively regulated by a number of metabolites. Gain of function mutations in GDH that abrogate GTP inhibition cause the hyperinsulinism/hyperammonemia syndrome (HHS), resulting in increased pancreatic β-cell responsiveness to leucine and susceptibility to hypoglycemia following high protein meals. We have previously shown that two of the polyphenols from green tea (epigallocatechin gallate (EGCG) and epicatechin gallate (ECG)) inhibit GDH in vitro and that EGCG blocks GDH-mediated insulin secretion in wild type rat islets. Using structural and site-directed mutagenesis studies, we demonstrate that ECG binds to the same site as the allosteric regulator, ADP. Perifusion assays using pancreatic islets from transgenic mice expressing a human HHS form of GDH demonstrate that the hyperresponse to glutamine caused by dysregulated GDH is blocked by the addition of EGCG. As observed in HHS patients, these transgenic mice are hypersensitive to amino acid feeding, and this is abrogated by oral administration of EGCG prior to challenge. Finally, the low basal blood glucose level in the HHS mouse model is improved upon chronic administration of EGCG. These results suggest that this common natural product or some derivative thereof may prove useful in controlling this genetic disorder. Of broader clinical implication is that other groups have shown that restriction of glutamine catabolism via these GDH inhibitors can be useful in treating various tumors. This HHS transgenic mouse model offers a highly useful means to test these agents in vivo.
Inhibition of the Na(+)-glucose cotransporter type 2 (SGLT2) is currently being pursued as an insulin-independent treatment for diabetes; however, the behavioral and metabolic consequences of SGLT2 deletion are unknown. Here, we used a SGLT2 knockout mouse to investigate the effect of increased renal glucose excretion on glucose homeostasis, insulin sensitivity, and pancreatic β-cell function.
SGLT2 knockout mice were fed regular chow or a high-fat diet (HFD) for 4 weeks, or backcrossed onto the db/db background. The analysis used metabolic cages, glucose tolerance tests, euglycemic and hyperglycemic clamps, as well as isolated islet and perifusion studies.
SGLT2 deletion resulted in a threefold increase in urine output and a 500-fold increase in glucosuria, as well as compensatory increases in feeding, drinking, and activity. SGLT2 knockout mice were protected from HFD-induced hyperglycemia and glucose intolerance and had reduced plasma insulin concentrations compared with controls. On the db/db background, SGLT2 deletion prevented fasting hyperglycemia, and plasma insulin levels were also dramatically improved. Strikingly, prevention of hyperglycemia by SGLT2 knockout in db/db mice preserved pancreatic β-cell function in vivo, which was associated with a 60% increase in β-cell mass and reduced incidence of β-cell death.
Prevention of renal glucose reabsorption by SGLT2 deletion reduced HFD- and obesity-associated hyperglycemia, improved glucose intolerance, and increased glucose-stimulated insulin secretion in vivo. Taken together, these data support SGLT2 inhibition as a viable insulin-independent treatment of type 2 diabetes.
Pancreatic beta-cells couple the oxidation of glucose to the secretion of insulin. Apart from the canonical K(ATP)-dependent glucose-stimulated insulin secretion (GSIS), there are important K(ATP)-independent mechanisms involving both anaplerosis and mitochondrial GTP (mtGTP). How mtGTP that is trapped within the mitochondrial matrix regulates the cytosolic calcium increases that drive GSIS remains a mystery. Here we have investigated whether the mitochondrial isoform of phosphoenolpyruvate carboxykinase (PEPCK-M) is the GTPase linking hydrolysis of mtGTP made by succinyl-CoA synthetase (SCS-GTP) to an anaplerotic pathway producing phosphoenolpyruvate (PEP). Although cytosolic PEPCK (PEPCK-C) is absent, PEPCK-M message and protein were detected in INS-1 832/13 cells, rat islets, and mouse islets. PEPCK enzymatic activity is half that of primary hepatocytes and is localized exclusively to the mitochondria. Novel (13)C-labeling strategies in INS-1 832/13 cells and islets measured substantial contribution of PEPCK-M to the synthesis of PEP. As high as 30% of PEP in INS-1 832/13 cells and 41% of PEP in rat islets came from PEPCK-M. The contribution of PEPCK-M to overall PEP synthesis more than tripled with glucose stimulation. Silencing the PEPCK-M gene completely inhibited GSIS underscoring its central role in mitochondrial metabolism-mediated insulin secretion. Given that mtGTP synthesized by SCS-GTP is an indicator of TCA flux that is crucial for GSIS, PEPCK-M is a strong candidate to link mtGTP synthesis with insulin release through anaplerotic PEP cycling.
Insulin exocytosis is regulated in pancreatic ß-cells by a cascade of intracellular signals translating glucose levels into
corresponding secretory responses. The mitochondrial enzyme glutamate dehydrogenase (GDH) is regarded as a major player in
this process, although its abrogation has not been tested yet in animal models. Here, we generated transgenic mice, named
ßGlud1–/–, with ß-cell-specific GDH deletion. Our results show that GDH plays an essential role in the full development of the insulin
secretory response. In situ pancreatic perfusion revealed that glucose-stimulated insulin secretion was reduced by 37% in ßGlud1–/–. Furthermore, isolated islets with either constitutive or acute adenovirus-mediated knock-out of GDH showed a 49 and 38%
reduction in glucose-induced insulin release, respectively. Adenovirus-mediated re-expression of GDH in ßGlud1–/– islets fully restored glucose-induced insulin release. Thus, GDH appears to account for about 40% of glucose-stimulated insulin
secretion and to lack redundant mechanisms. In ßGlud1–/– mice, the reduced secretory capacity resulted in lower plasma insulin levels in response to both feeding and glucose load,
while body weight gain was preserved. The results demonstrate that GDH is essential for the full development of the secretory
response in ß-cells. However, maximal secretory capacity is not required for maintenance of glucose homeostasis in normo-caloric
conditions.
Glutamate dehydrogenase (GDH) is found in all living organisms, but only animal GDH is regulated by a large repertoire of metabolites. More than 50 years of research to better understand the mechanism and role of this allosteric network has been frustrated by its sheer complexity. However, recent studies have begun to tease out how and why this complex behavior evolved. Much of GDH regulation probably occurs by controlling a complex ballet of motion necessary for catalytic turnover and has evolved concomitantly with a long antenna-like feature of the structure of the enzyme. Ciliates, the 'missing link' in GDH evolution, might have created the antenna to accommodate changing organelle functions and was refined in humans to, at least in part, link amino acid catabolism with insulin secretion.
Plasma amino acid concentrations were measured and selected amino acid ratios were calculated in 12 normal adults and 12 adults heterozygous for phenylketonuria (PKU) ingesting a hamburger and milk shake meal providing 1 g protein/kg body wt. Plasma concentrations of all amino acids increased significantly over baseline after meal ingestion in both groups, reaching the highest mean values 3-5 h after meal ingestion. Plasma phenylalanine concentrations were significantly higher in heterozygous than in normal subjects both before and at all times after meal ingestion. The absolute increase in plasma phenylalanine concentration over baseline and the area under the plasma phenylalanine concentration-time curve were approximately twice as large in heterozygous as in normal subjects. However, the molar ratio of the plasma phenylalanine concentration to the sum of the plasma concentrations of the other large neutral amino acids did not increase significantly over baseline, but rather decreased.
A new form of congenital hyperinsulinism characterized by hypoglycemia and hyperammonemia was described recently. We hypothesized that this syndrome of hyperinsulinism and hyperammonemia was caused by excessive activity of glutamate dehydrogenase, which oxidizes glutamate to alpha-ketoglutarate and which is a potential regulator of insulin secretion in pancreatic beta cells and of ureagenesis in the liver.
We measured glutamate dehydrogenase activity in lymphoblasts from eight unrelated children with the hyperinsulinism-hyperammonemia syndrome: six with sporadic cases and two with familial cases. We identified mutations in the glutamate dehydrogenase gene by sequencing glutamate dehydrogenase complementary DNA prepared from lymphoblast messenger RNA. Site-directed mutagenesis was used to express the mutations in COS-7 cells.
The sensitivity of glutamate dehydrogenase to inhibition by guanosine 5'-triphosphate was a quarter of the normal level in the patients with sporadic hyperinsulinism-hyperammonemia syndrome and half the normal level in patients with familial cases and their affected relatives, findings consistent with overactivity of the enzyme. These differences in enzyme insensitivity correlated with differences in the severity of hypoglycemia in the two groups. All eight children were heterozygous for the wild-type allele and had a mutation in the proposed allosteric domain of the enzyme. Four different mutations were identified in the six patients with sporadic cases; the two patients with familial cases shared a fifth mutation. In two clones of COS-7 cells transfected with the mutant sequence from one patient, the sensitivity of the enzyme to guanosine 5'-triphosphate was reduced, findings similar to those in the child's lymphoblasts.
The hyperinsulinism-hyperammonemia syndrome is caused by mutations in the glutamate dehydrogenase gene that impair the control of enzyme activity.
Glutamate dehydrogenase (GDH) is allosterically activated by the amino acid leucine to mediate protein stimulation of insulin secretion. Children with the hyperinsulinism/hyperammonemia (HI/HA) syndrome have symptomatic hypoglycemia plus persistent elevations of plasma ammonium. We have reported that HI/HA may be caused by dominant mutations of GDH that lie in a unique allosteric domain that is encoded within GDH exons 11 and 12. To examine the frequency of mutations in this domain, we screened genomic DNA from 48 unrelated cases with the HI/HA syndrome for exon 11 and 12 mutations in GDH. Twenty-five (52%) had mutations in these exons; 74% of the mutations were sporadic. Clinical manifestations included normal birth weight, late onset of hypoglycemia, diazoxide responsiveness, and protein-sensitive hypoglycemia. Enzymatic studies of lymphoblast GDH in seven of the mutations showed that all had reduced sensitivity to inhibition with GTP, consistent with an increase in enzyme activity. Mutations had little or no effect on enzyme responses to positive allosteric effectors, such as ADP or leucine. Based on the three-dimensional structure of GDH, the mutations may function by impairing the binding of an inhibitory GTP to a domain responsible for the allosteric and cooperativity properties of GDH.
The hyperinsulinism/hyperammonemia (HI/HA) syndrome is a form of congenital hyperinsulinism in which affected children have recurrent symptomatic hypoglycemia together with asymptomatic, persistent elevations of plasma ammonium levels. We have shown that the disorder is caused by dominant mutations of the mitochondrial enzyme, glutamate dehydrogenase (GDH), that impair sensitivity to the allosteric inhibitor, GTP. In 65 HI/HA probands screened for GDH mutations, we identified 19 (29%) who had mutations in a new domain, encoded by exons 6 and 7. Six new mutations were found: Ser(217)Cys, Arg(221)Cys, Arg(265)Thr, Tyr(266)Cys, Arg(269)Cys, and Arg(269)HIS: In all five mutations tested, lymphoblast GDH showed reduced sensitivity to allosteric inhibition by GTP (IC(50), 60--250 vs. 20--50 nmol/L in normal subjects), consistent with a gain of enzyme function. Studies of ATP allosteric effects on GDH showed a triphasic response with a decrease in high affinity inhibition of enzyme activity in HI/HA lymphoblasts. All of the residues altered by exons 6 and 7 HI/HA mutations lie in the GTP-binding domain of the enzyme. These data confirm the importance of allosteric regulation of GDH as a control site for amino acid-stimulated insulin secretion and indicate that the GTP-binding site is essential for regulation of GDH activity by both GTP and ATP.
Mutations of glutamate dehydrogenase cause the hyperinsulinism/hyperammonemia syndrome by desensitizing glutamate dehydrogenase to allosteric inhibition by GTP. Normal allosteric activation of glutamate dehydrogenase by leucine is thus uninhibited, leading us to propose that children with hyperinsulinism/hyperammonemia syndrome will have exaggerated acute insulin responses to leucine in the postabsorptive state. As hyperglycemia increases beta-cell GTP, we also postulated that high glucose concentrations would extinguish abnormal responsiveness to leucine in hyperinsulinism/hyperammonemia syndrome patients. After an overnight fast, seven hyperinsulinism/hyperammonemia syndrome patients (aged 9 months to 29 yr) had acute insulin responses to leucine performed using an iv bolus of L-leucine (15 mg/kg) administered over 1 min and plasma insulin measurements obtained at -10, -5, 0, 1, 3, and 5 min. The acute insulin response to leucine was defined as the mean increase in insulin from baseline at 1 and 3 min after an iv leucine bolus. The hyperinsulinism/hyperammonemia syndrome group had excessively increased insulin responses to leucine (mean +/- SEM, 73 +/- 21 microIU/ml) compared with the control children and adults (n = 17) who had no response to leucine (1.9 +/- 2.7 microU/ml; P < 0.05). Four hyperinsulinism/hyperammonemia syndrome patients then had acute insulin responses to leucine repeated at hyperglycemia (blood glucose, 150-180 mg/dl). High blood glucose suppressed their abnormal baseline acute insulin responses to leucine of 180, 98, 47, and 28 microU/ml to 73, 0, 6, and 19 microU/ml, respectively. This suppression suggests that protein-induced hypoglycemia in hyperinsulinism/hyperammonemia syndrome patients may be prevented by carbohydrate loading before protein consumption.
Glutamate dehydrogenase (GDH) is regulated by both positive (leucine and ADP) and negative (GTP and ATP) allosteric factors. We hypothesized that the phosphate potential of beta-cells regulates the sensitivity of leucine stimulation. These predictions were tested by measuring leucine-stimulated insulin secretion in perifused rat islets following glucose depletion and by tracing the nitrogen flux of [2-(15)N]glutamine using stable isotope techniques. The sensitivity of leucine stimulation was enhanced by long time (120-min) energy depletion and inhibited by glucose pretreatment. After limited 50-min glucose depletion, leucine, not alpha-ketoisocaproate, failed to stimulate insulin release. beta-Cells sensitivity to leucine is therefore proposed to be a function of GDH activation. Leucine increased the flux through GDH 3-fold compared with controls while causing insulin release. High glucose inhibited flux through both glutaminase and GDH, and leucine was unable to override this inhibition. These results clearly show that leucine induced the secretion of insulin by augmenting glutaminolysis through activating glutaminase and GDH. Glucose regulates beta-cell sensitivity to leucine by elevating the ratio of ATP and GTP to ADP and P(i) and thereby decreasing the flux through GDH and glutaminase. These mechanisms provide an explanation for hypoglycemia caused by mutations of GDH in children.
Homeostasis of blood glucose is maintained by hormone secretion from the pancreatic islets of Langerhans. Glucose stimulates insulin secretion from beta-cells but suppresses the release of glucagon, a hormone that raises blood glucose, from alpha-cells. The mechanism by which nutrients stimulate insulin secretion has been studied extensively: ATP has been identified as the main messenger and the ATP-sensitive potassium channel as an essential transducer in this process. By contrast, much less is known about the mechanisms by which nutrients modulate glucagon secretion. Here we use conventional pancreas perfusion and a transcriptional targeting strategy to analyse cell-type-specific signal transduction and the relationship between islet alpha- and beta-cells. We find that pyruvate, a glycolytic intermediate and principal substrate of mitochondria, stimulates glucagon secretion. Our analyses indicate that, although alpha-cells, like beta-cells, possess the inherent capacity to respond to nutrients, secretion from alpha-cells is normally suppressed by the simultaneous activation of beta-cells. Zinc released from beta-cells may be implicated in this suppression. Our results define the fundamental mechanisms of differential responses to identical stimuli between cells in a microorgan.
The mechanisms involved in the release of glucagon in response to hypoglycemia are unclear. Proposed mechanisms include the activation of the autonomic nervous system via glucose-sensing neurons in the central nervous system, via the regulation of glucagon secretion by intra-islet insulin and zinc concentrations, or via direct ionic control, all mechanisms that involve high-affinity sulfonylurea receptor/inwardly rectifying potassium channel-type ATP-sensitive K(+) channels. Patients with congenital hyperinsulinism provide a unique physiological model to understand glucagon regulation. In this study, we compare serum glucagon responses to hyperinsulinemic hypoglycemia versus nonhyperinsulinemic hypoglycemia. In the patient group (n = 20), the mean serum glucagon value during hyperinsulinemic hypoglycemia was 17.6 +/- 5.7 ng/l compared with 59.4 +/- 7.8 ng/l in the control group (n = 15) with nonhyperinsulinemic hypoglycemia (P < 0.01). There was no difference between the serum glucagon responses in children with diffuse, focal, and diazoxide-responsive forms of hyperinsulinism. The mean serum epinephrine and norepinephrine concentrations in the hyperinsulinemic group were 2,779 +/- 431 pmol/l and 2.9 +/- 0.7 nmol/l and appropriately rose despite the blunted glucagon response. In conclusion, the loss of ATP-sensitive K(+) channels and or elevated intraislet insulin cannot explain the blunted glucagon release in all patients with congenital hyperinsulinism. Other possible mechanisms such as the suppressive effect of prolonged hyperinsulinemia on alpha-cell secretion should be considered.
The cytoarchitecture of human islets has been examined, focusing on cellular associations that provide the anatomical framework for paracrine interactions. By using confocal microscopy and multiple immunofluorescence, we found that, contrary to descriptions of prototypical islets in textbooks and in the literature, human islets did not show anatomical subdivisions. Insulin-immunoreactive β cells, glucagon-immunoreactive α cells, and somatostatin-containing δ cells were found scattered throughout the human islet. Human β cells were not clustered, and most (71%) showed associations with other endocrine cells, suggesting unique paracrine interactions in human islets. Human islets contained proportionally fewer β cells and more α cells than did mouse islets. In human islets, most β, α, and δ cells were aligned along blood vessels with no particular order or arrangement, indicating that islet microcirculation likely does not determine the order of paracrine interactions. We further investigated whether the unique human islet cytoarchitecture had functional implications. Applying imaging of cytoplasmic free Ca²⁺ concentration, [Ca²⁺]i, we found that β cell oscillatory activity was not coordinated throughout the human islet as it was in mouse islets. Furthermore, human islets responded with an increase in [Ca²⁺]i when lowering the glucose concentration to 1 mM, which can be attributed to the large contribution of α cells to the islet composition. We conclude that the unique cellular arrangement of human islets has functional implications for islet cell function.
• α cell
• β cell
• cytoplasmic free Ca2+ concentration
• insulin
• glucagon
Glutamate dehydrogenase (GDH) plays an important role in insulin secretion as evidenced in children by gain of function mutations of this enzyme that cause a hyperinsulinism-hyperammonemia syndrome (GDH-HI) and sensitize beta-cells to leucine stimulation. GDH transgenic mice were generated to express the human GDH-HI H454Y mutation and human wild-type GDH in islets driven by the rat insulin promoter. H454Y transgene expression was confirmed by increased GDH enzyme activity in islets and decreased sensitivity to GTP inhibition. The H454Y GDH transgenic mice had hypoglycemia with normal growth rates. H454Y GDH transgenic islets were more sensitive to leucine- and glutamine-stimulated insulin secretion but had decreased response to glucose stimulation. The fluxes via GDH and glutaminase were measured by tracing 15N flux from [2-15N]glutamine. The H454Y transgene in islets had higher insulin secretion in response to glutamine alone and had 2-fold greater GDH flux. High glucose inhibited both glutaminase and GDH flux, and leucine could not override this inhibition. 15NH4Cl tracing studies showed 15N was not incorporated into glutamate in either H454Y transgenic or normal islets. In conclusion, we generated a GDH-HI disease mouse model that has a hypoglycemia phenotype and confirmed that the mutation of H454Y is disease causing. Stimulation of insulin release by the H454Y GDH mutation or by leucine activation is associated with increased oxidative deamination of glutamate via GDH. This study suggests that GDH functions predominantly in the direction of glutamate oxidation rather than glutamate synthesis in mouse islets and that this flux is tightly controlled by glucose.
Glucagon, secreted from pancreatic islet alpha cells, stimulates gluconeogenesis and liver glycogen breakdown. The mechanism regulating glucagon release is debated, and variously attributed to neuronal control, paracrine control by neighbouring beta cells, or to an intrinsic glucose sensing by the alpha cells themselves. We examined hormone secretion and Ca(2+) responses of alpha and beta cells within intact rodent and human islets. Glucose-dependent suppression of glucagon release persisted when paracrine GABA or Zn(2+) signalling was blocked, but was reversed by low concentrations (1-20 muM) of the ATP-sensitive K(+) (KATP) channel opener diazoxide, which had no effect on insulin release or beta cell responses. This effect was prevented by the KATP channel blocker tolbutamide (100 muM). Higher diazoxide concentrations (>/=30 muM) decreased glucagon and insulin secretion, and alpha- and beta-cell Ca(2+) responses, in parallel. In the absence of glucose, tolbutamide at low concentrations (<1 muM) stimulated glucagon secretion, whereas high concentrations (>10 muM) were inhibitory. In the presence of a maximally inhibitory concentration of tolbutamide (0.5 mM), glucose had no additional suppressive effect. Downstream of the KATP channel, inhibition of voltage-gated Na(+) (TTX) and N-type Ca(2+) channels (omega-conotoxin), but not L-type Ca(2+) channels (nifedipine), prevented glucagon secretion. Both the N-type Ca(2+) channels and alpha-cell exocytosis were inactivated at depolarised membrane potentials. Rodent and human glucagon secretion is regulated by an alpha-cell KATP channel-dependent mechanism. We propose that elevated glucose reduces electrical activity and exocytosis via depolarisation-induced inactivation of ion channels involved in action potential firing and secretion.
Acetyl–CoA carboxylase 2 (ACC)2 is a key regulator of mitochondrial fat oxidation. To examine the impact of ACC2 deletion on whole-body energy metabolism, we measured changes in substrate oxidation and total energy expenditure in Acc2 −/− and WT control mice fed either regular or high-fat diets. To determine insulin action in vivo, we also measured whole-body insulin-stimulated liver and muscle glucose metabolism during a hyperinsulinemic–euglycemic clamp in Acc2 −/− and WT control mice fed a high-fat diet. Contrary to previous studies that have suggested that increased fat oxidation might result in lower glucose oxidation, both fat and carbohydrate oxidation were simultaneously increased in Acc2 −/− mice. This increase in both fat and carbohydrate oxidation resulted in an increase in total energy expenditure, reductions in fat and lean body mass and prevention from diet-induced obesity. Furthermore, Acc2 −/− mice were protected from fat-induced peripheral and hepatic insulin resistance. These improvements in insulin-stimulated glucose metabolism were associated with reduced diacylglycerol content in muscle and liver, decreased PKCθ activity in muscle and PKCε activity in liver, and increased insulin-stimulated Akt2 activity in these tissues. Taken together with previous work demonstrating that Acc2 −/− mice have a normal lifespan, these data suggest that Acc2 inhibition is a viable therapeutic option for the treatment of obesity and type 2 diabetes.
• diet-induced obesity prevention
• intracellular diacylglycerol
• increased fat oxidation
• insulin resistance prevention
Background:
Plasma glucose levels are tightly regulated within a narrow physiologic range. Insulin-mediated glucose uptake by tissues must be balanced by the appearance of glucose from nutritional sources, glycogen stores, or gluconeogenesis. In this regard, a common pathway regulating both glucose clearance and appearance has not been described. The metabolism of glucose to produce ATP is generally considered to be the primary stimulus for insulin release from beta-cells. Similarly, gluconeogenesis from phosphoenolpyruvate (PEP) is believed to be the primarily pathway via the cytosolic isoform of phosphoenolpyruvate carboxykinase (PEPCK-C). These models cannot adequately explain the regulation of insulin secretion or gluconeogenesis.
Scope of review:
A metabolic sensing pathway involving mitochondrial GTP (mtGTP) and PEP synthesis by the mitochondrial isoform of PEPCK (PEPCK-M) is associated with glucose-stimulated insulin secretion from pancreatic beta-cells. Here we examine whether there is evidence for a similar mtGTP-dependent pathway involved in gluconeogenesis. In both islets and the liver, mtGTP is produced at the substrate level by the enzyme succinyl CoA synthetase (SCS-GTP) with a rate proportional to the TCA cycle. In the beta-cell PEPCK-M then hydrolyzes mtGTP in the production of PEP that, unlike mtGTP, can escape the mitochondria to generate a signal for insulin release. Similarly, PEPCK-M and mtGTP might also provide a significant source of PEP in gluconeogenic tissues for the production of glucose. This review will focus on the possibility that PEPCK-M, as a sensor for TCA cycle flux, is a key mechanism to regulate both insulin secretion and gluconeogenesis suggesting conservation of this biochemical mechanism in regulating multiple aspects of glucose homeostasis. Moreover, we propose that this mechanism may be important for regulating insulin secretion and gluconeogenesis compared to canonical nutrient sensing pathways.
Major conclusions:
PEPCK-M, initially believed to be absent in islets, carries a substantial metabolic flux in beta-cells. This flux is intimately involved with the coupling of glucose-stimulated insulin secretion. PEPCK-M activity may have been similarly underestimated in glucose producing tissues and could potentially be an unappreciated but important source of gluconeogenesis.
General significance:
The generation of PEP via PEPCK-M may occur via a metabolic sensing pathway important for regulating both insulin secretion and gluconeogenesis. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.
This report describes two patients from unrelated families with an unusual syndrome of hyperinsulinism plus hyperammonemia. The diagnosis of hyperinsulinism was based on the demonstration of fasting hypoglycemia with inappropriately elevated insulin levels, inappropriately low β-hydroxybutyrate and free fatty acid levels, and inappropriately large glycemic response to the administration of glucagon. In both patients, plasma ammonium levels were persistently elevated and unaffected by protein feeding, protein restriction, or benzoate therapy. Plasma and urinary amino acids, urinary organic acids, and urinary orotic acid levels were not consistent with any of the urea cycle enzyme defects or other hyperammonemic disorders. These two patients appear to represent a unique form of congenital hyperinsulinism distinct from the previously described autosomal dominant and autosomal recessive variants. We speculate that the underlying defect involves a site that is common to the amino acid regulation of both insulin secretion in pancreatic β-cells and urea synthesis in the liver. (J Pediatr 1997;130:661-4)
An important feature of glucose homeostasis is the effective release of glucagon from the pancreatic alpha cell. The molecular mechanisms regulating glucagon secretion are still poorly understood. We now demonstrate that human alpha cells express ionotropic glutamate receptors (iGluRs) that are essential for glucagon release. A lowering in glucose concentration results in the release of glutamate from the alpha cell. Glutamate then acts on iGluRs of the AMPA/kainate type, resulting in membrane depolarization, opening of voltage-gated Ca(2+) channels, increase in cytoplasmic free Ca(2+) concentration, and enhanced glucagon release. In vivo blockade of iGluRs reduces glucagon secretion and exacerbates insulin-induced hypoglycemia in mice. Hence, the glutamate autocrine feedback loop endows the alpha cell with the ability to effectively potentiate its own secretory activity. This is a prerequisite to guarantee adequate glucagon release despite relatively modest changes in blood glucose concentration under physiological conditions.
Nucleotide-specific isoforms of the tricarboxylic acid (TCA) cycle enzyme succinyl-CoA synthetase (SCS) catalyze substrate-level synthesis of mitochondrial GTP (mtGTP) and ATP (mtATP). While mtATP yield from glucose metabolism is coupled with oxidative phosphorylation and can vary, each molecule of glucose metabolized within pancreatic beta cells produces approximately one mtGTP, making mtGTP a potentially important fuel signal. In INS-1 832/13 cells and cultured rat islets, siRNA suppression of the GTP-producing pathway (DeltaSCS-GTP) reduced glucose-stimulated insulin secretion (GSIS) by 50%, while suppression of the ATP-producing isoform (DeltaSCS-ATP) increased GSIS 2-fold. Insulin secretion correlated with increases in cytosolic calcium, but not with changes in NAD(P)H or the ATP/ADP ratio. These data suggest a role for mtGTP in controlling pancreatic GSIS through modulation of mitochondrial metabolism, possibly involving mitochondrial calcium. Furthermore, in light of its tight coupling to TCA oxidation rates, mtGTP production may serve as an important molecular signal of TCA-cycle activity.
Unlabelled:
l-Leucine was administered as a primed continuous 3-4-h infusion in nonobese and obese subjects in the postabsorptive state and for 12 h in obese subjects after a 3-day and 4-wk fast. In nonobese and obese subjects studied in the post-absorptive state, the leucine infusion resulted in a 150-200% rise in plasma leucine above preinfusion levels, a small decrease in plasma glucose, and unchanged levels of plasma insulin and glucagon and blood ketones. Plasma isoleucine (60-70%) and valine (35-40%) declined to a greater extent than other amino acids (P < 0.001). After 3 days and 4 wk of fasting, equimolar infusions of leucine resulted in two- to threefold greater increments in plasma leucine as compared to post-absorptive subjects, a 30-40% decline in other plasma amino acids, and a 25-30% decrease in negative nitrogen balance. Urinary excretion of 3-methylhistidine was however, unchanged. Plasma glucose which declined in 3-day fasted subjects after leucine administration, surprisingly rose by 20 mg/100 ml after 4 wk of fasting. The rise in blood glucose occurred in the absence of changes in plasma glucagon and insulin and in the face of a 15% decline in endogenous glucose production (as measured by infusion of [3-(3)H]glucose). On the other hand, fractional glucose utilization fell by 30% (P < 0.001), thereby accounting for hyperglycemia. The estimated metabolic clearance rate of leucine fell by 48% after 3 days of fasting whereas the plasma delivery rate of leucine was unchanged, thereby accounting for a 40% rise in plasma leucine during early starvation. After a 4-wk fast, the estimated metabolic clearance rate of leucine declined further to 59% below base line. Plasma leucine nevertheless fell to postabsorptive levels as the plasma delivery rate of leucine decreased 65% below postabsorptive values.
Conclusions:
(a) Infusion of exogenous leucine in prolonged fasting results in a decline in plasma levels of other amino acids, improvement in nitrogen balance and unchanged excretion of 3-methylhistidine, thus suggesting stimulation of muscle protein synthesis, (b) leucine infusion also reduces glucose production and to an even greater extent, glucose consumption, thereby raising blood glucose concentration; and (c) the rise in plasma leucine in early starvation results primarily from a decrease in leucine clearance which drops progressively during starvation.
The effects of increasing concentrations of leucine (0.2, 2.0, and 15.0 mmol/liter) on glucagon secretion from the perfused rat pancreas were examined at various glucose levels (0, 3.3, or 8.3 mmol/liter) and in the absence or presence of either arginine (5.0 mmol/liter) or glutamine (10.0 mmol/liter). At a low glucose concentration (3.3 mmol/liter), leucine caused a dose-related biphasic increase in glucagon output in the absence of arginine, but only a transient increase in the presence of the latter amino acid. These positive responses were markedly reduced and, on occasion, abolished at a high glucose concentration (8.3 mmol/liter). Moreover, at a low glucose concentration (3.3 mmol/liter) and in the presence of arginine, the highest concentration of leucine (15.0 mmol/liter) provoked a sustained and reversible inhibition of glucagon release. Likewise, leucine (15.0 mmol/liter) reversibly inhibited glucagon secretion evoked by glutamine in the absence of glucose. Thus, leucine exerted a dual effect on the secretion of glucagon, the inhibitory effect of leucine prevailing at a high concentration of the branched chain amino acid and when glucagon secretion was already stimulated by arginine or glutamine. At a physiological concentration (0.2 mmol/liter), however, leucine was a positive stimulus for glucagon release, especially in the absence of another amino acid. Concomitantly, leucine was always a positive stimulus for both insulin and somatostatin secretion. The intimate mechanisms involved in the dual effect of leucine on glucagon secretion remain to be elucidated.
Arginine infusion is known to increase the plasma levels of both immunoreactive insulin and glucagon, while it changes the plasma glucose concentration only slightly. In order to clarify the metabolic consequences of glucagon and insulin mobilization induced by arginine, the effects β17w4=(FFA) were investigated of this amino acid on the turnover and plasma concentration of glucose, and on the plasma concentration of free fatty acids (FFA) in seven normal dogs. Infusion of arginine hydrochloride induced a biphasic release of insulin, which in turn increased the overall rate of glucose utilization by 40 per cent. There was, however, only a slight increase in plasma glucose concentration (4%) since the rate of glucose production increased to the same extent as the rate of glucose utilization. It is concluded that the biphasic pattern of insulin release not only reflects the secretory capacity of β cells, but also plays an essential metabolic role in maintaining a balance between the production and utilization of glucose during an arginine challenge. The observed increase in glucose production is attributed to the release of glucagon; this release is essential if normoglycemia is to be maintained. The concentration of FFA in plasma decreased by 50% during infusion of the amino acid. In the dog, arginine infusion rapidly reverses the pattern of fuel utilization characteristic of fasting; the release of free fatty acids from fat depots is inhibited, while the turnover of glucose is enhanced even though normoglycemia is maintained.
A direct inhibitory effect of leucine on hepatic glucose output has been suggested by previous observations in our laboratory. However, it has not been possible to separate the action of leucine from that of insulin. The current studies were undertaken in order to determine the effect of leucine on hepatic glucose formation in vitro, in the absence of insulin.
In liver slices from fasted mice, leucine was shown to inhibit the incorporation of C14 from various C14-labeled precursors into glucose. These results are consistent with the postulate that leucine inhibits the rate of gluconeogenesis, and thus hepatic glucose output, independent of insulin.
Accordingly, the effect of leucine on production of hypoglycemia cannot be regarded as a specific cause of hypoglycemia in certain infants. Assuming that the action of leucine is similar in all individuals, the induction of hypoglycemia by leucine in some individuals must reflect underlying defects in the intracellular regulation of the complex, multi-enzyme pathway of gluconeogenesis. Clarification of the mechanism of action of leucine on gluconeogenesis should contribute to current understanding of the means by which hepatic glucose output is regulated.
A case of a child with recurrent episodes of severe hypoglycemia since the age of 6 months is reported. Biochemical evaluation extended to the first-degree relatives is consistent with a familial form of hypoglycemia due to a leucine-sensitive hyperinsulinism. In addition, this patient has a persistent elevation of serum ammonia levels of uncertain etiology that is more pronounced after meals. Urea cycle defects, organic acidurias, and beta-oxidation defects have been ruled out, as well as a possible excessive deamination of glucogenetic amino acids. This unexpected hyperammonemia, which was also detected in the mother, might be related to leucine hypersensitivity.
Unlabelled:
Two cases of hyperinsulinaemic hypoglycaemia associated with persistent hyperammonaemia in unrelated infants of 7 days and 4 months of age are reported. Blood ammonia levels were 100-300 micromol/l (normal values <40 micromol/l). The hyperammonaemia was asymptomatic and not associated with any of the abnormalities of amino acids or organic acids observed in urea cycle enzyme defects. Orotic aciduria was normal. The hyperammonaemia was not influenced by the levels of blood glucose nor by subtotal pancreatectomy. On admission blood glucose was ca. 1.2 mmol/l (21.6 mg/dl) corresponding to blood insulin levels of 35 and 22 mU/l respectively in both infants. Continuous intravenous glucose perfusion was necessary to prevent hypoglycaemia. Furthermore 2-oxoglutaric acid in urine was increased in the second infant to 3.15 mg/mg creatinine (normal 0.41+/-0.12). This may point to mutations in the glutamate dehydrogenase gene.
Conclusion:
2-Oxoglutaric aciduria may be an important clue to the diagnose in this syndrome.
The combination of persistent hyperammonemia and hypoketotic hypoglycemia in infancy presents a diagnostic challenge. Investigation of the possible causes and regulators of the ammonia and glucose disposal may result in a true diagnosis and predict an optimum treatment.
Since the neonatal period, a white girl had been treated for hyperammonemia and postprandial hypoglycemia with intermittent hyperinsulinism. Her blood level of ammonia varied from 100 to 300 micromol/L and was independent of the protein intake.
Enzymes of the urea cycle as well as glutamine synthetase and glutamate dehydrogenase (GDH) were assayed in liver tissue and/or lymphocytes.
The activity of hepatic GDH was 874 nmol/(min.mg protein) (controls: 472-938). Half-maximum inhibition by guanosine triphosphate was reached at a concentration of 3.9 micromol/L (mean control values:.32). The ratio of plasma glutamine/blood ammonia was unusually low. Oral supplements with N-carbamylglutamate resulted in a moderate decrease of the blood level of ammonia. The hyperinsulinism was successfully treated with diazoxide.
A continuous conversion of glutamate to 2-oxoglutarate causes a depletion of glutamate needed for the synthesis of N-acetylglutamate, the catalyst of the urea synthesis starting with ammonia. In addition, the shortage of glutamate may lead to an insufficient formation of glutamine by glutamine synthetase. As GDH stimulates the release of insulin, the concomitant hyperinsulinism can be explained. This disorder should be considered in every patient with postprandial hypoglycemia and diet-independent hyperammonemia.
Because the hyperinsulinism/hyperammonemia (HI/HA) syndrome is associated with gain of function mutations in the leucine-stimulated insulin secretion pathway, we examined whether protein feeding or fasting was responsible for hypoglycemia in affected patients.
Patients with HI/HA (8 children and 6 adults) were studied. All had dominantly expressed mutations of glutamate dehydrogenase and plasma concentrations of ammonium that were 2 to 5 times normal. The responses to a 24-hour fasting test were determined in 7 patients. Responses to a 1.5 gm/kg oral protein tolerance test in 12 patients were compared with responses of 5 control subjects.
The median age at onset of hypoglycemia in the 14 patients was 9 months; diagnosis was delayed beyond age 2 years in 6 patients, and 4 were not given a diagnosis until adulthood. Fasting tests revealed unequivocal evidence of hyperinsulinism in only 1 of 7 patients. Three did not develop hypoglycemia until 12 to 24 hours of fasting; however, all 7 demonstrated inappropriate glycemic responses to glucagon that were characteristic of hyperinsulinism. In response to oral protein, all 12 patients with HI/HA showed a fall in blood glucose compared with none of 5 control subjects. Insulin responses to protein loading were similar in the patients with HI/HA and control subjects.
The postprandial blood glucose response to a protein meal is more sensitive than prolonged fasting for detecting hypoglycemia in the HI/HA syndrome.
Unlabelled:
Congenital hyperinsulinism is one of the most common causes of recurrent hypoglycaemia in early infancy. It is characterised by dysregulation of insulin secretion. Over the last few years, substantial progress has been made in understanding the molecular mechanisms of normal and pathological insulin secretion. Mutations in different genes (those for the sulphonylurea receptor, inward-rectifying potassium channel, glutamate dehydrogenase and glucokinase) are associated with different modes of inheritance. The clinical heterogeneity of the various forms is explained by different pathogenic mechanisms resulting in inappropriate, partly unregulated secretion of insulin. Early recognition of hypoglycaemia, correct differentiation between histological types (focal or diffuse), and maintenance of adequate glucose levels are of critical importance for the outcome of these patients.
Conclusion:
the recent advances in the knowledge of the basis of congenital hyperinsulinism have resulted in new diagnostic and treatment strategies. Application of these aspects to general clinical practice will lead to an improvement of the management and long-term outcome of affected patients.
Identification of regulatory mutations of glutamate dehydrogenase (GDH) in a form of congenital hyperinsulinism (GDH-HI) is providing a model for basal insulin secretion (IS) and amino acid (AA)-stimulated insulin secretion (AASIS) in which glutaminolysis plays a key role. Leucine and ADP are activators and GTP is an inhibitor of GDH. GDH-HI mutations impair GDH sensitivity to GTP inhibition, leading to fasting hypoglycemia, leucine hypersensitivity, and protein-induced hypoglycemia, indicating the importance of GDH in basal secretion and AASIS. The proposed model for glutaminolysis in IS is based on GDH providing NADH and alpha-ketoglutarate (alpha-KG) to the Krebs cycle, hence increasing the beta-cell ATP-to-ADP ratio to effect insulin release. The process operates with 1) sufficient lowering of beta-cell phosphate potential (i.e., fasting) and when 2) AAs provide leucine for allosteric activation and glutamate from transaminations. To test this hypothesis, IS studies were performed in rat and GDH-HI mouse models. In the rat study, rat islets were isolated, cultured, and then perifused in Krebs-Ringer bicarbonate buffer with 2 mmol/l glutamine using 10 mmol/l 2-aminobicyclo[2,2,1]-heptane-2-carboxylic acid (BCH) or a BCH ramp after 50 or 120 min of glucose deprivation. In the GDH-HI mouse study, the H454Y GDH-HI mutation driven by the rat insulin promoter was created for H454Y beta-cell-specific expression. Cultured, isolated islets were perifused in leucine 0-10 mmol/l with 2 mmol/l glutamine 0-25 mmol/l, AA 0-10 mmol/l, or glucose 0-25 mmol/l. Rat islets displayed enhanced BCH-stimulated IS after 120 min of glucose deprivation, but not when energized by fuel. H454Y and control islets had similar glucose-stimulated IS, but H454Y mice had lower random blood glucose. Leucine-stimulated IS and AASIS occurred at lower thresholds and were greater in H454Y versus control islets. Glutamine stimulated IS in H454Y but not control islets. The clinical manifestations of GDH-HI and related animal studies suggest that GDH regulates basal IS and AASIS. Energy deprivation enhanced GDH-mediated IS, and H454Y mice were hypoglycemic, substantiating roles for GDH and its regulation by the phosphate potential in basal IS. Excessive IS from H454Y islets upon exposure to GDH substrates or stimuli indicate that regulation of GDH by the beta-cell phosphate potential plays a critical role in AASIS. These findings provide a foundation for defining pathways of basal secretion and AASIS, augmenting our understanding of beta-cell function.
The response of the blood glucose to l-leucine was compared with control observations in each of 20 normal subjects. l-Leucine exerted an over-all hypoglycemic effect regardless of whether data were analyzed in terms of 1) the area between control and experimental curves, 2) the percentage difference between control and experimental curves at the time of maximal fall in blood glucose or 3) the percentage difference from base-line values of control and experimental values for all samples obtained between 30 and 100 min following l-leucine ingestion. In addition, a significant correlation was found between l-leucine sensitivity and glucose tolerance in this normal population. The possibility that such a correlation is part of a spectrum extending to both diabetes mellitus and hyperinsulinism is discussed.
Administration of leucine to normal miceproduced a fall in blood glucose concentration, which was significantly augmented in mice pretreated with ultralente insulin. The ability of leucine to produce hypoglycemia following insulin pretreatment was not associated with decreased activity of either hepatic glucose-6-phosphatase or fructose-1, 6-diphosphatase. Leucine did significantly decrease in vivo incorporation of glycine-1-C14, pyruvate-2-C14, and glycerol-2-C14 into glucose, but increased in vivo incorporation of glycerol-2-C14 into hepatic lipid. It is suggested that leucine may produce hypoglycemia by inhibition of the formation of hepatic glucose.
Glutamate dehydrogenase (GDH) is found in all organisms and catalyzes the oxidative deamination of glutamate to 2-oxoglutarate. While this enzyme does not exhibit allosteric regulation in plants, bacteria, or fungi, its activity is tightly controlled by a number of compounds in mammals. We have previously shown that this regulation plays an important role in insulin homeostasis in humans and evolved concomitantly with a 48-residue "antenna" structure. As shown here, the antenna and some of the allosteric regulation first appears in the Ciliates. This primitive regulation is mediated by fatty acids and likely reflects the gradual movement of fatty acid oxidation from the peroxisomes to the mitochondria as the Ciliates evolved away from plants, fungi, and other protists. Mutagenesis studies where the antenna is deleted support this contention by demonstrating that the antenna is essential for fatty acid regulation. When the antenna from the Ciliates is spliced onto human GDH, it was found to fully communicate all aspects of mammalian regulation. Therefore, we propose that glutamate dehydrogenase regulation of insulin secretion is a example of exaptation at the molecular level where the antenna and associated fatty acid regulation was created to accommodate the changes in organelle function in the Ciliates and then later used to link amino acid catabolism and/or regulation of intracellular glutamate/glutamine levels in the pancreatic beta cells with insulin homeostasis in mammals.
Excessive secretion of glucagon is a major contributor to the development of diabetic hyperglycemia. Secretion of glucagon is regulated by various nutrients, with glucose being a primary determinant of the rate of alpha cell glucagon secretion. The intra-islet action of insulin is essential to exert the effect of glucose on the alpha cells since, in the absence of insulin, glucose is not able to suppress glucagon release in vivo. However, the precise mechanism by which insulin suppresses glucagon secretion from alpha cells is unknown. In this study, we show that insulin induces activation of GABAA receptors in the alpha cells by receptor translocation via an Akt kinase-dependent pathway. This leads to membrane hyperpolarization in the alpha cells and, ultimately, suppression of glucagon secretion. We propose that defects in this pathway(s) contribute to diabetic hyperglycemia.
To characterize the clinical features and insulin regulation in infants with hypoglycemia due to prolonged neonatal hyperinsulinism.
Data were collected on 26 infants with hypoglycemia due to neonatal hyperinsulinism that later resolved. Acute insulin response (AIR) tests to calcium, leucine, glucose, and tolbutamide were performed in 11 neonates. Results were compared to children with genetic hyperinsulinism due to mutations of the adenosine triphosphate-dependent potassium (K(ATP)) channel and glutamate dehydrogenase (GDH).
Among the 26 neonates, there were significantly more males, small-for-gestational-age infants, and cesarean deliveries. Only 5 of the 26 had no identifiable risk factor. Hyperinsulinism was diagnosed at a median age of 13 days (range, 2 to 180 days) and resolved by a median age of 181 days (range, 18 to 403 days). Diazoxide was effective in 19 of the 21 neonates treated. In the 11 neonates tested, the AIRs to calcium, leucine, glucose, and tolbutamide resembled those in normal controls and differed from genetic hyperinsulinism due to K(ATP) channel and GDH mutations.
We define a syndrome of prolonged neonatal hyperinsulinism that is responsive to diazoxide, persists for several months, and resolves spontaneously. AIR tests suggest that both the K(ATP) channel and GDH have normal function.