Glucokinase is required for high-starch diet-induced ß-cell mass expansion in mice.

AIMS/INTRODUCTION: We aimed to determine whether glucokinase is required for ß-cell mass expansion induced by high-starch diet (HSTD)-feeding, as has been shown in its high-fat diet-induced expansion. MATERIALS AND METHODS: Eight-week-old male wild-type (Gck+/+ ) or glucokinase haploinsufficient (Gck+/- ) mice were fed either a normal chow (NC) or an HSTD for 15 weeks. The bodyweight, glucose tolerance, insulin sensitivity, insulin secretion and ß-cell mass were assessed. RESULTS: Both HSTD-fed Gck+/+ and Gck+/- mice had significantly higher bodyweight than NC-fed mice. Insulin and oral glucose tolerance tests revealed that HSTD feeding did not affect insulin sensitivity nor glucose tolerance in either the Gck+/+ or Gck+/- mice. However, during the oral glucose tolerance test, the 15-min plasma insulin concentration after glucose loading was significantly higher in the HSTD group than that in the NC group for Gck+/+ , but not for Gck+/- mice. ß-Cell mass was significantly larger in HSTD-fed Gck+/+ mice than that in NC-fed Gck+/+ mice. In contrast, the ß-cell mass of the HSTD-fed Gck+/- mice was not different from that of the NC-fed Gck+/- mice. CONCLUSIONS: The results showed that HSTD feeding would increase pancreatic ß-cell mass and insulin secretion in Gck+/+ , but not Gck+/- mice. This observation implies that glucokinase in ß-cells would be required for the increase in ß-cell mass induced by HSTD feeding.

Molecular reductions in glucokinase activity increase counter-regulatory responses to hypoglycemia in mice and humans with diabetes.

OBJECTIVE: Appropriate glucose levels are essential for survival; thus, the detection and correction of low blood glucose is of paramount importance. Hypoglycemia prompts an integrated response involving reduction in insulin release and secretion of key counter-regulatory hormones glucagon and epinephrine that together promote endogenous glucose production to restore normoglycemia. However, specifically how this response is orchestrated remains to be fully clarified. The low affinity hexokinase glucokinase is found in glucose-sensing cells involved in glucose homeostasis including pancreatic ß-cells and in certain brain areas. Here, we aimed to examine the role of glucokinase in triggering counter-regulatory hormonal responses to hypoglycemia, hypothesizing that reduced glucokinase activity would lead to increased and/or earlier triggering of responses. METHODS: Hyperinsulinemic glucose clamps were performed to examine counter-regulatory responses to controlled hypoglycemic challenges created in humans with monogenic diabetes resulting from heterozygous glucokinase mutations (GCK-MODY). To examine the relative importance of glucokinase in different sensing areas, we then examined responses to clamped hypoglycemia in mice with molecularly defined disruption of whole body and/or brain glucokinase. RESULTS: GCK-MODY patients displayed increased and earlier glucagon responses during hypoglycemia compared with a group of glycemia-matched patients with type 2 diabetes. Consistent with this, glucagon responses to hypoglycemia were also increased in I366F mice with mutated glucokinase and in streptozotocin-treated ß-cell ablated diabetic I366F mice. Glucagon responses were normal in conditional brain glucokinase-knockout mice, suggesting that glucagon release during hypoglycemia is controlled by glucokinase-mediated glucose sensing outside the brain but not in ß-cells. For epinephrine, we found increased responses in GCK-MODY patients, in ß-cell ablated diabetic I366F mice and in conditional (nestin lineage) brain glucokinase-knockout mice, supporting a role for brain glucokinase in triggering epinephrine release. CONCLUSIONS: Our data suggest that glucokinase in brain and other non ß-cell peripheral hypoglycemia sensors is important in glucose homeostasis, allowing the body to detect and respond to a falling blood glucose.

Nutrient sensing in pancreatic islets: lessons from congenital hyperinsulinism and monogenic diabetes.

Pancreatic beta cells sense changes in nutrients during the cycles of fasting and feeding and release insulin accordingly to maintain glucose homeostasis. Abnormal beta cell nutrient sensing resulting from gene mutations leads to hypoglycemia or diabetes. Glucokinase (GCK) plays a key role in beta cell glucose sensing. As one form of congenital hyperinsulinism (CHI), activating mutations of GCK result in a decreased threshold for glucose-stimulated insulin secretion and hypoglycemia. In contrast, inactivating mutations of GCK result in diabetes, including a mild form (MODY2) and a severe form (permanent neonatal diabetes mellitus (PNDM)). Mutations of beta cell ion channels involved in insulin secretion regulation also alter glucose sensing. Activating or inactivating mutations of ATP-dependent potassium (KATP ) channel genes result in severe but completely opposite clinical phenotypes, including PNDM and CHI. Mutations of the other ion channels, including voltage-gated potassium channels (Kv 7.1) and voltage-gated calcium channels, also lead to abnormal glucose sensing and CHI. Furthermore, amino acids can stimulate insulin secretion in a glucose-independent manner in some forms of CHI, including activating mutations of the glutamate dehydrogenase gene, HDAH deficiency, and inactivating mutations of KATP channel genes. These genetic defects have provided insight into a better understanding of the complicated nature of beta cell fuel-sensing mechanisms.

Regulatory T Cell Migration Is Dependent on Glucokinase-Mediated Glycolysis.

Migration of activated regulatory T (Treg) cells to inflamed tissue is crucial for their immune-modulatory function. While metabolic reprogramming during Treg cell differentiation has been extensively studied, the bioenergetics of Treg cell trafficking remains undefined. We have investigated the metabolic demands of migrating Treg cells in vitro and in vivo. We show that glycolysis was instrumental for their migration and was initiated by pro-migratory stimuli via a PI3K-mTORC2-mediated pathway culminating in induction of the enzyme glucokinase (GCK). Subsequently, GCK promoted cytoskeletal rearrangements by associating with actin. Treg cells lacking this pathway were functionally suppressive but failed to migrate to skin allografts and inhibit rejection. Similarly, human carriers of a loss-of-function GCK regulatory protein gene-leading to increased GCK activity-had reduced numbers of circulating Treg cells. These cells displayed enhanced migratory activity but similar suppressive function, while conventional T cells were unaffected. Thus, GCK-dependent glycolysis regulates Treg cell migration.

[Current status and issues of clinical development of novel anti-diabetic drugs].

Clinical development of new antidiabetic drugs such as SGLT2 inhibitor, GPR40 agonist, GPR119 agonist, and GKA has been progressing world wide. Action mechanism of each drug is unique and clearly distinguished from the existing drugs. The effect of SGLT2 inhibitors is independent of insulin action and characterized by inhibition of glucose reabsorption in the kidney accompanying with significant body weight reduction. It is known that GPR40 agonist stimulates insulin secretion in glucose independent manner. GKA potentiates glucose-stimulated insulin secretion from pancreatic beta cells and stimulates glucose uptake into the liver. Selective SGLT2 inhibitors and GPR40 agonist are expected to enter into the market in near future.

Interaction between the central and peripheral effects of insulin in controlling hepatic glucose metabolism in the conscious dog.

The importance of hypothalamic insulin action to the regulation of hepatic glucose metabolism in the presence of a normal liver/brain insulin ratio (3:1) is unknown. Thus, we assessed the role of central insulin action in the response of the liver to normal physiologic hyperinsulinemia over 4 h. Using a pancreatic clamp, hepatic portal vein insulin delivery was increased three- or eightfold in the conscious dog. Insulin action was studied in the presence or absence of intracerebroventricularly mediated blockade of hypothalamic insulin action. Euglycemia was maintained, and glucagon was clamped at basal. Both the molecular and metabolic aspects of insulin action were assessed. Blockade of hypothalamic insulin signaling did not alter the insulin-mediated suppression of hepatic gluconeogenic gene transcription but blunted the induction of glucokinase gene transcription and completely blocked the inhibition of glycogen synthase kinase-3ß gene transcription. Thus, central and peripheral insulin action combined to control some, but not other, hepatic enzyme programs. Nevertheless, inhibition of hypothalamic insulin action did not alter the effects of the hormone on hepatic glucose flux (production or uptake). These data indicate that brain insulin action is not a determinant of the rapid (<4 h) inhibition of hepatic glucose metabolism caused by normal physiologic hyperinsulinemia in this large animal model.

MODY2 caused by a novel mutation of GCK gene.

Maturity-onset diabetes of the young type 2 (MODY2) is an autosomal dominant inherited disease caused by heterozygous inactivating mutations in the glucokinase (GCK) gene and is characterized by mild noninsulin-dependent fasting hyperglycemia. It is treated with diet only, and complications are extremely rare. We present a report of a family with MODY2 caused by a novel NM_000162.3:c.878T>C mutation in exon 8 of the GCK gene. Testing for MODY2 and reporting all novel mutations are important to avoid difficulties in the interpretation of genetic test results and to provide fast and definitive diagnosis for all patients with this disease.

Alberto Sols, teacher and mentor of spanish biochemists (1917-1989).

Biochemistry in Spain owes much to the figure of Alberto Sols. In words of Nobel Prize winner Severo Ochoa: "He has been the first scientist to establish successfully biochemistry in Spain." His intellectual rigour, care in experimental design, emphasis on quality, and attention to the presentation of results permeated far beyond his inner circle to the then fledging Spanish biochemical community. It would be difficult to find some Spanish biochemist of the generation that now starts to retire who has not been influenced in a way or another by the work of Sols. However, it is also likely that the new generations of biochemists and molecular biologists in the country ignore who was Sols and what their field owns to him. The following lines try to highlight some key points of his scientific biography, the circumstances in which they took place and the state of the corresponding research area at that moment.

Functional characterization of MODY2 mutations highlights the importance of the fine-tuning of glucokinase and its role in glucose sensing.

Glucokinase (GK) acts as a glucose sensor in the pancreatic beta-cell and regulates insulin secretion. Heterozygous mutations in the human GK-encoding GCK gene that reduce the activity index increase the glucose-stimulated insulin secretion threshold and cause familial, mild fasting hyperglycaemia, also known as Maturity Onset Diabetes of the Young type 2 (MODY2). Here we describe the biochemical characterization of five missense GK mutations: p.Ile130Thr, p.Asp205His, p.Gly223Ser, p.His416Arg and p.Ala449Thr. The enzymatic analysis of the corresponding bacterially expressed GST-GK mutant proteins show that all of them impair the kinetic characteristics of the enzyme. In keeping with their position within the protein, mutations p.Ile130Thr, p.Asp205His, p.Gly223Ser, and p.His416Arg strongly decrease the activity index of GK, affecting to one or more kinetic parameters. In contrast, the p.Ala449Thr mutation, which is located in the allosteric activator site, does not affect significantly the activity index of GK, but dramatically modifies the main kinetic parameters responsible for the function of this enzyme as a glucose sensor. The reduced Kcat of the mutant (3.21±0.28 s(-1) vs 47.86±2.78 s(-1)) is balanced by an increased glucose affinity (S(0.5) = 1.33±0.08 mM vs 7.86±0.09 mM) and loss of cooperativity for this substrate. We further studied the mechanism by which this mutation impaired GK kinetics by measuring the differential effects of several competitive inhibitors and one allosteric activator on the mutant protein. Our results suggest that this mutation alters the equilibrium between the conformational states of glucokinase and highlights the importance of the fine-tuning of GK and its role in glucose sensing.

Brain glucose sensors play a significant role in the regulation of pancreatic glucose-stimulated insulin secretion.

As patients decline from health to type 2 diabetes, glucose-stimulated insulin secretion (GSIS) typically becomes impaired. Although GSIS is driven predominantly by direct sensing of a rise in blood glucose by pancreatic ß-cells, there is growing evidence that hypothalamic neurons control other aspects of peripheral glucose metabolism. Here we investigated the role of the brain in the modulation of GSIS. To examine the effects of increasing or decreasing hypothalamic glucose sensing on glucose tolerance and insulin secretion, glucose or inhibitors of glucokinase, respectively, were infused into the third ventricle during intravenous glucose tolerance tests (IVGTTs). Glucose-infused rats displayed improved glucose handling, particularly within the first few minutes of the IVGTT, with a significantly lower area under the excursion curve within the first 10 min (AUC0-10). This was explained by increased insulin secretion. In contrast, infusion of the glucokinase inhibitors glucosamine or mannoheptulose worsened glucose tolerance and decreased GSIS in the first few minutes of IVGTT. Our data suggest a role for brain glucose sensors in the regulation of GSIS, particularly during the early phase. We propose that pharmacological agents targeting hypothalamic glucose-sensing pathways may represent novel therapeutic strategies for enhancing early phase insulin secretion in type 2 diabetes.

UCP2 mRNA expression is dependent on glucose metabolism in pancreatic islets.

Uncoupling Protein 2 (UCP2) is expressed in the pancreatic ß-cell, where it partially uncouples the mitochondrial proton gradient, decreasing both ATP-production and glucose-stimulated insulin secretion (GSIS). Increased glucose levels up-regulate UCP2 mRNA and protein levels, but the mechanism for UCP2 up-regulation in response to increased glucose is unknown. The aim was to examine the effects of glucokinase (GK) deficiency on UCP2 mRNA levels and to characterize the interaction between UCP2 and GK with regard to glucose-stimulated insulin secretion in pancreatic islets. UCP2 mRNA expression was reduced in GK+/- islets and GK heterozygosity prevented glucose-induced up-regulation of islet UCP2 mRNA. In contrast to UCP2 protein function UCP2 mRNA regulation was not dependent on superoxide generation, but rather on products of glucose metabolism, because MnTBAP, a superoxide dismutase mimetic, did not prevent the glucose-induced up-regulation of UCP2. Glucose-stimulated insulin secretion was increased in UCP2-/- and GK+/- islets compared with GK+/- islets and UCP2 deficiency improved glucose tolerance of GK+/- mice. Accordingly, UCP2 deficiency increased ATP-levels of GK+/- mice. Thus, the compensatory down-regulation of UCP2 is involved in preserving the insulin secretory capacity of GK mutant mice and might also be implicated in limiting disease progression in MODY2 patients.

[Contribution of metabolic sensors on feeding behaviour and the control of body weight]. / Contribución de los sensores metabólicos sobre la conducta alimentaria y el control del peso corporal.

Metabolic sensors play an important role in the control of food intake, utilization of nutrients and demonstration of feeding behaviour. In this work we describe the study done in our laboratory on glucokinase (GK) as brain glucose sensor, the AMP kinase (AMPK) as detector of the fall of intracellular energy charge and as the S6K in the signaling pathway of mTOR with opposite effects to AMPK. Glucose sensors are molecular designs that detect with accuracy glucose concentrations, facilitating therefore the homeostasis of this hexose. We consider GK as a component of a glucose sensor system that might modulates the feeding behaviour and indirectly the control of body weight. Our findings indicate that GK and GLUT-2 mRNAs and proteins are coexpressed mainly in areas of the hypothalamus implied in the control of food intake. We have also found a high glucose phosphorylating activity with kinetic properties similar to that reported in the liver, with a high apparent Km for glucose that displays no product inhibition by glucose-6-phosphate. GK may be also regulated by the presence of glucokinase regulatory protein (GKRP), which has been identified in the same brain areas than GK. The coexpression of these molecules might play a role as glucose sensors in which GLUT-2 has a permissive role and the interactions of GK with GKRP made possible a real sensor activity. Furthermore, the effects of anorexigenic peptides in this system should facilitate the transduction of signals required to produce a state of satiety. Thus, GLP-1 reduced significantly the glucose metabolism in areas of the hypothalamus and brainstem related with food intake, which open new ways to the study of pathophysiologicals aspects of feeding behaviour. Besides we have studied the functions of AMPK and mTOR pathway in the hypothalamic areas ventromedial (VMH) and lateral (LH) under situations with alterations of the nutritional status and energy balance. Our results revealed that the activation of AMPK and S6K in VMH y LH occur in response to the changes of glucose concentrations or in the changes in the nutritional state, as well as GLP-1/exendin-4 act by counteracting the activation/inactivation of these kinases, which support a modulating role of these peptides on the kinases. On the other hand, GLP-1/exendin-4 might contribute to the normalization of the altered values of these kinases in pathophysiological states such as obesity.

The anorectic effect of GLP-1 in rats is nutrient dependent.

GLP-1-induced insulin secretion from the ß-cell is dependent upon glucose availability. The purpose of the current study was to determine whether CNS GLP-1 signaling is also glucose-dependent. We found that fasting blunted the ability of 3(rd) cerebroventricularly (i3vt)-administered GLP-1 to reduce food intake. However, fasted animals maintained the anorexic response to melanotan II, a melanocortin receptor agonist, indicating a specific effect of fasting on GLP-1 action. We also found that i3vt administration of leptin, which is also decreased with fasting, was not able to potentiate GLP-1 action in fasted animals. However, we did find that CNS glucose sensing is important in GLP-1 action. Specifically, we found that i3vt injection of 2DG, a drug that blocks cellular glucose utilization, and AICAR which activates AMPK, both blocked GLP-1-induced reductions in food intake. To examine the role of glucokinase, an important CNS glucose sensor, we studied glucokinase-heterozygous knockout mice, but found that they responded normally to peripherally administered GLP-1 and exendin-4. Interestingly, oral, but not i3vt or IP glucose potentiated GLP-1's anorectic action. Thus, CNS and peripheral fuel sensing are both important in GLP-1-induced reductions in food intake.

Management of type 2 diabetes: new and future developments in treatment.

The increasing prevalence, variable pathogenesis, progressive natural history, and complications of type 2 diabetes emphasise the urgent need for new treatment strategies. Longacting (eg, once weekly) agonists of the glucagon-like-peptide-1 receptor are advanced in development, and they improve prandial insulin secretion, reduce excess glucagon production, and promote satiety. Trials of inhibitors of dipeptidyl peptidase 4, which enhance the effect of endogenous incretin hormones, are also nearing completion. Novel approaches to glycaemic regulation include use of inhibitors of the sodium-glucose cotransporter 2, which increase renal glucose elimination, and inhibitors of 11ß-hydroxysteroid dehydrogenase 1, which reduce the glucocorticoid effects in liver and fat. Insulin-releasing glucokinase activators and pancreatic-G-protein-coupled fatty-acid-receptor agonists, glucagon-receptor antagonists, and metabolic inhibitors of hepatic glucose output are being assessed. Early proof of principle has been shown for compounds that enhance and partly mimic insulin action and replicate some effects of bariatric surgery.

Hypothalamic glucose sensing and glycaemic disease.

An adequate supply of nutrients is obligatory for life. Glucose is one of the main circulating substrates fuelling the mammalian body, particularly brain, and is normally maintained within a narrow range to ensure health. Given the challenge of maintaining glucose homeostasis, mammals have evolved specialized sensors for monitoring changes in glucose availability. Glucose sensors are distributed centrally and peripherally. Hypothalamic glucose sensors are arousing burgeoning research interest particularly as a result of increasing evidence that the hypothalamus plays an important role in the control of glucose homeostasis. During the last decade, knowledge of hypothalamic glucose sensors has greatly improved, especially as a result of transgenic mice technology and other advances in molecular genetic approaches. This review provides a broad overview of the relevance of hypothalamic glucose sensors in the physiological regulation of glucose homeostasis and putative pathophysiologic relevance to glycaemic diseases. The primary focus of this report has been to discuss recent data suggesting novel roles for hypothalamic glucose sensors in the control of hepatic glucose production, insulin secretion and hypoglycemia counterregulation. Finally, an improved understanding of hypothalamic glucose sensing pathways may be pertinent for an integral comprehension of the regulation of glucose homeostasis and associated disorders.

Mutations in pancreatic ß-cell Glucokinase as a cause of hyperinsulinaemic hypoglycaemia and neonatal diabetes mellitus.

Glucokinase is a key enzyme involved in regulating insulin secretion from the pancreatic ß-cell. The unique role of glucokinase in human glucose physiology is illustrated by the fact that genetic mutations in glucokinase can either cause hyperglycaemia or hypoglycaemia. Heterozygous inactivating mutations in glucokinase cause maturity-onset diabetes of the young (MODY), homozygous inactivating in glucokinase mutations result in permanent neonatal diabetes whereas heterozygous activating glucokinase mutations cause hyperinsulinaemic hypoglycaemia.

Glucose metabolism in Legionella pneumophila: dependence on the Entner-Doudoroff pathway and connection with intracellular bacterial growth.

Glucose metabolism in Legionella pneumophila was studied by focusing on the Entner-Doudoroff (ED) pathway with a combined genetic and biochemical approach. The bacterium utilized exogenous glucose for synthesis of acid-insoluble cell components but manifested no discernible increase in the growth rate. Assays with permeabilized cell preparations revealed the activities of three enzymes involved in the pathway, i.e., glucokinase, phosphogluconate dehydratase, and 2-dehydro-3-deoxy-phosphogluconate aldolase, presumed to be encoded by the glk, edd, and eda genes, respectively. Gene-disrupted mutants for the three genes and the ywtG gene encoding a putative sugar transporter were devoid of the ability to metabolize exogenous glucose, indicating that the pathway is almost exclusively responsible for glucose metabolism and that the ywtG gene product is the glucose transporter. It was also established that these four genes formed part of an operon in which the gene order was edd-glk-eda-ywtG, as predicted by genomic information. Intriguingly, while the mutants exhibited no appreciable change in growth characteristics in vitro, they were defective in multiplication within eukaryotic cells, strongly indicating that the ED pathway must be functional for the intracellular growth of the bacterium to occur. Curiously, while the deficient glucose metabolism of the ywtG mutant was successfully complemented by the ywtG(+) gene supplied in trans via plasmid, its defect in intracellular growth was not. However, the latter defect was also manifested in wild-type cells when a plasmid carrying the mutant ywtG gene was introduced. This phenomenon, resembling so-called dominant negativity, awaits further investigation.

Glucokinase and molecular aspects of liver glycogen metabolism.

Conversion of glucose into glycogen is a major pathway that contributes to the removal of glucose from the portal vein by the liver in the postprandial state. It is regulated in part by the increase in blood-glucose concentration in the portal vein, which activates glucokinase, the first enzyme in the pathway, causing an increase in the concentration of glucose 6-P (glucose 6-phosphate), which modulates the phosphorylation state of downstream enzymes by acting synergistically with other allosteric effectors. Glucokinase is regulated by a hierarchy of transcriptional and post-transcriptional mechanisms that are only partially understood. In the fasted state, glucokinase is in part sequestered in the nucleus in an inactive state, complexed to a specific regulatory protein, GKRP (glucokinase regulatory protein). This reserve pool is rapidly mobilized to the cytoplasm in the postprandial state in response to an elevated concentration of glucose. The translocation of glucokinase between the nucleus and cytoplasm is modulated by various metabolic and hormonal conditions. The elevated glucose 6-P concentration, consequent to glucokinase activation, has a synergistic effect with glucose in promoting dephosphorylation (inactivation) of glycogen phosphorylase and inducing dephosphorylation (activation) of glycogen synthase. The latter involves both a direct ligand-induced conformational change and depletion of the phosphorylated form of glycogen phosphorylase, which is a potent allosteric inhibitor of glycogen synthase phosphatase activity associated with the glycogen-targeting protein, GL [hepatic glycogen-targeting subunit of PP-1 (protein phosphatase-1) encoded by PPP1R3B]. Defects in both the activation of glucokinase and in the dephosphorylation of glycogen phosphorylase are potential contributing factors to the dysregulation of hepatic glucose metabolism in Type 2 diabetes.