Glucose-6-phosphatase dependent substrate transport in the glycogen storage disease type-1a mouse

Glucose-6-phosphatase catalytic subunit gene family

Glucose-6-phosphatase catalyzes the hydrolysis of glucose 6-phosphate (G6P) to glucose and inorganic phosphate. It is a multicomponent system located in the endoplasmic reticulum that comprises several integral membrane proteins, namely a catalytic subunit (G6PC) and transporters for G6P, inorganic phosphate, and glucose. The G6PC gene family contains three members, designated G6PC, G6PC2, and G6PC3. The tissue-specific expression patterns of these genes differ, and mutations in all three genes have been linked to distinct diseases in humans. This minireview discusses the disease association and transcriptional regulation of the G6PC genes as well as the biological functions of the encoded proteins.

Activation of glycolysis and apoptosis in glycogen storage disease type Ia

The deficiency of glucose-6-phosphatase (G6Pase) underlies glycogen storage disease type Ia (GSD-Ia, von Gierke disease; MIM 232200), an autosomal recessive disorder of metabolism associated with life-threatening hypoglycemia, growth retardation, renal failure, hepatic adenomas, and hepatocellular carcinoma. Liver involvement includes the massive accumulation of glycogen and lipids due to accumulated glucose-6-phosphate and glycolytic intermediates. Proteomic analysis revealed elevations in glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and other enzymes involved in glycolysis. GAPDH was markedly increased in murine G6Pase-deficient hepatocytes. The moonlighting role of GAPDH includes increasing apoptosis, which was demonstrated by increased TUNEL assay positivity and caspase 3 activation in the murine GSD-Ia liver. These analyses of hepatic involvement in GSD-Ia mice have implicated the induction of apoptosis in the pathobiology of GSD-Ia.

Emerging therapies for glycogen storage disease type I

Glycogen storage disease type I (GSD I) is caused by deficiency of the glucose-6-phosphatase catalytic subunit in type Ia or of glucose-6-phosphate transporter in type Ib. The cellular bases for disruptions of homeostasis have been increasingly understood in GSD I, including those for anemia, renal failure and neutropenia. Advances in the understanding of cellular abnormalities in GSD I have provided rationales for new therapy, and recent developments in gene therapy have led to potential curative treatments for GSD I. These advances will benefit patients with GSD I in the future, improving both quality of life and survival, as well as illuminating the molecular effects of altered metabolism upon multiple organ systems.

A syndrome with congenital neutropenia and mutations in G6PC3

BACKGROUND

The main features of severe congenital neutropenia are the onset of severe bacterial infections early in life, a paucity of mature neutrophils, and an increased risk of leukemia. In many patients, the genetic causes of severe congenital neutropenia are unknown.

METHODS

We performed genomewide genotyping and linkage analysis on two consanguineous pedigrees with a total of five children affected with severe congenital neutropenia. Candidate genes from the linkage interval were sequenced. Functional assays and reconstitution experiments were carried out.

RESULTS

All index patients were susceptible to bacterial infections and had very few mature neutrophils in the bone marrow; structural heart defects, urogenital abnormalities, and venous angiectasia on the trunk and extremities were additional features. Linkage analysis of the two index families yielded a combined multipoint lod score of 5.74 on a linkage interval on chromosome 17q21. Sequencing of G6PC3, the candidate gene encoding glucose-6-phosphatase, catalytic subunit 3, revealed a homozygous missense mutation in exon 6 that abolished the enzymatic activity of glucose-6-phosphatase in all affected children in the two families. The patients' neutrophils and fibroblasts had increased susceptibility to apoptosis. The myeloid cells showed evidence of increased endoplasmic reticulum stress and increased activity of glycogen synthase kinase 3beta (GSK-3beta). We identified seven additional, unrelated patients who had severe congenital neutropenia with syndromic features and distinct biallelic mutations in G6PC3.

CONCLUSIONS

Defective function of glucose-6-phosphatase, catalytic subunit 3, underlies a severe congenital neutropenia syndrome associated with cardiac and urogenital malformations.

Functional analysis of mutations in the glucose-6-phosphate transporter that cause glycogen storage disease type Ib

The glucose-6-phosphate transporter (G6PT) deficient in glycogen storage disease type Ib is a phosphate (P(i))-linked antiporter capable of G6P: P(i) and P(i):P(i) exchanges. We previously characterized G6PT mutations by measuring G6P uptake activities in microsomes co-expressing G6PT and glucose-6-phosphatase-alpha. Here we report a new assay, based on reconstituted proteoliposomes carrying only G6PT, and characterize G6P and P(i) uptake activities of 23 G6PT mutations. We show that co-expression and G6PT-only assays are equivalent in measuring G6PT activity. However, the p.Q133P mutation exhibits differential G6P and P(i) transport activities, suggesting that characterizing G6P and P(i) transport activities of G6PT mutations may yield insights to this genetic disorder.

Absence of the SRC-2 coactivator results in a glycogenopathy resembling Von Gierke's disease

Hepatic glucose production is critical for basal brain function and survival when dietary glucose is unavailable. Glucose-6-phosphatase (G6Pase) is an essential, rate-limiting enzyme that serves as a terminal gatekeeper for hepatic glucose release into the plasma. Mutations in G6Pase result in Von Gierke's disease (glycogen storage disease-1a), a potentially fatal genetic disorder. We have identified the transcriptional coactivator SRC-2 as a regulator of fasting hepatic glucose release, a function that SRC-2 performs by controlling the expression of hepatic G6Pase. SRC-2 modulates G6Pase expression directly by acting as a coactivator with the orphan nuclear receptor RORalpha. In addition, SRC-2 ablation, in both a whole-body and liver-specific manner, resulted in a Von Gierke's disease phenotype in mice. Our results position SRC-2 as a critical regulator of mammalian glucose production.

Mutations in the glucose-6-phosphatase-alpha (G6PC) gene that cause type Ia glycogen storage disease

Glucose-6-phosphatase-alpha (G6PC) is a key enzyme in glucose homeostasis that catalyzes the hydrolysis of glucose-6-phosphate to glucose and phosphate in the terminal step of gluconeogenesis and glycogenolysis. Mutations in the G6PC gene, located on chromosome 17q21, result in glycogen storage disease type Ia (GSD-Ia), an autosomal recessive metabolic disorder. GSD-Ia patients manifest a disturbed glucose homeostasis, characterized by fasting hypoglycemia, hepatomegaly, nephromegaly, hyperlipidemia, hyperuricemia, lactic acidemia, and growth retardation. G6PC is a highly hydrophobic glycoprotein, anchored in the membrane of the endoplasmic reticulum with the active center facing into the lumen. To date, 54 missense, 10 nonsense, 17 insertion/deletion, and three splicing mutations in the G6PC gene have been identified in more than 550 patients. Of these, 50 missense, two nonsense, and two insertion/deletion mutations have been functionally characterized for their effects on enzymatic activity and stability. While GSD-Ia is not more prevalent in any ethnic group, mutations unique to Caucasian, Oriental, and Jewish populations have been described. Despite this, GSD-Ia patients exhibit phenotypic heterogeneity and a stringent genotype-phenotype relationship does not exist.

Neutrophil stress and apoptosis underlie myeloid dysfunction in glycogen storage disease type Ib

Glycogen storage disease type Ib (GSD-Ib) is caused by a deficiency in the glucose-6-phosphate (G6P) transporter (G6PT) that works with a liver/kidney/intestine-restricted glucose-6-phosphatase-alpha (G6Pase-alpha) to maintain glucose homeostasis between meals. Clinically, GSD-Ib patients manifest disturbed glucose homeostasis and neutrophil dysfunctions but the cause of the latter is unclear. Neutrophils express the ubiquitously expressed G6PT and G6Pase-beta that together transport G6P into the endoplasmic reticulum (ER) lumen and hydrolyze it to glucose. Because we expected G6PT-deficient neutrophils to be unable to produce endogenous glucose, we hypothesized this would lead to ER stress and increased apoptosis. Using GSD-Ib mice, we showed that GSD-Ib neutrophils exhibited increased production of ER chaperones and oxidative stress, consistent with ER stress, increased annexin V binding and caspase-3 activation, consistent with an increased rate of apoptosis. Bax activation, mitochondrial release of proapoptotic effectors, and caspase-9 activation demonstrated the involvement of the intrinsic mitochondrial pathway in these processes. The results demonstrate that G6P translocation and hydrolysis are required for normal neutrophil functions and support the hypothesis that neutrophil dysfunction in GSD-Ib is due, at least in part, to ER stress and increased apoptosis.

The glucose-6-phosphate transporter is a phosphate-linked antiporter deficient in glycogen storage disease type Ib and Ic

Glycogen storage disease type Ib (GSD-Ib) is caused by deficiencies in the glucose-6-phosphate (G6P) transporter (G6PT) that have been well characterized. Interestingly, deleterious mutations in the G6PT gene were identified in clinical cases of GSD type Ic (GSD-Ic) proposed to be deficient in an inorganic phosphate (P(i)) transporter. We hypothesized that G6PT is both the G6P and P(i) transporter. Using reconstituted proteoliposomes we show that both G6P and P(i) are efficiently taken up into P(i)-loaded G6PT-proteoliposomes. The G6P uptake activity decreases as the internal:external P(i) ratio decreases and the P(i) uptake activity decreases in the presence of external G6P. Moreover, G6P or P(i) uptake activity is not detectable in P(i)-loaded proteoliposomes containing the p.R28H G6PT null mutant. The G6PT-proteoliposome-mediated G6P or P(i) uptake is inhibited by cholorgenic acid and vanadate, both specific G6PT inhibitors. Glucose-6-phosphatase-alpha (G6Pase-alpha), which facilitates microsomal G6P uptake by G6PT, fails to stimulate G6P uptake in P(i)-loaded G6PT-proteoliposomes, suggesting that the G6Pase-alpha-mediated stimulation is caused by decreasing G6P and increasing P(i) concentrations in microsomes. Taken together, our results suggest that G6PT has a dual role as a G6P and a P(i) transporter and that GSD-Ib and GSD-Ic are deficient in the same G6PT gene.

Necrotic foci, elevated chemokines and infiltrating neutrophils in the liver of glycogen storage disease type Ia

BACKGROUND/AIMS

Glycogen storage disease type Ia (GSD-Ia) patients manifest the long-term complication of hepatocellular adenoma (HCA) of unknown etiology. We showed previously that GSD-Ia mice exhibit neutrophilia and elevated serum cytokine levels. This study was conducted to evaluate whether human GSD-Ia patients exhibit analogous increases and whether in GSD-Ia mice a correlation exists between immune abnormalities and, biochemical and histological alterations in the liver.

METHODS

Differential leukocyte counts and cytokine levels were investigated in GSD-Ia patients. Hepatic chemokine production, neutrophil infiltration, and histological abnormalities were investigated in GSD-Ia mice.

RESULTS

We show that GSD-Ia patients exhibit increased peripheral neutrophil counts and serum interleukin-8 (IL-8). Compared to normal subjects, HCA-bearing GSD-Ia patients have a 2.8-fold higher serum IL-8 concentration, while GSD-Ia patients without HCA have a 1.4-fold higher concentration. Hepatic injury in GSD-Ia mice is evidenced by necrotic foci, markedly elevated infiltrating neutrophils, and increased hepatic production of chemokines.

CONCLUSIONS

Peripheral neutrophilia and elevated serum chemokines are characteristic of GSD-Ia with HCA-bearing GSD-Ia patients having the highest serum IL-8. In GSD-Ia mice these elevations correlate with elevated hepatic chemokine levels, neutrophil infiltration, and necrosis. Taken together, peripheral neutrophilia and increased serum chemokines may indicate hepatic injuries in GSD-Ia.

Angiotensin mediates renal fibrosis in the nephropathy of glycogen storage disease type Ia

Patients with glycogen storage disease type Ia (GSD-Ia) develop renal disease of unknown etiology despite intensive dietary therapies. This renal disease shares many clinical and pathological similarities to diabetic nephropathy. We studied the expression of angiotensinogen, angiotensin type 1 receptor, transforming growth factor-beta1, and connective tissue growth factor in mice with GSD-Ia and found them to be elevated compared to controls. While increased renal expression of angiotensinogen was evident in 2-week-old mice with GSD-Ia, the renal expression of transforming growth factor-beta and connective tissue growth factor did not increase for another week; consistent with upregulation of these factors by angiotensin II. The expression of fibronectin and collagens I, III, and IV was also elevated in the kidneys of mice with GSD-Ia, compared to controls. Renal fibrosis was characterized by a marked increase in the synthesis and deposition of extracellular matrix proteins in the renal cortex and histological abnormalities including tubular basement membrane thickening, tubular atrophy, tubular dilation, and multifocal interstitial fibrosis. Our results suggest that activation of the angiotensin system has an important role in the pathophysiology of renal disease in patients with GSD-Ia.

Glucocorticoids, metabolism and metabolic diseases

Since the discovery of the beneficial effects of adrenocortical extracts for treating adrenal insufficiency more than 80 years ago, glucocorticoids (GC) and their cognate, intracellular receptor, the glucocorticoid receptor (GR) have been characterized as critical components of the delicate hormonal control system that determines energy homeostasis in mammals. Whereas physiological levels of GCs are required for proper metabolic control, excessive GC action has been tied to a variety of pandemic metabolic diseases, such as type II diabetes and obesity. Highlighted by its importance for human health, the investigation of molecular mechanisms of GC/GR action has become a major focus in biomedical research. In particular, the understanding of tissue-specific functions of the GC-GR pathway has been proven to be of substantial value for the identification of novel therapeutic options in the treatment of severe metabolic disorders. Therefore, this review focuses on the role of the GC-GR axis for metabolic homeostasis and dysregulation, emphasizing tissue-specific functions of GCs in the control of energy metabolism.

11β-Hydroxysteroid Dehydrogenase Type 1 Regulation by Intracellular Glucose 6-Phosphate Provides Evidence for a Novel Link between Glucose Metabolism and Hypothalamo-Pituitary-Adrenal Axis Function

Microsomal glucose-6-phosphatase-alpha (G6Pase-alpha) and glucose 6-phosphate transporter (G6PT) work together to increase blood glucose concentrations by performing the terminal step in both glycogenolysis and gluconeogenesis. Deficiency of the G6PT in liver gives rise to glycogen storage disease type 1b (GSD1b), whereas deficiency of G6Pase-alpha leads to GSD1a. G6Pase-alpha shares its substrate (glucose 6-phosphate; G6P) with hexose-6-phosphate-dehydrogenase (H6PDH), a microsomal enzyme that regenerates NADPH within the endoplasmic reticulum lumen, thereby conferring reductase activity upon 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1). 11beta-HSD1 interconverts hormonally active C11beta-hydroxy steroids (cortisol in humans and corticosterone in rodents) to inactive C11-oxo steroids (cortisone and 11-dehydrocorticosterone, respectively). In vivo reductase activity predominates, generating active glucocorticoid. We hypothesized that substrate (G6P) availability to H6PDH in patients with GSD1b and GSD1a will decrease or increase 11beta-HSD1 reductase activity, respectively. We investigated 11beta-HSD1 activity in GSD1b and GSD1a mice and in two patients with GSD1b and five patients diagnosed with GSD1a. We confirmed our hypothesis by assessing 11beta-HSD1 in vivo and in vitro, revealing a significant decrease in reductase activity in GSD1b animals and patients, whereas GSD1a patients showed a marked increase in activity. The cellular trafficking of G6P therefore directly regulates 11beta-HSD1 reductase activity and provides a novel link between glucose metabolism and function of the hypothalamo-pituitary-adrenal axis.

Neutrophilia and elevated serum cytokines are implicated in glycogen storage disease type Ia

Glycogen storage disease type Ia (GSD-Ia) patients deficient in glucose-6-phosphatase-alpha manifest a disturbed glucose homeostasis. We hypothesized that disturbed glucose homeostasis might affect myeloid functions. Here, we show that GSD-Ia mice exhibit normal neutrophil activities but have elevated myeloid progenitor cells in the bone marrow and spleen. Interestingly, GSD-Ia mice exhibit a persistent increase in peripheral blood neutrophil counts along with elevated serum levels of granulocyte colony stimulating factor and cytokine-induced neutrophil chemoattractant. Taken together, our results suggest that a loss of glucose homeostasis can compromise the immune system, resulting in neutrophilia. This may explain some of the unexpected clinical manifestations seen in GSD-Ia.

Efficacy of Helper-dependent Adenovirus Vector-mediated Gene Therapy in Murine Glycogen Storage Disease Type Ia

Genetic deficiency of glucose-6-phosphatase (G6Pase) underlies glycogen storage disease type Ia (GSD-Ia, also known as von Gierke disease; MIM 232200), an autosomal recessive disorder of metabolism associated with life-threatening hypoglycemia and growth retardation. We tested whether helper-dependent adenovirus (HDAd)-mediated hepatic delivery of G6Pase would lead to prolonged survival and sustained correction of the metabolic abnormalities in G6Pase knockout (KO) mice, a model for a severe form of GSD-Ia. An HDAd vector encoding G6Pase was administered intravenously (2 or 5 x 10(12)vector particles/kg) to 2-week-old (w.o.) G6Pase-KO mice. Following HDAd vector administration survival was prolonged to a median of 7 months, in contrast to untreated affected mice that did not survive past 3 weeks of age. G6Pase levels increased more than tenfold between 3 days and 28 weeks after HDAd injection (P < 0.03). The weights of untreated 2 w.o. G6Pase-KO mice were approximately half those of their unaffected littermates, and treatment stimulated their growth to the size of wild-type mice. Severe hypoglycemia and hypercholesterolemia, which are hallmarks of GSD-Ia both in humans and in mice, were also restored to normalcy by the treatment. Glycogen accumulation in the liver was markedly reduced. The efficacy of HDAd-G6Pase treatment in reversing the physiological and biochemical abnormalities associated with GSD-Ia in affected G6Pase-KO mice justifies further preclinical evaluation in murine and canine models of GSD-Ia.

Gene therapy for type I glycogen storage diseases

The type I glycogen storage diseases (GSD-I) are a group of related diseases caused by a deficiency in the glucose-6-phosphatase-alpha (G6Pase-alpha) system, a key enzyme complex that is essential for the maintenance of blood glucose homeostasis between meals. The complex consists of a glucose-6-phosphate transporter (G6PT) that translocates glucose-6-phosphate from the cytoplasm into the lumen of the endoplasmic reticulum, and a G6Pase-alpha catalytic unit that hydrolyses the glucose-6-phosphate into glucose and phosphate. A deficiency in G6Pase-alpha causes GSD type Ia (GSD-Ia) and a deficiency in G6PT causes GSD type Ib (GSD-Ib). Both GSD-Ia and GSD-Ib patients manifest a disturbed glucose homeostasis, while GSD-Ib patients also suffer symptoms of neutropenia and myeloid dysfunctions. G6Pase-alpha and G6PT are both hydrophobic endoplasmic reticulum-associated transmembrane proteins that can not expressed in soluble active forms. Therefore protein replacement therapy of GSD-I is not an option. Animal models of GSD-Ia and GSD-Ib that mimic the human disorders are available. Both adenovirus- and adeno-associated virus (AAV)-mediated gene therapies have been evaluated for GSD-Ia in these model systems. While adenoviral therapy produces only short term corrections and only impacts liver expression of the gene, AAV-mediated therapy delivers the transgene to both the liver and kidney, achieving longer term correction of the GSD-Ia disorder, although there are substantial differences in efficacy depending on the AAV serotype used. Gene therapy for GSD-Ib in the animal model is still in its infancy, although an adenoviral construct has improved the metabolic profile and myeloid function. Taken together further refinements in gene therapy may hold long term benefits for the treatment of type I GSD disorders.

Specific reduction of hepatic glucose 6-phosphate transporter-1 ameliorates diabetes while avoiding complications of glycogen storage disease

D-Glucose-6-phosphatase is a key regulator of endogenous glucose production, and its inhibition may improve glucose control in type 2 diabetes. Herein, 2'-O-(2-methoxy)ethyl-modified phosphorothioate antisense oligonucleotides (ASOs) specific to the glucose 6-phosphate transporter-1 (G6PT1) enabled reduction of hepatic D-Glu-6-phosphatase activity in diabetic ob/ob mice. Treatment with G6PT1 ASOs decreased G6PT1 expression, reduced G6PT1 activity, blunted glucagon-stimulated glucose production, and lowered plasma glucose concentration in a dose-dependent manner. In contrast to G6PT1 knock-out mice and patients with glycogen storage disease, excess hepatic and renal glycogen accumulation, hyperlipidemia, neutropenia, and elevations in plasma lactate and uric acid did not occur. In addition, hypoglycemia was not observed in animals during extended periods of fasting, and the ability of G6PT1 ASO-treated mice to recover from an exogenous insulin challenge was not impaired. Together, these results demonstrate that effective glucose lowering by G6PT1 inhibitors can be achieved without adversely affecting carbohydrate and lipid metabolism.

Glycogen storage disease types I and II: treatment updates

Prior to 2006 therapy for glycogen storage diseases consisted primarily of dietary interventions, which in the case of glycogen storage disease (GSD) type II (GSD II; Pompe disease) remained essentially palliative. Despite improved survival and growth, long-term complications of GSD type I (GSD I) have not responded to dietary therapy with uncooked cornstarch or continuous gastric feeding. The recognized significant risk of renal disease and liver malignancy in GSD I has prompted efforts towards curative therapy, including organ transplantation, in those deemed at risk. Results of clinical trials in infantile Pompe disease with alglucosidase alfa (Myozyme) showed prolonged survival reversal of cardiomyopathy, and motor gains. This resulted in broad label approval of Myozyme for Pompe disease in 2006. Furthermore, the development of experimental therapies, such as adeno-associated virus (AAV) vector-mediated gene therapy, holds promise for the availability of curative therapy in GSD I and GSD II/Pompe disease in the future.

Impaired neutrophil activity and increased susceptibility to bacterial infection in mice lacking glucose-6-phosphatase-beta

Neutropenia and neutrophil dysfunction are common in many diseases, although their etiology is often unclear. Previous views held that there was a single ER enzyme, glucose-6-phosphatase-alpha (G6Pase-alpha), whose activity--limited to the liver, kidney, and intestine--was solely responsible for the final stages of gluconeogenesis and glycogenolysis, in which glucose-6-phosphate (G6P) is hydrolyzed to glucose for release to the blood. Recently, we characterized a second G6Pase activity, that of G6Pase-beta (also known as G6PC), which is also capable of hydrolyzing G6P to glucose but is ubiquitously expressed and not implicated in interprandial blood glucose homeostasis. We now report that the absence of G6Pase-beta led to neutropenia; defects in neutrophil respiratory burst, chemotaxis, and calcium flux; and increased susceptibility to bacterial infection. Consistent with this, G6Pase-beta-deficient (G6pc3-/-) mice with experimental peritonitis exhibited increased expression of the glucose-regulated proteins upregulated during ER stress in their neutrophils and bone marrow, and the G6pc3-/- neutrophils exhibited an enhanced rate of apoptosis. Our results define a molecular pathway to neutropenia and neutrophil dysfunction of previously unknown etiology, providing a potential model for the treatment of these conditions.

Immunocytochemical localization of glucose 6-phosphatase and cytosolic phosphoenolpyruvate carboxykinase in gluconeogenic tissues reveals unsuspected metabolic zonation

Immunohistochemical analysis was used to define the precise cell-specific localization of Glucose-6-phosphatase (Glc6Pase) and cytosolic form of the phosphoenolpyruvate carboxykinase (PEPCK-C) in the digestive system (liver, small intestine and pancreas) and the kidney. Co-expression of Glc6Pase and PEPCK-C was shown to take place in hepatocytes, in proximal tubules of the cortex kidney and at the top of the villi of the small intestine suggesting that these tissues are all able to perform complete gluconeogenesis. On the other hand, intrahepatic bile ducts, collecting tubes of the nephron and the urinary epithelium in the calices of the kidney, as well as the crypts of the small intestine, express Glc6Pase without significant levels of PEPCK-C. In such cases, the function of Glc6Pase could be related to the transepithelial transport of glucose characteristic of these tissues, rather than to the neoformation of glucose. Lastly, PEPCK-C expression in the absence of Glc6Pase was noted in both the exocrine pancreas and the endocrine islets of Langerhans. Possible roles of PEPCK-C in exocrine pancreas might be the provision of gluconeogenic intermediates for further conversion into glucose in the liver, whereas PEPCK-C would be instrumental in pyruvate cycling, which has been suggested to play a regulatory role in insulin secretion by the beta-cells of the islets.

Deletion of the gene encoding the ubiquitously expressed glucose-6-phosphatase catalytic subunit-related protein (UGRP)/glucose-6-phosphatase catalytic subunit-beta results in lowered plasma cholesterol and elevated glucagon

In liver, glucose-6-phosphatase catalyzes the hydrolysis of glucose-6-phosphate (G6P) to glucose and inorganic phosphate, the final step in the gluconeogenic and glycogenolytic pathways. Mutations in the glucose-6-phosphatase catalytic subunit (G6Pase) give rise to glycogen storage disease (GSD) type 1a, which is characterized in part by hypoglycemia, growth retardation, hypertriglyceridemia, hypercholesterolemia, and hepatic glycogen accumulation. Recently, a novel G6Pase isoform was identified, designated UGRP/G6Pase-beta. The activity of UGRP relative to G6Pase in vitro is disputed, raising the question as to whether G6P is a physiologically important substrate for this protein. To address this issue we have characterized the phenotype of UGRP knock-out mice. G6P hydrolytic activity was decreased by approximately 50% in homogenates of UGRP(-/-) mouse brain relative to wild type tissue, consistent with the ability of UGRP to hydrolyze G6P. In addition, female, but not male, UGRP(-/-) mice exhibit growth retardation as do G6Pase(-/-) mice and patients with GSD type 1a. However, in contrast to G6Pase(-/-) mice and patients with GSD type 1a, UGRP(-/-) mice exhibit no change in hepatic glycogen content, blood glucose, or triglyceride levels. Although UGRP(-/-) mice are not hypoglycemic, female UGRP(-/-) mice have elevated ( approximately 60%) plasma glucagon and reduced ( approximately 20%) plasma cholesterol. We hypothesize that the hyperglucagonemia prevents hypoglycemia and that the hypocholesterolemia is secondary to the hyperglucagonemia. As such, the phenotype of UGRP(-/-) mice is mild, indicating that G6Pase is the major glucose-6-phosphatase of physiological importance for glucose homeostasis in vivo.

Glucose-6-phosphate transporter gene therapy corrects metabolic and myeloid abnormalities in glycogen storage disease type Ib mice

Glycogen storage disease type Ib (GSD-Ib) is caused by a deficiency in the glucose-6-phosphate transporter (G6PT), an endoplasmic reticulum-associated transmembrane protein that is ubiquitously expressed. GSD-Ib patients suffer from disturbed glucose homeostasis and myeloid dysfunctions. To evaluate the feasibility of gene replacement therapy for GSD-Ib, we have infused adenoviral (Ad) vector containing human G6PT (Ad-hG6PT) into G6PT-deficient (G6PT(-/-)) mice that manifest symptoms characteristics of the human disorder. Ad-hG6PT infusion restores significant levels of G6PT mRNA expression in the liver, bone marrow and spleen, and corrects metabolic as well as myeloid abnormalities in G6PT(-/-) mice. The G6PT(-/-) mice receiving gene therapy exhibit improved growth; normalized serum profiles for glucose, cholesterol, triglyceride, uric acid and lactic acid; and reduced hepatic glycogen deposition. The therapy also corrects neutropenia and lowers the elevated serum levels of granulocyte colony-stimulating factor. The development of bone and spleen in the infused G6PT(-/-) mice is improved and accompanied by increased cellularity and normalized myeloid progenitor cell frequencies in both tissues. This effective use of gene therapy to correct metabolic imbalances and myeloid dysfunctions in GSD-Ib mice holds promise for the future of gene therapy in humans.

Long-term correction of murine glycogen storage disease type Ia by recombinant adeno-associated virus-1-mediated gene transfer

Glycogen storage disease type Ia (GSD-Ia) is caused by a deficiency in glucose-6-phosphatase-alpha (G6Pase-alpha), a nine-transmembrane domain, endoplasmic reticulum-associated protein expressed primarily in the liver and kidney. Previously, we showed that infusion of an adeno-associated virus (AAV) serotype 2 vector carrying murine G6Pase-alpha (AAV2-G6Pase-alpha) into neonatal GSD-Ia mice failed to sustain their life beyond weaning. We now show that neonatal infusion of GSD-Ia mice with an AAV serotype 1-G6Pase-alpha (AAV1-G6Pase-alpha) or AAV serotype 8-G6Pase-alpha (AAV8-G6Pase-alpha) results in hepatic expression of the G6Pase-alpha transgene and markedly improves the survival of the mice. However, only AAV1-G6Pase-alpha can achieve significant renal transgene expression. A more effective strategy, in which a neonatal AAV1-G6Pase-alpha infusion is followed by a second infusion at age one week, provides sustained expression of a complete, functional, G6Pase-alpha system in both the liver and kidney and corrects the metabolic abnormalities in GSD-Ia mice for the 57 week length of the study. This effective use of gene therapy to correct metabolic imbalances and disease progression in GSD-Ia mice holds promise for the future of gene therapy in humans.

Early, sustained efficacy of adeno-associated virus vector-mediated gene therapy in glycogen storage disease type Ia

The deficiency of glucose-6-phosphatase (G6Pase) underlies life-threatening hypoglycemia and growth retardation in glycogen storage disease type Ia (GSD-Ia). An adeno-associated virus (AAV) vector encoding G6Pase was pseudotyped as AAV8 and administered to 2-week-old GSD-Ia mice (n = 9). Median survival was prolonged to 7 months following vector administration, in contrast to untreated GSD-Ia mice that survived for only 2 weeks. Although GSD-Ia mice were initially growth-retarded, treated mice increased fourfold in weight to normal size. Blood glucose was partially corrected by 2 weeks following treatment, whereas blood cholesterol normalized. Glucose-6-phosphatase activity was partially corrected to 25% of the normal level at 7 months of age in treated mice, and blood glucose during fasting remained lower in treated, affected mice than in normal mice. Glycogen storage was partially corrected in the liver by 2 weeks following treatment, but reaccumulated to pre-treatment levels by 7 months old (m.o.). Vector genome DNA decreased between 3 days and 3 weeks in the liver following vector administration, mainly through the loss of single-stranded genomes; however, double-stranded vector genomes were more stable. Although CD8+ lymphocytic infiltrates were present in the liver, partial biochemical correction was sustained at 7 m.o. The development of efficacious AAV vector-mediated gene therapy could significantly reduce the impact of long-term complications in GSD-Ia, including hypoglycemia, hyperlipidemia and growth failure.

Increased cellular cholesterol efflux in glycogen storage disease type Ia mice: A potential mechanism that protects against premature atherosclerosis

Glycogen storage disease type Ia (GSD-Ia) patients manifest a pro-atherogenic lipid profile but are not at elevated risk for developing atherosclerosis. Serum phospholipid, which correlates positively with the scavenger receptor class B type I (SR-BI)-mediated cholesterol efflux, and apolipoprotein A-IV and E, acceptors for ATP-binding cassette transporter A1 (ABCA1)-mediated cholesterol transport, are increased in GSD-Ia mice. Importantly, sera from GSD-Ia mice are more efficient than sera from control littermates in promoting SR-BI- and ABCA1-mediated cholesterol effluxes. As the first step in reverse cholesterol transport, essential for cholesterol homeostasis, these observations provide one explanation why GSD-Ia patients are apparently protected against premature atherosclerosis.

Brain contains a functional glucose-6-phosphatase complex capable of endogenous glucose production

Glucose is absolutely essential for the survival and function of the brain. In our current understanding, there is no endogenous glucose production in the brain, and it is totally dependent upon blood glucose. This glucose is generated between meals by the hydrolysis of glucose-6-phosphate (Glc-6-P) in the liver and the kidney. Recently, we reported a ubiquitously expressed Glc-6-P hydrolase, glucose-6-phosphatase-beta (Glc-6-Pase-beta), that can couple with the Glc-6-P transporter to hydrolyze Glc-6-P to glucose in the terminal stages of glycogenolysis and gluconeogenesis. Here we show that astrocytes, the main reservoir of brain glycogen, express both the Glc-6-Pase-beta and Glc-6-P transporter activities and that these activities can couple to form an active Glc-6-Pase complex, suggesting that astrocytes may provide an endogenous source of brain glucose.

A Potential New Role for Muscle in Blood Glucose Homeostasis

The breakdown of tissue glycogen into glucose is critical for blood glucose homeostasis between meals. In the final steps of glycogenolysis, intracellular glucose 6-phosphate (Glc-6-P) is transported into the endoplasmic reticulum where it is hydrolyzed to glucose by glucose-6-phosphatase (Glc-6-Pase). Although the majority of body glycogen is stored in the muscle, the current dogma holds that Glc-6-Pase (now named Glc-6-Pase-alpha) is expressed only in the liver, kidney, and intestine, implying that muscle glycogen cannot contribute to interprandial blood glucose homeostasis. Recently we reported a second Glc-6-P hydrolase, Glc-6-Pase-beta. Glc-6-Pase-beta shares kinetic and structural similarities to Glc-6-Pase-alpha and couples with the Glc-6-P transporter to form an active Glc-6-Pase complex (Shieh, J.-J., Pan, C.-J., Mansfield, B. C., and Chou, J. Y. (2003) J. Biol. Chem. 278, 47098-47103). Here we demonstrate that muscle expresses both Glc-6-Pase-beta and Glc-6-P transporter and that they can couple to form an active Glc-6-Pase complex. Our data suggest that muscle may have a previously unrecognized role in interprandial glucose homeostasis.

Mice Lacking Thioredoxin-interacting Protein Provide Evidence Linking Cellular Redox State to Appropriate Response to Nutritional Signals

Thioredoxin-interacting protein (Txnip) is a ubiquitous protein that binds with high affinity to thioredoxin and inhibits its ability to reduce sulfhydryl groups via NADPH oxidation. HcB-19 mice contain a nonsense mutation in Txnip that eliminates its expression. Unlike normal animals, HcB-19 mice have approximately 3-fold increase in insulin levels when fasted. The C-peptide/insulin ratio is normal, suggesting that the hyperinsulinemia is due to increased insulin secretion. Fasted HcB-19 mice are hypoglycemic, hypertriglyceridemic, and have higher than normal levels of ketone bodies. Ablation of pancreatic beta-cells with streptozotocin completely blocks the fasting-induced hypoglycemia/hypertriglyceridemia, suggesting that these abnormalities are due to excess insulin secretion. This is supported by increased hepatic mRNA levels of the insulin-inducible, lipogenic transcription factor sterol-responsive element-binding protein-1c and two of its targets, acetyl-CoA carboxylase and fatty acid synthase. During a prolonged fast, the hyperinsulinemia up-regulates lipogenesis but fails to down-regulate hepatic phosphoenolpyruvate carboxykinase mRNA expression. Hepatic ratios of reduced:oxidized glutathione, established regulators of gluconeogenic/glycolytic/lipogenic enzymes, were elevated 30% in HcB-19 mice, suggesting a loss of Txnip-enhanced sulfhydryl reduction. The altered hepatic enzymatic profiles of HcB-19 mice divert phosphoenolpyruvate to glyceroneogenesis and lipogenesis rather than gluconeogenesis. Our findings implicate Txnip-modulated sulfhydryl redox as a central regulator of insulin secretion in beta-cells and regulation of many of the branch-points of gluconeogenesis/glycolysis/lipogenesis.

Functional silencing of hepatic microsomal glucose-6-phosphatase gene expression in vivo by adenovirus-mediated delivery of short hairpin RNA

An expression cassette containing mouse U6 polymerase III promoter directing expression of short hairpin RNA (shRNA) targeting murine microsomal glucose-6-phosphatase (G6P) transcript was generated. This construct was packaged into an adenoviral (AdV) backbone and viral stocks generated. Mice injected intravenously with AdV-G6PshRNA exhibited a significant reduction in postprandial glucose levels and had significantly elevated steady-state hepatic glycogen stores. Target gene silencing was confirmed by measurements demonstrating a significant reduction in both hepatic G6P transcript level and phosphohydrolase activity. These findings provide evidence that AdV delivery of expressed shRNA can be a productive tool to explore gene function in vivo.

A glucose-6-phosphate hydrolase, widely expressed outside the liver, can explain age-dependent resolution of hypoglycemia in glycogen storage disease type Ia

A fine control of the blood glucose level is essential to avoid hyper- or hypo-glycemic shocks associated with many metabolic disorders, including diabetes mellitus and type I glycogen storage disease. Between meals, the primary source of blood glucose is gluconeogenesis and glycogenolysis. In the final step of both pathways, glucose-6-phosphate (G6P) is hydrolyzed to glucose by the glucose-6-phosphatase (G6Pase) complex. Because G6Pase (renamed G6Pase-alpha) is primarily expressed only in the liver, kidney, and intestine, it has implied that most other tissues cannot contribute to interprandial blood glucose homeostasis. We demonstrate that a novel, widely expressed G6Pase-related protein, PAP2.8/UGRP, renamed here G6Pase-beta, is an acid-labile, vanadate-sensitive, endoplasmic reticulum-associated phosphohydrolase, like G6Pase-alpha. Both enzymes have the same active site structure, exhibit a similar Km toward G6P, but the Vmax of G6Pase-alpha is approximately 6-fold greater than that of G6Pase-beta. Most importantly, G6Pase-beta couples with the G6P transporter to form an active G6Pase complex that can hydrolyze G6P to glucose. Our findings challenge the current dogma that only liver, kidney, and intestine can contribute to blood glucose homeostasis and explain why type Ia glycogen storage disease patients, lacking a functional liver/kidney/intestine G6Pase complex, are still capable of endogenous glucose production.

Hypoglycemic agent YM440 suppresses hepatic glucose output via gluconeogenesis by reducing glucose-6-phosphatase activity in obese Zucker rats

Using a glucose clamp, we had shown that YM440, (Z)-1,4-bis[4-[(3,5-dioxo-1,2,4-oxadiazolidin-2-yl)methyl]phenoxy]but-2-ene, reduced the increased hepatic glucose output in obese Zucker rats. We further examined effects of YM440 on 14C-incorporation from [14C]bicarbonate into blood glucose via gluconeogenesis, and on gluconeogenic enzymatic activities. Fed obese Zucker rats showed a 4-fold increase of 14C-incorporation into blood glucose compared to that in lean rats. Glucose-6-phosphatase and fructose-1,6-bisphosphatase activities in obese rats were increased 1.4-fold and 1.6-fold compared with lean rats. YM440 (300 mg/kg for 2 weeks) decreased 14C-incorporation into blood glucose by 29% in obese rats. Glucose-6-phosphatase but not fructose-1,6-bisphosphatase activity was reduced by YM440 and closely correlated with 14C-incorporation into blood glucose, indicating a key role for glucose-6-phosphatase in hepatic glucose output. These results suggest that the increased gluconeogenesis in obese rats is mainly due to the increased activities of glucose-6-phosphatase and fructose-1,6-bisphosphatase and that YM440 suppresses hepatic glucose output by reducing glucose-6-phosphatase activity.

When translation meets metabolism: multiple links to diabetes

Type 2 diabetes is a polygenic disorder characterized by multiple biochemical defects including transcriptional, translational, and posttranslational abnormalities. Although major progress has been made in elucidation of factors at the transcriptional and posttranslational levels, defects at the translational level remain elusive. Mutation of a kinase that regulates translation initiation has been implicated in the etiology of a monogenic form of diabetes known as Wolcott-Rallison syndrome. Characterization of mice rendered deficient in eukaryotic initiation factors has provided model systems to study the involvement of translation in regulating insulin synthesis and secretion, hepatic function, peripheral insulin resistance, and diabetic complications. Recent progress in the understanding of endoplasmic reticulum overload by unfolded proteins has begun to uncover mechanisms leading to pancreatic beta-cell exhaustion. Future advances in this area may lead to identification of the missing links in the pathogenesis of beta-cell failures due to conditions such as hyperinsulinemia, hyperglycemia, and long-term treatment with sulfonylureas, and thus may identify novel therapeutic targets for diabetes.

The biochemistry and molecular biology of the glucose-6-phosphatase system

Progress has continued to be made over the past 4 years in our understanding of the glucose-6-phosphatase (G6Pase) system. The gene for a second component of the system, the putative glucose-6-P transporter (G6PT), was cloned, and mutations in this gene were found in patients diagnosed with glycogen storage disease type 1b. The functional characterization of this putative G6PT has been initiated, and the relationship between substrate transport via the G6PT and catalysis by the system's catalytic subunit continues to be explored. A lively debate over the feasibility of various aspects of the two proposed models of the G6Pase system persists, and the functional/structural relationships of the individual components of the system remain a hot topic of interest in G6Pase research. New evidence supportive of physiologic roles for the biosynthetic functions of the G6Pase system in vivo also has emerged over the past 4 years.

Gene expression during the priming phase of liver regeneration after partial hepatectomy in mice

Understanding the gene-expression patterns during liver regeneration may help to reveal how regenerative processes are initiated and controlled as well as shed new light onto processes that lead to liver disease. Using high-density oligonucleotide arrays, we have examined the gene-expression program in the livers of mice after partial hepatectomy. A time course was constructed for gene expression between 0 and 4 h after partial hepatectomy, corresponding to the priming phase of liver regeneration. The genomic program for liver regeneration involves transcription-factor generation, stress and inflammatory responses, cytoskeletal and extracellular matrix modification, and regulation of cell-cycle entry. The genome-wide changes that are observed provide a detailed and comprehensive map of the initial priming stage of liver regeneration.

Monitoring the correction of glycogen storage disease type 1a in a mouse model using [(18)F]FDG and a dedicated animal scanner

Monitoring gene therapy of glycogen storage disease type 1a in a mouse model was achieved using [(18)F]FDG and a dedicated animal scanner. The G6Pase knockout (KO) mice were compared to the same mice after infusion with a recombinant adenovirus containing the murine G6Pase gene (Ad-mG6Pase). Serial images of the same mouse before and after therapy were obtained and compared with wild-type (WT) mice of the same strain to determine the uptake and retention of [(18)F]FDG in the liver. Image data were acquired from heart, blood pool and liver for twenty minutes after injection of [(18)F]FDG. The retention of [(18)F]FDG was lower for the WT mice compared to the KO mice. The mice treated with adenovirus-mediated gene therapy had retention similar to that found in age-matched WT mice. These studies show that FDG can be used to monitor the G6Pase concentration in liver of WT mice as compared to G6Pase KO mice. In these mice, gene therapy returned the liver function to that found in age matched WT controls as measured by the FDG kinetics in the liver compared to that found in age matched wild type controls.

Evidence for glucose-6-phosphate transport in rat liver microsomes

The existence of glucose-6-phosphate transport across the liver microsomal membrane is still controversial. In this paper, we show that S3483, a chlorogenic acid derivative known to inhibit glucose-6-phosphatase in intact microsomes, caused the intravesicular accumulation of glucose-6-phosphate when the latter was produced by glucose-6-phosphatase from glucose and carbamoyl-phosphate. S3483 also inhibited the conversion of glucose-6-phosphate to 6-phosphogluconate occurring inside microsomes in the presence of electron acceptors (NADP or metyrapone). These data indicate that liver microsomal membranes contain a reversible glucose-6-phosphate transporter, which furnishes substrate not only to glucose-6-phosphatase, but also to hexose-6-phosphate dehydrogenase.

The glucose-6-phosphatase system

Glucose-6-phosphatase (G6Pase), an enzyme found mainly in the liver and the kidneys, plays the important role of providing glucose during starvation. Unlike most phosphatases acting on water-soluble compounds, it is a membrane-bound enzyme, being associated with the endoplasmic reticulum. In 1975, W. Arion and co-workers proposed a model according to which G6Pase was thought to be a rather unspecific phosphatase, with its catalytic site oriented towards the lumen of the endoplasmic reticulum [Arion, Wallin, Lange and Ballas (1975) Mol. Cell. Biochem. 6, 75--83]. Substrate would be provided to this enzyme by a translocase that is specific for glucose 6-phosphate, thereby accounting for the specificity of the phosphatase for glucose 6-phosphate in intact microsomes. Distinct transporters would allow inorganic phosphate and glucose to leave the vesicles. At variance with this substrate-transport model, other models propose that conformational changes play an important role in the properties of G6Pase. The last 10 years have witnessed important progress in our knowledge of the glucose 6-phosphate hydrolysis system. The genes encoding G6Pase and the glucose 6-phosphate translocase have been cloned and shown to be mutated in glycogen storage disease type Ia and type Ib respectively. The gene encoding a G6Pase-related protein, expressed specifically in pancreatic islets, has also been cloned. Specific potent inhibitors of G6Pase and of the glucose 6-phosphate translocase have been synthesized or isolated from micro-organisms. These as well as other findings support the model initially proposed by Arion. Much progress has also been made with regard to the regulation of the expression of G6Pase by insulin, glucocorticoids, cAMP and glucose.

CREB regulates hepatic gluconeogenesis through the coactivator PGC-1

When mammals fast, glucose homeostasis is achieved by triggering expression of gluconeogenic genes in response to glucagon and glucocorticoids. The pathways act synergistically to induce gluconeogenesis (glucose synthesis), although the underlying mechanism has not been determined. Here we show that mice carrying a targeted disruption of the cyclic AMP (cAMP) response element binding (CREB) protein gene, or overexpressing a dominant-negative CREB inhibitor, exhibit fasting hypoglycaemia [corrected] and reduced expression of gluconeogenic enzymes. CREB was found to induce expression of the gluconeogenic programme through the nuclear receptor coactivator PGC-1, which is shown here to be a direct target for CREB regulation in vivo. Overexpression of PGC-1 in CREB-deficient mice restored glucose homeostasis and rescued expression of gluconeogenic genes. In transient assays, PGC-1 potentiated glucocorticoid induction of the gene for phosphoenolpyruvate carboxykinase (PEPCK), the rate-limiting enzyme in gluconeogenesis. PGC-1 promotes cooperativity between cyclic AMP and glucocorticoid signalling pathways during hepatic gluconeogenesis. Fasting hyperglycaemia is strongly correlated with type II diabetes, so our results suggest that the activation of PGC-1 by CREB in liver contributes importantly to the pathogenesis of this disease.

Discriminant responses of the catalytic unit and glucose 6-phosphate transporter components of the hepatic glucose-6-phosphatase system in Ehrlich ascites-tumor-bearing mice

The effect of Ehrlich ascites tumor cells, in vivo, on the hepatic glucose-6-phosphatase (G6Pase) system was examined. The V(max) for glucose 6-phosphate hydrolysis by G6Pase was reduced by 40% and a greater than 15-fold decrease in mRNA encoding the catalytic unit of the G6Pase system was observed 8 days after injection with tumor cells. Blood glucose concentration was decreased from 169 +/- 17 to 105 +/- 9 mg/dl in tumor-bearing mice. There was no change in the G6P transporter (G6PT) mRNA level. However, there was a significant decrease in G6P accumulation into hepatic microsomal vesicles derived from tumor-bearing mice. Decreased G6P accumulation was also associated with a decrease in G6Pase hydrolytic activity in the presence of vanadate, a potent catalytic-unit inhibitor. In addition, G6P accumulation was nearly abolished in microsomes treated with N-bromoacetylethanolamine phosphate, an irreversible inhibitor of the G6PT. These results demonstrate that the catalytic unit and G6PT components of the G6Pase system can be discriminantly regulated, and that microsomal glucose 6-phosphate uptake is dependent on catalytic unit activity as well as G6PT action.

Probing into the function of the gene product responsible for glycogen storage disease type Ib

This study aimed at directly assessing glucose 6-phosphate (G6P) transport by intact rat liver microsomes. Tracer uptake from labeled G6P occurred with T(1/2) values that proved insensitive to unlabeled G6P or 100 microM vanadate, and could not be activated over background levels by intravesicular phosphate in the complete absence of G6P hydrolysis. [(32)P]Phosphate efflux was similarly unaffected by G6P or phosphate in the incubation medium. We conclude that the gene product responsible for glycogen storage disease type Ib is functionally distinct from the bacterial hexose phosphate transporter, which operates as an obligatory phosphate:phosphate or G6P:phosphate exchanger.

Acute inhibition of hepatic glucose-6-phosphatase does not affect gluconeogenesis but directs gluconeogenic flux toward glycogen in fasted rats. A pharmacological study with the chlorogenic acid derivative S4048

Effects of acute inhibition of glucose-6-phosphatase activity by the chlorogenic acid derivative S4048 on hepatic carbohydrate fluxes were examined in isolated rat hepatocytes and in vivo in rats. Fluxes were calculated using tracer dilution techniques and mass isotopomer distribution analysis in plasma glucose and urinary paracetamol-glucuronide after infusion of [U-(13)C]glucose, [2-(13)C]glycerol, [1-(2)H]galactose, and paracetamol. In hepatocytes, glucose-6-phosphate (Glc-6-P) content, net glycogen synthesis, and lactate production from glucose and dihydroxyacetone increased strongly in the presence of S4048 (10 microm). In livers of S4048-treated rats (0.5 mg kg(-1)min(-)); 8 h) Glc-6-P content increased strongly (+440%), and massive glycogen accumulation (+1260%) was observed in periportal areas. Total glucose production was diminished by 50%. The gluconeogenic flux to Glc-6-P was unaffected (i.e. 33.3 +/- 2.0 versus 33.2 +/- 2.9 micromol kg(-1)min(-1)in control and S4048-treated rats, respectively). Newly synthesized Glc-6-P was redistributed from glucose production (62 +/- 1 versus 38 +/- 1%; p < 0.001) to glycogen synthesis (35 +/- 5% versus 65 +/- 5%; p < 0.005) by S4048. This was associated with a strong inhibition (-82%) of the flux through glucokinase and an increase (+83%) of the flux through glycogen synthase, while the flux through glycogen phosphorylase remained unaffected. In livers from S4048-treated rats, mRNA levels of genes encoding Glc-6-P hydrolase (approximately 9-fold), Glc-6-P translocase (approximately 4-fold), glycogen synthase (approximately 7-fold) and L-type pyruvate kinase (approximately 4-fold) were increased, whereas glucokinase expression was almost abolished. In accordance with unaltered gluconeogenic flux, expression of the gene encoding phosphoenolpyruvate carboxykinase was unaffected in the S4048-treated rats. Thus, acute inhibition of glucose-6-phosphatase activity by S4048 elicited 1) a repartitioning of newly synthesized Glc-6-P from glucose production into glycogen synthesis without affecting the gluconeogenic flux to Glc-6-P and 2) a cellular response aimed at maintaining cellular Glc-6-P homeostasis.

Novel arguments in favor of the substrate-transport model of glucose-6-phosphatase

The purpose of this work was to discriminate between two models for glucose-6-phosphatase: one in which the enzyme has its catalytic site oriented toward the lumen of the endoplasmic reticulum, requiring transporters for glucose-6-phosphate, inorganic phosphate (Pi), and glucose (substrate-transport model), and a second one in which the hydrolysis of glucose-6-phosphate occurs inside the membrane (conformational model). We show that microsomes preloaded with yeast phosphoglucose isomerase catalyzed the detritiation of [2-(3)H]glucose-6-phosphate and that this reaction was inhibited by up to 90% by S3483, a compound known to inhibit glucose-6-phosphate hydrolysis in intact but not in detergent-treated microsomes. These results indicate that glucose-6-phosphate is transported to the lumen of the microsomes in an S3483-sensitive manner. Detritiation by intramicrosomal phosphoglucose isomerase was stimulated twofold by 1 mmol/l vanadate, a phosphatase inhibitor, indicating that glucose-6-phosphatase and the isomerase compete for the same intravesicular pool of glucose-6-phosphate. To investigate the site of release of Pi from glucose-6-phosphate, we incubated microsomes with Pb(2+), which forms an insoluble complex with Pi, preventing its rapid exit from the microsomes. Under these conditions, approximately 80% of the Pi that was formed after 5 min was intramicrosomal, compared with <10% in the absence of Pb(2+). We also show that, when incubated with glucose-6-phosphate and mannitol, glucose-6-phosphatase formed mannitol-1-phosphate and that this nonphysiological product was initially present within the microsomes before being released to the medium. These results indicate that the primary site of product release by glucose-6-phosphatase is the lumen of the endoplasmic reticulum.

Molecular genetics of type 1 glycogen storage disease

Glycogen storage disease type 1 (GSD 1) comprises a group of autosomal recessive inherited metabolic disorders caused by deficiency of the microsomal multicomponent glucose-6-phosphatase system. Of the two known transmembrane proteins of the system, malfunction of the catalytic subunit (G6Pase) characterizes GSD 1a. GSD 1 non-a is characterized by defective microsomal glucose-6-phosphate or pyrophosphate/phosphate transport due to mutations in G6PT (glucose-6-phosphate translocase gene) encoding a microsomal transporter protein. Mutations in G6Pase and G6PT account for approximately 80 and approximately 20% of GSD 1 cases, respectively. G6Pase and G6PT work in concert to maintain glucose homeostasis in gluconeogenic organs. Whereas G6Pase is exclusively expressed in gluconeogenic cells, G6PT is ubiquitously expressed and its deficiency generally causes a more severe phenotype. Rapid confirmation of clinically suspected diagnosis of GSD 1, reliable carrier testing, and prenatal diagnosis are facilitated by mutation analyses of the chromosome 11-bound G6PT gene as well as the chromosome 17-bound G6Pase gene.

Overexpression of the P46 (T1) translocase component of the glucose-6-phosphatase complex in hepatocytes impairs glycogen accumulation via hydrolysis of glucose 1-phosphate

The final step of gluconeogenesis and glycogenolysis is catalyzed by the glucose-6-phosphatase (Glc-6-Pase) enzyme complex, located in the endoplasmic reticulum. The complex consists of a 36-kDa catalytic subunit (P36), a 46-kDa glucose 6-phosphate translocase (P46), and putative glucose and inorganic phosphate transporters. Mutations in the genes encoding P36 or P46 have been linked to glycogen storage diseases type Ia and type Ib, respectively. However, the relative roles of these two proteins in control of the rate of glucose 6-phosphate hydrolysis have not been defined. To gain insight into this area, we have constructed a recombinant adenovirus containing the cDNA encoding human P46 (AdCMV-P46) and treated rat hepatocytes with this virus, or a virus encoding P36 (AdCMV-P36), or the combination of both viruses, resulting in large and equivalent increases in expression of the transgenes within 8-24 h of viral treatment. The overexpressed P46 protein was appropriately targeted to hepatocyte microsomes and caused a 58% increase in glucose 6-phosphate hydrolysis in nondetergent-treated (intact) microsomal preparations relative to controls, whereas overexpression of P36 caused a 3.6-fold increase. Overexpression of P46 caused a 50% inhibition of glycogen accumulation in hepatocytes from fasted rats incubated at 25 mm glucose relative to cells treated with a control virus (AdCMV-betaGAL). Furthermore, in hepatocytes from fed rats cultured at 25 mm glucose and then exposed to 15 mm glucose, AdCMV-P46 treatment activated glycogenolysis, as indicated by a 50% reduction in glycogen content relative to AdCMV-betaGAL-treated controls. In contrast, overexpression of P46 had only small effects on glycolysis, whereas overexpression of P36 had large effects on both glycogen metabolism and glycolysis, even in the presence of co-overexpressed glucokinase. Finally, P46 overexpression enhanced glucose 1-phosphate but not fructose 6-phosphate hydrolysis in intact microsomes, providing a mechanism by which P46 overexpression may exert its preferential effects on glycogen metabolism.

Canine model and genomic structural organization of glycogen storage disease type Ia (GSD Ia)

A canine model of glycogen storage disease Ia (GSD Ia), similar clinically, biochemically, and pathologically to the human disease, was established by crossbreeding Maltese and Beagle dogs carrying a mutated, defective glucose-6-phosphatase (G-6-Pase) gene. Ten puppies were born in three litters from these crossbreedings. Six were homozygous for the previously described M121I GSD Ia mutation. Of these six affecteds, two were stillborn, and one died at 2, 32, and 60 days of life, respectively (puppies A, B, C, D, E), while one is alive at age 15 months (puppy F). Affected puppies exhibited tremors, weakness, and neurologic signs when hypoglycemic. They had postnatal growth retardation and progressive hepatomegaly. Biochemical abnormalities included fasting hypoglycemia, hyperlactacidemia, hypercholesterolemia, hypertriglyceridemia, and hyperuricemia. Microscopic examination of tissues from affected puppies showed diffuse, marked hepatocellular vacuolation, with distended clear hepatocytes and central to marginally located rounded nuclei. In the kidneys of puppies D and E, there was segmental glomerular sclerosis and vacuolation of proximal convoluted tubular epithelium. Biochemical analysis revealed increased liver glycogen content and isolated markedly reduced G-6-Pase enzyme activity in liver and kidney. The canine G-6-Pase gene was characterized by screening a canine genomic library. It spans approximately 11.8 kb and consists of five exons with >90% amino acid sequence homology to the derived human sequence. The first 1.5 kb of the 5' region was sequenced and contains several putative response element motifs homologous to the human 5' region. Establishment of this canine colony of GSD Ia that closely resembles human disease and isolation of the canine genomic gene provides an excellent model for studying pathophysiology and long-term complications and an opportunity to develop novel therapeutic approaches such as drug and gene therapy.

Structural requirements for the stability and microsomal transport activity of the human glucose 6-phosphate transporter

Deficiencies in glucose 6-phosphate (G6P) transporter (G6PT), a 10-helical endoplasmic reticulum transmembrane protein of 429 amino acids, cause glycogen storage disease type 1b. To date, only three missense mutations in G6PT have been shown to abolish microsomal G6P transport activity. Here, we report the results of structure-function studies on human G6PT and demonstrate that 15 missense mutations and a codon deletion (delta F93) mutation abolish microsomal G6P uptake activity and that two splicing mutations cause exon skipping. While most missense mutants support the synthesis of G6PT protein similar to that of the wild-type transporter, immunoblot analysis shows that G20D, delta F93, and I278N mutations, located in helix 1, 2, and 6, respectively, destabilize the G6PT. Further, we demonstrate that G6PT mutants lacking an intact helix 10 are misfolded and undergo degradation within cells. Moreover, amino acids 415-417 in the cytoplasmic tail of the carboxyl-domain, extending from helix 10, also play a critical role in the correct folding of the transporter. However, the last 12 amino acids of the cytoplasmic tail play no essential role(s) in functional integrity of the G6PT. Our results, for the first time, elucidate the structural requirements for the stability and transport activity of the G6PT protein.