The molecular basis of the type 1 glycogen storage diseases

Glucose Transport and Transporters in the Endomembranes

Glucose is a basic nutrient in most of the creatures; its transport through biological membranes is an absolute requirement of life. This role is fulfilled by glucose transporters, mediating the transport of glucose by facilitated diffusion or by secondary active transport. GLUT (glucose transporter) or SLC2A (Solute carrier 2A) families represent the main glucose transporters in mammalian cells, originally described as plasma membrane transporters. Glucose transport through intracellular membranes has not been elucidated yet; however, glucose is formed in the lumen of various organelles. The glucose-6-phosphatase system catalyzing the last common step of gluconeogenesis and glycogenolysis generates glucose within the lumen of the endoplasmic reticulum. Posttranslational processing of the oligosaccharide moiety of glycoproteins also results in intraluminal glucose formation in the endoplasmic reticulum (ER) and Golgi. Autophagic degradation of polysaccharides, glycoproteins, and glycolipids leads to glucose accumulation in lysosomes. Despite the obvious necessity, the mechanism of glucose transport and the molecular nature of mediating proteins in the endomembranes have been hardly elucidated for the last few years. However, recent studies revealed the intracellular localization and functional features of some glucose transporters; the aim of the present paper was to summarize the collected knowledge.

Minireview: Genome Editing of Human Pluripotent Stem Cells for Modeling Metabolic Disease

The pathophysiology of metabolic diseases such as coronary artery disease, diabetes, and obesity is complex and multifactorial. Developing new strategies to prevent or treat these diseases requires in vitro models with which researchers can extensively study the molecular mechanisms that lead to disease. Human pluripotent stem cells and their differentiated derivatives have the potential to provide an unlimited source of disease-relevant cell types and, when combined with recent advances in genome editing, make the goal of generating functional metabolic disease models, for the first time, consistently attainable. However, this approach still has certain limitations including lack of robust differentiation methods and potential off-target effects. This review describes the current progress in human pluripotent stem cell-based metabolic disease research using genome-editing technology.

Glucose homeostasis in the newborn

Hepatic glucose production by glycogenolysis and gluconeogenesis is essential to maintain blood glucose levels, and the glucose-6-phosphatase system catalyses the terminal step of both pathways. Developmental delays in the postnatal up-regulation of hepatic glucose-6-phosphatase enzyme activity are common in preterm infants. Two groups of infants have been identified with failure of developmental regulation of glucose homeostasis. Firstly, up to 20% of preterm infants about to be discharged home are at risk of hypoglycaemia if a feed is delayed. Cortisol, corticotrophin and epinephrine levels are higher in the infants with severe and persistent hypoglycaemia, but insulin, glucagon and human growth hormone do not differ from normoglycaemic infants. Secondly, preterm infants with an inadequate glycaemic response to glucagon (30% at the time of discharge home) have relative fasting hyperglycaemia, hyperinsulinaemia, increased insulin:glucagon ratios and a lower insulin sensitivity index. Hormonal dysfunctions in preterm infants may contribute to failures in postnatal expression of hepatic enzymes.

Natural progress of blood glucose in full-term low-grade low-birthweight infants

BACKGROUND

Although various authors have suggested the risk of hypoglycemia in practical medicine for low-birthweight infants is exaggerated, convincing evidence using recent definitions of hypoglycemia is not documented.

METHODS

To evaluate the risk of hypoglycemia in low grade low-birthweight infants (LGLBWI) (2100 g < birthweight < 2500 g) whose only abnormality is low-birthweight, whole blood glucose (BGw) was measured five times (0, 0.5, 1, and 4 h after birth and just before the first bottle feeding) in 49 LGLBWI and 38 normal birthweight infants.

RESULTS

Whole blood glucose was not lower in LGLBWI with a gestational age of 38-40 weeks (GT38LGLBWI) than in normal birthweight individuals with a gestational age of 38-40 weeks at each of the five measuring times. No case of GT38LGLBWI, not even in small for gestational age infants, required treatment for hypoglycemia. The BGw was significantly lower in 37-week gestational age LGLBWI than in GT38LGLBWI at 0.5 h and 1 h after birth (P < 0.05). However, in all cases with low BGw value (below 30 mg/dL at 1 h after birth), BGw value increased naturally to the normal level 1.5 h after birth. No symptoms of hypoglycemia were observed.

CONCLUSIONS

In the care of hypoglycemia in LGLBWI, attention should be paid first to gestational age, namely, tendency to prematurity. In this study, however, no hypoglycemia that required treatment was found among full-term normal LGLBWI, even those who were small for gestational age. Frequent blood glucose measurement for those infants is therefore unnecessary.

Histone 2A stimulates glucose-6-phosphatase activity by permeabilization of liver microsomes

Histone 2A increases glucose-6-phosphatase activity in liver microsomes. The effect has been attributed either to the conformational change of the enzyme, or to the permeabilization of microsomal membrane that allows the free access of substrate to the intraluminal glucose-6-phosphatase catalytic site. The aim of the present study was the critical reinvestigation of the mechanism of action of histone 2A. It has been found that the dose-effect curve of histone 2A is different from that of detergents and resembles that of the pore-forming alamethicin. Inhibitory effects of EGTA on glucose-6-phosphatase activity previously reported in histone 2A-treated microsomes have been also found in alamethicin-permeabilized vesicles. The effect of EGTA cannot therefore simply be an antagonization of the effect of histone 2A. Histone 2A stimulates the activity of another latent microsomal enzyme, UDP-glucuronosyltransferase, which has an intraluminal catalytic site. Finally, histone 2A renders microsomal vesicles permeable to non-permeant compounds. Taken together, the results demonstrate that histone 2A stimulates glucose-6-phosphatase activity by permeabilizing the microsomal membrane.

Identification of two novel and potent competitive inhibitors of the glucose-6-phosphatase catalytic protein

AIM

In this study, we show that inhibitors of the glucose-6-phosphatase (G-6-Pase) catalytic protein could be an alternative approach to the recent G-6-Pase T1-translocase inhibitors to target this enzyme for the treatment of type 2 diabetes.

METHOD

The active enantiomers of 4-methoxyphenyl-[4-(4-methoxyphenyl)-4,5,6,7-tetrahydrothieno[3,2-c]pyridin-5-yl]methanone (Compound A-1) and 4-methoxyphenyl-[4-(4-trifluoromethoxyphenyl)-4,5,6,7-tetrahydrothieno[3,2-c]pyridin-5-yl]methanone (Compound B-1) were characterized as inhibitors of the G-6-Pase catalytic protein using pig and rat liver microsomes and cultured rat hepatocytes.

RESULTS

Both compounds were found to be potent competitive inhibitors of the G-6-Pase catalytic protein obtained from pig and rat liver microsomes. The K(i) values (microM) were calculated to be 0.61 +/- 0.02 and 0.63 +/- 0.08 for compound A-1 and B-1 on intact pig microsomes, and 0.27 +/- 0.02 and 0.29 +/- 0.06 on disrupted pig microsomes. The corresponding values for rat liver microsomes were found to be 3.3 +/- 0.6 and 4.0 +/- 1.2 for compound A-1 and B-1 on intact microsomes, and 1.54 +/- 0.1 and 1.21 +/- 0.1 on disrupted microsomes. Compound A-1 was also able to inhibit pyrophosphatase activities from both intact and disrupted microsomes with equal potency (IC50; 0.43-0.55 microm). Using cultured rat hepatocytes and glycerol as the substrate, these compounds were able to prevent glucose production up to 60% with a concomitant increase in the G-6-P content (2.3-fold) using compound A-1. No increase in glycogen levels was seen.

CONCLUSION

These data demonstrated that these compounds were more potent inhibitors on G-6-Pase obtained from pig microsomes and were able to penetrate the microsomal membrane. The hepatocyte data further support the kinetic data, and are also consistent with the evoked mechanism of action.

Therapeutic insulin and hepatic glucose-6-phosphatase activity in preterm infants

BACKGROUND

Hepatic glucose-6-phosphatase activity is low at birth, and in term infants rises rapidly to adult levels. In contrast, in most preterm infants, it remains low postnatally making them vulnerable to repeated hypoglycaemic episodes, resultant cerebral damage, or risk of sudden and unexpected death.

AIMS

To investigate the clinical features of preterm infants with low glucose-6-phosphatase enzyme activity to determine the influencing factors.

METHODS

Clinical data from 36 preterm infants were correlated by stepwise multiple regression analysis with V(max) of hepatic glucose-6-phosphatase as the dependent variable.

RESULTS

The most significant correlation was with the administration of insulin (units/kg/h postnatal life) with lesser effects of respiratory distress syndrome and dopamine administration. The V(max) changes reflected changes in the level of expression of the glucose-6-phosphatase protein.

CONCLUSION

In a variety of animal models, hepatic glucose-6-phosphatase levels have been shown to decrease in response to insulin, which also decreases transcription of the glucose-6-phosphatase gene. The association of insulin administration with high levels of hepatic glucose-6-phosphatase activity and protein expression was therefore most unexpected. Results from model systems, or adults, must be extrapolated to the metabolism of preterm infants with caution.

New lessons in the regulation of glucose metabolism taught by the glucose 6-phosphatase system

The operation of glucose 6-phosphatase (EC 3.1.3.9) (Glc6Pase) stems from the interaction of at least two highly hydrophobic proteins embedded in the ER membrane, a heavily glycosylated catalytic subunit of m 36 kDa (P36) and a 46-kDa putative glucose 6-phosphate (Glc6P) translocase (P46). Topology studies of P36 and P46 predict, respectively, nine and ten transmembrane domains with the N-terminal end of P36 oriented towards the lumen of the ER and both termini of P46 oriented towards the cytoplasm. P36 gene expression is increased by glucose, fructose 2,6-bisphosphate (Fru-2,6-P2) and free fatty acids, as well as by glucocorticoids and cyclic AMP; the latter are counteracted by insulin. P46 gene expression is affected by glucose, insulin and cyclic AMP in a manner similar to P36. Accordingly, several response elements for glucocorticoids, cyclic AMP and insulin regulated by hepatocyte nuclear factors were found in the Glc6Pase promoter. Mutations in P36 and P46 lead to glycogen storage disease (GSD) type-1a and type-1 non a (formerly 1b and 1c), respectively. Adenovirus-mediated overexpression of P36 in hepatocytes and in vivo impairs glycogen metabolism and glycolysis and increases glucose production; P36 overexpression in INS-1 cells results in decreased glycolysis and glucose-induced insulin secretion. The nature of the interaction between P36 and P46 in controling Glc6Pase activity remains to be defined. The latter might also have functions other than Glc6P transport that are related to Glc6P metabolism.

Glycogen storage diseases. Phenotypic, genetic, and biochemical characteristics, and therapy

The glycogen storage diseases are caused by inherited deficiencies of enzymes that regulate the synthesis or degradation of glycogen. In the past decade, considerable progress has been made in identifying the precise genetic abnormalities that cause the specific impairments of enzyme function. Likewise, improved understanding of the pathophysiologic derangements resulting from individual enzyme defects has led to the development of effective nutritional therapies for each of these disorders. Meticulous adherence to dietary therapy prevents hypoglycemia, ameliorates the biochemical abnormalities, decreases the size of the liver, and results in normal or nearly normal physical growth and development. Nevertheless, serious long-term complications, including nephropathy that can cause renal failure and hepatic adenomata that can become malignant, are a major concern in GSD-I. In GSD-III, the risk for hypoglycemia diminishes with age, and the liver decreases in size during puberty. Cirrhosis develops in some adult patients, and progressive myopathy and cardiomyopathy occur in patients with absent GDE activity in muscle. It remains unclear whether these complications of glycogen storage disease can be prevented by dietary therapy. Glycogen storage diseases caused by lack of phosphorylase activity are milder disorders with a good prognosis. The liver decreases in size, and biochemical abnormalities disappear by puberty.

Chemical modification of rat hepatic microsomes with N-ethylmaleimide results in inactivation of both UDP-N-acetylglucosamine-dependent stimulation of glucuronidation and UDP-glucuronic acid uptake

Chemical modification of rat hepatic microsomes with N-ethylmaleimide (NEM) resulted in inactivation of UDP-N-acetylglucosamine (UDP-GlcNAc)-dependent stimulation of glucuronidation of p-nitrophenol. Inactivation kinetics and pH dependence were in agreement with the modification of a single sulfhydryl group. NEM also inactivated the uptake of UDP-glucuronic acid (UDP-GlcUA) but not UDP-glucose. With various sulfhydryl-modifying reagents, the inactivation of UDP-GlcUA uptake was linked to that of glucuronidation. UDP-GlcUA protected against NEM-sensitive inactivation of both UDP-GlcNAc-dependent stimulation of glucuronidation and UDP-GlcUA uptake, suggesting that the sulfhydryl group is located within or near the UDP-GlcUA binding site of the microsomal protein involved in the stimulation. Using microsomes labeled with biotin-conjugated maleimide and immunopurification with anti-peptide antibody against UDP-glucuronosyltransferase family 1 (UGT1) isozymes, immunopurified UGT1s were found to be labeled with the maleimide and UDP-GlcUA protected against the labeling as it did with the NEM-sensitive inactivation. These data suggest the involvement of a sulfhydryl residue of microsomal protein in the UDP-GlcNAc-dependent stimulation mechanism via the stimulation of UDP-GlcUA uptake into microsomal vesicles.

Peroxyvanadium compounds inhibit glucose-6-phosphatase activity and glucagon-stimulated hepatic glucose output in the rat in vivo

The present investigation was undertaken to characterize the direct inhibitory action of the peroxyvanadium compounds oxodiperoxo(1, 10-phenanthroline) vanadate(V) (bpV(phen)) and oxodiperoxo(pyridine-2-carboxylate) vanadate(V) (bpV(pic)) on pig microsomal glucose-6-phosphatase (G-6-Pase) activity and on glucagon stimulated hyperglycemia in vivo. Both bpV(phen) and bpV(pic) were found to be potent competitive inhibitors of G-6-Pase with Ki values of 0.96 and 0.42 microM (intact microsomes) and 0.50 and 0.21 microM (detergent-disrupted microsomes). The corresponding values for ortho-vanadate were 20.3 and 20.0 microM. Administration of bpV(phen) to postprandial rats did not affect the basal glucose level although a modest and dose-dependent increase in plasma lactate levels was seen. Injection of glucagon raised the plasma glucose level from 5.5 mM to about 7.5 mM in control animals and this increase could be prevented dose-dependently by bpV(phen). The inhibition of the glucagon-mediated blood glucose increase was accompanied by a dose-dependent increase in plasma lactate levels from 2 mM to about 11 mM. In conclusion, the finding that vanadate and bpV compounds are potent inhibitors of G-6-Pase suggests that the blood-glucose-lowering effect of these compounds which is seen in diabetic animals may be partly explained by a direct effect on this enzyme rather than, as presently thought, being the result of inhibition of phosphoprotein tyrosine phosphatases and thereby insulin receptor dephosphorylation.

Failure to detect preterm infants at risk of hypoglycemia before discharge

In a series of 79 consecutive preterm infants who were ready for discharge, 14 (18%) infants were unable to maintain normal concentrations of blood glucose. This finding suggests that a significant number of preterm infants are at risk of hypoglycemia at home if a feed is omitted or delayed.

Structure and mutation analysis of the glycogen storage disease type 1b gene

Glycogen storage disease (GSD) 1b is the deficiency of endoplasmic reticulum glucose-6-phosphate (G6P) transport. We here report the structure of the gene encoding a protein likely to be responsible for G6P transport, and its mapping to human chromosome 11q23.3. The gene is composed of nine exons spanning a genomic region of approximately 4 kb. Primers based on the genomic sequence were used in single strand conformation polymorphism (SSCP) analysis and mutations were found in six out of seven GSD 1b patients analysed.

A different isoform of the transport protein mutated in the glycogen storage disease 1b is expressed in brain

There are differences in the kinetic properties of the liver and brain microsomal glucose-6-phosphate transport systems suggesting the possibility of tissue specific isoforms. The availability of a human liver cDNA sequence which is mutated in patients with deficiencies of liver microsomal glucose-6-phosphate transport (glycogen storage disease 1b) made it possible to determine if a brain isoform exists. Northern blots of liver and brain RNA revealed that the mRNA of the brain form is slightly longer than the liver one. Isolation and sequencing of the respective human brain cDNA revealed that the brain protein has an additional 22 amino acid sequence.

Three thiol groups are important for the activity of the liver microsomal glucose-6-phosphatase system. Unusual behavior of one thiol located in the glucose-6-phosphate translocase

Liver microsomal glucose-6-phosphatase (Glc-6-Pase) is a multicomponent system involving both substrate and product carriers and a catalytic subunit. We have investigated the inhibitory effect of N-ethylmaleimide (NEM), a rather specific sulfhydryl reagent, on rat liver Glc-6-Pase activity. Three thiol groups are important for Glc-6-Pase system activity. Two of them are located in the glucose-6-phosphate (Glc-6-P) translocase, and one is located in the catalytic subunit. The other transporters (phosphate and glucose) are not affected by NEM treatment. The NEM alkylation of the catalytic subunit sulfhydryl residue is prevented by preincubating the disrupted microsomes with saturating concentrations of substrate or product. This suggests either that the modified cysteine is located in the protein active site or that substrate binding hides the thiol group via a conformational change in the enzyme structure. Two other thiols important for the Glc-6-Pase system activity are located in the Glc-6-P translocase and are more reactive than the one located in the catalytic subunit. The study of the NEM inhibition of the translocase has provided evidence of the existence of two distinct areas in the protein that can behave independently, with conformational changes occurring during Glc-6-P binding to the transporter. The recent cloning of a human putative Glc-6-P carrier exhibiting homologies with bacterial phosphoester transporters, such as Escherichia coli UhpT (a Glc-6-P translocase), is compatible with the fact that two cysteine residues are important for the bacterial Glc-6-P transport.

Glycogen storage diseases and the liver

Carbohydrate metabolism in the liver is responsible for plasma glucose homeostasis. Liver glycogen storage diseases are metabolic disorders which result in abnormal storage amounts and/or forms of glycogen, and often (but not always) have hepatomegaly and hypoglycaemia as presenting features. To understand the clinical complexity of the glycogen storage diseases, it is necessary to understand the properties and regulation of the proteins involved in glycogen metabolism. Advances in treatment have greatly improved metabolic control and hence the quality of life and survival. However, the lack of understanding of the molecular basis of some of the clinical features of glycogen storage diseases makes it difficult logically to devise optimal treatment regimens to prevent some of the long-term complications. Recently, molecular biology has greatly advanced our understanding of the proteins and genes involved in liver glycogen metabolism and has led to better and less invasive methods of diagnosis of these disorders.

Liver microsomal transport of glucose-6-phosphate, glucose, and phosphate in type 1 glycogen storage disease

The transport of glucose-6-phosphate (G6P), glucose, and orthophosphate into liver microsomes, isolated from six patients with various subtypes of type 1 glycogen storage disease (GSD), was measured using a light-scattering method. We found that G6P, glucose, and phosphate could all cross the microsomal membrane, in four cases of type 1a GSD. In contrast, liver microsomal transport of G6P and phosphate was deficient in the GSD 1b and 1c patients, respectively. These results support the involvement of multiple proteins (and genes) in GSD type 1. The results obtained with the light-scattering method are in accordance with conventional kinetic analysis of the microsomal glucose-6-phosphatase system. Therefore, this technique could be used to directly diagnose type 1b and 1c GSD.

Glycogen storage disease: a basic understanding and guide to nursing care

Glycogen storage disease (GSD) is a group of genetic metabolic disorders resulting from a defect in the synthesis or degradation of glycogen. GSDs are classified according to the type of enzymatic defect and primary organs involved and are usually diagnosed in infancy or early childhood. Little is generally known about GSD and nurses may be unfamiliar with care requirements of the GSD patient. Nine distinct types of GSD are presented with the primary focus on type 1, the most common. Current treatment methods, roles of the nurse, and a sample nursing care plan are included.

Hepatic glucose-6-phosphatase development in preterm and full-term guinea-pigs: comparison with rat and human development

In term infants, hepatic glucose-6-phosphatase activity rises several-fold in the first few days after birth. In contrast, in many preterm infants, the postnatal rise in activity does not occur and the abnormally low levels can persist. In an attempt to determine if prematurity causes long-term changes in levels of glucose-6-phosphatase in liver of all mammals, we have studied the ontogeny of glucose-6-phosphatase in term guinea-pigs, and also in guinea-pigs delivered prematurely by Caesarean section. The activity of hepatic glucose-6-phosphatase in preterm guinea-pigs is about 5-fold lower than in term guinea-pigs at birth but the activity rises rapidly to very similar values to those found in term guinea-pigs. This indicates that prematurity alone does not result in abnormal development of hepatic glucose-6-phosphatase activity in guinea pigs. The changes in liver glucose-6-phosphatase activity in the postnatal period in term guinea-pigs were much smaller than those occurring in term postnatal rats or term infants.

Low levels of glucose-6-phosphate hydrolysis in the sarcoplasmic reticulum of skeletal muscle: involvement of glucose-6-phosphatase

Glucose-6-phosphate hydrolysis was measured in a fraction obtained from rabbit fast-twitch skeletal muscle and corresponding to total sarcoplasmic reticulum, as well as in three subfractions containing longitudinal tubules, terminal cisternae or both structures. In all cases the levels of hydrolysis measured both in native and disrupted membranes were approximately 60-100 times lower than the microsomal glucose-6-phosphatase activity of the corresponding livers. In contrast to liver microsomes, most (up to 80%) of the glucose-6-phosphate hydrolysing activity in muscle sarcoplasmic reticulum membranes was not inactivated by pH 5.0 pre-incubation indicating that it was not catalysed by the specific glucose-6-phosphatase enzyme. Osmotically induced changes in light-scattering intensity of sarcoplasmic reticulum vesicles revealed that, in contrast to liver microsomes, sarcoplasmic reticulum vesicles were not selectively permeable to glucose-6-phosphate as mannose-6-phosphate was also permeable and in addition they were poorly permeable to glucose. Immunoblot experiments using antibodies raised against the glucose-6-phosphatase enzyme, and liver endoplasmic reticulum glucose and Pi translocases, failed to detect the presence of these protein components in sarcoplasmic reticulum membranes. Southern blot analysis of reverse transcriptase-polymerase chain reaction products from rat muscle revealed that glucose-6-phosphatase mRNA is present in muscle. Quantification of Northern blot analysis of liver and muscle mRNA indicated that muscle contains less than 2% of the amount of glucose-6-phosphate mRNA found in corresponding livers. We conclude that very low levels of specific glucose-6-phosphatase (e.g. as in liver; E.C. 3.1.3.9) are present in muscle sarcoplasmic reticulum and that the muscle and liver glucose-6-phosphatase systems have several different properties.

Endoplasmic reticulum phosphate transport

The major role of the liver endoplasmic reticulum phosphate/pyrophosphate transport proteins is the regulation of blood glucose levels. The glucose-6-phosphatase enzyme is an endoplasmic reticulum enzyme system which hydrolyzes glucose-6-phosphate to glucose and phosphate. Glucose-6-phosphatase is the terminal step of both gluconeogenesis and glycogenolysis. The glucose-6-phosphatase enzyme is a very hydrophobic membrane protein and its active site is inside the lumen of the endoplasmic reticulum. The substrates and products of the enzyme therefore have to cross the endoplasmic reticulum membrane. The glucose-6-phosphatase enzyme is associated with a calcium binding protein (SP). There are also transport proteins for the substrate glucose-6-phosphate (T1) and the products phosphate (T2) and glucose (T3). There appear to be at least two different liver endoplasmic reticulum proteins that can transport phosphate. One of the proteins T2b can also transport pyrophosphate and carbamyl phosphate which are also substrates for the glucose-6-phosphatase enzyme. The metabolic regulation, genetic deficiencies, ontogeny and tissue distribution of the endoplasmic reticulum T2 proteins will be described.

Cloning and sequencing of the 5' region of the human glucose-6-phosphatase gene: transcriptional regulation by cAMP, insulin and glucocorticoids in H4IIE hepatoma cells

We have cloned and sequenced the first 1.2 kb of the 5' region of the human glucose-6-phosphatase gene. Transfection of H4IIE hepatoma cells with the 1.2 kb fragment fused to a luciferase reporter gene demonstrated both basal and hormone responsive luciferase activity. Dexamethasone increased and insulin decreased luciferase activity. Insulin and dibutyryl cyclic AMP both significantly decreased activity in the presence of dexamethasone.

The human embryonic-fetal kidney endoplasmic reticulum phosphate-pyrophosphate transport protein

Glucose-6-phosphatase is a multicomponent endoplasmic reticulum system comprising at least six different proteins, including a lumenal enzyme and several transport proteins. One of the transport proteins, T2beta, transports the substrate pyrophosphate and the product phosphate and its genetic deficiency is termed type 1c glycogen storage disease. We have used anti-T2beta antibodies for immunohistochemistry with image analysis and kinetic analysis of the glucose-6-phosphatase system to study for the temporal and spatial development of T2beta in human embryonic and fetal kidney. In metanephric kidney, there is an early predominance of T2beta expression in the ureteric bud derivatives and this changes with ontogeny such that developing nephrons, particularly proximal tubules, become dominant by mid-gestation. T2beta has the same spatial and temporal pattern as the glucose-6-phosphatase enzyme in both mesonephric and metanephric kidney. Pyrophosphate transport capacity is appropriate for the amount of glucose-6-phosphatase activity present in mid-gestation fetal kidney, in contrast to liver, where pyrophosphate transport capacity is developmentally delayed. Increasing knowledge of the temporal and spatial expression of the glucose-6-phosphatase proteins and their catalytic roles in early human development is essential for the elucidation of the aetiology of renal disease in both type I glycogen storage diseases and the developmental disorders of the glucose-6-phosphatase system.

The glucose-6-phosphatase enzyme in developing human trachea and oesophagus

Glucose-6-phosphatase is an endoplasmic reticulum system which is found primarily in liver and kidney. Recently, it has become clear that it is also present in lower amounts in a variety of other tissues. Previous histochemical studies of glucose-6-phosphate hydrolysis in trachea have given equivocal results and only one study on adult oesophagus has shown glucose-6-phosphatase, enzymatic activity but without cellular localization. We have now shown, using microassay techniques, that microsomes isolated from human foetal trachea and oesophagus both contain low levels of specific glucose-6-phosphatase activity (mean = 0.9 and 1.5 nmol min-1 mg-1 microsomal protein, respectively) which are less than 10% of the levels in microsomes of human foetal liver of similar age. In the developing trachea, glucose-6-phosphatase immunoreactivity has been found, using a monospecific antibody to the catalytic subunit of the glucose-6-phosphatase enzyme, to be first present at 10-11 weeks' gestation, and thereafter in foetal life, predominantly present in ciliated cells, with smaller amounts in non-ciliated secretory cells, duct lining cells, and occasional basal cells. The foetal oesophageal epithelium is transiently ciliated from 10 to 11 weeks' gestation, but ciliated cells are gradually replaced by squamous cells from 14 to 16 weeks onwards. Glucose-6-phosphatase immunoreactivity in human foetal oesophagus is predominantly confined to ciliated cells, but non-ciliated luminal cells are also reactive, as are occasional basal cells. Mucus secretory cells in foetal trachea and oesophagus are immunonegative, as is the entire epithelium of both organs in the embryo (up to 56 postovulatory days.

Glomerular and tubular function in glycogen storage disease

Urinary protein and calcium excretion were assessed in 77 patients with the hepatic glycogen storage diseases (GSD): 30 with GSD-I (median age 12.4 years, range 3.2-32.9 years), 25 with GSD-III (median age 10.5 years, range 4.2-31.3 years) and 22 with GSD-IX (median age 11.8 years, range 1.2-35.4 years). Inulin (Cinulin) and para-aminohippuric acid (CPAH) clearances were also measured in 33 of these patients. Those with GSD-I had significantly greater albumin (F = 15.07, P < 0.001), retinol-binding protein (RBP) (F = 14.66, P < 0.001), N-acetyl-beta-D-glucosaminidase (NAG) (F = 9.41, P < 0.001) and calcium (F = 7.41, P = 0.001) excretion than those with GSD-III and GSD-IX. GSD-I patients (n = 18) also had significantly higher Cinulin (F = 5.57, P = 0.009), but CPAH did not differ (F = 0.77, NS). Renal function was normal in GSD-III and GSD-IX patients. In GSD-I, Cinulin (r = -0.51, P = 0.03) and NAG excretion (r = -0.40, P = 0.03) were inversely correlated with age, whereas albumin excretion was positively correlated with age (r = +0.41, P = 0.03). RBP and calcium excretion were generally high throughout all age groups. Hyperfiltration in GSD-I is associated with renal tubular proteinuria that occurs before the onset of significant albuminuria. Deficiency of glucose-6-phosphatase within the proximal renal tubule may primarily cause tubular dysfunction, glomerular hyperfiltration being a secondary phenomenon.

The in vivo regulation of hepatic and renal glucose-6-phosphatase by thyroxine

The hepatic and renal microsomal glucose-6-phosphatase enzymes are situated with their active site in the lumen of the endoplasmic reticulum and for normal enzyme activity in vivo transport systems are needed for the substrates and products of the enzyme. We have shown that thyroxine activates the kidney glucose-6-phosphatase enzyme and the liver glucose 6-phosphate transport systems. In contrast, in hypophysectomised and adrenalectomised animals, thyroxine activates the transport systems and the enzyme in both liver and kidney.

Evidence for glucuronide (small molecule) sorting by human hepatic endoplasmic reticulum

The entry of substrates into, and the export of glururonides from, the lumen of hepatic endoplasmic reticulum (ER) in vitro (sealed microsomes) has been measured using radioactivity-labelled materials and a rapid filtration assay. Analysis of liver microsomes from a jaundiced patient showed the accumulation of bilirubin glucuronides within the lumen of the ER. Further analysis of these hepatic microsomes revealed that newly synthesized 1-naphthol glucuronide could exit from the microsomes whereas bilirubin glucuronide was accumulated within the microsomes. These results suggest the existence of mechanisms for the sorting of small molecules, destined for export through bile canalicular or basolateral plasma membranes, by ER. Furthermore, these sorting processes may be regulated by specific transporters within the ER.

The ontogeny of the glucose-6-phosphatase enzyme in human embryonic and fetal red blood cells

We have shown for the first time that the microsomal glucose-6-phosphatase enzyme protein is present in human embryonic and fetal red blood cells and the ontogeny of its expression has been determined. In the earliest embryos, red cells are predominantly of the primitive megaloblastic type. Circulating red cells in the primitive megaloblastic series are predominantly nucleated and glucose-6-phosphatase immunopositive. Non-nucleated, immunoreactive megaloblastic cells are in a minority. In fetuses > 12 weeks gestation, the erythrocytes are of the definitive normoblastic series and in the transitional period of switch-over in late embryonic-early fetal life, up to 30% of glucose-6-phosphatase immunopositive cells are definitive normoblastic in type, with a variable contribution from nucleated and non-nucleated cells. Thereafter, the number of immunopositive cells in the definitive normoblastic series decreases such that after 12 weeks gestation it is less than 5%. The fact that a predominantly hepatic protein in adults (glucose-6-phosphatase) is present in embryonic and fetal red blood cells, particularly nucleated red cells, raises the possibility of diagnosis of disorders of liver protein expression in nucleated fetal red cells isolated from the first trimester maternal circulation.

Bone mineralisation in type 1 glycogen storage disease

Radial bone mineral content (BMC) was measured using single photon absorptiometry in 11 prepubertal children, aged 3.4-12.6 years, with glycogen storage disease type 1 (GSD-1), 2 of whom were receiving granulocyte colony stimulating factor (G-CSF) therapy for chronic neutropenia. Patients were short (median height SD score -1.35, range -3.74 to -0.27), and had reduced BMC Z scores (median 1.79, range -6.35 to +0.27) and radial bone width Z scores (median -0.72, range -2.00 to +0.68). Those receiving G-CSF did not differ significantly from the rest of the group. Generally dietary calcium intake was low and urinary calcium excretion increased. Urinary lactate excretion was high but did not correlate with BMC Z scores. Factors regulating bone metabolism (parathyroid hormone and 25-hydroxy vitamin D concentrations) and markers of bone formation (osteocalcin and skeletal alkaline phosphatase) were not increased implying that there was no compensation for increased bone resorption.

CONCLUSION

Patients with GSD-1 may be at increased risk of fracture in later life and require close attention to metabolic control and calcium balance.

Fatty acyl-CoA esters inhibit glucose-6-phosphatase in rat liver microsomes

In native rat liver microsomes glucose 6-phosphatase activity is dependent not only on the activity of the glucose-6-phosphatase enzyme (which is lumenal) but also on the transport of glucose-6-phosphate, phosphate and glucose through the respective translocases T1, T2 and T3. By using enzymic assay techniques, palmitoyl-CoA or CoA was found to inhibit glucose-6-phosphatase activity in intact microsomes. The effect of CoA required ATP and fatty acids to form fatty acyl esters. Increasing concentrations (2-50 microM) of CoA (plus ATP and 20 microM added palmitic acid) or of palmitoyl-CoA progressively decreased glucose-6-phosphatase activity to 50% of the control value. The inhibition lowered the Vmax without significantly changing the Km. A non-hydrolysable analogue of palmitoyl-CoA also inhibited, demonstrating that binding of palmitoyl-CoA rather than hydrolysis produces the inhibition. Light-scattering measurements of osmotically induced changes in the size of rat liver microsomal vesicles pre-equilibrated in a low-osmolality buffer demonstrated that palmitoyl-CoA alone or CoA plus ATP and palmitic acid altered the microsomal permeability to glucose 6-phosphate, but not to glucose or phosphate, indicating that T1 is the site of palmitoyl-CoA binding and inhibition of glucose-6-phosphatase activity in native microsomes. The type of inhibition found suggests that liver microsomes may comprise vesicles heterogeneous with respect to glucose-6-phosphate translocase(s), i.e. sensitive or insensitive to fatty acid ester inhibition.

Multiple transport protein defects in a patient with glycogen storage disease type 1: GSD 1b/1c beta

A male child presented at 5 months of age with vomiting, diarrhoea, hypoglycaemia and hepatomegaly. Histology on a frozen liver biopsy suggested glycogen storage disease (GSD), while biochemical analyses confirmed an elevated glycogen content and normal activities of the GSD enzymes with the proviso that a variant of GSD 1 should be considered. The patient presented at 9 months of age with severe lactic acidosis and hypoglycaemia. A glucagon tolerance test and galactose load test on the patient produced no glycaemic response. A second biopsy was obtained and appropriately handled for the investigation of variants of the glucose-6-phosphatase enzyme (G6Pase) complex. Results showed that the patient had a deficiency of two transport proteins of the G6Pase complex, namely glucose-6-phosphate translocase and pyrophosphate translocase, i.e. GSD 1b/1c beta. These results were confirmed by additional kinetic analyses which provided confirmation of the double translocase deficiency. Evidence for inhibitors to these translocases was not found. The patient's treatment has resulted in the hypoglycaemia now being well controlled; however, at 3 years of age, height and weight are markedly lagging and he is moderately developmentally delayed. Neutropenia has not been found and neutrophil function is normal. Double enzyme deficiencies are very rare and possible explanations which might lead to this phenotype are considered. This, to the authors' knowledge, is the first report of a double translocase deficiency causing GSD type 1.

Glucose-6-phosphatase proteins of the endoplasmic reticulum

Hepatic glucose-6-phosphatase (G-6-Pase) catalyses the terminal step of hepatic glucose production and it plays a key role in the maintenance of blood glucose homeostasis. Hepatic G-6-Pase is an integral resident endoplasmic reticulum (ER) protein and it is part of a multicomponent system. Its active site is situated inside the lumen of the ER and transport proteins are needed to allow its substrates, glucose-6-phosphate (G-6-P) (and pyrophosphate), and its products, phosphate and glucose to cross the ER membrane. In addition, a calcium-binding protein is also associated with the G-6-Pase enzyme. Recent immunological studies have shown that G-6-Pase (which has conventionally been thought to be present only in the gluconeogenic organs) is present in minor cell types in a variety of human tissues and that its distribution changes dramatically during human development. In all the tissues, enzymatic analysis, direct transport assays and/or immunological detection of the ER glucose and phosphate transport proteins have been used to demonstrate the presence and activity of the whole G-6-Pase system. The G-6-Pase protein is very hydrophobic and has proved difficult to purify to homogeneity. Four proteins of the system have now been isolated and polyclonal antibodies have been raised against them; two have also been cloned. The available sequences, together with topological studies, have given some information about both the topology of the proteins in the ER and the probable mechanisms by which the proteins are retained in the ER.

Hyperlipidaemia does not impair vascular endothelial function in glycogen storage disease type 1a

Patients with glycogen storage disease type 1 (GSD-1) often have marked hyperlipidaemia with abnormal lipoprotein profiles. This metabolic abnormality improves, but is not fully corrected, with dietary therapy and therefore these patients may be at high risk for the development of atherosclerosis. Endothelial dysfunction is an early event in atherogenesis and can be detected in children and young adults at high risk. We studied endothelial function, using a non-invasive ultrasonographic method, in the brachial arteries of 6 adult GSD-1a patients (aged 23-33 years) with mean cholesterol of 7.9 mmol/l (range 4.7 to 14.6) and mean triglycerides of 9.1 mmol/l (range 4.1 to 21.3), and 12 age- and sex-matched normolipidaemic controls. Flow-mediated (endothelium-dependent) dilation was similar in patients and controls (8.2% vs. 10.5%; P = 0.20). Although the patient numbers are small, these results are consistent with the surprising lack of clinically evident atherosclerosis in GSD-1. The reasons these patients appear less susceptible to the damaging arterial effects of hyperlipidaemia are unknown. These results may have implications for others with secondary hyperlipidaemias.

Immunohistochemical localisation of glucose-6-phosphatase in developing human kidney

The objective of our study was to determine the cellular localisation of glucose-6-phosphatase in developing human kidney using monospecific antiserum and a standard immunohistochemical method (peroxidase-antiperoxidase, PAP) on formalin fixed and paraffin embedded tissue. In embryonic and early fetal development of the metanephric kidney, glucose-6-phosphatase is located primarily in derivatives of the ureteric bud such as the pelvis, calyces and collecting ducts. In mid-fetal life as nephrons evolve and develop they become increasingly immunoreactive to glucose-6-phosphatase, such that in mature metanephric kidney the proximal tubules are highly reactive for glucose-6-phosphatase with other elements of the nephron also immunopositive albeit at lower reactivities. In addition the parietal layer of Bowman's capsule and some cells of the visceral layer are immunopositive. Only with the development of nephrons does the early predominance of glucose-6-phosphatase immunoreactivity to ureteric bud derivatives change: in mature kidney the reactivity in the collecting ducts is a small proportion of the total. In proximal tubular cells the distribution of glucose-6-phosphatase immunoreactivity is relatively uniform throughout development in contrast to collecting ducts where in fetal life this reactivity is displaced to the apices and basal areas by intracellular glycogen deposits. The mesonephric kidney has a similar pattern of glucose-6-phosphatase immunoreactivity to that of metanephric kidney. The availability of monospecific antiserum to glucose-6-phosphatase and immunohistochemical methods now allows an alternative approach to cellular localisation. Many of the difficulties in the fixation of tissue and assay of glucose-6-phosphatase activity inherent in conventional histochemical methods are avoided by such methods.

Reliability of histological criteria in glycogen storage disease of the liver

The histological criteria for the diagnosis of the hepatic glycogen storage diseases (GSDs) are well recognized. However, some biopsies do not have the characteristic features peculiar to their type and not all biopsies with GSD changes are confirmed by enzyme analysis. We reviewed the liver biopsies of 59 patients with clinically suspected GSD. The enzyme defects in 31 of 40 patients with GSD morphology were demonstrated by enzyme analysis. We describe the history and histology of the 9 patients with GSD morphology not confirmed by enzyme analysis, present the diagnoses of the 19 patients shown not to have a GSD, and evaluate the reliability of the morphological criteria used to distinguish the types of hepatic GSD. In this study the predictive value of a biopsy with GSD changes was 90%. Mosaicism, the most sensitive criterion in the diagnosis of GSD, is not type-specific. Fibrosis does not reliably distinguish between the GSD types and although nuclear hyperglycogenation and lipid are characteristic of type I GSD, these features are not diagnostic of any particular enzyme deficiency. The lack of morphological specificity implies that a complete enzyme analysis be performed on each biopsy. A normal enzyme analysis does not exclude a GSD and careful long-term follow-up may be necessary.

The hepatic glucose-6-phosphatase system in Ehrlich-ascites-tumour-bearing mice

To examine the effects of the presence of Ehrlich ascites tumours on both the catalytic unit and the substrate/product translocase components of the glucose-6-phosphatase system in vivo, we isolated microsomes from the livers of control and tumour-bearing mice. Samples were analysed immunochemically for the quantity of catalytic unit, stabilizing protein and translocases T2 and T3 proteins. In comparison experiments, a variety of kinetic studies were performed. The most striking findings in tumour-bearing mice were: a 2.5-fold increase in the quantity of translocase T2 protein; increases in the Km and Vmax. for glucose 6-phosphate phosphohydrolase; and a decrease in the Km value for carbamoyl phosphate (carbamoyl-P) of carbamoyl-P:glucose phosphotransferase, all with intact microsomes. The percentage latency at Vmax. decreased for PPi phosphohydrolase and for glucose 6-phosphate phosphohydrolase, but was unaffected for carbamoyl-P:glucose phosphotransferase. These observations support a tumour-related increase in translocase T2 capacity in vivo, as it transports Pi from the microsomal lumen to the medium and carbamoyl-P or PPi from the medium to the microsomal lumen.