Molecular mechanisms governing offspring metabolic programming in rodent models of in utero stress

The results of different human epidemiological datasets provided the impetus to introduce the now commonly accepted theory coined as 'developmental programming', whereby the presence of a stressor during gestation predisposes the growing fetus to develop diseases, such as metabolic dysfunction in later postnatal life. However, in a clinical setting, human lifespan and inaccessibility to tissue for analysis are major limitations to study the molecular mechanisms governing developmental programming. Subsequently, studies using animal models have proved indispensable to the identification of key molecular pathways and epigenetic mechanisms that are dysregulated in metabolic organs of the fetus and adult programmed due to an adverse gestational environment. Rodents such as mice and rats are the most used experimental animals in the study of developmental programming. This review summarises the molecular pathways and epigenetic mechanisms influencing alterations in metabolic tissues of rodent offspring exposed to in utero stress and subsequently programmed for metabolic dysfunction. By comparing molecular mechanisms in a variety of rodent models of in utero stress, we hope to summarise common themes and pathways governing later metabolic dysfunction in the offspring whilst identifying reasons for incongruencies between models so to inform future work. With the continued use and refinement of such models of developmental programming, the scientific community may gain the knowledge required for the targeted treatment of metabolic diseases that have intrauterine origins.

Macro preemies: no thresholds for risks or concerns

PURPOSE OF REVIEW

To provide an overview of the literature regarding medical and developmental risks for moderate to late preterm infants (32-36 weeks gestation), with particular attention to the pediatrician's role in care during both inpatient and outpatient periods.

RECENT FINDINGS

Although the risks of medical issues and developmental delays decrease with increasing gestational age, research suggests that infants born after 32 weeks' gestation often exhibit significant morbidities associated with prematurity. These infants, often referred to as 'macro preemies', have been found to be at a greater risk for medical complications secondary to immature organ systems including impairments in temperature regulation, respiratory functioning, feeding coordination, bilirubin excretion, glucose control, and infection susceptibility. Recent studies of macro preemies also suggest a higher incidence of significant deficits noted in gross and fine motor skills, speech and communication, and learning and behavior compared to their full-term counterparts. Without careful attention from birth, macro preemie infants could be susceptible to both medical issues and developmental delays.

SUMMARY

Physicians should be aware of the research regarding increased medical and developmental risks for all infants born before term in order to provide their patients with comprehensive medical and neurodevelopmental follow-up care.

Glucose-6-phosphatase (G6PC1) promoter polymorphism associated with glycogen storage disease type 1a among the Indian population

Promoter polymorphism rs559748047 inG6PC1from GSD-1a among Indian population.

Neonatal liver physiology

In the neonate, the liver is relatively immature and undergoes several changes in its functional capacity during the early postnatal period. The essential liver functions can be classified into three categories: metabolism, detoxification, and bile synthesis. In general, the immature liver function has limited consequences on the healthy term neonate. However, preterm neonates are particularly susceptible to the effects of the immature liver function placing them at risk of hypoglycemia, hyperbilirubinemia, cholestasis, bleeding, and impaired drug metabolism. An appreciation of the dynamic changes in liver function during the neonatal period is essential for successful management of neonates who require medical and surgical interventions. This review will focus on the neonatal liver function as well as the changes that the liver undergoes as it matures.

Energy metabolism in the newborn farm animal with emphasis on the calf: endocrine changes and responses to milk-born and systemic hormones

Neonatal mammals need adaption to changes in nutrient supply because energy intake shifts from continuous parenteral supply of nutrients (mainly glucose, lactate, and amino acids) via the placenta to discontinuous colostrum and milk intake with lactose and fat as main energy sources. Besides ingested lactose, endogenous glucose production is essential in the neonate to assure sufficient glucose availability. Fetal endogenous glucose production is low, but endocrine changes (especially the prenatal rise of glucocorticoid production) promote maturation of metabolic pathways that enable marked glycogen synthesis before and enhanced gluconeogenesis after birth to establish an adequate glucose status during postnatal maturation. In preterm born farm animals gluconeogenic activity is low, mainly because of a low glucocorticoid and thyroid status. In full-term neonates, endogenous glucose production increases with age. Colostral bioactive components (such as growth factors, hormones, bioactive peptides, and cytokines) do not have a direct effect on endogenous glucose production. However, colostrum feeding stimulates intestinal growth and development, an effect at least in part mediated by bioactive substances. Increased nutrient and glucose absorption thus allows increased glucose supply and hepatic glycogen storage, which improves the glucose status. The improved energetic status of colostrum-fed neonates is reflected by an accelerated maturation of the somatotropic axis, leading especially to enhanced production of IGF-I in the neonate. Secretion and production of hormones involved in the regulation of glucose and fat metabolism in neonates depend on the developmental stage and the response to feeding. In addition, many such hormones have actions in the neonate that differ from adult animals. Endocrine action to support endogenous energy supply in neonates is probably not fully established, and therefore, needs postnatal maturation. Therefore, our knowledge on energy metabolism in the neonate needs to be extended to better understand the function and the failure and to assess endocrine responses during the neonatal period.

Functional analysis of the 5' flanking region of the human G6PC3 gene: regulation of promoter activity by glucose, pyruvate, AMP kinase and the pentose phosphate pathway

G6PC3 is a widely expressed isoform of glucose-6-phosphatase, found in many foetal and adult tissues. Mutations in this gene cause developmental abnormalities and severe neutropenia due to abolition of glucose recycling between the cytoplasm and endoplasmic reticulum. Low G6PC3 expression as a result of promoter polymorphisms or dysregulation could produce similar outcomes. Here we investigated the regulation of human G6PC3 promoter activity. HeLa and H4IIE cells were transiently transfected with G6PC3 promoter coupled to the firefly luciferase gene, and promoter activity was measured by dual luciferase assay. Activity was highest in a 453 bp segment of the G6PC3 promoter, from -455 to -3 relative to the transcriptional start site. This promoter was unresponsive to glucostatic hormones. Its activity increased significantly between 1 and 5.5 mM glucose, and was not elevated further by glucose concentrations up to 25 mM. Pyruvate increased its activity, but β-hydroxybutyrate and sodium acetate did not. Promoter activity was reduced by inhibitors of hexokinase, glyceraldehyde phosphate dehydrogenase and the oxidative branch of the pentose phosphate pathway, but not by a transketolase inhibitor. Deletion of two adjacent Enhancer-boxes (-274 to -279 and -299 to -304) reduced promoter activity and abolished the glucose effect, suggesting they could function as a glucose response element. Deletion of an additional downstream 140 bp (-140 to -306) restored activity, but not the glucose response, suggesting the presence of repressor elements in this region. 5-Aminoimidazole-4-carboxamide 1-β-D-ribofuranoside (AICAR) reduced promoter activity, showing dependence on AMP-kinase. Regulation of the G6PC3 promoter is thus radically different to that of the hepatic isoform, G6PC. It is sensitive to carbohydrate, but not to fatty acid metabolites, and at much lower physiological concentrations. Based on these findings, we speculate that reduced G6PC3 expression could occur during hypoglycemic episodes in vivo, which are common in utero and in the postnatal period. If such episodes lower G6PC3 expression they could place the foetus or infant at risk of impaired immune function and development, and this possibility requires further examination both in vitro and in vivo.

Genetic variation in hepatic glucose-6-phosphatase system genes in cases of sudden infant death syndrome

Genetic deficiencies of the hepatic glucose-6-phosphatase system, either of the enzyme (G6PC1) or of the glucose-6-phosphate transporter (G6PT1), result in fasting hypoglycaemia. Low hepatic G6PC1 activities were previously reported in a few term sudden infant death syndrome (SIDS) infants and assumed to be due to G6PC1 genetic deficiencies. In preterm infants, failures of postnatal activation of G6PC1 expression suggest disordered development as a novel cause of decreased G6PC1 activity in SIDS. G6PC1 and G6PT1 functional and mutational analysis was investigated in SIDS and non-SIDS infants. G6PC1 hepatic activity was abnormally low in 98 SIDS (preterm, n=13; term, n=85), and non-SIDS preterm infants (n=35) compared to term non-SIDS infants (n=29) and adults (n=9). Mean glycogen levels were elevated, except in term non-SIDS infants. A novel G6PT1 promoter polymorphism, 259C --> T was found; the - 259*T allele frequency was greater in term SIDS infants (n=140) than in term control infants (n=119) and preterm SIDS infants (n=30). Heterozygous and homozygous prevalence of 259C --> T was 38.6% and 7.1%, respectively, in term SIDS infants. In cell-based expression systems, the presence of - 259T in the promoter decreased basal luciferase activity by 3.2-fold compared to - 259C. Glucose-6-phosphatase latency in hepatic microsomes was elevated (indicating decreased G6PT1 function) in heterozygous and homozygous - 259T states. Delayed postnatal appearance of hepatic glucose-6-phosphatase in infants makes them vulnerable to hypoglycaemic episodes and this may occur in some SIDS infants. However, SIDS may be an association of more complex phenotypes in which several genes interact with multiple environmental factors. A UK-wide DNA Biobank of samples from all infant deaths, with an accompanying epidemiological database, should be established by pathologists to allow cumulative data to be collected from multiple genetic investigations on the same large cohort of samples, with the aim of selection of the best combination of genetic markers to predict unexpected infant death.

Isolation, characterization, and differentiation to hepatocyte-like cells of nonparenchymal epithelial cells from adult human liver

Activation and proliferation of human liver progenitor cells has been observed during acute and chronic liver diseases. Our goal was to investigate the presence of these putative progenitors in the liver of patients who underwent lobectomy for various reasons but did not show any hepatic insufficiency. Hepatic lesions were evaluated by histological analysis. Nonparenchymal epithelial (NPE) cells were isolated from samples of human liver resections located at a distance from the lesion that motivated the operation and were cultured and characterized. These cells exhibited a marked proliferative potential. They did not express the classic set of stem cell/progenitor markers (Oct-4, Rex-1, alpha-fetoprotein, CD90, c-kit, and CD34) and were faintly positive for albumin. When cultured at confluence in the presence of hepatocyte growth factor and either epidermal growth factor or fibroblast growth factor-4, they entered a differentiation process toward hepatocytes. Their phenotype was quantitatively compared with that of mature human hepatocytes in primary culture. Differentiated NPE cells expressed albumin; alpha1-antitrypsin; fibrinogen; hepatobiliary markers such as cytokeratins 7, 19, and 8/18; liver-enriched transcription factors; and genes characterized by either a fetal (cytochrome P4503A7 and glutathione S-transferase pi) or a mature (tyrosine aminotransferase, tryptophan 2,3-dioxygenase, glutathione S-transferase alpha, and cytochrome P4503A4) expression pattern. NPE cells could be isolated from the liver of several patients, irrespective of the absence or presence of lesions, and differentiated toward hepatocyte-like cells with an intermediate hepatobiliary and mature/immature phenotype. These cells are likely to represent a resident progenitor population of the adult human liver, even in the absence of hepatic failure. Disclosure of potential conflicts of interest is found at the end of this article.

Cold stress and hypoglycemia in the late preterm ("near-term") infant: impact on nursery of admission

Late preterm infants (34-37 weeks gestation) pose unique challenges to physicians and nurses involved in their care after birth. They may be cared for in different units within hospitals after birth, including Neonatal Intensive Care Units, Newborn Nurseries, or rooming in with the mother. As a result of their gestational age and birth weight, the late preterm infant is often assessed quickly and triaged identical to term infants. Such practice can potentially result in a lack of attention to important components for successful transition after birth. Cold stress and hypoglycemia are the two important problems in late preterm infants which require immediate treatment. Thus, surveillance of these and other physiological variables is needed to insure that they do not affect successful adaptation during the early hours and days after birth.

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.

Plasma catecholamines and the counterregulatory responses to hypoglycemia in infants: a critical role for epinephrine and cortisol

The purpose of this study was to define plasma catecholamine responses as part of the counterregulatory hormonal reaction to hypoglycemia in infants after a regular 3- to 4-h feed was omitted. Hormone levels were assessed once, at the end of the fast or at hypoglycemia. The 121 infants were subdivided into three groups for analysis: normoglycemia (n = 94, 78%); transient hypoglycemia (n = 11, 9%); or severe and persistent hypoglycemia (n = 16, 13%). The severe and persistent hypoglycemic group had significantly higher levels of cortisol and epinephrine than the normoglycemic group. Norepinephrine and glucagon levels did not differ between the groups. Human GH levels were higher in the transiently hypoglycemic group but not in the severe and persistent hypoglycemic group. Prefeed blood lactate levels differed significantly among the groups and were highest in the severe and persistent groups. Multiple regression analysis showed that cortisol levels were significantly higher in infants who had severe and persistent hypoglycemia. The counterregulatory hormonal response in infants to severe and persistent hypoglycemia was limited to elevations in only cortisol and epinephrine levels but did not involve glucagon or human GH. This limited hormonal response may also contribute to the frequent occurrence of hypoglycemia in these infants.

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.

Developmental disorders of glucose metabolism in infants

BACKGROUND

Developmental failures to adequately control postnatal blood glucose levels are common in the transition from fetal to infant life and can persist for many months. The standard method of functionally measuring hepatic glucose production and/or disordered glucose production is the response to a glucagon tolerance test.

METHOD

We adapted the standard glucagon tolerance test used for children and adults for use in preterm infants. 79 consecutive preterm infants gestational age range 25-36 weeks (mean 32.2 weeks), mean birth weight 1.66 kg admitted to the Neonatal Intensive Care Unit, Ninewells Hospital, Dundee and who survived to discharge home were recruited into the study. At the time of discharge home the characteristics of the group were as follows: adjusted mean gestational age 36.7 weeks, mean discharge weight 2.23 kg.

RESULTS

In this study of preterm infants the maximal increase in plasma glucose following administration of a glucagon tolerance test is 1.39 +/- 07 mmol/L, n = 78 (range 0-3.98 mmol/L).

CONCLUSIONS

An increase in plasma glucose of less than 4 mmol/L is considered abnormal in adults following administration of a fasting glucagon tolerance test. The responses of preterm infants and adults to glucagon are clearly different. The attenuated response to glucagon in the preterm infants is consistent with the low levels of hepatic glucose-6-phosphatase activity in premature infants as glucose-6-phosphatase is the terminal step of the two main pathways of liver glucose production.

Characterisation of hepatic microsomal glucose-6-phosphatase from broiler chickens

1. The hepatic glucose-6-phosphatase (G-6-Pase) kinetic variables from chickens were studied in intact and disrupted microsomes using two substrates: glucose-6-phosphate (G-6-P) and pyrophosphate (PPi). They were studied from embryonic life to 51 d of age. 2. The phosphohydrolase activity studied in the broiler chicken liver microsomes corresponds to a true glucose-6-phosphatase. 3. The enzyme VMAX with both substrates in intact and disrupted microsomes showed 2 maxima: one in 19-d-old embryos and the other in 9-d-old chickens. Pyrophosphatase (PPase) VMAX in intact microsomes was higher than that of the G-6-Pase at all ages studied, except in 12 d embryos and 3-d-old chicks. In disrupted microsomes the VMAX of both enzymatic activities were similar. The G-6-Pase latency was high in the 19-d-old embryos and 51-d-old chickens. 4. The KM for PPi and G-6-Pase decreased when microsomes were disrupted. In intact microsomes the G-6-P KM was low in embryos and 3-d-old chicks and later increased. On the other hand, the PPi KM in intact microsomes showed little change during the animal's life and was lower than that of G-6-P. In disrupted microsomes the KM for both substrates were similar. 5. These results suggest a sequential incorporation of the G-6-Pase system components in the endoplasmic reticulum.

The molecular background of glycogen metabolism disorders

The molecular pathology of classical glycogen storage disorders, glycogen synthase deficiency and Fanconi-Bickel syndrome is reviewed. The isolation of the respective cDNAs, the chromosomal localization of the genes and the elucidation of the genomic organization enabled mutation analysis in most disorders. The findings have shed light on the multi-protein structure of the glucose-6-phosphatase system, the phosphorylase kinase enzymatic complex and the molecular background of the differential tissue expression in debranching enzyme deficiency. The immediate practical benefit of these studies is our extending ability to predict the outcome of clinical variants and to offer genetic counseling to most families. The elucidation of the tertiary structure of these proteins and their structure-function relationship poses major challenges for the future.

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.

Conformational change of the catalytic subunit of glucose-6-phosphatase in rat liver during the fetal-to-neonatal transition

The glucose-6-phosphatase system was investigated in fetal rat liver microsomal vesicles. Several observations indicate that the orientation of the catalytic subunit is different in the fetal liver in comparison with the adult form: (i) the phosphohydrolase activity was not latent using glucose-6-phosphate as substrate, and in the case of other phosphoesters it was less latent; (ii) the intravesicular accumulation of glucose upon glucose-6-phosphate hydrolysis was lower; (iii) the size of the intravesicular glucose-6-phosphate pool was independent of the glucose-6-phosphatase activities; (iv) antibody against the loop containing the proposed catalytic site of the enzyme inhibited the phosphohydrolase activity in fetal but not in adult rat liver microsomes. Glucose-6-phosphate, phosphate, and glucose uptake could be detected by both light scattering and/or rapid filtration method in fetal liver microsomes; however, the intravesicular glucose-6-phosphate and glucose accessible spaces were proportionally smaller than in adult rat liver microsomes. These data demonstrate that the components of the glucose-6-phosphatase system are already present, although to a lower extent, in fetal liver, but they are functionally uncoupled by the extravesicular orientation of the catalytic subunit.

A gene on chromosome 11q23 coding for a putative glucose- 6-phosphate translocase is mutated in glycogen-storage disease types Ib and Ic

Glycogen-storage diseases type I (GSD type I) are due to a deficiency in glucose-6-phosphatase, an enzymatic system present in the endoplasmic reticulum that plays a crucial role in blood glucose homeostasis. Unlike GSD type Ia, types Ib and Ic are not due to mutations in the phosphohydrolase gene and are clinically characterized by the presence of associated neutropenia and neutrophil dysfunction. Biochemical evidence indicates the presence of a defect in glucose-6-phosphate (GSD type Ib) or inorganic phosphate (Pi) (GSD type Ic) transport in the microsomes. We have recently cloned a cDNA encoding a putative glucose-6-phosphate translocase. We have now localized the corresponding gene on chromosome 11q23, the region where GSD types Ib and Ic have been mapped. Using SSCP analysis and sequencing, we have screened this gene, for mutations in genomic DNA, from patients from 22 different families who have GSD types Ib and Ic. Of 20 mutations found, 11 result in truncated proteins that are probably nonfunctional. Most other mutations result in substitutions of conserved or semiconserved residues. The two most common mutations (Gly339Cys and 1211-1212 delCT) together constitute approximately 40% of the disease alleles. The fact that the same mutations are found in GSD types Ib and Ic could indicate either that Pi and glucose-6-phosphate are transported in microsomes by the same transporter or that the biochemical assays used to differentiate Pi and glucose-6-phosphate transport defects are not reliable.

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.

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 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.

Differential localisation of UDP-glucuronosyltransferase in kidney during human embryonic and fetal development

The aim of our study was to localise UDP-glucuronosyltransferase (UDPGT) in the developing mesonephric and metanephric kidneys of the human embryo and fetus, using immunohistochemical methods and an antibody preparation with broad specificity to the human isoforms. In embryonic and early fetal development of the metanephric kidney, UDPGT is located primarily in derivatives of the ureteric bud such as the ureter, pelvis, calyces and collecting ducts. This early predominance of UDPGT to ureteric bud derivatives declines by mid-fetal life: a) as nephrons evolve and develop they become increasingly UDPGT immunoreactive such that in mature metanephric kidney, the proximal tubules are highly UDPGT reactive, with other elements of the nephron also immunopositive (albeit at lower reactivities) and b) with the formation of an immunonegative transitional epithelium in ureter, pelvis and calyces, the reactivity retained in collecting ducts is only a small proportion of the total. The distribution of UDPGT immunoreactivity is relatively uniform in proximal tubular cells throughout development. This is in contrast to collecting ducts where, in fetal life, this reactivity is displaced to apices and bases by intracellular glycogen deposits. Parietal cells of Bowman's capsule are immunoreactive, but glomeruli are negative. In mesonephric kidney, as early as 32 days post-ovulation, tubules and the mesonephric duct are UDPGT immunoreactive and mesonephric immunopositivity overlaps with that in the developing metanephric kidney.

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.

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.

Abnormal expression of glucose-6-phosphatase in preterm infants

The hepatic microsomal glucose-6-phosphatase enzyme was studied in liver samples from 76 premature infants including 15 victims of sudden infant death syndrome. The data obtained were compared with glucose-6-phosphatase activity in liver samples from 95 term infants. In the majority of preterm infants up to 350 days of age the activity of the glucose-6-phosphatase enzyme was at or below the extreme low limit of the normal range in term infants. The premature infants with the lowest hepatic microsomal glucose-6-phosphatase activities are likely to be at risk of hypoglycaemic episodes during periods of relative starvation or stress.

The molecular basis of the genetic deficiencies of five of the components of the glucose-6-phosphatase system: improved diagnosis

The understanding of type 1 glycogen storage diseases (GSDs) has been greatly hindered by a lack of knowledge of the molecular basis of glucose-6-phosphatase (Glc-6-P'ase). The problem has been the complete failure of many laboratories, including our own, to purify to homogeneity a single polypeptide with high levels of Glc-6-P'ase activity. The best preparations to date all contain five or six different polypeptide bands and have specific activities in the range 17-50 mumoles/min per milligram. The two major reasons for failure have been that Glc-6-P'ase is extremely difficult to solubilise from the microsomal membrane (large amounts of detergents are needed) and that it is not a single polypeptide as originally thought, but a multicomponent system. Recent studies of patients with type 1 GSD have proved that Glc-6-P'ase comprises at least five different polypeptides. Four of the proteins have now been purified and three have been cloned. We have assayed the Glc-6-P'ase system in over 600 human biopsy samples and developed microassays to diagnose deficiencies of each of the proteins. Ways of avoiding possible problems which have the potential to lead to the wrong diagnosis will be discussed.

Impairment of the activity of the hepatic microsomal glucose-6-phosphatase system in three preterm infants

Three preterm infants born at 26-30 weeks' gestation who died between 103 and 266 days after birth were found to have elevated hepatic glycogen levels. Kinetic analysis of the hepatic microsomal glucose-6-phosphatase system demonstrated that one infant had abnormally low levels of activity of the glucose-6-phosphatase enzyme (partial type 1a glycogen storage disease) and two had deficiencies of T2, a microsomal phosphate/pyrophosphate transport protein (type 1c glycogen storage disease). In all three cases glycogen storage disease was not suspected prior to death even though both hypo- and hyperglycaemic episodes were recorded in the first 15 days after birth indicating that they had somewhat disordered blood glucose regulation. In the infant with low glucose-6-phosphatase enzyme activity, abnormal development of the glucose-6-phosphatase enzyme cannot be ruled out. This is the first description of abnormalities in the glucose-6-phosphatase system in preterm infants.

The molecular basis of the type 1 glycogen storage diseases

Microsomal glucose-6-phosphatase catalyses the last step in liver glucose production. Glucose-6-phosphatase deficiency, now termed type 1 glycogen storage disease, was first described almost 40 years ago but until recently very little was known about the molecular basis of the various type 1 glycogen storage diseases. Recently we have shown that at least six different proteins are needed for normal glucose-6-phosphatase activity in liver. Four of the proteins have been purified and three cloned. Study of the type 1 glycogen storage diseases has stimulated investigations of the mechanisms of small molecule transport across the endoplasmic reticulum membrane and demonstrated the existence of novel endoplasmic reticulum transport proteins for glucose and phosphate.