Glucose transporter gene expression in early mouse embryos

Nutrient Transporter Gene Expression in the Early Conceptus—Implications From Two Mouse Models of Diabetic Pregnancy

Maternal diabetes in early pregnancy increases the risk for birth defects in the offspring, particularly heart, and neural tube defects. While elevated glucose levels are characteristic for diabetic pregnancies, these are also accompanied by hyperlipidemia, indicating altered nutrient availability. We therefore investigated whether changes in the expression of nutrient transporters at the conception site or in the early post-implantation embryo could account for increased birth defect incidence at later developmental stages. Focusing on glucose and fatty acid transporters, we measured their expression by RT-PCR in the spontaneously diabetic non-obese mouse strain NOD, and in pregnant FVB/N mouse strain dams with Streptozotocin-induced diabetes. Sites of expression in the deciduum, extra-embryonic, and embryonic tissues were determined by RNAscope in situ hybridization. While maternal diabetes had no apparent effects on levels or cellular profiles of expression, we detected striking cell-type specificity of particular nutrient transporters. For examples, Slc2a2/Glut2 expression was restricted to the endodermal cells of the visceral yolk sac, while Slc2a1/Glut1 expression was limited to the mesodermal compartment; Slc27a4/Fatp4 and Slc27a3/Fatp3 also exhibited reciprocally exclusive expression in the endodermal and mesodermal compartments of the yolk sac, respectively. These findings not only highlight the significance of nutrient transporters in the intrauterine environment, but also raise important implications for the etiology of birth defects in diabetic pregnancies, and for strategies aimed at reducing birth defects risk by nutrient supplementation.

Hexokinase-2-Linked Glycolytic Overload and Unscheduled Glycolysis-Driver of Insulin Resistance and Development of Vascular Complications of Diabetes

The recent discovery of the glucose-induced stabilization of hexokinase-2 (HK2) to proteolysis in cell dysfunction in model hyperglycemia has revealed a likely key initiating factor contributing to the development of insulin resistance and vascular complications in diabetes. Consequently, the increased flux of glucose metabolism without a change in the expression and activity of glycolytic enzymes produces a wave of increased glycolytic intermediates driving mitochondrial dysfunction and increased reactive oxygen species (ROS) formation, the activation of hexosamine and protein kinase C pathways, the increased formation of methylglyoxal-producing dicarbonyl stress, and the activation of the unfolded protein response. This is called HK2-linked glycolytic overload and unscheduled glycolysis. The conditions required to sustain this are GLUT1 and/or GLUT3 glucose uptake and the expression of HK2. A metabolic biomarker of its occurrence is the abnormally increased deposition of glycogen, which is produced by metabolic channeling when HK2 becomes detached from mitochondria. These conditions and metabolic consequences are found in the vasculature, kidneys, retina, peripheral nerves, and early-stage embryo development in diabetes and likely sustain the development of diabetic vascular complications and embryopathy. In insulin resistance, HK2-linked unscheduled glycolysis may also be established in skeletal muscle and adipose tissue. This may explain the increased glucose disposal by skeletal uptake in the fasting phase in patients with type 2 diabetes mellitus, compared to healthy controls, and the presence of insulin resistance in patients with type 1 diabetes mellitus. Importantly, glyoxalase 1 inducer—trans-resveratrol and hesperetin in combination (tRES-HESP)—corrected HK2-linked glycolytic overload and unscheduled glycolysis and reversed insulin resistance and improved vascular inflammation in overweight and obese subjects in clinical trial. Further studies are now required to evaluate tRES-HESP for the prevention and reversal of early-stage type 2 diabetes and for the treatment of the vascular complications of diabetes.

Early cleaving embryos result in blastocysts with increased aspartate and glucose consumption, which exhibit different metabolic gene expression that persists in placental and fetal tissues

OBJECTIVES

Using time-lapse microscopy, previous research has shown that IVF mouse embryos that cleave earlier at the first division ('fast') develop into blastocysts with increased glucose consumption and lower likelihood of post-implantation loss as compared to slower cleaving embryos ('slow'). Further, metabolomics analysis employing LC-MS conducted on groups of 'fast' blastocysts revealed that more aspartate was consumed. With the worldwide adoption of single blastocyst transfer as the standard of care, the need for quantifiable biomarkers of viability, such as metabolism of specific nutrients, would greatly assist in embryo selection for transfer.

METHODS

Here we describe the development of a targeted enzymatic assay to quantitate aspartate uptake of single blastocysts.

RESULTS

Results demonstrate that the rates of aspartate and glucose consumption were significantly higher in individual 'fast' blastocysts. Blastocysts, together with placental and fetal liver tissue collected following transfer, were analysed for the expression of genes involved in aspartate and carbohydrate metabolism. In 'fast' blastocysts, expressions of B3gnt5, Slc2a1, Slc2a3, Got1 and Pkm2 were found to be significantly higher. In placental tissue derived from 'fast' blastocysts, expression of Slc2a1, Got1 and Pkm2 were significantly higher, while levels of Got1 and Pkm2 were lower in fetal liver tissue compared to tissue from 'slow' blastocysts.

CONCLUSIONS

Importantly, this study shows that genes regulating aspartate and glucose metabolism were increased in blastocysts that have higher viability, with differences maintained in resultant placentae and fetuses. Consequently, the analysis of aspartate uptake in combination with glucose represents biomarkers of development and may improve embryo selection efficacy and pregnancy rates.

Diabetic Embryopathy Susceptibility in Mice Is Associated with Differential Dependence on Glucosamine and Modulation of High Glucose-Induced Oxidative Stress

The high K_M glucose transporter, GLUT2 (SLC2A2), is expressed by embryos and causes high rates of glucose transport during maternal hyperglycemic episodes in diabetic pregnancies and causes congenital malformations (diabetic embryopathy). GLUT2 is also a low K_M transporter of the amino sugar, glucosamine (GlcN), which enters the hexosamine biosynthetic pathway (HBP) and provides substrate for glycosylation reactions. Exogenous GlcN also increases activity of the pentose phosphate pathway (PPP), which increases production of NADPH reducing equivalents. GLUT2-transported GlcN is inhibited by high glucose concentrations. Not all mouse strains are susceptible to diabetic embryopathy. The aim of this study was to test the hypothesis that susceptibility to diabetic embryopathy is related to differential dependence on exogenous GlcN for glycosylation or stimulation of the PPP. We tested this using murine embryonic stem cell (ESC) lines that were derived from embryopathy-susceptible FVB/NJ (FVB), and embryopathy-resistant C57Bl/6J (B6), embryos in the presence of low or high glucose, and in the presence or absence of GlcN. There were no significant differences in Glut2 expression, or of glucose or GlcN transport, between FVB and B6 ESC. GlcN effects on growth and incorporation into glycoproteins indicated that FVB ESC are more dependent on exogenous GlcN than are B6 ESC. GlcN stimulated PPP activity in FVB but not in B6 ESC. High glucose induced oxidative stress in FVB ESC but not in B6 ESC. These results indicate that FVB embryos are more dependent on exogenous GlcN for glycosylation, but also for stimulation of the PPP and NADPH production, than are B6 embryos, thereby rendering FVB embryos more susceptible to high glucose to induce oxidative stress.

Telfairia occidentalis stimulates hepatic glycolysis and pyruvate production via insulin-dependent and insulin-independent mechanisms

Background

Telfairia occidentalis (TO), a plant consumed for its nutritional and medicinal values, exhibits hypoglycaemic effect. However, the metabolic fate of the glucose following TO-induced insulin secretion and consequent hypoglycaemia is not clear.

Objective

This study determined the effect of ethyl acetate and n-hexane fractions of TO leaf extracts on some biochemical parameters in the glucose metabolic pathway to explain the possible fate of blood glucose following TO-induced hypoglycaemia.

Methods

Eighteen male Wistar rats (180-200 g) divided into control, n-hexane TO fraction- and ethyl acetate TO fraction-treated groups (n = 6/group) were used. The control animals received normal saline while the treated groups received TO at 100 mg/kg for seven days. After 24 h following the last dose, the animals were anaesthetised using ketamine; blood samples were collected and livers harvested to determine some biochemical parameters.

Results

Ethyl acetate TO fraction significantly increased plasma insulin, liver glucokinase activity and plasma pyruvate concentration, but significantly decreased plasma glucose and liver glycogen, without significant changes in plasma lactate, glucose-6-phosphate, liver glucose-6-phosphatase and lactate dehydrogenase activities when compared with control. N-hexane TO fraction significantly reduced liver glucose-6-phosphatase activity and glycogen but significantly increased plasma pyruvate, without significant changes in plasma glucose, insulin, glucose-6-phosphate and lactate concentrations; and liver glucokinase and lactate dehydrogenase activities.

Conclusion

The present study showed that insulin-mediated TO-induced hypoglycaemia resulted in the stimulation of glycolysis and pyruvate production via insulin-dependent and insulin-independent mechanisms.

Mechanisms of Congenital Malformations in Pregnancies with Pre-existing Diabetes

PURPOSE OF REVIEW

Fetuses of diabetic mothers are at increased risk for congenital malformations. Research in recent decades using animal and embryonic stem cell models has revealed many embryonic developmental processes that are disturbed by maternal diabetes. The aim of this review is to give clinicians a better understanding of the reasons for rigorous glycemic control in early pregnancy, and to provide background to guide future research.

RECENT FINDINGS

Mouse models of diabetic pregnancy have revealed mechanisms for altered expression of tissue-specific genes that lead to malformations that are more common in diabetic pregnancies, such as neural tube defects (NTDs) and congenital heart defects (CHDs), and how altered gene expression causes apoptosis that leads to malformations. Embryos express the glucose transporter, GLUT2, which confers susceptibility to malformation, due to high rates of glucose uptake during maternal hyperglycemia and subsequent oxidative stress; however, the teleological function of GLUT2 for mammalian embryos may be to transport the amino sugar glucosamine (GlcN) from maternal circulation to be used as substrate for glycosylation reactions and to promote embryo cell growth. Malformations in diabetic pregnancy may be not only due to excess glucose uptake but also due to insufficient GlcN uptake. Avoiding maternal hyperglycemia during early pregnancy should prevent excess glucose uptake via GLUT2 into embryo cells, and also permit sufficient GLUT2-mediated GlcN uptake.

EGFR mutation decreases FDG uptake in non‑small cell lung cancer via the NOX4/ROS/GLUT1 axis

[18F]fluoro‑2‑deoxyglucose (FDG) positron emission tomography (PET)‑computed tomography (CT) is a functional imaging modality based on glucose metabolism. The association between the maximum standardized uptake value (SUVmax) from 18F‑FDG PET‑CT scanning and epidermal growth factor receptor (EGFR) mutation status has, to the best of our knowledge, not previously been fully elucidated, and the potential mechanisms by which EGFR mutations alter FDG uptake are largely unknown. A total of 157 patients who were pathologically diagnosed with non‑small cell lung cancer (NSCLC) who underwent EGFR mutation testing and PET‑CT pretreatment between June 2015 and October 2017 were retrospectively analyzed. χ2 and univariate analyses were performed to identify the contributors to EGFR mutation. The receiver operating characteristic (ROC) curve was analyzed, and the area under the curve (AUC) was calculated. Glucose transporter 1 (GLUT1) and NADPH oxidase 4 (NOX4) expression, and reactive oxygen species (ROS) activity were detected in the A549 (wild‑type), PC‑9 (EGFR mutation‑positive, EGFR exon 19del) and NCI‑H1975 (EGFR mutation‑positive, combined with L858R and T790M substitution) cell lines. A total of 109 patients who met the criteria were enrolled, and 63 of those tested as EGFR mutation‑positive. The SUVmax values were significantly lower in patients with EGFR mutations (mean, 6.52±0.38) compared with in patients with wild‑type EGFR (mean, 9.37±0.31; P<0.001). Using univariate analysis, EGFR mutation status was significantly associated with sex, smoking status, tumor histology and SUVmax of the primary tumor. In the multivariate analysis, smoking status (never‑smoking), histopathology (adenocarcinoma) and SUVmax (≤9.91) were the statistically significant predictors of EGFR mutations. ROC curve analysis identified that the SUVmax cut‑off point was 9.92, for which the AUC was 0.75 (95% confidence interval, 0.68‑0.83). Reverse transcription‑polymerase chain reaction indicated that the GLUT1 mRNA decreased in the PC‑9 and NCI‑H1975 cell lines compared with the A549 cell line (0.82±0.07 and 0.72±0.04 vs. 0.98±0.04, respectively; P<0.05) and decreased ROS activity was observed in the PC‑9 cell line. Furthermore, the expression of NOX4 mRNA decreased by 20% in PC‑9 (P<0.01) and by 14% (P<0.05) in NCI‑H1975 cells. In addition, NOX4 protein expression decreased by 13% in PC‑9 and by 16% in NCI‑H1975 cells (both P<0.05) compared with the A549 cell line. The SUVmax could be considered to effectively predict EGFR mutation status of patients with NSCLC, and the EGFR mutation status may alter FDG uptake partially via the NOX4/ROS/GLUT1 axis.

Oocyte aging-induced Neuronatin (NNAT) hypermethylation affects oocyte quality by impairing glucose transport in porcine

DNA methylation plays important roles in regulating many physiological behaviors; however, few studies were focused on the changes of DNA methylation during oocyte aging. Early studies showed that some imprinted genes’ DNA methylation had been changed in aged mouse oocytes. In this study, we used porcine oocytes to test the hypothesis that oocyte aging would alter DNA methylation pattern of genes and disturb their expression in age oocytes, which affected the developmental potential of oocytes. We compared several different types of genes and found that the expression and DNA methylation of Neuronatin (NNAT) were disturbed in aged oocytes significantly. Additional experiments demonstrated that glucose transport was impaired in aged oocytes and injection of NNAT antibody into fresh oocytes led to the same effects on glucose transport. These results suggest that the expression of NNAT was declined by elevating DNA methylation, which affected oocyte quality by decreasing the ability of glucose transport in aged oocytes.

Embryonic Stem Cell Proliferation Stimulated By Altered Anabolic Metabolism From Glucose Transporter 2-Transported Glucosamine

The hexose transporter, GLUT2 (SLC2A2), which is expressed by mouse embryos, is important for survival before embryonic day 10.5, but its function in embryos is unknown. GLUT2 can transport the amino sugar glucosamine (GlcN), which could increase substrate for the hexosamine biosynthetic pathway (HBSP) that produces UDP-N-acetylglucosamine for O-linked N-acetylglucosamine modification (O-GlcNAcylation) of proteins. To understand this, we employed a novel murine embryonic stem cell (ESC) line that, like mouse embryos, expresses functional GLUT2 transporters. GlcN stimulated ESC proliferation in a GLUT2-dependent fashion but did not regulate pluripotency. Stimulation of proliferation was not due to increased O-GlcNAcylation. Instead, GlcN decreased dependence of the HBSP on fructose-6-PO₄ and glutamine. Consequently, glycolytic- and glutamine-derived intermediates that are needed for anabolic metabolism were increased. Thus, maternally obtained GlcN may increase substrates for biomass accumulation by embryos, as exogenous GlcN does for GLUT2-expressing ESC, and may explain the need for GLUT2 expression by embryos.

Interactions between mitochondrial reactive oxygen species and cellular glucose metabolism

Mitochondrial reactive oxygen species (ROS) production and detoxification are tightly balanced. Shifting this balance enables ROS to activate intracellular signaling and/or induce cellular damage and cell death. Increased mitochondrial ROS production is observed in a number of pathological conditions characterized by mitochondrial dysfunction. One important hallmark of these diseases is enhanced glycolytic activity and low or impaired oxidative phosphorylation. This suggests that ROS is involved in glycolysis (dys)regulation and vice versa. Here we focus on the bidirectional link between ROS and the regulation of glucose metabolism. To this end, we provide a basic introduction into mitochondrial energy metabolism, ROS generation and redox homeostasis. Next, we discuss the interactions between cellular glucose metabolism and ROS. ROS-stimulated cellular glucose uptake can stimulate both ROS production and scavenging. When glucose-stimulated ROS production, leading to further glucose uptake, is not adequately counterbalanced by (glucose-stimulated) ROS scavenging systems, a toxic cycle is triggered, ultimately leading to cell death. Here we inventoried the various cellular regulatory mechanisms and negative feedback loops that prevent this cycle from occurring. It is concluded that more insight in these processes is required to understand why they are (un)able to prevent excessive ROS production during various pathological conditions in humans.

The p38 MAPK signalling pathway is required for glucose metabolism, lineage specification and embryo survival during mouse preimplantation development

Preimplantation embryo development is an important and unique period and is strictly controlled. This period includes a series of critical events that are regulated by multiple signal-transduction pathways, all of which are crucial in the establishment of a viable pregnancy. The p38 mitogen-activated protein kinase (MAPK) signalling pathway is one of these pathways, and inhibition of its activity during preimplantation development has a deleterious effect. The molecular mechanisms underlying the deleterious effects of p38 MAPK suppression in early embryo development remain unknown. To investigate of the effect of p38 MAPK inhibition on late preimplantation stages in detail, we cultured 2-cell stage embryos in the presence of SB203580 for 48 h and analysed the 8-cell, morula, and blastocyst stages. We determined that prolonged inhibition of the p38 MAPK altered the expression levels of Glut1 and Glut4, decreased glucose uptake during the 8-cell to blastocyst transition, changed the expression levels of transcripts which will be important to lineage commitment, including Oct4/Pou5f1, Nanog, Sox2, and Gata6, and increased cell death in 8-16 cell stage embryos onwards. Strikingly, while the expression levels of Nanog, Gata6 and Oct4/Pou5f1 mRNAs were significantly decreased, Sox2 mRNA was increased in SB203580-treated blastocysts. Taken together, our results provide important insight into the biological processes controlled by the p38 MAPK pathway and its critical role during preimplantation development.

Association of facilitated glucose transporter 2 gene variants with the myelomeningocele phenotype

BACKGROUND

Neural tube defects (NTDs) remain the second most common cause of congenital malformations. Myelomeningocele (MM), the most common NTD compatible with survival, results from genetic and environmental factors. Epidemiologic studies and murine models support the hypotheses that obesity, diabetes and hyperglycemia confer increased risk of NTDs. Presence of wild-type facilitated glucose transporter, Glut2, in mouse embryos has been shown to increase risk for NTDs in hyperglycemic pregnancy.

METHODS

The GLUT2 gene of 96 MM patients was amplified, sequenced and compared with the reference sequence (NM_000340). Variants previously unreported in the single nucleotide polymorphisms (SNP) database were considered novel. Allele frequencies of reported SNPs were compared with reference populations using Fisher's exact test.

RESULTS

Analysis revealed three novel variants: a substitution in the core promoter region (c.-331c>t), a substitution (c.-182g>a) in the 5'-untranslated region, and a single base pair deletion (c.1441delT) in the coding sequences. Polymorphic alleles for 10 SNPs were also identified. Seven SNPs are significantly associated with MM in the Mexican American patients tested (p < 0.05) and two of the seven remained significant after Bonferroni correction.

CONCLUSION

We identified three novel variants and seven SNPs associated with MM. The novel variants in the core promoter and in the 5'-untranslated region could affect GLUT2 mRNA transcription and stability and translation efficiency. The c.1441delT variant is predicted to alter the reading frame and prematurely terminate translation of the GLUT2 protein at the C-terminus, affecting GLUT2 protein function. Presence of GLUT2 variants may disrupt GLUT2 activity and influence MM susceptibility.

Expression pattern of glucose metabolism genes correlate with development rate of buffalo oocytes and embryos in vitro under low oxygen condition

PURPOSE

This study evaluates the effect of low oxygen conditions (5 Vs 20%) on buffalo embryo development. Expression patterns of key glucose metabolism genes (HK, PFK, LDH, PDH, G6PDH and Glut1) were assessed in buffalo oocytes and embryos cultured at 5 and 20% oxygen and correlated with development rate.

METHODS

Maturation rate was observed by determining MII stages by Aceto-orcein method and blastocyst formation was observed at 7 day post insemination (dpi). Expression levels of genes were determined by real time PCR in oocytes / embryos at 5 and 20% O2.

RESULTS

Oocyte maturation and blastocyst formation rates were significantly higher at 5% O2 as compared to 20% O2 (P < 0.05). The expression pattern of glycolytic genes (HK, PFK and G6PDH) indicated that oocytes and embryos under 5% O2 tend to follow anaerobic glycolysis and pentose phosphate pathways to support optimum embryo development. Under 20% O2, oocytes and embryos had high expression of PDH indicating higher oxidative phosphorylation. Further, less G6PDH expression at 20% O2 was indicative of lower pentose phosphate activity. Higher expression of LDH was observed in oocytes and embryos under 20% O2 indicating sub-optimal culture conditions. High Glut1 activity was observed in the oocytes / embryos at 5% O2, indicative of high glucose uptake correlating with high expression of glycolytic genes.

CONCLUSION

The expression patterns of glucose metabolism genes could be a valuable indicator of the development potential of oocytes and embryos. The study indicates the importance of reduced oxygen conditions for production of good quality embryos.

Use of a murine embryonic stem cell line that is sensitive to high glucose environment to model neural tube development in diabetic pregnancy

BACKGROUND

Neural tube defects (NTDs) are significantly increased by maternal diabetes. Embryonic stem cells (ESC) that can differentiate into neuroepithelium and can sense supraphysiological glucose concentrations would be very valuable to simulate the effects of maternal diabetes on molecular and cellular processes during neural tube formation.

METHODS

LG-ESC, a recently established ESC line that expresses the glucose transporter, Scl2a2, and is sensitive to elevated glucose concentrations, were grown for up to 8 days in a three-dimensional culture to form neural cysts. We tested whether high glucose media inhibits expression of Pax3, a gene that is required for neural tube closure and whose expression is inhibited in embryos of diabetic mice, and inhibits formation of neural cysts.

RESULTS

Pax3 expression was detected after 4 days of culture and increased with time. Pax3 expression was inhibited by high glucose media, but not if cells had been cultured in low glucose media for the first 4 days of culture. Pax7, which is also expressed in dorsal neural tube, was not detected. Pax6, which is expressed in the ventral neural tube, was detected only after 8 days of culture, but was not inhibited by high glucose. High glucose media did not inhibit formation of neural cysts.

CONCLUSION

LG-ESC can be used as a model of embryonic exposure to a diabetic environment during neural tube development. While high glucose exposure inhibits expression of a gene required for neural tube closure, it may not inhibit all of the processes involved in formation of a neural tube-like structure.

Insulin-like growth factor-1 (IGF-1) enhances developmental competence of cat embryos cultured singly by modulating the expression of its receptor (IGF-1R) and reducing developmental block

OBJECTIVE

The aim of this study is to determine the effects of insulin-like growth factor-1 (IGF-1) and the mRNA expression of IGF-1 receptor (IGF-1R) during the in vitro development of cat embryos cultured in groups versus singly.

METHODS

Cumulus-oocyte complexes (COCs) were matured and fertilized in vitro with frozen-thawed semen. Cleaved embryos (48h post-fertilization) were randomly assigned to one of the following treatments: 1) group embryo culture without IGF-1 (10 embryos per 50μl droplet), 2) single-embryo culture without IGF-1, and 3) to 6) single-embryo culture (50μl droplet per embryo) supplemented with different concentrations of IGF-1 (5, 25, 50 and 100ng/ml, respectively). During in vitro culture, the embryos were analyzed for development to the morula, blastocyst and hatching blastocyst stage. Relative mRNA expression of IGF-1R was also examined by qPCR at the morula and blastocyst stages. In addition, the mRNA expression of IGF-1R in morula-stage embryos treated with IGF-1 was determined. The influence of IGF-1 to preimplantation embryo development was then explored by co-incubation with 0.5μM IGF-1R inhibitor (Picropodophyllin; PPP).

RESULTS

Group embryo culture led to a significantly higher blastocyst development rate compared with single-embryo culture (P<0.05). The poor development of singly cultured embryos coincided with the significantly lower IGF-1R expression in morulae than in group-cultured morulae. IGF-1 (25 or 50ng/ml) supplementation significantly improved the blastocyst formation rate of single embryos to a level similar to group culture by promoting the morula-to-blastocyst transition. IGF-1 supplementation (25 or 50ng/ml) of singly cultured embryos upregulated the expression of IGF-1R mRNA in morula-stage embryos to the same level as that observed in group-cultured embryos (without IGF-1). The beneficial effects of IGF-1 on singly cultured embryo were (P<0.05) suppressed by PPP even in the group culture embryo without growth factor supplementation.

CONCLUSION

IGF-1 supplementation improves the developmental competence of feline embryos cultured individually and also increases IGF-1R gene expression to levels similar to group-cultured embryos.

Maternal-fetal metabolic gene-gene interactions and risk of neural tube defects

Single-gene analyses indicate that maternal genes associated with metabolic conditions (e.g., obesity) may influence the risk of neural tube defects (NTDs). However, to our knowledge, there have been no assessments of maternal-fetal metabolic gene-gene interactions and NTDs. We investigated 23 single nucleotide polymorphisms among 7 maternal metabolic genes (ADRB3, ENPP1, FTO, LEP, PPARG, PPARGC1A, and TCF7L2) and 2 fetal metabolic genes (SLC2A2 and UCP2). Samples were obtained from 737 NTD case-parent triads included in the National Birth Defects Prevention Study for birth years 1999-2007. We used a 2-step approach to evaluate maternal-fetal gene-gene interactions. First, a case-only approach was applied to screen all potential maternal and fetal interactions (n = 76), as this design provides greater power in the assessment of gene-gene interactions compared to other approaches. Specifically, ordinal logistic regression was used to calculate the odds ratio (OR) and 95% confidence interval (CI) for each maternal-fetal gene-gene interaction, assuming a log-additive model of inheritance. Due to the number of comparisons, we calculated a corrected p-value (q-value) using the false discovery rate. Second, we confirmed all statistically significant interactions (q < 0.05) using a log-linear approach among case-parent triads. In step 1, there were 5 maternal-fetal gene-gene interactions with q < 0.05. The "top hit" was an interaction between maternal ENPP1 rs1044498 and fetal SLC2A2 rs6785233 (interaction OR = 3.65, 95% CI: 2.32-5.74, p = 2.09×10(-8), q=0.001), which was confirmed in step 2 (p = 0.00004). Our findings suggest that maternal metabolic genes associated with hyperglycemia and insulin resistance and fetal metabolic genes involved in glucose homeostasis may interact to increase the risk of NTDs.

Mouse embryonic stem cells established in physiological-glucose media express the high KM Glut2 glucose transporter expressed by normal embryos

Glut2 is one of the facilitative glucose transporters expressed by preimplantation and early postimplantation embryos. Glut2 is important for survival before embryonic day 10.5. The Glut2 KM (∼16 mmol/liter) is significantly higher than physiologic glucose concentrations (∼5.5 mmol/liter), suggesting that Glut2 normally performs some essential function other than glucose transport. Nevertheless, Glut2 efficiently transports glucose when extracellular glucose concentrations are above the Glut2 KM. Media containing 25 mmol/liter glucose are widely used to establish and propagate embryonic stem cells (ESCs). Glut2-mediated glucose uptake by embryos induces oxidative stress and can cause embryo cell death. Here we tested the hypothesis that low-glucose embryonic stem cells (LG-ESCs) isolated in physiological-glucose (5.5 mmol/liter) media express a functional Glut2 glucose transporter. LG-ESCs were compared with conventional D3 ESCs that had been cultured only in high-glucose media. LG-ESCs expressed Glut2 mRNA and protein at much higher levels than D3 ESCs, and 2-deoxyglucose transport by LG-ESCs, but not D3 ESCs, exhibited high Michaelis-Menten kinetics. Glucose at 25 mmol/liter induced oxidative stress in LG-ESCs and inhibited expression of Pax3, an embryo gene that is inhibited by hyperglycemia, in neuronal precursors derived from LG-ESCs. These effects were not observed in D3 ESCs. These findings demonstrate that ESCs isolated in physiological-glucose media retain a functional Glut2 transporter that is expressed by embryos. These cells are better suited to the study of metabolic regulation characteristic of the early embryo and may be advantageous for therapeutic applications.

Understanding diabetic teratogenesis: where are we now and where are we going?

Maternal pregestational diabetes (type 1 or type 2) poses an increased risk for a broad spectrum of birth defects. To our knowledge, this problem first came to the attention of the Teratology Society at the 14th Annual Meeting in Vancouver, B.C. in 1974, with a presentation by Lewis Holmes, "Etiologic heterogeneity of neural tube defects". Although advances in the control of diabetes in the decades since the discovery of insulin in the 1920's have reduced the risk for birth defects during diabetic pregnancy, the increasing incidence of diabetes among women of childbearing years indicates that this cause of birth defects is a growing public health concern. Major advances in understanding how a disease of maternal fuel metabolism can interfere with embryogenesis of multiple organ systems have been made in recent years. In this review, we trace the history of the study of diabetic teratogenesis and discuss a model in which tissue-specific developmental control genes are regulated at specific times in embryonic development by glucose metabolism. The major function of such genes is to suppress apoptosis, perhaps to preserve proliferative capability, and inhibit premature senescence.

Embryo Culture Media, Culture Techniques and Embryo Selection: A Tribute to Wesley Kingston Whitten

This review article gives a brief history of the classical experiments that led to the development of the embryo culture medium and in vitro embryo culture. It proposes that, in view of the outstanding and significant pioneering contributions of Wesley Kingston Whitten to the development of embryo culture medium, he be considered the “Father of Embryo Culture Medium”. Furthermore, it describes the nutritional requirements of early embryos and how these requirements with specific references to carbohydrates, amino acids, phosphates, growth factors, etc, have been utilized to formulate increasingly more complex embryo culture media. This has led to the development of progressively more efficacious embryo culture media including the formulation of completely defined and synthetic protein-free embryo culture medium. The review also describes physical factors, growth factors, insemination methods for the fertilization of oocytes and culture methods affecting embryo growth, development, metabolism, oxygen embryotoxicity and survival. In procedural terms, the review also summarizes the evolution of embryo culture techniques from tube culture to, microdrop culture under oil to co-culture to ultra microdrop culture techniques. It includes techniques of in vitro maturation and for the selection of potentially viable embryos of various developmental stages.

Diabetes and apoptosis: neural crest cells and neural tube

Birth defects resulting from diabetic pregnancy are associated with apoptosis of a critical mass of progenitor cells early during the formation of the affected organ(s). Insufficient expression of genes that regulate viability of the progenitor cells is responsible for the apoptosis. In particular, maternal diabetes inhibits expression of a gene, Pax3, that encodes a transcription factor which is expressed in neural crest and neuroepithelial cells. As a result of insufficient Pax3, cardiac neural crest and neuroepithelial cells undergo apoptosis by a process dependent on the p53 tumor suppressor protein. This, then provides a cellular explanation for the cardiac outflow tract and neural tube and defects induced by diabetic pregnancy.

Glucose transporters in gametes and preimplantation embryos

The oocyte, sperm and preimplantation embryo have unique metabolic needs that must be met to ensure successful pregnancy. The family of facilitative glucose transporters (GLUTs) plays a major role in providing metabolic substrates to these tissues. The variety of GLUTs expressed in these tissues allows for flexibility to adapt to a changing environment. Alterations in glucose transport and metabolism at the earliest stages of development can impact fetal development. Research into the mechanisms of normal glucose transport into cells is critical for improving outcomes in the increasingly common diabetic maternal environment. Here, we review the current understanding in the distribution and role of glucose transporters in gametes and preimplantation embryos under normal and diabetic conditions.

Expression of glucose transporter isoforms and the insulin receptor during hamster preimplantation embryo development

During preimplantation development, embryos of many species are known to express up to five isoforms of the facilitative glucose transporter proteins (GLUT). Development of hamster blastocysts is inhibited by glucose. We therefore investigated GLUT isoform and insulin receptor (IR) expression in hamster preimplantation embryos cultured in glucose-free medium from the 8-cell stage onwards. We show that GLUT1, 3 and 8 mRNA are constitutively expressed from the 8-cell to the blastocyst stage. The IR is expressed from the morula stage onwards. Messenger RNA of the insulin-responsive GLUT4 was not detected at any stage. GLUT1 and 3 were localised by immunocytochemistry. GLUT1 was expressed in both embryoblast and trophoblast, in the latter, mainly in basal and lateral membranes directed towards the blastocoel and embryoblast. GLUT3 was exclusively localised in the apical membrane of trophoblast cells. We show that hamster preimplantation embryos express several GLUT isoforms thus closely resembling embryos of other mammalian species. Despite endogenous IR expression, the insulin-sensitive isoform GLUT4 was not expressed, indicating that the insulin-mediated glucose uptake known from classical insulin target cells may not be relevant for hamster blastocysts.

Murine Glut-1 transporter haploinsufficiency: postnatal deceleration of brain weight and reactive astrocytosis

Glucose transporter type 1 (Glut-1) facilitates glucose flux across the blood-brain-barrier. In humans, Glut-1 deficiency causes acquired microcephaly, seizures and ataxia, which are recapitulated in our Glut-1 haploinsufficient mouse model. Postnatal brain weight deceleration and development of reactive astrogliosis were significant by P21 in Glut-1(+/-) mice. The brain weight differences remained constant after P21 whereas the reactive astrocytosis continued to increase and peaked at P90. Brain immunoblots showed increased phospho-mTOR and decreased phospho-GSK3-beta by P14. After fasting, the mature Glut-1(+/-) females showed a trend towards elevated phospho-GSK3-beta, a possible neuroprotective response. Lithium chloride treatment of human skin fibroblasts from control and Glut-1 DS patients produced a 45% increase in glucose uptake. Brain imaging of mature Glut-1(+/-) mice revealed a significantly decreased hippocampal volume. These subtle immunochemical changes reflect chronic nutrient deficiency during brain development and represent the experimental correlates to the human neurological phenotype associated with Glut-1 DS.

Hyperglycemia reduces mitochondrial content and glucose transporter expression in mouse embryos developing in vitro

The objective of this research was to examine the effects of high concentrations of glucose on mouse embryos developing in vitro by studying embryo viability, mitochondrial content and expression of glucose transporters. Addition of 55 mM glucose to the culture medium of two-cell stage embryos significantly reduced the formation of morulae and blastocysts, resulting in fewer cells in the blastocyst stage embryos and increased levels of apoptosis. Quantitative reverse transcriptase (RT) PCR analysis revealed that the expression levels of the pro-apoptotic genes Bax and Casp3 at the blastocyst stage were increased significantly by the addition of either 25 or 55 mM glucose to the culture medium. However, addition of 25 or 55 mM glucose to the culture medium did not change the copy numbers of the apoptosis-related miRNAs mmu-mir-15a, mmu-mir-16 and mmu-mir-21. MitoTracker Green fluorescence revealed a decrease in the mitochondrial mass. The expression levels of the mitochondrial DNA-encoded genes Cox1 and Cox2 decreased sharply with the addition of 25 or 55 mM glucose to the culture medium. Both transcripts and protein synthesis of the glucose transporters Glut1 and Glut3 were reduced in blastocysts cultured in the presence of either 25 or 55 mM glucose. These results suggest that hyperglycemia reduces both mitochondrial content and expression levels of glucose transporters in mouse embryos developing in vitro and that this may result in apoptosis in these embryos.

Essential role of glucose transporter GLUT3 for post-implantation embryonic development

Deletion of glucose transporter gene Slc2a3 (GLUT3) has previously been reported to result in embryonic lethality. Here, we define the exact time point of growth arrest and subsequent death of the embryo. Slc2a3(-/-) morulae and blastocysts developed normally, implanted in vivo, and formed egg-cylinder-stage embryos that appeared normal until day 6.0. At day 6.5, apoptosis was detected in the ectodermal cells of Slc2a3(-/-) embryos resulting in severe disorganization and growth retardation at day 7.5 and complete loss of embryos at day 12.5. GLUT3 was detected in placental cone, in the visceral ectoderm and in the mesoderm of 7.5-day-old wild-type embryos. Our data indicate that GLUT3 is essential for the development of early post-implanted embryos.

Role of glucose in cloned mouse embryo development

Cloned mouse embryos display a marked preference for glucose-containing culture medium, with enhanced development to the blastocyst stage in glucose-containing medium attributable mainly to an early beneficial effect during the first cell cycle. This early beneficial effect of glucose is not displayed by parthenogenetic, fertilized, or tetraploid nuclear transfer control embryos, indicating that it is specific to diploid clones. Precocious localization of the glucose transporter SLC2A1 to the cell surface, as well as increased expression of glucose transporters and increased uptake of glucose at the one- and two-cell stages, is also seen in cloned embryos. To examine the role of glucose in early cloned embryo development, we examined glucose metabolism and associated metabolites, as well as mitochondrial ultrastructure, distribution, and number. Clones prepared with cumulus cell nuclei displayed significantly enhanced glucose metabolism at the two-cell stage relative to parthenogenetic controls. Despite the increase in metabolism, ATP content was reduced in clones relative to parthenotes and fertilized controls. Clones at both stages displayed elevated concentrations of glycogen compared with parthenogenetic controls. There was no difference in the number of mitochondria, but clone mitochondria displayed ultrastructural alterations. Interestingly, glucose availability positively affected mitochondrial structure and localization. We conclude that cloned embryos may be severely compromised in terms of ATP-dependent processes during the first two cell cycles and that glucose may exert its early beneficial effects via positive effects on the mitochondria.

The facilitative glucose transporter GLUT3: 20 years of distinction

Glucose metabolism is vital to most mammalian cells, and the passage of glucose across cell membranes is facilitated by a family of integral membrane transporter proteins, the GLUTs. There are currently 14 members of the SLC2 family of GLUTs, several of which have been the focus of this series of reviews. The subject of the present review is GLUT3, which, as implied by its name, was the third glucose transporter to be cloned (Kayano T, Fukumoto H, Eddy RL, Fan YS, Byers MG, Shows TB, Bell GI. J Biol Chem 263: 15245-15248, 1988) and was originally designated as the neuronal GLUT. The overriding question that drove the early work on GLUT3 was why would neurons need a separate glucose transporter isoform? What is it about GLUT3 that specifically suits the needs of the highly metabolic and oxidative neuron with its high glucose demand? More recently, GLUT3 has been studied in other cell types with quite specific requirements for glucose, including sperm, preimplantation embryos, circulating white blood cells, and an array of carcinoma cell lines. The last are sufficiently varied and numerous to warrant a review of their own and will not be discussed here. However, for each of these cases, the same questions apply. Thus, the objective of this review is to discuss the properties and tissue and cellular localization of GLUT3 as well as the features of expression, function, and regulation that distinguish it from the rest of its family and make it uniquely suited as the mediator of glucose delivery to these specific cells.

Expression of the gene encoding the high-Km glucose transporter 2 by the early postimplantation mouse embryo is essential for neural tube defects associated with diabetic embryopathy

AIMS/HYPOTHESIS

Excess glucose transport to embryos during diabetic pregnancy causes congenital malformations. The early postimplantation embryo expresses the gene encoding the high-Km GLUT2 (also known as SLC2A2) glucose transporter. The hypothesis tested here is that high-Km glucose transport by GLUT2 causes malformations resulting from maternal hyperglycaemia during diabetic pregnancy.

MATERIALS AND METHODS

Glut2 mRNA was assayed by RT-PCR. The Km of embryo glucose transport was determined by measuring 0.5-20 mmol/l 2-deoxy[3H]glucose transport. To test whether the GLUT2 transporter is required for neural tube defects resulting from maternal hyperglycaemia, Glut2+/- mice were crossed and transient hyperglycaemia was induced by glucose injection on day 7.5 of pregnancy. Embryos were recovered on day 10.5, and the incidence of neural tube defects in wild-type, Glut2+/- and Glut2-/- embryos was scored.

RESULTS

Early postimplantation embryos expressed Glut2, and expression was unaffected by maternal diabetes. Moreover, glucose transport by these embryos showed Michaelis-Menten kinetics of 16.19 mmol/l, consistent with transport mediated by GLUT2. In pregnancies made hyperglycaemic on day 7.5, neural tube defects were significantly increased in wild-type embryos, but Glut2+/- embryos were partially protected from neural tube defects, and Glut2-/- embryos were completely protected from these defects. The frequency of occurrence of wild-type, Glut2+/- and Glut2-/- embryos suggests that the presence of Glut2 alleles confers a survival advantage in embryos before day 10.5.

CONCLUSIONS/INTERPRETATIONS

High-Km glucose transport by the GLUT2 glucose transporter during organogenesis is responsible for the embryopathic effects of maternal diabetes.

Tough beginnings: alterations in the transcriptome of cloned embryos during the first two cell cycles

Cloned embryos produced by somatic cell nuclear transfer (SCNT) display a plethora of phenotypic characteristics that make them different from fertilized embryos, indicating defects in the process of nuclear reprogramming by the recipient ooplasm. To elucidate the extent and timing of nuclear reprogramming, we used microarrays to analyze the transcriptome of mouse SCNT embryos during the first two cell cycles. We identified a large number of genes mis-expressed in SCNT embryos. We found that genes involved in transcription and regulation of transcription are prominent among affected genes, and thus may be particularly difficult to reprogram, and these likely cause a ripple effect that alters the transcriptome of many other functions, including oxidative phosphorylation, transport across membrane, and mRNA transport and processing. Interestingly, we also uncovered widespread alterations in the maternal (i.e., non-transcribed) mRNA population of SCNT embryos. We conclude that gene expression in early SCNT embryos is grossly abnormal, and that this is at least in part the result of incomplete reprogramming of transcription factor genes.

Differential expression of glucose transporter isoforms during embryonic stem cell differentiation

In mouse blastocysts six facilitative glucose transporter isoforms (GLUT)1-4, 8 and 9 are expressed. We have used the mouse embryonic stem (ES) cell line D3 and spontaneously differentiating embryoid bodies (EB) to investigate GLUT expression and the influence of glucose during differentiation of early embryonic cells. Both ES cells and EBs (2d-20d) expressed GLUT1, 3, and 8, whereas the isoforms 2 and 4 were detectable exclusively in EBs. Differentiation-associated expression of GLUT was analyzed by double staining with stage-specific embryonic antigen (SSEA-1), cytokeratins (CK18, 19), nestin, and desmin. Similar to trophoblast cells in mouse blastocysts the outer cell layer of endoderm-like cells showed a high GLUT3 expression in early EBs. In 20-day-old EBs no GLUT3 protein and only minor GLUT3 mRNA amounts could be detected. A minimal glucose concentration of 5 mM applied during 2 and 8 days of EB culture resulted in up-regulated GLUT4, Oct-4 and SSEA-1 levels and a delay in EB differentiation. We conclude that GLUT expression depends on cellular differentiation and that the expression is modulated by glucose concentration. The developmental and glucose-dependent regulation of GLUT strongly suggests a functional role of glucose and glucose transporters in ES cell differentiation and embryonic development.

Effects of in vitro maturation of monkey oocytes on their developmental capacity

The study of in vitro maturation (IVM) of rhesus monkey oocytes has important implications for biomedical research and human infertility treatment. In vitro-matured rhesus monkey oocytes show much less developmental potential than IVM oocytes of other species. Since about 1980 when rhesus monkey IVM, in vitro fertilization (IVF) and in vitro embryo culture (IVC) systems were established, numerous efforts have been made to improve the developmental competence of oocytes and to understand the mechanisms regulating oocyte maturation. This review describes recent progress in this area, particularly the effects of factors such as steroid hormones, energy substrates, amino acids, ovarian follicle status, maternal age and breeding season on the developmental competence, gene expression patterns and genome integrity of rhesus IVM oocytes.

Glucose utilization and the PI3-K pathway: mechanisms for cell survival in preimplantation embryos

The maintenance of optimal glucose utilization during the preimplantation period is critical for embryo survival. A decrease in glucose transport during preimplantation development has been linked to the early steps of programmed cell death in these embryos. Decreased glucose transport is not thought to be simply a consequence of cell death, rather it is thought to be a trigger that can initiate the apoptotic cascade. Extensive apoptosis during the preimplantation period may manifest later in pregnancy as a malformation – or miscarriage, if cell loss is excessive. Phosphatidylinositol 3-kinase (PI3-K) is a known regulator of a number of physiologic responses including cellular proliferation, growth, and survival as well as glucose metabolism. Studies performed in other cell systems have demonstrated that the PI3-K pathway plays a critical role in maintaining glucose transport and metabolism. This review will present the current evidence that suggests that PI3-K is vital for preimplantation embryo survival and development. In addition, data demonstrating that PI3-K activity is important for glucose metabolism during this early developmental period will be discussed.

High glucose-induced inhibition of 2-deoxyglucose uptake is mediated by cAMP, protein kinase C, oxidative stress and mitogen-activated protein kinases in mouse embryonic stem cells

Abnormally high glucose levels may play an important role in early embryo development and function. In the present study, we investigated the effect of high glucose on 2-deoxyglucose (2-DG) uptake and its related signalling pathway in mouse embryonic stem (ES) cells. 2. 2-Deoxyglucose uptake was maximally inhibited by 25 mmol/L glucose after 24 h treatment. However, 25 mmol/L mannitol and dextran did not affect 2-DG uptake. Indeed, 25 mmol/L glucose decreased GLUT-1 mRNA and protein levels. The glucose (25 mmol/L)-induced inhibition of 2-DG uptake was blocked by pertussis toxin (a G(i)-protein inhibitor; 2 ng/mL), SQ 22,536 (an adenylate cyclase inhibitor; 10(-6) mol/L) and the protein kinase (PK) A inhibitor myristoylated PKI amide-(14-22) (10(-6) mol/L). Indeed, 25 mmol/L glucose increased intracellular cAMP content. 3. Furthermore, 25 mmol/L glucose-induced inhibition of 2-DG uptake was prevented by 10(-4) mol/L neomycin or 10(-6) mol/L U 73,122 (phospholipase C (PLC) inhibitors) and staurosporine or bisindolylmaleimide I (protein kinase (PK) C inhibitors). At 25 mmol/L, glucose increased translocation of PKC from the cytoplasmic fraction to the membrane fraction. The 25 mmol/L glucose-induced inhibition of 2-DG uptake and GLUT-1 protein levels was blocked by SQ 22,536, bisindolylmaleimide I or combined treatment. In addition, 25 mmol/L glucose increased cellular reactive oxygen species and the glucose-induced inhibition of 2-DG uptake were blocked by the anti-oxidants N-acetylcysteine (NAC; 10(-5) mol/L) or taurine (2 yen 10(-3) mol/L). 4. Glucose (25 mmol/L) activated p38 mitogen-activated protein kinase (MAPK) and p44/42 MAPK. Staurosporine (10(-6) mol/L), NAC (10(-5) mol/L) and PD 98059 (10(-7) mol/L) attenuated the phosphorylation of p44/42 MAPK. Both SB 203580 (a p38 MAPK inhibitor; 10(-7) mol/L) and PD 98059 (a p44/42 MAPK inhibitor; 10(-7) mol/L) blocked 25 mmol/L glucose-induced inhibition of 2-DG uptake. 5. In conclusion, high glucose inhibits 2-DG uptake through cAMP, PLC/PKC, oxidative stress or MAPK in mouse ES cells.

PKC and MAPKs Pathways Mediate EGF-induced Stimulation of 2-Deoxyglucose Uptake in Mouse Embryonic Stem Cells

It has been reported that epidermal growth factor (EGF) and EGF receptor were highly expressed in embryo, suggesting that the EGF system is related to early embryo development in an autocrine and/or paracrine manner. Glucose becomes the preimplantation exogenous energy substrate and enters the blastocyst via glucose transporters. Thus, the effect of EGF on [3H]-2-deoxyglucose (2-DG) uptake and its related signaling pathways were examined in mouse embryonic stem (ES) cells. EGF significantly increased 2-DG uptake in time- and concentration- dependent manner (>12 hr, >10 ng/ ml) and increased mRNA and protein level of glucose transporter 1 (GLUT1) compared to control, respectively. Actinomycin D and cycloheximide completely blocked the effect of EGF on 2-DG uptake. EGF-induced increase of 2-DG uptake was blocked by AG1478 (EGF receptor tyrosine kinase blocker), genistein or herbimycin (tyrosine kinase inhibitors). In addition, EGF effect was blocked by neomycin and U 73122 [phospholipase C (PLC) inhibitors] as well as staurosporine and bisindolylmaleimide I [protein kinase C (PKC) inhibitors]. EGF was also observed to increase inositol phosphates (IPs) formation and activate a PKC translocation from the cytosolic to membrane fraction, suggesting a role of PLC and PKC. SB 203580 [p38 mitogen activated protein kinase (MAPK) inhibitor] or PD 98059 (p44/42 MAPKs inhibitor) blocked EGF-induced increase of 2-DG uptake. EGF also increased phosphorylation of p38 MAPK and p44/42 MAPKs, which was blocked by genistein or bisindolylmaleimide I, respectively. In conclusion, EGF partially increased 2-DG uptake via PKC, p38 MAPK, and p44/42 MAPKs in mouse ES cells.

Ependymal Cell Differentiation and GLUT1 Expression is a Synchronous Process in the Ventricular Wall

Ependymal cells appear to be totally differentiated during the first 3 weeks in the mouse brain. Early during postnatal development ependymal cells differentiate and undergo metabolic activation, which is accompanied by increased glucose uptake. We propose that ependymal cells induce an overexpression of the glucose transporter, GLUT1, during the first 2 weeks after delivery in order to maintain the early metabolic activation. During the first postnatal day, GLUT1 is strongly induced in the upper region of the third ventricle and in the ventral area of the rostral cerebral aqueduct. During the next 4 days, GLUT1 is expressed in all differentiated ependymal cells of the third ventricle and in hypothalamic tanycytes. At the end of the first week, ependymal cell differentiation and GLUT1 overexpression is concentrated in the latero-ventral area of the aqueduct. We propose that ependymal cell differentiation and GLUT1 overexpression is a synchronous process in the ventricular wall.

ANG II increases 2-deoxyglucose uptake in mouse embryonic stem cells

It is now suggested that all components of the renin-angiotensin system are present in many tissues, including the embryo and may play a major role in embryo development and differentiation. However, little is known regarding whether ANG II regulates glucose transport in mouse embryonic stem (ES) cells. Thus, the effects of ANG II on [3H]-2-deoxyglucose (2-DG) uptake and its related signal pathways were examined in mouse ES cells. ANG II significantly increased cell proliferation and 2-DG uptake in concentration- and time-dependent manner (>18 h, >10(-8) M) and increased mRNA and protein level of GLUT1 by 31+/-7% and 22+/-5% compared to control, respectively. Actinomycin D and cycloheximide completely blocked the effect of ANG II on 2-DG uptake. ANG II-induced increase of 2-DG uptake was blocked by losartan, an ANG II type 1 (AT1) receptor blocker, but not by PD 123319, an ANG II type 2 (AT2) receptor blocker. In addition, ANG II-induced stimulation of 2-DG uptake was attenuated by phospholipase C (PLC) inhibitors, neomycin and U 73122 and ANG II increased inositol phosphates (IPs) formation by 37+/-8% of control. Protein kinase C (PKC) inhibitors, staurosporine, bisindolylmaleimide I, and H-7 also blocked ANG II-induced stimulation of 2-DG uptake. Indeed, ANG II activated a PKC translocation from the cytosolic to membrane fraction, suggesting a role of PKC. A 23187 (Ca2+ ionophore) increased 2-DG uptake and nifedifine (L-type Ca2+ channel blocker) blocked it. In conclusion, ANG II increased 2-DG uptake by PKC activation via AT1 receptor in mouse ES cells.

Current perspectives on the causes of neural tube defects resulting from diabetic pregnancy

Maternal diabetes increases the risk for neural tube, and other, structural defects. The mother may have either type 1 or type 2 diabetes, but the diabetes must be existing at the earliest stages of pregnancy, during which organogenesis occurs. Abnormally high glucose levels in maternal blood, which leads to increased glucose transport to the embryo, is responsible for the teratogenic effects of maternal diabetes. Consequently, expression of genes that control essential developmental processes is disturbed. In this review, some of the biochemical pathways by which excess glucose metabolism disturbs neural tube formation are discussed. Research from the author's laboratory has shown that expression of Pax3, a gene required for neural tube closure, is significantly reduced by maternal diabetes, and this is associated with significantly increased neural tube defects (NTD). Pax3 encodes a transcription factor that has recently been shown to inhibit p53-dependent apoptosis. Evidence in support of this model, in which excess glucose metabolism inhibits expression of Pax3, thereby derepressing p53-dependent apoptosis of neuroepithelium and leading to NTD will be discussed.

Progressive elimination of microinjected trehalose during mouse embryonic development

Recently, sugars such as trehalose have been introduced into mammalian cells by overcoming the permeability barrier of cell membranes, and have provided improved tolerance against stresses associated with freezing and drying. However, the fate of the intracellular sugars has remained an open question. To address this issue, mouse oocytes were microinjected with 0.1 mol/l trehalose, and intracellular trehalose and glucose concentrations were determined during embryonic development using a high performance liquid chromatography and pulsed amperometric detection protocol. Trehalose was not detected in non-injected controls at any stage of development. In the microinjection group, the amount of intracellular trehalose progressively decreased as embryos developed. There was a corresponding increase in intracellular glucose concentration at the two-cell stage, suggesting cleavage of trehalose to two glucose molecules. In summary, this study presents a simple, highly sensitive protocol to determine intracellular sugars. The data reveal rapid elimination of microinjected trehalose during embryonic development. These findings have implications for designing osmolarity-optimized culture media for sugar-injected oocytes.

Identification of differentially expressed genes in murine embryos at the blastocyst stage using annealing control primer system

The identification of embryo-specific genes would provide insights into early embryonic development. However, the current methods employed to identify the genes that are expressed at a specific developmental stage are labor intensive and suffer from high rates of false positives. Here we employed a new and accurate reverse transcription-polymerase chain reaction (RT-PCR) method that involves annealing control primers (ACPs) to identify the genes that are specifically or prominently expressed in mouse blastocysts compared to 4-cell stage embryos. Using 120 ACPs, we identified and sequenced 74 of these differentially expressed genes (DEGs). Basic Local Alignment Search Tool (BLAST) searches revealed that 53 were known genes, 9 encoded ribosomal proteins, and 12 were unknown genes. Of the known genes, 14 were selected and further characterized using real-time quantitative PCR to assess their stage-specific expression in mouse embryos. This analysis suggests that the ACP system is a very good method for the identification of stage-specific genes in small numbers of mouse embryos. Further analysis of the differentially expressed blastocyst genes we have identified will provide insights into the molecular basis of preimplantation development.

Role of gap junctions during early embryo development

Gap junctional communication plays a central role in the maintenance of cellular homeostasis by allowing the passage of small molecules between adjacent cells. Gap junctions are composed of a family of proteins termed connexins. During preimplantation development several connexin proteins are expressed and assembled into gap junctions in the plasma membrane at compaction but the functional significance of connexin diversity remains controversial. Although, many of the connexin genes have been disrupted using homologous recombination in embryonic stem cells to obtain unique phenotypes, none of these studies has demonstrated a specific role for connexins during preimplantation development in the null mutants. This review surveys evidence for the involvement of gap junctional communication during embryo development highlighting discrepancies in the literature. Although some evidence suggests that gap junctions may be dispensable during preimplantation development this is difficult to envisage particularly for the process of cavitation and the maintenance of homeostasis between the differentiated trophectoderm cells and the pluripotent inner cell mass cells of the blastocyst.

Insulin acts via mitogen-activated protein kinase phosphorylation in rabbit blastocysts

The addition of insulin during in vitro culture has beneficial effects on rabbit preimplantation embryos leading to increased cell proliferation and reduced apoptosis. We have previously described the expression of the insulin receptor (IR) and the insulin-responsive glucose transporters (GLUT) 4 and 8 in rabbit preimplantation embryos. However, the effects of insulin on IR signaling and glucose metabolism have not been investigated in rabbit embryos. In the present study, the effects of 170 nM insulin on IR, GLUT4 and GLUT8 mRNA levels, Akt and Erk phosphorylation, GLUT4 translocation and methyl glucose transport were studied in cultured day 3 to day 6 rabbit embryos. Insulin stimulated phosphorylation of the mitogen-activated protein kinase (MAPK) Erk1/2 and levels of IR and GLUT4 mRNA, but not phosphorylation of the phosphatidylinositol 3-kinase-dependent protein kinase, Akt, GLUT8 mRNA levels, glucose uptake or GLUT4 translocation. Activation of the MAPK signaling pathway in the absence of GLUT4 translocation and of a glucose transport response suggest that in the rabbit preimplantation embryo insulin is acting as a growth factor rather than a component of glucose homeostatic control.

Two insulin-responsive glucose transporter isoforms and the insulin receptor are developmentally expressed in rabbit preimplantation embryos

Glucose is the most important energy substrate for mammalian blastocysts. Its uptake is mediated by glucose transporters (GLUT). In muscle and adipocyte cells insulin stimulates glucose uptake by activation of the insulin receptor (IR) pathway and translocation of GLUT4. GLUT4 is expressed in bovine preimplantation embryos. A new insulin-responsive isoform, GLUT8, was recently described in mouse blastocysts. Thus, potentially, two insulin-responsive isoforms are expressed in early embryos. The mechanism of insulin action on embryonic cells, however, is still not clear. In the present study expression of IR, GLUT1, 2, 3, 4, 5 and 8 was studied in rabbit preimplantation embryos using RT-PCR, Western blotting and immunohistochemistry. The rabbit mRNA sequences for the complete coding region of IR, GLUT4 and a partial GLUT8 sequence were determined by RACE-PCR and sequencing. GLUT4 was expressed in 3-day-old morulae and in 4- and 6-day-old blastocysts. IR and GLUT8 transcripts were detectable only in blastocysts. Blastocysts also expressed GLUT1 and 3, but not GLUT2 and 5. Transcript numbers of GLUT4 and 8 were higher in trophoblast than in embryoblast cells. Translation of IR, GLUT4 and 8 proteins in blastocysts was confirmed by Western blotting. GLUT4 was localized mainly in the membrane and in the perinuclear region in trophoblast cells while in embryoblast cells its localization was predominantly in the perinuclear cytoplasm. The possible function(s) of two insulin-responsive isoforms, GLUT4 and GLUT8, in rabbit preimplantation embryos needs further investigation. It may not necessarily be linked to insulin-stimulated glucose transport.

The cellular fate of glucose and its relevance in type 2 diabetes

Type 2 diabetes is a complex disorder with diminished insulin secretion and insulin action contributing to the hyperglycemia and wide range of metabolic defects that underlie the disease. The contribution of glucose metabolic pathways per se in the pathogenesis of the disease remains unclear. The cellular fate of glucose begins with glucose transport and phosphorylation. Subsequent pathways of glucose utilization include aerobic and anaerobic glycolysis, glycogen formation, and conversion to other intermediates in the hexose phosphate or hexosamine biosynthesis pathways. Abnormalities in each pathway may occur in diabetic subjects; however, it is unclear whether perturbations in these may lead to diabetes or are a consequence of the multiple metabolic abnormalities found in the disease. This review is focused on the cellular fate of glucose and relevance to human type 2 diabetes.

TRAIL and KILLER Are Expressed and Induce Apoptosis in the Murine Preimplantation Embryo1

TRAIL (tumor necrosis factor [TNF]-related apoptosis-inducing ligand) and KILLER are a death-inducing ligand and receptor pair that belong to the TNF and TNF-receptor superfamilies, respectively. To date, only one apoptosis-inducing TRAIL receptor (murine KILLER [MK]) has been identified in mice, and it is a homologue of human Death Receptor 5. Whereas the expression of other death receptors, such as Fas and TNF receptor 1 have been documented in mammalian preimplantation embryos, no evidence currently demonstrates either the presence or the function of TRAIL and its corresponding death receptor, MK. Using reverse transcription-polymerase chain reaction and confocal immunofluorescent microscopy, we found that both TRAIL and MK are expressed from the 1-cell through the blastocyst stage of murine preimplantation embryo development. These proteins are localized mainly at the cell surface from the 1-cell through the morula stage. At the blastocyst stage, both TRAIL and MK exhibit an apical staining pattern in the trophectoderm cells. Finally, using the TUNEL assay, we demonstrated that MK induces apoptosis in blastocysts sensitized to TRAIL via actinomycin D. Taken together, these data are the first to demonstrate the presence and function of TRAIL and MK, a death-inducing ligand and its receptor, in mammalian preimplantation embryos.

In vivo hyperglycemia and its effect on Glut-1 expression in the embryonic heart

BACKGROUND

Maternal diabetes exposes embryos to periods of hyperglycemia. Glucose is important for normal cardiogenesis, and Glut-1 is the predominant glucose transporter in the embryo.

METHODS

Pregnant mice were exposed to 6 or 12 hr hyperglycemia during organogenesis using intraperitoneal (IP) injections of D-glucose on gestational day (GD) 9.5 (plug = GD 0.5). Embryos were examined for morphology and total cardiac protein, and embryonic hearts were evaluated for Glut-1 protein and mRNA expression immediately after treatment (GD 9.75, GD 10.0), as well as on GD 10.5 and GD 12.5.

RESULTS

IP glucose injections were effective in producing sustained maternal hyperglycemia. Maternal hyperglycemia for 6 or 12 hr on GD 9.5, followed by normoglycemia, produced a decrease in overall size and total cardiac protein in embryos evaluated on GD 10.5 but no difference on GD 12.5. Cardiac Glut-1 expression was immediately upregulated in embryos exposed to 6 or 12 hr maternal hyperglycemia. On GD 10.5, cardiac Glut-1 expression was not different in embryos exposed to maternal hyperglycemia for 6 hr but was downregulated in embryos exposed for 12 hr. On GD 12.5, cardiac Glut-1 expression in embryos exposed to maternal hyperglycemia on GD 9.5 for 6 or 12 hr, followed by normoglycemia, was not different from controls. The temporal pattern was the same for Glut-1 protein and mRNA expression.

CONCLUSIONS

Hyperglycemia-induced alterations in Glut-1 expression likely interfere with balance of glucose available to the embryonic heart that may affect cardiac morphogenesis.