Oxidative DNA damage and embryo development

High glucose-increased miR-200c contributes to cellular senescence and DNA damage in neural stem cells

BACKGROUND

Maternal diabetes increases the risk for neural tube defects (NTDs). It is unclear if miRNAs, senescence, and DNA damage are involved in this process. In this study, we used neural stem cells as an in vitro proxy of embryonic neuroepithelium to investigate whether high glucose triggers neural stem cell senescence and DNA damage by upregulating miR-200c, which may be responsible for NTDs.

METHODS

C17.2 neural stem cells were cultured with normal glucose (5 mM) or high glucose (≥16.7 mM) at different doses and time points for detecting miR-200c levels, markers of senescence and DNA damage. Neural stem cells were exposed to antioxidant SOD1 mimetic Tempol and high glucose for 48 h to test roles of oxidative stress on the miR-200c, senescence, and DNA damage levels. An miR-200c mimic and an inhibitor were transfected into neural stem cells to increase or decrease miR-200c activities.

RESULTS

High glucose upregulated miR-200c in neural stem cells. A time course study of the effect of high glucose revealed that miR-200c initially increased at 12 h and reached its zenith at 18 h. Tempol reduced miR-200c levels caused by high glucose. High glucose induced markers of senescence and DNA damage in neural stem cells. Tempol abolished high glucose-induced markers of senescence and DNA damage. The miR-200c inhibitor suppressed high glucose-induced markers of senescence and DNA damage. Treatment with miR-200c mimic imitates high glucose-induced markers of senescence and DNA damage.

CONCLUSIONS

We show that high glucose increases miR-200c, which contributes to cellular senescence and DNA damage in neural stem cells and provides a potential pathway for maternal diabetes-induced neural tube defects.

Oxygen and placental development; parallels and differences with tumour biology

Human placentation involves the invasion of the conceptus into the wall of the uterus, and establishment of a blood supply from the maternal spiral arteries. The placenta has therefore been likened to a malignant tumour, albeit a highly regulated one. Oxygen plays an important role in controlling both placental development and tumour behaviour. In the placenta, early development takes place in a physiological low oxygen environment, which undergoes a transition with onset of the full maternal arterial circulation towards the end of the first trimester. By comparison, in tumours there is often a progressive hypoxia as the mass outgrows its blood supply. Both early placental tissues and tumour cells show high rates of proliferation, and the energy required to support these comes principally from glycolysis. Glycolysis is maintained in placental tissues by reoxidation of pyridine nucleotides through the polyol pathways, whereas in tumours there is fermentation to lactate, Warburg metabolism. In both cases, the reliance on glycolysis rather than oxidative phosphorylation preserves carbon skeletons that can be utilised in the synthesis of nucleotides, cell membranes and organelles, and that would otherwise be excreted as carbon dioxide. In the placenta, this reliance may also protect the embryo from free radical-mediated teratogenesis. Local oxygen gradients within both sets of tissues may influence the cell behaviour. In particular, they may induce an epithelial-mesenchymal transition, promoting extravillous trophoblast invasion in the placenta and metastasis in a tumour. Further investigations into the two scenarios may provide new insights of benefit to these contrasting, but similar, fields of cellular biology.

The status of diabetic embryopathy

Diabetic embryopathy is a theoretical enigma and a clinical challenge. Both type 1 and type 2 diabetic pregnancy carry a significant risk for fetal maldevelopment, and the precise reasons for the diabetes-induced teratogenicity are not clearly identified. The experimental work in this field has revealed a partial, however complex, answer to the teratological question, and we will review some of the latest suggestions.

Keap1-Nrf2 regulated redox signaling in utero: Priming of disease susceptibility in offspring

Intrauterine exposure to gestational diabetes, pre-eclampsia or intrauterine growth restriction alters the redox status of the developing fetus. Such pregnancy-related diseases in most cases do not have a readily identifiable genetic cause, and epigenetic 'priming' mechanisms in utero may predispose both mother and child to later-life onset of cardiovascular and metabolic diseases. The concept of 'fetal programing' or 'developmental priming' and its association with an increased risk of disease in childhood or adulthood has been reviewed extensively. This review focuses on adaptive changes in the in utero redox environment during normal pregnancy and the consequences of alterations in redox control associated with pregnancies characterized by oxidative stress. We evaluate the evidence that the Keap1-Nrf2 pathway is important for protecting the fetus against adverse conditions in utero and may itself be subject to epigenetic priming, potentially contributing to an increased risk of vascular disease and insulin resistance in later life.

2.45 GHz microwave irradiation-induced oxidative stress affects implantation or pregnancy in mice, Mus musculus

The present experiment was designed to study the 2.45 GHz low-level microwave (MW) irradiation-induced stress response and its effect on implantation or pregnancy in female mice. Twelve-week-old mice were exposed to MW radiation (continuous wave for 2 h/day for 45 days, frequency 2.45 GHz, power density=0.033549 mW/cm(2), and specific absorption rate=0.023023 W/kg). At the end of a total of 45 days of exposure, mice were sacrificed, implantation sites were monitored, blood was processed to study stress parameters (hemoglobin, RBC and WBC count, and neutrophil/lymphocyte (N/L) ratio), the brain was processed for comet assay, and plasma was used for nitric oxide (NO), progesterone and estradiol estimation. Reactive oxygen species (ROS) and the activities of ROS-scavenging enzymes- superoxide dismutase, catalase, and glutathione peroxidase-were determined in the liver, kidney and ovary. We observed that implantation sites were affected significantly in MW-irradiated mice as compared to control. Further, in addition to a significant increase in ROS, hemoglobin (p<0.001), RBC and WBC counts (p<0.001), N/L ratio (p<0.01), DNA damage (p<0.001) in brain cells, and plasma estradiol concentration (p<0.05), a significant decrease was observed in NO level (p<0.05) and antioxidant enzyme activities of MW-exposed mice. Our findings led us to conclude that a low level of MW irradiation-induced oxidative stress not only suppresses implantation, but it may also lead to deformity of the embryo in case pregnancy continues. We also suggest that MW radiation-induced oxidative stress by increasing ROS production in the body may lead to DNA strand breakage in the brain cells and implantation failure/resorption or abnormal pregnancy in mice.

Linkage study of embryopathy-polygenic inheritance of diabetes-induced skeletal malformations in the rat

We developed an inbred rat model of diabetic embryopathy, in which the offspring displays skeletal malformations (agnathia or micrognathia) when the mother is diabetic, and no malformations when she is not diabetic. Our aim was to find genes controlling the embryonic maldevelopment in a diabetic environment. We contrasted the fetal outcome in inbred Sprague-Dawley L rats (20% skeletal malformations in diabetic pregnancy) with that of inbred Wistar Furth rats (denotedW, no skeletal malformations in diabetic pregnancy). We used offspring from the backcross F(1)×L to probe for the genetic basis for malformation of the mandible in diabetic pregnancy. A set of 186 fetuses (93 affected, 93 unaffected) was subjected to a whole genome scan with 160 micro satellites. Analysis of genotype distribution indicated 7 loci on chromosome 4, 10 (3 loci), 14, 18, and 19 in the teratogenic process (and 14 other loci on 12 chromosomes with less strong association to the malformations), several of which contained genes implicated in other experimental studies of diabetic embryopathy. These candidate genes will be scrutinized in further experimentation. We conclude that the genetic involvement in rodent diabetic embryopathy is polygenic and predisposing for congenital malformations.

Congenital anomalies in diabetic pregnancy

Congenital malformations are more common in infants of diabetic women than in children of non-diabetic women. The etiology, pathogenesis and prevention of the diabetes-induced malformations have spurred considerable clinical and basic research efforts. The ultimate aim of these studies has been to obtain an understanding of the teratogenic process, which may enable precise preventive therapeutic measures in diabetic pregnancies. The results of the clinical and basic studies support the view of an early gestational induction of the malformations in diabetic pregnancy by a teratogenic process of multifactorial etiology. There may be possible targets for new therapeutic efforts revealed by the research work. Thus, future additions to the therapeutic efforts may include supplementation with antioxidants and/or folic acid, although more research is needed to delineate the dosages and compounds to be used. As the research into genetic predisposition for the teratogenic induction of malformations by maternal diabetes starts to reveal new genes and gene products involved in the etiology of the malformations, a set of new targets for intervention may arise.

Oxygen, the Janus gas; its effects on human placental development and function

The accumulation of oxygen in the earth's atmosphere enabled metabolic pathways based on high-energy electron transfers that were capable of sustaining complex multicellular organisms to evolve. This advance came at a price, however, for the high reactivity of oxygen posed a major challenge as biological molecules became susceptible to oxidative damage, resulting in potential loss of function. Many extant physiological systems are therefore adapted, and homeostatically regulated, to supply sufficient oxygen to meet energy demands whilst also protecting cells, and mitochondria in particular, from excessive concentrations that could lead to oxidative damage. The invasive form of implantation displayed by the human conceptus presents particular challenges in this respect. During the first trimester, the conceptus develops in a low oxygen environment that favours organogenesis in the embryo, and cell proliferation and angiogenesis in the placenta. Later in pregnancy, higher oxygen concentrations are required to support the rapid growth of the fetus. This transition, which appears unique to the human placenta, must be negotiated safely for a successful pregnancy. Normally, onset of the maternal placental circulation is a progressive periphery-centre phenomenon, and is associated with extensive villous regression to form the chorion laeve. In cases of miscarriage, onset of the circulation is both precocious and disorganized, and excessive placental oxidative stress and villous regression undoubtedly contribute to loss of the pregnancy. Comparison of experimental and in vivo data indicates that fluctuations in placental oxygen concentration are a more powerful stimulus for the generation of oxidative stress than chronic hypoxia alone. Placental oxidative and endoplasmic reticulum stress appear to play key roles in the pathophysiology of complications of pregnancy, such as intrauterine growth restriction and preeclampsia, through their adverse impacts on placental function and growth. Establishing an inviolable maternal blood supply for the second and third trimesters is therefore one of the most crucial aspects of human placentation.

Physiological implications of the materno-fetal oxygen gradient in human early pregnancy

This study evaluates the role of the fetal fluid cavities on materno-fetal oxygen diffusion in early pregnancy. Oxygen tension (pO2) was recorded using a multiparameter sensor inserted inside the exocoelomic cavity (ECC) or in the amniotic cavity. There was no correlation between coelomic pO(2) and gestational age, but a negative correlation was found between amniotic pO(2) and gestational age. The mean (SEM) pO(2) was 19.5 mm Hg (1.83) in the ECC at 7-11 weeks and 15.4 mm Hg (1.36) in the amniotic cavity at 11-16 weeks. The volume of the ECC changed little between 7 and 10 weeks of gestation, indicating that coelomic pO(2) results from passive oxygen diffusion through the placenta and is an indicator of the overall pO(2) inside the gestational sac during the first trimester. By contrast, the amniotic cavity volume increases exponentially, whereas amniotic pO(2) decreases with gestational age, suggesting that the increase in uterine blood flow is not sufficient to compensate for the rapid increase in amniotic fluid volume during the first half of pregnancy.

The human first trimester gestational sac limits rather than facilitates oxygen transfer to the foetus--a review

Oxygen (O2) free radicals are a potential teratologic threat to the foetal tissues and are known to be involved in the pathophysiology of common human pregnancy disorders such as miscarriage and pre-eclampsia. During the first two months of human gestation, the placenta surrounds the whole gestational sac, the villi contain only a few capillaries located mainly within the centre of the mesenchymal core, the trophoblastic layer is twice the thickness it will be in the second trimester, the foetal red cells are nucleated and the exocoelomic cavity (ECC) occupies most of the space inside the gestational sac. The ECC contains no oxygen transport system, but anti-oxidant molecules that may provide additional protection to the embryo from oxidative damage are present. Ultrasound and anatomical studies have also demonstrated that the intervillous circulation starts in the periphery of the placenta at around 9 weeks of gestation, and that it becomes continuous and diffuse in the entire placenta only after 12 weeks. Overall, these anatomical features provide indirect evidence that the architecture of the human first trimester gestational sac limits foetal exposure to O2 to what is strictly necessary for its development. These results are in agreement with the concept that the placenta and foetus develop in a physiologically low O2 environment and that its metabolism must be essentially anaerobic. Because of these anatomical arrangements, different nutritional pathways to those operating during most of pregnancy must serve the first-trimester foetus. Up to 9 weeks of gestation, foetal nutrition appears to depend on uterine glandular secretions that are delivered into the intervillous space, supplemented by maternal plasma proteins and other molecules that may percolate through the trophoblastic shell. These molecules diffuse through, or are transported by, the trophoblast of the villi and the chorionic plate into the ECC. From here they are absorbed by the secondary yolk sac (SYS), in which the extraembryonic circulation is probably first established. At the end of the first trimester, the SYS and two-thirds of the placental mass degenerate, and the ECC is progressively obliterated by the enlarging amniotic cavity. The trophoblastic plugs occluding the utero-placental arteries are gradually dislocated, allowing maternal blood to flow into the intervillous space, and the uterine glands involute. These major anatomical transformations modify considerably the spatial relationships between the maternal tissues and the developing embryo, and, consequently, the materno-embryonic exchange pathways. Overall the comparison of morphological features with physiological findings reveals that the architecture of the human first trimester gestational sac is designed to limit foetal exposure to oxygen to that which is strictly necessary for its development, and that during early pregnancy alternative nutritional pathways are in use.

Oxygen, early embryonic metabolism and free radical-mediated embryopathies

Free radicals, once the preserve of chemists, are now recognized as playing a central role in many biological systems. They are formed as an inevitable by-product of aerobic respiration and various cytoplasmic processes at a rate dependent upon the prevailing oxygen tension. At physiological concentrations, oxygen and nitrogen free radical species play key roles in intracellular signalling, regulating many homeostatic mechanisms and mediating stress responses. If concentrations exceed cellular defences, however, then indiscriminate damage may occur to lipids, proteins and DNA. Cell function may be perturbed, and in the most severe cases apoptosis may result. Although there are significant species differences, many aspects of early mammalian development, from fertilization through to differentiation of the principal organ systems, take place in vivo in a low oxygen environment. This may serve to protect the embryo from free radical damage, for exposure of early embryos to ambient oxygen concentrations or the products of maternal metabolic disorders is often associated with reduced viability and an increased rate of congenital malformations. Administration of free radical scavengers, including vitamins C and E, can mitigate many of these effects, indicating the importance of a balanced maternal diet to successful reproduction.

Uterine glands provide histiotrophic nutrition for the human fetus during the first trimester of pregnancy

Providing adequate nutrition to the fetus is key to a successful pregnancy. The interstitial form of implantation displayed by the human blastocyst is generally associated with early onset of maternal blood flow to the developing placenta, and hence hemotrophic exchange. However, the recent finding that the maternal intraplacental circulation is not fully established until the third month of gestation suggests that human fetal nutrition may be initially histiotrophic. We therefore investigated activity of the uterine glands during early pregnancy. We demonstrate here that these glands remain active until at least wk 10 of pregnancy, and that their secretions are delivered freely into the placental intervillous space. We also demonstrate phagocytic uptake by the placental syncytiotrophoblast of two glycoproteins, the mucin MUC-1 and glycodelin A, synthesized in the maternal glands. Glycodelin was also detected within the epithelium of the secondary yolk sac lining the exocoelomic cavity, indicating that the yolk sac may play an important role in nutrient exchange before vascularisation of the chorionic villi. Our findings demonstrate that the uterine glands are an important source of nutrients during organogenesis, when metabolism is essentially anaerobic.

DNA damage: air-breaks?

Cells deficient in repairing DNA double-strand breaks have an increased level of spontaneous chromosomal aberrations. Modulating the level of molecular oxygen and its reactive metabolites demonstrates that oxygen metabolism is a major source of genomic instability.

Maternal vascularisation of the human placenta: does the embryo develop in a hypoxic environment?

Compared to other mammals, implantation in the human establishes a precocious and intimate apposition between the maternal and fetal tissues. In the past it has been assumed that this relationship permits early onset of haemotrophic exchange, which in turn confers evolutionary advantage by supporting higher development. However, there is now strong evidence from a number of different disciplines to suggest that human pregnancy comprises two contrasting periods. During the first trimester there is little maternal bloodflow to the placenta, the oxygen tension within the feto-placental unit is low, and the uterine glands may provide much of the nutrient supply. At the start of the second trimester the maternal circulation within the intervillous space becomes fully established, the oxygen tension rises and haemotrophic nutrition becomes dominant. During the transition period there is a period of placental oxidative stress, and the response of the tissues to the changing oxygen concentration may play a pivotal role in determining the success or otherwise of the pregnancy.

Nutrition of the human fetus during the first trimester--a review

In all mammalian species nutrition of the conceptus is initially histiotrophic, with the trophectoderm phagocytosing first oviductal and then uterine secretions. Following implantation and establishment of the chorioallantoic placenta there is a transition to haemotrophic nutrition, with exchange between the maternal and fetal circulations. It has long been assumed that the transition occurs soon after implantation in the human, due to the invasive nature of this process. However, the recent realization that the maternal circulation to the placenta is not fully established until the end of the first trimester casts doubt on the validity of this assumption. There is new evidence that the uterine glands discharge secretions into the intervillous space until at least 8 weeks of pregnancy, and that these are taken up by the syncytiotrophoblast. Also, during early pregnancy selected maternal proteins accumulate within the fluid of the coelomic cavity, from which they may be transported to the fetus by the secondary yolk sac. Histiotrophic nutrition may be advantageous to the fetus during the first trimester as it provides nutrients under a low oxygen concentration, so reducing the risk of free radical mediated damage during the sensitive period of organogenesis. Once this is complete, and fetal oxygen requirements rise, there is a transition to haemotrophic nutrition at the start of the second trimester, when the maternal placental circulation is fully established.

Effect of oxidative stress on development and DNA damage in in-vitro cultured bovine embryos by comet assay

The correlation of oxidative stress on development and DNA damage in bovine embryos was investigated by the comet assay (single-cell microgel electrophoresis), an effective technique for detecting single-strand DNA breakage. After in vitro maturation and fertilization, one-cell stage embryos without cumulus cells were cultured for 8 days in SOF medium containing amino acids plus 5% FCS under low (5%) and atmospheric (20% ) oxygen concentration. After 8 days of culture, the extent of blastocyst formation was significantly decreased (P<0.001) when embryos were cultured under 20% oxygen concentration (5.8 +/- 2.4%) when compared to embryos cultured under 5% oxygen concentration (35.1 +/- 6.7%). At the day 3 of development, DNA damage of individual embryos cultured under 5% or 20% oxygen concentration was measured by the comet assay, which entails microgel electrophoresis that can readily detect damaged DNA. After measuring the DNA damage in individual embryos by the comet assay, the length (149.9 +/- 15.3 microm) of the migrating DNA fragment that is indicative of damaged DNA was significantly increased (P<0.001) in the embryos cultured under 20% oxygen concentration when compared to embryos cultured in 5% oxygen concentration (42.3 +/- 7 microm). The length of damaged DNA in more than 50% of embryos was less than 50 microm. when embryos were cultured under 5% oxygen concentration. In contrast, the distribution of damaged DNA shifted to the more damaged extent when embryos were cultured under 20% oxygen concentration. These results demonstrate that the retardation in bovine embryo development than in likely due oxidative stress as a consequence of the higher atmospheric oxygen concentration is positively correlated with an increase in the extent of DNA damage. Moreover, these results demonstrate that the comet assay is a useful method to evaluate embryo culture conditions.

Pathogenesis of diabetes-induced congenital malformations

The increased rate of fetal malformation in diabetic pregnancy represents both a clinical problem and a research challenge. In recent years, experimental and clinical studies have given insight into the teratological mechanisms and generated suggestions for improved future treatment regimens. The teratological role of disturbances in the metabolism of inositol, prostaglandins, and reactive oxygen species has been particularly highlighted, and the beneficial effect of dietary addition of inositol, arachidonic acid and antioxidants has been elucidated in experimental work. Changes in gene expression and induction of apoptosis in embryos exposed to a diabetic environment have been investigated and assigned roles in the teratogenic processes. The diabetic environment appears to simultaneously induce alterations in several interrelated teratological pathways. The complex pathogenesis of diabetic embryopathy has started to unravel, and future research efforts will utilize both clinical intervention studies and experimental work that aim to characterize the human applicability and the cell biological components of the discovered teratological mechanisms.