Oxygen and lack of oxygen in fetal and placental development, feto-placental coupling, and congenital heart defects

Integrated Transcriptomic and Metabolomic Profiling of the Placenta in a Dexamethasone-Induced Cleft Palate Rabbit Model

BACKGROUND

Cleft palate is a congenital malformation influenced by both genetic and environmental factors. Although environmental contributors have been extensively studied, the placenta-an essential organ that mediates maternal-fetal interactions and offers protection against environmental insults-remains poorly understood in this context. This study aimed to explore transcriptomic and metabolomic alterations in the placenta following maternal exposure to corticosteroids, using a dexamethasone-induced cleft palate rabbit model.

METHODS

Untargeted metabolomics and transcriptomics analyses were conducted on placental and amniotic fluid samples from fetuses with and without dexamethasone-induced cleft palate. Histopathological examination was performed to assess structural abnormalities in the placenta.

RESULTS

The cleft palate group exhibited marked placental pathologies, including fibrosis, calcification, and necrosis. Transcriptomic analysis identified 4744 differentially expressed genes, enriched in pathways related to hormone signaling, vascular development, and inflammation. Metabolomic profiling revealed significant changes in both placenta and amniotic fluid, especially in the urea cycle, aspartate metabolism, and nicotinate and nicotinamide metabolism. The urea cycle was particularly disrupted in the cleft palate group.

CONCLUSION

These findings reveal a strong association between placental structural and functional abnormalities and cleft palate formation in the dexamethasone-induced model, offering novel insights into the potential role of the placenta in cleft palate pathogenesis.

Heme oxygenase/carbon monoxide system and development of the heart

Progressive differentiation controlled by intercellular signaling between pharyngeal mesoderm, foregut endoderm, and neural crest-derived mesenchyme is required for normal embryonic and fetal development. Gasotransmitters (criteria: 1) a small gas molecule; 2) freely permeable across membranes; 3) endogenously and enzymatically produced and its production regulated; 4) well-defined and specific functions at physiologically relevant concentrations; 5) functions can be mimicked by exogenously applied counterpart; and 6) cellular effects may or may not be second messenger-mediated, but should have specific cellular and molecular targets) are integral to gametogenesis and subsequent embryogenesis, fetal development, and normal heart maturation. Important for in utero development, the heme oxygenase/carbon monoxide system is expressed during gametogenesis, by the placenta, during embryonic development, and by the fetus. Complex sequences of biochemical pathways result in the progressive maturation of the human heart in utero . The resulting myocardial architecture, consisting of working myocardium, coronary arteries and veins, epicardium, valves and cardiac skeleton, endocardial lining, and cardiac conduction system, determines function. Oxygen metabolism in normal and maldeveloping hearts, which develop under reduced and fluctuating oxygen concentrations, is poorly understood. "Normal" hypoxia is critical for heart formation, but "abnormal" hypoxia in utero affects cardiogenesis. The heme oxygenase/carbon monoxide system is important for in utero cardiac development, and other factors also result in alterations of the heme oxygenase/carbon monoxide system during in utero cardiac development. This review will address the role of the heme oxygenase/carbon monoxide system during cardiac development in embryo and fetal development.

Alteration in Cortical Structure Mediating the Impact of Blood Oxygen-Carrying Capacity on Gross Motor Skills in Infants With Complex Congenital Heart Disease

Congenital heart disease (CHD) is the most common congenital anomaly, leading to an increased risk of neurodevelopmental abnormalities in many children with CHD. Understanding the neurological mechanisms behind these neurodevelopmental disorders is crucial for implementing early interventions and treatments. In this study, we recruited 83 infants aged 12-26.5 months with complex CHD, along with 86 healthy controls (HCs). We collected multimodal data to explore the abnormal patterns of cerebral cortex development and explored the complex interactions among blood oxygen-carrying capacity, cortical development, and gross motor skills. We found that, compared to healthy infants, those with complex CHD exhibit significant reductions in cortical surface area development, particularly in the default mode network. Most of these developmentally abnormal brain regions are significantly correlated with the blood oxygen-carrying capacity and gross motor skills of infants with CHD. Additionally, we further discovered that the blood oxygen-carrying capacity of infants with CHD can indirectly predict their gross motor skills through cortical structures, with the left middle temporal area and left inferior temporal area showing the greatest mediation effects. This study identified biomarkers for neurodevelopmental disorders and highlighted blood oxygen-carrying capacity as an indicator of motor development risk, offering new insights for the clinical management CHD.

Prevalence of fetal anomalies, stillbirth, neonatal morbidity, or mortality in pregnancies complicated by placenta accreta spectrum disorders

INTRODUCTION

Placenta accreta spectrum disorders (PAS) lead to major complications in pregnancy. While the maternal morbidity associated with PAS is well known, there is less information regarding neonatal morbidity in this setting. The aim of this study is to describe the neonatal outcomes (fetal malformations, neonatal morbidity, twin births, stillbirth, and neonatal death), using an international multicenter database of PAS cases.

MATERIAL AND METHODS

This was a prospective, multicenter cohort study based on prospectively collected cases, using the international multicenter database of the International Society for PAS, carried out between January 2020 and June 2022 by 23 centers with experience in PAS care. All PAS cases were included, regardless of whether singleton or multiple pregnancies and were managed in each center according to their own protocols. Data were collected via chart review. Local Ethical Committee approval and Data Use Agreements were obtained according to local policies.

RESULTS

There were 315 pregnancies eligible for inclusion, with 12 twin pregnancies, comprising 329 fetuses/newborns; 2 cases were excluded due to inconsistency of data regarding fetal abnormalities. For the calculation of neonatal morbidity and mortality, all elective pregnancy terminations were excluded, hence 311 pregnancies with 323 newborns were analyzed. In our cohort, 3 neonates (0.93%) were stillborn; of the 320 newborns delivered, there were 10 cases (3.13%) of neonatal death. The prevalence of major congenital malformations was 4.64% (15/323 newborns), most commonly, cardiovascular, central nervous system, and gastrointestinal tract malformations. The overall prevalence of major neonatal morbidity in pregnancies complicated by PAS was 47/311 (15.1%). There were no stillbirths, neonatal deaths, or fetal malformations in reported twin gestations.

CONCLUSIONS

Although some outcomes may be too rare to detect within our cohort and data should be interpreted with caution, our observational data supports reassuring neonatal outcomes for women with PAS.

Moderate altitude as a risk factor for isolated congenital malformations. Results from a case-control multicenter-multiregional study

BACKGROUND

Living in high-altitude regions has been associated with a higher prevalence of some birth defects. Moderate altitudes (1500-2500 m) have been associated with some congenital heart diseases and low birth weight. However, no studies have been conducted for other isolated congenital malformations.

OBJECTIVES

To estimate the prevalence at birth of isolated congenital malformations in low and moderate altitudes and to determine if moderate altitudes are a risk factor, such as high altitudes, for isolated congenital malformations adjusted for other factors.

METHODS

The study consisted of a case-control multicenter-multiregional study of 13 isolated congenital malformations. Cases included live births with isolated congenital malformations and controls at low (10-1433 m) and moderate altitudes (1511-2426 m) from a Mexican registry from January 1978 to December 2019. Prevalence per 10,000 (95% CI) per altitude group was estimated. We performed unadjusted and adjusted logistic regression models (adjusted for maternal age, parity, malformed relatives, socioeconomic level, and maternal diabetes) for each isolated congenital malformation.

RESULTS

Hydrocephaly and microtia had a higher at-birth prevalence, and spina bifida, preauricular tag, and gastroschisis showed a lower at-birth prevalence in moderate altitudes. Moderate altitudes were a risk factor for hydrocephaly (aOR 1.39), microtia (aOR 1.60), cleft-lip-palate (aOR 1.27), and polydactyly (aOR 1.32) and a protective effect for spina bifida (aOR 0.87) compared with low altitudes.

CONCLUSIONS

Our findings provide evidence that moderate altitudes as higher altitudes are an associated risk or protective factor to some isolated congenital malformations, suggesting a possible gradient effect.

Atrial natriuretic peptide signaling co-regulates lipid metabolism and ventricular conduction system gene expression in the embryonic heart

It has been shown that atrial natriuretic peptide (ANP) and its high affinity receptor (NPRA) are involved in the formation of ventricular conduction system (VCS). Inherited genetic variants in fatty acid oxidation (FAO) genes are known to cause conduction abnormalities in newborn children. Although the effect of ANP on energy metabolism in noncardiac cell types is well documented, the role of lipid metabolism in VCS cell differentiation via ANP/NPRA signaling is not known. In this study, histological sections and primary cultures obtained from E11.5 mouse ventricles were analyzed to determine the role of metabolic adaptations in VCS cell fate determination and maturation. Exogenous treatment of E11.5 ventricular cells with ANP revealed a significant increase in lipid droplet accumulation, FAO and higher expression of VCS marker Cx40. Using specific inhibitors, we further identified PPARγ and FAO as critical downstream regulators of ANP-mediated regulation of metabolism and VCS formation.

Effect and mechanism of hypoxia on differentiation of porcine-induced pluripotent stem cells into vascular endothelial cells

Pigs are similar to humans in organ size and physiological function, and are considered as good models for studying cardiovascular diseases. The study of porcine-induced pluripotent stem cells (piPSC) differentiating into vascular endothelial cells (EC) is expected to open up a new way of obtaining high-quality seed cells. Given that the hypoxic environment has an important role in the differentiation process of vascular EC, this work intends to establish a hypoxia-induced differentiation system of piPSC into vascular EC. There is evidence that the hypoxia microenvironment in the initial stage could significantly improve differentiation efficiency. Further study suggests that the hypoxia culture system supports a combined effect of hypoxia inducible factors and their associated regulatory molecules, such as HIF-1α, VEGFA, FGF2, LDH-A, and PDK1, which can efficiently promote the lineage-specific differentiation of piPSC into EC. Most notably, the high level of ETV2 after 4 d of hypoxic treatment indicates that it possibly plays an important role in the promoting process of EC differentiation. The research is expected to help the establishment of new platforms for piPSC directional induction research, so as to obtain adequate seed cells with ideal phenotype and functionality.

Risk of birth defects in pregnant persons with sleep-disordered breathing during pregnancy

INTRODUCTION

As many as one in four pregnant women may experience sleep-disordered breathing (SDB) during pregnancy. The same sequelae of SDB, such as insulin resistance and inflammation, have been implicated in the development of certain birth defects.

METHODS

This is a secondary analysis of the SDB substudy of the Nulliparous Pregnancy Outcomes Study: Monitoring Mothers-to-Be study, which included 2106 participants who had a sufficiency sleep study at two visits at different time points in pregnancy. SDB was based on a self-administered home sleep apnea test with data scored by trained, blinded research polysomnologists. SDB was defined as an apnea-hypopnea index (AHI) ≥5. The primary outcome of this analysis was any of the 45 non-chromosomal birth defects included in the National Birth Defects Prevention Network Annual Report.

RESULTS

In this cohort, the overall rate of birth defects was 3.1%. The prevalence was similar between those without SDB (3.0%) and those with only mid-pregnancy SDB (3.4%), but was higher in those with early-pregnancy SDB (6.7%). After adjusting for maternal age, chronic hypertension, pregestational diabetes, and body mass index (BMI), there were no statistically significant differences in the risk of birth defects by subject SDB status.

CONCLUSIONS

Further studies to evaluate the effect of prepregnancy and early-pregnancy SDB on the fetus, as well as the risk of specific birth defects and neonatal outcomes in those with an objectively measured diagnosis of SDB, are still needed.

Shared developmental pathways of the placenta and fetal heart

Congenital heart defects (CHD) remain the most common class of birth defect worldwide, affecting 1 in every 110 live births. A host of clinical and morphological indicators of placental dysfunction are observed in pregnancies complicated by fetal CHD and, with the recent emergence of single-cell sequencing capabilities, the molecular and physiological associations between the embryonic heart and developing placenta are increasingly evident. In CHD pregnancies, a hostile intrauterine environment may negatively influence and alter fetal development. Placental maldevelopment and dysfunction creates this hostile in-utero environment and may manifest in the development of various subtypes of CHD, with downstream perfusion and flow-related alterations leading to yet further disruption in placental structure and function. The adverse in-utero environment of CHD-complicated pregnancies is well studied, however the specific etiological role that the placenta plays in CHD development remains unclear. Many mouse and rat models have been used to characterize the relationship between CHD and placental dysfunction, but these paradigms present substantial limitations in the assessment of both the heart and placenta. Improvements in non-invasive placental assessment can mitigate these limitations and drive human-specific investigation in relation to fetal and placental development. Here, we review the clinical, structural, and molecular relationships between CHD and placental dysfunction, the CHD subtype-dependence of these changes, and the future of Placenta-Heart axis modeling and investigation.

Preeclampsia and Fetal Congenital Heart Defects

Endothelial dysfunction, impaired implantation and placental insufficiency have been identified as mechanisms behind the development of pre-eclampsia, resulting in angiogenic factors' alteration. Angiogenic imbalance is also associated with congenital heart defects, and this common physiologic pathway may explain the association between them and pre-eclampsia. This review aims to understand the physiology shared by these two entities and whether women with pre-eclampsia have an increased risk of fetal congenital heart defects (or the opposite). The present research has highlighted multiple vasculogenic pathways associated with heart defects and preeclampsia, but also epigenetic and environmental factors, contributing both. It is also known that fetuses with a prenatal diagnosis of congenital heart disease have an increased risk of several comorbidities, including intrauterine growth restriction. Moreover, the impact of pre-eclampsia goes beyond pregnancy as it increases the risk for following pregnancies and for diseases later in life in both offspring and mothers. Given the morbidity and mortality associated with these conditions, it is of foremost importance to understand how they are related and its causative mechanisms. This knowledge may allow earlier diagnosis, an adequate surveillance or even the implementation of preventive strategies.

Can Erythropoietin Reduce Hypoxemic Neurological Damages in Neonates With Congenital Heart Defects?

Congenital heart defects (CHD), the most common cause of birth defects with increasing birth prevalence, affect nearly 1% of live births worldwide. Cyanotic CHD are characterized by hypoxemia, with subsequent reduced oxygen delivery to the brain, especially critical during brain development, beginning in the fetus and continuing through the neonatal period. Therefore, neonates with CHD carry a high risk for neurological comorbidities, even more frequently when there are associated underlying genetic disorders. We review the currently available knowledge on potential prevention strategies to reduce brain damage induced by hypoxemia during fetal development and immediately after birth, and the role of erythropoietin (EPO) as a potential adjunctive treatment. Maternal hyper-oxygenation had been studied as a potential therapeutic to improve fetal oxygenation. Despite demonstrating some effectiveness, maternal hyper-oxygenation has proven to be impractical for extensive clinical application, thus prompting the investigation of specific pathways for pharmacological intervention. Among those, the role of antioxidant pathways and Hypoxia Inducible Factors (HIF) have been studied for their involvement in the protective response to hypoxic injury. One of the proteins induced by HIF, EPO, has properties of being anti-apoptotic, antioxidant, and protective for neurons, astrocytes, and oligodendrocytes. In human trials, EPO administration in neonates with hypoxic ischemic encephalopathy (HIE) significantly reduced the neurological hypoxemic damages in several reported studies. Currently, it is unknown if the mechanisms of pathophysiology of cyanotic CHD are like HIE. Neonates with cyanotic CHD are exposed to both chronic hypoxemia and episodes of acute ischemia-reperfusion injury when undergo cardiopulmonary bypass surgery requiring aortic cross-clamp and general anesthesia. Our review supports future trials to evaluate the potential efficiency of EPO in reducing the hypoxemic neurologic damages in neonates with CHD. Furthermore, it suggests the need to identify early biomarkers of hypoxia-induced neurological damage, which must be sensitive to the neuroprotective effects of EPO.

The Association of Fetal Congenital Cardiac Defects and Placental Vascular Malperfusion

OBJECTIVES

Abnormal early angiogenesis appears to impact both placental disorders and fetal congenital heart defects (CHD). We sought to assess the association of placental perfusion defects (PPD) and fetal (CHD).

METHODS

Singleton pregnancies with isolated severe fetal CHD were compared to controls without congenital anomalies or maternal malperfusion (MVM) risk factors. CHD was categorized into group 1: single left ventricle morphology and transposition of the great vessels (TGA) and group 2: single right ventricle and two ventricle morphology. Malperfusion was defined as fetal vascular malperfusion (FVM), MVM, and both FVM and MVM.

RESULTS

PPD was increased for all CHD (n = 47), CHD with or without risk factors, and CHD groups compared to controls (n = 92). Overall CHD cases and CHD with risk factors had an increased risk of FVM (30% and 80% vs 14%), and MVM (43% and 50% vs 21%), respectively. MVM rates were similar in CHD with and without maternal risk factors. FVM (38% vs 14%) and MVM (44% vs 21%) were increased in Group 1. MVM (42% vs 21%) and both FVM and MVM (16% vs 3%) were increased in Group 2.

CONCLUSIONS

PPD risk is increased in severe isolated fetal CHD. The highest risk is seen in fetal CHD with maternal risk factors.

Hypoxia and Mitochondrial Dysfunction in Pregnancy Complications

Hypoxia is a common and severe stress to an organism's homeostatic mechanisms, and hypoxia during gestation is associated with significantly increased incidence of maternal complications of preeclampsia, adversely impacting on the fetal development and subsequent risk for cardiovascular and metabolic disease. Human and animal studies have revealed a causative role of increased uterine vascular resistance and placental hypoxia in preeclampsia and fetal/intrauterine growth restriction (FGR/IUGR) associated with gestational hypoxia. Gestational hypoxia has a major effect on mitochondria of uteroplacental cells to overproduce reactive oxygen species (ROS), leading to oxidative stress. Excess mitochondrial ROS in turn cause uteroplacental dysfunction by damaging cellular macromolecules, which underlies the pathogenesis of preeclampsia and FGR. In this article, we review the current understanding of hypoxia-induced mitochondrial ROS and their role in placental dysfunction and the pathogenesis of pregnancy complications. In addition, therapeutic approaches selectively targeting mitochondrial ROS in the placental cells are discussed.

Activation of amino acid metabolic program in cardiac HIF1-alpha-deficient mice

HIF1-alpha expression defines metabolic compartments in the developing heart, promoting glycolytic program in the compact myocardium and mitochondrial enrichment in the trabeculae. Nonetheless, its role in cardiogenesis is debated. To assess the importance of HIF1-alpha during heart development and the influence of glycolysis in ventricular chamber formation, herein we generated conditional knockout models of Hif1a in Nkx2.5 cardiac progenitors and cardiomyocytes. Deletion of Hif1a impairs embryonic glycolysis without influencing cardiomyocyte proliferation and results in increased mitochondrial number and transient activation of amino acid catabolism together with HIF2α and ATF4 upregulation by E12.5. Hif1a mutants display normal fatty acid oxidation program and do not show cardiac dysfunction in the adulthood. Our results demonstrate that cardiac HIF1 signaling and glycolysis are dispensable for mouse heart development and reveal the metabolic flexibility of the embryonic myocardium to consume amino acids, raising the potential use of alternative metabolic substrates as therapeutic interventions during ischemic events.

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Loss of cardiac Hif1a does not preclude heart development or cardiac function

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Embryonic Hif1a-deficient hearts transiently upregulate amino acid catabolism

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Amino acid catabolism activation sustains heart growth in the absence of glycolysis

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HIF2α and ATF4 are transiently upregulated in the developing heart upon Hif1a loss

miR-219a suppresses human trophoblast cell invasion and proliferation by targeting vascular endothelial growth factor receptor 2 (VEGFR2)

OBJECTIVE

Vascular endothelial growth factor (VEGF) plays a critical role in regulating trophoblast cell invasion and proliferation, involved in a variety of pregnancy complications, such as spontaneous abortion and pre-eclampsia. Numerous studies have revealed that microRNAs (miRNAs) are participated in a series of molecular processes that regulate cell function, such as cell invasion, proliferation, and apoptosis. Vascular endothelial growth factor receptor 2 (VEGFR2), a receptor of VEGF, has been shown to be involved in trophoblast function. However, the relation between miRNA and VEGFR2 and their role in trophoblast function remain to be elucidated.

METHODS

The effect of miR-219a on the trophoblast function has been explored using luciferase reporter, transwell, qRT-PCR, western blot, bromodeoxyuridine (BrdU), ELISA, immunofluorescent staining, and tube formation assays.

RESULTS

In the current study, we observed that through targeted inhibition of VEGFR2 expression by miR-219a, the function of VEGFR2 as well as the downstream PI3K/AKT/NF-κB signaling pathway were suppressed, leading to suppression of trophoblastic proliferation and invasion. Moreover, upregulation of VEGFR2 restored the miR-219a-inhibited cell proliferation, invasion, and tube formation.

CONCLUSIONS

These results revealed that miR-219a played crucial roles in negatively regulating trophoblastic proliferation and invasion by suppression of the PI3K/AKT/NF-κB signaling pathway by targeting VEGFR2, therefore serving as a potential treatment method for the complications of pregnancy caused by trophoblastic dysregulation.

Hypoxia as a Driving Force of Pluripotent Stem Cell Reprogramming and Differentiation to Endothelial Cells

Inadequate supply of oxygen (O₂) is a hallmark of many diseases, in particular those related to the cardiovascular system. On the other hand, tissue hypoxia is an important factor regulating (normal) embryogenesis and differentiation of stem cells at the early stages of embryonic development. In culture, hypoxic conditions may facilitate the derivation of embryonic stem cells (ESCs) and the generation of induced pluripotent stem cells (iPSCs), which may serve as a valuable tool for disease modeling. Endothelial cells (ECs), multifunctional components of vascular structures, may be obtained from iPSCs and subsequently used in various (hypoxia-related) disease models to investigate vascular dysfunctions. Although iPSC-ECs demonstrated functionality in vitro and in vivo, ongoing studies are conducted to increase the efficiency of differentiation and to establish the most productive protocols for the application of patient-derived cells in clinics. In this review, we highlight recent discoveries on the role of hypoxia in the derivation of ESCs and the generation of iPSCs. We also summarize the existing protocols of hypoxia-driven differentiation of iPSCs toward ECs and discuss their possible applications in disease modeling and treatment of hypoxia-related disorders.

The association of maternal hypertensive disorders with neonatal congenital heart disease: analysis of a United States cohort

OBJECTIVE

To examine the association of any type of maternal hypertensive disorders of pregnancy (HDP) and neonatal congenital heart diseases (CHD).

STUDY DESIGN

We compared the prevalence of CHD between neonates born to mothers with HDP to those delivered to mothers without HDP among 24,525,889 hospital records of living infants, from a national database. We controlled for multiple confounding factors by using multiple logistic regression analysis.

RESULTS

Infants delivered to mothers with HDP had higher prevalence of CHD compared to infants born to mothers without HDP [5.20% vs. 1.47%; aOR: 2.51(2.38-2.64), p < 0.001]. Maternal diabetes was more frequent among infants born to mothers with HDP and was independently associated with CHD [aOR 5.14 (5.04-5.23), p < 0.001].

CONCLUSION

Infants born to mothers with hypertension had almost a threefold increase in CHD compared with those born to mothers without hypertension. Further studies are needed to investigate the underlying mechanism and direction of this association.

Association between metal cobalt exposure and the risk of congenital heart defect occurrence in offspring: a multi-hospital case-control study

Background

Many studies have investigated heavy metal exposure could increase the occurrence of congenital heart defects (CHDs). However, there are limited data regarding the relationship between cobalt exposure and CHD occurrence in offspring. The aim of this study was to analyze the association between cobalt exposure in mothers and the risk of CHDs in offspring.

Materials and methods

In order to explore the association between cobalt exposure and occurrence of congenital heart defect (CHD), a case-control study with 490 controls and 399 cases with CHDs in China were developed. The concentrations of cobalt in hair of pregnant woman and fetal placental tissue were measured and processed by a logistic regression analysis to explore the relationship between cobalt exposure and risk of CHDs.

Results

The median concentration of hair cobalt in the control and case group was 0.023 ng/mg and 0.033 ng/mg (aOR, 1.837; 95% CI, 1.468–2.299; P < 0.001), respectively. And the median (5–95% range) fetal placental cobalt concentrations were 19.350 ng/g and 42.500 ng/g (aOR, 2.924; 95% CI, 2.211–3.868; P < 0.001) in the control and case groups, respectively. Significant differences in the middle level of cobalt in hair were found in the different CHD subtypes, including septal defects, conotruncal defects, right ventricular outflow tract obstruction, and left ventricular outflow tract obstruction (P < 0.001). Dramatically, different cobalt concentrations in fetal placental tissue were found in all subtypes of cases with CHDs (P < 0.01).

Conclusions

The finding suggested that the occurrence of CHDs may be associated with cobalt exposure.

Generation of a hypoxia-sensing mouse model

Oxygen (O₂ ) homeostasis is essential to the metazoan life. O₂ -sensing or hypoxia-regulated molecular pathways are intimately involved in a wide range of critical cellular functions and cell survival from embryogenesis to adulthood. In this report, we have designed an innovative hypoxia sensor (O₂ CreER) based on the O₂ -dependent degradation domain of the hypoxia-inducible factor-1α and Cre recombinase. We have further generated a hypoxia-sensing mouse model, R26-O₂ CreER, by targeted insertion of the O₂ CreER-coding cassette in the ROSA26 locus. Using the ROSA^mTmG mouse strain as a reporter, we have found that this novel hypoxia-sensing mouse model can specifically identify hypoxic cells under the pathological condition of hind-limb ischemia in adult mice. This model can also label embryonic cells including vibrissal follicle cells in E13.5-E15.5 embryos. This novel mouse model offers a valuable genetic tool for the study of hypoxia and O₂ sensing in mammalian systems under both physiological and pathological conditions.