The impact of maternal obesity in pregnancy on placental glucocorticoid and macronutrient transport and metabolism

Maternal obesity is the most common metabolic disturbance in pregnancy affecting >1 in 5 women in some countries. Babies born to obese women are heavier with more adiposity at birth, and are vulnerable to obesity and metabolic disease across the lifespan suggesting offspring health is ‘programmed’ by fetal exposure to an obese intra-uterine environment. The placenta plays a major role in dictating the impact of maternal health on prenatal development. Maternal obesity impacts the function of integral placental receptors and transporters for glucocorticoids and nutrients, key drivers of fetal growth, though mechanisms remain poorly understood. This review aims to summarise current knowledge in this area, and considers the impact of obesity on the epigenetic machinery of the placenta at this vital juncture in offspring development. Further research is required to advance understanding of these areas in the hope that the trans-generational cycle of obesity can be alleviated.     

Reduction of biliverdin and placental transfer of bilirubin and biliverdin in the pregnant guinea pig.

Biliverdin was reduced to bilirubin in pregnant and foetal guinea pigs, and the 100000 g supernatant from homogenates of foetal liver, placenta and maternal liver showed high biliverdin reductase activity. The placental transport of unconjugated bilirubin and biliverdin was compared by injecting unlabelled and radiolabelled pigments into the foetal or maternal circulation and analysing blood collected from the opposite side of the placenta. Injected bilirubin crossed the placenta from foetus to mother and vice versa, but injected biliverdin did not appear to cross without prior reduction to bilirubin. The guinea-pig placenta is apparently more permeable to bilirubin than biliverdin. Reduction of biliverdin to bilirubin in the foetus may, therefore, be essential for efficient elimination of haem catabolites from the foetus in placental mammals.     

Programming of the endocrine pancreas by the early nutritional environment

A substantial body of evidence now suggests that poor intrauterine milieu elicited by maternal nutritional disturbance or placental insufficiency may programme susceptibility in the foetus to later develop chronic degenerative diseases, such as obesity, hypertension, cardiovascular diseases and diabetes. Further data showing the developmental programming of the metabolic syndrome are now available thanks to animal studies in which the foetal environment has been manipulated. This review examines the developmental programming of glucose intolerance by disturbed intrauterine metabolic condition in rats. It focuses on the alteration of the endocrine pancreas at birth. Long-term consequences, deterioration of glucose tolerance and even transgenerational effects are reported. Maternal protein, caloric restriction and diabetes during gestation/lactation lead to altered beta-cell mass. This review also tempts to identify cellular and molecular mechanisms involved in this process.     

Hormonal regulation of the Menkes and Wilson copper-transporting ATPases in human placental Jeg-3 cells

Copper deficiency during pregnancy results in early embryonic death and foetal structural abnormalities including skeletal, pulmonary and cardiovascular defects. During pregnancy, copper is transported from the maternal circulation to the foetus by mechanisms which have not been clearly elucidated. Two copper-transporting ATPases, Menkes (ATP7A; MNK) and Wilson (ATP7B; WND), are expressed in the placenta and both are involved in placental copper transport, as copper accumulates in the placenta in both Menkes and Wilson disease. The regulatory mechanisms of MNK and WND and their exact role in the placenta are unknown. Using a differentiated polarized Jeg-3 cell culture model of placental trophoblasts, MNK and WND were shown to be expressed within these cells. Distinct roles for MNK and WND are suggested on the basis of their opposing responses to insulin. Insulin and oestrogen increased both MNK mRNA and protein levels, altered the localization of MNK towards the basolateral membrane in a copper-independent manner, and increased the transport of copper across this membrane. In contrast, levels of WND were decreased in response to insulin, and the protein was located in a tight perinuclear region, with a corresponding decrease in copper efflux across the apical membrane. These results are consistent with a model of copper transport in the placenta in which MNK delivers copper to the foetus and WND returns excess copper to the maternal circulation. Insulin and oestrogen stimulate copper transport to the foetus by increasing the expression of MNK and reducing the expression of WND. These data show for the first time that MNK and WND are differentially regulated by the hormones insulin and oestrogen in human placental cells.     

Cadherins in maternal-foetal interactions: red queen with a green beard?

Cadherins are homophilic cell surface adhesion proteins, some of which mediate interactions between maternal and foetal tissues during mammalian pregnancy. David Haig suggested that these proteins may exhibit 'green-beard gene' effects, whereby the nature of binding between identical alleles in mother and foetus leads to differential levels of resource transfer. The selfish effects of such self-recognizing alleles should, however, be suppressed over evolutionary time by unlinked genes, which is expected to lead to antagonistic coevolution between placentally expressed cadherins and unlinked modifiers. Such molecular coevolution should leave a signature of positive selection, with high ratios of non-synonymous to synonymous amino acid substitution. We present evidence that three placentally expressed cadherin genes, E-cadherin, P-cadherin and VE-cadherin, have been subject to positive selection. By contrast, a 'control' cadherin that is not expressed in the placenta, H-cadherin, showed no evidence of selection. These results provide support for the hypothesis that the cadherin genes involved in maternal-foetal interactions have been subject to green-beard-effect mutations over the course of evolutionary history, leading to antagonistic coevolution with suppressing elements from the parliament of genes.     

Enriched Environment-induced Maternal Weight Loss Reprograms Metabolic Gene Expression in Mouse Offspring

The global prevalence of weight loss is increasing, especially in young women. However, the extent and mechanisms by which maternal weight loss affects the offspring is still poorly understood. Here, using an enriched environment (EE)-induced weight loss model, we show that maternal weight loss improves general health and reprograms metabolic gene expression in mouse offspring, and the epigenetic alterations can be inherited for at least two generations. EE in mothers induced weight loss and its associated physiological and metabolic changes such as decreased adiposity and improved glucose tolerance and insulin sensitivity. Relative to controls, their offspring exhibited improved general health such as reduced fat accumulation, decreased plasma and hepatic lipid levels, and improved glucose tolerance and insulin sensitivity. Maternal weight loss altered gene expression patterns in the liver of offspring with coherent down-regulation of genes involved in lipid and cholesterol biosynthesis. Epigenomic profiling of offspring livers revealed numerous changes in cytosine methylation depending on maternal weight loss, including reproducible changes in promoter methylation over several key lipid biosynthesis genes, correlated with their expression patterns. Embryo transfer studies indicated that oocyte alteration in response to maternal metabolic conditions is a strong factor in determining metabolic and epigenetic changes in offspring. Several important lipid metabolism-related genes have been identified to partially inherit methylated alleles from oocytes. Our study reveals a molecular and mechanistic basis of how maternal lifestyle modification affects metabolic changes in the offspring.     

Placental triglyceride accumulation in maternal type 1 diabetes is associated with increased lipase gene expression

Maternal diabetes can cause fetal macrosomia and increased risk of obesity, diabetes, and cardiovascular disease in adulthood of the offspring. Although increased transplacental lipid transport could be involved, the impact of maternal type 1 diabetes on molecular mechanisms for lipid transport in placenta is largely unknown. To examine whether maternal type 1 diabetes affects placental lipid metabolism, we measured lipids and mRNA expression of lipase-encoding genes in placentas from women with type 1 diabetes (n = 27) and a control group (n = 21). The placental triglyceride (TG) concentration and mRNA expression of endothelial lipase (EL) and hormone-sensitive lipase (HSL) were increased in placentas from women with diabetes. The differences were more pronounced in women with diabetes and suboptimal metabolic control than in women with diabetes and good metabolic control. Placental mRNA expression of lipoprotein lipase and lysosomal lipase were similar in women with diabetes and the control group. Immunohistochemistry showed EL protein in syncytiotrophoblasts facing the maternal blood and endothelial cells facing the fetal blood in placentas from both normal women and women with diabetes. These results suggest that maternal type 1 diabetes is associated with TG accumulation and increased EL and HSL gene expression in placenta and that optimal metabolic control reduces these effects.     

Up regulation of the maternal immune response in the placenta of cattle naturally infected with Neospora caninum

Neospora caninum is an intracellular protozoan parasite which is a major cause of abortion in cattle worldwide. It forms persistent infections which recrudesce during pregnancy leading to foetal infection and in a proportion of cases, abortion. The mechanisms underlying abortion are not understood. In this study, recrudescence of a persistent infection in eight naturally infected cows occurred between 20 and 33 weeks of gestation. Animals were killed at the time of recrudescence and parasites were detected in the placentae and foetuses. An active maternal immune response consisting of an infiltration of CD4+ and CD8+ T cells and a 46-49 fold increase in interferon-γ and interleukin-4 mRNA was detected. Other cytokines, notably interleukin-12 p40, interleukin-10 and tumour necrosis factor-α were also significantly increased and Major Histocompatibility Class II antigen was expressed on maternal and foetal epithelial and stromal fibroblastoid cells. Significantly, despite the presence of an active maternal immune response in the placenta, all the foetuses were alive at the time of maternal euthanasia. There was evidence of parasites within foetal tissues; their distribution was restricted to the central nervous system and skeletal muscle and their presence was associated with tissue necrosis and a non-suppurative inflammatory response involving lymphocytes and macrophages, irrespective of the gestational age of the foetus. Whilst an active maternal immune response to a pathogen in the placenta is generally considered to be damaging to the foetal trophoblast, our findings suggest that the presence of a parasite-induced maternal immune response in the placenta is not detrimental to foetal survival but may contribute to the control of placental parasitosis.     

Maternal obesity and diabetes induces latent metabolic defects and widespread epigenetic changes in isogenic mice

Intrauterine nutrition can program metabolism, creating stable changes in physiology that may have significant health consequences. The mechanism underlying these changes is widely assumed to involve epigenetic changes to the expression of metabolic genes, but evidence supporting this idea is limited. Here we have performed the first study of the epigenomic consequences of exposure to maternal obesity and diabetes. We used a mouse model of natural-onset obesity that allows comparison of genetically identical mice whose mothers were either obese and diabetic or lean with a normal metabolism. We find that the offspring of obese mothers have a latent metabolic phenotype that is unmasked by exposure to a Western-style diet, resulting in glucose intolerance, insulin resistance and hepatic steatosis. The offspring show changes in hepatic gene expression and widespread but subtle alterations in cytosine methylation. Contrary to expectation, these molecular changes do not point to metabolic pathways but instead reside in broadly developmental ontologies. We propose that, rather than being adaptive, these changes may simply produce an inappropriate response to suboptimal environments; maladaptive phenotypes may be avoidable if postnatal nutrition is carefully controlled.     

Placental contribution to the origins of sexual dimorphism in health and diseases: sex chromosomes and epigenetics

Sex differences occur in most non-communicable diseases, including metabolic diseases, hypertension, cardiovascular disease, psychiatric and neurological disorders and cancer. In many cases, the susceptibility to these diseases begins early in development. The observed differences between the sexes may result from genetic and hormonal differences and from differences in responses to and interactions with environmental factors, including infection, diet, drugs and stress. The placenta plays a key role in fetal growth and development and, as such, affects the fetal programming underlying subsequent adult health and accounts, in part for the developmental origin of health and disease (DOHaD). There is accumulating evidence to demonstrate the sex-specific relationships between diverse environmental influences on placental functions and the risk of disease later in life. As one of the few tissues easily collectable in humans, this organ may therefore be seen as an ideal system for studying how male and female placenta sense nutritional and other stresses, such as endocrine disruptors. Sex-specific regulatory pathways controlling sexually dimorphic characteristics in the various organs and the consequences of lifelong differences in sex hormone expression largely account for such responses. However, sex-specific changes in epigenetic marks are generated early after fertilization, thus before adrenal and gonad differentiation in the absence of sex hormones and in response to environmental conditions. Given the abundance of X-linked genes involved in placentogenesis, and the early unequal gene expression by the sex chromosomes between males and females, the role of X- and Y-chromosome-linked genes, and especially those involved in the peculiar placenta-specific epigenetics processes, giving rise to the unusual placenta epigenetic landscapes deserve particular attention. However, even with recent developments in this field, we still know little about the mechanisms underlying the early sex-specific epigenetic marks resulting in sex-biased gene expression of pathways and networks. As a critical messenger between the maternal environment and the fetus, the placenta may play a key role not only in buffering environmental effects transmitted by the mother but also in expressing and modulating effects due to preconceptional exposure of both the mother and the father to stressful conditions.     

Extra-embryonic-specific imprinted expression is restricted to defined lineages in the post-implantation embryo

A subset of imprinted genes in the mouse have been reported to show imprinted expression that is restricted to the placenta, a short-lived extra-embryonic organ. Notably, these so-called “placental-specific” imprinted genes are expressed from both parental alleles in embryo and adult tissues. The placenta is an embryonic-derived organ that is closely associated with maternal tissue, and as a consequence, maternal contamination can be mistaken for maternal-specific imprinted expression. The complexity of the placenta, which arises from multiple embryonic lineages, poses additional problems in accurately assessing allele-specific repressive epigenetic modifications in genes that also show lineage-specific silencing in this organ. These problems require that extra evidence be obtained to support the imprinted status of genes whose imprinted expression is restricted to the placenta. We show here that the extra-embryonic visceral yolk sac (VYS), a nutritive membrane surrounding the developing embryo, shows a similar “extra-embryonic–lineage-specific” pattern of imprinted expression. We present an improved enzymatic technique for separating the bilaminar VYS and show that this pattern of imprinted expression is restricted to the endoderm layer. Finally, we show that VYS “extra-embryonic–lineage-specific” imprinted expression is regulated by DNA methylation in a similar manner as shown for genes showing multi-lineage imprinted expression in extra-embryonic, embryonic, and adult tissues. These results show that the VYS is an improved model for studying the epigenetic mechanisms regulating extra-embryonic–lineage-specific imprinted expression.     

Suboptimal in vitro culture conditions: an epigenetic origin of long-term health effects

The foetal origins of adult diseases or Barker hypothesis suggests that there can be adverse in uterus effects on the foetus that can lead to certain diseases in adults. Extending this hypothesis to the early stages of embryo development, in particular, to preimplantation stages, it was recently demonstrated that, long-term programming of postnatal development, growth and physiology can be irreversibly affected during this period of embryo development by suboptimal in vitro culture (IVC). As an example, it was found in two recent studies that, mice derived from embryos cultured in suboptimal conditions can suffer from obesity, increased anxiety, and deficiencies on their implicit memory system. In addition, it was observed that suboptimal IVC can cause disease in mature animals by promoting alterations in their genetic imprinting during preimplantation development. Imprinting and other epigenetic mechanisms control the establishment and maintenance of gene expression patterns in the embryo, placenta and foetus. The previously described observations, suggest that the loss of epigenetic regulation during preimplantation development may lead to severe long-term effects. Although mostly tested in rodents, the hypothesis that underlies these studies can also fit assisted reproductive technology (ART) procedures in other species, including humans. The lack of information on how epigenetic controls are lost during IVC, and on the long-term consequences of ART, underscore the necessity for sustained epigenetic analysis of embryos produced in vitro and long-term tracking of the health of the human beings conceived using these procedures.     

PHLDA2 is an imprinted gene in cattle

Genomic imprinting is an epigenetic non-Mendelian phenomenon found predominantly in placental mammals. Imprinted genes display differential expression in the offspring depending on whether the gene is maternally or paternally inherited. Currently, some 100 imprinted genes have been reported in mammals, and while some of these genes are imprinted across most mammalian species, others have been shown to be imprinted in only a few species. The PHLDA2 gene that codes for a pleckstrin homology-like domain, family A (member 2), protein has to date been shown to be a maternally expressed imprinted gene in humans, mice and pigs. Genes subject to imprinting can have major effects on mammalian growth, development and disease. For instance, disruption of imprinted genes can lead to aberrant growth syndromes in cloned domestic mammals, and it has been demonstrated that PHLDA2 mRNA expression levels are aberrant in the placenta of somatic clones of cattle. In this study, we demonstrate that PHLDA2 is expressed across a range of cattle foetal tissues and stages and provide the first evidence that PHLDA2 is a monoallelically expressed imprinted gene in cattle foetal tissues, and also in the bovine placenta.     

Placental structure in gestational diabetes mellitus

The placenta is a transitory organ, located between the mother and the foetus, which supports intrauterine life. This organ has nutritional, endocrine and immunologic functions to support foetal development. Several factors are related to the correct functioning of the placenta including foetal and maternal blood flow, appropriate nutrients, expression and function of receptors and transporters, and the morphology of the placenta itself. Placental morphology is crucial for understanding the pathophysiology of the organ as represents the physical structure where nutrient exchange occurs. In pathologies of pregnancy such as diabetes mellitus in humans and animal models, several changes in the placental morphology occur, related mainly with placental size, hypervascularization, higher branching capillaries of the villi and increased glycogen deposits among others. Gestational diabetes mellitus is associated with modifications in the structure of the human placenta including changes in the surface area and volume, as well as histological changes including an increased volume of intervillous space and terminal villi, syncytiotrophoblast number, fibrinoid areas, and glycogen deposits. These modifications may result in functional changes in this organ thus limiting the wellbeing of the developing foetus. This review gives an overview of recurrent morphological changes at macroscopic and histological levels seen in the placenta from gestational diabetes in humans and animal models. This article is part of a Special Issue entitled: Membrane Transporters and Receptors in Pregnancy Metabolic Complications edited by Luis Sobrevia.     

Impact of foetus and mother on IFN-gamma-induced indoleamine 2,3-dioxygenase and inducible nitric oxide synthase expression in murine placenta following Toxoplasma gondii infection

IFN-gamma production is a hallmark of acute infection with the protozoan parasite Toxoplasma gondii. The tryptophan-catabolising enzyme indoleamine 2,3-dioxygenase (IDO), as well as inducible nitric oxide synthase (NOS2) are induced by IFN-gamma and can play extremely diverse roles in immune regulation, defence against pathogens and physiological homeostasis. We investigated the regulation of these two central enzymes in the placenta during acute infection of pregnant female mice. Using IFN-gamma receptor knockout (IFNgammaR-/-) mice, we showed that IDO is not constitutively expressed in term placentas. In contrast, NOS2 expression was observed, largely dependent on IFN-gamma signalling. Upon infection with the avirulent PRU strain of T. gondii, IDO mRNA expression was induced in an IFNgammaR-dependent manner. Surprisingly, NOS2 mRNA was severely suppressed. Importantly, we showed in crossing experiments of heterozygote (IFNgammaR+/-) mothers with IFNgammaR-/- males and vice versa that IDO expression largely depends on the presence of IFN-gamma receptors on foetal cells, and to a lesser extent on maternal cells. Immunohistochemical analysis localised foetal IDO production to invasive trophoblasts within the maternal part of the placenta. The placental vascular endothelium only stained positive when the mothers possessed functional IFN-gamma receptors. In contrast, placental NOS2 expression, but also its suppression following infection, seems to be largely dependent on IFN-gamma signalling in maternal cells. Neither factor appears to regulate placental T. gondii growth, as we observed no difference in parasite numbers between (+/-) and (-/-) foetuses. Taken together, our results demonstrate the crucial role of the foetus in placental IDO, but not NOS2, production following T. gondii infection.     

The early pregnancy placenta foreshadows DNA methylation alterations of solid tumors

The placenta relies on phenotypes that are characteristic of cancer to successfully implant the embryo in the uterus during early pregnancy. Notably, it has to invade its host tissues, promote angiogenesis—while surviving hypoxia—, and escape the immune system. Similarities in DNA methylation patterns between the placenta and cancers suggest that common epigenetic mechanisms may be involved in regulating these behaviors. We show here that megabase-scale patterns of hypomethylation distinguish first from third trimester chorionic villi in the placenta, and that these patterns mirror those that distinguish many tumors from corresponding normal tissues. We confirmed these findings in villous cytotrophoblasts isolated from the placenta and identified a time window at the end of the first trimester, when these cells come into contact with maternal blood, as the likely time period for the methylome alterations. Furthermore, the large genomic regions affected by these patterns of hypomethylation encompass genes involved in pathways related to epithelial-mesenchymal transition, immune response, and inflammation. Analyses of expression profiles corresponding to genes in these hypomethylated regions in colon adenocarcinoma tumors point to networks of differentially expressed genes previously implicated in carcinogenesis and placentogenesis, where nuclear factor kappa B is a key hub. Taken together, our results suggest the existence of epigenetic switches involving large-scale changes of methylation in the placenta during pregnancy and in tumors during neoplastic transformation. The characterization of such epigenetic switches might lead to the identification of biomarkers and drug targets in oncology as well as in obstetrics and gynecology.     

Endocrine regulation of phosphorus and calcium metabolism during pregnancy in domestic ruminants

In pregnant domestic ruminants (cows, ewes, goats) foetal plasma calcium and inorganic phosphorus concentrations are higher than those measured in the dam. The foetus regulates its own calcaemia and phosphataemia. Changes in maternal plasma calcium levels have no significant effect on foetal calcaemia. Calcium and phosphorus are transported from the dam to the foetus according to a one-way process, the transport from the foetus to the dam being negligible. An important part of the calcium transferred to the foetus comes from the maternal skeleton. The true molecular mechanisms involved in placental transport of calcium are still unknown. This is an active transport, stimulated by vitamin D metabolites (of maternal, foetal or placental origin) and maternal prolactin. Maternal calcitonin protects the skeleton of the pregnant (and lactating) female ruminant against excessive demineralization, partly by modulating placental transport of calcium during periods of intense mineralization of foetal skeleton.     

Maternally-inherited Grb10 reduces placental size and efficiency

The control of foetal growth is poorly understood and yet it is critically important that at birth the body has attained appropriate size and proportions. Growth and survival of the mammalian foetus is dependent upon a functional placenta throughout most of gestation. A few genes are known that influence both foetal and placental growth and might therefore coordinate growth of the conceptus, including the imprinted Igf2 and Grb10 genes. Grb10 encodes a signalling adapter protein, is expressed predominantly from the maternally-inherited allele and acts to restrict foetal and placental growth. Here, we show that following disruption of the maternal allele in mice, the labyrinthine volume was increased in a manner consistent with a cell-autonomous function of Grb10 and the enlarged placenta was more efficient in supporting foetal growth. Thus, Grb10 is the first example of a gene that acts to limit placental size and efficiency. In addition, we found that females inheriting a mutant Grb10 allele from their mother had larger litters and smaller offspring than those inheriting a mutant allele from their father. This grandparental effect suggests Grb10 can influence reproductive strategy through the allocation of maternal resources such that offspring number is offset against size.