Embryonic and fetal programming of physiological disorders in adulthood

In the past decade, data from numerous epidemiological studies have indicated strong inverse associations between birth weight and risk of coronary heart disease, hypertension, type 2-diabetes, and other diseases in adulthood. The "Barker hypothesis" thus postulates that a number of organ structures and functions undergo programming during embryonic and fetal life. This developmental programming determines the set points of physiological and metabolic responses in adult life. Alterations of nutrient availability during gestation may lead to developmental adaptations, via hormonal maneuvers by the embryo and fetus that readjust these set points. These adaptive measures have short-term benefits to the embryo and fetus, so that the newborn will be better prepared for the adverse environment (e.g., undernutrition). However, adequate nutritional support during postnatal life that enables catch-up growth may create metabolic conflicts that predispose the adult to aberrant physiological functions and, ultimately, increased risk of disease. It is plausible that other adverse in utero conditions, including exposure to developmental toxicants, may similarly alter adult disease susceptibility. This article provides an overview of the Barker hypothesis, its supporting evidence, the current advances in understanding the biological mechanisms underlying this phenomenon, and its implications for developmental toxicology.     

Adaptive responses by mouse early embryos to maternal diet protect fetal growth but predispose to adult onset disease

Poor maternal nutrition during pregnancy can alter postnatal phenotype and increase susceptibility to adult cardiovascular and metabolic diseases. However, underlying mechanisms are largely unknown. Here, we show that maternal low protein diet (LPD), fed exclusively during mouse preimplantation development, leads to offspring with increased weight from birth, sustained hypertension, and abnormal anxiety-related behavior, especially in females. These adverse outcomes were interrelated with increased perinatal weight being predictive of later adult overweight and hypertension. Embryo transfer experiments revealed that the increase in perinatal weight was induced within blastocysts responding to preimplantation LPD, independent of subsequent maternal environment during later pregnancy. We further identified the embryo-derived visceral yolk sac endoderm (VYSE) as one mediator of this response. VYSE contributes to fetal growth through endocytosis of maternal proteins, mainly via the multiligand megalin (LRP2) receptor and supply of liberated amino acids. Thus, LPD maintained throughout gestation stimulated VYSE nutrient transport capacity and megalin expression in late pregnancy, with enhanced megalin expression evident even when LPD was limited to the preimplantation period. Our results demonstrate that in a nutrient-restricted environment, the preimplantation embryo activates physiological mechanisms of developmental plasticity to stablize conceptus growth and enhance postnatal fitness. However, activation of such responses may also lead to adult excess growth and cardiovascular and behavioral diseases.     

The intrauterine programming of adult and children's disesases

The intrauterine programming hypothesis proposed that cardiovascular and metabolic disease originate through developmental plasticity and fetal adaptation arising from failure of the materno-placental supply of nutrients. The hypothesis is supported by experimental data in animals indicating that maternal nutrition can program long term effects on the offspring. The intrauterine conditions in which the fetus develops have an important role in regulating the function of its physiological systems later in life. Intrauterine programming of physiological systems occurs at the gene, cell, tissue, organ, and system levels and causes permanent structural and functional changes, which can lead to overt disease, particularly with increasing age. The physiological prevention of disease programming is discussing.     

Prenatal exposure to glucocorticoids and adult disease

There is evidence to suggest that an individual's susceptibility to cardiovascular disease cannot be entirely explained by differences in life style factors (i.e., low physical activity, high fat/salt diet), or genetic causes, but may also be influenced by factors encountered during intrauterine life. Epidemiological studies found the link between low birth weight for gestational age (a broad index of sub-optimal intrauterine environment) and increased incidence of cardiovascular and metabolic diseases in adulthood. Many animal models in which the intrauterine environment was altered during early/late or throughout gestation demonstrated long-term effects on adult health. In general stress in early gestation is more likely to be associated with adult cardiovascular disease including hypertension, whereas late gestation stress may also be associated with adult hypotension in addition to metabolic/endocrine abnormalities. Two systems have been widely hypothesised to serve as mechanisms via which adverse prenatal influences impinge on adult cardiovascular and metabolic disease; hippocampal-hypothalamo-pituitary-adrenal axis (HHPA) and renin-angiotensin system (RAS). Interestingly, at least in our animal model of adult hypertension after brief/early prenatal glucocorticoid exposure, HHPA axis is not altered when studied either in late gestation or at several stages during adulthood. However, our more recent results, using the same animal model, suggest a major role for the central and renal RAS. This review will mainly focus on animal models and potential mechanisms via which a perturbed intrauterine environment (undernutrition or steroid exposure) lead to adult cardiovascular and/or metabolic disease.     

The influence of leptin on early life programming of obesity

Epidemiological evidence together with experimental models shows a direct relationship between fetal and early postnatal growth patterns and an increased risk of adult metabolic disease. Maternal health and nutrition are key determinants in influencing infant growth but the precise molecular mechanisms underlying this relationship are unclear, although it is evident that there are critical time windows when these effects are important. Animal models show mechanistic parallels with human populations and highlight that the early environment represents a therapeutic window for protection from obesity and metabolic disease. The observation that developmental programming can be reversed has been demonstrated in studies in which both maternal and neonatal leptin treatment prevents the induction of the adverse metabolic phenotype. Given that orally administered peptides are absorbed intact by the new born, the prospect of providing supplemental leptin either as drops or in milk deserves serious consideration as a means of reducing or reversing the obesity and type 2 diabetes epidemic.     

Maternal adaptations and inheritance in the transgenerational programming of adult disease

Adverse exposures in utero have long been linked with an increased susceptibility to adult cardio-renal and metabolic diseases. Clear gender differences exist, whereby growth-restricted females, although exhibiting some phenotypic modifications, are often protected from overt disease outcomes. One of the greatest physiological challenges facing the female gender, however, is that of pregnancy; yet little research has focused on the outcomes associated with this, as a potential 'second-hit' for those who were small at birth. We review the limited evidence suggesting that pregnancy may unmask cardio-renal and metabolic disease states and the consequences for long-term maternal health in females who were born small. Additionally, a growing area of research in this programming field is in the transgenerational transmission of low birth weight and disease susceptibility. Pathways for transmission might include an abnormal adaptation to pregnancy by the growth-restricted mother and/or inheritance via the parental germline. Strategies to optimise the pregnancy environment and/or prevent the consequences of inheritance of programmed deficits and dysfunction are of critical importance for future generations.     

Maternal exposure to dexamethasone or cortisol in early pregnancy differentially alters insulin secretion and glucose homeostasis in adult male sheep offspring

An adverse intrauterine environment increases the risk of developing various adult-onset diseases, whose nature varies with the timing of exposure. Maternal undernutrition in humans can increase adiposity, and the risk of coronary heart disease and impaired glucose tolerance in adult life, which may be partly mediated by maternal or fetal endocrine stress responses. In sheep, dexamethasone in early pregnancy impairs cardiovascular function, but not glucose homeostasis in adult female offspring. However, male offspring are often more susceptible to early life "programming". Pregnant sheep were infused intravenously with saline (0.19 ml/h), dexamethasone (0.48 mg/h), or cortisol (5 mg/h), for 2 days from 26 to 28 days of gestation. In male offspring, size at birth and postnatal growth were measured, and glucose tolerance [intravenous glucose tolerance test (IVGTT)], insulin secretion, and insulin sensitivity of glucose, alpha-amino nitrogen, and free fatty acid metabolism were assessed at 4 yr of age. We show that cortisol, but not dexamethasone, treatment of mothers causes fasting hyperglycemia in adult male offspring. Maternal cortisol induced a second-phase hyperinsulinemia during IVGTT, whereas maternal dexamethasone induced a first-phase hyperinsulinemia. Dexamethasone improved glucose tolerance, while cortisol had no impact, and neither affected insulin sensitivity. This suggests that maternal glucocorticoid exposure in early pregnancy alters glucose homeostasis and induces hyperinsulinemia in adult male offspring, but in a glucocorticoid-specific manner. These consequences of glucocorticoid exposure in early pregnancy may lead to pancreatic exhaustion and diabetes longer term and are consistent with stress during early pregnancy contributing to such outcomes in humans.     

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.     

Epigenetic regulation of the placental HSD11B2 barrier and its role as a critical regulator of fetal development

“Fetal programming” is a term used to describe how early-life experience influences fetal development and later disease risk. In humans, prenatal stress-induced fetal programming is associated with increased risk of preterm birth, and a heightened risk of metabolic and neurological diseases later in life. A critical determinant of this is the regulation of fetal exposure to glucocorticoids by the placenta. Glucocorticoids are the mediators through which maternal stress influences fetal development. Excessive fetal glucocorticoid exposure during pregnancy results in low birth weight and abnormalities in a number of tissues. The amount of fetal exposure to maternal glucocorticoids depends on the expression of HSD11B2, an enzyme predominantly produced by the syncytiotrophoblast in the placenta. This protects the fetus by converting active glucocorticoids into inactive forms. In this review we examine recent findings regarding placental HSD11B2 that suggest that its epigenetic regulation may mechanistically link maternal stress and long-term health consequences in affected offspring.     

Intergenerational consequences of fetal programming by in utero exposure to glucocorticoids in rats

Epidemiological studies linking low birth weight and subsequent cardiometabolic disease have given rise to the hypothesis that events in fetal life permanently program subsequent cardiovascular risk. The effects of fetal programming may not be limited to the first-generation offspring. We have explored intergenerational effects in the dexamethasone-programmed rat, a model in which fetal exposure to excess glucocorticoid results in low birth weight with subsequent adult hyperinsulinemia and hyperglycemia underpinned by increased activity of the key hepatic gluconeogenic enzyme, phosphoenolpyruvate carboxykinase (PEPCK). We found that the male offspring of female rats that had been exposed prenatally to dexamethasone, but were not manipulated in their own pregnancy, also had reduced birth weight (5.66 +/- 0.06 vs. 6.12 +/- 0.06 g, P < 0.001), glucose intolerance, and elevated hepatic PEPCK activity (5.7 +/- 0.6 vs. 3.3 +/- 0.2 nmol.min(-1).mg protein(-1), P < 0.001). These effects resolved in a third generation. Similar intergenerational programming was observed in offspring of male rats exposed prenatally to dexamethasone mated with control females. The persistence of such programming effects through several generations, transmitted by either maternal or paternal lines, indicates the potential importance of epigenetic factors in the intergenerational inheritance of the "programming phenotype" and provides a basis for the inherited association between low birth weight and cardiovascular risk factors.     

In utero programming of cardiovascular disease

Low birth weight, thinness and short body length at birth are now known to be associated with increased rates of cardiovascular disease and non-insulin dependent diabetes in adult life. The fetal origins hypothesis proposes that these diseases originate through adaptations which the fetus makes when it is undernourished. These adaptations may be cardiovascular, metabolic or endocrine. They permanently change the structure and function of the body. Prevention of the diseases may depend on prevention of imbalances in fetal growth or imbalances between pre- and post-natal growth, or imbalances in nutrient supply to the fetus.     

The relation of small head circumference and thinness at birth to death from cardiovascular disease in adult life.

OBJECTIVE--To determine how fetal growth is related to death from cardiovascular disease in adult life. DESIGN--A follow up study of men born during 1907-24 whose birth weights, head circumferences, and other body measurements were recorded at birth. SETTING--Sheffield, England. SUBJECTS--1586 Men born in the Jessop Hospital. MAIN OUTCOME MEASURE--Death from cardiovascular disease. RESULTS--Standardised mortality ratios for cardiovascular disease fell from 119 in men who weighed 5.5 pounds (2495 g) or less at birth to 74 in men who weighed more than 8.5 pounds (3856 g). The fall was significant for premature cardiovascular deaths up to 65 years of age (chi 2 = 5.0, p = 0.02). Standardised mortality ratios also fell with increasing head circumference (chi 2 = 4.6, p = 0.03) and increasing ponderal index (weight/length3) (chi 2 = 3.8, p = 0.05; for premature deaths chi 2 = 6.0, p = 0.01). They were not related to the duration of gestation. Among men for whom the ratio of placental weight to birth weight was in the highest fifths the standardised mortality ratio was 137. CONCLUSION--These findings show that reduced fetal growth is followed by increased mortality from cardiovascular disease. They suggest that reduction in growth begins early in gestation. They are further evidence that cardiovascular disease originates through programming of the body''s structure, physiology, and metabolism by the environment during fetal life. Maternal nutrition may have an important influence on programming.     

Maternal diet modulates placenta growth and gene expression in a mouse model of diabetic pregnancy

Unfavorable maternal diet during pregnancy can predispose the offspring to diseases later in life, such as hypertension, metabolic syndrome, and obesity. However, the molecular basis for this phenomenon of "developmental programming" is poorly understood. We have recently shown that a diet nutritionally optimized for pregnancy can nevertheless be harmful in the context of diabetic pregnancy in the mouse, associated with a high incidence of neural tube defects and intrauterine growth restriction. We hypothesized that placental abnormalities may contribute to impaired fetal growth in these pregnancies, and therefore investigated the role of maternal diet in the placenta. LabDiet 5015 diet was associated with reduced placental growth, commencing at midgestation, when compared to pregnancies in which the diabetic dam was fed LabDiet 5001 maintenance chow. Furthermore, by quantitative RT-PCR we identify 34 genes whose expression in placenta at midgestation is modulated by diet, diabetes, or both, establishing biomarkers for gene-environment interactions in the placenta. These results implicate maternal diet as an important factor in pregnancy complications and suggest that the early phases of placenta development could be a critical time window for developmental origins of adult disease.     

Prenatal Adversity Modulates the Quality of Maternal Care Via the Exposed Offspring

Adversities in pregnancy, including poor diet and stress, are associated with increased risk of developing both metabolic and mental health disorders later in life, a phenomenon described as fetal programming or developmental origins of disease. Predominant hypotheses proposed to explain this relationship suggest that the adversity imposes direct changes to the developing fetus which are maintained after birth resulting in an increased susceptibility to ill health. However, during pregnancy the mother, the developing fetus, and the placenta are all exposed to the adversity. The same adversities linked to altered offspring outcome can also result in suboptimal maternal care, which is considered an independent adverse exposure for the offspring. Recent key experiments in mice reveal the potential of prenatal adversity to drive alterations in maternal care through abnormal maternal–pup interactions and via alterations in placental signaling. Together, these data highlight the critical importance of viewing fetal programming holistically paying attention to the intimate, bidirectional, and reiterative relationship between mothers and their offspring.     

Intrauterine programming of hypertension: the role of the renin-angiotensin system

From experiments with prenatal undernutrition in the rat, it is clear that fetal exposure to glucocorticoids of maternal origin is a key first step in the programming of hypertension and perhaps coronary heart disease. The chain of events leading from glucocorticoid action in the fetal tissues to hypertension in adulthood involves the development of hypersensitivity to glucocorticoids in adult life (Scheme 1). This has the effect of activating the RAS through induction of key genes such as ACE, which, in turn, may increase sensitivity of the blood vessels to the actions of ANGII. Another consequence of prenatal undernutrition, which may or may not involve glucocorticoids, is the abnormal development of the kidney [35]. Impaired nephrogenesis must surely have an impact upon lifelong renal function and cardiovascular control. Progress has been made in demonstrating that hypertension can be prenatally programmed through maternal dietary manipulation and some of the putative mechanisms involved have been identified. The priorities in this field of research must now be to clarify the role of maternal diet as a programming stimulus in order to generate an effective series of public health guidelines for pregnant women. Although the identification of metabolic mechanisms might suggest possible pharmacological interventions in early life as a means of reducing cardiovascular risk in adult life [49], it will always be more desirable to optimize maternal diet.     

Sedentary behavior during postnatal life is determined by the prenatal environment and exacerbated by postnatal hypercaloric nutrition

The discovery of a link between in utero experience and later metabolic and cardiovascular disease is one of the most important advances in epidemiology research of recent years. There is now increasing evidence that alterations in the fetal environment have long-term consequences on metabolic and endocrine pathophysiology in adult life. This process has been termed "fetal programming," and we have shown that undernutrition of the mother during gestation leads to obesity, hypertension, hyperphagia, hyperinsulinemia, and hyperleptinemia in offspring. Using this model of maternal undernutrition throughout pregnancy, we investigated whether prenatal influences may lead to alterations in postnatal locomotor behavior, independent of postnatal nutrition. Virgin Wistar rats were time mated and randomly assigned to receive food either ad libitum (ad libitum group) or at 30% of ad libitum intake (undernourished group). Offspring from UN mothers were significantly smaller at birth than AD offspring. At weaning, offspring were assigned to one of two diets [control or hypercaloric (30% fat)]. At ages of 35 days, 145 days, and 420 days, voluntary locomotor activity was assessed. At all ages studied, offspring from undernourished mothers were significantly less active than offspring born of normal birth weight for all parameters measured, independent of postnatal nutrition. Sedentary behavior in programmed offspring was exacerbated by postnatal hypercaloric nutrition. This work is the first to clearly separate prenatal from postnatal effects and shows that lifestyle choices themselves may have a prenatal origin. We have shown that predispositions to obesity, altered eating behavior, and sedentary activity are linked and occur independently of postnatal hypercaloric nutrition. Moreover, the prenatal influence may be permanent as offspring of undernourished mothers were still significantly less active compared with normal offspring at an advanced adult age, even in the presence of a healthy diet throughout postnatal life.     

Epigenetic regulation of the placental HSD11B2 barrier and its role as a critical regulator of fetal development

“Fetal programming” is a term used to describe how early-life experience influences fetal development and later disease risk. In humans, prenatal stress-induced fetal programming is associated with increased risk of preterm birth, and a heightened risk of metabolic and neurological diseases later in life. A critical determinant of this is the regulation of fetal exposure to glucocorticoids by the placenta. Glucocorticoids are the mediators through which maternal stress influences fetal development. Excessive fetal glucocorticoid exposure during pregnancy results in low birth weight and abnormalities in a number of tissues. The amount of fetal exposure to maternal glucocorticoids depends on the expression of HSD11B2, an enzyme predominantly produced by the syncytiotrophoblast in the placenta. This protects the fetus by converting active glucocorticoids into inactive forms. In this review we examine recent findings regarding placental HSD11B2 that suggest that its epigenetic regulation may mechanistically link maternal stress and long-term health consequences in affected offspring.     

Methylation changes at NR3C1 in newborns associate with maternal prenatal stress exposure and newborn birth weight

Early life experiences, including those in utero, have been linked to increased risk for adult-onset chronic disease. The underlying assumption is that there is a critical period of developmental plasticity in utero when selection of the fetal phenotype that is best adapted to the intrauterine environment occurs. The current study is the first to test the idea that extreme maternal psychosocial stressors, as observed in the Democratic Republic of Congo, may modify locus-specific epigenetic marks in the newborn resulting in altered health outcomes. Here we show a significant correlation between culturally relevant measures of maternal prenatal stress, newborn birth weight and newborn methylation in the promoter of the glucocorticoid receptor NR3C1. Increased methylation may constrain plasticity in subsequent gene expression and restrict the range of stress adaptation responses possible in affected individuals, thus increasing their risk for adult-onset diseases.     

Methylation changes at NR3C1 in newborns associate with maternal prenatal stress exposure and newborn birth weight

Early life experiences, including those in utero, have been linked to increased risk for adult-onset chronic disease. The underlying assumption is that there is a critical period of developmental plasticity in utero when selection of the fetal phenotype that is best adapted to the intrauterine environment occurs. The current study is the first to test the idea that extreme maternal psychosocial stressors, as observed in the Democratic Republic of Congo, may modify locus-specific epigenetic marks in the newborn resulting in altered health outcomes. Here we show a significant correlation between culturally relevant measures of maternal prenatal stress, newborn birth weight and newborn methylation in the promoter of the glucocorticoid receptor NR3C1. Increased methylation may constrain plasticity in subsequent gene expression and restrict the range of stress adaptation responses possible in affected individuals, thus increasing their risk for adult-onset diseases.     

The thrifty phenotype hypothesis: thrifty offspring or thrifty mother?

Medical research is increasingly focusing on the contribution of nutritional programming to disease in later life. Programming is a process whereby a stimulus during a critical window of time permanently affects subsequent structure, function or developmental schedule of the organism. The thrifty phenotype hypothesis is widely used to interpret such studies, with early growth restriction seen as adaptation to environmental deprivation. However, such permanent adjustment is less beneficial than maintaining flexibility so as to recover from early growth deficits if the environment improves. Thus, the existing thrifty phenotype hypothesis fails to explain why plasticity is lost so early in development in species with extended growth. One explanation is that the developing organism simply cannot maintain phenotypic plasticity throughout the period of organ growth. This article adds a life history perspective, arguing that programming of the offspring may in some species benefit maternal fitness more than it does that of individual offspring. Closing the critical window early in development allows the preservation of maternal strategy in offspring phenotype, which in humans benefits the mother by constraining offspring demand after weaning. The offspring gains by being buffered against environmental fluctuations during the most sensitive period of development, allowing coherent adaptation of organ growth to the state of the environment. The critical window is predicted to close when offspring physiology becomes independent of maternal physiology, the timing of which depends on offspring trait. Because placental nutrition and lactation buffer against short-term environmental fluctuations, maternal strategy is predicted to derive from long-term experience, encapsulated in maternal size and nutritional status. Such an approach implies that public health programmes for improving birth weight may be more effective if they target maternal development rather than nutrition during pregnancy. Equally, aggressive nutritional management of infants born small or pre-term may induce the very environmental fluctuations that are naturally softened by maternal nutrition.