Intergenerational effects of early life programming: The role of glucocorticoids and maternal obesity
Hypertension and type two diabetes mellitus (Type 2 DM) are serious chronic illnesses that impact on the lives of millions of people around the world. Various epidemiological studies have shown a relationship between early life events such as intrauterine growth retardation (IUGR) resulting in low birth weight and the development of these chronic illnesses in adult life. To explain the link between these two events, it has been suggested that an ‘insult’ at a critical time point of development can ‘program’ alterations in gene expression, organ size, and cell number. This has been termed “the early life origins of disease’. There is also evidence that these programmed effects are not limited to the first generation but can also be passed to subsequent generations. With changes in lifestyle in modern society, the prevalence of obesity is increasing, in association with problems such as type 2 DM, hypertension, fatty liver, atherosclerosis and the metabolic syndrome. Obesity during pregnancy is linked to problems such as gestational diabetes, hypertension and early miscarriage as well as a higher risk of congenital malformations. Maternal obesity has also been recognised as one of the factors capable of ‘programming’ the offspring, increasing the risk of childhood and adult disorders such as obesity and hypertension. In this thesis I have used two animal models to explore the underlying mechanisms of programming and its intergenerational effects: i) a rat model of prenatal glucocorticoid over-exposure (the dexamethasone-programmed rat) and ii) a mouse model of obesity during pregnancy.Using the dexamethasone-programmed rat, I have shown that prenatal glucocorticoid overexposure reduces fetal and placental weight in the first generation (F1) offspring, in association with alterations in gene expression in placenta and liver. In addition, I have shown effects on fetal and placental weights and gene expression in the second generation (F2) offspring. The observed changes in gene expression in the F2 offspring differ from those in the first generation. Thus, although effects on fetal growth are seen in both generations, the underlying mechanisms appear to be different. We also observed marked parent of origin effects on fetal and placental growth and gene expression in the second generation. In the mouse model of maternal obesity, birth weight was decreased in the F1 offspring. At weaning, the offspring of obese mothers were heavier than controls, however this difference in weight was not persistent. At three months of age, F1 female offspring of obese mothers showed altered expression of hepatic genes important in lipid regulation and metabolism. More striking changes were seen in the F2 generation in which there was an effect of paternal exposure to maternal obesity to decrease birth weight. There were also parent of origin effects on organ weights and insulin levels at six months of age. These results provide evidence for the transmission of programming effects to a second generation in two different programming models and suggest that the mechanisms leading to these effects differ between generations.