Physiological signals critical periods and hypothalamic mechanisms underlying neonatal programming of adult hyperphagia by maternal over-nutrition

Extensive epidemiological and animal-based studies have demonstrated that maternal obesity not only heightens the complications commonly associated with pregnancy, but also increases the long-term risks of metabolic and cardiovascular disease in offspring. This effect is independent of any direct genetic cause. Inheritance of a characteristic which has been acquired by the mother raises important biological issues; consequently, the full significance of this phenomenon extends beyond the immediate context of energy balance. Nevertheless, the immediate implications of these findings provide important perspectives on the escalating problems of obesity, which threaten to overwhelm our healthcare services. We have recently developed an experimental protocol in which female rats or mice are given a highly palatable diet, rich in sugar and fat, before conception and throughout gestation and lactation. This diet results in overt obesity and mimics the obesogenic food which is commonly eaten throughout the western hemisphere. Although the pups of these rodents are raised on a normal diet from weaning, by adulthood they show increased appetite and obesity, with signs of type-2 diabetes and cardiovascular disease. Fifteen years ago a novel hormone secreted by fat cells was discovered and named leptin (from the Greek leptos, meaning thin). When administered in adulthood to normal rodents or to mice which are deficient of this hormone, leptin suppresses feeding. In chronic cases of obesity, however, leptin is ineffective; under such conditions, humans and rodents are said to be leptin-resistant. During the first ten days after birth, rodents produce a surge of leptin in the blood. This enters the brain and promotes the growth of pathways in the hypothalamus, the part of the brain which controls appetite. One of the most striking changes we have found in the rodents programmed for high appetite by maternal over-nutrition is a massively raised and prolonged surge of leptin before weaning. We have also found leptin-resistance in these animals by postnatal day 30 (9 days after weaning), long before they are overtly obese. These animals show a loss of leptin's ability to suppress feeding behaviour, and also a reduction in the cellular activity normally induced by leptin within the appetite-regulating part of the hypothalamus. These findings lead us to hypothesise that chemical signals in the mother's milk promote the abnormal surge of leptin in the pup's blood. We also hypothesise that the consequent over-stimulation of leptin-sensitive circuits in the hypothalamus results in chronic leptin-resistance and a persistently unrestrained appetite. The proposed research will elucidate the chemical signals received from the mother and the critical period during which those signals influence the infant's future appetite by excess secretion of leptin. The therapeutic possibilities of increasing the ratio of beneficial fatty acids in the maternal diet will be addressed. We will also elucidate the processes causing the hypothalamus to become leptin-resistant and the effects on appetite-regulating circuits. These studies will increase our understanding of the biological mechanisms which underlie the ways in which maternal obesity may influence the long-term health of offspring.

We have developed an experimental protocol in rodents which shows that maternal diet-induced obesity programmes offspring hyperphagia. This is associated with an amplified and prolonged leptin surge in the offspring before weaning. We hypothesise that the latter half of the suckling period is critical for programming adult hyperphagia via excess leptin, which is driven by factors in the dam's milk. Leptin resistance is seen in the offspring by postnatal day 30. In response to leptin, they fail to exhibit anorexia and show reduced pSTAT-3 in the arcuate nucleus (ARC). We will investigate the signals and processes underlying the ontogeny of these phenomena. Central to our objectives is the physiological and pathophysiological regulation of neonatal leptin and its subsequent effects on leptin-signalling and peptidergic pathways intrinsic to or projecting from the ARC. This multidisciplinary project will investigate the significance of maternal obesity versus diet in programming hyperphagia. To this end, we will study wildtype offspring of heterozygous MC4 receptor deficient mice (which are hyperphagic irrespective of diet). The neonatal leptin surge will be characterised and related to adipocyte maturation in different fat depots. Interventions will be made to amplify and extend the surge in control pups or curtail it in offspring of obese dams. The ontogeny and nature of the leptin resistance will be studied in terms of leptin receptor expression and leptin-signalling pathways. Our model will also address findings which have led others to identify an apparent paradox, in which reduced orexigenic AgRP is associated with hyperphagia. Since we hypothesise that this paradox is illusory because AgRP reduction reflects increased turnover from an impaired pathway, we must quantity its expression. Our model offers a tractable opportunity to address this paradox. It is unencumbered by the genetic lesions of ob/ob mice or rats bred to be susceptible to diet-induced obesity.

The results from this project will be communicated to the general scientific community through presentation at scientific meetings spanning the breadth of the disciplines involved to achieve a maximal target audience (e.g. Developmental Origins of Health and Disease International Congress; Society for Gynecologic Investigation, Physiological Society, European Congress on Obesity, Society for Neuroscience Annual Meeting). Publication will be in high impact peer reviewed journals with fast on-line access (e.g. PLoS One, Cell Metabolism, Nature Medicine). Locally results will be presented at departmental research meetings and local special interest meetings (e.g. Divisional and Departmental Developmental Programming Meetings). The applicants are regularly invited to speak at national (e.g. Nutrition Society, Biochemical Society, Physiological Society) and international meetings (e.g. World Congress on the Developmental Origins of Health and Disease, Endocrine Society, Society for Gynaecological Investigation; International Obesity Congress) and the applicants (LP and PT) have recently been invited to join the British Nutrition Foundation Task Force on Obesity. The major beneficiaries in London, nationally and internationally will be: (1) researchers in hypothalamic neurobiology and appetite regulation, (2) researchers in developmental programming, (3) researchers in metabolism and obesity (4) pharmaceutical companies involved in identifying anti-obesity drug targets. In the longer term the work should inform those giving advice to pregnant and nursing mothers. The broader involvement of both academic (basic scientists and clinicians) and industrial collaborators (eg Tate & Lylle) will maximise the dissemination of results. Dissemination will be enhanced further by the inclusion of our American collaborator, Sebastien Bouret, CA, USA. The proposal provides a good fit with the BBSRC priority areas of Ageing Research: Lifelong Health and Wellbeing and Developmental Origins of Health, offering an integrative approach to basic and comparative physiology applying molecular, and cellular information to whole animal physiology and epigenetics.