Maternal environmental contribution to adult sensitivity and resistance to obesity in Long Evans rats

Rodent models of metabolic disorders: considerations for use in studies of neonatal programming

Epidemiologically, metabolic disorders have garnered much attention, perhaps due to the predominance of obesity. The early postnatal life represents a critical period for programming multifactorial metabolic disorders of adult life. Though altricial rodents are prime subjects for investigating neonatal programming, there is still no sufficiently generalised literature on their usage and methodology. This review focuses on establishing five approach-based models of neonatal rodents adopted for studying metabolic phenotypes. Here, some modelled interventions that currently exist to avoid or prevent metabolic disorders are also highlighted. We also bring forth recommendations, guidelines and considerations to aid research on neonatal programming. It is hoped that this provides a background to researchers focused on the aetiology, mechanisms, prevention and treatment of metabolic disorders.

Maternal pineal melatonin in gestation and lactation physiology, and in fetal development and programming

Pregnancy and lactation are reproductive processes that rely on physiological adaptations that should be timely and adequately triggered to guarantee both maternal and fetal health. Pineal melatonin is a hormone that presents daily and seasonal variations that synchronizes the organism's physiology to the different demands across time through its specific mechanisms and ways of action. The reproductive system is a notable target for melatonin as it actively participates on reproductive physiology and regulates the hypothalamus-pituitary-gonads axis, influencing gonadotropins and sexual hormones synthesis and release. For its antioxidant properties, melatonin is also vital for the oocytes and spermatozoa quality and viability, and for blastocyst development. Maternal pineal melatonin blood levels increase during pregnancy and triggers the maternal physiological alterations in energy metabolism both during pregnancy and lactation to cope with the energy demands of both periods and to promote adequate mammary gland development. Moreover, maternal melatonin freely crosses the placenta and is the only source of this hormone to the fetus. It importantly times the conceptus physiology and influences its development and programing of several functions that depend on neural and brain development, ultimately priming adult behavior and energy and glucose metabolism. The present review aims to explain the above listed melatonin functions, including the potential alterations observed in the progeny gestated under maternal chronodisruption and/or hypomelatoninemia.

Effects of early postnatal environment on hypothalamic gene expression in OLETF rats

Previous reports have shown that the early postnatal environment has the ability to modify the obesity phenotype of Otsuka Long-Evans Tokushima Fatty (OLETF) rats. To determine whether this early postnatal environment affects hypothalamic signaling systems involved in energy balance, OLETF pups and lean Long-Evans Tokushima Otsuka (LETO) pups were cross-fostered to same or opposite strain Dams (designated as LdLp: LETO pups with LETO dams; LdOp: OLETF pups with LETO dams; OdLp: LETO pups with OLETF dams; and OdOp: OLETF pups with OLETF dams). Hypothalamic gene expression was examined at postnatal day 23 (PND 23) and PND 90 as OdOp rats started to gain more body weight at PND 23 and developed obesity at PND 90 relative to lean control LdLp rats. On PND 23, neuropeptide Y (Npy) gene expression was significantly increased in the dorsomedial hypothalamus (DMH) in both LdOp and OdOp pups compared to LdLp pups. Maternal environment did not affect DMH Npy expression in LETO weanlings. On PND 90, maternal environment during the cross-fostering period had a major effect on DMH Npy expression. Levels were significantly increased in both OdOp and OdLp rats relative to those in LdOp rats and LdLp controls. Reduced expression of Npy in the DMH of LdOp rats was consistent with their reduction of body weight compared to OdOp rats. In contrast to DMH Npy, gene expression for Npy and proopiomelanocortin in the arcuate nucleus appeared to appropriately respond to alterations in body weight and plasma leptin levels. Levels of oxytocin gene expression in the paraventricular nucleus were lower in offspring raised by LETO dams apparently responding to the higher DMH NPY levels. Together, our results demonstrate effects of both genotype and early postnatal environment on obesity of OLETF rats and further suggest an important role of DMH NPY in the development of obesity of OLETF rats.

Thyroid Hormone-Dependent Epigenetic Regulation of Melanocortin 4 Receptor Levels in Female Offspring of Obese Rats

Maternal obesity is a risk factor for offspring obesity. The melanocortin 4 receptor (Mc4r) is one of the mediators of food intake and energy balance. The present study examined the epigenetic mechanisms underlying altered Mc4r levels in the hypothalamic paraventricular nucleus in the offspring of high-fat diet (HFD)-induced obese dams and sought to elucidate the role of thyroid hormones in epigenetic regulation and tagging of their nucleosome at the Mc4r promoter. Female Wistar rats were fed an HFD or standard chow from weaning through gestation and lactation. Epigenetic alterations were analyzed in the offspring on postnatal day 21 at the Mc4r promoter using chromatin immunoprecipitation and bisulfite sequencing. To study the role of triiodothyronine (T3) in Mc4r downregulation, dams received methimazole (MMI), an inhibitor of thyroid hormone production. Offspring of HFD-fed dams had a greater body weight, elevated plasma T3 concentrations, and lower Mc4r messenger RNA levels than controls. At the Mc4r promoter, offspring of HFD-fed mothers demonstrated increased histone 3 lysine 27 acetylation (H3K27ac) with a greater association to thyroid hormone receptor-β (TRβ), an inhibitor of Mc4r transcription. Moreover, TRβ coimmunoprecipitated with H3K27ac, supporting their presence in the same complex. Maternal MMI administration prevented the HFD reduction in Mc4r levels, the increase in TRβ, and the increase in the TRβ-H3K27ac association, providing further support for the role of T3 in downregulating Mc4r levels. These findings demonstrate that a perinatal HFD environment affects Mc4r regulation through a T3 metabolic pathway involving histone acetylation of its promoter.

Cross-fostering: Elucidating the effects of gene×environment interactions on phenotypic development

Cross-fostering of litters from soon after birth until weaning is a valuable tool to study the ways in which gene×environment interactions program the development of neural, physiological and behavioral characteristics of mammalian species. In laboratory mice and rats, the primary focus of this review, cross-fostering of litters between mothers of different strains or treatment groups (intraspecific) or between mothers of different species (interspecific) has been conducted over the past 9 decades. Areas of particular interest have included maternal effects on emotionality, social preferences, responses to stressful stimulation, nutrition and growth, blood pressure regulation, and epigenetic effects on brain development and behavior. Results from these areas of research highlight the critical role of the postnatal maternal environment in programming the development of offspring phenotypic characteristics. In addition, experimental paradigms that have included cross-fostering have permitted investigators to tease apart prenatal versus postnatal effects of various treatments on offspring development and behavior.

Post-weaning diet determines metabolic risk in mice exposed to overnutrition in early life

BACKGROUND

Maternal overnutrition during pregnancy is associated with an increased risk of obesity and cardiometabolic disease in the offspring; a phenomenon attributed to 'developmental programming'. The post-weaning development of obesity may associate with exacerbation of the programmed metabolic phenotype. In mice, we have previously shown that exposure to maternal overnutrition causes increased weight gain in offspring before weaning, but exerts no persistent effects on weight or glucose tolerance in adulthood. In order to determine whether post-weaning exposure to a cafeteria diet might lead to an exacerbation of programmed effects, offspring born and raised by mothers on control (CON) or cafeteria (DIO) diets were transferred onto either CON or DIO diets at weaning.

FINDINGS

Post-weaning DIO caused the development of obesity, with hyperglycaemia and hyperinsulinaemia in males; and obesity with hyperinsulinaemia in females and with increased cholesterol levels in both sexes. Exposure to maternal overnutrition during pregnancy and lactation caused only subtle additional effects on offspring phenotype.

CONCLUSIONS

These results suggest that post-weaning exposure to a high-fat high-sugar diet has a more profound effect on offspring weight gain and glucose tolerance than exposure to maternal overnutrition. These data emphasise the importance of optimising early life nutrition in offspring of both obese and lean mothers.

Maternal regulation of offspring development in mammals is an ancient adaptation tied to lactation

The developmental origins of health and disease (DOHaD) is a paradigm for understanding metabolic diseases of modern humans. Vulnerability to disease is linked to perturbations in development during critical time periods in fetal and neonatal life. These perturbations are caused by environmental signals, often generated or transduced by the mother. The regulation of mammalian development depends to a large extent on maternal biochemical signals to her offspring. We argue that this adaptation is ancient, and originated with the evolution of lactation. Lactation evolved earlier than live birth and before the extensive placental development of modern eutherian mammals. Milk contains a host of signaling molecules including nutrients, immunoglobulins, growth factors and metabolic hormones. As evidenced by marsupials, lactation originally served to supply the biochemical factors for growth and development for what is essentially a fetus to a weanling transitioning to independent existence. In placental mammals maternal signaling in earliest life is accomplished through the maternal-placental-fetal connection, with more of development shifted to in utero life. However, significant development occurs postpartum, supported by milk. Mothers of all taxa provide biochemical signals to their offspring, but for non-mammalian mothers the time window is short. Developing mammals receive maternal biochemical signals over an extended period. These signals serve to guide normal development, but also can vary in response to environmental conditions. The ancient adaptation of lactation resulted in a lineage (mammals) in which maternal regulation of offspring development evolved to a heightened degree, with the ability to modify development at multiple time points. Modern metabolic diseases may arise due to a mismatch between maternal regulation and eventual circumstances of the offspring, and due to a large proportion of mothers that exceed past evolutionary norms in body fat and pregnancy weight gain such that maternal signals may no longer be within the adaptive range.

Selective leptin insensitivity and alterations in female-reproductive patterns linked to hyperleptinemia during infancy

The dramatic increase in the prevalence of childhood obesity worldwide makes the investigation of its early developmental stages and effective prevention strategies an urgent issue. CCK₁ deficient OLETF rats are a model of obesity previously used to study the early phases of this disorder. Here, we exposed wild type (LETO) females to an early obesogenic environment and genetically obese OLETF females to a lean postnatal environment, to assess long term alterations in leptin sensitivity, predisposition to diet induced obesity and adult female health. We found that genetically lean females reared by obese mothers presented early postnatal hyperleptemia, selectively reduced response to leptin and sensitivity to diet induced obesity when exposed to a high palatable diet as adults. The estrous cycle structure and intake profile were permanently disrupted, despite presenting normal adiposity/body weight/food intake. Genetically obese females reared by lean dams showed normalized early levels of leptin and reduced body weight, food intake and body fat at adulthood; normalized estrous cycle structure and food intake across the cycle, improved hormonal profile and peripheral leptin sensitivity and a remarkable progress in self-control when exposed to a high fat/palatable diet. Altogether, it appears that the early postnatal environment plays a critical role in determining later life coping with metabolic challenges and has an additive effect on the genetic predisposition that makes OLETF females morbidly obese as adults. This work also links, for the first time, alterations in the leptin system during early development to later life abnormalities related to female reproduction and health.