Metabolic imprinting: critical impact of the perinatal environment on the regulation of energy homeostasis

Testicular germ cell tumors in mice: new ways to study a genetically complex trait

Testicular germ cell tumors (TGCTs) are the most common cancer affecting young men. Although TGCTs are common and the genetic component of susceptibility is unusually strong, discovery of TGCT susceptibility genes in humans has been challenging. The 129/Sv inbred mouse strain is an important experimental model for studying the genetic control of TGCT susceptibility. It is the only inbred mouse strain with an appreciable frequency of spontaneous TGCTs. TGCTs in 129/Sv males share various developmental and histological characteristics with human pediatric TGCTs. As in humans, susceptibility in 129/Sv is a genetically complex trait that is too complex for conventional genetic approaches. However, several genetic variants, when congenic or isogenic on the 129/Sv background, act as genetic modifiers of TGCT susceptibility. Alternative experimental approaches based on these modifier genes can be used to unravel the complex genetic control of TGCT susceptibility. We discuss the application of modifier genes in genetic interaction tests and sensitized polygenic trait analyses toward the understanding of the complex genetics and biology of TGCT susceptibility in mice.

Diet-induced obesity in female mice leads to offspring hyperphagia, adiposity, hypertension, and insulin resistance: a novel murine model of developmental programming

Maternal obesity is increasingly prevalent and may affect the long-term health of the child. We investigated the effects of maternal diet-induced obesity in mice on offspring metabolic and cardiovascular function. Female C57BL/6J mice were fed either a standard chow (3% fat, 7% sugar) or a palatable obesogenic diet (16% fat, 33% sugar) for 6 weeks before mating and throughout pregnancy and lactation. Offspring of control (OC) and obese dams (OO) were weaned onto standard chow and studied at 3 and 6 months of age. OO were hyperphagic from 4 to 6 weeks of age compared with OC and at 3 months locomotor activity was reduced and adiposity increased (abdominal fat pad mass; P<0.01). OO were heavier than OC at 6 months (body weight, P<0.05). OO abdominal obesity was associated with adipocyte hypertrophy and altered mRNA expression of beta-adrenoceptor 2 and 3, 11 beta HSD-1, and PPAR-gamma 2. OO showed resistance artery endothelial dysfunction at 3 months, and were hypertensive, as assessed by radiotelemetry (nighttime systolic blood pressure at 6 months [mm Hg] mean+/-SEM, male OO, 134+/-1 versus OC, 124+/-2, n=8, P<0.05; female OO, 137+/-2 versus OC, 122+/-4, n=8, P<0.01). OO skeletal muscle mass (tibialis anterior) was significantly reduced (P<0.01) OO fasting insulin was raised at 3 months and by 6 months fasting plasma glucose was elevated. Exposure to the influences of maternal obesity in the developing mouse led to adult offspring adiposity and cardiovascular and metabolic dysfunction. Developmentally programmed hyperphagia, physical inactivity, and altered adipocyte metabolism may play a mechanistic role.

Poverty, obesity, and malnutrition: an international perspective recognizing the paradox

In the year 2000, multiple global health agencies and stakeholders convened and established eight tenets that, if followed, would make our world a vastly better place. These tenets are called the Millennium Development Goals. Most of these goals are either directly or indirectly related to nutrition. The United Nations has led an evaluation team to monitor and assess the progress toward achieving these goals until 2015. We are midway between when the goals were set and the year 2015. The first goal is to "eradicate extreme poverty and hunger." Our greatest responsibility as nutrition professionals is to understand the ramifications of poverty, chronic hunger, and food insecurity. Food insecurity is complex, and the paradox is that not only can it lead to undernutrition and recurring hunger, but also to overnutrition, which can lead to overweight and obesity. It is estimated that by the year 2015 noncommunicable diseases associated with overnutrition will surpass undernutrition as the leading causes of death in low-income communities. Therefore, we need to take heed of the double burden of malnutrition caused by poverty, hunger, and food insecurity. Informing current practitioners, educators, and policymakers and passing this information on to future generations of nutrition students is of paramount importance.

Hypothalamic substrates of metabolic imprinting

The mammalian brain develops according to intrinsic genetic programs that are influenced by a variety of environmental factors. Developing neural circuits take shape in two major environments: one in utero and a second during postnatal life. Although an abundance of epidemiological and experimental evidence indicates that nutritional variables during perinatal life have a lasting effect on metabolic phenotype, the underlying mechanisms remain unclear. Peripheral hormones are widely regarded as effective signals that reflect the state of peripheral environments and can directly influence the development of a variety of functional neural systems. Recent findings suggest that the adipocyte-derived hormone leptin may play an important role in directing formation of hypothalamic neural pathways that control body weight. The arcuate nucleus of the hypothalamus (ARH) is a key site for the regulatory actions of leptin in adults, and this same hormone is required for the normal development of ARH projections to other parts of the hypothalamus. In this review, the neurobiological role of leptin is considered within the context of hypothalamic development and the possibility that variations in both prenatal and postnatal nutritional environments may impact development of neural circuits that control energy metabolism through an indirect action on leptin secretion, or signaling, during key developmental critical periods.

Neuropeptide Y in normal eating and in genetic and dietary-induced obesity

Neuropeptide Y (NPY) is one the most potent orexigenic peptides found in the brain. It stimulates food intake with a preferential effect on carbohydrate intake. It decreases latency to eat, increases motivation to eat and delays satiety by augmenting meal size. The effects on feeding are mediated through at least two receptors, the Y1 and Y5 receptors. The NPY system for feeding regulation is mostly located in the hypothalamus. It is formed of the arcuate nucleus (ARC), where the peptide is synthesized, and the paraventricular (PVN), dorsomedial (DMN) and ventromedial (VMN) nuclei and perifornical area where it is active. This activity is modulated by the hindbrain and limbic structures. It is dependent on energy availability, e.g. upregulation with food deprivation or restriction, and return to baseline with refeeding. It is also sensitive to diet composition with variable effects of carbohydrates and fats. Leptin signalling and glucose sensing which are directly linked to diet type are the most important factors involved in its regulation. Absence of leptin signalling in obesity models due to gene mutation either at the receptor level, as in the Zucker rat, the Koletsky rat or the db/db mouse, or at the peptide level, as in ob/ob mouse, is associated with increased mRNA abundance, peptide content and/or release in the ARC or PVN. Other genetic obesity models, such as the Otsuka-Long-Evans-Tokushima Fatty rat, the agouti mouse or the tubby mouse, are characterized by a diminution in NPY expression in the ARC nucleus and by a significant increase in the DMN. Further studies are necessary to determine the exact role of NPY in these latter models. Long-term exposure to high-fat or high-energy palatable diets leads to the development of adiposity and is associated with a decrease in hypothalamic NPY content or expression, consistent with the existence of a counter-regulatory mechanism to diminish energy intake and limit obesity development. On the other hand, an overactive NPY system (increased mRNA expression in the ARC associated with an upregulation of the receptors) is characteristic of rats or rodent strains sensitive to dietary-induced obesity. Finally, NPY appears to play an important role in body weight and feeding regulation, and while it does not constitute the only target for drug treatment of obesity, it may nevertheless provide a useful target in conjunction with others.

Developmental programming and adult obesity: the role of leptin

PURPOSE OF REVIEW

An adverse prenatal environment may induce long-term metabolic consequences, in particular obesity and insulin resistance. Although the mechanisms are unclear, this 'programming' has generally been considered an irreversible change in developmental trajectory. Recent work has highlighted the importance of the hormone leptin during critical windows of development in the pathogenesis of programming related disorders.

RECENT FINDINGS

Maintaining a critical leptin level during development may allow the normal maturation of tissues and pathways involved in metabolic homeostasis and a period of relative hypo or hyperleptinemia may induce some of the metabolic adaptations which underlie developmental programming. Furthermore, nutritional or therapeutic intervention in postnatal life can ameliorate the consequences of developmental malprogramming and, at least in the rodent, developmental programming is potentially reversible by intervention with leptin late in the phase of developmental plasticity.

SUMMARY

Inappropriate growth during pregnancy or lactation can result in individuals with an increased risk of later obesity and related metabolic sequelae. Taken together, recent studies highlight the importance of leptin in disorders manifest as a consequence of developmental programming and offer exciting new strategies for therapeutic intervention, whether it be maternal or neonatal intervention or targeted nutritional manipulation in postnatal life.

Developmental programming of hypothalamic feeding circuits

The hypothalamus plays a critical role in the regulation of food intake and body weight, and recent work has defined a core circuitry in the hypothalamus that appears to mediate many of the effects of the adipocyte-derived hormone leptin on feeding and glucose homeostasis. However, until recently, little was known about the development of these critical pathways. This review summarizes recent advances regarding the post-natal development of 'metabolic' projections from the arcuate nucleus of the hypothalamus. Evidence accumulated primarily in mice indicates that these circuits develop after birth and remain both structurally and functionally immature until the second week of life. Recent studies have begun to identify cues governing development of these pathways, and leptin appears to play a crucial neurotrophic role in the development of the hypothalamic circuits regulating food intake and adiposity. The neurodevelopmental actions of leptin appear specifically to be restricted to a neonatal critical period that coincides with the naturally occurring surge in leptin. In addition, the timing and amplitude of the post-natal leptin surge has important consequences for normal body weight regulation and glucose homeostasis later in life. Ultimately, these data promise to provide new insight into the mechanisms by which alteration of perinatal nutrition may have long-term consequences on body weight regulation and adiposity in the offspring.