Location: Children's Nutrition Research Center2013 Annual Report
1a. Objectives (from AD-416):
1) determine role of circadian clock in regulation of food intake and interaction between diet composition and circadian rhythms of food intake on body weight control during post-weaning and adult life; determine specific role of central and peripheral clocks, as well as circadian output pathways in maintaining homeostasis of food intake; (2-3 removed; SYs left); 4) investigate impact of prematurity on GI and metabolic response to perinatal nutrition; 5) compare impact of continuous vs. intermittent bolus delivery of nutrients provided enterally or parenterally on protein synthesis and accretion in neonatal models and identify intracellular signaling mechanism involved; 8) investigate changes of SIRT3 gene expression in the liver, and study effects of SIRT3 expression on hepatic metabolism, oxidative stress, and fat deposition; 9) determine role of protein kinase C interacting cousin of thioredoxin in insulin-mediated growth, macronutrient metabolism, and insulin resistance in the liver; 10) define action of glucagon-like peptide-2 (GLP-2) receptor on food intake and inter-organ macronutrient flux; 11) study ghrelin peptide expression profile under different diet regimes; 12) conduct mechanistic analyses of differences in metabolic profile between WT and null mice; 13) confirm predicted lipotropic effects of lecithin, choline and betaine in high-fat-fed mouse models of the metabolic syndrome; 14) test impact of liver specific LRH-1 knockout on the lipotropic effects of lecithin, choline and betaine in high-fat-fed mouse models of the metabolic syndrome; 15)identify genes that show epigenetic dysregulation in obesity; 16) determine if methylation and expression of specific genes in hypothalamus and/or adipose tissue differ between lean and obese animals and determine if maternal obesity and/or nutrition before and during pregnancy persistently alters epigenetic regulation in offspring; 17) determine if maternal obesity and/or nutrition before and during pregnancy persistently alters epigenetic regulation in offspring hypothalamus or adipose tissue; 18) identify placental epigenetic mechanisms that affect fetal nutrition, growth and development; 19) determine how programming of glucose intolerance, obesity, and the epigenetic dysregulation of skeletal muscle-growth in mice is affected by maternal diet during development; 20) determine if epigenetic programming and reprogramming contribute to lineage-specific patterns of gene expression; 21) develop targested knock-in mouse model to determine if nutrients can modulate hypermethylation, epigenetic silencing and increase susceptibility to disease; 22) evaluate leukocyte patterns, gene expression profiles, and inflammatory mediators in adipose tissue under influence of diatary manipulation that leads to obesity.
1b. Approach (from AD-416):
The research will be accomplished using a variety of animal models and scientific tools to simulate the human newborn and/or child. Animal models will be used to understand the central and peripheral circadian clock mechanisms that influence eating behavior, metabolism, and energy balance. Newborm animal models will be used to examine the effect of chronic parenteral nutrition during the neonatal period on glucose tolerance, insulin sensitivity, and body composition during late infancy and adolescence. Researchers will investigate the effects of intermittent bolus feeding versus continuous feeding, delivered either enterally or parenterally, on protein synthesis in neonatal animal models. This will allow our team to determine the long-term impact of these feeding modalities on growth and body composition. Various models will be placed on obesigenic diets at 5-6 weeks of age and evaluated at 7 days, 5 weeks, and 6 months to define a blood leukocyte expression profile at these time points. Children's Nutrition Research Center scientists will also characterize the functions of intracellular systems in the liver and their influences on the onset of fatty liver disease and glucose homeostasis. Additional investigation will occur on the intracellular signaling pathways of GLP-2 and their metabolic effects on food intake, energy expenditure, and glucose homeostasis. Various mouse models, and a human model of epigenetic dysregulation compromising placental development, will be used to test if maternal obesity and fetal nutrition during development affects the establishment of gene-specific DNA methylation patterns in the developing fetus, which would cause permanent changes in gene expression, metabolism, food intake regulation, and body weight. Additionally we will investigate the mechanisms regulating DNA methylation during development, and characterize their involvement in nutritional programming during critical ontogenic periods. We will characterize the role of ghrelin and its receptor in nutritional regulation of energy and glucose homeostasis.
3. Progress Report:
Significant research progress was accomplished during the year. To review the progress, please refer to project 6250-51000-055-10S (Project 1), 6250-51000-055-20S (Project 2), 6250-51000-055-30S (Project 3), 6250-51000-055-40S (Project 4), and 6250-51000-055-50S (Project 5)
1. Leucine supplementation enhances protein synthesis in neonates. The growth rate of low birth weight infants is frequently suboptimal, thus better strategies are needed to optimize the nutritional management of these infants whose growth has been compromised. On the basis of studies in cells and rodents demonstrating positive effects of the branched-chain amino acid leucine, scientists at the Children's Nutrition Research Center in Houston, Texas, examined whether leucine may stimulate the synthesis of muscle proteins in the neonate. Because an increase in muscle mass requires that more proteins be synthesized than degraded, a nutritional supplement that promotes protein synthesis can lead to an increase in muscle mass. We found that leucine rapidly stimulates protein synthesis in the muscle of neonatal piglets and that this stimulation can be sustained for at least 24 hours, provided that the supply of other amino acids is not limiting. The response to leucine occurs whether the leucine is provided parenterally by infusion or enterally in the formula. These results have implications for the pediatric nutrition community as they suggest that leucine supplementation has the potential to enhance lean growth in early life.
2. GLP-2R action on POMC neurons in food intake and glycemic control. Uncontrolled blood glucose is a hallmark of diabetic patients. However, little is known about whether Glucagon-like peptide-2 (GLP-2) can act like insulin to suppress blood glucose. Using genetic mouse models and pharmacological approaches, scientists at the Children's Nutrition Research Center in Houston, Texas, found that GLP-2 is a key satiety signal for the physiological control of food intake and gastric emptying, and this contributes to the control of energy balance, body weight, and blood glucose. We also learned that GLP-2 directly modulates activities of POMC neurons, enhances the PI3K signaling pathway, and suppresses glucose production in the liver. These findings provided insight into the gut-brain-liver axis in the control of blood glucose and food intake.
3. SIRT3 gene increases liver synthesis of glucose. Liver produces glucose and releases it into bloodstream. This process contributes to the high glucose levels in diabetics. Researchers at the Children's Nutrition Research Center in Houston, Texas, have generated genetically engineered mice with elevated expression of the SIRT3 gene in all of their tissues and found that these animals had increased glucose production. However, other factors that play important roles in liver glucose production were not changed in these mice. Evidence suggests that the SIRT3 gene may directly modify liver glucose synthetic enzymes to regulate their activity. These findings shed light on novel mechanisms of liver glucose production and possibly new ways to treat diabetes.
4. Diverse functions of glutaredoxins. A group of proteins, called Glutaredoxins (Grxs), are important for protecting the life of cells from damage caused by free radicals. However, how these proteins act in the cell and whether these proteins are directly involved in the process of damage repair are still largely unknown. Children's Nutrition Research Center researchers in Houston, Texas, have identified a new member of Grxs that comprises two distinct functional regions, in which one region is similar to other members of Grxs, but another one is unique and its function is unknown. They further determined that this unique region had enzymatic activities and could be crucial for repairs of damaged genetic material in the cell. Their findings may provide insights into strategies to prevent free radical induced damages in human disorders, including aging, cancer, and chronic diseases.
5. Generation of GHS-R knockout mice to unravel Ghrelin's unknowns. Ghrelin is a hormone within your body that is responsible for stimulating appetite and satiety. But, despite an enormous body of literature documenting ghrelin's multifaceted functions, the exact molecular mechanisms responsible for its function remain largely elusive. A major obstacle hindering progress of the field is insufficient knowledge of the sites of ghrelin's action, due to the absence of tissue-specific knockout models for ghrelin signaling. Scientists at the Children's Nutrition Research Center in Houston, Texas, have successfully generated the first set of tissue-specific knockout mice for the ghrelin receptor (GHS-R). These novel mouse models will serve as powerful tools for unraveling the tissue-specific roles of GHS-R, thus filling critical knowledge gaps in understanding of biological functions of GHS-R, and providing a foundation for future theraputic developments targeting the ghrelin receptor.
6. NLRP7 and genomic imprinting. NLRP7, a protein that is expressed in germ cells, which are key for reproduction, and the early embryo causes abnormal imprinting in the placenta. Using various types of cells, scientists at the Children's Nutrition Research Center in Houston, Texas, found that NLRP7 affects DNA methylation at multiple genes. Using a model system, we also found that NLRP7 plays a role in the earliest stages of development of the placenta and affects methylation at several genes. These findings are important and provide a new understanding about imprinting regulation in the placenta, which is secondarily relevant to understanding how placental genes regulate nutrient transfer to the fetus.
7. GLP-2 treatment does not prevent NEC in preterm neonates. Premature infants are at an increased risk for an intestinal disease called necrotizing enterocolitis (NEC) that has devastating effects on the health and long-term development of the infant. Based on previous findings, researchers at the Children's Nutrition Research Center in Houston, Texas, tested whether infusing GLP-2 intravenously immediately after birth could augment intestinal growth and prevent NEC in premature piglets fed infant formula. We found that GLP-2 treatment delayed the onset of NEC but did not prevent the ultimate incidence of the disease, such that approximately 70% of pigs developed NEC. We found that GLP-2 treatment increased intestinal growth as expected, but did not reduce the inflammation that occurs in association with NEC. Our findings suggest that GLP-2 treatment was not effective in the prevention of NEC.
8. Frequent disruption of circadian homeostasis increases the risk of leptin resistance. Research is lacking on whether circadian disruption is an independent risk factor for the development of leptin resistance, which is common in overweight and obese people where the brain consistently receives hunger messages that lead to overeating and weight gain. Scientists at the Children's Nutrition Research Center in Houston, Texas, have demonstrated that disruption of circadian homeostasis induces leptin resistance, a hallmark of obesity for humans, in the absence of gene mutations or changes in food-intake, diet choice, and daily activity. We have also characterized the molecular pathways for the circadian clock to control the homeostasis of leptin signaling. This discovery is novel and helps to explain the obesity prevalence in the modern societies, and will have significant impact on both basic research and clinic practice for human obesity prevention and treatment in the future.
Burrin, D.G., Davis, T.A. 2013. Mechanism of nutrient sensing. In: Ross, A.C., Caballero, B., Cousins, R.J., Tucker, K.L., Ziegler, T.R., editors. Modern Nutrition in Health and Disease, 11th edition, Part II. Nutritional roles in integrated biologic systems, Section B. Digestive, endocrine, immune, and neural mechanisms, Chapter 47. Philadelphia, PA:Wolters Kluwar-Lippincott, Williams & Wilkins Publishers. p. 626-632.