Location: Children's Nutrition Research Center
2023 Annual Report
Objectives
Objective 1: Determine the effect of enteral nutrition on FGF19 secretion and the activation of FGF19 receptors and downstream signaling pathways and metabolism in various tissues in neonatal pigs.
Objective 2: Determine whether increased FGF19 availability using parenteral administration of porcine FGF19 and oral FXR agonist treatment controls the rate of growth, tissue protein synthesis and intestinal development in neonatal pigs.
Objective 3: Determine whether being born prematurely blunts the protein and glucose metabolic responses to the feeding-induced rise in amino acids and insulin and identify the mechanisms involved.
Objective 4: Identify the mechanisms by which amino acids, particularly leucine and its metabolites, regulate protein synthesis, degradation, and accretion and how these responses change with development.
Objective 5: removed due to investigator departure
Objective 6: Establish the molecular mechanisms and functional significance of differences in gene expression identified in satellite cell-derived myoblasts isolated from the offspring of dams fed a low-protein versus an adequate protein diet over critical windows of postnatal development.
Objective 7: Determine the impact of maternal dietary protein level during lactation on biomarkers of one-carbon metabolism in their offspring and establish if the observed effects translate into differences for DNA methylation and/or histone post-translational modifications in satellite cell-derived myoblasts isolated from the skeletal muscles of these offspring.
Approach
Despite improvements in their nutritional management, most premature and low birth weight infants have experienced growth faltering by discharge. Many remain small to adulthood and are at an increased risk for developing metabolic diseases such as obesity and type 2 diabetes. The goal of this project is to identify the mechanisms that regulate the diminished growth and altered metabolic responses to nutrition in premature and low birth weight infants and to develop new nutritional strategies to optimize their growth and development. Our approach will be to use neonatal piglet and rodent models to fill these knowledge gaps. We will determine whether being born prematurely blunts the anabolic response to feeding and identify mechanisms by which amino acids, particularly leucine, regulate lean growth. We will determine the role of the enterokine, FGF19, in the anabolic response to enteral feeding in the preterm and whether augmentation of its secretion will enhance growth and metabolic function. We will establish the mechanisms by which undernutrition during critical windows of postnatal development impacts proliferation of skeletal muscle stem cells and the mature muscle nuclear number. Further we will test whether methyl group deficiency induced by inadequate amino acid supply results in permanent epigenetic modifications that impact muscle growth. This project is expected to have a positive impact by providing novel information that will be directly useful in optimizing the nutritional management of premature and low birth weight infants and improving their long-term metabolic health and growth.
Progress Report
As part of Objective 1, we completed studies demonstrating how the stage of pregnancy and mode of birth (vaginal vs. cesarean delivery) influence the secretion of a novel gut hormone called fibroblast growth factor 19 (FGF19). FGF19 is a gut hormone that is important in controlling the amount of bile made in the liver. Hepatic bile production is critical for fat digestion in newborn infants. Fat digestion is not well developed in preterm infants and may explain why they experience poor growth. FGF19 is secreted mainly from epithelial cells that line the intestinal tissue. We used neonatal piglets that were delivered via cesarean section at either 10 days before birth (preterm) or at near the normal term birth date (term). We studied neonatal piglets that were born naturally via vaginal delivery at term. Strikingly, we found that piglets born vaginally at term have a significantly higher blood level of FGF19 at birth compared to those that are born via cesarean delivery at preterm or term gestational age. This finding prompted us to measure the blood levels of several hormones that change markedly during the process of labor during birth. Our results showed that cortisol, a key stress-related glucocorticoid hormone, was most highly correlated with the plasma FGF19 levels at birth among all three groups. To test whether glucocorticoids are directly involved to stimulate intestinal FGF19 secretion, we treated intestinal tissue explants from piglets with dexamethasone (synthetic glucocorticoid). Dexamethasone is a drug used to promote development in premature infants. Our results showed that dexamethasone increased FGF19 production in pig intestinal explants, but we also showed that it boosted FGF19 in human intestinal enteroids, or miniguts, made from human intestine tissue collected by biopsies during hospital visits. Our results suggest that the surge in cortisol that occurs in late pregnancy and during labor triggers an increase in the expression and secretion of FGF19 in neonatal piglets. These results indicate that treating preterm pigs with glucocorticoid hormones prior to birth may correct the deficiency levels of FGF19 and restore normal production of FGF19 in preterm pigs. This is clinically relevant since expectant mothers of preterm infants and newborn preterm infants are often treated with glucocorticoids to promote maturation of lung function in the infant.
As part of Objective 2, we completed animal studies with premature pigs to test the effect of intravenous infusion of FGF19 and oral treatment with a drug to increase intestinal FGF19 production. Our results showed that FGF19 infused into the blood decreases rapidly and has a negative effect on the production of bile acids in the liver. We also showed that a novel drug that mimics the bile acid tropifexor, increased blood levels of FGF19 when fed orally to preterm pigs. We are completing the analysis of these studies.
Some infants are not able to ingest food normally and must be fed by orogastric tube either on an intermittent bolus schedule or continuously. We have previously shown that intermittent bolus feeding promotes greater weight gain and skeletal muscle growth than continuous feeding in a piglet model born at term. As part of Objective 3, we investigated the molecular mechanisms responsible for the enhanced skeletal muscle growth with intermittent bolus feeding in piglets born at term. We identified specific components in both the amino acid and insulin signaling pathways that regulate the synthesis of protein in skeletal muscle and found that their activation was enhanced when nutrition was administered as intermittent bolus feeds rather than continuously. We further investigated the effects of feeding frequency on the growth of organs including the heart, liver, intestine, and kidney. We found that intermittent bolus feeding promotes greater organ growth than continuous feeding, and that it does so by activating the intracellular signaling pathways that stimulate the synthesis of proteins in these organs. The increase in the growth of most organs was proportional to the growth of the whole body. These data, together with our previous findings in neonatal pigs born at term, suggest that the intermittent bolus pattern of feeding may be more advantageous than continuous feeding for providing nutritional support to neonates born at term who are unable to feed normally.
Premature infants frequently grow slowly and have reduced lean mass compared with infants born at term. We have previously shown that supplementation with the branched-chain amino acid leucine can promote lean growth by stimulating the intracellular mechanisms that regulate protein synthesis. As part of Objective 4, we investigated whether supplementation with leucine, when nutrition is delivered continuously by orogastric tube, can promote the synthesis of protein in skeletal muscle of preterm pigs. We showed that supplementation with leucine enhances the activation of the intracellular signaling pathways that regulate translation initiation and stimulates the synthesis of skeletal muscle proteins. However, the dose required in preterm pigs is higher than in pigs born at term. These findings are directly relevant to the clinical care of premature infants.
After completing the development of the infant pig calorimeters, our lab took responsibility for Objective 4C with full-term pigs. The animal experimentation for this study has been completed. We fed newborn pigs for 3 weeks with isocaloric and isonitrogenous milk-replacement diets supplemented with either the amino acid, leucine, or a derivative, beta-hydroxy beta-methyl butyrate. We determined the effects on their energy metabolism (using the calorimeters), body composition, and the growth of their skeletal muscles and fat stores. To understand the molecular mechanism responsible for the observed changes we undertook a series of biochemical, molecular, and histological measurements. The tissue and data analyses will be undertaken next year.
For Objective 6 we have begun to repopulate the mouse colony necessary for performing the described studies. Additionally, we have re-established the cell culture facility, and are updating and optimizing the protocols required to perform the planned studies to take advantages of new instrumentation and technologies that are now available.
Accomplishments
1. Stress hormone promotes gut development in preterm pigs. Bile production is critical for fat digestion in newborn infants. Researchers at the Children's Nutrition Research Center in Houston, Texas, have discovered that a stress hormone regulates the production of a novel gut hormone, Fibroblast Growth Factor 19 (FGF19) that is important in controlling the amount of bile made in the liver. Preterm piglets were used as a model for human infants to show that prematurity reduces the secretion of FGF19. The findings show that vaginal birth and production of the stress hormone, cortisol, during labor strongly stimulate the production of FGF19 in the blood. Treatment of intestinal cells with this stress hormone induce secretion of FGF19. Glucocorticoids, like cortisol, are frequently given to expectant preterm mothers and their newborn infants after birth to accelerate lung maturation. This finding points to cortisol treatment as a potential opportunity to also regulate FGF19, which may be valuable to improve growth, metabolic and hepatic outcomes in preterm babies. These findings are exciting because they could be an important step toward improved neonate care, particularly preterm neonates.
