Physiological roles of System A amino acid transporter in fetal growth and development

Three babies, in every 100, are born small. Small babies have less reserve and ability to withstand the stress of labour than normal-sized babies. They are at a higher risk of death and illness in the first few days of life and also of developing diseases in later life including diabetes, high blood pressure and heart disease. Fetal growth restriction is therefore a major health problem. Why babies fail to reach their growth potential in the womb and indeed how it happens is unclear for most cases. We are studying how fetal restriction occurs. We think that the answer lies in genes that control the growth of certain organs in the baby. The placenta is a particularly important organ for how well the baby grows given that it provides the maternal nutrients and oxygen that the baby needs. We believe that proteins that sit at the membrane of placental cells and specialize in transport of nutrients across the placenta are important in supplying nutrients for growth. We predict that a decrease in number or activity of these proteins will mean that the baby will be less nourished and not able to grow properly. We plan to test our hypothesis by using mouse models where pups are born small due to the removal of amino-acid System A transporters. We plan to accurately measure how well the nutrients are transferred across the placenta in these animals, how well organs such as liver, brain and placenta cope with this and what diseases these mice develop soon after birth and in later life. These experiments will provide important information about how fetal restriction comes about and of the role of amino-acid transporters in development and disease. Because transporter proteins are often used as drug targets or delivery systems our work could also have diagnostic and therapeutical applications.

Amino acid transporters deliver amino acids to intracellular amino acid sensors such as those of the mTOR and GCN2 signalling pathways. The Sodium-coupled neutral amino acid transporters (SNAT1-SNAT5) of the Slc38 family are widely expressed and account for System A (SNAT1, 2 and 4)- and System N (SNAT3 and 5)-type amino acid transport in mammalian cells. System A utilizes the Na+ electrochemical gradient to concentrate amino acids into the cytosol. Amino acids accumulated in this manner may be used for protein syntesis, metabolism, or to provide a driving force for the transmembrane transport of other amino acids by amino acid exchangers. System A is the major amino acid transport system subject to regulation by environmental conditions, proliferative stimuli, developmental changes, hormones and growth factors. Although System A has been investigated for nearly four decades, and is one of the major transporter systems, the physiological roles that they may play remain largely unknown. The intracellular signaling pathways also remain uncertain. We have generated the first conditional mouse knock-out models for System A genes: a double knock-out of the family isoforms SNAT1 and SNAT2, and a knock-out of the imprinted SNAT4 isoform. Both sets of knock-outs result in significant fetal and placental growth restriction, with SNAT1+SNAT2 embryos showing severe anaemia and a neural tube defect phenotype of variable penetrance. The overall aim of this project is to elucidate the mechanisms leading to fetal growth restriction and developmental phenotypes. We hypothesize that the loss-of-function of System A activity in the placenta is the major contributing factor. We will use an integrated cellular, molecular and whole animal approach to determine: 1- the contribution of placental SNAT1+SNAT2 to fetal growth and development; 2- the contribution of placental SNAT4 to fetal growth and maternal metabolism in late gestation.

Fetal growth is an area of interest to a diverse group of beneficiaries including the general public, scientists, health care professionals and policy makers. Intra-uterine growth restriction (IUGR) is a complication that affects 3% of all pregnancies and results from the fetus failing to achieve its genetically determined growth potential. Low birth weight is a major cause of neonatal morbidity and mortality and is associated with later life risk of type 2 diabetes and cardiovascular disease. Little is known about how, or indeed why, so many human babies fail to reach their growth potential in the womb. Historically, IUGR has been attributed to reduction in placental blood flow. However, a growing body of recent evidence suggests that alterations in expression and activity of placental nutrient transporters contributes to IUGR. However, it remains unclear if these alterations are a cause, rather than a response, to altered fetal growth. Identifying the key mechanisms by which fetal growth trajectories are normally maintained and regulated, and of the pathological processes leading to restricted growth, would be of considerable benefit to improving health. This project will address the relative contributions of placental and fetal System A amino acid transporter for growth, using mouse models, with expected significant new insights for human IUGR with diagnostic and potentially therapeutic value. Pregnant women are therefore likely important beneficiaries. The findings of the current study will be disseminated through multiple channels and to multiple target audiences to ensure maximum cost benefit. The general public will be informed via press and broadcast media following joint press releases by the University of Cambridge press office in consultation with the BBSRC. The applicants have considerable experience in public dissemination of their findings. Dr Constancia, Prof. Sibley and Dr. Glazier have frequent contact with the media including TV, radio and magazines (including those aimed at pregnant women) and have given public lectures. Dr Constancia's group, and the Manchester groups, are involved in activities run by their respective Universities during National Science week and every year they host a couple of undergraduate students in their laboratories. Peer reviewed publications in the scientific literature as well as international conference presentations will be the main route by which findings are disseminated to the scientific community. Dr Constancia is a member of the Epigenome Network of Excellence that will provide a further route for dissemination to individuals with an interest in imprinting, epigenetics and fetal growth. Dr. Constancia, Prof. Sibley, and Dr. Glazier are regularly invited to present lectures at international conferences, including those aimed at research scientist as well as clinicians and health care professionals. They also give lectures to both undergraduate and graduate students. Prof. Sibley and Dr. Glazier have participated in number of public engagement activities, either as individuals or as part of broader Maternal and Fetal Health Research Centre (MFHRC) fora. MFHRC has developed a User Group to improve the development of research within the maternity services at St Mary's hospital. This User Group is formed from local representatives of service users including women from the 'baby café', National Childbirth Trust and general and teenage parent craft groups.