Glutamine transport in human and rat placenta

Regulation of placental amino acid transport in health and disease

Abnormal fetal growth, i.e., intrauterine growth restriction (IUGR) or fetal growth restriction (FGR) and fetal overgrowth, is associated with increased perinatal morbidity and mortality and is strongly linked to the development of metabolic and cardiovascular disease in childhood and later in life. Emerging evidence suggests that changes in placental amino acid transport may contribute to abnormal fetal growth. This review is focused on amino acid transport in the human placenta, however, relevant animal models will be discussed to add mechanistic insights. At least 25 distinct amino acid transporters with different characteristics and substrate preferences have been identified in the human placenta. Of these, System A, transporting neutral nonessential amino acids, and System L, mediating the transport of essential amino acids, have been studied in some detail. Importantly, decreased placental Systems A and L transporter activity is strongly associated with IUGR and increased placental activity of these two amino acid transporters has been linked to fetal overgrowth in human pregnancy. An array of factors in the maternal circulation, including insulin, IGF-1, and adiponectin, and placental signaling pathways such as mTOR, have been identified as key regulators of placental Systems A and L. Studies using trophoblast-specific gene targeting in mice have provided compelling evidence that changes in placental Systems A and L are mechanistically linked to altered fetal growth. It is possible that targeting specific placental amino acid transporters or their upstream regulators represents a novel intervention to alleviate the short- and long-term consequences of abnormal fetal growth in the future.

Assessment of Placental Sodium-Independent Leucine Uptake and Transfer in Trophoblast Cells

The placenta maintains the balance between nutrition and growth control of the fetus through selective and regulated supply of macronutrients such as carbohydrates, proteins, lipids, and critical micronutrients. Perturbations in the balanced supply of nutrients as found in gestational diseases and altered fetal development have been associated with changes in amino acid transport proteins, such as the System L amino acid heterodimeric exchangers LAT1/SLC7A5 and LAT2/SLC7A8. Syncytiotrophoblasts (STB) form the crucial cell layer at the placental barrier coordinating the transfer of essential amino acids such as leucine from the maternal to the fetal circulation. The System L-mediated leucine transport across the placental barrier is a Na+-independent process against a counter-directed gradient, maintained by a tightly regulated interplay between accumulative transporters, exchangers, and facilitators.The two methods described here allow to standardize and characterize the uptake kinetics of leucine in conventionally cultured BeWo cells and the transfer of leucine across the placental cell barrier using a BeWo monolayer in the Transwell® system.

Maternal immune activation and role of placenta in the prenatal programming of neurodevelopmental disorders

Maternal infection during pregnancy, leading to maternal immune activation (mIA) and cytokine release, increases the offspring risk of developing a variety of neurodevelopmental disorders (NDDs), including schizophrenia. Animal models have provided evidence to support these mechanistic links, with placental inflammatory responses and dysregulation of placental function implicated. This leads to changes in fetal brain cytokine balance and altered epigenetic regulation of key neurodevelopmental pathways. The prenatal timing of such mIA-evoked changes, and the accompanying fetal developmental responses to an altered in utero environment, will determine the scope of the impacts on neurodevelopmental processes. Such dysregulation can impart enduring neuropathological changes, which manifest subsequently in the postnatal period as altered neurodevelopmental behaviours in the offspring. Hence, elucidation of the functional changes that occur at the molecular level in the placenta is vital in improving our understanding of the mechanisms that underlie the pathogenesis of NDDs. This has notable relevance to the recent COVID-19 pandemic, where inflammatory responses in the placenta to SARS-CoV-2 infection during pregnancy and NDDs in early childhood have been reported. This review presents an integrated overview of these collective topics and describes the possible contribution of prenatal programming through placental effects as an underlying mechanism that links to NDD risk, underpinned by altered epigenetic regulation of neurodevelopmental pathways.

Expression and role of SNAT3 in the placenta

Glutamine is the most versatile amino acid and its plasma concentration is the highest of all amino acid. Many transporters are therefore involved in glutamine uptake or efflux. Glutamine is actively released from the placenta into fetal circulation. In this study, we examined the alteration of transporters that transport glutamine into fetal circulation as gestation progresses. High expression levels of system A and y(+)L were found in the rat placenta in the late period of pregnancy and the expression levels of these transporters increased as gestation progressed (p<0.05). On the other hand, the expression of SNAT3, the system N transporter, was detected in the early period of pregnancy and its expression level decreased as gestation progressed (p<0.05). SNAT3 was also found to be expressed in isolated human primary cytotrophoblast cells and its expression level was decreased by their differentiation into syncytiotrophoblast cells (p<0.05). Since this regulation is closely related to glutamine synthetase expression, SNAT3 may play a key role in providing glutamine corresponding to glutamine synthetase function in the early period of gestation. This is the first report on the expression of SNAT3 in the placenta in the early stage of pregnancy.

Transport of amino acids through the placenta and their role

Amino acids are transported across the human placenta mediated by transporter proteins that differ in structure, mechanism and substrate specificity. Some of them are Na+-dependent systems, whereas others are Na+-independent. Among these there are transporters composed of a heavy chain, a glycoprotein, and a light chain. Moreover, they can be differently distributed in the two membranes forming the syncytiotrophoblast. The transport mechanisms involved and their regulation are only partially known. In the placenta itself, part of the amino acids is metabolized to form other compounds important for the fetus. This occurs for instance for arginine, which gives rise to polyamines and to NO. Interconversion occurs among few other amino acids Transport is altered in pregnancy complications, such as restricted fetal growth.

SNAT expression in rat placenta

Amino acid transport System A (SysA) activity is present within the rodent and human placentas. Inhibition of this transport system is associated with fetal growth retardation. Several cDNAs encoding SysA transport proteins have been discovered, and their presence documented within the human placenta. We have demonstrated the presence of mRNA encoding three of these transporters, SNAT1, 2, and 4 within the rat placenta over the final third of gestation. Abundance of these mRNA species increases from day 14 to day 20 of gestation. Immunohistochemistry demonstrates the presence of SNAT1 and 2 within the placental labyrinth at both days 14 and 20. Transport proteins are also present within marginal giant cells and, for SNAT1, within fetal endothelium. In conclusion, several proteins capable of SysA transport activity are present within the rodent placenta. mRNA expression increases over the final third of gestation, coincident with the period of greatest need for fetal amino acid delivery.

Fetoplacental transport and utilization of amino acids in IUGR — a review

Amino acids have multiple functions in fetoplacental development. The supply of amino acids to the fetus involves active transport across and metabolism within the trophoblast. Transport occurs through various amino acid transport systems located on both the maternal and fetal facing membranes, many of which have now been documented to be present in rat, sheep and human placentas. The capacity of the placenta to supply amino acids to the fetus develops during pregnancy through alterations in such factors as surface area and specific time-dependent transport system expression. In intrauterine growth restriction (IUGR), placental surface area and amino acid uptakes are decreased in human and experimental animal models. In an ovine model of IUGR, produced by hyperthermia-induced placental insufficiency (PI-IUGR), umbilical oxygen and essential amino acid uptake rates are significantly reduced in the most severe cases in concert with decreased fetal growth. These changes indicate that severe IUGR is likely associated with a shift in amino acid transport capacity and metabolic pathways within the fetoplacental unit. After transport across the trophoblast in normal conditions, amino acids are actively incorporated into tissue proteins or oxidized. In the sheep IUGR fetus, however, which is hypoxic, hypoglycemic and hypoinsulinemic, there appear to be net effluxes of amino acids from the liver and skeletal muscle, suggesting changes in amino acid metabolism. Potential changes may be occurring in the insulin/IGF-I signaling pathway that includes decreased production and/or activation of specific signaling proteins leading to a reduced protein synthesis in fetal tissues. Such observations in the placental insufficiency model of IUGR indicate that the combination of decreased fetoplacental amino acid uptake and disrupted insulin/IGF signaling in liver and muscle account for decreased fetal growth in IUGR.

Isoforms of amino acid transporters in placental syncytiotrophoblast: plasma membrane localization and potential role in maternal/fetal transport

Many cell proteins exist as isoforms arising either from gene duplication or alternate RNA splicing. There is growing evidence that isoforms with different, but closely related, functional characteristics are often directed to discrete cellular locations. Thus, specialized functions may be carried out by proteins of similar evolutionary origin in different membrane compartments. In polarized epithelial cells, this mechanism allows the cell to control amino acid transport independently at each of its specialized apical and basolateral plasma membrane domains. Investigations of isoform localization in these membranes have generally been performed in epithelia other than the placental trophoblast.This review of placental amino acid transporter isoforms first provides an overview of their properties and preliminary plasma membrane localization. We then discuss studies suggesting various roles of isoform localization in trophoblast function. To provide insights into the molecular basis of this localization in trophoblast, we present a review of current knowledge of plasma membrane protein localization as derived from investigations with a widely used epithelial model cell line. Finally, we discuss a potential approach using cultured trophoblast-derived cells for studies of transporter isoform localization and function. We hope that this review will stimulate investigation of the properties of trophoblast transporter isoforms, their membrane localization and their contribution to the cellular mechanism of maternal-fetal nutrient transport.

Effects of maturation on RNA transcription and protein expression of four MRP genes in human placenta and in BeWo cells

The placenta is a multifunctional organ that protects the fetus from toxic compounds and the MRPs contribute to this function. The expression of MRP1, MRP2, MRP3, and MRP5 was compared in human placental tissue and in BeWo cells by real-time RT-PCR analysis; protein expression was assessed by Western blot. MRP1 and MRP3 were the most abundantly expressed genes in placenta but only MRP1 was highly expressed in the BeWo cells. Expression of MRP1 increased 4-fold in the third as compared with first trimester placental samples, and increased 20-fold with polarization of BeWo cells. MRP2, MRP3, and MRP5 were weakly expressed both in placenta and BeWo cells. Protein expression followed mRNA quantification for MRP1 and MRP5 but not for MRP2 and MRP3. These data indicated that MRP1 and MRP5 increase with trophoblast maturation, suggesting a particular role for these proteins in the organ functional development.

Recent molecular advances in mammalian glutamine transport

Much has been learned about plasma membrane glutamine transporter activities in health and disease over the past 30 years, including their potential regulatory role in metabolism. Since the 1960s, discrimination among individual glutamine transporters was based on functional characteristics such as substrate specificity, ion dependence, and kinetic and regulatory properties. Within the past two years, several genes encoding for proteins with these defined activities (termed "systems") have been isolated from human and rodent cDNA libraries and found to be distributed among four distinct gene families. The Na(+)-dependent glutamine transporter genes isolated thus far are System N (SN1), System A (ATA1, ATA2), System ASC/B(0) (ASCT2 or ATB(0)), System B(0,+) (ATB(0,+)) and System y(+)L (y(+)LAT1, y(+)LAT2). Na(+)-independent glutamine transporter genes encoding for System L (LAT1, LAT2) and System b(0,+) (b(0,+)AT) have also been recently isolated, and similar to y(+)L, have been shown to function as disulfide-linked heterodimers with the 4F2 heavy chain (CD98) or rBAT (related to b(0,+) amino acid transporter). In this review, the molecular features, catalytic mechanisms and tissue distributions of each are addressed. Although most of these transporters mediate the transmembrane movement of several other amino acids, their potential roles in regulating interorgan glutamine flux are discussed. Most importantly, these newly isolated transporter genes provide the long awaited tools necessary to study their molecular regulation during the catabolic states in which glutamine is considered to be "conditionally essential."

Amino acid transporters in the human placenta

Amino acid transport across the human placenta is active, mediated by specific transporters in syncytiotrophoblast plasma membranes. Using functional criteria such as substrate specificity and sodium dependence, approximately 15 transport systems have been identified in the human placenta. Recently, the area of molecular biology of amino acid transporters has evolved rapidly and at least 25 cDNA clones coding for mammalian amino acid transporters or transporter subunits have been identified. The primary objective of this review is to integrate the available functional data on placental amino acid transport systems with recent molecular information on mammalian amino acid transporters. Furthermore, models for the mechanisms for net materno-fetal transfer of amino acids are discussed. Finally, the evidence to suggest that alterations in placental amino acid transport systems may play a crucial role in the regulation of fetal growth are presented briefly.

Placental transport and metabolism of amino acids

This review examines the placental transport and metabolism of amino acids, with a special emphasis on unifying and interpreting in-vivo and in-vitro data. For a variety of technical reasons, in-vivo studies, which quantify placental amino-acid fluxes and metabolism, have been relatively limited, in comparison to in-vitro studies using various placental preparations. Following an introduction to placental amino-acid uptake and transfer to the fetus, the review attempts to reconcile in-vitro placental transport data with in-vivo placental data. Data are discussed with reference to the measured delivery rates of amino acids into the fetal circulation and the contribution of placental metabolism to this rate for many amino acids. The importance of exchange transporters in determining efflux from the placenta into the fetal circulation is presented with special reference to in-vivo studies of non-metabolizable and essential amino acids. The data which illustrate the interconversion and nitrogen exchange of three groups of amino acids, glutamine-glutamate, BCAAs and serine-glycine, within the placenta are discussed in terms of the potential role such pathways may serve for other placenta functions. The review also presents comparisons of the sheep and human placentae in terms of their in-vivo amino-acid transport rates.

Amino acid transport regulation and early embryo development

Amino acids are essential components of media utilized to culture fertilized human eggs to the blastocyst stage in vitro. Use of such media has led to a significant increase in the proportion of embryos that implant upon transfer to the uterus and to a decrease in the number that need to be transferred to achieve pregnancy. Little is known about the mechanisms by which amino acids foster development of healthy human blastocysts. Indications are, however, that many of these mechanisms are the same in human and mouse embryos. Both essential and nonessential amino acid transport benefit preimplantation mouse embryo development, albeit at different stages. Nonessential amino acid transport improves development primarily during cleavage, whereas essential amino acid transport supports development of more viable embryos, especially subsequent to the eight-cell stage. This review discusses likely mechanisms for these beneficial effects.

Anionic amino acid transporter expression in late gestation rodent yolk sac

The yolk sac plays an important role in fetal nutrition. Transport of amino acids by the rodent visceral yolk sac has been shown previously. We have demonstrated the presence of several amino acid transport proteins capable of the Na(+)-dependent transport of anionic amino acids within late gestation mouse visceral yolk sac and uterine epithelium. We speculate that these proteins may be involved in the efflux of glutamate from the fetal to the maternal circulations.