The mTOR (mammalian target of rapamycin) signalling pathway functions as a nutrient sensor, both in individual cells and, more globally, in organs such as the fat body in Drosophila and the hypothalamus in the rat. The activity of placental amino acid transporters is decreased in IUGR (intrauterine growth restriction), and recent experimental evidence suggests that these changes contribute directly to the restricted fetal growth. We have shown that mTOR regulates the activity of the placental L-type amino acid transporter system and that placental mTOR activity is decreased in IUGR. The present review summarizes the emerging evidence implicating placental mTOR signalling as a key mechanism linking maternal nutrient and growth factor concentrations to amino acid transport in the human placenta. Since fetal growth is critically dependent on placental nutrient transport, placental mTOR signalling plays an important role in the regulation of fetal growth.
- amino acid
- fetal growth restriction
- mammalian target of rapamycin (mTOR)
- membrane transporter
mTOR (mammalian target of rapamycin) is a serine/threonine protein kinase which has been shown to regulate cell growth (reviewed in ). It is found in two complexes in the cell: mTORC (mTOR complex) 1 and 2. mTORC1 is inhibited by the drug rapamycin, whereas mTORC2 has been suggested to be unaffected by rapamycin . However, it was reported recently that long-term incubation with rapamycin also inhibits mTORC2 , and rictor (rapamycin-insensitive companion of mTOR), one of the mTORC2-associated proteins, may also be regulated by rapamycin . mTORC1 stimulates translation through the phosphorylation of 4E-BP1 (eukaryotic initiation factor 4E-binding protein 1) and the ribosomal protein S6K1 (S6 kinase 1), whereas mTORC2 has been shown to activate Akt . In individual cells, mTOR has been shown to regulate cell growth primarily by affecting protein translation in response to nutrient and growth factor availability . mTOR in the fat body in Drosophila, which is comparable with vertebrate liver and adipose tissue, can sense amino acid deprivation and induce a general reduction in growth, a function that overrides cell-autonomous control . In the rat, hypothalamic mTOR signalling has been shown to regulate appetite . These data suggest that mTOR functions as a nutrient sensor. In the present paper, we review emerging evidence implicating placental mTOR signalling as a key mechanism linking maternal nutrient availability to fetal growth by the regulation of placental nutrient transporters.
mTOR is a central integrator of various extracellular signals, such as growth factors, nutrients, energy and stress . Growth-factor- and hormone-induced mTOR activation is the best characterized and is mediated by the activation of PI3K (phosphoinositide 3-kinase). Nutrient levels, especially amino acids, represent another major upstream regulator of mTOR. The exact mechanism of mTOR activation by nutrient availability remains unclear. It is believed that the class III PI3K hVps34 (human vacuolar protein sorting 34) communicates amino acid sufficiency to mTOR [7,8]. Rag (Ras-related GTPase) proteins were recently shown to mediate the amino acid signal to TORC1 (target of rapamycin 1) [9,10]. The relationship between the Rag proteins and hVps34 remains unclear. Nutrients, such as glucose, might regulate mTOR signalling through energy production in the form of ATP: low ATP levels activate AMPK (AMP-activated protein kinase) and this leads to mTOR inactivation . It has also been proposed that mTOR itself serves as a homoeostatic ATP sensor . Whether glucose levels alone activate mTOR remains unknown. Various environmental stresses also lead to down-regulation of mTOR signalling. For example, mTOR is inhibited by hypoxia .
The placenta and fetal growth
The placenta represents the main interface between the mother and the fetus. The placenta performs three primary functions, which are to (i) provide an immunological barrier between the mother and fetus, (ii) produce and secrete hormones and cytokines, and (iii) mediate the transfer of nutrients, oxygen and waste products. Optimal growth is of critical importance to the fetus, but as many as 15% of all pregnancies are complicated by aberrant fetal growth, either IUGR (intrauterine growth restriction) or fetal overgrowth, resulting in the delivery of an LGA (large-for-gestational-age) baby [14,15]. IUGR and fetal overgrowth represent two important pregnancy complications, because babies subjected to abnormal intrauterine growth are at risk of short- as well as long-term complications. Fetal overgrowth is associated with traumatic birth injuries and development of the metabolic syndrome in childhood, as well as metabolic abnormalities in adult life. IUGR babies have increased perinatal morbidity and are at risk of developing Type 2 diabetes and cardiovascular disease as adults (reviewed in ). To date, there is no specific treatment for abnormal fetal growth.
Placental nutrient transport in abnormal fetal growth
The main determinant of fetal growth is placental nutrient transport. For nutrients to pass from the maternal to the fetal blood, they have to cross the syncytiotrophoblast and the endothelium of the fetal capillaries. This type of endothelium allows for relatively unrestricted passage of nutrients, such as amino acids, through pores within the interendothelial cleft. Therefore transplacental transport is primarily limited by the two polarized plasma membranes of the syncytiotrophoblast: the MVM (microvillous membrane) and the BM (basal plasma membrane). The transport of amino acids across the placental barrier is active, resulting in plasma concentrations of amino acids that are higher in the fetus than in the mother . Amino acids are transported across plasma membranes by transporter proteins (Figure 1), of which three have been shown to be associated with changes in activity when fetal growth is altered. Taurine transporter activity is reduced in MVM isolated from IUGR placentas . The activity of the system A transporter, which transports small neutral amino acids, such as alanine, serine and glutamine, has been reported to be up-regulated in MVM isolated from pregnancies associated with fetal overgrowth  and down-regulated in IUGR . System L is a sodium-independent transporter which transports large neutral amino acids with bulky side chains, such as leucine. This transporter has been reported to have reduced activity in both MVM and BM in association with IUGR , and to have increased activity in MVM from diabetic pregnancies giving rise to LGA babies . These in vitro data are compatible with in vivo data showing a reduction of the transfer of the essential amino acids leucine and phenylalanine in IUGR . The down-regulation of amino acid transporters in IUGR could be the cause of the reduced fetal plasma amino acid concentrations seen in this pregnancy complication . The hypothesis that changes in placental nutrient transport are a cause, rather than a response, to altered fetal growth is supported by experimental evidence. For example, in pregnant rats subjected to protein malnutrition during gestation, it is likely that the down-regulation of placental system A amino acid activity contributes directly to the development of IUGR . These data show that fetal growth is critically dependent on placental nutrient transport, hence factors that regulate placental nutrient transport will influence fetal growth.
mTOR and nutrient transporters
There is existing evidence that amino acid transporters are downstream targets of the mTOR pathway (reviewed in ). For example, treatment of the BJAB cell line with rapamycin decreases the mRNA expression of five amino acid transporters . The down-regulation of 4F2hc in FL5.12 cells upon growth factor withdrawal has been reported to be dependent on mTOR [27,28], perhaps in a rapamycin-insensitive fashion . Recently, it was found that TOR activity stimulates the surface expression of a cationic amino acid transporter in the fat body of Drosophila . The mRNA expression of LAT1, one of the isoforms of the light chain of the L-type amino acid transporter, has been shown to be induced by PDGF (platelet-derived growth factor) in an mTOR-dependent manner in rat vascular smooth muscle cells . Furthermore, leucine-stimulated increase in system A amino acid transporter activity in L6 myotubes is inhibited by rapamycin . The growth of a yeast strain dependent on leucine is sensitive to rapamycin, which inhibits both growth and leucine uptake . Rapamycin has also been shown to inhibit the import of tryptophan in yeast, a decrease caused by vacuolar degradation of the transporters .
The role of placental mTOR in nutrient sensing and regulation of nutrient transporters
A few studies have investigated the role of mTOR in the placenta and it has been shown that mTOR is essential for early growth and proliferation, as disruption of the mTOR gene is lethal [35,36]. Similarly, amino acid signalling through mTOR leads to the development of trophoblast cell motility and initiation of implantation . In the mature placenta, mTOR is expressed at the mRNA level  and we reported recently that the mTOR protein is expressed in the human placenta .
We have also reported that the system L amino acid transporter is stimulated by mTOR in the human placenta , and our preliminary data indicate that mTOR also stimulates the placental system A and taurine transporters . It has been reported by us  and others  that placentas from pregnancies complicated by IUGR have reduced mTOR activity. Therefore we suggest that the down-regulation of placental amino acid transport in IUGR  and the decreased fetal plasma concentrations of leucine and taurine in this pregnancy complication  may be due to a decreased mTOR activity. The factors down-regulating placental mTOR signalling in IUGR remain to be fully established. However, IUGR fetuses may be hypoglycaemic  and have reduced plasma concentrations of insulin and IGF-I (insulin-like growth factor 1) [44,45]. Maternal concentrations of IGF-I and glucose are also reduced in this complication [43,46], suggesting that the IUGR placenta is exposed to decreased levels of glucose and growth factors, signalling parameters known to regulate mTOR. Studies in immortalized cell lines originating from human trophoblasts suggest that glucose- and growth-factor-induced trophoblast cell proliferation is mediated through mTOR activation . Our preliminary data in cultured primary trophoblast cells suggest that glucose and growth factors are upstream regulators of placental mTOR (S. Roos, O. Lagerlöf, M. Wennergren, T. L. Powell and T. Jansson, unpublished work).
In summary, these data are in line with our hypothesis that the mTOR pathway in the placenta functions as a nutrient-sensing pathway. Since the mTOR pathway regulates amino acid transporters, and amino acid transporters are important regulators of fetal growth, this suggests that mTOR de facto determines fetal growth and that placental mTOR constitutes a mechanistic link between maternal nutrient availability and fetal growth (Figure 2).
This study was supported by grants from the Swedish Research Council [grant numbers 10838 and 14555], the Swedish Diabetes Association, the Frimurare–Barnhus Direktionen, the Magnus Bergvall Foundation, the Åhlens Foundation, the Wilhelm and Martina Lundgren Foundation, the Anders Otto Swärd Foundation, the Royal Society of Arts and Sciences in Gothenburg, the Herbert and Karin Jacobsson Foundation and the Royal Swedish Academy of Sciences.
mTOR Signalling, Nutrients and Disease: Biochemical Society Focused Meeting held at Medical Sciences Teaching Centre, University of Oxford, U.K., 15–16 September 2008. Organized and Edited by Richard Boyd (Oxford, U.K.), Deborah Goberdhan (Oxford, U.K.) and Richard Lamb (Cancer Research UK, London, U.K.).
Abbreviations: BM, basal plasma membrane; 4E-BP1, eukaryotic initiation factor 4E-binding protein 1; hVps34, human vacuolar protein sorting 34; IGF-I, insulin-like growth factor 1; IUGR, intrauterine growth restriction; LGA, large-for-gestational-age; mTOR, mammalian target of rapamycin; mTORC, mTOR complex; MVM, microvillous plasma membrane; PI3K, phosphoinositide 3-kinase; Rag, Ras-related GTPase; S6K1, ribosomal protein S6 kinase 1
- © The Authors Journal compilation © 2009 Biochemical Society