Placental transport of leucine and lysine is reduced in intrauterine growth restriction

Isolation of plasma membrane vesicles from mouse placenta at term and measurement of system A and system beta amino acid transporter activity

Placental amino acid transport is essential for optimal fetal growth and development, with a reduced fetal provision of amino acids being implicated as a potential cause of fetal growth restriction (FGR). Understanding placental insufficiency related FGR has been aided by the development of mouse models that have features of the human disease. However, to take maximal advantage of these, methods are required to study placental function in the mouse. Here, we report a method to isolate plasma membrane vesicles from mouse placenta near-term and have used these to investigate two amino acid transporters, systems A and β, the activities of which are reduced in human placental microvillous plasma membrane (MVM) vesicles from FGR pregnancies. Plasma membrane vesicles were isolated at embryonic day 18 by a protocol involving homogenisation, MgCl₂ precipitation and centrifugation. Vesicles were enriched 11.3 ± 0.5-fold in alkaline phosphatase activity as compared to initial homogenate, with minimal intracellular organelle contamination as judged by marker analyses. Cytochemistry revealed alkaline phosphatase was localised between trophoblast layers I and II, with intense reaction product deposited on the maternal-facing plasma membrane of layer II, suggesting that vesicles were derived from this trophoblast membrane. System A and system β activity in mouse placental vesicles, measured as Na⁺-dependent uptake of ¹⁴C-methylaminoisobutyric acid (MeAIB) and ³H-taurine respectively confirmed localisation of these transporters to the maternal-facing plasma membrane of layer II. Comparison to human placental MVM showed that system A activity was comparable at initial rate between species whilst system β activity was significantly lower in mouse. This mirrored the lower expression of TAUT observed in mouse placental vesicles. We conclude that syncytiotrophoblast layer II-derived plasma membrane vesicles can be isolated and used to examine transporter function.

Prolonged maternal amino acid infusion in late-gestation pregnant sheep increases fetal amino acid oxidation

Protein supplementation during human pregnancy does not improve fetal growth and may increase small-for-gestational-age birth rates and mortality. To define possible mechanisms, sheep with twin pregnancies were infused with amino acids (AA group, n = 7) or saline (C group, n = 4) for 4 days during late gestation. In the AA group, fetal plasma leucine, isoleucine, valine, and lysine concentrations were increased (P < 0.05), and threonine was decreased (P < 0.05). In the AA group, fetal arterial pH (7.365 +/- 0.007 day 0 vs. 7.336 +/- 0.012 day 4, P < 0.005), hemoglobin-oxygen saturation (46.2 +/- 2.6 vs. 37.8 +/- 3.6%, P < 0.005), and total oxygen content (3.17 +/- 0.17 vs. 2.49 +/- 0.20 mmol/l, P < 0.0001) were decreased on day 4 compared with day 0. Fetal leucine disposal did not change (9.22 +/- 0.73 vs. 8.09 +/- 0.63 micromol x min(-1) x kg(-1), AA vs. C), but the rate of leucine oxidation increased 43% in the AA group (2.63 +/- 0.16 vs. 1.84 +/- 0.24 micromol x min(-1) x kg(-1), P < 0.05). Fetal oxygen utilization tended to be increased in the AA group (327 +/- 23 vs. 250 +/- 29 micromol x min(-1) x kg(-1), P = 0.06). Rates of leucine incorporation into fetal protein (5.19 +/- 0.97 vs. 5.47 +/- 0.89 micromol x min(-1) x kg(-1), AA vs. C), release from protein breakdown (4.20 +/- 0.95 vs. 4.62 +/- 0.74 micromol x min(-1) x kg(-1)), and protein accretion (1.00 +/- 0.30 vs. 0.85 +/- 0.25 micromol x min(-1) x kg(-1)) did not change. Consistent with these data, there was no change in the fetal skeletal muscle ubiquitin ligases MaFBx1 or MuRF1 or in the protein synthesis regulators 4E-BP1, eEF2, eIF2alpha, and p70(S6K). Decreased concentrations of certain essential amino acids, increased amino acid oxidation, fetal acidosis, and fetal hypoxia are possible mechanisms to explain fetal toxicity during maternal amino acid supplementation.

Understanding placental nutrient transfer--why bother? New biomarkers of fetal growth

The placenta, in general and the physiology of maternofetal nutrient transfer is under-researched compared to other organs with epithelial transport function, as evidenced, for example, by publication numbers. This report provides reasons why more researchers should become involved in this topic. First, the syncytiotrophoblast, the transporting epithelium of the placenta, though having many basic cell physiology properties similar to those of other transporting epithelia, has several properties which are markedly different. Better information on these might help fundamental understanding of how epithelia in general function as well as improving knowledge of how the syncytiotrophoblast operates. Second, the synctiotrophoblast has a key role in controlling fetal growth, not only by transporting nutrients and waste products of metabolism but also because it increasingly appears to be one site, perhaps even the dominant site, in which integration of, sometimes conflicting, signals between mother and fetus takes place. Finally, better understanding of placental nutrient transfer and especially of how it is regulated by maternal and fetal signals could provide better information on the placental phenotype in fetal growth disorders--information which might contribute to providing better biomarkers which the obstetrician could use to improve early diagnosis of these disorders.

Regulation of amino acid transporters by glucose and growth factors in cultured primary human trophoblast cells is mediated by mTOR signaling

Inhibition of mammalian target of rapamycin (mTOR) signaling in cultured human primary trophoblast cells reduces the activity of key placental amino acid transporters. However, the upstream regulators of placental mTOR are unknown. We hypothesized that glucose, insulin, and IGF-I regulate placental amino acid transporters by inducing changes in mTOR signaling. Primary human trophoblast cells were cultured for 24 h with media containing various glucose concentrations, insulin, or IGF-I, with or without the mTOR inhibitor rapamycin, and, subsequently, the activity of system A, system L, and taurine (TAUT) transporters was measured. Glucose deprivation (0.5 mM glucose) did not significantly affect Thr172-AMP-activated protein kinase phosphorylation or REDD1 expression but decreased S6 kinase 1 phosphorylation at Thr389. The activity of system L decreased in a dose-dependent manner in response to decreasing glucose concentrations. This effect was abolished in the presence of rapamycin. Glucose deprivation had two opposing effects on system A activity: 1) an "adaptive" upregulation mediated by an mTOR-independent mechanism and 2) downregulation by an mTOR-dependent mechanism. TAUT activity was increased after incubating cells with glucose-deprived media, and this effect was largely independent of mTOR signaling. Insulin and IGF-I increased system A activity and insulin stimulated system L activity, effects that were abolished by rapamycin. We conclude that the mTOR pathway represents an important intracellular regulatory link between nutrient and growth factor concentrations and amino acid transport in the human placenta.

Expression of enzymes regulating placental ammonia homeostasis in human fetal growth restricted pregnancies

Functional placental insufficiency results in impaired feto-placental exchange, and subsequently in fetal growth restriction (FGR). We hypothesized that reductions in placental amino acid transporter activities in FGR pregnancies may be accompanied by abnormal expression of placental ammonia-handling enzymes. Term placentas were obtained from growth restricted (N=11) and normal (N=17) human pregnancies, and examined for glutamate dehydrogenase (GDH), glutamine synthetase (GS) and glutaminase (GA) mRNA and protein expression. Northern and Western blots were normalized on human actin mRNA and protein expression. For GA, the presence of mRNA coding the kidney isoform, and the absence of mRNA coding the liver isoform of the enzyme were demonstrated in the human placenta. In FGR pregnancies, placental expression of GDH mRNA was reduced (P<0.05) compared to normal pregnancies (1.576+/-0.144 vs. 2.092+/-0.177, respectively; mean+/-SE), whereas GS and GA mRNA expression was not different between the two types of pregnancy. GDH protein expression were also reduced (P<0.05) in FGR placentas compared to normal placentas (1.055+/-0.079 vs. 1.322+/-0.053, respectively; mean+/-SE). The GS and GA protein expression was not different in FGR pregnancies. Our data indicate that in cases of FGR, glutamate-to-oxoglutarate transformation in the placenta is limited, yet glutamine synthesis from and decomposition to glutamate seems to be preserved. This may reflect down-regulation of GDH in response to decreased fetal liver output and reduced umbilical artery glutamate concentrations in human FGR pregnancies.

Placental mTOR links maternal nutrient availability to fetal growth

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.

Leptin affects system A amino acid transport activity in the human placenta: evidence for STAT3 dependent mechanisms

BACKGROUND

Amino acids are important nutrients during fetal development, and the activity of placental amino acid transporters is crucial in the regulation of fetal growth. Leptin, an adipocyte- and placenta-derived hormone, has been proposed to act as a peripheral signal in reproduction in humans. Leptin is elevated during pregnancy and elevated further in pathologic pregnancies such as preeclampsia. However, the role of leptin in placental function has not been fully elucidated. We hypothesize that leptin plays a role in the regulation of placental amino acid transport by activation of the JAK-STAT pathway.

METHODS

Placental amino acid transport, specifically system A transport was studied in placental villous fragments using the amino acid analog, methylaminoisobutyric acid (MeAIB). Specific inhibitors of the JAK-STAT signal transduction pathway were used to further elucidate their role in leptin-mediated effects on amino acid transport activity. Western blotting was performed to identify STAT3 phosphorylation as a measure of leptin receptor activation.

RESULTS

Leptin significantly increased system A amino acid transporter activity by 22-42% after 1h of incubation. Leptin activated JAK-STAT signaling pathway as evidenced by STAT3 phosphorylation, and inhibition of STAT3 or JAK2 resulted in 36-45% reduction in system A amino acid transporter activity. Furthermore, blocking endogenously produced leptin also decreased system A transport by 45% comparable to STAT3 inhibition.

CONCLUSIONS

These data demonstrate that leptin stimulates system A by JAK-STAT dependent pathway in placental villous fragments. Our findings support the autocrine/paracrine role of leptin in regulating amino acid transport in the human placenta.

The transplacental transport of essential amino acids in uncomplicated human pregnancies

OBJECTIVE

The purpose of this study was to assess the placental transport of the essential amino acids (EAAs) in normal pregnancies.

STUDY DESIGN

Nine ((13)C or (2)H) EAAs were infused simultaneously as a bolus into the maternal circulation of 12 patients with uncomplicated pregnancy before cesarean delivery. Maternal samples were collected before and after the bolus; umbilical blood was collected at delivery. The fetal/maternal molar percent enrichment for each EAA was calculated for both the umbilical vein and artery. Plasma amino acids enrichments were analyzed by gas chromatography mass spectrometry and concentrations by high performance liquid chromatography. Data were analyzed with paired and unpaired t-test.

RESULTS

The umbilical arterial enrichments were significantly lower than the venous. Fetal/maternal ratios for leucine, isoleucine, methionine, and phenylalanine were > 0.80, with no significant differences among their molar percent enrichment ratios, whereas fetal/maternal ratios of the other 5 EAAs were significantly lower (< 0.60).

CONCLUSION

The EAAs showed significant umbilical uptake and striking differences in their transport rates in vivo.

Regulation of placental amino acid transporter activity by mammalian target of rapamycin

The activity of placental amino acid transporters is decreased in intrauterine growth restriction (IUGR), but the underlying regulatory mechanisms have not been established. Inhibition of the mammalian target of rapamycin (mTOR) signaling pathway has been shown to decrease the activity of the system L amino acid transporter in human placental villous fragments, and placental mTOR activity is decreased in IUGR. In the present study, we used cultured primary trophoblast cells to study mTOR regulation of placental amino acid transporters in more detail and to test the hypothesis that mTOR alters amino acid transport activity by changes in transporter expression. Inhibition of mTOR by rapamycin significantly reduced the activity of system A (−17%), system L (−28%), and taurine (−40%) amino acid transporters. mRNA expression of isoforms of the three amino acid transporter systems in response to mTOR inhibition was measured using quantitative real-time PCR. mRNA expression of l-type amino acid transporter 1 (LAT1; a system L isoform) and taurine transporter was reduced by 13% and 50%, respectively; however, mTOR inhibition did not alter the mRNA expression of system A isoforms (sodium-coupled neutral amino acid transporter-1, -2, and -4), LAT2, or 4F2hc. Rapamycin treatment did not significantly affect the protein expression of any of the transporter isoforms. We conclude that mTOR signaling regulates the activity of key placental amino acid transporters and that this effect is not due to a decrease in total protein expression. These data suggest that mTOR regulates placental amino acid transporters by posttranslational modifications or by affecting transporter translocation to the plasma membrane.

Amino acid transport systems beta and A in fetal T lymphocytes in intrauterine growth restriction and with tumor necrosis factor-alpha treatment

Intrauterine growth restriction (IUGR) is associated with reduced activity of placental amino acid transport systems beta and A. Whether this phenotype is maintained in fetal cells outside the placenta is unknown. In IUGR, cord blood tumor necrosis factor (TNF)-alpha concentrations are raised, potentially influencing amino acid transport in fetal cells. We used fetal T lymphocytes as a model to study systems beta and A amino acid transporters in IUGR compared with normal pregnancy. We also studied the effect of TNF-alpha on amino acid transporter activity. In fetal lymphocytes from IUGR pregnancies, taurine transporter mRNA expression encoding system beta transporter was reduced, but there was no change in system beta activity. No significant differences were observed in system A mRNA expression (encoding SNAT1 and SNAT2) or system A activity between the two groups. After 24 or 48 h TNF-alpha treatment, fetal T lymphocytes from normal pregnancies showed no significant change in system A or system beta activity, although cell viability was compromised. This study represents the first characterization of amino acid transport in a fetal cell outside the placenta in IUGR. We conclude that the reduced amino acid transporter activity found in placenta in IUGR is not a feature of all fetal cells.

The fetal response to chronic placental insufficiency

Fetal growth restriction is most commonly caused by failure of the placenta to meet the increasing demands for oxygen and substrate of the developing fetus, resulting in common fetal compensatory responses. Understanding these responses is helpful in developing a management strategy that will optimize pregnancy outcome.

The mechanisms and regulation of placental amino acid transport to the human foetus

The mechanisms by which amino acids are transferred across the human placenta are fundamental to our understanding of foetal nutrition. Amino acid transfer across the human placenta is dependent on transport across both the microvillous and basal plasma membranes of the placental syncytiotrophoblast, and on metabolism within the syncytiotrophoblast. Although the principles underlying uptake of amino acids across the microvillous plasma membrane are well understood, the extent to which amino acids are metabolised within human placenta and the mechanisms by which amino acids are transported out of the placenta across the basal plasma membrane are not well understood. Understanding the mechanisms and regulation of amino acid transport is necessary to understand the causes of intrauterine growth restriction in human pregnancy.

The pregnant sheep as a model for human pregnancy

Successful outcome of human pregnancy not only impacts the quality of infant life and well-being, but considerable evidence now suggests that what happens during fetal development may well impact health and well-being into adulthood. Consequently, a thorough understanding of the developmental events that occur between conception and delivery is needed. For obvious ethical reasons, many of the questions remaining about the progression of human pregnancy cannot be answered directly, necessitating the use of appropriate animal models. A variety of animal models exist for the study of both normal and compromised pregnancies, including laboratory rodents, non-human primates and domestic ruminants. While all of these animal models have merit, most suffer from the inability to repetitively sample from both the maternal and fetal side of the placenta, limiting their usefulness in the study of placental or fetal physiology under non-stressed in vivo conditions. No animal model truly recapitulates human pregnancy, yet the pregnant sheep has been used extensively to investigate maternal-fetal interactions. This is due in part to the ability to surgically place and maintain catheters in both the maternal and fetal vasculature, allowing repeated sampling from non-anesthetized pregnancies. Considerable insight has been gained on placental oxygen and nutrient transfer and utilization from use of pregnant sheep. These findings were often confirmed in human pregnancies once appropriate technologies became available. The purpose of this review is to provide an overview of human and sheep pregnancy, with emphasis placed on placental development and function as an organ of nutrient transfer.

Role of the placenta in fetal programming: underlying mechanisms and potential interventional approaches

Adverse influences during fetal life alter the structure and function of distinct cells, organ systems or homoeostatic pathways, thereby ‘programming’ the individual for an increased risk of developing cardiovascular disease and diabetes in adult life. Fetal programming can be caused by a number of different perturbations in the maternal compartment, such as altered maternal nutrition and reduced utero–placental blood flow; however, the underlying mechanisms remain to be fully established. Perturbations in the maternal environment must be transmitted across the placenta in order to affect the fetus. Here, we review recent insights into how the placenta responds to changes in the maternal environment and discuss possible mechanisms by which the placenta mediates fetal programming. In IUGR (intrauterine growth restriction) pregnancies, the increased placental vascular resistance subjects the fetal heart to increased work load, representing a possible direct link between altered placental structure and fetal programming of cardiovascular disease. A decreased activity of placental 11β-HSD-2 (type 2 isoform of 11β-hydroxysteroid dehydrogenase) activity can increase fetal exposure to maternal cortisol, which programmes the fetus for later hypertension and metabolic disease. The placenta appears to function as a nutrient sensor regulating nutrient transport according to the ability of the maternal supply line to deliver nutrients. By directly regulating fetal nutrient supply and fetal growth, the placenta plays a central role in fetal programming. Furthermore, perturbations in the maternal compartment may affect the methylation status of placental genes and increase placental oxidative/nitrative stress, resulting in changes in placental function. Intervention strategies targeting the placenta in order to prevent or alleviate altered fetal growth and/or fetal programming include altering placental growth and nutrient transport by maternally administered IGFs (insulin-like growth factors) and altering maternal levels of methyl donors.

Modification of fetal plasma amino acid composition by placental amino acid exchangers in vitro

Fetal growth is dependent on both the quantity and relative composition of amino acids delivered to the fetal circulation, and impaired placental amino acid supply is associated with restricted fetal growth. Amino acid exchangers can alter the composition, but not the quantity, of amino acids in the intra- and extracellular amino acid pools. In the placenta, exchangers may be important determinants of the amino acid composition in the fetal circulation. This study investigates the substrate specificity of exchange between the placenta and the feto-placental circulation. Maternal-fetal transfer of radiolabelled amino acids and creatinine were measured in the isolated perfused human placental cotyledon. Transfer of L-[14C]serine or L-[14C]leucine, and [3H]glycine, were measured in the absence of amino acids in the fetal circulation (transfer by non-exchange mechanisms) and following 10-20 micromol boluses of unlabelled amino acids into the fetal circulation to provide substrates for exchange (transfer by exchange and non-exchange mechanisms). The ability of fetal arterial boluses of L-alanine and L-leucine to stimulate release of amino acids from the placenta was also determined using HPLC in order to demonstrate the overall pattern of amino acid release. Experiments with radiolabelled amino acids demonstrated increased maternal-fetal transfer of L-serine and L-leucine, but not glycine, following boluses of specific amino acids into the fetal circulation. L-[14C]Leucine, but not L-[14C]serine or [3H]glycine, was transferred from the maternal to the fetal circulation by non-exchange mechanisms also (P<0.01). HPLC analysis demonstrated that fetal amino acid boluses stimulated increased transport of a range of different amino acids by 4-7 micromol l(-1) (P<0.05). Amino acid exchange provides a mechanism to supply the fetus with amino acids that it requires for fetal growth. This study demonstrates that these transporters have the capacity to exchange micromolar amounts of specific amino acids, and suggests that they play an important role in regulating fetal plasma amino acid composition.

Mammalian target of rapamycin in the human placenta regulates leucine transport and is down-regulated in restricted fetal growth

Pathological fetal growth is associated with perinatal morbidity and the development of diabetes and cardiovascular disease later in life. Placental nutrient transport is a primary determinant of fetal growth. In human intrauterine growth restriction (IUGR) the activity of key placental amino acid transporters, such as systems A and L, is decreased. However the mechanisms regulating placental nutrient transporters are poorly understood. We tested the hypothesis that the mammalian target of rapamycin (mTOR) signalling pathway regulates amino acid transport in the human placenta and that the activity of the placental mTOR pathway is reduced in IUGR. Using immunohistochemistry and culture of trophoblast cells, we show for the first time that the mTOR protein is expressed in the transporting epithelium of the human placenta. We further demonstrate that placental mTOR regulates activity of the l-amino acid transporter, but not system A or taurine transporters, by determining the mediated uptake of isotope-labelled leucine, methylaminoisobutyric acid and taurine in primary villous fragments after inhibition of mTOR using rapamycin. The protein expression of placental phospho-S6K1 (Thr-389), a measure of the activity of the mTOR signalling pathway, was markedly reduced in placentas obtained from pregnancies complicated by IUGR. These data identify mTOR as an important regulator of placental amino acid transport, and provide a mechanism for the changes in placental leucine transport in IUGR previously demonstrated in humans. We propose that mTOR functions as a placental nutrient sensor, matching fetal growth with maternal nutrient availability by regulating placental nutrient transport.

Brief hyperglycaemia in the early pregnant rat increases fetal weight at term by stimulating placental growth and affecting placental nutrient transport

In pregnant women with type 1 diabetes, suboptimal glucose control in the first trimester is a strong predictor for giving birth to a large fetus. However, the mechanisms underlying this association are unknown. We hypothesized that transient hyperglycaemia in early pregnancy results in (1) increased placental growth and (2) an up-regulation of placental nutrient transport capacity, which leads to fetal overgrowth at term. In order to test this hypothesis, pregnant rats were given intraperitoneal injections of glucose (2 g kg(-1), resulting in a 50-100% increase in blood glucose level during 90 min) or saline (control) in either early or late gestation using four different protocols: one single injection on gestational day (GD) 10 (n=5), three injections on GD 10 (n=8-9), six injections on GD 10 and 11 (n=9-11) or three injections on GD 19 (n=7-8). Multiple injections were given approximately 4 h apart. Subsequently, animals were studied on GD 21. Three glucose injections in early pregnancy significantly increased placental weight by 10%, whereas fetal weight was found to be increased at term in response to both three (9% increase in fetal weight, P<0.05) and six glucose injections (7%, P=0.05) in early gestation. A single glucose injection on GD 10 or three injections of glucose on GD 19 had no effect on placental or fetal growth. In groups where a change in feto-placental growth was observed, we measured placental system A and glucose transport activity in the awake animals on GD 21 and placental expression of the glucose and amino acid transporters GLUT1, GLUT3, SNAT2 (system A), LAT1 and LAT 2 (system L). Placental system A transport at term was down-regulated by six glucose injections in early pregnancy (by -33%, P<0.05), whereas placental mRNA and protein levels were unchanged. No long-term alterations in maternal metabolic status were detected. In conclusion, we demonstrate that transient hyperglycaemia in early pregnancy is sufficient to increase fetal weight close to term. In contrast, brief hyperglycaemia in late pregnancy did not stimulate fetal growth. Increased fetal growth may be explained by a larger placenta, which would allow for more nutrients to be transferred to the fetus. These data suggest that maternal metabolic control in early pregnancy is an important determinant for feto-placental growth and placental function throughout the remainder of gestation. We speculate that maternal metabolism in early pregnancy represents a key environmental cue to which the placenta responds in order to match fetal growth rate with the available resources of the mother.

Down-regulation of placental transport of amino acids precedes the development of intrauterine growth restriction in rats fed a low protein diet

Intrauterine growth restriction (IUGR) represents an important risk factor for perinatal complications and for adult disease. IUGR is associated with a down-regulation of placental amino acid transporters; however, whether these changes are primary events directly contributing to IUGR or a secondary consequence is unknown. We investigated the time course of changes in placental and fetal growth, placental nutrient transport in vivo and the expression of placental nutrient transporters in pregnant rats subjected to protein malnutrition, a model for IUGR. Pregnant rats were given either a low protein (LP) diet (n = 64) or an isocaloric control diet (n = 66) throughout pregnancy. Maternal insulin, leptin and IGF-I levels decreased, whereas maternal amino acid concentrations increased moderately in response to the LP diet. Fetal and placental weights in the LP group were unaltered compared to control diet at gestational day (GD) 15, 18 and 19 but significantly reduced at GD 21. Placental system A transport activity was reduced at GD 19 and 21 in response to a low protein diet. Placental protein expression of SNAT2 was decreased at GD 21. In conclusion, placental amino acid transport is down-regulated prior to the development of IUGR, suggesting that these placental transport changes are a cause, rather than a consequence, of IUGR. Reduced maternal levels of insulin, leptin and IGF-1 may link maternal protein malnutrition to reduced fetal growth by down-regulation of key placental amino acid transporters.

Endocrine regulation of human fetal growth: the role of the mother, placenta, and fetus

The environment in which the fetus develops is critical for its survival and long-term health. The regulation of normal human fetal growth involves many multidirectional interactions between the mother, placenta, and fetus. The mother supplies nutrients and oxygen to the fetus via the placenta. The fetus influences the provision of maternal nutrients via the placental production of hormones that regulate maternal metabolism. The placenta is the site of exchange between mother and fetus and regulates fetal growth via the production and metabolism of growth-regulating hormones such as IGFs and glucocorticoids. Adequate trophoblast invasion in early pregnancy and increased uteroplacental blood flow ensure sufficient growth of the uterus, placenta, and fetus. The placenta may respond to fetal endocrine signals to increase transport of maternal nutrients by growth of the placenta, by activation of transport systems, and by production of placental hormones to influence maternal physiology and even behavior. There are consequences of poor fetal growth both in the short term and long term, in the form of increased mortality and morbidity. Endocrine regulation of fetal growth involves interactions between the mother, placenta, and fetus, and these effects may program long-term physiology.

Placental adaptive responses and fetal programming

Fetal programming occurs when the normal pattern of fetal development is disrupted by an abnormal stimulus or 'insult' applied at a critical point in in utero development. This then leads to an effect, for example diabetes or hypertension, which manifests itself in adult life. As the placenta is the regulator of nutrient composition and supply from mother to fetus and the source of hormonal signals that affect maternal and fetal metabolism, appropriate development of the placenta is crucial to normal fetal development. Placental function evolves in a carefully orchestrated developmental cascade throughout gestation. Disruption of this cascade can lead to abnormal development of the placental vasculature or of the trophoblast. Timing of a developmental 'insult' will be critical in consequent placental function and hence programming of the fetus. The 'insults' that alter placental development include hypoxia and abnormal maternal nutrient status, to which the placenta may adapt by alterations in transporter expression and activity to maintain fetal growth or by epigenetic regulation of placental gene expression. Hypoxia is physiological for organogenesis and placental tissue normally exists in a relatively hypoxic environment, but intrauterine growth restriction (IUGR) and pre-eclampsia are associated with a greater degree of trophoblast hypoxia. The metabolic activity of placental mitochondria leads to oxidative stress even in normal pregnancy which is exacerbated further in IUGR, diabetic and pre-eclamptic pregnancies and may also give nitrative stress known to lead to covalent modification and hence altered activity of proteins. Hypoxia, oxidative and nitrative stress all alter placenta development and may be a general underlying mechanism that links altered placental function to fetal programming.

Placental phenotypes of intrauterine growth

The placenta is essential to nutrition before birth. Recent work has shown that a range of clearly defined alterations can be found in the placentas of infants with intrauterine growth restriction (IUGR). In the mouse, a placental specific knockout of a single imprinted gene, encoding IGF-2, results in one pattern of alterations in placenta structure and function which leads to IUGR. We speculate that the alterations in the human placenta can also be grouped into patterns, or phenotypes, that are associated with specific patterns of fetal growth. Identifying the placental phenotypes of different fetal growth patterns will improve the ability of clinicians to recognize high-risk patients, of laboratory scientists to disentangle the complexities of IUGR, and of public health teams to target interventions aimed at ameliorating the long-term adverse effects of inadequate intrauterine growth.

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.

Higher urinary excretion of essential amino acids in preterm infants fed protein hydrolysates

AIM

Protein hydrolysates have been introduced in preterm formulae, but it is not clear whether they are needed for the feeding of preterm infants. We designed a randomized, controlled trial to test the effects of a preterm formula with hydrolysed cow's milk proteins on short-term growth and urinary and plasma amino acids levels.

METHODS

Infants with a birthweight < or = 1750 g and gestational age < or = 34 wk fed a conventional preterm infant formula (formula B) or a hydrolysed formula (formula A). Weight was measured daily; length, head circumference, mid-arm circumference and total skinfold thickness were measured weekly. Blood and urine were analysed for amino acid concentrations at start, 14 and 28 d.

RESULTS

Twenty-one infants met the criteria for randomization. The daily feeding volumes were: formula A 172.8 +/- 5.6 vs formula B 170.1 +/- 2.8 ml/kg/d. Infants fed with formula A showed slower weight gain (17.4 +/- 3.4 vs 20.5 +/- 3.3 g/kg/d; p = 0.045) and lower mean change in Z-scores for weight (-0.18 +/- 0.16 vs 0.00 +/- 0.09; p = 0.009) and for head circumference (-0.06 +/- 0.13 vs 0.06 +/- 0.13; p = 0.049). After 14 d, infants receiving formula A had statistically significant higher urinary levels of essential amino acids compared to infants receiving formula B.

CONCLUSION

Our results support the hypothesis of less nutritional value of hydrolysed versus conventional preterm formulae. Higher renal excretion of essential amino acids may be one of the mechanisms involved. These findings must be confirmed by further studies with larger sample sizes and protein hydrolysates with different degrees of hydrolysis.

Polarized lactate transporter activity and expression in the syncytiotrophoblast of the term human placenta

We investigated the polarization of l-lactate transport in human syncytiotrophoblast by measuring uptake of [(14)C] l-lactate by both microvillous (maternal-facing; MVM) and basal (fetal-facing; BM) plasma membranes. [(14)C] l-lactate uptake by MVM and BM was stimulated in the presence of an inwardly directed H(+)gradient, with a significantly higher uptake in MVM than in BM at initial rate (15.4+/-2.3 vs 5.6+/-0.6 pmol/mg protein/20 sec). Stereospecific inhibition was observed in MVM, with a higher affinity for l-lactate compared with d-lactate. In BM, there was no difference in the inhibition by these two stereoisomers. Inhibition of lactate uptake in both MVM and BM by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), an inhibitor of monocarboxylate transporter (MCT) activity, indicated MCT-mediated mechanisms across both membranes. Kinetic modelling supported a two-transporter model as the best fit for both MVM and BM, the K(m)of the major component being 6.21 mm and 25.01 mm in MVM and BM respectively. Western blotting and immunolocalization examining the distribution of MCT1 and MCT4, showed that MCT expression was polarized, MCT1 being predominantly localized to BM and MCT4 showing greater abundance on MVM. CD147, a chaperone protein for MCT1 and MCT4, was equally expressed by both membranes. These studies demonstrate that the opposing plasma membranes of human syncytiotrophoblast are polarized with respect to both MCT activity and expression.

Placental transport of leucine in a porcine model of low birth weight

Low birth weight is a major factor in neonatal morbidity and mortality in humans and domestic species and is a predictor of physiological disorders in adulthood. This study utilised the naturally occurring variation in pig fetal size within a uterus to test the hypothesis that placental amino acid transport capability is associated with fetal growth. Leucine uptake by trophoblast vesicles prepared from placentas supplying an average-sized fetus and the smallest fetus in the uterus was assessed. On days 45 and 65 of gestation, uptake of leucine by the porcine placenta was predominantly sodium independent and was inhibited by the non-metabolised leucine analogue 2-amino-2-norbornane-carboxylic acid, indicating that uptake occurs via system L. By day 100 the uptake of leucine by placentas supplying average-sized fetuses had changed from being predominantly sodium independent to involving both sodium-dependent (system B0) and -independent (system L) pathways. This change was not seen in placentas supplying the smallest fetus, which continued to display predominantly sodium-independent uptake. In conclusion, these data show gestational- and fetal size-dependent changes in the transport of leucine across the porcine placenta.

Placental uptake and transport of ACP, a neutral nonmetabolizable amino acid, in an ovine model of fetal growth restriction

Reductions in fetal plasma concentrations of certain amino acids and reduced amino acid transport in vesicle studies suggest impaired placental amino acid transport in human fetal growth restriction (FGR). In the present study, we tested the hypothesis of an impairment in amino acid transport in the ovine model of hyperthermia-induced FGR by determining transplacental and placental retention and total placental clearance of a branched-chain amino acid (BCAA) analog, the nonmetabolizable neutral amino acid aminocyclopentane-1-carboxylic acid (ACP), in singleton control (C) and FGR pregnancies at 135 days gestation age (dGA; term 147 dGA). At study, based on the severity of the placental dysfunction, FGR fetuses were allocated to severe (sFGR, n = 6) and moderate FGR (mFGR, n = 4) groups. Fetal (C, 3,801.91 +/- 156.83; mFGR, 2,911.33 +/- 181.35; sFGR, 1,795.99 +/- 238.85 g; P < 0.05) and placental weights (C, 414.38 +/- 38.35; mFGR, 306.23 +/- 32.41; sFGR, 165.64 +/- 28.25 g; P < 0.05) were reduced. Transplacental and total placental clearances of ACP per 100 g placenta were significantly reduced in the sFGR but not in the mFGR group, whereas placental retention clearances were unaltered. These data indicate that both entry of ACP into the placenta and movement from the placenta into fetal circulation are impaired in severe ovine FGR and support the hypothesis of impaired placental BCAA transport in severe human FGR.

Glucose Metabolism in the Human Preterm and Term Placenta of IUGR Fetuses

Many fetuses suffering from intrauterine growth restriction (IUGR) are hypoglycaemic. However, the underlying mechanisms are not well established. An increased placental glucose consumption in IUGR could impair glucose transfer across the placenta. In this study we used two different approaches to investigate glucose metabolism in preterm and term placentae of IUGR fetuses. We determined activity and protein expression of the three rate-limiting glycolytic enzymes phosphofructo kinase (PFK), pyruvate kinase (PK) and hexokinase (HXK) in a cytoplasmic fraction of homogenates of placentae obtained from IUGR and appropriate for gestational age (AGA) pregnancies. Protein expression was assessed using Western blot and enzyme activities were determined in a spectrophotometer by measuring the rate of NADH oxidation (PFK and PK) or NADP reduction (HXK) in enzyme reactions coupled to the respective enzyme. To determine the distribution of the glycolytic enzymes immunocytochemistry was performed. We also measured glucose consumption and lactate production in fresh placental villous tissue using a perifusion system. The expression of PFK, PK and HXK as well as the activity of PK and HXK was unaltered in IUGR placentae. The activity of PFK on the other hand was 32 per cent lower in IUGR placentae (n=24, P<0.05). Immunocytochemistry confirmed the distribution of the enzymes to the cytoplasm of the syncytiotrophoblast. Placental glucose consumption in IUGR [0.06+/-0.01 micromol/(min*g), n=5] was not different from AGA [0.06+/-0.005 micromol/(min*g), n=12], whereas lactate production was decreased by 28 per cent in IUGR. These results do not support the hypothesis of increased placental glucose consumption but suggest an altered glycolytic pathway in the IUGR placenta.

Nitric oxide and superoxide impair human placental amino acid uptake and increase Na+ permeability: implications for fetal growth

Based on evidence that thiol and tyrosine reagents inhibit some amino acid transporters, we tested the hypothesis that NO- and O2- -derived free radicals would impair nutrient uptake by the human placenta. Syncytiotrophoblast microvillous plasma membrane vesicles (MVM) and placental villous fragments were exposed to the drug SIN-1 in the presence or absence of superoxide dismutase (SOD) and hemoglobin (Hb). The uptake of [3H]arginine, [3H]taurine, and [3H]leucine; [14C]MeAIB; and 22Na was studied in MVM, whereas the uptake of [3H]taurine was examined in villous fragments. Nitrotyrosine formation was assessed by Western blotting and quantified by ELISA. In MVM, SIN-1 caused an inhibition of [3H]arginine, [3H]taurine, and [14C]MeAIB uptake but had no significant effect on equilibrium [3H]leucine uptake. These effects were prevented by SOD or Hb, implying that both NO and O2- radicals were essential. In contrast, 22Na+ uptake was significantly increased, and this effect was prevented by SOD. In villous fragments, SIN-1 impaired Na+-dependent [3H]taurine uptake, with no effect on Na+-independent uptake. Increased nitrotyrosine formation was observed in MVM after SIN-1 treatment. Endogenous NO- and O2- -derived free radicals may alter human placental nutrient transfer in vivo, with implications for fetal growth.

Placental transport of amino acids in normal and growth-restricted pregnancies

In human pregnancies placental amino acid transport has been studied at the time of delivery and also by in utero fetal blood sampling (FBS). A significant reduction in amino acid fetal-maternal gradients and in umbilical veno-arterial differences has been demonstrated in intrauterine growth-restricted (IUGR) pregnancies. Fetal-maternal transfer rates have been further investigated in vivo by stable isotope methodologies. Following a maternal bolus infusion of [1-13C]-glycine and [1-13C]-leucine performed at fetal blood sampling, the transfer rate of the non-essential amino acid glycine is significantly lower than that for the essential amino acid leucine, suggesting that glycine can be newly synthesized in the feto-placental unit. Moreover, in growth-restricted pregnancies the fetal/maternal ratio of [1-13C]-leucine is significantly lower, and proportional to the degree of severity. In vitro studies have described a variety of transport systems for amino acids within the microvillous membrane (MVM) and the basal membrane (BM) of the placenta and significant differences have been reported in growth-restricted pregnancies for system A, system L, and taurine transporters. These changes are significantly associated to both biophysical and biochemical parameters of severity. Moreover, significant relationships can be found in arginine transport system and uterine oxygenation, suggesting a role in nitric oxide (NO) synthesis.

Errata

Intrauterine growth restriction (IUGR) represents an important risk factor for perinatal complications and for adult disease. IUGR is associated with a down‐regulation of placental amino acid transporters; however, whether these changes are primary events directly contributing to IUGR or a secondary consequence is unknown. We investigated the time course of changes in placental and fetal growth, placental nutrient transport in vivo and the expression of placental nutrient transporters in pregnant rats subjected to protein malnutrition, a model for IUGR. Pregnant rats were given either a low protein (LP) diet (n= 64) or an isocaloric control diet (n= 66) throughout pregnancy. Maternal insulin, leptin and IGF‐I levels decreased, whereas maternal amino acid concentrations increased moderately in response to the LP diet. Fetal and placental weights in the LP group were unaltered compared to control diet at gestational day (GD) 15, 18 and 19 but significantly reduced at GD 21. Placental system A transport activity was reduced at GD 19 and 21 in response to a low protein diet. Placental protein expression of SNAT2 was decreased at GD 21. In conclusion, placental amino acid transport is down‐regulated prior to the development of IUGR, suggesting that these placental transport changes are a cause, rather than a consequence, of IUGR. Reduced maternal levels of insulin, leptin and IGF‐1 may link maternal protein malnutrition to reduced fetal growth by down‐regulation of key placental amino acid transporters.

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.

ATP dependent Ca2+ transport across basal membrane of human syncytiotrophoblast in pregnancies complicated by intrauterine growth restriction or diabetes

Neonates born after pregnancies complicated by diabetes or intrauterine growth restriction (IUGR) have increased incidence of hypocalcaemia. Furthermore, IUGR is associated with reduced bone mineralization in infancy and osteoporosis in adult life. We tested the hypothesis that placental calcium transport is altered in these pregnancy complications. Transport of calcium into syncytiotrophoblast basal plasma membrane (BM) vesicles was studied by rapid filtration and protein expression of Ca(2+) ATPase by Western blot. In IUGR Ca(2+) ATPase activity was increased by 48 per cent (n=13; P< 0.05) whereas protein expression was 15 per cent lower (n=13; P< 0.05) than in controls (n=16). Basal membrane ATP dependent calcium transport was unaltered in gestational diabetes (GDM) but increased by 54 per cent in insulin dependent diabetes (IDDM) compared to controls (P< 0.05; n =14). Diabetes did not affect Ca(2+) ATPase expression in BM. We have previously shown that the mid-molecular fragment of parathyroid hormone related peptide (PTHrP midmolecule) stimulates BM Ca(2+) ATPase in vitro. PTHrP midmolecule concentrations in umbilical cord plasma were measured using radioimmunoassay. The concentrations in umbilical cord plasma were increased in IUGR, but unaltered in diabetes. In conclusion, placental calcium pump is activated in IUGR and IDDM, which may be secondary to increased foetal calcium demand. We speculate that PTHrP midmolecule may be one mechanism for activating BM Ca(2+) ATPase in IUGR.

Leptin stimulates the activity of the system A amino acid transporter in human placental villous fragments

The activity and expression of placental nutrient transporters are primary determinants for the supply of nutrients to the fetus, and these nutrients in turn regulate fetal growth. We developed an experimental system to assess amino acid uptake in single primary villous fragments to study hormonal regulation of the amino acid transporter system A in term human placenta. Validation of the method, using electron microscopy and studies of hormone production, indicated that fragments maintained ultrastructural and functional integrity for at least 3 h. The activity of system A was measured as the Na(+)-dependent uptake of methylaminoisobutyric acid (MeAIB), and the effect of 1 h incubation in various hormones was investigated. Uptake of MeAIB into villous fragments in the presence of Na(+) was linear up to at least 30 min. Insulin (300 ng/ml, n = 14) increased system A activity by 56% (P < 0.05). This effect was also present at insulin concentrations in the physiological range (+47% at 0.6 ng/ml, n = 10, P < 0.05). Leptin (500 ng/ml, n = 14) increased Na(+)-dependent MeAIB uptake by 37% (P < 0.05). System A activity increased in a concentration-dependent fashion in response to leptin (n = 10). However, neither epidermal GF (600 ng/ml), cortisol (340 ng/ml), nor GH (500 ng/ml) altered system A activity significantly (n = 14). We conclude that primary single isolated villous fragments can be used in studies of hormonal regulation of nutrient uptake into the syncytiotrophoblast. These data suggest that leptin regulates system A, a key amino acid transporter.

The effect of a maternal infusion of amino acids on umbilical uptake in pregnancies complicated by intrauterine growth restriction

OBJECTIVE

To establish whether, in human pregnancies complicated by intrauterine growth restriction (IUGR), the maternal intravenous infusion of amino acids can increase fetal amino acid concentrations and umbilical uptake.

STUDY DESIGN

Before elective cesarean delivery, a solution of amino acids was infused into a maternal vein in 8 patients with pregnancies complicated by IUGR (experimental group). At cesarean delivery, maternal, umbilical venous, and arterial blood samples were obtained. Ten comparable IUGR pregnancies were compared with those in the experimental group.

RESULTS

In the experimental group, all maternal amino acid concentrations were increased significantly. In the umbilical vein, valine, methionine, isoleucine, leucine, phenylalanine, arginine, serine, glycine, and proline concentrations were elevated. Umbilical venoarterial differences of amino acid per mole of oxygen for leucine, isoleucine, methionine, arginine, glycine, serine, and proline were elevated but not for lysine, histidine, threonine, valine, and phenylalanine.

CONCLUSION

In pregnancies complicated by IUGR, increasing the maternal concentration of amino acids leads to an increased umbilical uptake of some of the amino acids to the fetus. There was no evidence of a change in the uptake of 3 essential amino acids: lysine, histidine, and threonine.

Alterations in the activity of placental amino acid transporters in pregnancies complicated by diabetes

Alterations in placental transport may contribute to accelerated fetal growth in pregnancies complicated by diabetes. We studied the activity of the syncytiotrophoblast amino acid transporter system A and the transport of the essential amino acids leucine, lysine, and taurine. Syncytiotrophoblast microvillous plasma membranes (MVMs) and basal plasma membranes (BMs) were isolated from placentas obtained from normal pregnancies and pregnancies complicated by gestational diabetes mellitus (GDM) and type 1 diabetes, with and without large-for-gestational-age (LGA) fetuses. Amino acid transport was assessed using radio-labeled substrates and rapid filtration techniques. System A activity in MVM was increased (65-80%, P < 0.05) in all groups with diabetes independent of fetal overgrowth. However, MVM system A activity was unaffected in placentas of normal pregnancies with LGA fetuses. MVM leucine transport was increased in the GDM/LGA group. In BMs, amino acid transport was unaffected by diabetes. In conclusion, diabetes in pregnancy is associated with an increased system A activity in MVM, and MVM leucine transport is increased in the GDM/LGA group. We suggest that these changes result in an increased uptake of neutral amino acids across MVM, which may be used in placental metabolism or be delivered to the fetus. The increased MVM leucine uptake in the GDM/LGA group may contribute to accelerated fetal growth in these patients.

Nutrition in early life. How important is it?

While few would argue the importance of nutrition during adult life, temporary excess or deficiency has typically been thought to be of little long-term consequence. Recent data, summarized above, suggests that this may not be the case during in utero life, when alterations in the quantity or quality of nutrients provided may have life-long consequences. Perhaps even more surprisingly, decisions made in the neonatal period, such as whether to breastfeed or bottle feed, may have impacts on later health that, while small individually, have huge public health implications. Clarification of the links between adult health and fetal/neonatal nutrition are clearly required. Prospective studies, though difficult because of the time involved, will play a key role in this process, as will more basic research on the mechanisms underlying both normal and pathologic fetal development.

Glucose transport and system A activity in syncytiotrophoblast microvillous and basal plasma membranes in intrauterine growth restriction

The mechanisms underlying the reduced fetal plasma concentrations of amino acids and glucose associated with intrauterine growth restriction (IUGR) remain to be fully established. The activity of the amino acid transporter system A has been shown to be reduced in the syncytiotrophoblast microvillous membrane (MVM) in IUGR, however the impact of these changes on transplacental transport is difficult to assess without information on system A activity in the basal plasma membrane (BM). In this study we measured system A activity and mediated D-glucose uptake using radiolabelled substrates and rapid filtration techniques, and glucose transporter isoform 1 (GLUT 1) protein expression using Western blots in MVM and BM isolated from human placentas. In term IUGR (n=11) MVM system A activity was unaltered compared to controls (n=9). In contrast, system A activity in MVM was reduced by 50 per cent (P< 0.05) in preterm IUGR (n=8, gestational age 28-36 weeks) as compared to controls (n=8, gestational age 28-35 weeks). BM system A activity was unaltered in both IUGR groups. Similarly, MVM and BM GLUT 1 expression and mediated D-glucose uptake was not affected by IUGR. In all preterm IUGR pregnancies signs of severe fetal compromise were present whereas term IUGR fetuses were less affected. These data support the view that MVM system A activity is related to the severity of compromise in IUGR. The markedly reduced system A activity in MVM in preterm IUGR together with the unaltered activity in BM is consistent with a decreased transplacental transport of neutral amino acids in this pregnancy complication. The hypoglycemia present in utero in some IUGR fetuses is not caused by a decreased glucose transport capacity across the syncytiotrophoblast plasma membranes.

Cationic amino acid transporter activity in the syncytiotrophoblast microvillous plasma membrane and oxygenation of the uteroplacental unit

The purpose of this study was to determine whether there is any relationship between the activity of cationic amino acid transporters in the microvillous plasma membrane (MVM) of the syncytiotrophoblast and the oxygenation of the uteroplacental unit. Oxygenation data were obtained at the time of caesarean section from the uterine veins, the maternal radial artery and the umbilical vessels of 7 normal (AGA) and 13 intrauterine growth restricted (IUGR) pregnancies. Microvillous plasma membranes were isolated from the same placentas and the activity of the system y(+) and y(+)L cationic amino acid transporters determined by measuring (3)H- l -arginine uptake in the presence and absence of l -glutamine. In IUGR pregnancies uterine venous Po(2) was significantly higher (AGA=44.7+/-8.0 mmHg; IUGR=57.2+/-2.3 mmHg, P< 0.05) and umbilical venous Po(2) was significantly lower (AGA=33.4+/-3.0 mmHg; IUGR=25.1+/-2.0 mmHg, P< 0.05) than in AGA pregnancies. System y(+)L activity, but not system y(+) activity, was inversely correlated with uterine venous Po(2) (P< 0.01; r(2)=0.4) in AGA and IUGR pregnancies. In IUGR pregnancies without associated maternal pre-eclampsia, y(+)L activity, but not y(+) activity, was also directly related to the umbilical O(2) content difference (P< 0.01; r(2)=0.9). A significant negative correlation was found between system y(+) and the umbilical O(2) content difference in AGA pregnancies (P< 0.01; r(2)=0.9). Our data are consistent with the hypothesis that in IUGR fetuses uterine oxygenation is not reduced and can be increased. The inverse correlation between system y(+)L activity and uterine venous Po(2) and the correlations with umbilical venous-arterial O(2) content difference suggest a relationship between cationic amino acid transporter activity and oxygen tension in the uteroplacental unit.

Placental development in normal and compromised pregnancies-- a review

Intrauterine growth restriction (IUGR) is a significant cause of infant mortality and morbidity. It is now clear that IUGR infants exhibit higher rates of coronary heart disease, type 2-diabetes, hypertension and stroke as adults. Therefore, fetal growth not only impacts the outcome of the perinatal period, but also impacts adult well-being. The etiologies of IUGR are numerous, but are often associated with abnormalities in placental structure and function. The process of implantation and placentation requires the production of a plethora of growth factors, cell-adhesion molecules, extracellular matrix proteins, hormones and transcription factors. Many of these exhibit altered expression within the placenta of IUGR pregnancies. However, it has been difficult to fully assess their role during the development of placental insufficiency (PI) in the human, underscoring the need for animal models. Using an ovine model of PI-IUGR we have observed changes in the expression of vascular endothelial growth factor, placental growth factor, their common receptors, as well as angiopoietin 2 and its receptor, Tie 2. We found that changes in these growth factors can be associated with both acute and chronic changes in placental vascular structure and function. These studies and others are providing needed insight into the developmental chronology of placental insufficiency.

Regulation of placental transfer: the Na(+)/H(+) exchanger--a review

This review article considers the purposes and mechanisms of regulation of placental transfer in general terms and then illustrates some key points with reference to the Na(+)/H(+) exchanger (NHE), a transport protein found in the syncytiotrophoblast. NHE probably has a role in the homeostasis of syncytiotrophoblast intracellular pH and may also be involved in syncytiotrophoblast cell volume regulation as well as H(+) loss from and Na(+) transfer to the fetus. The activity and expression of NHE in the microvillous plasma membrane of the syncytiotrophoblast is reduced in placentas from preterm, growth restricted babies as compared to their gestationally matched normally grown counterparts. There are differential effects of gestation in normal pregnancy on NHE mRNA, NHE protein and NHE activity. There is also evidence of acute modulation of NHE activity. Regulation of NHE in syncytiotrophoblast is therefore complex with control at transcription, post transcription and post translational loci.

Placental transport of leucine, phenylalanine, glycine, and proline in intrauterine growth-restricted pregnancies

L-[1-13C]Leucine, [1-13C]glycine, L-[1-13C]phenylalanine, and L-[1-13C]proline were infused as a bolus into the maternal circulation of seven appropriate for gestational age at 30.3 +/- 3.0 wk and 7 intrauterine growth-restricted pregnancies at 26.5 +/- 1.0 wk gestation to investigate placental transport in vivo. Umbilical venous samples were obtained at the time of in utero fetal blood sampling at 450 +/- 74 sec from the bolus injection. In normal pregnancies the fetal/maternal (F/M) enrichment ratios for leucine (0.76 +/- 0.06) and phenylalanine (0.77 +/- 0.06) were higher (P < 0.01) than the F/M ratios for glycine (0.18 +/- 0.04) and proline (0.22 +/- 0.02). This suggests that these two essential amino acids rapidly cross the placenta in vivo. Compared with the essentials, both glycine and proline had significantly lower F/M enrichment ratios, which were not different from each other. The results support the hypothesis that amino acids with high affinity for exchange transporters cross the placenta most rapidly. In intrauterine growth-restricted pregnancies, the F/M enrichment ratio was significantly lower (P < 0.01) for L-[1-13C]leucine (0.76 +/- 0.06 vs. 0.48 +/- 0.07) and for L-[1-13C]phenylalanine (0.77 +/- 0.06 vs. 0.46 +/- 0.07) compared with appropriate for gestational age pregnancies reflecting impaired transplacental flux. The F/M enrichment ratio did not differ for [1-13C]glycine (0.18 +/- 0.04 vs. 0.17 +/- 0.03), and L-[1-13C]proline (0.22 +/- 0.02 vs. 0.18 +/- 0.04).

Placental transport rather than maternal concentration of amino acids regulates fetal growth in monochorionic twins: implications for fetal origin hypothesis

OBJECTIVE

The cause of discordant growth in monochorionic twins is not clear. We hypothesize that growth restriction of 1 monochorionic twin is due to fetal under-nutrition.

STUDY DESIGN

We measured plasma amino acid concentrations by high performance liquid chromatography in maternal venous and fetal umbilical cord venous blood samples that were collected at birth from gestational age-matched monochorionic twins (n = 14) with a birth weight discordance of > or =20%. The concordant monochorionic twins with birth weight differences of < or =10% constitute a control group.

RESULTS

In the intrauterine growth-restricted twins, fetal concentrations of essential amino acids valine (P <.01), leucine (P <.01), isoleucine (P <.01), phenylalanine (P <.01), and L-arginine (P <.05) were lower than the co-twins and concordant monochorionic twin pairs. Fetal concentrations of the nonessential amino acids taurine (P <.001), serine (P <.01), glycine (P <.01), tyrosine (P <.01), and aspartic acid (P <.01) were lower in the intrauterine growth-restricted twin than the co-twin or concordant monochorionic twins. No such differences were found between concordant monochorionic twin pairs. Maternal amino acid concentrations were similar between discordant and concordant groups.

CONCLUSION

Concentrations of certain essential and nonessential amino acids in the intrauterine growth-restricted twins were lower than the co-twins. These differences support the hypothesis that intrauterine growth-restriction that affects 1 of the monochorionic twins is due to the impaired placental transport of amino acids rather than intertwin transfusion of blood.

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.