Lysine and alanine transport in the perfused guinea-pig placenta

Evolution of Placental Function in Mammals: The Molecular Basis of Gas and Nutrient Transfer, Hormone Secretion, and Immune Responses

Placenta has a wide range of functions. Some are supported by novel genes that have evolved following gene duplication events while others require acquisition of gene expression by the trophoblast. Although not expressed in the placenta, high-affinity fetal hemoglobins play a key role in placental gas exchange. They evolved following duplications within the beta-globin gene family with convergent evolution occurring in ruminants and primates. In primates there was also an interesting rearrangement of a cassette of genes in relation to an upstream locus control region. Substrate transfer from mother to fetus is maintained by expression of classic sugar and amino acid transporters at the trophoblast microvillous and basal membranes. In contrast, placental peptide hormones have arisen largely by gene duplication, yielding for example chorionic gonadotropins from the luteinizing hormone gene and placental lactogens from the growth hormone and prolactin genes. There has been a remarkable degree of convergent evolution with placental lactogens emerging separately in the ruminant, rodent, and primate lineages and chorionic gonadotropins evolving separately in equids and higher primates. Finally, coevolution in the primate lineage of killer immunoglobulin-like receptors and human leukocyte antigens can be linked to the deep invasion of the uterus by trophoblast that is a characteristic feature of human placentation.

Early pregnancy maternal endocrine insulin-like growth factor I programs the placenta for increased functional capacity throughout gestation

In early pregnancy, the concentrations of IGFs increase in maternal blood. Treatment of pregnant guinea pigs with IGFs in early to midpregnancy enhances placental glucose transport and fetal growth and viability near term. In the current study, we determined whether exogenous IGFs altered placental gene expression, transport, and nutrient partitioning during treatment, which may then persist. Guinea pigs were infused with IGF-I, IGF-II (both 1 mg/kg x d) or vehicle sc from d 20-35 of pregnancy and killed on d 35 (term is 70 d) after administration of [(3)H]methyl-D-glucose (MG) and [(14)C]amino-isobutyric acid (AIB). IGF-I increased placental and fetal weights (+15 and +17%, respectively) and MG and AIB uptake by the placenta (+42 and +68%, respectively) and fetus (+59 and +90%, respectively). IGF-I increased placental mRNA expression of the amino acid transporter gene Slc38a2 (+780%) and reduced that of Igf2 (-51%), without altering the glucose transporter Slc2a1 or Vegf and Igf1 genes. There were modest effects of IGF-I treatment on MG and AIB uptake by individual maternal tissues and no effect on plasma glucose, total amino acids, free fatty acids, triglycerides, and cholesterol concentrations. IGF-II treatment of the mother did not alter any maternal, fetal or placental parameter. In conclusion, exogenous IGF-I, but not IGF-II, in early pregnancy increases placental transport of MG and AIB, enhancing midgestational fetal nutrient uptake and growth. This suggests that early pregnancy rises in maternal circulating IGF-I play a major role in regulating placental growth and functional development and thus fetal growth throughout gestation.

Expression and adaptive regulation of amino acid transport system A in a placental cell line under amino acid restriction

Trans-placental transport of amino acids is vital for the developing fetus. Using the BeWo cell line as a placental model, we investigated the effect of restricting amino acid availability on amino acid transport system type A. BeWo cells were cultured either in amino acid-depleted (without non-essential amino acids) or control media for 1, 3, 5 or 6 h. System A function was analysed using alpha(methyl-amino)isobutyric acid (MeAIB) transcellular transport studies. Transporter (sodium coupled neutral amino acid transporter (SNAT1/2)) expression was analysed at mRNA and protein level by Northern and Western blotting respectively. Localisation was carried out using immunocytochemistry. MeAIB transcellular transport was significantly (P < 0.05) increased by incubation of the cells in amino acid-depleted medium for 1 h, and longer incubation times caused further increases in the rate of transfer. However, the initial response was not accompanied by an increase in SNAT2 mRNA; this occurred only after 3 h and further increased for the rest of the 6-h incubation. Similarly, it took several hours for a significant increase in SNAT2 protein expression. In contrast, relocalisation of existing SNAT2 transporters occurred within 30 min of amino acid restriction and continued throughout the 6-h incubation. When the cells were incubated in medium with even lower amino acid levels (without non-essential plus 0.5 x essential amino acids), SNAT2 mRNA levels showed further significant (P < 0.0001) up-regulation. However, incubation of cells in depleted medium for 6 h caused a significant (P = 0.014) decrease in the expression of SNAT1 mRNA. System L type amino acid transporter 2 (LAT2) expression was not changed by amino acid restriction, indicating that the responses seen in the system A transporters were not a general cell response. These data have shown that placental cells adapt in vitro to nutritional stress and have identified the physiological, biochemical and genomic mechanisms involved.

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.

In vitro modulation of L-arginine transport in trophoblast cells by glucose

BACKGROUND

The uptake of the semi-essential amino acid, L-arginine, into trophoblast cells was measured with the aim of determining the effect of different glucose concentrations on L-arginine influx kinetics.

METHODS

This study used a novel superfused microcarrier culture system of BeWo cells (an established choriocarcinoma cell line with many characteristics of normal human trophoblast) and a rapid, paired-tracer dilution technique to measure unidirectional influx into the cells.

RESULTS

At 10 mmol L-1 D-glucose, L-arginine unidirectional influx across the microvillous border of the cells was saturable with a Km of 1.14 +/- 0.14 mmol L-1 and a Vmax of 121. 36 +/- 5.89 nmol mg-1 protein min-1. When cells were preincubated for 24 h in the presence of 30 mmol L-1 D-glucose, there was a significant increase in the Vmax for L-arginine of nearly 30%. Similarly, preincubation in the presence of 1 mmol L-1 D-glucose and 12.5 mIU mL-1 human insulin reduced the Km for L-arginine influx by over 55%.

CONCLUSION

These data suggest that the modulation of placental transport of L-arginine by glucose and insulin could contribute to the fetal macrosomia observed in diabetic mothers.

Transporters for cationic amino acids in animal cells: discovery, structure, and function

The structure and function of the four cationic amino acid transporters identified in animal cells are discussed. The systems differ in specificity, cation dependence, and physiological role. One of them, system y+, is selective for cationic amino acids, whereas the others (B[0,+], b[0,+], and y+ L) also accept neutral amino acids. In recent years, cDNA clones related to these activities have been isolated. Thus two families of proteins have been identified: 1) CAT or cationic amino acid transporters and 2) BAT or broad-scope transport proteins. In the CAT family, three genes encode for four different isoforms [CAT-1, CAT-2A, CAT-2(B) and CAT-3]; these are approximately 70-kDa proteins with multiple transmembrane segments (12-14), and despite their structural similarity, they differ in tissue distribution, kinetics, and regulatory properties. System y+ is the expression of the activity of CAT transporters. The BAT family includes two isoforms (rBAT and 4F2hc); these are 59- to 78-kDa proteins with one to four membrane-spanning segments, and it has been proposed that these proteins act as transport regulators. The expression of rBAT and 4F2hc induces system b[0,+] and system y+ L activity in Xenopus laevis oocytes, respectively. The roles of these transporters in nutrition, endocrinology, nitric oxide biology, and immunology, as well as in the genetic diseases cystinuria and lysinuric protein intolerance, are reviewed. Experimental strategies, which can be used in the kinetic characterization of coexpressed transporters, are also discussed.

Demonstration of system y+L activity on the basal plasma membrane surface of rat placenta and developmentally regulated expression of 4F2HC mRNA

Na(+)-independent cationic amino acid transport in the rat placenta occurs by leucine-sensitive and leucine-insensitive pathways. The ontogeny of these transport mechanisms within the rat placenta has been described recently. To assign the leucine-inhibitable portion of uptake definitively the uptake of [3H]arginine was studied in the presence of both BCH (to inhibit system Bo,+) and varied concentrations of leucine. Uptake of arginine into basal-enriched membrane vesicles derived from rat placenta was, in the presence of sodium, inhibited by micromolar concentrations of leucine, consistent with assignment of this activity to system y+L. In contrast, the majority of arginine uptake into apical-enriched membrane vesicles was leucine insensitive. Messenger RNA derived from rat placenta at days 14, 16, 18 and 20 of gestation was hybridized with full-length rat cDNA probes against NBAT and 4F2HC (thought to encode proteins associated with system bo,+ and y+L activities, respectively). No NBAT mRNA was detected, whereas 4F2HC mRNA was present at all gestational stages, increasing 12-fold over the last third of gestation. It is concluded that system y+L is present in the basal plasma membrane of the rat placenta syncytium and is subject to developmental regulation by a mechanism that alters the steady content of 4F2HC mRNA.

Identification of two leucine-sensitive lysine transport activities in human placental basal membrane

The recent demonstration of multiple high-affinity leucine-sensitive cationic transport systems prompted this investigation of their role in lysine uptake in basal cell membrane. Transport of lysine by basal membrane was saturable at both 22 and 37 degrees C and linear in time to 1 min and 30 sec, respectively. At 22 degrees C, at least two systems were active. The portion of uptake inhibited by the sulphydryl binding reagent N-ethylmaleimide (NEM) but not by leucine in the absence of sodium had a high K(m) and high Vmax and was attributed to system y+. NEM-insensitive uptake was fitted by a one-system model with K(m) (+/- s.e.) of 4 +/- 1 microM and a Vmax of 0.9 +/- 0.1 pmol/mg protein/min. This component was completely inhibited by leucine in the absence of sodium but not by glutamine in the presence of sodium. Therefore, it was attributed to system bo,+. At 37 degrees C, at least three systems were active. For essentially the same reasons as above the NEM inhibitable uptake was attributed to system y+. NEM-insensitive uptake was fitted by a one-system model with K(m) of 26 +/- 7 microM and Vmax of 11.1 +/- 2.8 pmol/mg protein/30 sec. Inhibition studies, however, indicated its heterogeneity. NEM-insensitive saturable uptake was only partially inhibited by either leucine in the absence of sodium (system bo,+) or by glutamine in the presence of sodium (system y+L). It is concluded that the NEM-insensitive portion of lysine uptake at 37 degrees C represents activity of both system bo,+ and the temperature-sensitive system y+L. As a previous investigation indicates, only one of these (system y+L) is present in the more specialized microvillous membrane. The demonstration of functional differences in the high affinity leucine transporters of basal and microvillous membrane in this and our previous investigations suggest that the two membranes possess different transport or modifier proteins.

Placental amino acid transport

Normal fetal growth and development depend on a continuous supply of amino acids from the mother to the fetus. The placenta is responsible for the transfer of amino acids between the two circulations. The human placenta is hemomonochorial, meaning that the maternal and fetal circulations are separated by a single layer of polarized epithelium called the syncytiotrophoblast, which is in direct contact with maternal blood. Transport proteins located in the microvillous and basal membranes of the syncytiotrophoblast are the principal mechanism for transfer from maternal blood to fetal blood. Knowledge of the function and regulation of syncytiotrophoblast amino acid transporters is of great importance in understanding the mechanism of placental transport and potentially improving fetal and newborn outcomes. The development of methods for the isolation of microvillous and basal membrane vesicles from human placenta over the past two decades has contributed greatly to this understanding. Now a primary cultured trophoblast model is available to study amino acid transport and regulation as the cells differentiate. The types of amino acid transporters and their distribution between the syncytiotrophoblast microvillous and basal membranes are somewhat unique compared with other polarized epithelia. These differences may reflect the unusual circumstance of this epithelium that is exposed to blood on both sides. The current state of knowledge as to the types of transport systems present in syncytiotrophoblast, their regulation, and the effects of maternal consumption of drugs on transport are discussed.

Lysine uptake by human placental microvillous membrane: comparison of system y+ with basal membrane

Transport of lysine by microvillous membranes was investigated by characterization of L-[3H]lysine uptake in membrane vesicles isolated from human placentas. At least one Na(+)-independent system was observed at 22 degrees C and two systems at 37 degrees C. Lysine concentration dependence data were fit by a one- or two-system model with a Michaelis-Menten constant (Km) of 124 +/- 28 microM and a maximum velocity (Vmax) of 33.1 +/- 7.7 pmol.mg protein-1.min-1 at 22 degrees C and with Km values of 1 +/- 0.6 and 245 +/- 51 microM and Vmax values of 0.14 +/- 0.07 and 45.8 +/- 8.7 pmol.mg protein-1.30 s-1 at 37 degrees C. In the presence of N-ethylmaleimide, the uptake (37 degrees C) data were fit by a one-system model with kinetic parameters similar to the lower Km system. Uptake of L-lysine in the absence of Na+ was inhibited completely by L-arginine, L-histidine, and L-homoarginine. In the presence of Na+, uptake was inhibited completely by these same three amino acids and L-leucine but only partially by other neutral amino acids. To compare directly microvillous and basal membrane from the same placenta, we examined the inhibition of 20 microM lysine uptake in the presence of Na+. Inhibition by L-leucine was similar in the two membranes. However, L-homoserine, L-alanine, and L-phenylalanine over a wide concentration range inhibited substantially less in microvillous (at both temperatures) than in basal membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogeny of cationic amino acid transport systems in rat placenta

Gestational regulation of the placental transfer of amino acids from maternal to fetal circulations is essential for the proper development of the fetus. The cationic amino acid transport systems of the microvillous (maternal facing) and basal (fetal facing) membranes of the rat placental syncytiotrophoblast were examined. Inhibition analysis documented the presence of three kinetically distinct cationic amino acid transport mechanisms: a single Na(+)-dependent mechanism in the microvillous membrane, which increased in activity from 14 to 20 days gestation but was absent from the basal membrane throughout the entire gestational period (system Bo,+), and two Na(+)-independent transport systems in both membrane domains, one that is completely inhibited by leucine, which increased in activity in both the microvillous and basal membrane domains, and the other that is leucine insensitive, which remained fairly constant in the basal membrane and increased throughout gestation in the microvillous membrane (system y1+). Northern analysis with the system y1+ cDNA revealed a specific band of approximately 7.4-7.9 kb, which increased with increasing gestational age.

Cis-inhibition and trans-stimulation of cationic amino acid transport in the perfused rat pancreas

Transport of cationic amino acids in the isolated perfused rat pancreas was studied using dual-isotope dilution techniques. At 50 microM substrate concentration, unidirectional tracer uptakes for L-arginine (56 +/- 1%), L-lysine (49 +/- 2%), and L-ornithine (44 +/- 3%) were followed by rapid tracer efflux. In the presence of Na+, influx of L-arginine [Michaelis constant (Km) = 1.74 +/- 0.15 mM, maximum velocity (Vmax) = 1.97 +/- 0.07 mumol.min-1.g-1] and L-lysine (Km = 2.48 +/- 0.17 mM, Vmax = 2.42 +/- 0.08 mumol.min-1.g-1) was mediated by a common transport system, sensitive to cis-inhibition by L-ornithine, 2,4-L-diaminobutyric acid, D-lysine, and D-arginine. Substrates for system A [alpha-(methylamino)isobutyric acid] and an anionic carrier (L-aspartate) were poor cis-inhibitors of L-arginine entry. Removal of Na+ resulted in a 40% reduction in cationic amino acid influx. After cell loading (20 min), L-[3H]-lysine cleared predominantly from a slowly exchanging pool with a rate constant of 5.97 +/- 0.67 min. An influx/efflux permeability ratio of 14.5 +/- 1.6 was determined, and efflux of L-lysine was trans-stimulated by vascular challenges with cationic or large neutral amino acids. The specificity, relative Na+ independence, and exchange properties of this saturable cationic amino acid transporter in the pancreatic epithelium resemble those reported for system y+ in cultured fibroblasts and hepatocytes.

Two cationic amino acid transport systems in human placental basal plasma membranes

Transport of cationic amino acids in basal (fetal facing) plasma membranes was investigated by characterization of L-[3H]lysine and L-[3H]arginine uptake in membrane vesicles isolated from term human placentas. At least two Na(+)-independent systems were present. Lysine concentration dependence data were fit by a two-system model with Km values of 1.0 +/- 0.8 and 223 +/- 57 microM and Vmax values of 0.06 +/- 0.03 and 24.0 +/- 5.8 pmol.mg protein-1.min-1. In the presence of either 10 mM L-leucine or Na+ plus 10 mM L-homoserine, the data were fit by single system models with kinetic parameters similar to the higher and lower Km systems seen in the absence of inhibitors. Uptake of 10 or 20 microM L-lysine in the absence of Na+ showed the higher Km system was inhibited completely by L-arginine, L-homoarginine, and L-histidine. In the presence of Na+, the higher Km system was inhibited completely by L-alanine, L-homoserine, L-leucine, L-phenylalanine, and L-norleucine. The lower Km system was inhibited completely by L-arginine, L-homoarginine, L-histidine, L-leucine, and L-methionine. Time course studies of uptake demonstrated that uptake by either system alone filled the total vesicular space. The basal membrane of human placental syncytiotrophoblast possesses two transport systems for lysine and arginine, resembling the ubiquitous y+ system and the bo,+ system previously described in mouse blastocysts. The higher Vmax of the y+ system suggests that in utero it may mediate transfer of cationic amino acids from the syncytiotrophoblast to the fetus. The role of the high-affinity low-capacity bo,+ system remains to be determined.

Characterization of amino acid transport systems in human placental basal membrane vesicles

The amino acid transport systems have been characterized in basal membrane vesicles prepared from human full-term placental syncytiotrophoblasts. Transport of amino acids across basal membranes occurred via passive diffusion and Na(+)-independent and Na(+)-dependent carrier-mediated systems. Passive diffusion was responsible for a substantial fraction of transport. L-Glutamate and alpha-(methylamino)isobutyrate were transported only Na(+)-independently, while the transport of L-alanine was dependent solely on an Na+ gradient from the outside to the inside of the vesicles. L-Methionine, L-leucine, glycine and L-proline transport were supported by both Na(+)-independent and Na(+)-dependent systems. L-Lysine transport was decreased in the presence of cations, an inwardly directed Na+ gradient was much more effective than a K+ gradient at slowing L-lysine transport. A cross-inhibition analysis of these amino acids indicates that at least three Na(+)-independent and five Na(+)-dependent carrier-mediated systems exist in the human placental syncytiotrophoblast basal membranes. One Na(+)-independent system interacts with all substrates tested. Another Na(+)-independent system carries glycine, L-methionine, L-leucine and L-lysine; it is sensitive to L-glutamate, but not to L-proline or alpha-(methylamino)isobutyrate. The third system is selective for L-lysine, which is inhibited by L-methionine, glycine and L-leucine, but inaccessible to L-glutamate, L-proline and alpha-(methylamino)isobutyrate. One Na(+)-dependent system carries L-alanine, glycine, L-methionine and L-leucine, and it is sensitive to L-proline. The second system mediates transport of L-alanine, glycine, L-methionine and L-proline, but is not sensitive to L-leucine. The third system carries L-alanine, glycine and L-proline, and is inaccessible to L-methionine and L-leucine. The fourth system is responsible for L-methionine and L-leucine; it is sensitive to L-alanine and glycine, but not to L-proline. The fifth system is selective for L-proline.