Glutamate cycling may drive organic anion transport on the basal membrane of human placental syncytiotrophoblast

Identification of HTRA4 as a Transcriptional Target of p63 in Trophoblast

The placenta plays a crucial role in pregnancy success. ΔNp63α (p63), a transcription factor from the TP53 family, is highly expressed in villous cytotrophoblasts (CTBs), the epithelial stem cells of the human placenta, and is involved in CTB maintenance and differentiation. We examined the mechanisms of action of p63 by identifying its downstream targets. Gene expression changes were evaluated following overexpression and knockdown of p63 in the JEG3 choriocarcinoma cell line, using microarray-based RNA profiling. High-temperature requirement A4 (HTRA4), a placenta-specific serine protease involved in trophoblast differentiation and altered in preeclampsia, was identified as a gene reciprocally regulated by p63, and its expression was characterized in primary human placental tissues by RNA-sequencing and in situ hybridization. Potential p63 DNA-binding motifs were identified in the HTRA4 promoter, and p63 occupancy at some of these sites was confirmed using chromatin immunoprecipitation, followed by quantitative PCR in both JEG3 and trophoblast stem cells. These data begin to identify members of the transcriptional network downstream of p63, thus laying the groundwork for probing mechanisms by which this important transcription factor regulates trophoblast stemness and differentiation.

Association between aberrant amino acid metabolism and nonchromosomal modifications fetal structural anomalies: A cohort study

BACKGROUND

More than half of the cases of fetal structural anomalies have no known cause with standard investigations like karyotype testing and chromosomal microarray. The differential metabolic profiles of amniotic fluid (AF) and maternal blood may reveal valuable information about the physiological processes of fetal development, which may provide valuable biomarkers for fetal health diagnostics.

METHODS

This cohort study of singleton-pregnant women had indications for amniocentesis, including structural anomalies and a positive result from maternal serum screening or non-invasive prenatal testing, but did not have any positive abnormal karyotype or chromosomal microarray analysis results. A total of 1580 participants were enrolled between June 2021 and March 2022. Of the 1580 pregnant women who underwent amniocentesis, 294 were included in the analysis. There were 137 pregnant women in the discovery cohort and 157 in the validation cohort.

RESULTS

High-coverage untargeted metabolomic analysis of AF revealed distinct metabolic signatures with 321 of the 602 metabolites measured (53%) (false discovery rate, q < 0.005), among which amino acids predominantly changed in structural anomalies. Targeted metabolomics identified glutamate and glutamine as novel predictive markers for structural anomalies, their vital role was also confirmed in the validation cohort with great predictive ability, and the area under the receiver operating characteristic curves (AUCs) were 0.862 and 0.894 respectively. And AUCs for glutamine/glutamate were 0.913 and 0.903 among the two cohorts.

CONCLUSIONS

Our results suggested that the aberrant glutamine/glutamate metabolism in AF is associated with nonchromosomal modificantions fetal structural anomalies. Based on our findings, a novel screening method could be established for the nonchromosomal modificantions fetal structural anomalies. And the results also indicate that monitoring fetal metabolic conditions (especially glutamine and glutamine metabolism) may be helpful for antenatal diagnosis and therapy.

Role of Uptake Transporters OAT4, OATP2A1, and OATP1A2 in Human Placental Bio-disposition of Pravastatin

Pravastatin is currently under evaluation for prevention of preeclampsia. Factors contributing to placental disposition of pravastatin are important in assessment of potential undesirable fetal effects. The purpose of this study was to identify the uptake transporters that contribute to the placental disposition of pravastatin. Our data revealed the expression of organic anion transporting polypeptide 1A2 (OATP1A2) and OATP2A1 in the apical, and OATP2B1 and OATP5A1 in the basolateral membranes of the placenta, while organic anion transporter 4 (OAT4) exhibited higher expression in basolateral membrane but was detected in both membranes. Preloading placental membrane vesicles with glutarate increased the uptake of pravastatin suggesting involvement of glutarate-dependent transporters such as OAT4. In the HEK293 cells overexpressing individual uptake transporters, OATP2A1, OATP1A2 and OAT4 were determined to accept pravastatin as a substrate at physiological pH, while the uptake of pravastatin by OATP2B1 (known to interact with pravastatin at acidic pH) and OATP5A1 was not detected at pH 7.4. These findings led us to propose that OATP1A2 and OATP2A1 are responsible for the placental uptake of pravastatin from the maternal circulation, while OAT4 mediates the passage of the drug across placental basolateral membrane in the fetal-to-maternal direction.

Overview of Drug Transporters in Human Placenta

The transport of drugs across the placenta is a point of great importance in pharmacotherapy during pregnancy. However, the knowledge of drug transport in pregnancy is mostly based on experimental clinical data, and the underlying biological mechanisms are not fully understood. In this review, we summarize the current knowledge of drug transporters in the human placenta. We only refer to human data since the placenta demonstrates great diversity among species. In addition, we describe the experimental models that have been used in human placental transport studies and discuss their availability. A better understanding of placental drug transporters will be beneficial for the health of pregnant women who need drug treatment and their fetuses.

N-acetylcysteine, xCT and suppression of Maxi-chloride channel activity in human placenta

INTRODUCTION

Placental oxidative stress features in pregnancy pathologies but in clinical trials antioxidant supplementation has not improved outcomes. N-acetylcysteine (NAC) stimulates glutathione production and is proposed as a therapeutic agent in pregnancy. However, key elements of N-acetylcysteine biology, including its cellular uptake mechanism, remains unclear. This study explores how the cystine/glutamate transporter xCT may mediate N-acetylcysteine uptake and how N-acetylcysteine alters placental redox status.

METHODS

The involvement of xCT in NAC uptake by the human placenta was studied in perfused placenta and Xenopus oocytes. The effect of short-term N-acetylcysteine exposure on the placental villous proteome was determined using LC-MS. The effect of N-acetylcysteine on Maxi-chloride channel activity was investigated in perfused placenta, villous fragments and cell culture.

RESULTS

Maternoplacental N-acetylcysteine administration stimulated intracellular glutamate efflux suggesting a role of the exchange transporter xCT, which was localised to the microvillous membrane of the placental syncytiotrophoblast. Placental exposure to a bolus of N-acetylcysteine inhibited subsequent activation of the redox sensitive Maxi-chloride channel independently of glutathione synthesis. Stable isotope quantitative proteomics of placental villi treated with N-acetylcysteine demonstrated changes in pathways associated with oxidative stress, apoptosis and the acute phase response.

DISCUSSION

This study suggests that xCT mediates N-acetylcysteine uptake into the placenta and that N-acetylcysteine treatment of placental tissue alters the placental proteome while regulating the redox sensitive Maxi-chloride channel. Interestingly N-acetylcysteine had antioxidant effects independent of the glutathione pathway. Effective placental antioxidant therapy in pregnancy may require maintaining the balance between normalising redox status without inhibiting physiological redox signalling.

Nutrition and Metabolic Adaptations in Physiological and Complicated Pregnancy: Focus on Obesity and Gestational Diabetes

Pregnancy offers a window of opportunity to program the future health of both mothers and offspring. During gestation, women experience a series of physical and metabolic modifications and adaptations, which aim to protect the fetus development and are closely related to both pre-gestational nutritional status and gestational weight gain. Moreover, pre-gestational obesity represents a challenge of treatment, and nowadays there are new evidence as regard its management, especially the adequate weight gain. Recent evidence has highlighted the determinant role of nutritional status and maternal diet on both pregnancy outcomes and long-term risk of chronic diseases, through a transgenerational flow, conceptualized by the Development Origin of Health and Diseases (Dohad) theory. In this review we will analyse the physiological and endocrine adaptation in pregnancy, and the metabolic complications, thus the focal points for nutritional and therapeutic strategies that we must early implement, virtually before conception, to safeguard the health of both mother and progeny. We will summarize the current nutritional recommendations and the use of nutraceuticals in pregnancy, with a focus on the management of pregnancy complicated by obesity and hyperglycemia, assessing the most recent evidence about the effects of ante-natal nutrition on the long-term, on either maternal health or metabolic risk of the offspring.

Human placental uptake of glutamine and glutamate is reduced in fetal growth restriction

Fetal growth restriction (FGR) is a significant risk factor for stillbirth, neonatal complications and adulthood morbidity. Compared with those of appropriate weight for gestational age (AGA), FGR babies have smaller placentas with reduced activity of amino acid transporter systems A and L, thought to contribute to poor fetal growth. The amino acids glutamine and glutamate are essential for normal placental function and fetal development; whether transport of these is altered in FGR is unknown. We hypothesised that FGR is associated with reduced placental glutamine and glutamate transporter activity and expression, and propose the mammalian target of rapamycin (mTOR) signaling pathway as a candidate mechanism. FGR infants [individualised birth weight ratio (IBR) < 5th centile] had lighter placentas, reduced initial rate uptake of ¹⁴C-glutamine and ¹⁴C-glutamate (per mg placental protein) but higher expression of key transporter proteins (glutamine: LAT1, LAT2, SNAT5, glutamate: EAAT1) versus AGA [IBR 20th-80th]. In further experiments, in vitro exposure to rapamycin inhibited placental glutamine and glutamate uptake (24 h, uncomplicated pregnancies) indicating a role of mTOR in regulating placental transport of these amino acids. These data support our hypothesis and suggest that abnormal glutamine and glutamate transporter activity is part of the spectrum of placental dysfunction in FGR.

Effects of Bisphenol A on expression of genes related to amino acid transporters, insulin- like growth factor, aquaporin and amino acid release by porcine trophectoderm cells

The peri-implantation period of pregnancy is critical for conceptus development, implantation, and signaling for establishment of pregnancy. This study evaluated the effects of bisphenol A (BPA) on proliferation, adhesion, and migration of porcine trophectoderm (pTr2) cells, expression of transporters of arginine and synthesis of amino acids. All concentrations of BPA decreased proliferation and adhesion of pTr2 cells after 96 h compared to the control group. Lower concentrations of BPA (1 × 10^-9, 1 × 10^-8, 10^-7M) increased (P < 0.05), but higher concentrations of BPA (1 × 10^-5, 1 × 10^-4 M) decreased migration of pTr2 cells. BPA increased expression of SLC7A1 mRNA at lower concentrations (1 × 10^-9 to 1 × 10^-6M) and SL7A6, another cationic acid transporter, at higher concentrations (1 × 10^-5, 1 × 10^-4 M). BPA also down-regulated the expression of IGF1 and IGF1 receptor at concentrations of 1 × 10^-7 to 1 × 10^-4 M compared to the control group. The expression of mRNAs for aquaporins (AQP) 3 and 4 were reduced at all concentrations of BPA, but at lower concentrations of BPA, (1 × 10^-9 to 1 × 10^-8M) expression of AQP9 mRNA increased and the expression of AQP11 was not affected by BPA (P > 0.05). There was an inhibitory effect of BPA on the release of synthesis of asparagine, threonine, taurine, tryptophan, and ornithine into the culture medium by pTr2 cells. Collectively, BPA adversely affected the expression of transporters for cationic amino acids like arginine, as well as AQPs, IGF1, and IGF1R associated with proliferation, migration, and adhesion of pTr2 cells. Those adverse effects would likely increase pregnancy losses during the peri-implantation period of pregnancy.

Interplay of drug transporters P-glycoprotein (MDR1), MRP1, OATP1A2 and OATP1B3 in passage of maraviroc across human placenta

Special attention is required when pharmacological treatment is indicated for a pregnant woman. P-glycoprotein (MDR1) is a well-known transporter localized in the maternal blood-facing apical membrane of placental syncytiotrophoblast and is considered to play an important role in protecting the developing fetus. Maraviroc, a MDR1 substrate that is registered for treatment of HIV infection, shows a low toxicity profile, suggesting favorable tolerability also if administered to pregnant women. Nevertheless, there is only poor understanding to date regarding the extent to which it permeates across the placental barrier and what are the transport mechanisms involved. Endeavoring to clarify the passage of maraviroc across placenta, we used in this study the method of closed-circuit perfusion of maraviroc across human placental cotyledon. The data obtained confirmed slight involvement of MDR1, but they also suggest possible interaction with other transport system(s) working in the opposite direction from that of MDR1. Complementary in vitro studies, including cellular experiments on choriocarcinoma BeWo cells as well as transporter-overexpressing MDCKII and A431 cell lines and accumulation in placental fresh villous fragments, revealed maraviroc transport by MRP1, OATP1A2, and OATP1B3 transporters. Based on mRNA expression data in the placental tissue, isolated trophoblasts, and fetal endothelial cells, especially MRP1 and OATP1A2 seem to play a crucial role in cooperatively driving maraviroc into placental tissue. By the example of maraviroc, we show here the important interplay of transporters in placental drug handling and its possibility to overcome the MDR1-mediated efflux.

Glibenclamide transfer across the perfused human placenta is determined by albumin binding not transporter activity

The placenta mediates the transfer of maternal nutrients into the fetal circulation while removing fetal waste products, drugs and environmental toxins that may otherwise be detrimental to fetal development. This study investigated the role of drug transporters and protein binding in the transfer of the antidiabetic drug glibenclamide across the human placental syncytiotrophoblast using placental perfusion experiments and computational modeling. In the absence of albumin, placental glibenclamide uptake from the fetal circulation was not affected by competitive inhibition with bromosulphothalein (BSP), indicating that OATP2B1 does not mediate placental glibenclamide uptake from the fetus. In the presence of maternal and fetal albumin, BSP increased placental glibenclamide uptake from the fetal circulation by displacing glibenclamide from BSA, increasing the free fraction of glibenclamide driving diffusive transport. The P-gp and BCRP inhibitor GF120918 did not affect placental glibenclamide uptake from the maternal circulation and as such this study did not find any evidence for the apical efflux transporters in placental glibenclamide transfer. Computational modeling confirmed that albumin binding and not transporter activity, is the dominant factor in the transfer of glibenclamide across the human placenta. The effect of BSP binding to albumin on promoting the diffusive transfer of glibenclamide highlights the importance of drug-protein binding interactions and their interpretation using computational modeling.

Contributions of Drug Transporters to Blood-Placental Barrier

The placenta is the only organ linking two different individuals, mother and fetus, termed as blood-placental barrier. The functions of the blood-placental barrier are to regulate material transfer between the maternal and fetal circulation. The main functional units are the chorionic villi within which fetal blood is separated by only three or four cell layers (placental membrane) from maternal blood in the surrounding intervillous space. A series of drug transporters such as P-glycoprotein (P-GP), breast cancer resistance protein (BCRP), multidrug resistance-associated proteins (MRP1, MRP2, MRP3, MRP4, and MRP5), organic anion-transporting polypeptides (OATP4A1, OATP1A2, OATP1B3, and OATP3A1), organic anion transporter 4 (OAT4), organic cation transporter 3 (OCT3), organic cation/carnitine transporters (OCTN1 and OCTN2), multidrug and toxin extrusion 1 (MATE1), and equilibrative nucleoside transporters (ENT1 and ENT2) have been demonstrated on the apical membrane of syncytiotrophoblast, some of which also expressed on the basolateral membrane of syncytiotrophoblast or fetal capillary endothelium. These transporters are involved in transport of most drugs in the placenta, in turn, affecting drug distribution in fetus. Moreover, expressions of these transporters in the placenta often vary along with the gestational ages and are also affected by pathophysiological factor. This chapter will mainly illustrate function and expression of these transporters in placentas, their contribution to drug distribution in fetus, and their clinical significance.

Evidence of adaptation of maternofetal transport of glutamine relative to placental size in normal mice, and in those with fetal growth restriction

Key points

Fetal growth restriction (FGR) is a major risk factor for stillbirth and has significant impact upon lifelong health.

A small, poorly functioning placenta, as evidenced by reduced transport of nutrients to the baby, underpins FGR. It remains unclear how a small but normal placenta differs from the small FGR placenta in terms of ability to transfer nutrients to the fetus.

Placental transport of glutamine and glutamate, key amino acids for fetal growth, was assessed in normal mice and those with FGR.

Glutamine and glutamate transport was greater in the lightest versus heaviest placenta in a litter of normally grown mice. Placentas of mice with FGR had increased transport capacity in mid‐pregnancy, but this adaptation was insufficient in late pregnancy.

Placental adaptations, in terms of increased nutrient transport (per gram) to compensate for small size, appear to achieve appropriate fetal growth in normal pregnancy. Failure of this adaptation might contribute to FGR.

Abstract

Fetal growth restriction (FGR), a major risk factor for stillbirth, and neonatal and adulthood morbidity, is associated with reduced placental size and decreased placental nutrient transport. In mice, a small, normal placenta increases its nutrient transport, thus compensating for its reduced size and maintaining normal fetal growth. Whether this adaptation occurs for glutamine and glutamate, two key amino acids for placental metabolism and fetal growth, is unknown. Additionally, an assessment of placental transport of glutamine and glutamate between FGR and normal pregnancy is currently lacking. We thus tested the hypothesis that the transport of glutamine and glutamate would be increased (per gram of tissue) in a small normal placenta [C57BL6/J (wild‐type, WT) mice], but that this adaptation fails in the small dysfunctional placenta in FGR [insulin‐like growth factor 2 knockout (P0) mouse model of FGR]. In WT mice, comparing the lightest versus heaviest placenta in a litter, unidirectional maternofetal clearance (K_mf) of ¹⁴C‐glutamine and ¹⁴C‐glutamate (^glutamineK_mf and ^glutamateK_mf) was significantly higher at embryonic day (E) 18.5, in line with increased expression of LAT1, a glutamine transporter protein. In P0 mice, ^glutamineK_mf and ^glutamateK_mf were higher (P0 versus wild‐type littermates, WTL) at E15.5. At E18.5, ^glutamineK_mf remained elevated whereas ^glutamateK_mf was similar between groups. In summary, we provide evidence that ^glutamineK_mf and ^glutamateK_mf adapt according to placental size in WT mice. The placenta of the growth‐restricted P0 fetus also elevates transport capacity to compensate for size at E15.5, but this adaptation is insufficient at E18.5; this may contribute to decreased fetal growth.

Fetal growth restriction (FGR) is a major risk factor for stillbirth and has significant impact upon lifelong health.

A small, poorly functioning placenta, as evidenced by reduced transport of nutrients to the baby, underpins FGR. It remains unclear how a small but normal placenta differs from the small FGR placenta in terms of ability to transfer nutrients to the fetus.

Placental transport of glutamine and glutamate, key amino acids for fetal growth, was assessed in normal mice and those with FGR.

Glutamine and glutamate transport was greater in the lightest versus heaviest placenta in a litter of normally grown mice. Placentas of mice with FGR had increased transport capacity in mid‐pregnancy, but this adaptation was insufficient in late pregnancy.

Placental adaptations, in terms of increased nutrient transport (per gram) to compensate for small size, appear to achieve appropriate fetal growth in normal pregnancy. Failure of this adaptation might contribute to FGR.

A systems perspective on placental amino acid transport

Placental amino acid transfer is a complex process that is essential for fetal development. Impaired amino acid transfer causes fetal growth restriction, which may have lifelong health consequences. Transepithelial transfer of amino acids across the placental syncytiotrophoblast requires accumulative, exchange and facilitated transporters on the apical and basal membranes to work in concert. However, transporters alone do not determine amino acid transfer and factors that affect substrate availability, such as blood flow and metabolism, may also become rate-limiting for transfer. In order to determine the rate-limiting processes, it is necessary to take a systems approach which recognises the interdependence of these processes. New technologies have the potential to deliver targeted interventions to the placenta and help poorly growing fetuses. While many factors are necessary for amino acid transfer, novel therapies need to target the rate-limiting factors if they are going to be effective. This review will outline the factors which determine amino acid transfer and describe how they become interdependent. It will also highlight the role of computational modelling as a tool to understand this process.

Estrone sulphate uptake by the microvillous membrane of placental syncytiotrophoblast is coupled to glutamate efflux

Organic anion transporters (OATs) and organic anion transporting polypeptides (OATPs) are transport proteins that mediate exchange of metabolites, hormones and waste products. Directional transport by these transporters can occur when exchange is coupled to the gradients of other substrates. This study investigates whether the activity of OATP4A1 and OATP2A1 on the maternal facing microvillus membrane of the placental syncytiotrophoblast is coupled to the glutamate gradient. OAT and OATP transporter proteins were over expressed in Xenopus oocytes to study their transport characteristics. Further transport studies were performed in term human placental villous fragments. Xenopus oocytes expressing OATP4A1 mediated glutamate uptake. No glutamate transport was observed in oocytes expressing OAT1, OAT3, OAT7 or OATP2A1. In oocytes expressing OATP4A1, uptake of estrone sulphate, thyroid hormones T3 and T4 and the bile acid taurocholate stimulated glutamate efflux. In term placental villous fragments addition of estrone sulphate and taurocholate trans-stimulated glutamate efflux. Coupling of OATP4A1 to the glutamate gradient may drive placental uptake of estrone-sulphate and thyroid hormone while also facilitating uptake of potentially harmful bile acids. In contrast, if OATP2A1 is not coupled to a similar gradient, it may function more effectively as an efflux transporter, potentially mediating efflux of prostaglandins to the mother. This study provides further evidence for glutamate as an important counter-ion driving transport into the placenta.

•

OATP4A1 and OATP2A1 are present on the maternal facing surface of the placenta.

•

OATP4A1, but not OATP2A1, activity was coupled to glutamate efflux.

•

Placental estrone-sulphate and thyroid hormone uptake is coupled to glutamate efflux.

•

Glutamate acts as a counter-ion for OATP4A1, driving transport into the placenta.

Placental control of drug delivery

The placenta serves as the interface between the maternal and fetal circulations and regulates the transfer of oxygen, nutrients, and waste products. When exogenous substances are present in the maternal bloodstream-whether from environmental contact, occupational exposure, medication, or drug abuse-the extent to which this exposure affects the fetus is determined by transport and biotransformation processes in the placental barrier. Advances in drug delivery strategies are expected to improve the treatment of maternal and fetal diseases encountered during pregnancy.

Placental transporter localization and expression in the Human: the importance of species, sex, and gestational age differences†

The placenta is a critical organ during pregnancy, essential for the provision of an optimal intrauterine environment, with fetal survival, growth, and development relying on correct placental function. It must allow nutritional compounds and relevant hormones to pass into the fetal bloodstream and metabolic waste products to be cleared. It also acts as a semipermeable barrier to potentially harmful chemicals, both endogenous and exogenous. Transporter proteins allow for bidirectional transport and are found in the syncytiotrophoblast of the placenta and endothelium of fetal capillaries. The major transporter families in the human placenta are ATP-binding cassette (ABC) and solute carrier (SLC), and insufficiency of these transporters may lead to deleterious effects on the fetus. Transporter expression levels are gestation-dependent and this is of considerable clinical interest as levels of drug resistance may be altered from one trimester to the next. This highlights the importance of these transporters in mediating correct and timely transplacental passage of essential compounds but also for efflux of potentially toxic drugs and xenobiotics. We review the current literature on placental molecular transporters with respect to their localization and ontogeny, the influence of fetal sex, and the relevance of animal models. We conclude that a paucity of information exists, and further studies are required to unlock the enigma of this dynamic organ.

A novel mode of operation of SLC22A11: Membrane insertion of estrone sulfate versus translocation of uric acid and glutamate

Estrone sulfate alias estrone-3-sulfate (E3S) is considerably larger and much more hydrophobic than typical substrates of SLC22 transporters. It is puzzling that many otherwise unrelated transporters have been reported to transport E3S. Here we scrutinized the mechanism of transport of E3S by SLC22A11 (alias OAT4), by direct comparison with uric acid (UA), an important physiological substrate. Heterologous expression of SLC22A11 in human 293 cells gave rise to a huge unidirectional efflux of glutamate (Glu) and aspartate, as determined by LC-MS/MS. The uptake of E3S was 20-fold faster than the uptake of UA. Yet, the outward transport of Glu was inhibited by extracellular E3S, but not by UA. The release of E3S after preloading was trans-stimulated by extracellular dehydroepiandrosterone sulfate (DHEAS), but neither by UA nor 6-carboxyfluorescein (6CF). The equilibrium accumulation of E3S was enhanced 3-fold by replacement of chloride with gluconate, but the opposite effect was observed for UA. These results establish that SLC22A11 provides entirely different transport mechanisms for E3S and UA. Therefore, E3S must not be used as a substitute for UA to assay the function of SLC22A11. In equilibrium accumulation experiments, the transporter-mediated uptake was a linear function of the concentration of UA and 6CF. By contrast, in the same concentration range the graph for E3S was hyperbolic. This suggests that SLC22A11 inserts E3S into a small volume with limited capacity, the plasma membrane. Our data support the notion that the reverse process, extraction from the membrane, is also catalyzed by the carrier.