Cannabidiol disrupts tryptophan metabolism in the human term placenta

The increasing use of cannabis during pregnancy raises concerns about its impact on fetal development. While cannabidiol (CBD) shows therapeutic promise, its effects during pregnancy remain uncertain. We investigated CBD's influence on tryptophan (TRP) metabolism in the human placenta. TRP is an essential amino acid that is metabolized via the serotonin and kynurenine (KYN) pathways, which are critical for fetal neurodevelopment. We used human term villous placental explants, an advanced ex vivo model, to study CBD's impact on key TRP metabolic enzymes. In addition, vesicles isolated from the microvillous membrane (MVM) of the human placenta were used to assess CBD's effect on placental serotonin uptake. Explants were exposed to CBD at therapeutic (0.1, 1, 2.5 μg/ml) and non-therapeutic (20 and 40 μg/ml) concentrations to determine its effects on the gene and protein expression of key enzymes in TRP metabolism and metabolite release. CBD upregulated TRP hydroxylase (TPH) and downregulated monoamine oxidase (MAO-A), resulting in reduced levels of 5-hydroxyindoleacetic acid (HIAA). It also downregulated serotonin transporter expression and inhibited serotonin transport across the MVM by up to 60% while simultaneously enhancing TRP metabolism via the kynurenine pathway by upregulating indoleamine-pyrrole 2,3-dioxygenase (IDO-1). Among kynurenine pathway enzymes, kynurenine 3 monooxygenase (KMO) was upregulated while kynurenine aminotransferase 1 (KAT-1) was downregulated; the former is associated with neurotoxic metabolite production, while the latter is linked to reduced neuroprotective metabolite levels. Overall, these results indicate that CBD modulates TRP catabolism in the human placenta, potentially disrupting the tightly regulated homeostasis of the serotonin and KYN pathways.

Pathological shifts in tryptophan metabolism in human term placenta exposed to LPS or poly I:C†

Maternal immune activation during pregnancy is a risk factor for offspring neuropsychiatric disorders. Among the mechanistic pathways by which maternal inflammation can affect fetal brain development and programming, those involving tryptophan (TRP) metabolism have drawn attention because various TRP metabolites have neuroactive properties. This study evaluates the effect of bacterial (lipopolysaccharides/LPS) and viral (polyinosinic:polycytidylic acid/poly I:C) placental infection on TRP metabolism using an ex vivo model. Human placenta explants were exposed to LPS or poly I:C, and the release of TRP metabolites was analyzed together with the expression of related genes and proteins and the functional activity of key enzymes in TRP metabolism. The rate-limiting enzyme in the serotonin pathway, tryptophan hydroxylase, showed reduced expression and functional activity in explants exposed to LPS or poly I:C. Conversely, the rate-limiting enzyme in the kynurenine pathway, indoleamine dioxygenase, exhibited increased activity, gene, and protein expression, suggesting that placental infection mainly promotes TRP metabolism via the kynurenine (KYN) pathway. Furthermore, we observed that treatment with LPS or poly I:C increased activity in the kynurenine monooxygenase branch of the KYN pathway. We conclude that placental infection impairs TRP homeostasis, resulting in decreased production of serotonin and an imbalance in the ratio between quinolinic acid and kynurenic acid. This disrupted homeostasis may eventually expose the fetus to suboptimal/toxic levels of neuroactive molecules and impair fetal brain development.

Developmental expression of catecholamine system in the human placenta and rat fetoplacental unit

Catecholamines norepinephrine and dopamine have been implicated in numerous physiological processes within the central nervous system. Emerging evidence has highlighted the importance of tightly regulated monoamine levels for placental functions and fetal development. However, the complexities of synthesis, release, and regulation of catecholamines in the fetoplacental unit have not been fully unraveled. In this study, we investigated the expression of enzymes and transporters involved in synthesis, degradation, and transport of norepinephrine and dopamine in the human placenta and rat fetoplacental unit. Quantitative PCR and Western blot analyses were performed in early-to-late gestation in humans (first trimester vs. term placenta) and mid-to-late gestation in rats (placenta and fetal brain, intestines, liver, lungs, and heart). In addition, we analyzed the gene expression patterns in isolated primary trophoblast cells from the human placenta and placenta-derived cell lines (HRP-1, BeWo, JEG-3). In both human and rat placentas, the study identifies the presence of only PNMT, COMT, and NET at the mRNA and protein levels, with the expression of PNMT and NET showing gestational age dependency. On the other hand, rat fetal tissues consistently express the catecholamine pathway genes, revealing distinct developmental expression patterns. Lastly, we report significant transcriptional profile variations in different placental cell models, emphasizing the importance of careful model selection for catecholamine metabolism/transport studies. Collectively, integrating findings from humans and rats enhances our understanding of the dynamic regulatory mechanisms that underlie catecholamine dynamics during pregnancy. We identified similar patterns in both species across gestation, suggesting conserved molecular mechanisms and potentially shedding light on shared biological processes influencing placental development.

Metabolomic analysis of the human placenta reveals perturbations in amino acids, purine metabolites, and small organic acids in spontaneous preterm birth

Spontaneous preterm delivery presents one of the most complex challenges in obstetrics and is a leading cause of perinatal morbidity and mortality. Although it is a common endpoint for multiple pathological processes, the mechanisms governing the etiological complexity of spontaneous preterm birth and the placental responses are poorly understood. This study examined placental tissues collected between May 2019 and May 2022 from a well-defined cohort of women who experienced spontaneous preterm birth (n = 72) and healthy full-term deliveries (n = 30). Placental metabolomic profiling of polar metabolites was performed using Ultra-High Performance Liquid Chromatography/Mass Spectrometry (UHPLC/MS) analysis. The resulting data were analyzed using multi- and univariate statistical methods followed by unsupervised clustering. A comprehensive metabolomic evaluation of the placenta revealed that spontaneous preterm birth was associated with significant changes in the levels of 34 polar metabolites involved in intracellular energy metabolism and biochemical activity, including amino acids, purine metabolites, and small organic acids. We found that neither the preterm delivery phenotype nor the inflammatory response explain the reported differential placental metabolome. However, unsupervised clustering revealed two molecular subtypes of placentas from spontaneous preterm pregnancies exhibiting differential enrichment of clinical parameters. We also identified differences between early and late preterm samples, suggesting distinct placental functions in early spontaneous preterm delivery. Altogether, we present evidence that spontaneous preterm birth is associated with significant changes in the level of placental polar metabolites. Dysregulation of the placental metabolome may underpin important (patho)physiological mechanisms involved in preterm birth etiology and long-term neonatal outcomes.

Characterization of a human placental clearance system to regulate serotonin levels in the fetoplacental unit

BACKGROUND

Serotonin (5-HT) is a biogenic monoamine with diverse functions in multiple human organs and tissues. During pregnancy, tightly regulated levels of 5-HT in the fetoplacental unit are critical for proper placental functions, fetal development, and programming. Despite being a non-neuronal organ, the placenta expresses a suite of homeostatic proteins, membrane transporters and metabolizing enzymes, to regulate monoamine levels. We hypothesized that placental 5-HT clearance is important for maintaining 5-HT levels in the fetoplacental unit. We therefore investigated placental 5-HT uptake from the umbilical circulation at physiological and supraphysiological levels as well as placental metabolism of 5-HT to 5-hydroxyindoleacetic acid (5-HIAA) and 5-HIAA efflux from trophoblast cells.

METHODS

We employed a systematic approach using advanced organ-, tissue-, and cellular-level models of the human placenta to investigate the transport and metabolism of 5-HT in the fetoplacental unit. Human placentas from uncomplicated term pregnancies were used for perfusion studies, culturing explants, and isolating primary trophoblast cells.

RESULTS

Using the dually perfused placenta, we observed a high and concentration-dependent placental extraction of 5-HT from the fetal circulation. Subsequently, within the placenta, 5-HT was metabolized to 5-hydroxyindoleacetic acid (5-HIAA), which was then unidirectionally excreted to the maternal circulation. In the explant cultures and primary trophoblast cells, we show concentration- and inhibitor-dependent 5-HT uptake and metabolism and subsequent 5-HIAA release into the media. Droplet digital PCR revealed that the dominant gene in all models was MAO-A, supporting the crucial role of 5-HT metabolism in placental 5-HT clearance.

CONCLUSIONS

Taken together, we present transcriptional and functional evidence that the human placenta has an efficient 5-HT clearance system involving (1) removal of 5-HT from the fetal circulation by OCT3, (2) metabolism to 5-HIAA by MAO-A, and (3) selective 5-HIAA excretion to the maternal circulation via the MRP2 transporter. This synchronized mechanism is critical for regulating 5-HT in the fetoplacental unit; however, it can be compromised by external insults such as antidepressant drugs.

Assessment of the role of nucleoside transporters, P-glycoprotein, breast cancer resistance protein, and multidrug resistance-associated protein 2 in the placental transport of entecavir using in vitro, ex vivo, and in situ methods

The nucleoside analog entecavir (ETV) is a first-line pharmacotherapy for chronic hepatitis B in adult and pediatric patients. However, due to insufficient data on placental transfer and its effects on pregnancy, ETV administration is not recommended for women after conception. To expand knowledge of safety, we focused on evaluating the contribution of nucleoside transporters (NBMPR sensitive ENTs and Na⁺ dependent CNTs) and efflux transporters, P-glycoprotein (ABCB1), breast cancer resistance protein (ABCG2), and multidrug resistance-associated transporter 2 (ABCC2), to the placental kinetics of ETV. We observed that NBMPR and nucleosides (adenosine and/or uridine) inhibited [³H]ETV uptake into BeWo cells, microvillous membrane vesicles, and fresh villous fragments prepared from the human term placenta, while Na⁺ depletion had no effect. Using a dual perfusion study in an open-circuit setup, we showed that maternal-to-fetal and fetal-to-maternal clearances of [³H]ETV in the rat term placenta were decreased by NBMPR and uridine. Net efflux ratios calculated for bidirectional transport studies performed in MDCKII cells expressing human ABCB1, ABCG2, or ABCC2 were close to the value of one. Consistently, no significant decrease in fetal perfusate was observed in the closed-circuit setup of dual perfusion studies, suggesting that active efflux does not significantly reduce maternal-to-fetal transport. In conclusion, ENTs (most likely ENT1), but not CNTs, ABCB1, ABCG2, and ABCC2, contribute significantly to the placental kinetics of ETV. Future studies should investigate the placental/fetal toxicity of ETV, the impact of drug-drug interactions on ENT1, and interindividual variability in ENT1 expression on the placental uptake and fetal exposure to ETV.

Perfused rat term placenta as a preclinical model to investigate placental dopamine and norepinephrine transport

The placenta represents a non-neuronal organ capable of transporting and metabolizing monoamines. Since these bioactive molecules participate in numerous processes essential for placental and fetal physiology, any imbalance in their levels during pregnancy may affect brain development, projecting a higher risk of behavioral disorders in childhood or adulthood. Notably, the monoamine system in the placenta is a target of various psychoactive drugs and can be disrupted in several pregnancy pathologies. As research in pregnant women poses significant ethical restrictions, animal models are widely employed to study monoamine homeostasis as a mechanism involved in fetal programming. However, detailed knowledge of monoamine transport in the rat placenta is still lacking. Moreover, relatability to the human placental monoamine system is not examined. The present study provides insights into the transplacental monoamine dynamics between maternal and fetal circulation. We show that norepinephrine maternal-to-fetal transport is <4% due to high metabolism within the trophoblast. In contrast, dopamine maternal-to-fetal transport exceeds 25%, likely through passive transport across the membrane. In addition, we show high clearance of norepinephrine and dopamine from the fetal circulation mediated by the organic cation transporter 3 (OCT3). Altogether, we present transcriptional and functional evidence that the in situ rat placenta perfusion represents a suitable model for (patho)physiological investigation of dopamine and norepinephrine homeostasis in the fetoplacental unit. With the rapid advancements in drug discovery and environmental toxicity, the use of rat placenta as a preclinical model could facilitate screening of possible xenobiotic effects on monoamine homeostasis in the placenta.

Precision-cut rat placental slices as a model to study sex-dependent inflammatory response to LPS and Poly I:C

Introduction

Maternal inflammation in pregnancy represents a major hallmark of several pregnancy complications and a significant risk factor for neurodevelopmental and neuropsychiatric disorders in the offspring. As the interface between the mother and the fetus, the placenta plays a crucial role in fetal development and programming. Moreover, studies have suggested that the placenta responds to an inflammatory environment in a sex-biased fashion. However, placenta-mediated immunoregulatory mechanisms are still poorly understood.

Methods

Therefore, we have developed a model of ex vivo precision-cut placental slices from the rat term placenta to study acute inflammatory response. Rat placental slices with a precise thickness of 200 µm were generated separately from male and female placentas. Inflammation was stimulated by exposing the slices to various concentrations of LPS or Poly I:C for 4 and 18 hours.

Results

Treatment of placental slices with LPS significantly induced the expression and release of proinflammatory cytokines TNF-α, IL-6, and IL-1β. In contrast, Poly I:C treatment resulted in a less-pronounced inflammatory response. Interestingly, the female placenta showed higher sensitivity to LPS than male placenta. Anti-inflammatory agents, curcumin, 1α,25- dihydroxyvitamin D3, and progesterone attenuated the LPS-induced proinflammatory cytokine response at both mRNA and protein levels.

Discussion

We conclude that rat placental slices represent a novel alternative model to study the role of sexual dimorphism in the acute inflammatory response and immune activation in pregnancy.

Functional reorganization of monoamine transport systems during villous trophoblast differentiation: evidence of distinct differences between primary human trophoblasts and BeWo cells

BACKGROUND

Three primary monoamines-serotonin, norepinephrine, and dopamine-play major roles in the placenta-fetal brain axis. Analogously to the brain, the placenta has transport mechanisms that actively take up these monoamines into trophoblast cells. These transporters are known to play important roles in the differentiated syncytiotrophoblast layer, but their status and activities in the undifferentiated, progenitor cytotrophoblast cells are not well understood. Thus, we have explored the cellular handling and regulation of monoamine transporters during the phenotypic transitioning of cytotrophoblasts along the villous pathway.

METHODS

Experiments were conducted with two cellular models of syncytium development: primary trophoblast cells isolated from the human term placenta (PHT), and the choriocarcinoma-derived BeWo cell line. The gene and protein expression of membrane transporters for serotonin (SERT), norepinephrine (NET), dopamine (DAT), and organic cation transporter 3 (OCT3) was determined by quantitative PCR and Western blot analysis, respectively. Subsequently, the effect of trophoblast differentiation on transporter activity was analyzed by monoamine uptake into cells.

RESULTS

We present multiple lines of evidence of changes in the transcriptional and functional regulation of monoamine transporters associated with trophoblast differentiation. These include enhancement of SERT and DAT gene and protein expression in BeWo cells. On the other hand, in PHT cells we report negative modulation of SERT, NET, and OCT3 protein expression. We show that OCT3 is the dominant monoamine transporter in PHT cells, and its main functional impact is on serotonin uptake, while passive transport strongly contributes to norepinephrine and dopamine uptake. Further, we show that a wide range of selective serotonin reuptake inhibitors affect serotonin cellular accumulation, at pharmacologically relevant drug concentrations, via their action on both OCT3 and SERT. Finally, we demonstrate that BeWo cells do not well reflect the molecular mechanisms and properties of healthy human trophoblast cells.

CONCLUSIONS

Collectively, our findings provide insights into the regulation of monoamine transport during trophoblast differentiation and present important considerations regarding appropriate in vitro models for studying monoamine regulation in the placenta.

Trophoblast Differentiation Affects Crucial Nutritive Functions of Placental Membrane Transporters

Cytotrophoblasts are progenitor cells that proliferate and fuse to form the multinucleated syncytiotrophoblast layer, implicated in placental endocrine and transport functions. While membrane transporters play a critical role in the distribution of nutrients, hormones, and xenobiotics at the maternal-fetal interface, their selectivity to the syncytiotrophoblast layer is poorly characterized. We aimed to evaluate the regulation of placental transporters in response to trophoblast differentiation in vitro. Experiments were carried out in isolated primary human trophoblast cells before and after syncytialization. Gene expression of six molecular markers and thirty membrane transporters was investigated by qPCR analysis. Subsequently, functional expression was evaluated for proteins involved in the transplacental transfer of essential nutrients i.e., cholesterol (ABCA1, ABCG1), glucose (SLC2A1), leucine (SLC3A2, SLC7A5), and iron (transferrin receptor, TfR1). We identified that human chorionic gonadotropin, placental lactogen, endoglin, and cadherin-11 serve as optimal gene markers for the syncytialization process. We showed that trophoblast differentiation was associated with differential gene expression (mostly up-regulation) of several nutrient and drug transporters. Further, we revealed enhanced protein expression and activity of ABCG1, SLC3A2, SLC7A5, and TfR1 in syncytialized cells, with ABCA1 and GLUT1 displaying no change. Taken together, these results indicate that the syncytiotrophoblast has a dominant role in transporting essential nutrients cholesterol, leucine, and iron. Nonetheless, we present evidence that the cytotrophoblast cells may also be linked to transport functions that could be critical for the cell fusion processes. Our findings collectively yield new insights into the cellular functions associated with or altered by the trophoblast fusion. Importantly, defective syncytialization could lead to nutrient transfer imbalance, ultimately compromising fetal development and programming.

Effect of Selected Antidepressants on Placental Homeostasis of Serotonin: Maternal and Fetal Perspectives

Depression is a prevalent condition affecting up to 20% of pregnant women. Hence, more than 10% are prescribed antidepressant drugs, mainly serotonin reuptake inhibitors (SSRIs) and selective serotonin and noradrenaline reuptake inhibitors (SNRIs). We hypothesize that antidepressants disturb serotonin homeostasis in the fetoplacental unit by inhibiting serotonin transporter (SERT) and organic cation transporter 3 (OCT3) in the maternal- and fetal-facing placental membranes, respectively. Paroxetine, citalopram, fluoxetine, fluvoxamine, sertraline, and venlafaxine were tested in situ (rat term placenta perfusion) and ex vivo (uptake studies in membrane vesicles isolated from healthy human term placenta). All tested antidepressants significantly inhibited SERT- and OCT3-mediated serotonin uptake in a dose-dependent manner. Calculated half-maximal inhibitory concentrations (IC₅₀) were in the range of therapeutic plasma concentrations. Using in vitro and in situ models, we further showed that the placental efflux transporters did not compromise mother-to-fetus transport of antidepressants. Collectively, we suggest that antidepressants have the potential to affect serotonin levels in the placenta or fetus when administered at therapeutic doses. Interestingly, the effect of antidepressants on serotonin homeostasis in rat placenta was sex dependent. As accurate fetal programming requires optimal serotonin levels in the fetoplacental unit throughout gestation, inhibition of SERT-/OCT3-mediated serotonin uptake may help explain the poor outcomes of antidepressant use in pregnancy.

Prenatal inflammation as a link between placental expression signature of tryptophan metabolism and preterm birth

Spontaneous preterm birth is a serious medical condition responsible for substantial perinatal morbidity and mortality. Its phenotypic characteristics, preterm labor with intact membranes (PTL) and preterm premature rupture of the membranes (PPROM), are associated with significantly increased risks of neurological and behavioral alterations in childhood and later life. Recognizing the inflammatory milieu associated with PTL and PPROM, here, we examined expression signatures of placental tryptophan metabolism, an important pathway in prenatal brain development and immunotolerance. The study was performed in a well-characterized clinical cohort of healthy term pregnancies (n = 39) and 167 preterm deliveries (PTL, n = 38 and PPROM, n = 129). Within the preterm group, we then investigated potential mechanistic links between differential placental tryptophan pathway expression, preterm birth and both intra-amniotic markers (such as amniotic fluid interleukin-6) and maternal inflammatory markers (such as maternal serum C-reactive protein and white blood cell count). We show that preterm birth is associated with significant changes in placental tryptophan metabolism. Multifactorial analysis revealed similarities in expression patterns associated with multiple phenotypes of preterm delivery. Subsequent correlation computations and mediation analyses identified links between intra-amniotic and maternal inflammatory markers and placental serotonin and kynurenine pathways of tryptophan catabolism. Collectively, the findings suggest that a hostile inflammatory environment associated with preterm delivery underlies the mechanisms affecting placental endocrine/transport functions and may contribute to disruption of developmental programming of the fetal brain.

Revisiting Steroidogenic Pathways in the Human Placenta and Primary Human Trophoblast Cells

Steroid hormones play a crucial role in supporting a successful pregnancy and ensuring proper fetal development. The placenta is one of the principal tissues in steroid production and metabolism, expressing a vast range of steroidogenic enzymes. Nevertheless, a comprehensive characterization of steroidogenic pathways in the human placenta and potential developmental changes occurring during gestation are poorly understood. Furthermore, the specific contribution of trophoblast cells in steroid release is largely unknown. Thus, this study aimed to (i) identify gestational age-dependent changes in the gene expression of key steroidogenic enzymes and (ii) explore the role of trophoblast cells in steroid biosynthesis and metabolism. Quantitative and Droplet Digital PCR analysis of 12 selected enzymes was carried out in the first trimester (n = 13) and term (n = 20) human placentas. Primary trophoblast cells (n = 5) isolated from human term placentas and choriocarcinoma-derived cell lines (BeWo, BeWo b30 clone, and JEG-3) were further screened for gene expression of enzymes involved in placental synthesis/metabolism of steroids. Finally, de novo steroid synthesis by primary human trophoblasts was evaluated, highlighting the functional activity of steroidogenic enzymes in these cells. Collectively, we provide insights into the expression patterns of steroidogenic enzymes as a function of gestational age and delineate the cellular origin of steroidogenesis in the human placenta.

Profiling of Tryptophan Metabolic Pathways in the Rat Fetoplacental Unit During Gestation

Placental homeostasis of tryptophan is essential for fetal development and programming. The two main metabolic pathways (serotonin and kynurenine) produce bioactive metabolites with immunosuppressive, neurotoxic, or neuroprotective properties and their concentrations in the fetoplacental unit must be tightly regulated throughout gestation. Here, we investigated the expression/function of key enzymes/transporters involved in tryptophan pathways during mid-to-late gestation in rat placenta and fetal organs. Quantitative PCR and heatmap analysis revealed the differential expression of several genes involved in serotonin and kynurenine pathways. To identify the flux of substrates through these pathways, Droplet Digital PCR, western blot, and functional analyses were carried out for the rate-limiting enzymes and transporters. Our findings show that placental tryptophan metabolism to serotonin is crucial in mid-gestation, with a subsequent switch to fetal serotonin synthesis. Concurrently, at term, the close interplay between transporters and metabolizing enzymes of both placenta and fetal organs orchestrates serotonin homeostasis and prevents hyper/hypo-serotonemia. On the other hand, the placental production of kynurenine increases during pregnancy, with a low contribution of fetal organs throughout gestation. Any external insult to this tightly regulated harmony of transporters and enzymes within the fetoplacental unit may affect optimal in utero conditions and have a negative impact on fetal programming.

Dynamics of Tryptophan Metabolic Pathways in Human Placenta and Placental-Derived Cells: Effect of Gestation Age and Trophoblast Differentiation

L-Tryptophan is an essential amino acid and a precursor of several physiologically active metabolites. In the placenta, the serotonin and kynurenine metabolic pathways of tryptophan metabolism have been identified, giving rise to various molecules of neuroactive or immunoprotective properties, such as serotonin, melatonin, kynurenine, kynurenic acid, or quinolinic acid. Current literature suggests that optimal levels of these molecules in the fetoplacental unit are crucial for proper placenta functions, fetal development and programming. Placenta is a unique endocrine organ that, being equipped with a battery of biotransformation enzymes and transporters, precisely orchestrates homeostasis of tryptophan metabolic pathways. However, because pregnancy is a dynamic process and placental/fetal needs are continuously changing throughout gestation, placenta must adapt to these changes and ensure proper communication in the feto-placental unit. Therefore, in this study we investigated alterations of placental tryptophan metabolic pathways throughout gestation. Quantitative polymerase chain reaction (PCR) analysis of 21 selected genes was carried out in first trimester (n = 13) and term (n = 32) placentas. Heatmap analysis with hierarchical clustering revealed differential gene expression of serotonin and kynurenine pathways across gestation. Subsequently, digital droplet PCR, Western blot, and functional analyses of the rate-limiting enzymes suggest preferential serotonin synthesis early in pregnancy with a switch to kynurenine production toward term. Correspondingly, increased function and/or protein expression of serotonin degrading enzyme and transporters at term indicates efficient placental uptake and metabolic degradation of serotonin. Lastly, gene expression analysis in choriocarcinoma-derived cell lines (BeWo, BeWo b30, JEG-3) revealed dissimilar expression patterns and divergent effect of syncytialization compared to primary trophoblast cells isolated from human term placentas; these findings show that the commonly used in vitro placental models are not suitable to study placental handling of tryptophan. Altogether, our data provide the first comprehensive evidence of changes in placental homeostasis of tryptophan and its metabolites as a function of gestational age, which is critical for proper placental function and fetal development.

Serotonin homeostasis in the materno-foetal interface at term: Role of transporters (SERT/SLC6A4 and OCT3/SLC22A3) and monoamine oxidase A (MAO-A) in uptake and degradation of serotonin by human and rat term placenta

AIM

Serotonin is crucial for proper foetal development, and the placenta has been described as a 'donor' of serotonin for the embryo/foetus. However, in later stages of gestation the foetus produces its own serotonin from maternally-derived tryptophan and placental supply is no longer needed. We propose a novel model of serotonin homeostasis in the term placenta with special focus on the protective role of organic cation transporter 3 (OCT3/SLC22A3).

METHODS

Dually perfused rat term placenta was employed to quantify serotonin/tryptophan transport and metabolism. Placental membrane vesicles isolated from human term placenta were used to characterize serotonin transporters on both sides of the syncytiotrophoblast.

RESULTS

We obtained the first evidence that serotonin is massively taken up from the foetal circulation by OCT3. This uptake is concentration-dependent and inhibitable by OCT3 blockers of endogenous (glucocorticoids) or exogenous (pharmaceuticals) origin. Population analyses in rat placenta revealed that foetal sex influences placental extraction of serotonin from foetal circulation. Negligible foetal serotonin levels were detected in maternal-to-foetal serotonin/tryptophan transport and metabolic studies.

CONCLUSION

We demonstrate that OCT3, localized on the foetus-facing membrane of syncytiotrophoblast, is an essential component of foeto-placental homeostasis of serotonin. Together with serotonin degrading enzyme, monoamine oxidase-A, this offers a protective mechanism against local vasoconstriction effects of serotonin in the placenta. However, this system may be compromised by OCT3 inhibitory molecules, such as glucocorticoids or antidepressants. Our findings open new avenues to explore previously unsuspected/unexplained complications during pregnancy including prenatal glucocorticoid excess and pharmacotherapeutic risks of treating pregnant women with OCT3 inhibitors.

The inhibitory effect of antiretroviral drugs on the L-carnitine uptake in human placenta

In spite of remarkable reduction in the number of children born with HIV due to antiretroviral therapy, concerns remain on the short- and long-term effects of antiretroviral drugs at the feto-placental unit. Cardio- and skeletal myopathies have been reported in children exposed to antiretroviral drugs prenatally. These conditions have also been described in perturbed placental transfer of l-carnitine, an essential co-factor in fatty acid oxidation. Due to limited fetal and placental synthesis, carnitine supply is maintained through the placental carnitine uptake from maternal blood by the organic cation/carnitine transporters OCTN1 and OCTN2 (SLC22A4 and SLC22A5, respectively). The aim of our study was to investigate potential inhibition of placental carnitine uptake by a broad range of antiretroviral drugs comprising nucleoside/nucleotide reverse transcriptase inhibitors (lamivudine, zidovudine, abacavir, tenofovir disoproxil fumarate), non-nucleoside reverse transcriptase inhibitors (rilpivirine, efavirenz, etravirine), protease inhibitors (ritonavir, lopinavir, atazanavir, saquinavir, tipranavir), integrase inhibitors (raltegravir, dolutegravir, elvitegravir) and viral entry inhibitor, maraviroc. Studies in choriocarcinoma BeWo cells and human placenta-derived models confirmed predominant expression and function of OCTN2 above OCTN1 in l-carnitine transport. Subsequent screenings in BeWo cells and isolated MVM vesicles revealed seven antiretroviral drugs as inhibitors of the Na⁺-dependent l-carnitine uptake, corresponding to OCTN2. Ritonavir, saquinavir and elvitegravir showed the highest inhibitory potential which was further confirmed for ritonavir and saquinavir in placental fresh villous fragments. Our data indicate possible impairment in placental and fetal supply of l-carnitine with ritonavir and saquinavir, while suggesting retained placental carnitine transport with the other antiretroviral drugs.

Equilibrative Nucleoside Transporter 1 (ENT1, SLC29A1) Facilitates Transfer of the Antiretroviral Drug Abacavir across the Placenta

Abacavir is a preferred antiretroviral drug for preventing mother-to-child human immunodeficiency virus transmission; however, mechanisms of its placental transfer have not been satisfactorily described to date. Because abacavir is a nucleoside-derived drug, we hypothesized that the nucleoside transporters, equilibrative nucleoside transporters (ENTs, SLC29A) and/or Na⁺-dependent concentrative nucleoside transporters (CNTs, SLC28A), may play a role in its passage across the placenta. To test this hypothesis, we performed uptake experiments using the choriocarcinoma-derived BeWo cell line, human fresh villous fragments, and microvillous plasma membrane (MVM) vesicles. Using endogenous substrates of nucleoside transporters, [³H]-adenosine (ENTs, CNT2, and CNT3) and [³H]-thymidine (ENTs, CNT1, and CNT3), we showed significant activity of ENT1 and CNT2 in BeWo cells, whereas experiments in the villous fragments and MVM vesicles, representing a model of the apical membrane of a syncytiotrophoblast, revealed only ENT1 activity. When testing [³H]-abacavir uptakes, we showed that of the nucleoside transporters, ENT1 plays the dominant role in abacavir uptake into placental tissues, whereas contribution of Na⁺-dependent transport, most likely mediated by CNTs, was observed only in BeWo cells. Subsequent experiments with dually perfused rat term placentas showed that Ent1 contributes significantly to overall [³H]-abacavir placental transport. Finally, we quantified the expression of SLC29A in first- and third-trimester placentas, revealing that SLC29A1 is the dominant isoform. Neither SLC29A1 nor SLC29A2 expression changed over the course of placental development, but there was considerable interindividual variability in their expression. Therefore, drug-drug interactions and the effect of interindividual variability in placental ENT1 expression on abacavir disposition into fetal circulation should be further investigated to guarantee safe and effective abacavir-based combination therapies in pregnancy.