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.

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.

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.

Background: Maternal immune activation is a major risk factor for neurodevelopmental disorders; however, the molecular mechanisms linking gestational inflammation to fetal brain vulnerability remain incompletely defined. The tryptophan–kynurenine pathway is highly responsive to immune signaling and represents a potential mediator of placenta–fetal brain communication. Methods: Pregnant rats received Poly I:C (5 or 15 mg/kg) or lipopolysaccharide (1 mg/kg) on gestational days 17–18. Maternal plasma, amniotic fluid, placenta, decidua, and fetal brain were collected 4, 24, and 48 h after the final injection. Cytokines, gene expression, protein levels, and kynurenine-pathway metabolites were quantified, and multivariate analyses were performed to assess temporal and compartment-specific responses. Results: Maternal inflammation was confirmed by increased IL-6 and IL-1β in plasma, together with elevated CRP and TNF-α in amniotic fluid. The placenta showed rapid remodeling of the kynurenine pathway, including induction of indoleamine 2,3-dioxygenase 1 protein and time-dependent changes in downstream enzymes and in the expression of amino acid transporters. These effects were sex dependent at the protein levels, with female placentas showing increased kynurenine aminotransferase expression, while placental kynurenine and quinolinic acid concentrations increased with treatment independent of sex. The decidua displayed limited and non–sex-dimorphic responses. In the fetal brain, transcriptional changes emerged later and were strongly sex dependent, with broader downregulation of kynurenine-pathway and transporter genes in females. Multivariate analyses indicated that placental responses preceded and were associated with fetal brain transcriptional remodeling. Conclusion: Following maternal immune activation, the placenta showed early and pronounced remodeling of the kynurenine pathway that preceded and was statistically associated with sex-specific changes in the fetal brain. This temporal and associative pattern identifies placental kynurenine metabolism as a candidate mediator linking maternal inflammation to differential neurodevelopmental vulnerability.