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.

L -色氨酸是一种必需氨基酸，也是几种生理活性代谢物的前体。在胎盘中，色氨酸代谢的血清素和犬尿氨酸代谢途径已被确定，产生各种具有神经活性或免疫保护特性的分子，例如血清素、褪黑激素、犬尿氨酸、犬尿酸或喹啉酸。目前的文献表明，胎儿胎盘单位中这些分子的最佳水平对于适当的胎盘功能、胎儿发育和编程至关重要。胎盘是一种独特的内分泌器官，配备了一组生物转化酶和转运蛋白，可精确协调色氨酸代谢途径的稳态。然而，由于妊娠是一个动态过程，并且胎盘/胎儿的需求在整个妊娠过程中不断变化，因此胎盘必须适应这些变化并确保胎儿-胎盘单位中的适当沟通。因此，在这项研究中，我们研究了整个妊娠期间胎盘色氨酸代谢途径的变化。在妊娠早期对 21 个选定基因进行了定量聚合酶链反应 (PCR) 分析（ n = 13) 和项 ( n = 32) 胎盘。层次聚类热图分析揭示了妊娠期间血清素和犬尿氨酸途径的差异基因表达。随后，数字液滴 PCR、蛋白质印迹和限速酶的功能分析表明，在妊娠早期优先合成血清素，并在足月前转向犬尿氨酸的产生。 相应地，足月时血清素降解酶和转运蛋白的功能和/或蛋白质表达增加表明胎盘有效吸收和代谢降解血清素。最后，绒毛膜癌来源的细胞系（BeWo、BeWo b30、JEG-3）中的基因表达分析揭示了与从人足月胎盘中分离的原代滋养层细胞相比，不同的表达模式和不同的合胞化效应。这些发现表明，常用的体外胎盘模型不适合研究胎盘对色氨酸的处理。总而言之，我们的数据提供了第一个全面的证据，证明色氨酸及其代谢物的胎盘稳态随胎龄的变化而变化，这对于正常的胎盘功能和胎儿发育至关重要。