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

Abstract

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.

Effects of Maren Pills on the Intestinal Microflora and Short-Chain Fatty Acid Profile in Drug-Induced Slow Transit Constipation Model Rats

“…Recently, it was shown that the secretion of 5-HT in the gut is regulated by the intestinal microflora, and the SCFAs produced by the intestinal microflora could promote 5-HT levels in enterochromaffin cells in the epithelia ( Yano et al, 2015 ; El Aidy et al, 2017 ). Tryptophan hydroxylase (TPH) is a rate-limiting enzyme for 5-HT synthesis and controlled the biosynthesis rate of 5-HT ( Kim et al, 2018 ; Abad et al, 2020 ). Moreover, when 5-HT is uptaken in the cell, 5-HT is degraded into 5-hydroxyindoleacetic acid (5-HIAA) by monoamine oxidases ( Manzella et al, 2018 ).…”

The Function of the Kynurenine Pathway in the Placenta: A Novel Pharmacotherapeutic Target?

“…Nevertheless, LAT1, LAT2 and ASCT2 proteins were all detected with distinct spatial patterns using immunohistochemistry [ 57 ], suggesting that new synthesis of these transporters is limited, and that turnover is low as they remain persistently present as a membrane bound transporter. Expression of Slc7a5 and Slc7a8 increased throughout gestation in mice, with a concomitant increase in placental tryptophan content [ 58 ]. In human chorionic plate arteries, LAT1 expression is higher compared with LAT2 and ASCT2 [ 19 ].…”

“…In mice, IDO activity is very high around conception but disappears at the end of pregnancy, opposite to the human situation [ 126 ]. In rat placentas, Ido1 was not detected, but placental expression of Ido2 increased towards term, resembling the human IDO1 enzyme [ 58 ]. The choriocarcinoma cancer cell lines which are regularly used in placental research also seem unsuitable to investigate the kynurenine pathway, because of dissimilar gene expression of kynurenine pathway associated genes compared to primary trophoblast cells [ 27 ].…”

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

“…Additionally, trophoblast cells are fully equipped with 5-HT-producing and 5-HT-degrading enzymes: tryptophan hydroxylase (TPH) and monoamine oxidase A (MAO-A), respectively [ 32 ]. The placenta is thus capable of regulating 5-HT levels in the fetoplacental unit in accordance with placental and fetal demands, which are related to gestational age in both humans [ 33 ] and rats [ 34 ]. Specifically, at the beginning of pregnancy, the embryo/fetus lacks the capacity to synthesize 5-HT and is fully dependent on maternal or placental sources [ 35 ].…”

“…Towards term, the fetus can synthesize 5-HT from maternally derived tryptophan [ 36 , 37 ] and placental synthesis of 5-HT declines [ 33 ]. Importantly, term placenta shows increased expression of SERT, OCT3, and MAO-A, the three most important components of a “5-HT-detoxification mechanism”, in the fetoplacental unit [ 33 , 34 ].…”