Impact of intraamniotic inflammation on tryptophan metabolism in the placenta-fetal brain axis in rats

Abstract

In brief: Intrauterine inflammation disrupts tryptophan metabolism in both the placenta and the fetal brain, leading to a shift toward neurotoxic metabolites. These findings highlight the critical role of placental function in neurodevelopment and suggest that inflammation-induced metabolic changes may contribute to neurodevelopmental disorders.

Abstract: The placenta plays a crucial role beyond nutrient transfer, acting as a dynamic endocrine organ that significantly influences maternal physiology and fetal development. It responds rapidly to even slight changes in the in utero environment to promote fetal survival. Disruptions in placental function are increasingly recognized as key contributors to the origins of neurodevelopmental disorders. In this study, we employed advanced technology to induce intrauterine inflammation through ultrasound-guided administration of LPS into gestational sacs. We then evaluated its effects on the gene expression of enzymes involved in TRP metabolism and conducted a comprehensive LC/MS analysis of the metabolome in the placenta and fetal brain of Wistar rats. Our results show that intraamniotic injection of LPS induces a robust inflammatory response leading to significant alterations in TRP metabolism, including downregulation of tryptophan hydroxylase (TPH) in the placenta, resulting in a decrease in serotonin (5-HT) levels. Similarly, in the fetal brain, exposure to LPS led to reduced Tph expression and increased monoamine oxidase expression, suggesting a decrease in 5-HT synthesis and an increase in its degradation. Furthermore, an upregulation of the kynurenine pathway was observed in both the placenta and fetal brain. Moreover, we detected a shift toward neurotoxicity, evidenced by an imbalance between neuroprotective and neurotoxic metabolites, including decreased levels of kynurenic acid and upregulation of kynurenine monooxygenase in the fetal brain. In conclusion, our findings reveal significant alterations in TRP metabolism following intrauterine inflammation, potentially contributing to neurodevelopmental disorders.

Keywords: fetal brain; intrauterine inflammation; neurodevelopmental disorders; placenta; tryptophan metabolism.

Figure 1

Tryptophan metabolic pathway. TRP is metabolized by two main pathways: the serotonin pathway and the kynurenine pathway. The main catabolic pathways of TRP are shown with the enzyme metabolizing the corresponding catabolites. TPH, tryptophan hydroxylase; DDC, tryptophan decarboxylase; AANAT, aralkylamine N-acetyltransferase; ASMT, N-acetylserotonin O-methyltransferase; MAO, monoamine oxidase; IDO, indoleamine 2,3-dioxygenase; AFMID, arylformamidase; KAT, kynurenine amino transferase; KMO, kynurenine 3-monooxygenase; KYNU, kynureninase; HAAO, 3-hydroxyanthranilate 3,4-dioxygenase; QPRT, quinolinate phosphoribosyltransferase.

Figure 2

Gene expression of proinflammatory cytokines in rat placenta and fetal brain exposed to intraamniotic LPS. Proinflammatory cytokines Il-1b, Tnf-α and Il-6 from rat placenta (A) and fetal brain (B) exposed to 10 μg LPS were analyzed by qPCR. Data are presented as individual points, with EMMs shown as dotted lines; n = 4 dams per group. Statistical significance was evaluated using a LMM to account for litter effects; *P ≤ 0.05, **P ≤ 0.01 and ***P ≤ 0.0001.

Figure 3

Gene expression of the main enzymes and transporters involved in TRP metabolic pathways in rat placenta and fetal brain exposed to intraamniotic LPS. Enzymes and transporters of the 5-HT pathway (Tph1, Tph2, Mao-a, Scl6a4 and Scl22a3) and enzymes of the KYN pathway (Ido, Kat1, Kmo and Kynu) from rat placenta (A) and fetal brain (B) exposed to intraamniotic LPS (10 μg) were analyzed by qPCR. Data are presented as individual points, with EMMs shown as dotted lines; n = 4 dams per group. Statistical significance was evaluated using a LMM to account for litter effects; *P ≤ 0.05, **P ≤ 0.01 and ***P ≤ 0.001.

Figure 4

Relationship between amniotic fluid IL-6 concentration and gene expression of the main enzyme involved in the 5-HT and KYN pathways of TRP metabolism. Pearson correlation between relative gene expression and amniotic fluid IL-6 concentration in the placenta (A) and fetal brain (B) after 24 h of exposure to intraamniotic LPS (10 µg). Normalized gene expression data were log2-transformed *P ≤ 0.05 and **P ≤ 0.01.

Figure 5

Changes in the placental and fetal brain metabolomic profile in LPS-triggered intraamniotic inflammation. Bubble plot highlighting metabolites that are dysregulated in placentas and brain exposed to intraamniotic LPS compared to PBS. The node color gradient indicates the difference in metabolite levels between LPS and PBS, expressed as a fold change, while the node radius reflects the negative log of the P-value.

Figure 6

Metabolites and product/substrate ratios related to TRP metabolism in rat placenta and fetal brain. Graphs showing differences between LPS- and PBS-treated rats for metabolites related to the 5-HT and KYN pathways in the placenta (A) and fetal brain (B). Data are presented as individual points, with EMMs shown as dotted lines; n = 4 dams per group. Statistical analysis was performed using a LMM to account for litter. *P ≤ 0.05 and **P ≤ 0.01.

Figure 7

Schematic illustration of the effects of intraamniotic LPS exposure on the TRP metabolic pathways in the placenta and fetal brain. Created in BioRender.com.