Introduction: The microbiome changes during pregnancy and is implicated in pregnancy complications which contribute to nearly 5 million maternal, and neonatal deaths yearly. Impaired placental development primarily accounts for the majority of pregnancy complications although the mechanisms involved are poorly understood. Optimal placental (trophoblast) development depends on the fine balance between the renewal of the cytotrophoblast stem cells and their differentiation into the invasive extravillus trophoblast and the multinucleated syncytiotrophoblast (ST) epithelial cells. While pregnancy is punctuated by finely timed fluctuations in various hormones, that regulate trophoblast differentiation, the reasons for the concomitant changes in the microbiome with advancing gestation are unclear. As the microbiome does not reach the systemic circulation, but its metabolites do, it is possible that the metabolites may mediate microbial-host interactions reflected in microbial changes observed in pregnancy. We show that several microbial-derived tryptophan metabolites dose-dependently decrease energy metabolism and trophoblast differentiation via G-protein coupled receptor mediated cAMP production.

Methods and results: By treating term human primary trophoblast cells and measuring oxygen consumption using Mito stress test, we demonstrate that tryptophan metabolites (tryptamine, Indole-3-carbaldehyde, indole-3-acetic-acid (IAA) mediate differential effects on both energy metabolism and trophoblast differentiation. Tryptamine dose dependently reduced βhCG production (marker of trophoblast differentiation) at 12.6 µM (p ≤0.0001, n = 5) and 50µM (p ≤0.0001, n = 5). At similar doses (IAA) had no effect on either metabolism or trophoblast differentiation. Tryptamine reduced cyclic adenosine monophosphate (cAMP) production by 20% (p ≤0.05, N=3) and shifted the EC₅₀ of forskolin induced cAMP production dose response from 2.6 µM to 4.0 µM.

Discussion: These results demonstrate how microbial metabolites are likely to influence pregnancy outcomes by interacting with signalling pathways important to trophoblast differentiation. As GPCR signalling is an important pathway for various developmental processes including stem cell pluripotency, differentiation, and cell polarity in embryogenesis, these results emphasise the importance of understanding the role of microbial derived metabolites in pregnancy in health and disease.