Role of Serotonin on Gene Expression and Physiology in Human Cytotrophoblasts and Placenta

Serotonin (5-hydroxytryptamine; 5-HT) is transported into the human placenta through the serotonin transporter (SERT/SLC6A4) on the surface of the syncytiotrophoblast. During this transit, a significant amount of 5-HT becomes concentrated in the cytotrophoblast nucleus. We used immunochemistry, inhibitors of SERT and transglutaminase 2, and RNA sequencing to elucidate the mechanism and consequences of this nuclear localization. Exogenous 5-HT recapitulated the uptake of 5-HT into the trophoblasts and its preferential concentration in cytotrophoblast nuclei we observed in the intact placenta. Cystamine eliminated the staining of the nuclei in placental explants by exogenous 5-HT, suggesting that serotonylation mediated this phenomenon. This was confirmed by Western blots and immunoprecipitation that identified histone 3, and specifically the 5th glutamine residue in histone 3, as a site of serotonylation. Inhibiting SERT with escitalopram or transglutaminase 2 with cystamine blocked cytotrophoblast differentiation in vitro and led to marked changes in RNA expression. Of the 38 524 mRNAs identified in these trophoblasts, cystamine changed the expression of 1986 and escitalopram significantly altered 374. Both treatments altered the expression of 155 mRNAs either positively or negatively. The downregulated genes were involved with cell proliferation, morphogenesis, motility, and growth, whereas genes that were upregulated controlled cell survival and protection pathways. These findings suggest that maternal 5-HT promotes placental, embryonic/fetal, and organismal development through histone serotonylation and consequent alterations in gene expression. They raise the possibility that alterations in 5-HT flux in the placenta affect placental and fetal growth, as well as organismal somatic, neurologic developmental, and pathological trajectories.