TGFb-induced SMAD1/5 activation drives transient epithelial-to-mesenchymal transition in trophoblasts

Successful human placental development requires fetal trophoblasts to undergo epithelial-to-mesenchymal and mesenchymal-to-endothelial transitions, enabling invasion and remodeling of maternal spiral arteries for proper placental perfusion. These trans-differentiation processes are disrupted in preeclampsia, a major cause of maternal and fetal morbidity worldwide. Despite its clinical significance, the molecular mechanisms governing these differentiation trajectories at the fetal-maternal interface remain poorly defined. While Transforming Growth Factor-beta (TGF{beta}) has been implicated in trophoblast invasion, its role remains controversial. In this study, we report increased SMAD2/3 phosphorylation (pSMAD2/3), but reduced pSMAD1/5 and SNAIL levels in preeclamptic placentas. Mechanistically, we show that TGF{beta}1 induces transient SMAD1/5 phosphorylation via dual engagement of the TGF{beta} type I receptor ALK5 and the BMP type I receptor ALK2 in both HTR8/SVneo cells and human trophoblast stem cells. Moreover, single-nucleus transcriptomics of first-trimester placentas revealed co-expression of ACVR1, TGFBR1, SMAD1 and SMAD5 at the transition zone between cytotrophoblasts and extravillous trophoblasts. Our findings suggest that transient TGF{beta}-induced ALK5/ALK2-SMAD1/5 signaling is critical for EMT initiation, a prerequisite for proper placental differentiation.