The decidua is a hallmark of reproduction in many placental mammals. Differentiation of decidual stromal cells is known to be induced by progesterone and the cyclic AMP/protein kinase A (cAMP/PKA) pathway. Several candidates have been identified as the physiological stimulus for adenylyl cyclase activation, but their relative importance remains unclear. To bypass this uncertainty, the standard approach for in vitro experiments uses membrane-permeable cAMP and progestin. We phylogenetically infer that prostaglandin E₂ (PGE₂) likely was the signal that ancestrally induced decidualization in conjunction with progesterone. This suggests that PGE₂ and progestin should be able to activate the core gene regulatory network of decidual cells. To test this prediction, we performed a genome-wide study of gene expression in human endometrial fibroblasts decidualized with PGE₂ and progestin. Comparison to a cAMP-based protocol revealed shared activation of core decidual genes and decreased induction of senescence-associated genes. Single-cell transcriptomics of PGE₂-mediated decidualization revealed a distinct, early-activated state transitioning to a differentiated decidual state. PGE₂-mediated decidualization was found to depend upon progestin-dependent induction of PGE₂ receptor 2 (PTGER2) which in turn leads to PKA activation upon PGE₂ stimulation. Progesterone-dependent induction of PTGER2 is absent in opossum, an outgroup taxon of placental mammals which is incapable of decidualization. Together, these findings suggest that the origin of decidualization involved the evolution of progesterone-dependent activation of the PGE₂/PTGER2/PKA axis, facilitating entry into a PKA-dominant rather than AKT-dominant cellular state. We propose the use of PGE₂ for in vitro decidualization as an alternative to 8-Br-cAMP.

Summary sentence

In vitro decidualization of human endometrial stromal fibroblasts with PGE₂ and progestin induces a different but genuine decidual cell state through PTGER2-dependent PKA activation when compared to the conventional cyclic AMP/MPA protocol.

Graphical Abstract