Certain cell types must expand their exocytic pathway to guarantee efficiency and fidelity of protein secretion. A spectacular case is offered by decidualizing human endometrial stromal cells (EnSCs). In the midluteal phase of the menstrual cycle, progesterone stimulation induces proliferating EnSCs to differentiate into professional secretors releasing proteins essential for efficient blastocyst implantation. Here, we describe the architectural rearrangements of the secretory pathway of a human EnSC line (TERT-immortalized human endometrial stromal cells (T-HESC)). As in primary cells, decidualization entails proliferation arrest and the coordinated expansion of the entire secretory pathway without detectable activation of unfolded protein response (UPR) pathways. Decidualization proceeds also in the absence of ascorbic acid, an essential cofactor for collagen biogenesis, despite also the secretion of some proteins whose folding does not depend on vitamin C is impaired. However, even in these conditions, no overt UPR induction can be detected. Morphometric analyses reveal that the exocytic pathway does not increase relatively to the volume of the cell. Thus, differently from other cell types, abundant production is guaranteed by a coordinated increase of the cell size following arrest of proliferation.

Upon progesterone stimulation, Endometrial Stromal Cells (EnSCs) undergo a differentiation program into secretory cells (decidualization) to release in abundance factors crucial for embryo implantation. We previously demonstrated that decidualization requires massive reshaping of the secretory pathway and, in particular, of the Golgi complex. To decipher the underlying mechanisms, we performed a time-course transcriptomic analysis of in vitro decidualizing EnSC. Pathway analysis shows that Gene Ontology terms associated with vesicular trafficking and early secretory pathway compartments are the most represented among those enriched for upregulated genes. Among these, we identified a cluster of co-regulated genes that share CREB3L1 and CREB3L2 binding elements in their promoter regions. Indeed, both CREB3L1 and CREB3L2 transcription factors are up-regulated during decidualization. Simultaneous downregulation of CREB3L1 and CREB3L2 impairs Golgi enlargement, and causes dramatic changes in decidualizing EnSC, including Golgi fragmentation, collagen accumulation in dilated Endoplasmic Reticulum cisternae, and overall decreased protein secretion. Thus, both CREB3L1 and CREB3L2 are required for Golgi reshaping and efficient protein secretion, and, as such, for successful decidualization.