Single-cell RNA sequencing and lineage tracing confirm mesenchyme to epithelial transformation (MET) contributes to repair of the endometrium at menstruation

Abstract

The human endometrium experiences repetitive cycles of tissue wounding characterised by piecemeal shedding of the surface epithelium and rapid restoration of tissue homeostasis. In this study we used a mouse model of endometrial repair and three transgenic lines of mice to investigate whether epithelial cells that become incorporated into the newly formed luminal epithelium have their origins in one or more of the mesenchymal cell types present in the stromal compartment of the endometrium. Using scRNAseq we identified a novel population of PDGFRb+ mesenchymal stromal cells that developed a unique transcriptomic signature in response to endometrial breakdown/repair. These cells expressed genes usually considered specific to epithelial cells and in silico trajectory analysis suggested they were stromal fibroblasts in transition to becoming epithelial cells. To confirm our hypothesis we used a lineage tracing strategy to compare the fate of stromal fibroblasts (PDGFRa+) and stromal perivascular cells (NG2/CSPG4+). We demonstrated that stromal fibroblasts can undergo a mesenchyme to epithelial transformation and become incorporated into the re-epithelialised luminal surface of the repaired tissue. This study is the first to discover a novel population of wound-responsive, plastic endometrial stromal fibroblasts that contribute to the rapid restoration of an intact luminal epithelium during endometrial repair. These findings form a platform for comparisons both to endometrial pathologies which involve a fibrotic response (Asherman’s syndrome, endometriosis) as well as other mucosal tissues which have a variable response to wounding.

A perivascular niche supports endometrial epithelial regeneration

“…Classical studies suggested that newly regenerated epithelia are mainly derived from proliferation of the remaining epithelia close to denuded stromal cells 55 . SSEA-1+ epithelial stem cells serve as a resident pool of progenitors for epithelial regeneration in uterus 6,16,37,54,56 . They have the potential to proliferate and restore epithelial integrity in case of cell death nearby 8 .…”

Incorporation of Decidual Stromal Cells Derived Exosomes in Sodium Alginate Hydrogel as an Innovative Therapeutic Strategy for Advancing Endometrial Regeneration and Reinstating Fertility

“…[76] Studies have demonstrated that MET is crucial in the process of endometrial decidualization and plays a significant role in supporting embryonic development. [77,78] Furthermore, the activation of MET has been shown to counteract the development of uterine fibrosis in cases of IUA. [67,79] The MET process is known to occur during decidualization, [41,80] leading to the hypothesis that DSC-exos may encapsulate factors that promote MET and thus reverse the process of EMT in the injured uterus and inhibit excessive fibrosis.…”

Cellular heterogeneity and dynamics of the human uterus in healthy premenopausal women

“…Next, to gauge the global relationship among major cell types, we projected the stromal, epithelial, and endothelial cells together, and observed a strong "flow" where stromal cells are upstream of both epithelial and endothelial cells (Figure 3F). This suggests that, unlike mice, human uterus stromal cells 14 have the potential to undergo a mesenchymal-to-epithelial transition to contribute to the epithelial and endothelial cell compartments during the normal menstrual cycle (22)(23)(24).…”

“…First, in hematopoietic stem-cell (HSC) transplant recipients (XX females receiving XY male HSC) bone marrow-derived human mesenchymal stem cells may contribute to uterine tissue homeostasis (3,6,20,21). Second, it was shown that perivascular or stromal cells in mouse myometrium may undergo a mesenchymal-to-epithelial transition (MET), playing a crucial role in the regeneration of epithelial cells in the uterus of mice after birth (22)(23)(24). While molecular markers for these stem/progenitor cells have been identified, our understanding of the cellular diversity in the uterus, especially the dynamics and interdependency of different cell types, remains limited.…”