circRNA3669 promotes goat endometrial epithelial cells proliferation via miR-26a/RCN2 to activate PI3K/AKT-mTOR and MAPK pathways1

The development of receptive endometrium (RE) from pre-receptive endometrium (PE) for successful embryo implantation is a complex dynamic process in which the morphology and physiological states of the endometrial epithelium undergo a series of significant changes, including cell proliferation and apoptosis. However, the molecular mechanisms are not yet fully understood. In this study, a higher circRNA3669 level was observed in PE than in RE of goats. Functional assays revealed that this overexpression promoted the proliferation of goat endometrial epithelial cells (GEECs) by activating the expression of genes related to the PI3K/AKT-mTOR and MAPK pathways, thereby inhibiting apoptosis in vitro. Furthermore, circRNA3669 functioned as a competing endogenous RNA (ceRNA) to upregulate Reticulocalbin-2 (RCN2) expression at the post-transcriptional level by interacting with and downregulating miR-26a in GEECs. In addition, RCN2, which is highly expressed in the PE of goats, was found to be regulated by β-estradiol (E2) and progesterone (P4). Our results demonstrated that RCN2 also affected the key proteins PI3K, AKT, mTOR, JNK, and P38 in the PI3K/AKT-mTOR and MAPK pathways, thereby facilitating GEECs proliferation and suppressing their apoptosis in vitro. Collectively, we constructed a new circRNA3669-miR-26a-RCN2 regulatory network in GEECs, which further provides strong evidence that circRNA could potentially play a crucial regulatory role in the development of RE in goats.