Embryo-maternal interaction is associated with the location of the embryo during the mobility phase in mares

Despite advances in the molecular analysis of the endometrium during early equine pregnancy, little is known about embryo-maternal interaction during the embryo mobility phase. It has been demonstrated that the embryo stimulates upregulation of prostaglandin synthase (PTGES) locally in the endometrium. This suggests that endometrial gene expression is influenced by the location of the embryo in the uterus. The aim of this study was to characterize the transcriptome of the endometrium of mares based on the location of the mobile embryo and to compare the gene expression of the endometrium between the uterine horns of pregnant (n = 8) and non-bred (n = 8) mares. On day 12 (D0 = ovulation), a cytobrush was used to collect endometrial samples from the middle of the uterine horn of pregnant mares that did (PE) and did not (PWE) contain the embryo, and from the middle of both uterine horns of non-bred mares (NB). RNA was extracted, followed by library preparation and total RNA-sequencing (PE-150bp). On average, 40 million reads were generated per sample. Reads were trimmed and mapped to the current equine reference genome (Ecab 3.0). Differentially expressed genes (DEGs) were identified using Deseq2 (P value < 0.01) and gene ontology (GO) analysis was performed using Shinny Go. Based on the location of the embryo (PE vs PWE), 56 DEGs were identified, 41 upregulated and 15 downregin PWE relative to PE. The DEGs included two genes (LOC100064092, FADS3) involved with fatty acid metabolism, and seven genes (RIPOR2, SMAD6, ROBO1, TENM1, SELL, ADORA2A, and AGR2) involved in cell-to-cell interaction. Based on GO, the DEGs in the pregnant group were mainly associated with cell adhesion, response to chemokines, inflammatory responses, connective tissue replacement involved in inflammatory responses, and connective tissue replacement. Additionally, there were 669 and 707 DEGs, in PE vs. NB and PWE vs. NB comparisons, respectively. The DEGs were mainly involved in cell metabolism and signaling. Moreover, there were 13 overlapping genes among the PE vs. NB and PE vs. PWE groups. There were two genes (PLIN5, LOC100064092) involved with fatty acid metabolism, one with hematopoiesis (SERPINB10), one with vascularization and development (ATOH8), one gene related to osmotic pressure (MUC5B), and one gene related to endometrial remodeling (TIMP1). In conclusion, these results suggest that the embryo initiates local maternal-embryo dialogue, embryo development and immune system regulation during the early pre-fixation phase of pregnancy. This project was supported by the John P. Hughes Endowment and the Center for Equine Health with funds provided by the State of California satellite wagering fund and contributions by private donors.