In contrast to other domestic mammals, the embryo-derived signal(s) leading to maternal recognition of pregnancy (MRP) are still unknow in the mare. We hypothesize that these embryonic signals could be packed into uterine extracellular vesicles (uEVs), acting as multi-signal messengers between the conceptus and the maternal tract, and contributing to MRP. To unveil these signals, the RNA and protein cargos of uEVs isolated from uterine lavages collected from pregnant mares (P; day 10, 11, 12 and 13 after ovulation) and cyclic control mares (C; day 10 and 13 after ovulation) were analyzed. Our results showed a fine-tuned regulation of the uEV cargo (RNAs and proteins), by the day of pregnancy, the estrous cycle, and even the size of the embryo. A particular RNA pattern was identified with specific increase on P12 related to immune system and hormonal response. Besides, a set of proteins as well as RNAs was highly enriched in EVs on P12 and P13. Differential abundance of miRNAs was also identified in P13-derived uEVs. Their target genes were linked to down- or upregulated genes in the embryo and the endometrium, exposing their potential origin. Our study identified for first time specific molecules packed in uEVs, which were previously associated to MRP in the mare, and thus bringing added value to the current knowledge. Further integrative and functional analyses will help to confirm the role of these molecules in uEVs during MRP in the mare.
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