Extracellular vesicles from early pregnant uterine fluids promote expression of implantation-related markers in ovine endometrium

Extracellular vesicles (EVs) are crucial for intercellular communications and they may play important role in the delivery of molecular messages during the pre-implantation period of pregnancy. EVs have been isolated from uterine luminal fluid (ULF) in human and sheep. Recent data have provided evidence that EVs contained in ovine ULF can penetrate endometrial epithelial cells after a 6-days infusion in vivo. Nevertheless, embryo implantation involves rapid and dynamic changes in molecular interactions with the endometrium. Our present work aims to determine whether EVs collected from ULF interact with endometrial epithelial cells and modify cell physiology after a short time of in vitro and in vivo incubation conditions.Primary cultures of endometrial epithelial cells were derived from ovine uteri on day 12 post-oestrus. EVs were purified from ovine ULFs on Day 14 of pregnancy (2 days before conceptus implantation). The presence of EVs in ULF was confirmed by transmission electron microscopic observation. ULF EVs were labeled with lipophilic PKH26 fluorescent dye and then incubated with primary cultures of epithelial cells during 30 min to 24h. Confocal microscopy analyses revealed an uptake of EVs as early as 30 minutes after incubation, followed by a progressive increase of intracellular fluorescence up to 6 hours.For the in vivo study, ovine ULF EVs isolated on Day 14 of pregnancy were labeled with PKH26 fluorescent dye and infused into the uterine lumen of cyclic ewes on Day 12 post-oestrus. After 24h, numerous epithelial cells from the luminal layer and superficial glands exhibited an intensive fluorescence signal, whereas deep endometrial glands displayed few fluorescent cells. No signal was detectable in the stroma. The impact of EVs on endometrial function was investigated by quantifying transcript expression of a selection of endometrial genes. First data pointed out that expression of two genes, including the Myxovirus-Resistance Protein (MX1) was up-regulated following EVs uptake by the endometrial epithelium. This work provides first evidence that ovine EVs from pregnant ULF can (i) enter in endometrial epithelial cells within 30 min in vitro or 24h in vivo (ii) modulate expression of endometrial gene expression known to be critical for embryo implantation. These results suggest a critical role for EVs in the preparation of endometrium when implantation iniates.