Follicular fluid extracellular vesicles support energy metabolism of bovine oocytes, improving blastocyst development and quality^†

Extracellular vesicles (EVs), from follicular fluid seem to play a significant role in communication within ovarian follicles in several species. The present study aimed to examine the supporting effect of follicular fluid derived small EVs (FF-sEVs) during in vitro maturation (IVM) of bovine cumulus-oocyte complexes (COCs) under conditions of disturbed energy metabolism. Bovine COCs were matured in vitro with inhibitors targeting lipid metabolism (etomoxir) or glucose metabolism (iodoacetate combined with dehydroepiandrosterone), in the presence or absence of FF-sEVs. Following maturation, oocytes and cumulus cells were analyzed by real-time qPCR and stained to visualize lipid droplets. The uptake of FF-sEVs was visualised by fluorescent labelling. In vitro fertilization and embryo culture were followed by mass spectrometry analysis of hatched blastocysts.

We demonstrate for the first time that FF-sEVs are transported from the medium into the oocytes, via the cumulus cells and through transzonal projections (TZPs) into the perivitelline space and ooplasm. Cumulus cells under metabolic stress conditions exhibit an increased FF-sEVs uptake from the maturation medium. FF-sEVs supplementation during metabolic stress conditions enhances the MII rate in oocytes and positively affects subsequent embryo development and quality revealed by altered metabolic activity. Lipid droplet parameters and gene expression in cumulus cells and oocytes are affected by FF-sEVs supplementation, which is more pronounced in cumulus cells.

Our findings show that FF-sEVs supplementation during IVM under metabolic stress conditions significantly affects COCs, with a positive effect on further blastocyst quality. We provide novel insights into the role of FF-sEVs in oocyte maturation and blastocyst development.