In the bovine placenta, fetal cotyledons and maternal caruncles together form the placentomes [1]. Within these placentomes, fetal stem villi branch into smaller terminal villi [2], expanding into maternal crypts to ensure an efficient exchange. Fetal binucleate trophoblast cells (BNCs) fuse with maternal epithelial cells to form trinucleate feto-maternal hybrid cells [3] and express pregnancy associated glycoproteins (PAGs) [4] and placental lactogen [5], which are stored within their secretory granules. Lectins specifically binding to BNCs and their secretory granules are Phaseolus vulgaris leucoagglutinin (PHA-L) and Dolichos biflorus agglutinin (DBA) [6], [7]. These secretory granules contain intraluminal vesicles [8] with a mean diameter of 50 – 60 nm [9]. The content of the granules is released by exocytosis into the maternal connective tissue after fusion of the BNC [10]. The vesicles are exocytosed along with the protein contents of the secretory granules and can be found as extracellular vesicles (EVs) in the maternal connective tissue close to trinucleate feto-maternal hybrid cells [8], [9].

Bovine placental EVs isolated from primary trophoblast cells can influence the expression of uterine receptivity-related proteins in bovine endometrial cells [11]. Such observations indicate the participation of bovine placental EVs in feto-maternal communication and their potential to affect maternal cells locally and possibly systemically.

EVs are membrane-bound vesicles classified into three main subtypes among others [12], [13]: microvesicles and exosomes [14], and apoptotic bodies, which are released from dying cells [15]. Microvesicles (50 - 1000 nm), are shed by outward budding from the plasma membrane [16]. Exosomes (30 – 150 nm), are formed when multivesicular bodies (MVBs) fuse with the cell membrane and release their intraluminal vesicles into the extracellular space [17]. EVs transport various cargo molecules, such as proteins and nucleic acids, and thereby participate in cellular communication [18].

In the human placenta, EVs deriving from villous explant cultures carry ERVW-1 (syncytin-1), a human endogenous retroviral envelope protein [19]. ERVW-1 induces syncytialization and thus the formation of the syncytiotrophoblast through cell fusion [20]. It is found on EVs derived from villous cytotrophoblast explants and enhances the integration of these EVs into BeWo cells [21]. This phenomenon might be explained by the specific function of retroviral envelope proteins. The surface unit of retroviral envelope proteins interact with corresponding receptors, hASCT2 in the case of syncytin-1 [22], leading to conformational changes within the transmembrane unit and triggering membranal fusion [23]. Like ERVW-1, the bovine endogenous retroviral envelope protein K1 (BERV-K1) also has fusogenic capacities [24]. BERV-K1 is predominantly expressed in BNCs and was integrated into the genome of Bovinae, a subfamily of the Bovidae containing mostly larger-sized ungulates such as domestic cattle [24], [25].

However, it is unknown if the EVs from BNCs carry BERV-K1 and which functions they might have. The migration of BNCs and their fusion with a maternal epithelial cell suggest its possible involvement in feto-maternal communication. The degranulation of the intraluminal vesicles contained in their secretory granules upon fusion, suggests a purpose of these vesicles at the feto-maternal interface. If present on the EV surface, fusogenic BERV-K1 might assist their function by facilitating their internalization into target cells, through its inherent ability to fuse membranes. In this study, we isolated EVs from bovine placental tissue and evaluated the presence of BERV-K1 on these EVs.