Extracellular vesicles, syntaxin 2 and SNAP23 collectively play a role in the receptive uterine microenvironment

Uterine luminal fluid (ULF) composition plays a major role in cell-to-cell communication between the receptive endometrium and an invading blastocyst. ULF is made up of secretions from the uterine glands and uterine epithelial cells (UEC). However, the cellular mechanisms regulating these exocytotic secretions are not yet understood. This study investigated the role of extracellular vesicles (EVs) during early pregnancy using Transmission Electron Microscopy (TEM). TEM analysis at time of fertilisation (TOF) Day 1 and at time of receptivity (TOR) Day 5.5 revealed EVs present, with an abundance at TOR. Exocytosis signalling in UECs, by SNARE proteins syntaxin 2 (syn2) and SNAP23, was also examined. Immunofluorescence microscopy showed both syn2 and SNAP23 to be present in the apical area of UECs at TOR. Western blot and immunofluorescence quantification revealed a significant increase in syn2 and SNAP23 at TOR compared to TOF. SNAP23 colocalization with apical actin showed SNAP23 was in the luminal space contributing to ULF. Overall, this data shows EVs, syn2 and SNAP23 (potential receptivity marker) are present in ULF and may together create a favourable microenvironment for blastocyst implantation. Summary statement During uterine receptivity SNAREs participate in the secretion of ULF. EVs and SNAP23 are present in the uterine luminal space during uterine receptivity. SNAP23 has the potential to be used as a receptivity marker.