Extracellular vesicles, syntaxin 2 and SNAP23 in the uterine microenvironment of the rat.

IN BRIEF: During uterine receptivity, exocytosis from uterine epithelial cells (UECs) contributes to uterine fluid composition, playing a role in communication with an implanting blastocyst. SNAREs, a family of proteins involved in exocytosis, are increased in the receptive uterine epithelium of the rat. ABSTRACT: Uterine luminal fluid is composed of secretions from the uterine luminal and glandular epithelial cells. The fluid composition plays a role in cell-to-cell communication between the receptive endometrium and an invading blastocyst. Part of this fluid is released from the epithelial cells via exocytosis, mechanisms regulating this are not yet understood. Using transmission electron microscopy, this study identified extracellular vesicles in the uterine lumen at the time of fertilisation and uterine receptivity. Immunofluorescence microscopy showed SNARE proteins syntaxin 2 and SNAP23 in the apical area of UECs at the time of receptivity. SNAP23 was also found in the uterine fluid on day 5.5 of early pregnancy. Western blotting of isolated UECs demonstrated a significant increase in syntaxin 2 and SNAP23 at the time of uterine receptivity compared to the time of fertilisation. The morphological evidence of extracellular vesicles in the uterine lumen and the presence of SNARE proteins syntaxin 2 and SNAP23 in the apical part of the luminal epithelium at the time of uterine receptivity suggests that exocytosis contributes to the composition of the uterine luminal fluid, a potential component of maternal-foetal communication during early pregnancy.