Implantation of the blastocyst into the uterine wall involves coordinated changes in uterine epithelial cells (UECs), trophoblastic cells and uterine luminal fluid volume and composition. One way the luminal fluid is altered is via UEC exocytosis. The SNARE protein complex including t-SNAREs (Syntaxin-2 and SNAP23) and v-SNAREs (VAMP) with the help of regulatory protein (Munc 18), regulate exocytosis in a variety of cell types. Currently regulation of exocytosis in UECs is unknown however the secretin hormone, which is released from decidual cells during early pregnancy, has been shown to regulate epithelial cell exocytosis in other tissues.
Immunofluorescence microscopy and western blotting identified changes in localisation and quantity of SNARE complex proteins SNAP23, syntaxin 2, VAMP and Munc 18-2 in uterine epithelial cells and luminal fluid at the time of uterine receptivity in rats. For the first time, secretin receptor was localised apically in rat UECs in vivo and found in receptive human endometrial epithelial cells in vitro.
An increase in apical vesicles within UECs is one of the many changes seen at the time of uterine receptivity. Apical secretin receptors in UECs could initiate an exocytosis event coordinating release of these vesicles into the uterine luminal fluid. Munc 18-2 localised to the perinuclear/Golgi area could be involved in movement of vesicles through the Golgi complex. SNARE proteins SNAP23, syntaxin-2 and VAMP, were found apically in UECs and regulate apical vesicle docking with the UEC apical plasma membrane. The presence of SNAP23 in uterine luminal fluid specifically at the time of receptivity may also play a novel role in uterine fluid. This study describes how exocytosis could contribute to uterine luminal fluid composition – an essential requirement for uterine receptivity and successful implantation.