During early pregnancy the uterine epithelial cells (UECs) undergo a number of changes in preparation for blastocyst implantation. We have recently shown that this process includes an increase in caveolae in the basal plasma membrane (PM) of UECs at the time of implantation. Internalisation of caveolae at the time of implantation facilitates the removal of focal adhesion components from the basal PM, reducing the adhesiveness of UECs to the underlying matrix to permit blastocyst implantation.
Recent research suggested that the formation of caveolae is prevented by the transmembrane protein prominin-2, through the sequestration of plasma membrane cholesterol. The present study utilises quantitative transmission electron microscopy analysis and immunofluorescence microscopy to demonstrate a correlation between the localisation of prominin-2 and the abundance of caveolae in UECs. At the time of fertilisation prominin-2 is present in the basal PM, which appears flattened and contains relatively few caveolae. However, at the time of implantation prominin-2 is lost from the basal PM of UECs, which becomes highly tortuous and exhibits a significant increase in the abundance of caveolae, concurrent with the loss of focal adhesions.
Previous research has demonstrated that uterine receptivity and blastocyst implantation are negatively impacted by ovarian hyperstimulation (OH). The present study demonstrated that OH treatment caused a retention of prominin-2 at the basal PM of UECs at the time of implantation. This basal PM also remained flattened and there was no increase in caveolae. As a result, these UECs remained firmly anchored to the underlying matrix, due to the retention of focal adhesions. This study indicates that the loss of prominin-2 from the basal PM of UECs is an important step in preparation for blastocyst implantation. The disruption of this process by OH provides a potential mechanism for the reduction in uterine receptivity after such treatment.