Trophinin, tastin, and bystin: A complex mediating unique attachment between trophoblastic and endometrial epithelial cells at their respective apical cell membranes

Embryo implantation is a complex process consisting of multiple cross-talks between maternal and embryonic cells. Defining the mechanisms underlying implantation at molecular level is challenging task in reproductive biology. In order to identify molecules involved in cellular interactions between trophoblastic and endometrial epithelial cells, we have established two human cell lines, trophoblastic HT-H and endometrial epithelial SNG-M. These two cell types exhibit cell adhesion at their respective apical cell membranes. Molecules involved in this unique cell adhesion were identified by expression complementary DNA cloning and were named trophinin, tastin, and bystin. Trophinin is a membrane protein thought to have self-binding activity and thus mediates homophilic cell adhesion. Tastin and bystin are cytoplasmic proteins required for trophinin to exhibit cell adhesion activity. Trophinin is strongly expressed in trophectoderm of monkey blastocysts. In human endometrium, trophinin is expressed for a limited period in the luminal epithelium at the time expected for implantation. In human placenta, trophinin, tastin, and bystin are strongly expressed in trophoblast and endometrium at the uteroplacental interface at an early stage in pregnancy. All these molecules disappear from the human placenta in the second trimester. The unique expression pattern and cell adhesion activity exhibited by trophinin, tastin, and bystin suggest strongly the involvement of these molecules in the initial attachment of blastocyst to uterus.