Cell-cell-communication during placental development and possible implications for trophoblast proliferation and differentiation.

Since direct cell-cell-communication plays a crucial role in the coordination of proliferation and differentiation processes during development we have focused on the expression patterns of gap junctions and their functional implication in the human placenta. The gap junction protein connexin40 (Cx40) is expressed in the proximal extravillous trophoblast of cell islands and columns. In accordance with these observations, isolated trophoblast cells from first and second trimester placentae and choriocarcinoma cells (Jeg-3) reveal Cx40 expression. This channel is not only characteristic of the trophoblast cells along the invasive pathway but also of endothelial cells. To elucidate the functional role of this channel for proliferation and invasion, the non-coupled Jeg-3 cells have been transfected with Cx26, Cx40 and Cx43, respectively. In contrast to Cx40, the Cx26 channel was more potent in reducing proliferation and inducing differentiation indicated by hCG-beta secretion. Using the nude mouse model to study invasion properties of choriocarcinoma cells, we demonstrated that malignant trophoblast cells were able to invade host vessels and to replace endothelial cells. Upregulation of endogeneous connexin genes in tumours grown in nude mice enforces further experimental strategies to investigate the importance of the different channels to fake the cell biological program of endothelial cells.