Cadherin-mediated differentiation and fusion of human trophoblastic cells in vitro

The placenta supports fetal growth and development during pregnancy. A key step in human placentation involves the terminal differentiation and fusion of villous cytotrophoblasts to form the multinucleated syncytial trophoblast. To date, the cellular mechanism(s) that mediate this developmental process remain poorly characterized. We have determined that the expression of the calcium-dependent cell adhesion molecule, known as cadherin-11, increases during the terminal differentiation and fusion of villous cytotrophoblasts isolated from the human term placenta, BeWo choriocarcinoma cells cultured in the presence of cyclic AMP, and primary cultures of human extravillous cytotrophoblasts treated with transforming growth factor-β1. These observations led us to hypothesize that cadherin-11 mediates the formation of multinucleated syncytium from mononucleate trophoblastic cells in vitro. As cadherin function is regulated by interactions with the cytoplasmic proteins, known as the catenins, we first examined α-, β-, γ-catenin, and p120ctn expression in primary cultures of human villous cytotrophoblasts. The terminal differentiation and fusion of these trophoblastic cells was associated with a reduction in the expression of these four catenin subtypes. In contrast, α-, β-, γ-catenin, and p120ctn were maintained in non-fusing JEG-3 choriocarcinoma cells. These four catenin subtypes were subsequently immunolocalized to the mononucleate cells but not the multinucleated syncytium present in these trophoblastic cell cultures and the villous cytotrophoblasts of the human placenta. To better define the role(s) of cadherin-11 in the terminal differentiation and fusion of human trophoblastic cells in vitro, we examined the effects of ectopic cadherin- 11 expression on the morphological differentiation of non-fusing JEG-3 cells. Cadherin-11 expression in mononucleate JEG-3 cells promoted the terminal differentiation and fusion of these cells with time in culture. In contrast, a reduction in cadherin-11 expression was capable of inhibiting the formation of multinucleated syncytium in primary cultures of human villous cytotrophoblasts. Collectively, these studies demonstrate a critical role for cadherin-11 in the terminal differentiation and fusion of human mononucleate trophoblastic cells in vitro. These observations further our understanding of the adhesive mechanisms operative during the formation and organization of the human placenta and provide insight into the cell biology of cadherin-11.Medicine, Faculty ofObstetrics and Gynaecology, Department ofGraduat