Calreticulin regulates syncytialization through control of the synthesis and transportation of E-cadherin in BeWo cells

During placental development, mononuclear cytotrophoblasts differentiate and fuse to syncytiotrophoblasts (STBs) to form syncytia, which secrete human chorionic gonadotropin (hCG). Decreased maternal perfusion of the placenta, which leads to placental dysfunction, induces changes in trophoblast syncytialization. Our previous study showed that calreticulin (CRT), a Ca2+-binding molecular chaperone found in the endoplasmic reticulum, is expressed in the human placenta and is involved in regulating extravillous trophoblast invasion, although its role in villous trophoblasts remains unclear. In this study, we investigated the functional role of CRT in trophoblast differentiation using the human trophoblast-like cell line BeWo, in which CRT gene expression was knocked down. We found that CRT was highly expressed in human third-trimester placentas and mainly localized in STBs. The fetal growth restriction group exhibited significantly lower CRT expression levels than did the control group. In BeWo cells, CRT knockdown markedly suppressed forskolin-induced cell fusion and β-hCG secretion. As for the mechanism responsible for these effects, the cell surface expression of E-cadherin, a key adhesion molecule related to syncytialization, was decreased, and E-cadherin accumulated adjacent to the Golgi apparatus in the CRT-knockdown cells, which led to dysfunctional cell-to-cell adhesion. Additionally, metabolic labeling and a pulse-chase study revealed that the protein expression of E-cadherin was suppressed at the translational level in the CRT-knockdown cells. Collectively, these results demonstrate that CRT regulates syncytialization by ensuring appropriate control of both the synthesis and transportation of E-cadherin, suggesting that CRT expression is important for placental development during pregnancy.