Polarity and fusion of JAR choriocarcinoma cells as assessed by enveloped viral glycoproteins.

Placental trophoblasts are epithelial cells which undergo physiological fusion and generate syncytia. In this study, a placental choriocarcinoma cell line JAR was infected with two enveloped viruses, Parainfluenza type 3 (P3) or vesicular stomatitis virus (VSV). Both viruses possess a fusion glycoprotein known to be able to induce polykaryon formation in nonpolarized cells. The P3 virus fusion protein was localized on the apical as well as on the basal plasma membrane domains of the infected JAR cells. Infection of the JAR cell monolayer with P3 virus, whose fusion protein is active at pH 7.0, resulted in syncytia formation. Furthermore, the actin ring structure surrounding individual cells disappeared during the P3 virus induced cell-cell fusion. On the contrary, the VSV glycoprotein was found preferentially on the apical plasma membrane domain. To activate the VSV fusogen, the cells were subjected to pH 5.0. However, no syncytia formation peculiar to VSV-infected fibroblasts was observed, and the actin ring structures remained intact. We conclude that in JAR cells the VSV fusion protein exhibits a polarized expression while the P3 virus fusion glycoprotein is distributed between the two membrane domains. Our results suggest that an apically situated fusogen is not sufficient to mediate cell-cell fusion of JAR cells.