A PKA-ezrin-connexin 43 signaling complex controls gap junction communication and thereby trophoblast cell fusion

Cell fusion occurs as part of the differentiation of some cell types including myotubes in muscle and osteoclasts in remodeling bone. In the human placenta, mononuclear cytotrophoblasts in a human chorionic gonadotropin (hCG)-driven process fuse to form multinucleated syncytia that allow exchange of nutrients and gases between the maternal and fetal circulation. Experiments displacing protein kinase A (PKA) from A kinase anchoring proteins (AKAPs) or depleting specific AKAPs by siRNA-mediated knock down pointed to ezrin as a scaffold required for hCG-, cAMP and PKA-mediated regulation of the fusion process. By a variety of immunoprecipitation and immunolocalization experiments, we show that ezrin directs PKA to a molecular complex of connexin 43 (Cx43) and zona occludens-1 (ZO-1). A combination of knock down and reconstitution experiments with ezrin or Cx43 with or without the ability to bind its interaction partner or PKA demonstrated that ezrin-mediated coordination of PKA and Cx43 localization is necessary for discrete control of Cx43 phosphorylation and hCG-stimulated gap junction communication which triggers cell fusion in cytotrophoblasts.