Trophoblasts, placental cells of epithelial lineage, undergo extensive differentiation to form the cellular components of the placenta. Trophoblast progenitor cell differentiation into the multinucleated syncytiotrophoblast is a key developmental process required for placental function, where defects in syncytiotrophoblast formation and turnover associate with placental pathologies and link to poor pregnancy outcomes. The cellular and molecular processes governing syncytiotrophoblast formation are poorly understood, but require the activation of pathways that direct cell fusion. The protease, A Disintegrin and Metalloproteinase 12 (ADAM12), controls cell fusion in myoblasts and is highly expressed in the placenta localizing to multiple trophoblast populations. However, the importance of ADAM12 in regulating trophoblast fusion is unknown. Here, we describe a function for ADAM12 in regulating trophoblast fusion. Using two distinct trophoblast models of cell fusion, we show that ADAM12 is dynamically upregulated and is under the transcriptional control of protein kinase A. siRNA-directed loss of ADAM12 impedes spontaneous fusion of primary cytotrophoblasts, whereas overexpression of the secreted variant, ADAM12S, potentiates cell fusion in the Bewo trophoblast cell line. Mechanistically, both ectopic and endogenous levels of ADAM12 were shown to control trophoblast fusion through E-cadherin ectodomain shedding and remodeling of intercellular boundaries. This study describes a novel role for ADAM12 in placental development, specifically highlighting its importance in controlling the differentiation of villous cytotrophoblasts into multinucleated cellular structures. Moreover, this work identifies E-cadherin as a novel ADAM12 substrate, and highlights the significance that cell adhesion molecule ectodomain shedding has in normal development. Cell Death and Differentiation (2015) 22, 1970-1984 doi:10.1038/cdd.2015 published online 24 April 2015 In mammalian development, the placenta forms the mechanical and physiological link between maternal and fetal circulations. In rodents and higher order primates, including humans, this transfer is achieved through extensive uterine infiltration by fetal-derived cells of epithelial lineage called trophoblasts.1 Trophoblast differentiation is essential for optimal placental function, where the underlying molecular processes regulating specific functions of distinct trophoblast populations are strictly controlled. Defects in trophoblast differentiation associate with impaired placental function and directly impact fetal and maternal health. Within the placenta, progenitor trophoblasts (also called villous cytotrophoblasts; vCTs) overlie a well-defined basal lamina forming an organized, mitotically active epithelial layer. vCT differentiation into an overlying multinucleated structure called the syncytiotrophoblast (synCT) is a key event in placental development. 3 The synCT, composed of millions of nuclei sharing one common cytoplasm, directs nutrient and gas exchange between m...