Essential Roles of L-Type Amino Acid Transporter 1 in Syncytiotrophoblast Development by Presenting Fusogenic 4F2hc

The layers of the epithelial syncytium, i.e., syncytiotrophoblasts, differentiate from chorionic trophoblasts via cell fusion and separate maternal and fetal circulations in hemochorial placentas.L-type amino acid transporter 1 (LAT1) and its covalently linked ancillary subunit 4F2hc are colocalized on both maternal and fetal surfaces of syncytiotrophoblasts, implying their roles in amino acid transfer through the placental barrier.In this study, LAT1 knockout, in addition, revealed a novel role of LAT1 in syncytiotrophoblast development.LAT1 at midgestation was selectively expressed in trophoblastic lineages in the placenta, exclusively as a LAT1-4F2hc heterodimer.In LAT1 homozygous knockout mice, chorionic trophoblasts remained largely mononucleated, and the layers of syncytiotrophoblasts were almost completely absent.The amount of 4F2hc protein, which possesses a fusogenic function in trophoblastic cells, as well as in virus-infected cells, was drastically reduced by LAT1 knockout, with less affecting the mRNA level.Knockdown of LAT1 in trophoblastic BeWo cells also reduced 4F2hc protein and suppressed forskolin-induced cell fusion.These results demonstrate a novel fundamental role of LAT1 to support the protein expression of 4F2hc via a chaperone-like function in chorionic trophoblasts and to promote syncytiotrophoblast formation by contributing to cell fusion in the developing placenta.KEYWORDS 4F2 antigen, amino acid transporter, cell fusion, placenta, syncytiotrophoblast T he placenta serves as a site of maternofetal exchange of various substances, as well as an immunological barrier and an endocrine organ.Although the overall tissue organization varies among species, the importance of the epithelial syncytium, i.e., syncytiotrophoblasts, that separates maternal and fetal circulations is common to hemochorial placentas in which the maternal blood directly contacts with the fetal chorion.In the labyrinth of mouse placentas, two syncytiotrophoblast layers start to emerge at midgestation by cell fusion of chorionic trophoblasts (1, 2).Two retrovirusderived murine proteins, syncytins, have been identified to mediate syncytialization of trophoblastic cells in the developing placenta (3, 4).Although the in vivo relevance has not been well addressed, several other proteins, including the single-membranespanning type II membrane glycoprotein 4F2hc/CD98hc/FRP-1, have been also suggested to be involved in the cell fusion of trophoblastic cells (5-12).4F2hc is the heavy chain subunit of 4F2 antigen (CD98) and is covalently linked to a nonglycosylated light chain.The suppression of 4F2hc expression in human trophoblastic BeWo cells reduces forskolin-induced cell fusion, suggesting its significant implication in the syncytiotro-