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

ABSTRACT

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

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ACKNOWLEDGMENTS

We thank Miyuki Kurauchi, Yuka Miyoshi, Satori Matsumoto, Kaori Nakano, Yoshie Hayamizu, and Yui Kurokawa for technical assistance. We also thank Yuewi Li for helping with preliminary experiments at the initial stage and Masanori Uchikawa (Osaka University, Osaka, Japan) and Naoto Hayasaka (Yamaguchi University, Yamaguchi, Japan) for generous technical advice.

R. Ohgaki, T. Ohmori, S. Hara, S. Nakagomi, and M. Kanai-Azuma performed experiments and analyzed data. R. Ohgaki and Y. Kanai designed the experiments and wrote the manuscript with the participation of M. Kanai-Azuma, K. Kaneda-Nakashima, S. Okuda, and S. Nagamori. Y. Kanai conceived the project.

This study was supported by Grants-in-Aid for Scientific Research on Priority Areas (17081016 to Y. Kanai) and by the Regional Innovation Strategy Support Program to Y. Kanai from the Ministry of Education, Culture, Sports, Science, and Technology of Japan; Grants-in-Aid for Scientific Research (15H04685 and 22659052 to Y. Kanai) from the Japan Society for the Promotion of Science; and the Advanced Research for Medical Products Mining Program of the National Institute of Biomedical Innovation (12-02 to Y. Kanai).