ST6GALNAC1-mediated sialylation in uterine endometrial epithelium facilitates the epithelium-embryo attachment

Successful embryo implantation necessitates a synergistic interaction between the receptive uterine endometrium and the competent embryo. During the "window of implantation" stage, the endometrial epithelium is coated with a layer of glycocalyx, where terminal sialic acid and fucose on glycans play a crucial role in facilitating embryo attachment. In this article, we study the role of ST6 N-acetylgalactosaminide alpha-2,6-sialyltransferase 1 (ST6GALNAC1) and its regulated Sialyl Thomsen-nouvelle (sTn) glycan structure in controlling endometrial function. IHC results showed that ST6GALNAC1 and sTn are mainly located in the endometrial epithelium, and their expression is elevated in receptive endometrium relative to non-receptive endometrium. In vitro embryo implantation model verified that manipulating the endometrial sTn by over- or down-regulation of ST6GALNAC1 influenced the receptiveness of AN3CA and Ishikawa cells to JAR-spheroids. Immunoprecipitation combined with mass spectrometry identified certain adhesion molecules (CD44 and integrin β1) are highly modified with sTn by ST6GALNAC1 in endometrial cells. We next focus on sialic acid-binding immunoglobulin-like lectin 6 (Siglec-6) since it recognizes sTn and analysis results of published single-cell sequencing dataset showed that SIGLEC6 was mainly concentrated in the polar trophectoderm of the human embryo. Ectopic expression of Siglec-6 promotes the adhesiveness of HTR8/Svneo-spheroids on endometrial cells, and more importantly, we confirmed that Siglec-6 binds with sTn-modified CD44. This study demonstrates that endometrial sTn regulated by ST6GALNAC1 facilitates embryo attachment via binding with trophoblastic Siglec-6, which gives a better understanding of the glycobiology of implantation.