Human embryonic stem cells derived trophoblastic spheroids (BAP-EB) as human embryo surrogate for identification of attachment-related endometrial surface molecules

Abstract

Study question: Does BAP-EB induce expression of attachment-related molecules in endometrial epithelial cells? Summary answer: BAP-EB produced soluble factor(s) that induced expression of surface proteins on endometrial cells involving in BAP-EB-endometrial cells attachment. What is known already: Implantation failure can be caused by impaired endometrial receptivity. However, mechanistic study of the implantation process in humans is limited by lack of an appropriate implantation model. Trophoblastic spheroids (BAP-EB) derived from human embryonic stem cells by bone morphogenic protein 4, A83-01 and PD173074 treatment possess human blastocoel-like structure and attach specifically onto receptive endometrial epithelial cell lines. The use of BAP-EB as human embryo surrogate for mechanistic study of early implantation is explored in the current report. Study design, size, duration: Human embryonic stem cells (H9) were induced to differentiate into BAPEB for 48h and 72h. Endometrial epithelial cells (EEC) were isolated from endometrial biopsies obtained from infertile women on day 2 (hCG+2, n=20) or day 7 (hCG+7, n=13) post-hCG induction of ovulation. The BAP-EB spent media were collected before (0h) and after BAP differentiation for 48h and 72h. Participants/materials, setting, methods: The attachment rates of H9-BAP-EB onto the cultured EEC were compared after 3 hours of coculture. The spent media of BAP-EB-48h (attachment incompetent) or BAP-EB-72h (attachment competent) were used to treat the receptive endometrial Ishikawa cells for 3 hours, and the differentially expressed cell surface proteins were identified by mass spectrometry. The functional roles of the target surface proteins were studied by antibody blocking during the attachment assays. Main results and the role of chance: The attachment rates of BAP-EB derived from H9 onto EEC isolated from receptive (hCG+7 day) phase were significantly higher than those of prereceptive (hCG+2 day) phase (hCG+7: 26.7±14.7% vs hCG+2: 2.3±3.0%, p<0.001) endometria. After incubation with the spent media of attachment competent (72h) or incompetent (0h, 48h) BAP-EB for 3 hours, cell surface proteins of treated endometrial Ishikawa cells were biotinylated and identified by liquid chromatography-mass spectrometry. In total, eight cell surface proteins were induced by the spent medium of attachment competent BAP-EB-72h when compared to that of the incompetent BAP-EBs. Five of them namely Catenin alpha-1, Catenin delta-1, Plexin-B2, Extended synaptotagmin-2 and CD98hc were selected for further study. Their expressions were significantly higher in the receptive endometrial cell lines (RL95, Ishikawa) when compared to the non-receptive ones (HEC-1B, AN3CA). The BAP-EB attachment rates were significantly reduced after treating Ishikawa cells with antibodies against CD98hc (42%) and Catenin delta-1 (36%) when compared to normal IgG control (62%, p<0.01, chi-square test). Limitations, reasons for caution: The soluble factor(s) that secreted from BPA-EB have not been identified. BAP-EB and the endometrial cell lines used may not fully represent the in vivo developed human blastocysts, and the primary endometrial cells, respectively. Wider implications of the findings: The number of human embryos donated for research use is limited. The results obtained in this study demonstrated the usefulness of BAP-EB as human embryo surrogate for studying human implantation, and the role of human embryos in inducing attachment related endometrial epithelial cell surface molecules.