Expression of Syncytin 1 (HERV-W), in the preimplantation human blastocyst, embryonic stem cells and trophoblast cells derived in vitro

STUDY QUESTION: As Syncytin 1 (human endogenous retrovirus (HERV-W)) is crucial for human embryo placentation is it expressed during preimplantation embryo development? SUMMARY ANSWER: Syncytin 1 was expressed mainly in trophoblast cells of the blastocyst particularly in cells underlying the inner cell mass (ICM). WHAT IS KNOWN ALREADY: Syncytin 1 (along with HERV-FRD or Syncytin 2) is expressed in first-trimester placenta and required for cell-cell fusion to enable formation of syncytiotrophoblast and effective placentation. STUDY DESIGN, SIZE AND DURATION: Preimplantation human embryos donated for research were cultured in vitro and protein expression of Syncytin 1 at the blastocyst stage of development investigated. Comparisons were made with protein (Syncytin 1) and mRNA (Syncytin 1 and 2) expression in human embryonic stem cells (hESCs) undergoing differentiation to trophoblast-like cells in vitro. In total, 10 blastocysts (x3 or 4 replicates) were analysed and 4 hESC lines. The study was terminated after consistent observations of embryos were made. PARTICIPANTS/MATERIALS, SETTING, METHODS: Donated embryos were thawed and cultured to blastocyst, fixed with 4% w/v paraformaldehyde. Syncytin 1 protein expression was determined by immunofluorescent localisation and confocal microscopy. Additionally, hESCs were differentiated to trophoblast-like cells in standard and conditioned culture medium with growth factors (bone morphogenetic protein 4 (BMP4) or fibroblast growth factor 4 (FGF4) and assessed for mRNA (Syncytin 1 and 2) by quantitative polymerase chain reaction (qPCR) and protein expression by immunolocalization and western blot. MAIN RESULTS AND ROLE OF CHANCE: Syncytin 1 was expressed in cytoplasm and on the cell surface of some trophoblast cells, and consistently the trophectodermunderlying the ICM of the blastocyst. There was weak but consistent expression of Syncytin 1 in cells on the periphery of the ICM also displaying pluripotency antibody marker (Tra-1-60). Three-dimensional reconstruction of confocal slice data provided good visualization of expression. The time course of expression of Syncytin 1 was replicated in hESCs differentiated in vitro confirming the embryo observations and providing statistically significant differences in protein and mRNA level (P = 0.002) and (P < 0.05), respectively. LIMITATION, REASONS FOR CAUTION: Culture of a limited number of embryos to blastocyst in vitro may not replicate the range and quality of development in situ. Probes (antibodies, PCR) were tested for specificity, but might have non-specific reactions. WIDER IMPLICATIONS OF FINDINGS: Syncytin expression is a prerequisite for embryo implantation and placentation. Understanding when expression first occurs during embryo development may be informative for understanding conditions of abnormal gestations such as pre-clampsia. STUDY FUNDING/COMPETING INTERESTS: The study was supported partly by an ERASMUS training grant and grant G0801059 from the Medical Research Council, U.K. There were no competing interests.