Syncytins expressed in human placental trophoblast

Abstract

Three versions of syncytiotrophoblast exist in the human placenta: an invasive type associated with the implanting conceptus, non-invasive villous type of definitive placenta, and placental bed giant cells. Syncytins are encoded by modified env genes of endogenous retroviruses (ERV), but how they contribute functionally to placental syncytial structures is unclear. A minimum of eight genes (ERVW1, ERVFRD-1, ERVV-1, ERVV-2, ERVH48-1, ERVMER34-1, ERV3-1, & ERVK13-1) encoding syncytin family members are expressed in human trophoblast, the majority from implantation to term. ERVW1 (Syncytin 1) and ERVFRD-1 (Syncytin 2) are considered the major fusogens, but, when the expression of their genes is analyzed by single cell RNAseq in first trimester placenta, their transcripts are distinctly patterned and also differ from those of their proposed binding partners, SLC1A5 and MFSD2A, respectively. ERVRH48-1 (suppressyn or SUPYN) and ERVMER34-1 are probable negative regulators of fusion and co-expressed, primarily in cytotrophoblast. The remaining genes and their products have been little studied. Syncytin expression is a feature of placental development in almost all eutherian mammals studied, in at least one marsupial, and in viviparous lizards, which lack the trophoblast lineage. Their expression has been inferred to be essential for pregnancy success in the mouse. All the main human ERV genes arose following independent retroviral insertion events, none of which trace back to the divergence of eutherians and metatherians (marsupials). While syncytins may be crucial for placental development, it seems unlikely that they helped orchestrate the divergence of eutherians and marsupials.

Keywords: Endogenous retrovirus; Extravillous trophoblast; Fusogen; Placental evolution; Suppressyn; Syncytiotrophoblast.

Fig. 1:

First trimester human placental development (left to right). The human blastocyst (far left), comprises an inner cell mass (ICM) completely surrounded by a layer of trophectoderm (TE) cells, and resides within the endometrial cavity just prior to implantation [days 5–6 post-fertilization (pf)]. At the time of implantation (days 8–9 pf), the ICM has developed the beginnings of an amniotic cavity (amnion) and the leading edge of the implanting embryo is characterized by an inner group of proliferative cytotrophoblast cells (CytoTB) and a deeper, non-proliferative, invasive, multinucleated, thickened mass of primitive syncytium. By day 13 pf, the embryo is fully implanted in the maternal decidua (MD). The primitive syncytium now completely surrounds the embryo. Primary villi have begun to form as invaginations of the cytotrophoblast layer. The primitive syncytium can invade the uterine glands (UG) and contacts maternal vessels. It contains lacunae that will grow and coalesce; they are filled with endometrial secretions and small amounts of maternal blood. By late in the first trimester (right panel), the villous placenta has been established with maternal blood now filling the intervillous space (IVS). Villi contain a discontinuous layer of proliferative CytoTB surrounded by multinucleated villous syncytiotrophoblast (SyncytioTB). The anchoring villi extend across the IVS to attach to the MD at their tips. Extending from these anchoring villi are the HLA-G-positive extravillous cytotrophoblast cells (EVTB). EVTB will invade deeply into the MD and remodel maternal spiral arteries. Although not depicted here, they will also invade into maternal veins, uterine glands and lymphatic spaces and reach well into the uterine myometrium. Multinucleated trophoblast giant cells (Giant cell) can be found deep in the MD and in the maternal uterine myometrium (MM).

Fig. 2:

Placental cell clusters from 10x Genomics and Smart-seq2 (SS2) scRNA-seq analysis [1] derived from five first trimester placentas visualized by Uniform Manifold Approximation and Projection (UMAP), visualizing the distribution of syncytin gene transcripts for ERVW1, ERVFRD-1, ERVV-1, ERVV-2, ERVH48–1, ERVMER34–1, ERV3–1, & ERVK3–1, as well as for two putative receptors, SLC1A5 and MFSD2A, in the human placenta. Additionally, we map the location of the mRNA encoding the pan-TB marker KR7, the mature syncytiotrophoblast marker CGB8, and the EVT marker HLA-G, as well as CCNB1 (to indicate replicating cells). Relative expression of genes within individual cells (dots) is on an indigo-yellow scale (Bottom Right). Also shown is a map of cell types (HC: Hoffbauer cells). All data were derived by use of the web site provided by Vento-Tormo et al. https://maternal-fetal-interface.cellgeni.sanger.ac.uk/. Importantly, CGB8 expression is confined to a small cluster of cells in the syncytiotrophoblast region, a pattern consistent with the hypothesis that mature syncytiotrophoblast is probably poorly represented in the diagrams.