Syncytin-1

Syncytin-1 mediated trophoblast fusion is essential for normal placental development. The early placental barrier is composed of two placenta specific cell layers: cytotrophoblast and syncytiotrophoblast layer. Cytotrophoblasts are continually dividing, non-differentiated cells and the syncytiotrophoblast is one, fully differentiated, non dividing, fused cell syncytium. Syncytin-1 expression on the surface of cytotrophoblasts and syncytiotrophoblast mediate fusion. The syncytiotrophoblast layer is the interface between the developing fetus and the maternal blood supply, which forms, together with the underlying basal membrane and the fetal endothelium the placental barrier. The placental barrier enables nutrient and waste exchange, while blocking maternal immune and other cells, particles and molecules from passing into the fetal blood circulation. Cytotrophoblasts are forced into senescence by fusion into the syncytiotrophoblast. Therefore, cytotrophoblast proliferation is necessary for growth and maintenance of the syncytiotrophoblast layer. Syncytin-1 expression in cytotrophoblasts promotes G1/S transition and proliferation thereby ensuring continual replenishment of the cytotrophoblast pool. The name syncytin derives from its involvement in the formation of syncytium, the multinucleated syncytiotrophoblast protoplasm. There is another endogenous retroviral envelope protein expressed in the placenta from a different ERV family: syncytin-2 (of HERV-FRD). == Receptor ==

The syncytin-1 receptor is the Na-dependent amino acid transporter 2 (ASCT2 or SLC1A5). This receptor places syncytin-1 in a large viral interference group called retroviral mammalian type D receptor (RDR) interference group. Syncytin-1 has been shown to interfere with viral infection in-vitro by RDR interference group member spleen necrosis virus. Syncytin-1 can also recognize ASCT1 or SLC1A4, but this receptor is not a receptor for the RDR interference group. Mutation studies of syncytin-1 and of ASCT2 have provided insight into potential receptor binding domains and determinants. A putative receptor binding domain was identified in syncytin-1 at residues 117-144. The amino acid sequence at this region is well conserved amongst RDR interference group members. The motif SDGGGX2DX2R is present in all RDR interference group members within this conserved region and may play an important role in binding. Preliminary evidence with syncytin-1 and spleen necrosis virus indicate this motif contains the ASCT2 binding determinants. The largest ectodomain of ASCT1 and ASCT2, extracellular loop 2 (ECL2), contains at its C-terminus a 21 residue hypervariable region between human, mouse, and hamster receptors. This region was shown to confer specificity to receptor binding by most RDR interference group members. Both glycosylation pattern and amino acid sequence differences between human and rodent receptors are determinants in susceptibility to infection by RDR interference group members. Murine (mouse) ASCT1 expressing cells are only susceptible to syncytin-1 and another endogenous retroviral env protein (that of Baboon Endogenous Retrovirus) and human ASCT1 has only been shown to bind syncytin-1. Further research is needed to elucidate ASCT and RDR binding determinants. == Structure ==

Syncytin-1 shares many structural elements with class I retroviral glycoproteins (such as, Murine Leukemia Virus gp, and HIV gp120, gp41). It is composed of a surface subunit (SU) and transmembrane subunit (TM), separated by a furin cleavage site. The two subunits form a heterodimer and are likely linked by a disulfide bond between two conserved cysteine rich motifs: CXXC in SU and CX6CC in TM. == Clinical significance ==

Pre-eclampsia Hypoxic conditions characteristic of Pre-eclampsia and IUGR are associated with abnormal expression of syncytin-1 in trophoblast cells and pre-eclamptic placental tissue has reduced levels of syncytin-1 expression. Abnormal syncytin-1 expression likely plays an important role in placental pathologies. Neurological pathologies ERVW-1 is a single locus within the HERV-W family encoding a fully functional env protein. mRNA and protein expression of the ERVW-1 locus in neural tissue is implicated in neurodegeneration and development of multiple sclerosis. Multiple sclerosis retrovirus like particle (MSRV) envelope protein shares high sequence similarity to ERVW-1 encoded syncytin-1 and has long been studied as an important factor in MS pathogenesis. The gene locus of MSRV env has not been determined. Preliminary evidence implicates aberrant expression of ERVW-1 in neuron and glial cells and HERV-W LTR mediated aberrant cellular protein expression in the pathogenesis of bipolar disorder and schizophrenia. == References ==