Subcellular localization of PP5/TFPI-2 in human placenta: a possible role of PP5/TFPI-2 as an anti-coagulant on the surface of syncytiotrophoblasts

Abstract

Placental protein 5 (PP5)/tissue factor pathway inhibitor-2 (TFPI-2), a serine proteinase inhibitor, is homologous to tissue factor pathway inhibitor (TFPI) and commonly found in peripheral blood of pregnant woman. Although TFPI is well known to be synthesized primarily in endothelium and to play an important role in regulation of the extrinsic pathway of blood coagulation, the function of PP5/TFPI-2 remains unclear. Our previous report demonstrated that PP5/TFPI-2 expression is ubiquitous and extremely high in growing placental tissue. Using newly generated polyclonal anti-PP5/TFPI-2 antibody, and by immunohistochemistry and immunoelectromicroscopy, we examined precise localization of PP5/TFPI-2 in placenta especially in syncytiotrophoblasts, which had been shown to produce PP5/TFPI-2 mRNA by our previous study using in situ hybridization. Immunoelectromicroscopy revealed PP5/TFPI-2 localizing on the surface of the microvilli and the membrane of endoplasmic reticulum of syncytiotrophoblasts. To confirm the cell surface association of PP5/TFPI-2, placental villi were incubated with heparin and resultant soluble fraction was analysed by Western blotting. Heparin liberating PP5/TFPI-2 from villi suggested that PP5/TFPI-2 might be retained on the microvilli surface through the binding to membrane-anchored proteoglycans such as glypican and/or syndecan family members. We also examined the relationship between the presence of cell layer of syncytiotrophoblasts and the coagulation using clinical specimens, and revealed that the fibrin depositions were specifically observed on the regions lacking syncytiotrophoblasts cell layer in placental villi. Therefore, it is likely that during pregnancy PP5/TFPI-2 may be retained on the surface of placental villi via proteoglycans, and may play an important role to maintain intervillous blood flow and the patency of microvasculature in feto-maternal blood system mediated by the inhibition of serine proteinases involved in the blood coagulation.