Pregnancy-specific glycoproteins: complex gene families regulating maternal-fetal interactions

The pregnancy-specific glycoproteins (PSGs) are the most abundant trophoblastic proteins in maternal blood during human pregnancy and they appear to be exclusive to species with hemochorial placentation. There are ten protein-coding human PSG genes (PSG1 - PSG9, PSG11) and also multiple PSG genes in non-human primates, rodents and bats. Several studies indicate that PSGs have immunoregulatory, pro-angiogenic, and anti-platelet functions. Some PSGs have been shown to bind different moieties on the surface of cells, including the tetraspanin CD9, heparan sulphate, and specific integrins. Recently, PSG1 was shown to associate with and activate the anti-inflammatory cytokines transforming growth factor (TGF)-β1 and TGF- β2 making PSG1 one of the few known biological activators of these important cytokines. TGF-βs regulate many biological processes essential for pregnancy success including trophoblast invasion and proliferation, angiogenesis, extracellular matrix formation and tolerance to the fetal semi-allograft. As summarized in this review, progress has been made in recent years towards a better understanding of the functions of these proteins which were originally described in the early 1970s, but more research will likely contribute to demonstrate their importance for a successful pregnancy.