Abstract
Pregnancy-specific glycoproteins (PSGs) are immunoglobulin superfamily members encoded by multigene families in rodents and primates. In human pregnancy, PSGs are secreted by the syncytiotrophoblast, a fetal tissue, and reach a concentration of up to 400 ug/ml in the maternal bloodstream at term. Human and mouse PSGs induce release of anti-inflammatory cytokines such as IL-10 and TGFβ1 from monocytes, macrophages, and other cell types, suggesting an immunoregulatory function. RGD tri-peptide motifs in the majority of human PSGs suggest that they may function like snake venom disintegrins, which bind integrins and inhibit interactions with ligands. We noted that human PSG1 has a KGD, rather than an RGD motif. The presence of a KGD in barbourin, a platelet integrin αIIbβ3 antagonist found in snake venom, suggested that PSG1 may be a selective αIIbβ3 ligand. Here we show that human PSG1 binds αIIbβ3 and inhibits the platelet – fibrinogen interaction. Unexpectedly, however, the KGD is not critical as multiple PSG1 domains independently bind and inhibit αIIbβ3 function. Human PSG9 and mouse Psg23 are also inhibitory suggesting conservation of this function across primate and rodent PSG families. Our results suggest that in species with haemochorial placentation, in which maternal blood is in direct contact with fetal trophoblast, the high expression level of PSGs reflects a requirement to antagonise abundant (3 mg/ml) fibrinogen in the maternal circulation, which may be necessary to prevent platelet aggregation and thrombosis in the prothrombotic maternal environment of pregnancy.
Highlights
Pregnancy-specific glycoproteins (PSGs) are expressed throughout human pregnancy and are the most abundant fetal proteins secreted by the placental syncytiotrophoblast into the maternal bloodstream in mid to late pregnancy (,200 - 400 mg/ml) [1,2,3]
PSG1 is the only PSG with a KGD, rather than RGD, motif on the exposed F-G loop of the protein N-domain, a motif with selective activity towards platelet integrin aIIbb3 in the barbourin disintegrin [25]. We tested whether it exhibits anti-thrombotic activity by determining whether recombinant PSG1 made in HEK293T cells inhibits binding of an Oregon Green conjugate of human fibrinogen to Thrombin Receptor-Activating Peptide (TRAP)-activated washed human platelets
We show that human PSG1 and PSG9 and mouse Psg23 proteins inhibit the platelet – fibrinogen interaction suggesting that this may be a conserved function of all PSG proteins during pregnancy
Summary
Pregnancy-specific glycoproteins (PSGs) are expressed throughout human pregnancy and are the most abundant fetal proteins secreted by the placental syncytiotrophoblast into the maternal bloodstream in mid to late pregnancy (,200 - 400 mg/ml) [1,2,3]. Human PSG proteins consist of four immunoglobulin (Ig)-like domains – an amino-terminal Ig variable-like (N) domain and three Ig constant-like (C) domains and, like the closely related CEACAMs, are heavily N-glycosylated, on the amino-terminal N-domain [5]. The lack of discernible orthologous relationships between primate and rodent PSGs suggests that these gene families evolved independently from a common ancestor in the different mammalian lineages, suggesting strong selection pressure driving rapid evolution [6]. Notwithstanding the lack of orthology and the different domain contents of human and mouse PSGs, they share conserved functions, as outlined below. The human PSG Ndomains and the mouse N1-domains exhibit evidence of conserved or convergent evolution [6]
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