Functional analyses of placental protein 13/galectin‐13

Abstract

Placental protein 13 (PP13) was cloned from human term placenta. As sequence analyses, alignments and computational modelling showed its conserved structural and functional homology to members of the galectin family, the protein was designated galectin-13. Similar to human eosinophil Charcot-Leyden crystal protein/galectin-10 but not other galectins, its weak lysophospholipase activity was confirmed by 31 P-NMR. In this study, recombinant PP13/ galectin-13 was expressed and specific monoclonal antibody to PP13 was developed. Endogenous lysophospholipase activity of both the purified and also the recombinant protein was verified. Sugar binding assays revealed that N-acetyl-lactosamine, mannose and N-acetyl-glucosamine residues widely expressed in human placenta had the strongest binding affinity to both the purified and recombinant PP13/galectin-13, which also effectively agglutinated erythrocytes. The protein was found to be a homodimer of 16 kDa subunits linked together by disulphide bonds, a phenomenon differing from the noncovalent dimerization of previously known prototype galectins. Furthermore, reducing agents were shown to decrease its sugar binding activity and abolish its haemagglutination. Phosphorylation sites were computed on PP13/galectin-13, and phosphorylation of the purified protein was confirmed. Using affinity chromatography, PAGE, MALDI-TOF MS and post source decay, annexin II and beta/gamma actin were identified as proteins specifically bound to PP13/galectin-13 in placenta and fetal hepatic cells. Perinuclear staining of the syncytiotrophoblasts showed its expression in these cells, while strong labelling of the syncytiotrophoblasts' brush border membrane confirmed its galectin-like externalization to the cell surface. Knowing its colocalization and specific binding to annexin II, PP13/galectin-13 was assumed to be secreted to the outer cell surface by ectocytosis, in microvesicles containing actin and annexin II. With regard to our functional and immunomorphological results, PP13/galectin-13 may have special haemostatic and immunobiological functions at the lining of the common feto-maternal blood-spaces or developmental role in the placenta.Keywords: brush border membrane; carbohydrate binding; galectin; lysophospholipase; placental protein.Placental protein 13 (PP13) is a member of the group of the so-called Ôpregnancy-related proteinsÕ [1] that might be highly expressed in placenta and some maternal/fetal tissues during pregnancy. The structural and functional characteristics of these proteins and their possible role in placental development and regulation pathways are receiving increased interest at present. PP13 was first isolated from human placenta and characterized by Bohn et al. in 1983. It was found to be comprised of two identical 16 kDa subunits held together by disulfide bonds, and to have the lowest carbohydrate content (0.6%) of any known placental proteins [2]. Later, cloning of PP13 was performed in parallel by two research groups [3,4], and its sequence was deposited separ...

First trimester screening of maternal placental protein 13 for predicting preeclampsia and small for gestational age: In-house study and systematic review

“…[2,3] Moreover, expression is induced by syncytialisation of trophoblast cell lines in vitro suggesting that the expression is regulated by terminal differentiation. [2,[4][5][6] The presence of PP13 on the surface of the placenta indicates that PP13 may have haemostatic and immunobiological functions in the placenta. In support of this, aggregates rich in PP13 are associated with zones of T-cell, neutrophil and macrophage accumulation.…”

Embryo–Placento–Maternal Interaction and Biomarkers: From Diagnosis to Therapy – A Workshop Report

“…Detailed molecular biological analyses revealed its characteristic structural and functional features, identifying PP13 as galectin-13, a placental member of a beta-galactoside binding protein family. PP13 was shown to be localized at the apical membrane of the placenta, on the lining of the common feto-maternal blood-spaces [18]. In parallel, clinical tests identified PP13 as a promising first trimester biomarker for the prediction of PE [19].…”

T cell apoptosis at the maternal–fetal interface in early human pregnancy, involvement of galectin-1

“…This redundancy may be important only in genetic backgrounds that pose an increased immunological stress. As multiple galectins are present in the human decidua (28)(29)(30)(31), multiple galectins present in the murine placenta (39) could, conceivably, compensate for each other.…”

“…Fifteen galectins have been described in mammals, of which gal1, gal3, gal9, and gal13 occur in the human placenta (4,(27)(28)(29)(30)(31). Cycling endometrium produces gal1 during the secretory phase, and its expression is augmented during early pregnancy.…”