Sancai Xie

Pregnancy-Associated Bovine and Ovine Glycoproteins Exhibit Spatially and Temporally Distinct Expression Patterns During Pregnancy

Abstract The pregnancy-associated glycoproteins (PAG) constitute a large family of recently duplicated genes. They show structural resemblance to pepsin and related aspartic proteinases. A total of 21 bovine (bo) PAG and 9 ovine (ov) PAG cDNA have been identified. Phylogenetic analysis indicated that the PAG are divided into two main groupings that accurately reflect their tissue expression, as determined by in situ hybridization. In the first pattern, represented by ovPAG-2 and boPAG-2, -8, -10, and -11 (where the numbering is arbitrary and reflects order of discovery within species), expression occurred throughout the outer epithelial layer of the placenta (trophectoderm). The second pattern was predominant localization to binucleate cells. Ribonuclease protection assays, which allow discrimination between closely related transcripts, have shown that the expression of PAG varies in a temporal manner over pregnancy. Of those bovine PAG expressed predominantly in binucleate cells, boPAG-1, -6, and -7 are expressed weakly, if at all, by Day 25 placenta, but are present at the middle and end of pregnancy. Others, such as boPAG-4, -5, and -9, are expressed at Day 25 and at earlier stages. Although not among the earliest PAG produced by the trophoblast, boPAG-1 has been used for pregnancy diagnosis, particularly in dairy cows, where there is a major need for a sensitive method capable of detecting pregnancy within 1 mo of conception. It seems likely that some of the newly discovered PAG will be better candidates than PAG-1 for pregnancy diagnosis.

Glycoproteins of the aspartyl proteinase gene family secreted by the developing placenta

Pregnancy in cattle and sheep can be diagnosed by the presence of placentally-derived antigens (pregnancy-associated glycoproteins or PAG-1) in maternal serum soon after implantation begins at about Day 20 following conception. Molecular cloning of their cDNA has revealed that PAG-1 belong to the aspartic proteinase gene family and have about 50% amino acid sequence identity to pepsin. However, critical amino acid substitutions at the active site regions suggest that both bovine and ovine PAG-1 are enzymatically inactive. PAG-1 expression has been shown by in situ hybridization and immunocytochemistry to be localized to the trophoblast binucleate cells, which invade maternal uterine endometrium during implantation. The glycoproteins are concentrated in dense cytoplasmic granules that are discharged after the binucleate cells have migrated to the maternal side of the placental barrier. We suggest, therefore, that the PAG-1 might have an endocrine function either as carriers of other bioactive peptides or by acting as hormones themselves. Recently screening of placental libraries with nucleic acid probes has identified additional cDNA that are very abundant and code for polypeptides (PAG-2 and PAG-3) related to, but antigenically and structurally distinct from PAG-1 described above. These molecules have sequences of amino acids at their catalytic centers that are consistent with their being potentially functional proteinases but their role during pregnancy, like that of PAG-1, is unclear.

An Aspartic Proteinase Expressed in the Equine Placenta

This manuscript describes the cloning of a novel aspartic proteinase expressed in the placenta of the horse (order Perrisodactyla). Evidence for similar genes in the cat (Carnivora) and ruminants (Artiodactyla), indicates that these molecules have been conserved within widely divergent species with distinct types of placentation. Since ePAG is produced by the outer cell layer (trophoblast) of the placenta, it can tentatively be grouped with the pregnancy-associated glycoproteins (PAG) of cattle, sheep, and pig. The high sequence identity that ePAG shares with pepsinogens as well as the PAG, indicates that ePAG may be the evolutionary bridge that links these two groups of aspartic proteinases.

EXPRESSION OF MULTIPLE GENES FOR PREGNANCY-ASSOCIATED GLYCOPROTEINS IN THE SHEEP PLACENTA

Together these experiments have showed that there are many PAG genes in the ovine genome and many of these are expressed in the placenta. These PAG genes vary as much as 40% in nucleotide sequence identity. Thus, a wide range of ovPAG molecules are secreted at the placental-uterine surface during pregnancy.

Pregnancy Associated Glycoproteins

Publisher Summary This chapter describes the structural chemistry and the biological aspects of Pregnancy-Associated Glycoproteins (PAGs). A combination of southern genomic blotting and cDNA screening has indicated that the PAGs are present in all species of the suborder Ruminantia and probably throughout the entire Artiodactyla (even-toed ungulates) order of mammals, which includes pigs and their relatives. This diversification of the lineage by a series of duplications appears to have coincided with the separation of Artiodactyla and Perissodactyla (horses, rhinos, and their relatives), an event that occurred at least 80 million years ago. Another unusual feature of the PAGs is that they have accumulated mutations that lead to amino acid changes at high rates. In general, such mutations tend to be deleterious and are eliminated. Thus, the great variability in PAGs has not only occurred rapidly, but also must have been aided by positive selection. The PAGs are transcribed from an extensive gene family that probably arose via a single duplication event from a pepsinogen F-like progenitor gene about 80 million years ago. They are secretory products of the outer cell layer (trophectoderm) of the placentas of mammals within the Artiodactyla order.