Characterization of pregnancy-associated glycoproteins extracted from zebu (Bos indicus) placentas removed at different gestational periods

A multiplexed parallel reaction monitoring assay to monitor bovine pregnancy-associated glycoproteins throughout pregnancy and after gestation

Bovine pregnancy-associated glycoproteins (boPAGs) are extensively glycosylated secretory proteins of trophoblast cells. Roughly 20 different boPAG members are known but their distribution patterns and degree of glycosylation during pregnancy are not well characterized. The objective of the present study was the development of a parallel reaction monitoring-based assay for the profiling of different boPAGs during pregnancy and after gestation. Furthermore, we investigated the effects of N-glycosylation on our analytical results. BoPAGs were purified from cotyledons of four different pregnancy stages. The assay detects 25 proteotypic peptides from 18 boPAGs in a single run. The highest abundances were found for boPAG 1 in both, glycosylated and deglycosylated samples. Strongest effects of glycosylation were detected during mid and late pregnancy as well as in afterbirth samples. Furthermore, we identified different boPAG-clusters based on the observed relative protein abundances between glycosylated and deglycosylated samples. A linkage between the impact of glycosylation and potential N-glycosylation sites or phylogenetic relation was not detected. In conclusion, the newly developed parallel reaction monitoring-based assay enables for the first time a comprehensive semi-quantitative profiling of 18 different boPAGs during pregnancy and post-partum on protein level, thereby investigating the influence of glycosylation. The results of this study provide new and important starting points to address further research on boPAGs to better understand their physiological role during pregnancy and for the development of new pregnancy detection tests.

Using Pregnancy-Associated Glycoproteins (PAGs) to Improve Reproductive Management: From Dairy Cows to Other Dairy Livestock

Simple Summary

Pregnancy loss is a major cause of infertility in dairy animals, particularly in cattle, which affects the productivity and profitability of farms. Detecting these unsuccessful pregnancies could offer farmers the opportunity to reduce the economic damage caused by pregnancy loss. The determination of proteins secreted by the placenta and related to the presence of a viable conceptus called pregnancy-associated glycoproteins (PAGs) represents a diagnostic tool to identify pregnant or non-pregnant animals and to predict early pregnancy failures. This review describes the state of the art related to PAGs’ function, pregnancy profile, and use in reproductive management in bovine and other dairy livestock.

Abstract

Pregnancy success represents a major issue for the economic income of cattle breeders. Early detection of pregnant and non-pregnant animals, as well as the prediction of early pregnancy failure, can influence farm management decisions. Several diagnostic tools for pregnancy are currently available. Among these, pregnancy-associated glycoproteins (PAGs) have been shown to be useful for identifying the presence of vital embryos and for pregnancy follow-up monitoring. This review presents an overview of the PAGs’ functions, their pregnancy trends, and their use as a tool to improve reproductive management in bovine and other dairy livestock, such as small ruminants and buffalos.

Identification of pregnancy-associated glycoprotein family (PAG) in the brown bear (Ursus arctos L.)

Pregnancy-associated glycoproteins (PAGs) are abundant embryo-originated products expressed in the pre-placental trophoblast and, later, in the post-implantational chorionic epithelium of some mammalian species. This paper describes the identification and cellular immunolocalization of the chorionic PAG family in the discoidal-type placenta of the brown bear (Ursus arctos L. - Ua), in which the PAGs were named 'UaPAG-Ls'. The study used: 1) Western blot for total placental glycoproteins; and 2) cross-species heterologous double fluorescent immunohistochemistry (IHC) for cellular immune-localization of the PAGs. This is the first study reporting the identification and immunolocalization of the UaPAG-L family in placental cells during early pregnancy in the brown bear. Our Western analysis revealed a dominant mature 72 kDa UaPAG-L isoform was expressed in all Ua placentas during early pregnancy. Various other UaPAG-L isoforms (16-66 kDa) were also identified. Using IHC, the UaPAG-L proteins were localized to trophectodermal cells (TRD), where signal intensity resembled intense TRD proliferation within developing placenta. The data increases our general knowledge of PAG proteins localized in discoidal-type placenta during early pregnancy in the brown bear.

Investigation of PAG2 mRNA Expression in Water Buffalo Peripheral Blood Mononuclear Cells and Polymorphonuclear Leukocytes from Maternal Blood at the Peri-Implantation Period

The main objective of this study was to assess PAG2 mRNA expression in maternal blood cells at the peri-implantation period in water buffalo; moreover, we wanted to evaluate the earliest time in which PAG-2 could be detected in maternal blood. Thirty-two lactating buffaloes artificially inseminated (AI) were utilized. Blood was collected at Days 0, 14, 18, 28, 40 after AI (AI = day 0). Pregnancy was diagnosed by ultrasound at Days 28 and 40 post AI. Out of 32 buffaloes, 14 were pregnant (P group) and 18 were not pregnant (NP group). The plasma PAG-2 threshold of 1.0 ng/mL in the P group was reached at day 40 post AI. PAG2 mRNA expression differed between the P and NP groups, and was either evaluated in Peripheral Blood Mononuclear Cells (PBMC) or Polymorphonuclear Leukocytes (PMN), starting from day 14. However, both the estimated marginal means and multiple comparisons showed that PAG2 mRNA expression was higher in PMN than PBMC. In the present study, PAG-2 appeared in the blood (40 Days post AI), and an early expression of PAG2 mRNA at Day 14 post AI was also observed. Although further research is undoubtedly required, PAG2 mRNA in peripheral blood leukocytes could be using to better understand the role that PAGs play during pregnancy in buffalo.

Primary structures of different isoforms of buffalo pregnancy-associated glycoproteins (BuPAGs) during early pregnancy and elucidation of the 3-dimensional structure of the most abundant isoform BuPAG 7

Pregnancy-associated glycoproteins (PAGs) are expressed during pregnancy by the trophoectodermal cells of fetus. Presence of PAGs in dam's circulation has been widely used in pregnancy diagnosis. The present study reports the identification and characterization of different PAG isoforms in buffalo during early stages of pregnancy. The PAG mRNAs isolated from fetal cotyledons (Pregnancy stages: 45, 75 and 90 days) were successfully cloned in pJET1.2 vector and transformed in E. coli. A total of 360 random clones were sequenced and correlated with their stages of expression. A total of 12 isoforms namely, BuPAG 1, 2, 4, 6, 7, 8, 9, 13, 15, 16, 18 and one new isoform were identified. BuPAG 7 was found as the most abundant isoform in all three stages followed by BuPAG 18. Further, a large number of variants were found for most of these isoforms. Phylogenetic relationship of identified BuPAGs showed that BuPAG 2 belonged to an ancient group while other members clustered with modern group. Three-dimensional (3D) structure of BuPAG 7 was determined by homology modeling and molecular dynamic (MD) simulations which displayed a typical fold represented by other aspartic proteinase (AP) family members. Molecular docking of Pepstatin inhibitor with BuPAG 7 revealed to interact through various hydrogen bonding and hydrophobic interactions. Various amino acid substitutions were observed in peptide-binding cleft of BuPAG 7. Superimposition of BuPAG 7 with homologous structures revealed the presence of a 35-41 amino acid long insertion (alpha helix connected by two loops) near the N- terminus which seems to be a unique feature of BuPAG 7 in AP family. This is the first report on identification and sequence characterization of PAG isoforms in buffalo with unique finding that these isoforms represent many transcript variants. We also report 3D structure of the most abundant isoform BuPAG 7 for the first time.

Expression of pregnancy-associated glycoprotein family in the epitheliochorial placenta of two Camelidae species (C. dromedarius and C. bactrianus)

This study describes placental morphology and immunolocalization of the placental pregnancy associated glycoprotein-like family (PAGs) identified in two selected taxa of Old-World camels of the Camelidae family: Camelus dromedarius (Cd) and Camelus bactrianus (Cb). Placental tissues of Cd from days 140-293 post-coitum (dpc), term (404 dpc); and of Cb from term (440 dpc) were examined. Histological staining (hematoxylin/eosin and propidium iodine) revealed the development of the placental structure, while chorionic folding increased the feto-placental surface during the progress of pregnancy. The camelid placenta during early pregnancy is similar to the diffuse epitheliochorial type, and during later stages of pregnancy resembles the synepitheliochorial (cotyledonary) type. Placental expression of the PAGs was detected (Alexa 488 - green) within camelid trophectoderm cells (TRD - chorionic epithelium as outer layer of embryonic cells) among all placental cells with nuclei stained by propidium iodide (red). The PAGs, identified in both Camelidae taxa, were named CbPAGs and CdPAGs. Placental CbPAG and CdPAG expression is restricted to the TRD cells, which are differentially developed throughout gestation. Cross-reactivity of polyvalent anti-pPAG polyclonals with the CbPAGs and CdPAGs revealed high structural similarities of the PAG-like epitopes in pigs and camels. This is the first study identifying PAG expression in chorionic cells of the camel placenta.

Purification of pregnancy-associated glycoproteins from late-pregnancy Bubalus bubalis placentas and development of a radioimmunoassay for pregnancy diagnosis in water buffalo females

Background

Pregnancy-associated glycoproteins (PAGs) were first described as placental antigens present in the blood serum of the mother soon after implantation. Here, we describe the purification of several pregnancy-associated glycoproteins from water buffalo placenta (wbPAGs). A specific radioimmunoassay (RIA) was developed for early pregnancy diagnosis in buffalo species.

Results

Amino-terminal microsequencing of immunoreactive placental proteins allowed the identification of eleven wbPAGs sequences [Swiss-Prot accession numbers: P86369 to P86379]. Three polyclonal antisera (AS#858, AS#859 and AS#860) were raised in rabbits against distinct wbPAG fractions. A new RIA (RIA-860) was developed and used to distinguish between pregnant (n = 33) and non-pregnant (n = 26) water buffalo females.

Conclusions

Our results confirmed the multiplicity of PAG expression in buffalo placenta. In addition, the RIA-860 system was shown to be sensitive, linear, reproducible, accurate and specific in measuring PAG concentrations in buffalo plasma samples from Day 37 of gestation onwards.

Assessment of pregnancy-associated glycoprotein (PAG) concentrations in swamp buffalo samples from fetal and maternal origins by using interspecies antisera

Pregnancy-associated glycoproteins (PAG) constitute a large family of glycoproteins found in the outer placental epithelial cell layer of the placenta in Eutherian species. In ruminants, they are noted to be structurally closely related among the different species. This study was designed to determine PAG concentrations in maternal and fetal plasma, allantoic and amniotic fluids in buffalo species. Antisera (AS) generated in rabbits against distinct PAG molecules were used in three radioimmunoassay (RIA)-PAG systems: RIA-1 (antiserum raised against bovine PAG67kDa; AS#497), RIA-2 (antiserum raised against caprine PAG55 + 62 kDa; AS#706) or RIA-3 (antiserum raised against buffalo PAG; AS#859). Samples were collected at a slaughterhouse (n = 67). PAG concentrations determined by RIA-2 gave significantly higher results in both allantoic and amniotic fluids (12.7 ± 2.1 ng/mL and 24.0 ± 7.3 ng/mL, respectively). Regarding maternal and fetal plasma, PAG concentrations obtained by RIA-2 (21.8 ± 2.4 ng/mL and 20.2 ± 2.5 ng/mL, respectively) and RIA-3 (25.0 ± 2.2 ng/mL and 21.9 ± 3.2 ng/mL, respectively) were higher than those obtained by RIA-1 (15.5 ± 1.4 ng/mL and 16.1 ± 1.8 ng/mL, respectively). The correlation among the three systems was very high. The study clearly reveals the ability of different PAG-RIA systems to measure PAG concentration in swamp buffalo samples.

Pregnancy-associated glycoprotein (PAG) family localized in chorionic cells within the epitheliochorial/diffuse placenta of the alpaca (Lama pacos)

Pregnancy-associated glycoproteins (PAGs) are abundant embryo-originated products expressed in the pre-placental trophoblast and later in the post-implantational chorionic epithelium of some ungulate species. This paper describes the cellular immunolocalization of the chorionic PAG family in the epitheliochorial placenta type of the alpaca (Lama pacos-Lp), in which the PAGs were named 'LpPAGs'. Placental Lp sections (5 μm) of different females near mid-pregnancy (150 days post coitum; dpc), advanced pregnancy (244-263 dpc) and late pregnancy (347 dpc) were used for cross-species (heterologous-ht) double fluorescent immunohistochemistry (htdF-IHC). The htdF-IHC was performed with primary rabbit polyvalent anti-porcine PAG polyclonals. The LpPAG immuno-complexes were visualized with secondary goat anti-rabbit immunoglobulins-conjugated with Alexa 488 fluorophore (green), among all nuclei of placental cells stained with propidium iodide (red). This is the first study reporting the immunolocalization of the LpPAG family identified by htdF-IHC at the feto/maternal interface during different pregnancy stages of the alpaca. The most dominant and strongest immune-positive LpPAG signals were found in the well-developed chorionic cell layer. Our htdF-IHC indicated relatively high epitope resemblance to that of the PAGs in camelids and pigs. These data increase our general knowledge of chorionic PAG localization during pregnancy-stage dependent development of the epitheliochorial diffuse placenta type in the alpaca.

Isolation of new pregnancy-associated glycoproteins from water buffalo (Bubalus bubalis) placenta by Vicia villosa affinity chromatography

The present study describes the isolation and characterization of new pregnancy-associated glycoprotein molecules (PAG) from midpregnancy and late-pregnancy placentas in the water buffalo (Bubalus bubalis). After extraction, the homogenates are subjected to acid and ammonium sulfate precipitations followed by DEAE chromatography. Subsequently, the water buffalo PAG (wbPAG) from these solutions are enriched by Vicia villosa agarose (VVA) affinity chromatography. As determined by western blotting with anti-PAG sera, the apparent molecular masses of the immunoreactive bands from the VVA peaks range from 59.5 to 75.8kDa and from 57.8 to 73.3kDa in the midpregnancy and late-pregnancy placentas, respectively. Amino-terminal microsequencing of the immunoreactive proteins has allowed the identification of three distinct wbPAG sequences, which have been deposited in the SwissProt database: RGSXLTIHPLRNIRDFFYVG (acc. no. P85048), RGSXLTILPLRNIID (acc. no. P85049), and RGSXLTHLPLRNI (acc. no. P85050). Their comparison to previously identified proteins has shown that two of them are new because they have not been described before. Our results confirm the suitability of VVA chromatography for the enrichment of the multiple PAG molecules expressed in buffalo placenta.

Identification of novel pregnancy-associated glycoproteins (PAG) expressed by the peri-implantation conceptus of domestic ruminants

Pregnancy-associated glycoproteins (PAG) are a multigene family, related to aspartic proteinases, expressed in the placenta of artiodactyls. Ruminant PAG expression varies spatially as well as temporally during gestation. The aim of this study was the identification of novel PAG expressed around implantation in domestic ruminants and to study their phylogenetic relationships. Day 12, 14, 17 (ovine); day 15, 18 (bovine); and day 12, 14, 15, 16, 18 (caprine) conceptuses were used for reverse transcription-polymerase chain reaction (RT-PCR) performed on RNA extracted from conceptuses pooled on each day of recovery. In ovine, a single PAG was detected on day 14, ovPAG2, from the PAG II group. On day 17, three more PAG were identified. One of them similar to ovPAG1 and two unknown named as ovPAG10 and ovPAG11, apparently from the PAG I group. In bovine, PAG expression was detected by day 15 and two transcripts were identified on day 18. One, boPAG2, from the PAG II group and other was previously unknown, boPAG22, closest to boPAG2. In goats, PAG expression was not detected on non-elongated conceptuses (days 12, 14 and 15). Two different transcripts were identified in both day 16 and 18 elongated conceptuses, one similar to caPAG2, from the PAG II group, whereas the other, caPAG12, apparently from the PAG I group, was previously unknown. Interestingly, the three new early PAG identified in sheep and goats are clustered together within the PAG I group. In summary, a complement of new PAGs expressed at implantation in domestic ruminants has been cloned.

Isolation of pregnancy-associated glycoproteins from placenta of the American bison (Bison bison) at first half of pregnancy

This paper describes the successful purification and characterisation of pregnancy-associated glycoproteins (PAG) extracted from placenta (3-4 months) of American bisons (Amb). Chorionic AmbPAG proteins were purified from foetal cotyledonary tissues (CT) and liquid cotyledonary-carrying proteins (LCP) leaking from damaged cells. Our protocols successfully indicated the usefulness of AmbPAG protein identification, especially from LCP fraction. The AmbPAGs were extracted, precipitated and eluted during DEAE cellulose chromatography. The richest protein fractions were further chromatographed on VVA (Vicia villosa agglutinin affinity column), then characterised by mono- and bi-dimensional electrophoresis, Western blot and N-terminal amino acid (aa) sequence. After being transferred to PVDF membranes, three selected VVA-purified AmbPAG isoforms differing in molecular masses and isoelectric points (Ip 4-4.6) were selected for sequencing. One identified N-terminal 25aa sequence of AmbPAG72kDa CT form was identified as completely new (RGSNLTSLPLQNVIDLFYVGNITIG). Two other AmbPAG proteins purified from different sources (74kDa CT and 76kDa LCP forms; RGSNLTIHPLRNIRDIFYVGNITIG) were identical or corresponded to N-terminus of various bovine PAGs (boPAG). The two AmbPAGs (74kDa CT and 76kDa LCP) revealed identical micro-sequence to boPAG7; and were similar mainly to bovine PAG4, -6, -15 and -17 precursors that were identified by full-length sequencing derived from cDNA cloning. The novel sequence of the AmbPAG (72kDa CT) was related to some boPAG and various other ruminant PAG precursors (caprine and ovine). All three identified AmbPAG sequences were also relatively similar to mature forms of purified native boPAG(56-75kDa) proteins. This is the first report indicating aa sequences of native AmbPAG proteins purified from placenta (CT and LCP) of bison species. The N-terminal sequences of the AmbPAGs have been deposited in the EMBL-EBI database (UniProtKB; Accession Nos.: P84916, P84917 and P84918).

Pregnancy-associated glycoprotein family (PAG)--as chorionic signaling ligands for gonadotropin receptors of cyclic animals

The chorionic pregnancy-associated glycoprotein (PAG) family was identified in pigs, cattle and other eutherian mammals. The objective of this study was to examine whether secretory chorionic proteins (including PAGs), produced in vitro by explants of porcine and bovine placental membranes, may interact with other proteins, i.e. gonadal and extragonadal binding sites. Trophoblast (TRF) and trophectoderm (TRD) explants of pigs (n=38; 14-61 dpc-day post coitum) or cotyledons (CT) of cows (n=5; 40-110 dpc) were long-term cultured. Released chorionic proteins were ultra-fractionated from media (>10 kDa) or precipitated [20-75% of (NH(4))(2)SO(4)]. The PAGs were monitored by Western/PAGE (30-73 kDa). Secretory TRF/TRD/CT (+PAG) proteins (0.78-25 microg/ligand) were examined by radioreceptor assay (RRA) with iodinated hCG ((125)I-hCG) for binding-effectiveness by gonadotropin receptors of cyclic pigs and cows (cRc). Gonadal and extragonadal cRc isolated from luteal-phase corpora lutea and uteri (cCLRc, cMYORc and cENDRc) were tested with positive control ligands: porcine LH and hCG (0.39-50 ng/ml). Control proteins produced in vitro by endometrial (END) explants of cyclic (cEND), pseudopregnant (PsEND) and pregnant (pEND) gilts were utilised as negative ligands (0.78-25 microg/ligand). Positive control ligands competed with (125)I-hCG for binding by cCLRc, cMYORc and cENDRc (18-61%/B(0) for hCG and 27-57%/B(0) for LH). Negative ligands (cEND, PsEND and pEND) did not show cRc bindings. This is the first RRA report indicating that in vitro produced porcine TRF/TRD proteins (+PAG) competed (P< or =0.05) with (125)I-hCG for binding by cCLRc, cMYORc and cENDRc in a concentration- and pregnancy stage-dependent manner. The highest competition with (125)I-hCG (up to P< or =0.001) was found for ultra-fractionated TRF/TRD proteins (>10 kDa) during early pregnancy (<22 dpc). The greatest competition (P< or =0.05) of precipitated porcine TRD proteins (>30 dpc) was detected for fractions obtained by saturation with use of 20% of (NH(4))(2)SO(4). Bovine CT proteins revealed lower competition of (125)I-hCG for bovine cCLRc (during 45 dpc only) that was more efficient with CT (up to 71%) than with non-labelled hCG (82%). The PAG proteins may play a role as potential "signal molecules", because they were able to interact with gonadotropin receptors of luteal-phase animals. It seems that the pPAG proteins may be luteoprotective chorionic-origin signals during implantation and placentation, according to binding-effectiveness of the chorionic ligands that was comparable to LH/hCG ligands with gonadal and extragonadal receptors of cyclic animals.

Chorionic mRNA expression and N-glycodiversity of pregnancy-associated glycoprotein family (PAG) of the European bison (Bison bonasus)

Placental PAG mRNA expression and N-glycodiversity of multiple PAG proteins secreted in vitro by trophectoderm (chorion epithelium) of wild pecoran Bovidae taxons was not examined previously. The study on European bison (Eb) aimed: (1) to determine placental PAG mRNA expression by in situ hybridisation; (2) to identify a profile of pecoran PAG protein family secreted in vitro by cotyledonary (CT) explants; (3) to examine N-glycodiversity of the PAG proteins in this wild taxon. In addition, we compared (4) a profile and N-glycodiversity of the PAG protein family secreted in vitro by CT and interCT-trophectoderm (intCT-TRD) explants of domestic ruminants. Cotyledonary sections of the Eb were used for in situ hybridisation (ISH) with (35)S-labelled probes produced with porcine PAG cDNA as templates. Various CT and intCT-TRD explants were long-term cultured in vitro. Chorionic proteins were isolated from media, ultra-filtrated (>10 kDa MWCO) and analysed by PAGE-Western blotting with various polyclonal anti-PAG sera. Protein samples with or without enzymatic deglycosylation were examined after different times of explant cultures. Released chorionic proteins were deglycosylated by N-glycanase F (PNGase F+) and compared to glycosylated forms (PNGase F-). This is the first paper demonstrating the PAG-like mRNA transcript expression (by ISH) and N-glycodiversity of immuno-reactive PAG-like proteins (produced in vitro by chorionic explants) of European bison. Various PAG proteins of Eb (EbPAG) were secreted by CT explants during long-term in vitro studies. Major approximately 78, approximately 67 and approximately 65 kDa EbPAG-like proteins were reduced by enzymatic deglycosylation (at least by 10 kDa). Considerably smaller amounts of approximately 45 kDa EbPAG-like proteins were also observed. In addition, we have found that various PAG proteins (30-73 kDa) were secreted by bovine CT explants, during long-term in vitro cultures. Corresponding amounts of PAG proteins, similar in M(r), were also secreted by intCT-TRD explants, whose tissues were not utilised for PAG protein extraction during other scientists' previous studies. It seems that the M(r)-heterogeneity and N-glycodiversity of the PAG protein family can play very important role during feto-placental interactions in Bovidae species.

Pregnancy associated glycoprotein-1, -6, -7, and -17 are major products of bovine binucleate trophoblast giant cells at midpregnancy

Pregnancy associated glycoproteins (PAGs) are extensively glycosylated secretory proteins of ruminant trophoblast cells. In cattle placenta several PAG cDNAs are expressed, but the variety of correspondent proteins and their degree of glycosylation are not well characterized. Thus, we purified PAGs by using a protocol which included a lectin (Vicia villosa agglutinin) affinity chromatography. Due to their specific glycosylation pattern, PAGs derived from binucleate trophoblast giant cells were highly enriched by this protocol. PAGs were purified from cotyledons of 2 day 100 placentas and from a single placenta at day 155 and 180. In all samples three major bands (75; 66; 56 kDa) were detected by one-dimensional SDS-PAGE. Mass-spectrometric analysis identified the 75 kDa band as a mixture of PAG-7 and PAG-6, the 66 kDa band as PAG-1 and the 56 kDa band as PAG-17. N-terminal sequencing of the day 100 sample confirmed the mass spectrometric identifications. Enzymatic release of N-glycans with peptide-N-glycanase-F from PAGs reduced the molecular weight to approximately 37 kDa which corresponds to the theoretical molecular mass of PAGs. Limited peptide-N-glycanase-F treatment revealed that all four N-glycosylation sites are quantitatively occupied in PAG-1. Compared to PAG-1 the number of potential N-glycosylation sites is lower in PAG-17 (three sites) and higher in PAG-6 and -7 (five and six sites, respectively). This suggests that the number of attached N-glycans is the main determinant of molecular mass of bovine PAGs. The degree of glycosylation may be a major factor regulating the plasma half life of PAGs.