Cloning, expression, purification, and immunocharacterization of placental protein-14

Affinity purification of native glycodelin from amniotic fluid for biological investigations and development of a glycodelin ELISA for clinical studies

INTRODUCTION

Glycodelin is a glycoprotein with different oligosaccharides that are responsible for its diverse biological functions in contraception and immunosuppression. Therefore, it is necessary to have access to adequate amounts of glycodelin with retained carbohydrate structure for functional studies because the carbohydrate part can be lacking or be insufficient in recombinant glycodelin from prokaryotic and eukaryotic cell systems.

METHODS AND RESULTS

Native glycodelin was purified from amniotic fluid by a series of affinity chromatography steps and had many glycosylated forms verified by mass spectrometry. About 7.5 mg glycodelin was obtained from 1.5 L amniotic fluid. No high molecular mass forms of glycodelin were found in amniotic fluid. Aliquots of the purified glycodelin were used as an immunogen in rabbits for antibody production against glycodelin and a calibrator in a highly sensitive glycodelin enzyme-linked immunosorbent assay (ELISA) with a detection limit of about 1 μg/L.

CONCLUSIONS

Native glycodelin was purified from amniotic fluid and used as an immunogen for raising a rabbit antibody against glycodelin and a calibrator in a highly sensitive glycodelin ELISA. We found no high molecular mass forms of glycodelin in amniotic fluid. Aliquots of the purified glycodelin were set aside for functional studies which are in progress.

The dimeric crystal structure of the human fertility lipocalin glycodelin reveals a protein scaffold for the presentation of complex glycans

Structural study of human glycodelin, which is involved in reproduction, reveals a homodimeric structure ideally suited as scaffold for the presentation of distinct glycans, whereas bioinformatic analysis indicated exclusive occurrence in suborders of primates that have a menstrual cycle.

Sex hormone regulation of tear lipocalin in the rabbit lacrimal gland

Tear lipocalin (TL) (approximately 18 kDa), a member of the lipocalin superfamily, has been identified as one of the major proteins present in rabbit lacrimal fluid. The concentration of TL has been found to be decreased in the tears of patients with dry eye disease. Lacrimal gland insufficiency, one of the major causes of dry eye disease, is known to affect mainly postmenopausal women, where there is a significant decrease in the production of androgen and estrogen. These observations suggest that sex hormones might influence dry eye indirectly by regulating the expression of TL. The purpose of this study was to determine: (1) the effect of sexual maturation on the expression of TL; and (2) if the expression of TL is regulated by the estrogen, 17beta-estradiol, and/or the androgen, dihydrotestosterone, in sexually mature female rabbits. Lacrimal fluid (LF) and lacrimal gland soluble fraction (Si) was collected from juvenile (2 kg) and sexually mature (4 kg) male and female New Zealand white (NZW) rabbits. In addition, LF and Si were collected from 4 kg rabbits, 7 days after being either sham operated (control), ovariectomized (OVX), ovariectomized treated with estrogen (OVX+E) or ovariectomized treated with dihydrotestosterone (OVX+DHT). Samples were analyzed for protein levels of TL by SDS-PAGE and Western blotting using a polyclonal rat anti-rabbit TL antibody. Densitometry analysis showed that TL protein levels in both LF and Si increased with age in male and female rabbits. In addition, TL protein levels were significantly higher in the sexually mature 4 kg male compared with the 4 kg female, while no significant difference in TL protein levels were seen among the juvenile male and female rabbits. Furthermore, ovariectomy decreased the protein levels of TL in LF and Si fraction by 50% and 20% respectively, compared with control values. Estrogen treatment increased TL protein levels by 30% and 50% in the LF and Si fraction respectively, compared with the sham operated group. DHT treatment also increased TL protein levels by approximately 150% in both LF and Si fraction compared with control values. These results support the hypothesis that sex hormones influence TL protein levels in rabbit lacrimal glands. The possibility of a role of TL in dry eye needs to be further investigated.

Comparative ligand-binding analysis of ten human lipocalins

At least ten different lipocalins occur in the human body: retinol-binding protein (RBP), alpha1-acid glycoprotein, alpha1-microglobulin, apolipoprotein D, beta-trace protein, complement component 8gamma, glycodelin, neutrophil gelatinase-associated lipocalin, odorant-binding protein, and tear lipocalin. Although many of these lipocalins seem to play an important physiological role, their precise biological function is not always clear. Especially the interpretation of their diverse ligand-binding activities has been hampered by the fact that the natural lipocalins were prepared from different sources and with varying purity. Here we present a generic expression and purification strategy for the recombinant lipocalins, which is based on secretion into the periplasm of E. coli, where disulphide bonds are readily formed, followed by affinity purification via the Strep-tag II and gel filtration. The ten human lipocalins were successfully prepared and their ligand-binding activities were compared via fluorescence titration with a set of typical ligands: retinol, retinoic acid (RA), 11-(5-(dimethylamino)-1-naphthalene-sulfonylamino)undecanoic acid (DAUDA), and 8-anilino-1-naphtalene-sulfonic acid (ANS). As result, merely two lipocalins, RBP and beta-trace, revealed high affinities both for retinol and for RA, which probably reflects a specialized physiological function in retinoid complexation. Surprisingly, the strongest retinol affinity was detected for apolipoprotein D, whereas this lipocalin exhibits much weaker binding activity for retinoic acid. Binding studies with the two spectroscopic probes DAUDA and ANS revealed mixed patterns, which demonstrates that the affinity for lipophilic substances varies considerably among human lipocalins. Notably, RBP with its perfectly moulded retinol-binding site did not show any detectable binding activity for both compounds. Hence, our recombinant expression and purification system should be useful for further structural and functional studies of lipocalins from human origin and beyond.

Glycodelin A, not glycodelin S, is apoptotically active. Relevance of sialic acid modification

Glycodelin, previously known as PP14 (placental protein-14), is a kernel lipocalin secreted by the glandular epithelium of the endometrium upon progesterone stimulation and by the seminal vesicles. The isoform of the protein present in female reproductive tissue, glycodelin A (GdA), and the male counterpart, glycodelin S (GdS), have identical amino acid sequences, but strikingly different N-linked glycans. It is well documented in literature that GdA is an immunosuppressive protein, and we have shown that this activity is due to its ability to induce apoptosis in activated T cells. The precise role of GdS in seminal plasma is not known. In this study, we report that GdS is not apoptotically active. We observe that the apoptotic activity requires the presence of sialic acid residues on the complex glycans, as in the case of GdA; however, complex glycans of GdS are non-sialylated. We have expressed the wild-type protein in Pichia pastoris, which does not add sialic acid to the secreted proteins, and confirmed our observations that the protein is apoptotically inactive in the non-sialylated form. Our results indicate that differential glycosylation modulates the function of the different glycodelin isoforms.

Glycodelin: a major lipocalin protein of the reproductive axis with diverse actions in cell recognition and differentiation

Glycodelin is a glycoprotein that belongs to the lipocalin superfamily. Depending on glycosylation, glycodelin appears in various isoforms. In the uterus, glycodelin-A is the major progesterone-regulated glycoprotein secreted into uterine luminal cavity by secretory/decidualized endometrial glands. The other tissues expressing glycodelin include fallopian tubes, ovary, breast, seminal vesicle, bone marrow, and eccrine glands. Glycodelin-A potently and dose-dependently inhibits human sperm-egg binding, whereas differently glycosylated glycodelin-S from seminal plasma has no such effect. Absence of contraceptive glycodelin-A in the uterus during periovulatory midcycle is consistent with an open "fertile window." Glycodelin induced by local or systemic administration of progestogens may potentially reduce the fertilizing capacity of sperm in any phase of the menstrual cycle. Glycodelin also has immunosuppressive activity. Its high concentration at the fetomaternal interface may contribute to protection of the embryonic semiallograft. Besides being an epithelial differentiation marker, glycodelin appears to play a role in glandular morphogenesis, as transfection of glycodelin cDNA into a glycodelin-negative breast cancer cells resulted in formation of gland-like structures, restricted proliferation, and induction of other epithelial markers. These various properties, as well as the chemistry, biology, and clinical aspects of glycodelin, continue to be areas of active investigation reviewed in this communication.

Placental protein 14 induces apoptosis in T cells but not in monocytes

Substantial evidence exists in literature to suggest that placental protein 14 (PP14) (recently renamed glycodelin A), exhibits immunosuppressive properties and is an indispensable macromolecule in the maternal system for the establishment, maintenance, and progression of pregnancy. Though there are several reports substantiating the above, the mechanism of its action at the molecular level has not been elucidated as yet. In this paper we provide data that suggest that amniotic fluid PP14 and recombinant PP14 expressed in Pichia pastoris induce apoptosis in human peripheral blood lymphocytes upon activation, independent of monocytes. That PP14 has a direct apoptotic action on T cells but not on monocytes was also demonstrated by utilizing human cell lines. PP14 was shown to induce apoptosis in the human T cell lines, Jurkat and MOLT-4 cells, but not in the human monocytic cell line, U937.

Characterization of glycosylated variants of beta-lactoglobulin expressed in Pichia pastoris

Glycosylated variants of beta-lactoglobulin (BLG) were produced in the methylotrophic yeast Pichia pastoris to mimic the glycosylation pattern of glycodelin, a homologue of BLG found in humans. Glycodelin has three sites for glycosylation, corresponding to amino acids 63-65 (S1), 85-87 (S2) and 28-30 (S3) of BLG. These three sites were engineered into BLG to produce the variants S2, S12 and S123, which carried one, two and three glycosylation sites, respectively. The oligosaccharides on these BLG variants ranged from (mannose)(9)(N-acetylglucosamine)(2) (Man(9)GN(2)) to Man(15)GN(2) and were of the alpha-linked high mannose type. The variant S123 exhibited highest levels of glycosylation, with the range of glycans being Man(9-14)GN(2). Digestion of S123 with alpha-1,2 linkage specific mannosidase resulted in a single product corresponding to Man(6)GN(2). These results indicated a glycosylation pattern consisting of a Man(5)GN(2) structure extended by 4-9 mannose residues attached mainly by alpha-1,2 linkages. The results also indicated extension of the Man(5)GN(2) structure by a single alpha-1,6-linked mannose. The N-linked glycosylation pathway in P.pastoris is significantly different from that in Saccharomyces cerevisiae, with the addition of shorter outer chains to the core and no alpha-1,3 outer extensions.

Characterization of monoclonal antibodies to glycodelin and recombinant glycodelin

Glycodelin-A belongs to the lipocalin superfamily. Although it is associated with normal endometrial growth during the menstrual cycle, fertilization and normal pregnancy in humans, the molecular mechanism of its biological action has not been elucidated. To undertake studies to understand the functional relevance of any molecule, obtaining large quantities of the protein becomes essential. With the ultimate aim of purifying glycodelin either from its natural sources (human amniotic fluid) or the recombinant glycodelin from bacterial recombinant lysates, we raised monoclonal antibodies to this protein. As immunogens, recombinant glycodelin expressed in E. coli and Pichia pastoris as well as glycodelin from amniotic fluid were used. The monoclonal antibodies generated were characterized with respect to binding to both the native as well as the recombinant proteins using ELISA, immunoblotting, and immunohistochemistry.

Recombinant protein expression in Pichia pastoris

The methylotrophic yeast Pichia pastoris is now one of the standard tools used in molecular biology for the generation of recombinant protein. P. pastoris has demonstrated its most powerful success as a large-scale (fermentation) recombinant protein production tool. What began more than 20 years ago as a program to convert abundant methanol to a protein source for animal feed has been developed into what is today two important biological tools: a model eukaryote used in cell biology research and a recombinant protein production system. To date well over 200 heterologous proteins have been expressed in P. pastoris. Significant advances in the development of new strains and vectors, improved techniques, and the commercial availability of these tools coupled with a better understanding of the biology of Pichia species have led to this microbe's value and power in commercial and research labs alike.

Heterologous protein expression in the methylotrophic yeast Pichia pastoris

During the past 15 years, the methylotrophic yeast Pichia pastoris has developed into a highly successful system for the production of a variety of heterologous proteins. The increasing popularity of this particular expression system can be attributed to several factors, most importantly: (1) the simplicity of techniques needed for the molecular genetic manipulation of P. pastoris and their similarity to those of Saccharomyces cerevisiae, one of the most well-characterized experimental systems in modern biology; (2) the ability of P. pastoris to produce foreign proteins at high levels, either intracellularly or extracellularly; (3) the capability of performing many eukaryotic post-translational modifications, such as glycosylation, disulfide bond formation and proteolytic processing; and (4) the availability of the expression system as a commercially available kit. In this paper, we review the P. pastoris expression system: how it was developed, how it works, and what proteins have been produced. We also describe new promoters and auxotrophic marker/host strain combinations which extend the usefulness of the system.