Biochemical characterization of hamster oviductin as a sulphated zona pellucida-binding glycoprotein

The role of oviduct-specific glycoprotein (OVGP1) in modulating biological functions of gametes and embryos

Diverse lines of evidence indicate that the mammalian oviduct makes important contributions to the complex process of reproduction other than being simply a conduit for the transport of gametes and embryos. The cumulative synthesis and transport of proteins secreted by oviductal secretory cells into the oviductal lumen create a microenvironment supporting important reproductive events, including sperm capacitation, fertilization, and early embryo development. Among the components that have been identified in the oviductal fluid is a family of glycosylated proteins known collectively as oviduct-specific glycoprotein (OVGP1) or oviductin. OVGP1 has been identified in several mammalian species, including humans. The present review summarizes the work carried out, in various mammalian species, by many research groups revealing the synthesis and secretion of OVGP1, its fate in the female reproductive tract upon secretion by the oviductal epithelium, and its role in modulating biological functions of gametes and embryos. The production and functions of recombinant human OVGP1 and recombinant OVGP1 of other mammalian species are also discussed. Some of the findings obtained with immunocytochemistry will be highlighted in the present review. It is hoped that the findings obtained from recent studies carried out with recombinant OVGP1 from various species will rekindle researchers’ interest in pursuing further the role of the oviductal microenvironment, of which OVGP1 is a major component, in contributing to the successful occurrence of early reproductive events, and the potential use of OVGP1 in improving the current assisted reproductive technology in alleviating infertility.

A Comparative View on the Oviductal Environment during the Periconception Period

The oviduct plays important roles in reproductive events: sperm reservoir formation, final gamete maturation, fertilization and early embryo development. It is well known that the oviductal environment affects gametes and embryos and, ultimately, the health of offspring, so that in vivo embryos are better in terms of morphology, cryotolerance, pregnancy rates or epigenetic profile than those obtained in vitro. The deciphering of embryo-maternal interaction in the oviduct may provide a better understanding of the embryo needs during the periconception period to improve reproductive efficiency. Here, we perform a comparative analysis among species of oviductal gene expression related to embryonic development during its journey through the oviduct, as described to date. Cross-talk communication between the oviduct environment and embryo will be studied by analyses of the secreted or exosomal proteins of the oviduct and the presence of receptors in the membrane of the embryo blastomeres. Finally, we review the data that are available to date on the expression and characterization of the most abundant protein in the oviduct, oviductin (OVGP1), highlighting its fundamental role in fertilization and embryonic development.

Structural and functional characterization of buffalo oviduct-specific glycoprotein (OVGP1) expressed during estrous cycle

Oviduct-specific glycoprotein (OVGP1) is a high molecular weight chitinase-like protein belonging to GH18 family. It is secreted by non-ciliated epithelial cells of oviduct during estrous cycle providing an essential milieu for fertilization and embryo development. The present study reports the characterization of buffalo OVGP1 through structural modeling, carbohydrate-binding properties and evolutionary analysis. Structural model displayed the typical fold of GH18 family members till the boundary of chitinase-like domain further consisting of a large (β/α)₈ TIM barrel sub-domain and a small (α+β) sub-domain. Two critical catalytic residues were found substituted in the catalytic centre (Asp to Phe118, Glu to Leu120) compared with the active chitinase. The carbohydrate-binding groove in TIM barrel was lined with various conserved aromatic residues. Molecular docking with different sugars revealed the involvement of various residues in hydrogen-bonding and non-bonded contacts. Most of the substrate-binding residues were conserved except for a few replacements (Ser13, Lys48, Asp49, Pro50, Asp167, Glu199, Gln272 and Phe275) in comparison with other GH18 members. The residues Trp10, Trp79, Asn80, Gln272, Phe275 and Trp334 were involved in recognition of all six ligands. The α+β sub-domain participated in sugar-binding through Thr270, Gln272, Tyr242 and Phe275. The binding assays revealed significant sugar-binding with purified native and recombinant OVGP1. Phylogenetic analysis revealed that OVGP1 was closely related to AMCases followed by other CLPs and evolution of OVGP1 occurred through several gene duplications. This is the first study describing the structural characteristics of OVGP1 that will further help to understand its interaction with gametes to perform crucial reproductive functions.

Identification of 56 Proteins Involved in Embryo-Maternal Interactions in the Bovine Oviduct

The bovine embryo develops in contact with the oviductal fluid (OF) during the first 4–5 days of pregnancy. The aim of this study was to decipher the protein interactions occurring between the developing embryo and surrounding OF. In-vitro produced 4–6 cell and morula embryos were incubated or not (controls) in post-ovulatory OF (OF-treated embryos) and proteins were then analyzed and quantified by high resolution mass spectrometry (MS) in both embryo groups and in OF. A comparative analysis of MS data allowed the identification and quantification of 56 embryo-interacting proteins originated from the OF, including oviductin (OVGP1) and several annexins (ANXA1, ANXA2, ANXA4) as the most abundant ones. Some embryo-interacting proteins were developmental stage-specific, showing a modulating role of the embryo in protein interactions. Three interacting proteins (OVGP1, ANXA1 and PYGL) were immunolocalized in the perivitelline space and in blastomeres, showing that OF proteins were able to cross the zona pellucida and be taken up by the embryo. Interacting proteins were involved in a wide range of functions, among which metabolism and cellular processes were predominant. This study identified for the first time a high number of oviductal embryo-interacting proteins, paving the way for further targeted studies of proteins potentially involved in the establishment of pregnancy in cattle.

The golden (Syrian) hamster as a model for the study of reproductive biology: Past, present, and future

BACKGROUND

The golden (Syrian) hamster (Mesocricetus auratus) is a small rodent that belongs to the Cricetidae family. It has several unique features that are advantageous for the study of reproductive and developmental biology, including a consistent estrous cycle (4 days), high responsiveness to conventional superovulation regimens, and the short gestation period (16 days).

METHODS

Based on the published reports, the development in assisted reproductive technology (ART) in the golden hamsters was summarized.

MAIN FINDINGS

The technical ease of in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) in this species has contributed to our understanding of the basic mechanisms of mammalian fertilization in the last century. However, a strong developmental block in vitro of hamster embryos and unavailability of gene-modified strains has hampered its broader use in biomedical fields. A recently developed in vivo transfection method has enabled us to generate gene knockout hamsters without any major obstacles. It would be interesting to revisit the genes whose functions could not be identified using mouse models.

CONCLUSION

The authors expect that gene knockout hamsters might be able to substitute for mice-at least in part-for better understanding of gene functions in mammals including humans.

Recombinant hamster oviductin is biologically active and exerts positive effects on sperm functions and sperm-oocyte binding

Studies carried out in several mammalian species suggest that oviductin, also known as oviduct-specific glycoprotein or OVGP1, plays a key role in sperm capacitation, fertilization, and development of early embryos. In the present study, we used recombinant DNA technology to produce, for the first time, recombinant hamster OVGP1 (rHamOVGP1) in human embryonic kidney 293 (HEK293) cells. rHamOVGP1 secreted in the culture medium was purified by affinity chromatography. The resulting protein migrated as a poly-dispersed band of 160-350 kDa on SDS-PAGE corresponding to the molecular mass of the native HamOVGP1. Subsequent mass spectrometric analysis of the purified rHamOVGP1 confirmed its identity as HamOVGP1. Immunocytochemistry demonstrated binding of rHamOVGP1 to the mid-piece and head of hamster sperm and to the zona pellucida (ZP) of ovarian oocytes. In vitro functional experiments showed that addition of rHamOVGP1 in the capacitation medium further enhanced tyrosine phosphorylation of two sperm proteins of approximately 75 kDa and 83 kDa in a time-dependent manner. After 3 hours of incubation in the presence of rHamOVGP1, a significant increase in acrosome reaction was measured. Pretreatment of either sperm or oocyte with 20 μg/ml of rHamOVGP1 prior to sperm-egg binding assay significantly increased the number of sperm bound to the ZP. Addition of rHamOVGP1 in the medium during sperm-egg binding with either oocyte or sperm pretreated with rHamOVGP1 also saw an increase in the number of sperm bound to ZP. In all experimental conditions, the effect of rHamOVGP1 on sperm-oocyte binding was negated by the addition of monoclonal anti-HamOVGP1 antibody. The successful production and purification of a biologically active rHamOVGP1 will allow further exploration of the function of this glycoprotein in reproductive function.

Purification, sequence characterization and effect of goat oviduct-specific glycoprotein on in vitro embryo development

Oviduct-specific glycoprotein (oviductin) plays an important role during fertilization and early embryonic development. The oviductin cDNA was successfully cloned and sequenced in goat, which possessed an open reading frame of 1620 nucleotides representing 539 amino acids. Predicted amino acid sequence showed very high identity with sheep (97%) followed by cow (94%), porcine (77%), hamster (69%), human (66%), rabbit (65%), mouse (64%) and baboon (62%). The bioinformatics analysis of the sequences revealed the presence of a signal sequence of 21 amino acids, one potential N-linked glycosylation site at position 402, 21 potential O-linked glycosylation sites and 36 potential phosphorylation sites. The native oviductin was purified from the oviductal tissue, which showed three distinct bands on SDS-PAGE and western blot (MW ~60-95 kDa). The predicted molecular weight of goat oviductin was 57.5 kDa, calculated from the amino acid sequences. The observed higher molecular weight has been attributed to the presence of large number of potential O-linked glycosylation sites. The lower concentration (10 μg/mL) of oviductin increased the cleavage rate, morula and blastocyst yield significantly (P < 0.05) as compared to higher concentration (100 μg/mL). Goat oviductin retarded the activity of pronase (0.1%) on zona solubility of oocytes significantly (P < 0.01).

Oviductal secretions: will they be key factors for the future ARTs?

A variety of evolutionary processes has led to the development of different organs to ensure that internal fertilization occur successfully. Fallopian tubes are a particularly interesting example of such organs. Some of the key events during fertilization and early embryo development occur in the oviduct. Knowledge of the different components described in the oviduct is extensive. Oviductal components include hormones, growth factors and their receptors that have important roles in the physiology of the oviduct and embryo development. Other oviductal factors protect the gamete and the embryos against oxidative stress and pathogens. Different proteins and enzymes are present in the oviductal fluid and have the ability to interact with the oocyte and the sperm before the fertilization occurs. Of special interest is the oviduct-specific glycoprotein (OVGP1), a glycoprotein that is conserved in different mammals, and its association with the zona pellucida (ZP). Interaction of the oocyte with oviductal secretions leads us to emphasize the concept of 'ZP maturation' within the oviduct. The ZP changes produced in the oviduct result in an increased efficiency of the in vitro fertilization technique in some animal models, contributing in particular to the control of polyspermy and suggesting that a similar role could be played by oviductal factors in human beings. Finally, attention should be given to the presence in the oviductal fluid of several embryotrophic factors and their importance in relation to the in vivo versus in vitro developmental ability of the embryos.

What controls polyspermy in mammals, the oviduct or the oocyte?

A block to polyspermy is required for successful fertilisation and embryo survival in mammals. A higher incidence of polyspermy is observed during in vitro fertilisation (IVF) compared with the in vivo situation in several species. Two groups of mechanisms have traditionally been proposed as contributing to the block to polyspermy in mammals: oviduct-based mechanisms, avoiding a massive arrival of spermatozoa in the proximity of the oocyte, and egg-based mechanisms, including changes in the membrane and zona pellucida (ZP) in reaction to the fertilising sperm. Additionally, a mechanism has been described recently which involves modifications of the ZP in the oviduct before the oocyte interacts with spermatozoa, termed "pre-fertilisation zona pellucida hardening". This mechanism is mediated by the oviductal-specific glycoprotein (OVGP1) secreted by the oviductal epithelial cells around the time of ovulation, and is reinforced by heparin-like glycosaminoglycans (S-GAGs) present in oviductal fluid. Identification of the molecules contributing to the ZP modifications in the oviduct will improve our knowledge of the mechanisms of sperm-egg interaction and could help to increase the success of IVF systems in domestic animals and humans.

Surface mapping of binding of oviductin to the plasma membrane of golden hamster spermatozoa during in vitro capacitation and acrosome reaction

Oviductins are high-molecular-weight glycoproteins synthesized and secreted by nonciliated oviductal epithelial cells and have been shown to play a role in fertilization and early embryo development. The present study was carried out to examine the in vitro binding capacity of hamster oviductin to homologous sperm and to determine the sites of its localization in untreated, capacitated, and acrosome-reacted spermatozoa. Freshly prepared epididymal and capacitated sperm as well as acrosome-reacted sperm were incubated with oviductal fluid prepared from isolated hamster oviducts, fixed and then probed with a monoclonal antibody against hamster oviductin. Results obtained with pre-embedding immunolabeling experiments revealed binding of oviductin to the acrosomal cap and the apical aspect of the postacrosomal region. Immunolabeling of both regions appeared to be more intense in capacitated spermatozoa. Acrosome-reacted sperm showed an immunoreaction of moderate intensity over the postacrosomal region. The plasma membrane overlying the equatorial segment also exhibited a weak labeling. Quantitative analysis obtained with the surface replica technique indicated that oviductin had a higher binding affinity for the acrosomal cap than the postacrosomal region and that the binding of oviductin to the latter plasma membrane domain was enhanced during capacitation. Binding of oviductin to the postacrosomal region, however, was attenuated after acrosome reaction. Immunolabeling for oviductin was found to be the weakest over the equatorial segment regardless of the experimental conditions. The binding of hamster oviductin to specific membrane domains of the homologous sperm and the changes in its distribution during capacitation and acrosome reaction may be important for the function of hamster oviductin preceding and during fertilization.

Detection of glycosyltransferases in the golden hamster (Mesocricetus auratus) oviduct and evidence for the regulation of O-glycan biosynthesis during the estrous cycle

Recently, we provided evidence that the glycosylation of hamster oviductin, a member of the mucin family of glycoproteins, is regulated during the estrous cycle. In order to further elucidate the glycosylation process of oviductal glycoproteins, we identified biosynthetic pathways involved in the assembly of mucin-type O-linked oligosaccharide (O-glycan) chains in the hamster oviduct. Our results demonstrated that the hamster oviduct has high activities of glycosyltransferases that synthesize O-glycans with core 1, 2, 3 and 4 structures as well as elongated structures. Oviduct therefore represents a typical mucin-secreting tissue. Our results also showed that specific glycosyltransferase activities are regulated during the estrous cycle. Mucin-type core 2 beta6-GlcNAc-transferase (C2GnT2) is responsible for synthesizing core 2 and core 4 structures in the oviduct. Specific assays for C2GnT2 revealed a cyclical pattern throughout the estrous cycle with high activity at the stages of proestrus and estrus and low activity at diestrus 1. Using semiquantitative RT-PCR, the mRNA levels for C2GnT2 in the estrous cycle stages could be correlated with the enzyme activities. An increase in glycosyltransferase activity in the hamster oviduct at the time of ovulation suggests that glycosylation of oviductal glycoproteins may be necessary for these proteins to exert their functions during the process of fertilization.

Detection of nascent and/or mature forms of oviductin in the female reproductive tract and post-ovulatory oocytes by use of a polyclonal antibody against recombinant hamster oviductin

Oviductins belong to a family of glycoproteins that have been suggested to play several roles during the early processes of reproduction. Recently, a polyclonal antibody was raised against recombinant hamster oviductin (rhaOv(m)). Here the anti-rhaOv(m) antibody was used to investigate the sites of localization of oviductin in the female golden hamster. In the hamster oviduct, immunolabeling was restricted to the content of the Golgi saccules and secretory granules of the non-ciliated oviduct cells. After its release into the lumen, oviductin becomes associated with the zona pellucida of post-ovulatory oocytes. In unfertilized oocytes, oviductin was also detected in membrane invaginations along the oolemma and in some vesicles within the ooplasm. Furthermore, oviductin was detected over the microvilli and within multivesicular bodies of uterine epithelial cells. Western blotting analysis revealed the presence of oviductin in the hamster oviduct but not in the uterus or ovary. In the oviduct, the anti-rhaOv(m) antibody detected a polydispersed band corresponding to native oviductin (160-350 kD) and several lower molecular weight bands (<100 kD) corresponding to nascent and partially glycosylated forms of oviductin. The anti-rhaOv(m) antibody provides an additional tool for investigation into the cytochemical and biochemical properties of different forms of hamster oviductin in the female reproductive tract.

Evidence for the Regulation of Glycosylation of Golden Hamster (Mesocricetus auratus) Oviductin During the Estrous Cycle1

The oviduct contributes to the reproductive environment by secreting various factors, including a family of glycoproteins termed oviductins. Although many studies have demonstrated that ovarian hormones modulate oviductin gene expression in several mammalian species, there has been controversy surrounding the regulation of golden hamster oviductin. The current study was undertaken to investigate the transcriptional and translational modifications of hamster oviductin during the estrous cycle. First, we verified that hamster oviductin mRNA expression remains constant throughout the estrous cycle by semiquantitative reverse transcription polymerase chain reaction. We then developed a polyclonal antibody against recombinant hamster oviductin (rhaOvm). The anti-rhaOvm antibody was subsequently used in conjunction with quantitative immunocytochemistry to investigate the oviductin levels in the hamster oviduct during the estrous cycle. Quantification of immunolabeling revealed a high, consistent level of glycoprotein throughout the estrous cycle. Therefore, it appears that the production of oviductin is not regulated differentially during the estrous cycle. Size variations in hamster oviductin expression were also investigated by Western blot analysis. The oviduct contains several forms of oviductin at each stage of the estrous cycle, the native glycosylated form(s) of 160-350 kDa, and several precursor forms of 70-100 kDa. Although variations in the intensities of the polydispersed band were not evident during the estrous cycle, additional bands ranging from 90 to 100 kDa were detected in the estrus, metestrus, and diestrus 1 stages. The results from the present investigations suggest that whereas ovarian hormones do not appear to influence the hamster oviductin mRNA and protein expressions, glycosylation of hamster oviductin appears to be differentially regulated during the estrous cycle.

A review of in vitro gamete maturation and embryo culture and potential impact on future animal biotechnology

This review considers the relationship of in vitro gamete maturation and embryo culture to the future development of animal biotechnology. The areas reviewed are oocyte maturation in vitro and embryo culture and their importance for successful in vitro embryo production. The rapidly developing area of spermatogonial cell transplantation and culture is also reviewed. The scientific milestones leading to the development of each area, the problems and prospects for future development and the possible significance of major advances in each area are discussed.

Deglycosylation of glycoproteins with trifluoromethanesulphonic acid: elucidation of molecular structure and function

The alteration of proteins by post-translational modifications, including phosphorylation, sulphation, processing by proteolysis, lipid attachment and glycosylation, gives rise to a broad range of molecules that can have an identical underlying protein core. An understanding of glycosylation of proteins is important in clarifying the nature of the numerous variants observed and in determining the biological roles of these modifications. Deglycosylation with TFMS (trifluoromethanesulphonic acid) [Edge, Faltynek, Hof, Reichert, and Weber, (1981) Anal. Biochem. 118, 131-137] has been used extensively to remove carbohydrate from glycoproteins, while leaving the protein backbone intact. Glycosylated proteins from animals, plants, fungi and bacteria have been deglycosylated with TFMS, and the most extensively studied types of carbohydrate chains in mammals, the N-linked, O-linked and glycosaminoglycan chains, are all removed by this procedure. The method is based on the finding that linkages between sugars are sensitive to cleavage by TFMS, whereas the peptide bond is stable and is not broken, even with prolonged deglycosylation. The relative susceptibility of individual sugars in glycosidic linkage varies with the substituents at C-2 and the occurrence of amido and acetyl groups, but even the most stable sugars are removed under conditions that are sufficiently mild to prevent scission of peptide bonds. The post-translational modifications of proteins have been shown to be required for diverse biological functions, and selective procedures to remove these modifications play an important role in the elucidation of protein structure and function.

Variable expression of oviductin mRNA at different stages of human reproductive cycle

PURPOSE

To examine the in vivo expression of oviductin mRNA at different stages of the human female reproductive cycle including pregnancy and after menopause.

METHODS

Oviducts were obtained from 25 women in normal menstrual cycle, 5 in early pregnancy, 5 undergoing postpartum sterilization, and 4 menopausal women. The oviductal mucosal tissue was isolated and oviductin mRNA was assessed using reverse-transriptase-polymerase chain reaction (RT-PCR); its correlation with various hormones was assessed.

RESULTS

Oviductin mRNA was detected throughout the menstrual cycle, highest in the periovulatory period. It continued to be expressed in early pregnancy but was absent in the postpartum period and after menopause.

CONCLUSIONS

The production and function of oviductin at different stages of human reproductive cycle including pregnancy is not well known. Its highest expression at the time of ovulation is consistent with a supportive role in fertilization and early embryo development.

Distribution of lectin-binding glycosidic residues in the hamster follicular oocytes and their modifications in the zona pellucida after ovulation

In the present study, we have employed a battery of colloidal gold-tagged lectins as probes in conjunction with quantitative analysis to demonstrate the distribution and changes of carbohydrate residues in the hamster zona pellucida (ZP) during ovarian follicular development and during transit of the oocyte through the oviduct after ovulation. High-resolution lectin-gold cytochemistry performed on thin sections of LR White-embedded ovaries revealed a moderate to strong reactivity to WGA, PNA, DSA, AAA, and MAA over the entire thickness of the ZP of ovarian oocytes at different stages of follicular development. Labeling intensity over the ZP progressively increased as follicles matured in the ovary. In parallel, there was an association of labeling by gold particles with cortical granules, stacks of Golgi saccules, and complex structures called vesicular aggregates in the oocyte proper especially during the late stages of follicular growth. In contrary, labeling with each of HPA, DBA, and BSAIB(4) was absent in the ovary but was found to be localized over Golgi complexes and secretory granules in the non-ciliated secretory cells of the oviduct. When ovulated oocytes were labeled with each of HPA, WGA, RCA-I, PNA, DSA, BSAIB(4), AAA, MAA, and DBA, the ZP and several organelles in the oocyte proper presented a differential distribution of lectin-binding sites. Quantitative analysis was also performed on labeling by lectin-gold complexes that bind specifically to the ZP of mature follicular and ovulated oocytes. Quantitative evaluation revealed heterogeneous labeling between the inner and the outer zone of the ZP. A significant increase in the labeling densities in both inner and outer ZP was noted when tissue sections of ovulated oocytes were labeled with RCA-I or AAA. Tissue sections of ovaries labeled with WGA demonstrated a significant increase in the density of labeling in the outer layer of the ZP. Labeling by PNA, DSA, and MAA, however, showed a significant decrease in both the inner and outer portions of the ZP. Together, these results suggest that in the hamster, glycoproteins carrying specific sugar residues are added to the ZP of ovarian follicles during the early stages of folliculogenesis and are processed through a common secretory machinery, and that there is a significant change in both the sugar moieties and distribution of glycoproteins in the ZP following ovulation. Our results also showed that the hamster oviduct plays an important role in contributing certain glycoproteins to the ZP suggesting that the sugar moieties of these oviductal glycoproteins may have functional significance in fertilization.

Characterization, fate, and function of hamster cortical granule components

Little is known about the composition and function of mammalian cortical granules. In this study, lectins were used as tools to: (1) estimate the number and molecular weight of glycoconjugates in hamster cortical granules and show what sugars are associated with each glycoconjugate; (2) identify cortical granule components that remain associated with the oolemma, cortical granule envelope, and/or zona pellucida of fertilized oocytes and preimplantation embryos; and (3) examine the role of cortical granule glycoconjugates in preimplantation embryogenesis. Microscopic examination of unfertilized oocytes revealed that the lectins PNA, DBA, WGA, RCA(120), Con A, and LCA bound to hamster cortical granules. Moreover, LCA and Con A labeled the zona pellucida, cortical granule envelope, and plasma membrane of fertilized and artificially activated oocytes and two and eight cell embryos. Lectin blots of unfertilized oocytes had at least 12 glycoconjugates that were recognized by one or more lectins. Nine of these glycoconjugates are found in the cortical granule envelope and/or are associated with the zona pellucida and plasma membrane following fertilization. In vivo functional studies showed that the binding of Con A to one or more mannosylated cortical granule components inhibited blastomere cleavage in two-cell embryos. Our data show that hamster cortical granules contain approximately 12 glycoconjugates of which nine remain associated extracellularly with the fertilized oocyte after the cortical reaction and that one or more play a role in regulating cleavage divisions.

Mouse oviduct-specific glycoprotein gene: genomic organization and structure of the 5'-flanking regulatory region

A member of the chitinase protein family, oviduct-specific glycoprotein (OGP), can directly associate with gametes or with the early embryo in the oviduct. Although the glycoprotein is widely distributed among mammalian species and there is indirect evidence concerning the involvement of the molecule in the fertilization process, its physiological functions are far from completely understood. To understand the fundamental mechanisms that direct gene expression as well as to know the physiological significance of OGP, we have isolated and characterized a mouse OGP gene (mogp-1). The gene was found to span 13.4 kilobases (kb) including 11 exons and 10 introns. The genomic organization of mogp-1 is well conserved compared to the other members of the chitinase family. Two transcription initiation sites were found at positions 18 and 14 upstream from the first ATG codon. Fluorescence in situ hybridization analysis demonstrated that the mogp-1 was located on the R-positive F3 band of mouse chromosome 3. Although the putative promoter region of mogp-1 lacked typical TATA, CAAT, or GC box sequences, the region contained several motif sequences of transcription factor binding sites including 10 half-palindromic estrogen responsive elements (ERE) and an imperfect ERE. Transient transfection experiments demonstrated that promoter activity could be modulated by various sequences within the 2.2 kb of the 5'-flanking region, and that the mogp-1 promoter was transactivated in an estrogen receptor-positive cell line, MCF-7, by the addition of estradiol-17beta (E(2)). In addition, relevant promoter activity for E(2) responsiveness resides within the first 270 base pairs upstream of the mogp-1. These findings should facilitate our understanding of the regulation of OGP gene expression, and they may be helpful for designing experiments to unravel the role of OGP in the process of mammalian fertilization.

Steroid regulation of retinol-binding protein in the ovine oviduct

Two studies were conducted to identify retinol-binding protein (RBP) expression in the ovine oviduct and to determine the role of ovarian steroids in its regulation. Ewes were salpingectomized on Days 1, 5, or 10 of their respective estrous cycles, and oviducts were homogenized for RNA analysis, fixed for immunocytochemistry (ICC), or cultured for 24 h for protein analysis. ICC localized RBP to the epithelium of all oviducts. RBP synthesis was demonstrated by immunoprecipitation of radiolabeled RBP from the medium of oviductal explant cultures. Explant culture medium from oviducts harvested on Day 1 contained significantly more RBP than medium from oviducts collected on Days 5 or 10. Slot-blot analysis demonstrated that steady-state RBP mRNA levels were significantly higher on Day 1 than Day 5 or 10. In the second experiment, ovariectomized ewes were treated with estradiol-17beta (E2), progesterone (P4), E2+P4 (E2+P4), or vehicle control, and oviducts were analyzed as above. P4 alone or in combination with E2 significantly reduced steady-state RBP mRNA levels compared to those in E2-treated animals. Oviductal explants from E2- and E2+P4-treated animals released 3- to 5-fold more RBP into the medium than control and P4 treatments as determined by ELISA. RBP synthesis of metabolically labeled RBP was increased by E2 and E2+P4 treatments. This study demonstrates that P4 applied on an estradiol background negatively regulates RBP gene expression in the oviduct whereas estradiol appears to stimulate RBP synthesis and secretion.

Allelic polymorphism and chromosomal localization of the human oviductin gene (MUC9)

OBJECTIVE

To determine if the gene coding for human oviductin (the estrogen-dependent, oviduct-specific glycoprotein with an affinity for the zona pellucida) shows length polymorphism in the region of tandem repeats. To determine the frequencies of the length alleles in health and disease.

DESIGN

Descriptive fundamental and clinical studies.

SETTING

Fertility clinic and research center, university teaching hospital.

PATIENT(S)

Fertile women, women with a history of ectopic pregnancy or tubal disease, and women with stage I or II endometriosis.

INTERVENTION(S)

Blood samples were drawn for DNA analysis.

MAIN OUTCOME MEASURE(S)

Length and sequence of the region of tandem repeats.

RESULT(S)

Four different length alleles of the human oviductin gene were identified. Their relative frequencies in pathologic cases were not statistically significant compared with those found in normal fertile women. The human genome contains a single copy of the oviductin gene located on chromosome 1p13.

CONCLUSION(S)

The human oviductin gene codes for a glycoprotein that shares the characteristics of epithelial mucins. Because eight epithelial mucin genes have been identified so far, we therefore propose to name this gene MUC9. The biologic function of the protein is likely to include protection of the early embryo and of the fallopian tube itself.

Fate of hamster oviductin in the oviduct and uterus during early gestation

Oviductins are a family of glycoproteins which are synthesized and secreted by oviductal secretory cells and which, upon their secretion in the lumen of the oviduct, become associated with postovulatory oocytes and developing embryos. Recently, we showed that hamster oviductin is maximally secreted in the oviduct at the time of ovulation and is later associated with a certain population of uterine epithelial cells, where it is subsequently endocytosed and degraded. In light of these results, this study was conducted to follow the fate of hamster oviductin in the oviduct and uterus during early gestation. Using a monoclonal antibody against hamster oviductin, immunofluorescence and immunogold labeling revealed that during early gestation, immunoreactivity to oviductin in the uterus gradually diminished to an almost total disappearance at time of implantation. However, the strong labeling intensity remained unchanged in the oviduct. Biochemical analyses demonstrated that a degradation of oviductin occurs in the uterus, and a loss of immunoreactivity was also observed as gestation progressed, so that by the time of implantation, immunoreactivity to oviductin was barely detectable. The decrease of oviductin along the uterine epithelium at the time of blastocyst attachment and its final disappearance at implantation suggest that this glycoprotein could be a potential modulator of uterine receptivity.

Biochemical changes in the equine capsule following prostaglandin-induced pregnancy failure

The equine embryonic capsule, an acellular covering that envelops the conceptus during the second and third weeks of pregnancy, is composed of mucin-like glycoproteins. Its structure is consistent with a dual role during early pregnancy: protection of the conceptus, and communication between the embryo and the mother. Loss of sialic acid from the capsular glycoproteins at day 16 correlates with the time of "fixation," or loss of conceptus mobility throughout the uterine horns. This study investigated how the structure of the capsule is linked to the maintenance of pregnancy. Six pregnancies, confirmed by ultrasound, were terminated by prostaglandin injection on day 14, prior to the time of embryo fixation. These "defective" conceptuses were collected at day 17, and the structure and molecular properties of their capsules were compared to those of day 17 conceptuses collected from 5 normal pregnancies. Defective capsules were not significantly different from normal capsules in terms of dry weight, amino acid composition, and content of neutral and amino sugars. However, defective capsules failed to show the loss of sialic acid normally occurring around the time of embryo fixation. Analysis of the capsular mucins following trypsin digestion was carried out by radioactive labeling with 3H on sialyl-oligosaccharides and 125I on tyrosine residues, followed by fast protein liquid chromatography and sodium dodecyl sulfate polyacrylamide gel electrophoresis. Differences in the trypsin fragmentation patterns indicated increased susceptibility of the defective capsules to proteolysis. We conclude that there is a temporal association between desialylation of the equine capsule and embryonic survival, and that failure to desialylate alters the properties of the capsule.

Immunohistochemical localization of oviductin in the endometrial lining of the golden hamster (Mesocricetus auratus) during the estrous cycle and early gestation

Oviductal non-ciliated secretory epithelial cells, under hormonal stimulation, synthesize and secrete a family of glycoproteins referred to as oviductins. These glycoproteins are found in oviductal fluid in several mammalian species, and have been localized in the oviduct, and in the zona pellucida of ovulated oocytes. In the golden hamster, this glycoprotein is named hamster oviductin-I. Recently, an immunofluorescent study on hamster uterine tissue has revealed the presence of the glycoprotein in luminal epithelial cells in a heterogeneous labelling pattern during the estrous cycle. The mechanism of endometrial epithelial cell receptivity to hamster oviductin-1 is not known. In this study, immunohistochemical studies were performed using a monoclonal antibody against the oviductin in conjunction with silver enhancement technique, in an attempt to determine further the factors playing a role in uterine receptivity to oviductin-1. Paraffin sections of hamster uterus obtained from different stages of the estrous cycle and from days 1-6 of gestation, and paraffin sections of hamster oviduct obtained from days 1-6 of gestation were used in this study. The results we obtained using the silver enhancement technique show that hamster uterus luminal epithelial cells exhibit a homogeneous, high intensity immunolabelling pattern throughout the estrous cycle, whereas, during gestation, labelling intensity decreases as the period for blastocyst implantation approaches. Oviduct epithelial cells revealed no definite fluctuating pattern in immunolabelling intensities during gestation, indicating no change in synthesis and secretion of the glycoprotein during this period. It is speculated that receptors for hamster oviductin-1 are present at the apical cell surface of endometrial cells and that implantation of the developing blastocyst into the uterine wall is possible only following downregulation of these receptors. The use of the silver enhancement technique proves to be an effective tool in immunohistochemical studies at the light microscope level, as seen through this study.

Allelic polymorphism in the hamster oviductin gene is due to a variable number of mucin-like tandem repeats

Oviductins are high-molecular-weight glycoproteins specifically secreted by the oviduct. These proteins bind to the zona pellucida of the ovulated oocyte and remain associated with the embryo during its transit in the oviduct. They may be involved in fertilization and early embryonic development. In order to explore their putative biological function, the cDNA sequence corresponding to oviductin in the golden hamster was determined. We found that the deduced amino acid sequence of this heavily O-glycosylated protein presents characteristics typical of mucins, including serine- or threonine-rich tandem repeats. Analysis of several cDNA clones and of genomic DNA revealed the presence of a single copy gene with two frequent alleles differing in the number of repeats. Comparison with oviductin sequences from other mammals indicates a high degree of conservation amongst species, except for the repeat region which shows divergence, notably in the number of repeats. Based on its biochemical and genetic properties, hamster oviductin can now be classified as a secretory mucin. This concept provides a new insight in the elucidation of its biological role: oviductin could possibly provide the oviduct and the oocyte with a protective coating ensuring normal tubal function and embryonic development.

Hormonal control of the biosynthesis of hamster oviductin

In several mammalian species, the epithelial secretory cells of the oviduct synthesize and secrete specific glycoproteins that become associated with the zona pellucida of the ovulated oocyte. These glycoproteins are collectively designated as oviductins. A monoclonal antibody directed against hamster oviductin was used to study the ontogeny of this glycoprotein. Indirect immunofluorescence experiments performed on sections of hamster oviduct revealed that the glycoprotein begins to be secreted in 10-day-old females and that all of the oviductal secretory cells showed fluorescent staining by day 14. The intensity of the immunofluorescence reaction reached a maximum in the 28-day-old females. The oviducts of the 7-day-old hamster incorporated [35S]methionine in vitro into several proteins; however, the production and secretion of detectable amounts of radiolabeled oviductin only began at 14 days of age and reached a maximum at day 28 of age. It appears that the ontogeny of oviductin parallels the hormone dependent changes leading to sexual maturation and that its maximum secretion is already established at the time of the first ovulatory cycle. These results are substantiated by the fact that the production of oviductin is induced in estradiol-treated, but not progesterone or non-treated prepubertal animals, as determined by indirect immunofluorescence experiments.

Elaboration of an oviductin by the oviductal epithelium in relation to embryo development as visualized by immunocytochemistry

The hamster oviduct secretes a high molecular weight antigen that belongs to the family of glycoproteins known as oviductins. In the present study, using immuno-electron microscopy, we examined the location of this hamster oviductin-1 (Hm Ov-1) in hamster oviductal oocytes and early embryos up to the blastocyst stage. The immunoreactive pattern of Hm Ov-1 changes markedly during the embryo development. In oviductal oocytes prior to fertilization, Hm Ov-1 was associated exclusively with the zona pellucida. Following fertilization, immunolabeling was detected in the perivitelline space and over the plasma membrane of 2-cell, 4-cell, and 8-cell embryos as well as young blastocysts. The change of the immunoreactive pattern was accompanied by the formation of an abundant number of coated pits, endocytic vesicles, multivesicular bodies, and lysosomal-like structures which were strongly labeled by gold particles. These immunogold-labeled cytoplasmic organelles characteristic of the endosomal-lysosomal apparatus were particularly evident in 2-cell, 4-cell, and 8-cell embryos and showed a decrease in number in the blastocysts. The close resemblance between the labeled flocculent material detected in the perivitelline space and that found in the zona matrix of early embryos and blastocysts suggested that the Hm Ov-1-associated electron-dense, flocculent material in the perivitelline space originated from the zona pellucida and was later endocytosed by the blastomeres through coated pits and endocytic vesicles. The detection of Hm Ov-1 in numerous multivesicular bodies and lysosomal structures indicated that the oviductin is eventually degraded. Although the exact functional role of Hm Ov-1 is not known, the presence of a copious amount of Hm Ov-1 in early hamster embryos may be ascribed to a special relationship between this particular oviductin and embryo development.

Oviductins possess chitinase- and mucin-like domains: a lead in the search for the biological function of these oviduct-specific ZP-associating glycoproteins

Over the last 10 years considerable progress has been made in the immunological and biochemical characterization of oviduct-specific glycoproteins. It is now well established that a subclass of these secretory products, designated as oviductins, associate with the zona pellucida of the ovulated oocyte and with the early embryo. Recent reports on the cloning of cDNAs of oviductins from various species, including that of golden hamster (Mesocricetus auratus) oviductin by our laboratory, allowed us to compare their deduced amino acid sequences with those of other proteins. Optimal alignment analysis showed that oviductins contain regions of significant similarity with catalytically inactive mammalian members of the bacterial and microfilarial chitinase protein family. Most importantly, a close examination of the hamster and human deduced amino acid sequences revealed that both glycoproteins possess contiguous Ser/Thr rich repeated units, clustered in their carboxy-terminal portions. These mucin-type motifs are similar in the hamster and human glycoprotein, although hamster oviductin contains more of these complete units. This striking feature might indicate that these molecules play a similar role to mucin-type glycoproteins, e.g., in protecting the oocyte and early embryo against attacks from their environment. We propose a model whereby oviductins are targeted to the oocyte via the interaction of their chitinase-like domains with specific oligosaccharide moieties of the zona pellucida. Once localized to this structure, oviductin molecules would act as a protective shield around the oocyte and early embryo by virtue of their densely glycosylated mucin-type domains.

Biochemical characterization of a bovine oviduct-specific sialo-glycoprotein that sustains sperm viability in vitro

A bovine oviduct-specific glycoprotein (BOGP) that sustained the viability of bovine spermatozoa in vitro was purified from an extract of bovine oviducts. The amino-terminal amino acid sequence of BOGP was found to be a homologous with that of oviductin, a protein from hamster that was recently characterized by Mallete and Bleau (1993: Biochem. J. 295, 437-445). Purified BOGP was characterized as a sialo-glycoprotein containing N-linked and O-linked sialo-oligosaccharides side chains with galactose, mannose, N-acetyl-D-galactosamine, N-acetyl-D-glucosamine, fucose and sialic acids in its core protein (57 kDa). Intact BOGP has a wide range of isoelectric points (pIs) from 6.5 to 3.0 but a narrow range of molecular masses around 95 kDa. On isoelectric focusing of neuraminidase-treated BOGP (AS-BOGP), a narrow band with a pI of 9.3 was observed, and the ability of AS-BOGP to maintain sperm viability was negligible. We propose that BOGP is a mucin-type sialo-glycoprotein with a molecular mass of 72 kDa that contains one N-linked and approx. 15 O-linked sialo-oligosaccharide chains. These side chains appear to be important for the maintenance of sperm viability.