Protein-carbohydrate interactions during fertilization

Characterization of plasma membrane associated type II α-D-mannosidase and β-N-acetylglucosaminidase of Aquarius remigis sperm

For successful fertilization to occur, molecules on the surface of male and female gametes must recognize each other in a complementary manner. In some organisms, sperm possess a glycosidase on the plasma membrane overlying the head while eggs have glycoproteins that are recognized by those glycosidases resulting in sperm-egg recognition. In this study, two glycosidases, mannosidase and β-N-acetylglucosaminidase, were identified and biochemically characterized in Aquarius remigis sperm. The mannosidase had a Km of 2.36 ± 0.19 mM, a Vmax of 27.49 ± 0.88 pmol/min and a Hill coefficient of 0.94 ± 0.18 at its optimal pH of 7.0. The mannosidase was extracted most efficiently with CHAPSO but was also efficiently extracted with sodium chloride. Mannosidase activity was effectively inhibited by swainsonine, but not by kifunesine, and was significantly reduced in the presence of Mn(2+) and Mg(2+), but not Zn(2+). N-acetylglucosaminidase had a Km of 0.093 ± 0.01 mM, a Vmax of 153.80 ± 2.97 pmol/min and a Hill coefficient of 0.96 ± 0.63 at its optimal pH of 7.0. N-acetylglucosaminidase was extracted most efficiently with potassium iodide but was also efficiently extracted with Triton X-100 and Zn(2+), but not Ca(2+), Co(2+), Mn(2+) or Mg(2+), significantly inhibited its activity. Taken together, these results indicate that the A. remigis sperm surface contains at least two glycosidases that may recognize complementary glycoconjugates on the surface of water strider eggs.

Structures and biosynthesis of the N- and O-glycans of recombinant human oviduct-specific glycoprotein expressed in human embryonic kidney cells

Oviduct-specific glycoprotein (OVGP1) is a major mucin-like glycoprotein synthesized and secreted exclusively by non-ciliated secretory cells of mammalian oviduct. In vitro functional studies showed that OVGP1 plays important roles during fertilization and early embryo development. We have recently produced recombinant human oviduct-specific glycoprotein (rhOVGP1) in human embryonic kidney 293 (HEK293) cells. The present study was undertaken to characterize the structures and determine the biosynthetic pathways of the N- and O-glycans of rhOVGP1. Treatment of the stable rhOVGP1-expressing HEK293 cells with either GalNAcα-Bn to block O-glycan extension, tunicamycin to block N-glycosylation, or neuraminidase increased the electrophoretic mobility of rhOVGP1. A detailed analysis of O- and N-linked glycans of rhOVGP1 by mass spectrometry showed a broad range of many simple and complex glycan structures. In order to identify the enzymes involved in the glycosylation of rhOVGP1, we assayed glycosyltransferase activities involved in the assembly of O- and N-glycans in HEK293 cells, and compared these to those from the immortalized human oviductal cells (OE-E6/E7). Our results demonstrate that HEK293 and OE-E6/E7 cells exhibit a similar spectrum of glycosyltransferase activities that can synthesize elongated and sialylated O-glycans with core 1 and 2 structures, as well as complex multiantennary N-glycans. It is anticipated that the knowledge gained from the present study will facilitate future studies of the role of the glycans of human OVGP1 in fertilization and early embryo development.

The expression of fucose isoforms of amniotic and plasma alpha-1-acid glycoprotein derived from 2nd and 3rd trimester normal pregnancies

OBJECTIVES

To analyse modifications in AGP fucosylation in relation to different stages of human pregnancy.

DESIGN AND METHODS

The relative amounts of three fucosyl-glycotopes on AGP were analysed by lectin-ELISA using fucose-specific biotinylated lectins in 169 plasma and 178 amniotic fluid samples from normal pregnancies with gestational ages of 14 to 42 weeks.

RESULTS

The plasma AGPs of all the pregnant women and amniotic AGPs from the 2nd trimester lacked fucoses. In contrast, in the 3rd trimester the amniotic AGPs were highly decorated by the innermost alpha1,6-fucose as well as alpha1,2- and alpha1,3-fucoses of the outer arms, reaching the highest expression around the perinatal period. At delivery the relative amounts of the alpha1,3- and alpha1,2-AGP isoforms, but not the alpha1,6 isoform, significantly decreased.

CONCLUSIONS

The highly fucosylated amniotic AGP isoforms could be implicated in regulatory processes to ensure homeostasis during pregnancy and to protect the fetus. They have the potential of becoming laboratory markers in obstetrics to monitor pregnancy.

Polypeptide backbone derived from carboxyl terminal of mouse ZP3 inhibits sperm-zona binding

For mammalian organism, fertilization begins with species-specific recognition between sperm and egg, a process depending upon egg zona pellucida glycoproteins and putative sperm interacting protein(s). In mouse, zona pellucida glycoprotein ZP3 is believed to be the primary receptor for sperm and inducer of sperm acrosomal reaction, and its function has been attributed to the specific O-linked oligosaccharides attached to polypeptide backbone. While lots of reports have focused on the role of ZP3's oligosaccharides in fertilization, there are few concerning its polypeptide backbone. To investigate whether mZP3 polypeptide backbone is involved in sperm-egg recognition, three partially overlapping cDNA fragments, together covering entire mouse ZP3, were cloned, expressed and purified under denaturing condition. Although all three refolded proteins possess native conformation, only one derived from the carboxyl terminal showed inhibitory effect to the sperm-zona binding during in vitro fertilization. This phenomenon could not be explained by enhanced acrosomal exocytosis rate, in that the acrosomal reaction assay demonstrated its inability to induce the acrosomal reaction. Our results suggest that the carboxyl terminal of mZP3 polypeptide backbone interacts with sperm and such interaction plays a significant role in sperm-zona binding, ultimately successful fertilization.

Clinical significance of sperm protein 17 expression and immunogenicity in esophageal cancer

We recently identified sperm protein 17 (Sp17) transcripts in esophageal squamous cell carcinomas (ESCCs) by differential display. This study was designed to determine the clinical significance of Sp17 protein in different stages of esophageal tumorigenesis and to test the hypothesis that aberrant localization of Sp17 protein to immunosurveillant site may lead to production of anti-Sp17 antibodies in serum, which may be of clinical relevance in ESCCs. Sp17 transcripts were detected by RT-PCR in 26 of 30 (86%) ESCCs, while no transcripts were detected in normal esophageal tissues. Rabbit polyclonal antibody was raised against an immunogenic peptide of Sp17 and used to evaluate protein expression by immunohistochemistry. Expression of Sp17 protein was observed in 60/80 (75%) of ESCCs and 27/30 (90%) dysplastic tissues, while no detectable Sp17 expression was observed in 13 distant histologically normal epithelia. Sixteen of the 60 immunopositive ESCCs showed nuclear expression in addition to cytoplasmic localization of the protein. The circulating levels of anti-Sp17 antibodies, determined by ELISA, were significantly elevated in ESCC patients when compared with normal subjects (p < 0.001). Increasing Sp17 antibody titers were observed to be associated with the progressive disease in 4 patients. In conclusion, the study demonstrates expression of Sp17 protein in esophageal tumor as well as dysplastic tissues, suggesting it to be an early event in the development of ESCC. To our knowledge, this is the first report showing elevated levels of anti-Sp17 antibodies in ESCC patients.

Cancer immunotherapy targeting Sp17: when should the laboratory findings be translated to the clinics?

Despite advances in chemotherapeutic agents, the prognosis for some cancers remains extremely poor, suggesting the need for other treatment modalities. Immunotherapy appears an ideal approach because the mechanisms of tumor cell killing induced by tumor vaccines are different from those from chemotherapy. Various investigations are ongoing to identify suitable targets for this purpose. Sperm protein 17 (Sp17) was originally identified by our group as a novel cancer-testis antigen in various malignancies, including multiple myeloma. Sp17 is a highly immunogenic protein and the observation that more than 90% of vasectomized males develop immunity against Sp17 suggests the opportunity and safety of Sp17 for tumor vaccines. Recent works by other workers suggest a low level of expression of Sp17 in some normal tissues, and investigators have questioned whether Sp17 is in fact a suitable target for immunotherapy. In this paper, we review the general principles of immunotherapy and provide evidence supporting the highly immunogenic nature of Sp17. We also address the discrepancies between the objectives of oncologists involved in treating cancer patients and their familiarity with acceptable levels of toxicity of any effective therapy and those of pure laboratory-based investigators. Finally, we present some early clinical data supporting the rationale for further investigations of Sp17 for tumor vaccines.

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.

Clinical Laboratory Evaluation of Male Subfertility

Male subfertility is a common problem with a complex etiology, requiring a complete andrological work-up for proper diagnosis. The male reproductive tract is controlled by a well-balanced hormonal system, in which hypothalamic (GnRH), pituitary (LH, FSH) and testicular hormones (androgens, inhibin B) participate. Any disturbance of this hormonal system may therefore lead to testicular dysfunction and interfere with the spermatogenesis process. In addition, also other components along the ductal system, such as epididymis, prostate and seminal vesicles, that improve sperm fertility by contributing their secretions to the semen, might function inadequately and thus fail to enhance the fertilizing capacity of the sperm cells. External factors (heat, chemicals, life style) and anatomical abnormalities (varicocele) were shown to have a negative influence on male fertility. In a number of patients genetic defects can be identified as the cause of their infertility. Laboratory tests are available to assess hormone concentrations, semen composition, accessory gland function and sperm cell function. Conventional semen analysis includes the determination of sperm concentration, semen volume, sperm motility (qualitative and quantitative), sperm morphology, sperm cell vitality, pH, leucocytes and antibodies. The usefulness of the determination of these parameters as predictor of fertility appears to be rather limited, however. Therefore, alternative tests, some based on more functional aspects (sperm penetration, capacitation, acrosome reaction), have been developed. Furthermore, there is an increasing attention for the assessment of DNA integrity, for instance by the flowcytometer-based Sperm Chromation Structure Assay (SCSA), as an additional or alternative parameter of sperm quality. It is likely and desirable that further assays with better predictive value are being developed in the near future.

Contribution of mouse egg zona pellucida glycoproteins to gamete recognition during fertilization

For sperm to fertilize eggs, they must first bind to the thick zona pellucida (ZP) that surrounds the plasma membrane of all unfertilized mammalian eggs. An extensive literature suggests that mouse sperm recognize and bind to a specific ZP glycoprotein called mZP3. However, the role of individual ZP glycoproteins in binding of mouse sperm to eggs has been called into question by recent transgenic experiments with null mice. Results of such experiments have been interpreted to mean that binding of sperm depends on the supramolecular structure of the ZP, not on an individual ZP glycoprotein. Here, it is argued that results of these transgenic experiments actually are consistent with the prevailing view of gamete recognition that implicates a specific ZP glycoprotein in both binding of mouse sperm to eggs and induction of the acrosome reaction.

Crystal Structure of Thermotoga maritima α-l-Fucosidase

Fucosylated glycoconjugates are involved in numerous biological events, and alpha-l-fucosidases, the enzymes responsible for their processing, are therefore of crucial importance. Deficiency in alpha-l-fucosidase activity is associated with fucosidosis, a lysosomal storage disorder characterized by rapid neurodegeneration, resulting in severe mental and motor deterioration. To gain insight into alpha-l-fucosidase function at the molecular level, we have determined the crystal structure of Thermotoga maritima alpha-l-fucosidase. This enzyme assembles as a hexamer and displays a two-domain fold, composed of a catalytic (beta/alpha)(8)-like domain and a C-terminal beta-sandwich domain. The structures of an enzyme-product complex and of a covalent glycosyl-enzyme intermediate, coupled with kinetic and mutagenesis studies, allowed us to identify the catalytic nucleophile, Asp(244), and the Brønsted acid/base, Glu(266). Because T. maritima alpha-l-fucosidase occupies a unique evolutionary position, being far more closely related to the mammalian enzymes than to any other prokaryotic homolog, a structural model of the human enzyme was built to document the structural consequences of the genetic mutations associated with fucosidosis.

Expression of sperm protein 17 (Sp17) in ovarian cancer

Sperm protein 17 (Sp17) is an antigenic protein highly expressed in spermatozoa. Sp17 expression was demonstrated recently in multiple myeloma, suggesting that it may be a novel cancer-testis antigen. Expression of Sp17 mRNA and protein was examined in human ovarian tumors. Sp17 mRNA was evaluated by reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot analysis of RNA derived from epithelial ovarian tumors and normal tissues. RT-PCR analysis detected Sp17 transcripts in 15 of 18 (83%) primary ovarian tumors. The transcript was not detected in RNA derived from normal uterus or cervix, whereas weak expression was noted in some normal ovarian tissue samples. Northern blot analysis showed no detectable Sp17 mRNA expression in normal tissues, including normal ovary, but showed Sp17 expression in 17 of 25 ovarian tumors (68%). To evaluate protein expression, mouse monoclonal antibodies were produced against recombinant Sp17 protein and used in Western blot and immunohistochemical analyses of normal reproductive tissue and primary ovarian tumor samples. Sp17 protein was detected by Western blot analysis in normal spermatozoa and in 8 of 19 ovarian tumor samples. Immunohistochemical studies showed Sp17 expression in spermatozoa, ciliated cells of the female reproductive tract, and most ovarian tumors evaluated. Tumors showed a predominantly nuclear localization of Sp17 expression, with some cytoplasmic staining. These results demonstrate that Sp17, a protein with restricted expression in somatic tissues, is expressed in ovarian tumors. Because Sp17 is immunogenic, it may represent a novel target for immunotherapeutic interventions for ovarian cancer patients.

Embryo-maternal communication in bovine - strategies for deciphering a complex cross-talk

Early embryonic development, implantation and maintenance of a pregnancy are critically dependent on an intact embryo-maternal communication. So far, only few signals involved in this dialogue have been identified. In bovine and other ruminants, interferon tau is the predominant embryonic pregnancy recognition signal, exhibiting antiluteolytic activity. However, this is just one aspect of the complex process of embryo-maternal signalling, and a number of other systems are more likely to be involved. To gain a more comprehensive understanding of these important mechanisms, integrated projects involving specialists in embryology, reproductive biotechnology and functional genome research are necessary to perform a systematic analysis of interactions between pre-implantation stage embryos and oviduct or uterine epithelial cells, respectively. State-of-the-art transcriptomic and proteomic technologies will identify reciprocal signals between embryos and their maternal environment and the respective downstream reaction cascades. For in vivo studies, the use of monozygotic twins as recipient animals provides elegant model systems, thus eliminating genetic variability as a cause of differential gene expression. In addition, suitable systems for the co-culture of oviduct epithelial or endometrium cells with the respective embryonic stages need to be established for functional validation of candidate genes potentially involved in the dialogue between embryos and their maternal environment. The knowledge of these mechanisms should help to increase the pregnancy rate following embryo transfer and to avoid embryonic losses. Candidate genes involved in embryo-maternal communication will also be used to define new quality criteria for the selection of embryos for transfer to recipients. Another application is the supplementation of embryotrophic factors or components of embryo-maternal signalling in optimized formulations, such as bioartificial matrices. As a long-term goal, signalling mechanisms identified in bovine will also be functionally evaluated in other species, including the human.

Detection of ligand- and solvent-induced shape alterations of cell-growth-regulatory human lectin galectin-1 in solution by small angle neutron and x-ray scattering

The bioactivity of galectin-1 in cell growth regulation and adhesion prompted us to answer the questions whether ligand presence and a shift to an aprotic solvent typical for bioaffinity chromatography might alter the shape of the homodimeric human lectin in solution. We used small angle neutron and synchrotron x-ray scattering studies for this purpose. Upon ligand accommodation, the radius of gyration of human galectin-1 decreased from 19.1 +/- 0.1 A in the absence of ligand to 18.2 +/- 0.1 A. In the aprotic solvent dimethyl sulfoxide, which did not impair binding capacity, galectin-1 formed dimers of a dimer, yielding tetramers with a cylindrical shape. Intriguingly, no dissociation into subunits occurred. In parallel, NMR monitoring was performed. The spectral resolution was in accord with these data. In contrast to the properties of the human protein, a nonhomologous agglutinin from mistletoe sharing galactose specificity was subject to a reduction in the radius of gyration from approximately 62 A in water to 48.7 A in dimethyl sulfoxide. Evidently, the solvent caused opposite responses in the two tested galactoside-binding lectins with different folding patterns. We have hereby proven that ligand presence and an aprotic solvent significantly affect the shape of galectin-1 in solution.

Zona-binding inhibitory factor-1 from human follicular fluid is an isoform of glycodelin

Zona-binding inhibitory factor-1 (ZIF-1), a glycoprotein in human follicular fluid, reduces the binding of spermatozoa to the zona pellucida. ZIF-1 has a number of properties similar to those of glycodelin-A from human follicular fluid. The objective of this study was to compare the biochemical characteristics of these two glycoproteins. N-terminal sequencing and protease-digested peptide mapping showed that ZIF-1 and glycodelin-A have the same protein core. However, these glycoproteins differ in their oligosaccharide chains, as demonstrated by fluorophore-assisted carbohydrate electrophoresis, lectin-binding ability, and isoelectric focusing. ZIF-1 inhibited spermatozoa-zona pellucida binding slightly more than did glycodelin-A and significantly suppressed progesterone-induced acrosome reaction of human spermatozoa. Indirect immunofluorescence staining revealed specific binding of glycodelin-A and ZIF-1 to the acrosome region of human spermatozoa, where ZIF-1 produced a stronger signal than did glycodelin-A at the same protein concentration. These data suggest that ZIF-1 is a differentially glycosylated isoform of glycodelin that potently inhibits human sperm-egg interaction. Future study on the function role of ZIF-1 would provide a better understanding of the regulation of fertilization in humans.

Binding of zona binding inhibitory factor-1 (ZIF-1) from human follicular fluid on spermatozoa

Previous studies showed that zona binding inhibitory factor-1 (ZIF-1) was the glycoprotein mainly responsible for the spermatozoa zona binding inhibitory activity of human follicular fluid. ZIF-1 has a number of properties similar to glycodelin-A. A binding kinetics experiment in the present study demonstrated the presence of two binding sites of ZIF-1 on human spermatozoa. These binding sites were saturable, reversible, and bound to (125)I-ZIF-1 in a time-, concentration-, and temperature-dependent manner. Glycodelin-A shared one common binding site with ZIF-1 on spermatozoa, and it could displace only 70% of the (125)I-ZIF-1 bound on human spermatozoa. ZIF-1 and glycodelin-A formed complexes with the soluble extract of human spermatozoa. Coincubation of solubilized zona pellucida proteins reduced the binding of ZIF-1 to two complexes of the extract, suggesting that the ZIF-1 binding sites and zona pellucida protein receptors on human spermatozoa were closely related. ZIF-1, but not glycodelin-A, significantly suppressed progesterone-induced acrosome reaction of human spermatozoa. The carbohydrate moieties derived from ZIF-1 reduced the binding of native ZIF-1 on human spermatozoa as well as the zona binding inhibitory activity of the glycoprotein, although the intensity of the effects are lower when compared with the native protein. These effects are not due to the action of the molecules on the motility, viability, and acrosomal status of the treated spermatozoa. Deglycosylated ZIF-1 had no inhibitory effect on both ZIF-1 binding and zona binding capacity of spermatozoa. We concluded that the carbohydrate part of ZIF-1 was critical for the functioning of the glycoprotein.

On the species specificity of sperm binding and sperm penetration of the zona pellucida

Sperm binding and sperm penetration of the zona pellucida (zp) are regarded as species-specific. In this investigation, the interactions between bovine oocytes and porcine, respectively, equine spermatozoa have been studied under in vitro conditions and compared with the normal in vitro fertilization of bovine oocytes by bovine sperm. Surprisingly, many of the heterologous spermatozoa adhered firmly to the bovine oocytes and could not be removed by intense washing. On average, more than 100 boar or equine spermatozoa were bound to the zp of bovine oocytes. Electron microscopic studies clearly demonstrated that porcine sperm attached to the zona and underwent the acrosome reaction. Equine spermatozoa displayed a similar binding affinity, but unlike the porcine spermatozoa even penetrated the zp and were taken up into the oocyte after a longer period of co-incubation. Considering these new results the dogma of a strict species specificity of sperm zona interactions under in vitro conditions has to be reconsidered.

Functional morphology of the zona pellucida

The zona pellucida (ZP) is an extracellular matrix surrounding the oocyte and the early embryo that exerts several important functions during fertilization and early embryonic development. The ZP of most mammalian species is composed of three major glycoproteins that show considerable heterogeneity due to extensive post-translational modifications. Sodium dodecyl sulphate polyacrylamide gel electrophoresis of the ZP reveals three to four glycoproteins which have been nominated ZPI. ZP2, ZP3 and ZP4. As cloning and characterization of the ZP genes of a variety of mammalian species including domestic animals show a high homology, three classes of ZP genes, ZPA, ZPB and ZPC can be discerned. The corresponding proteins were named ZPA, ZPB and ZPC. Whereas in the mouse ZPB is the primary sperm receptor. the situation is more complicated in other species. For instance, in the pig ZPA has been shown to possess receptor activity. Interaction between gametes during fertilization is at least in part regulated by carbohydrate moieties of the ZP and carbohydrate-binding proteins of the sperm surface. In domestic animals zona proteins are expressed in both the oocyte and granulosa cells in a stage-specific pattern and may play a role in granulosa cell differentiation. The role of ZP glycoproteins in immunocontraception is briefly discussed.

Molecular basis of human sperm-zona pellucida interaction

The recognition of carbohydrate sequences by complimentary receptors has been shown to be a critical factor in gamete interaction in many different animal species. We have proposed the hypothesis that, in the human, sperm binding to the zona pellucida requires a 'selectin-like' interaction. We have used the hemizona assay (a unique internally controlled bioassay that evaluates tight binding of human sperm to the homologous zona pellucida) and advanced methods of carbohydrate analysis to test this hypothesis. We provide compelling evidence that demonstrates that oligosaccharide recognition is also required for specific, tight human gamete binding. Hapten inhibition tests, zona pellucida lectin-binding studies and removal/modification of functional carbohydrates by chemical and enzymatic methods provide evidence for the involvement of defined carbohydrate moieties in initial binding. Our studies suggest the existence of distinct zona-binding proteins on human sperm that can bind to selectin ligands. Additionally, results suggest a possible convergence in the types of carbohydrate sequences recognized during initial human gamete binding and immune/inflammatory cell interactions. Full characterization of the glycoconjugates that manifest selectin-ligand activity on the human zona pellucida will allow for a better understanding of human gamete interaction in physiological and pathological situations.

Localization of sperm ligand carbohydrate chains in pig zona pellucida glycoproteins

Neutral N-linked carbohydrate chains from pig ZPB/ZPC mixture are shown to possess sperm ligand activity. Of these complex-type chains, triantennary/tetraantennary chains exhibit the activity stronger than that of diantennary chains. Intact ZPB and ZPC cannot be separated from each other unless acidic N-acetyllactosamine regions of their carbohydrate chains are removed by endo-beta-galactosidase digestion. The endo-beta-galactosidase-digested ZPB and its N-terminal fragment of 111 residues retain the sperm ligand activity. Three glycopeptides, having one Asn residue to which the carbohydrate chain is linked, are obtained by lysylendopeptidase digestion of the heat-solubilized zonae containing intact ZPB, and lysylendopeptidase and chymotryptic digestions of endo-beta-galactosidase-digested ZPB. On the basis of sugar mapping analysis of the N-linked chains from these glycopeptides and comparison with the carbohydrate structures of the main intact neutral N-linked chains of ZPB/ZPC, the triantennary and tetraantennary chains are shown to be localized mainly at Asn220 of ZPB, whereas diantennary chains are present on all the three N-glycosylation sites (Asn203, Asn220 and Asn333). These results suggest that the carbohydrate chains linked to Asn220 of ZPB participate predominantly in sperm-egg binding. ZPC has been shown to support the expression of sperm ligand activity of ZPB. Three glycopeptides, each having one of the N-glycosylation sites, are obtained by tryptic digestion of endo-beta-galactosidase-digested ZPC. Triantennary and tetraantennary chains are found mainly at Asn271 of ZPC, whereas diantennary chains are present at all three N-glycosylation sites (Asn124, Asn146 and Asn271). Thus, the localization of triantennary and tetraantennary chains in ZPC is different from that in ZPB.

Towards defining the role of glycans as hardware in information storage and transfer: basic principles, experimental approaches and recent progress

The term 'code' in biological information transfer appears to be tightly and hitherto exclusively connected with the genetic code based on nucleotides and translated into functional activities via proteins. However, the recent appreciation of the enormous coding capacity of oligosaccharide chains of natural glycoconjugates has spurred to give heed to a new concept: versatile glycan assembly by the genetically encoded glycosyltransferases endows cells with a probably not yet fully catalogued array of meaningful messages. Enciphered by sugar receptors such as endogenous lectins the information of code words established by a series of covalently linked monosaccharides as letters for example guides correct intra- and intercellular routing of glycoproteins, modulates cell proliferation or migration and mediates cell adhesion. Evidently, the elucidation of the structural frameworks and the recognition strategies within the operation of the sugar code poses a fascinating conundrum. The far-reaching impact of this recognition mode on the level of cells, tissues and organs has fueled vigorous investigations to probe the subtleties of protein-carbohydrate interactions. This review presents information on the necessarily concerted approach using X-ray crystallography, molecular modeling, nuclear magnetic resonance spectroscopy, thermodynamic analysis and engineered ligands and receptors. This part of the treatise is flanked by exemplarily chosen insights made possible by these techniques.

Towards the molecular basis of sperm and egg interaction during mammalian fertilization

During the past 2 decades, a number of genes have been cloned from mammals which encode polypeptides that participate in the process of fertilization. Among these are glycoproteins ZP1-3 that constitute the zona pellucida of eggs from mice to human beings. In mice, one of these glycoproteins, mZP3, acts as a primary sperm receptor and acrosome reaction-inducer. The evidence suggests that acrosome-intact sperm recognize and bind to a specific class of mZP3 oligosaccharides present on two serine residues (O-linked) located near the carboxy-terminus of the polypeptide. Mutagenesis of either of these residues results in the synthesis of an inactive form of the receptor. Therefore, mammalian fertilization is a carbohydrate-mediated event. It is possible that changes in the structure of these oligosaccharides (e.g., composition, sequence, linkages, modifications, etc.) could account for species-specific binding of sperm to eggs. Stably transfected somatic cells, null mutant animals, and DNA constructs are now available to test this possibility both in vivo and in vitro.

Biosynthesis and expression of zona pellucida glycoproteins in mammals

The zona pellucida (ZP) is an extracellular matrix surrounding the oocyte and the early embryo that exerts several important functions during fertilization and early embryonic development. The ZP of most mammalian species is composed of three glycoproteins (ZPA, ZPB, ZPC), products of the gene families ZPA, ZPB and ZPC that have been found to be highly homologous within mammalian species. Most data on the structure and function of the ZP are obtained from studies in mouse. New data from pig and other domestic animals, however, indicate that the mouse model does not hold for all other species. Whereas in the mouse ZPB is the primary sperm receptor, in the pig ZPA has been shown to possess receptor activity. Contrary to the mouse, where the growing oocyte is the only source of zona glycoproteins, in domestic animals these proteins are expressed in both the oocyte and granulosa cells in a stage-specific pattern and may play also a role in granulosa cell differentiation. In several mammalian species, the epithelial secretory cells of the oviduct synthesize and secrete specific glycoproteins (oviductins) that become closely associated with the ZP of the ovulated oocyte. Once bound to the ZP, oviductin molecules could act as a protective layer around the oocyte and early embryo by virtue of their densely glycosylated mucin-type domains.

Glycohistochemistry: the why and how of detection and localization of endogenous lectins

The central dogma of molecular biology limits the downstream flow of genetic information to proteins. Progress from the last two decades of research on cellular glycoconjugates justifies adding the enzymatic production of glycan antennae with information-bearing determinants to this famous and basic pathway. An impressive variety of regulatory processes including cell growth and apoptosis, folding and routing of glycoproteins and cell adhesion/migration have been unravelled and found to be mediated or modulated by specific protein (lectin)-carbohydrate interactions. The conclusion has emerged that it would have meant missing manifold opportunities not to recruit the sugar code to cellular information transfer. Currently, the potential for medical applications in anti-adhesion therapy or drug targeting is one of the major driving forces fuelling progress in glycosciences. In histochemistry, this concept has prompted the introduction of carrier-immobilized carbohydrate ligands (neoglycoconjugates) to visualize the cells' capacity to be engaged in oligosaccharide recognition. After their isolation these tissue lectins will be tested for ligand analysis. Since fine specificities of different lectins can differ despite identical monosaccharide binding, the tissue lectins will eventually replace plant agglutinins to move from glycan profiling and localization to functional considerations. Namely, these two marker types, i.e. neoglycoconjugates and tissue lectins, track down accessible binding sites with relevance for involvement in interactions in situ. The documented interplay of synthetic organic chemistry and biochemistry with cyto- and histochemistry nourishes the optimism that the application of this set of innovative custom-prepared tools will provide important insights into the ways in which glycans can act as hardware in transmitting information during normal tissue development and pathological situations.

Galactosyltransferase function during mammalian fertilization

Gamete recognition has been studied extensively in the mouse. In this system, it is generally believed that sperm bind to a class of O-linked oligosaccharides on the zona pellucida glycoprotein, ZP3. The best characterized sperm receptor for ZP3 is beta1, 4-galactosyltransferase (GalT), which functions in a lectin-like capacity by binding to N-terminal N-acetylglucosamine residues on ZP3 oligosaccharides. Multivalent oligosaccharides on ZP3, as well as synthetic polymers terminating in N-acetylglucosamine aggregate GalT, leading to activation of a heterotrimeric G protein cascade and culminating in the acrosome reaction. Following fertilization, cortical granules release N-acetylglucosaminidase, which removes the binding site for sperm GalT and facilitates the zona block to polyspermic binding. Genetic manipulation of GalT expression has confirmed its function as a ZP3 receptor. Overexpressing GalT on sperm leads to increased binding of ZP3, increased G protein activation, and precocious acrosome reactions. In contrast, sperm from mice made null for GalT by homologous recombination are refractory to ZP3, in that they are unable to bind soluble ZP3 and fail to undergo the acrosome reaction in response to zona glycoproteins. Surprisingly, GalT null sperm still bind to the zona and achieve low rates of fertilization in vitro. This then suggests that sperm-egg binding involves receptor-ligand interactions independent of GalT and ZP3. The current model suggests that GalT functions as the ZP3 receptor that is responsible for inducing the acrosome reaction, whereas initial sperm-zona binding is dictated by other sperm surface receptors. Consistent with this, at least three other zona pellucida monosaccharides have been implicated in sperm binding, and novel sperm surface glycoproteins have been suggested to function in gamete binding. A large scaffolding protein has been identified that associates with the GalT cytoplasmic domain and may be responsible for orchestrating its signal transduction capacities that lead to the acrosome reaction.

Strategies for glycoconjugate analysis

Glycoconjugates such as glycoproteins and glycosphingolipids exhibit multiple functions in biological systems. A correlation of functional features with defined structural parameters, however, presupposes detailed information on the glycoconjugates' carbohydrate moieties including the type of linkage of the oligosaccharide chain(s), monosaccharide composition as well as sequence, linkage positions and anomeric configurations of individual sugar monomers. Chemical and biochemical analyses are often impeded by the limited amounts of sample available and the vast structural heterogeneity of glycoconjugate glycans, thus requiring highly sensitive and efficient methods for detection, separation and structural elucidation of these carbohydrates. The aim of the article is to present suitable strategies for structural characterization of glycoconjugate glycans and to briefly review some of the techniques commonly used in this field.

Animal lectins as cell adhesion molecules

Protein-carbohydrate interaction is exploited in cell adhesion mechanisms besides the recognition of peptide motifs. The sugar code thus significantly contributes to the intriguing specificity of cellular selection of binding partners. Focusing on two classes of lectins (selectins and galectins), it is evident that their functionality for mediation of adhesive contacts is becoming increasingly appreciated, as is the integration of this type of interaction with other recognition modes to yield the noted specificity. The initial contact formation between leukocytes and activated endothelium makes use of selectins to guide lymphocyte trafficking. In addition to the three selectins which bind a distinct array of ligands, galectin-1 and galectin-3 and possibly other members of this family are involved in cell-cell or cell-matrix interactions. This review summarizes structural and functional aspects of these two classes of endogenous lectins relevant for cell adhesion.

Glycoconjugate expression and cartilage development of the cranial skeleton

Only few detailed investigations have focused on the glycobiology of cranial development. The functional elements in most inductive and morphogenetic processes are not individual cells, but rather collectives of interacting populations and extracellular matrix components that give rise to specific tissues and organs. Experimental evidence strongly suggests that sugar chains not only confer morphological characteristics. Complex carbohydrate molecules and their corresponding receptors are involved in recognition processes decoding biological information during cranial morphogenesis. The distribution patterns of glycoconjugates are highly dynamic and show a clear correlation with characteristic structural modifications. However, due to the intricate interactions in vivo the definitive physiological impact of these observations has to be established in further studies. In this review, the spatial and temporal patterns of lectin-reactive epitopes and selected receptor types are presented and discussed with regard to their potential importance to physiological craniogenesis.

Structural dynamics and function of early embryonic coats

The extracellular embryonic coats (embryo/blastocyst coverings) that surround early mammalian embryos are most often still referred to as zona pellucida. Accumulating evidence from a number of earlier and recent studies clearly indicates that this is an oversimplification which cannot be defended anymore, at least in many species. Structural modifications of the coats occur during cleavage and blastocyst stages; these are most obvious in a number of species with the central type of implantation and, related to this, a high degree of blastocyst expansion, notably the rabbit and the horse. In this contribution, formation and transformation of the various layers of coats (zona pellucida, mucoprotein layer, neozona and gloiolemma in the rabbit, capsule in the horse) will be reviewed, as will be comparable structures seen, e.g., in the fur seal (subzonal layer) and the baboon. These phenomena will be discussed in the context of structurally more subtle changes found in other species, including those with small blastocysts and other types of implantation, in particular with biochemical modifications which may be physiologically quite important. The molecular mechanisms of deposition of coats and their transformation and shedding/dissolution will be briefly addressed. The possible functional significance of the coats and their transformation will be discussed (mechanical, morphogenetic and immunological role, molecular transport control, blastocyst positioning, implantation, trap and reservoir function for signalling molecules).

Temporal and spatial regulation of expression of two galectins during kidney development of the chicken

Organogenesis and the establishment of the mature phenotype require an interplay between diverse recognition systems. Concerning protein-carbohydrate interactions, galectins are known to be involved in several extra- and intracellular functions. Due to the occurrence of two avian galectins in liver (chicken galectin-16 CG-16) and intestine (chicken galectin-14; CG-14) with different developmental regulation. the questions addressed are to what extent and where these galectins are present during chicken kidney development. Using Western blot analysis, the presence of both activities in tissue extracts was ascertained. A solid-phase assay showed peak levels at day 12 followed by a decline. A histochemical analysis was carried out in combination with routine staining. Epithelial cells of the mesonephric proximal tubules were immunoreactive in the cytoplasm for CG-14 from day 5 of incubation onwards. Additionally, epithelial cells of the metanephric collecting ducts were stained. For CG-16 a rather similar pattern of staining was seen, additional positivity in early glomerular podocytes being notable. At the electron microscopical level, a diffuse staining for CG-14 was seen in the cytoplasm, whereas immunoreactivity for CG-16 was observed mainly in mitochondria. These results demonstrate quantitative differences in the developmental regulation of the two avian galectins with obvious similarities in the cell-type pattern but with a disparate intracellular localisation profile.

Fucose in N-glycans: from plant to man

Fucosylated oligosaccharides occur throughout nature and many of them play a variety of roles in biology, especially in a number of recognition processes. As reviewed here, much of the recent emphasis in the study of the oligosaccharides in mammals has been on their potential medical importance, particularly in inflammation and cancer. Indeed, changes in fucosylation patterns due to different levels of expression of various fucosyltransferases can be used for diagnoses of some diseases and monitoring the success of therapies. In contrast, there are generally at present only limited data on fucosylation in non-mammalian organisms. Here, the state of current knowledge on the fucosylation abilities of plants, insects, snails, lower eukaryotes and prokaryotes will be summarised.