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

Identification of Sperm-Binding Sites in the N-Terminal Domain of Bovine Egg Coat Glycoprotein ZP4

The species-selective interaction between sperm and egg at the beginning of mammalian fertilisation is partly mediated by a transparent envelope called the zona pellucida (ZP). The ZP is composed of three or four glycoproteins (ZP1-ZP4). The functions of the three proteins present in mice (ZP1-ZP3) have been extensively studied. However, the biological role of ZP4, which was found in all other mammals studied so far, has remained largely unknown. Previously, by developing a solid support assay system, we showed that ZP4 exhibits sperm-binding activity in bovines and the N-terminal domain of bovine ZP4 (bZP4 ZP-N1 domain) is a sperm-binding region. Here, we show that bovine sperm bind to the bZP4 ZP-N1 domain in a species-selective manner and that N-glycosylation is not required for sperm-binding activity. Moreover, we identified three sites involved in sperm binding (site I: from Gln-41 to Pro-46, site II: from Leu-65 to Ser-68 and site III: from Thr-108 to Ile-123) in the bZP4 ZP-N1 domain using chimeric bovine/porcine and bovine/human ZP4 recombinant proteins. These results provide in vitro experimental evidence for the role of the bZP4 ZP-N1 domain in mediating sperm binding to the ZP.

Rheotaxis-based sperm separation using a biomimicry microfluidic device

Sperm selection is crucial to assisted reproduction, influencing the success rate of the treatment cycle and offspring health. However, in the current clinical sperm selection practices, bypassing almost all the natural selection barriers is a major concern. Here, we present a biomimicry microfluidic method, inspired by the anatomy of the female reproductive tract, that separates motile sperm based on their rheotaxis behavior to swim against the flow into low shear rate regions. The device includes micropocket geometries that recall the oval-shaped microstructures of the female fallopian tube to create shear protected zones for sperm separation. Clinical tests with human samples indicate that the device is capable of isolating viable and highly motile sperm based on their rheotaxis responses, resulting in a separation efficiency of 100%. The device presents an automated alternative for the current sperm selection practices in assisted reproduction.

Ligands and Receptors Involved in the Sperm-Zona Pellucida Interactions in Mammals

Sperm-zona pellucida (ZP) interaction, involving the binding of sperm surface ligands to complementary carbohydrates of ZP, is the first direct gamete contact event crucial for subsequent gamete fusion and successful fertilization in mammals. It is a complex process mediated by the coordinated engagement of multiple ZP receptors forming high-molecular-weight (HMW) protein complexes at the acrosomal region of the sperm surface. The present article aims to review the current understanding of sperm-ZP binding in the four most studied mammalian models, i.e., murine, porcine, bovine, and human, and summarizes the candidate ZP receptors with established ZP affinity, including their origins and the mechanisms of ZP binding. Further, it compares and contrasts the ZP structure and carbohydrate composition in the aforementioned model organisms. The comprehensive understanding of sperm-ZP interaction mechanisms is critical for the diagnosis of infertility and thus becomes an integral part of assisted reproductive therapies/technologies.

Cathepsin D in prawn reproductive system: its localization and function in actin degradation

Cathepsin D (CAT-D) is a well-known aspartic protease that serves a function as house-keeping lysosomal enzyme in all somatic cells. Its existence in reproductive tissues is highly variable, even in the somatic derived epithelial cells of reproductive tract. In Macrobrachium rosenbergii, existence of MrCAT-D and its translational product was detected in both somatic cells (Sertoli-like supporting cells) and developing spermatogenic cells as well as along accessory spermatic ducts. Specifically, MrCAT-D was localized onto the sperm surface rather than within the acrosomal matrix, as evident by similar staining pattern of anti-CAT-D on live and aldehyde fixed sperm. MrCAT-D in testicular extracts and sperm isolates showed active enzyme activities towards its specific fluorogenic substrate (MCA-Gly-Lys-Pro-Ile-Leu-Phe-Phe-Arg-Leu-Lys (Dnp)-D-Arg-NH₂). MrCAT-D also exerted its function towards hydrolyzing filamentous actin, the meshwork of which is shown to be localized at the junction between germ cells and supporting cells and spermatogonia in M. rosenbergii testicular epithelium. Together, we have localized MrCAT-D transcript and its translational product in both supporting and germ cells of testis and claimed its enzymatic function towards actin degradation, which may be related to sperm release from the epithelial cell interaction.

Identification of lipid raft glycoproteins obtained from boar spermatozoa

The surface of the spermatozoa is coated with glycoproteins the redistribution of which during in vitro capacitation plays a key role in the subsequent fertilization process. Lipid rafts are membrane microdomains involved in signal transduction through receptors and include or recruit specific types of proteins and glycoproteins. Few studies have focused on identifying glycoproteins resident in the lipid rafts of spermatozoa. Proteins associated with lipid rafts modify their localization during capacitation. The objective of the study was to identify the glycoproteins associated with lipid rafts of capacitated boar spermatozoa through a lectin-binding assay coupled to mass spectrometry approach. From the proteomic profiles generated by the raft proteins extractions, we observed that after capacitation the intensity of some bands increased while that of others decreased. To determine whether the proteins obtained from lipid rafts are glycosylated, lectin blot assays were performed. Protein bands with a good resolution and showing significant glycosylation modifications after capacitation were analyzed by mass spectrometry. The bands of interest had an apparent molecular weight of 64, 45, 36, 34, 24, 18 and 15 kDa. We sequenced the 7 bands and 20 known or potential glycoproteins were identified. According to us, for ten of them this is the first time that their association with sperm lipid rafts is described (ADAM5, SPMI, SPACA1, Seminal plasma protein pB1, PSP-I, MFGE8, tACE, PGK2, SUCLA2, MDH1). Moreover, LYDP4, SPAM-1, HSP60, ZPBP1, AK1 were previously reported in lipid rafts of mouse and human spermatozoa but not in boar spermatozoa. We also found and confirmed the presence of ACR, ACRBP, AWN, AQN3 and PRDX5 in lipid rafts of boar spermatozoa. This paper provides an overview of the glycosylation pattern in lipid rafts of boar spermatozoa before and after capacitation. Further glycomic analysis is needed to determine the type and the variation of glycan chains of the lipid rafts glycoproteins on the surface of spermatozoa during capacitation and acrosome reaction.

Rheotaxis-based separation of sperm with progressive motility using a microfluidic corral system

The separation of motile sperm from semen samples is sought after for medical infertility treatments. In this work, we demonstrate a high-throughput microfluidic device that can passively isolate motile sperm within corrals inside a fluid channel, separating them from the rest of the diluted sample. Using finite element method simulations and proposing a model for sperm motion, we investigated how flow rate can provide a rheotaxis zone in front of the corral for sperm to move upstream/downstream depending on their motility. Using three different flow rates that provided shear rates above the minimum value within the rheotaxis zone, we experimentally tested the device with human and bovine semen. By taking advantage of the rheotactic behavior of sperm, this microfluidic device is able to corral motile sperm with progressive velocities in the range of 48-93 μm⋅s^-1 and 51-82 μm⋅s^-1 for bovine and human samples, respectively. More importantly, we demonstrate that the separated fractions of both human and bovine samples feature 100% normal progressive motility. Furthermore, by extracting the sperm swimming distribution within the rheotaxis zone and sperm velocity distribution inside the corral, we show that the minimum velocity of the corralled sperm can be adjusted by changing the flow rate; that is, we are able to control the motility of the separated sample. This microfluidic device is simple to use, is robust, and has a high throughput compared with traditional methods of motile sperm separation, fulfilling the needs for sperm sample preparation for medical treatments, clinical applications, and fundamental studies.

Lipid raft dynamics linked to sperm competency for fertilization in mice

It is well known that mammalian sperm acquires fertilization ability after several maturation processes, particularly within the female reproductive tract. In a previous study, we found that both glycosylphosphatidylinositol (GPI)-anchored protein (GPI-AP) release and lipid raft movement occur during the sperm maturation process. In several genetic studies, release of GPI-AP is a crucial step for sperm fertilization ability in the mouse. Here, we show that lipid raft movement is also fundamental for sperm to be competent for fertilization by comparing the sperm maturation process of two mouse inbred strains, C57BL/6 and BALB/c. We found that ganglioside GM1 movement was exclusively reduced in BALB/c compared with C57BL/6 among other examined sperm maturation parameters, such as GPI-AP release, sperm migration to the oviduct, cholesterol efflux, protein tyrosine phosphorylation and acrosome reaction, and was strongly linked to sperm fertility phenotype. The relationship between GM1 movement and in vitro fertilization ability was confirmed in other mouse strains, suggesting that lipid raft movement is one of the important steps for completing the sperm maturation process.

The egg coat zona pellucida 3 glycoprotein - evolution of its putative sperm-binding region in Old World murine rodents (Rodentia: Muridae)

In eutherian mammals, before fertilisation can occur the spermatozoon has to bind to, and penetrate, the egg coat, the zona pellucida (ZP). In the laboratory mouse there is good evidence that the primary sperm-binding site is a protein region encoded by Exon 7 of the ZP3 gene and it has been proposed that binding is species specific and evolves by sexual selection. In the present study we investigate these hypotheses by comparing Exon 6 and 7 sequences of ZP3 in 28 species of murine rodents of eight different divisions from Asia, Africa and Australasia, in which a diverse array of sperm morphologies occurs. We found considerable nucleotide (and corresponding amino acid) sequence divergence in Exon 7, but not in Exon 6, across these species, with evidence for positive selection at five codon positions. This molecular divergence does not appear to be due to reinforcement to reduce hybridisation, nor does it correlate with divergence in sperm head morphology or tail length, thus it is unlikely to be driven by inter-male sperm competition. Other forms of post-copulatory sexual selection therefore appear to have resulted in the molecular divergence of this region of ZP3 in this highly speciose group of mammals.

Prion protein 2 (dublet) gene (PRND): role in ovine semen capacitation, cryopreservation and fertility

The aim of the present study was to examine the role of Doppel protein in the capacitation process and fertilising ability of both fresh and frozen–thawed (FT) spermatozoa from rams carrying different prion protein 2 (dublet) (PRND) gene polymorphisms. The detection efficacy of new anti-Doppel monoclonal antibodies and PRND mRNA quantification were also explored in ovine spermatozoa. Three different genotypes (AA, GA, GG) were identified for codon 26 of ovine PRND-c.78G>A. Using flow cytometry, a higher fluorescence was detected in fresh compared with FT sperm samples incubated with anti-Doppel primary and fluorescein isothiocyanate-conjugated secondary antibodies (P < 0.05). Capacitation was affected by semen treatment (fresh and FT) and male PRND genotype (P < 0.05). After IVF, the use of fresh semen resulted in a higher cleavage rate than the use of FT spermatozoa (P = 0.004). IVF using spermatozoa from individuals classified as carriers of the AA or GA PRND genotypes resulted in higher cleavage rates than seen using spermatozoa from GG carriers (P ≤ 0.0006). Finally, using semen from rams with the AA PRND genotype resulted in the highest Day 6 and Day 8 embryo rates (P ≤ 0.04). In conclusion, the results of the present study confirm that the identification of different PRND genotypes is important for studying the sperm capacitation process and for improving sperm cryoresistance and embryo production. Furthermore, the detection of Doppel in ejaculated ovine spermatozoa, along with its low expression after cryopreservation, strongly suggests an important physiological function of this protein in male fertility.

Biological roles of glycans

Simple and complex carbohydrates (glycans) have long been known to play major metabolic, structural and physical roles in biological systems. Targeted microbial binding to host glycans has also been studied for decades. But such biological roles can only explain some of the remarkable complexity and organismal diversity of glycans in nature. Reviewing the subject about two decades ago, one could find very few clear-cut instances of glycan-recognition-specific biological roles of glycans that were of intrinsic value to the organism expressing them. In striking contrast there is now a profusion of examples, such that this updated review cannot be comprehensive. Instead, a historical overview is presented, broad principles outlined and a few examples cited, representing diverse types of roles, mediated by various glycan classes, in different evolutionary lineages. What remains unchanged is the fact that while all theories regarding biological roles of glycans are supported by compelling evidence, exceptions to each can be found. In retrospect, this is not surprising. Complex and diverse glycans appear to be ubiquitous to all cells in nature, and essential to all life forms. Thus, >3 billion years of evolution consistently generated organisms that use these molecules for many key biological roles, even while sometimes coopting them for minor functions. In this respect, glycans are no different from other major macromolecular building blocks of life (nucleic acids, proteins and lipids), simply more rapidly evolving and complex. It is time for the diverse functional roles of glycans to be fully incorporated into the mainstream of biological sciences.

Egg envelopes of Armadillidium vulgare (Latreille, 1804) (Crustacea, Isopoda Oniscidea): Ultrastructure and lectins binding

The ultrastructural study carried out on (a) oocytes of Armadillidium vulgare during vitellogenesis, (b) mature eggs taken from the ovaries during the parturial moult of the posterior half of the body, and (c) fertilized eggs collected within a few hours of their release into the brood pouch, has clearly demonstrated that before the fertilization the chorion is the only envelope present in the egg of oniscidean isopods. In the mature eggs, the chorion appears as a uniformly electron-dense lamina, about 0.4-0.5 µm thick, which does not show any specialized area. A second envelope, described by other authors as vitelline envelope, is formed above the oolemma only right after fertilization and appears separated from the chorion by a space full of liquid. The ways in which the genesis of this envelope is realized are not yet clear; it could be interpreted rather as a fertilization membrane. The investigations carried out with the aid of a battery of FITC-lectins have highlighted the presence at the chorion surface of unfertilized eggs of various saccharide residues distributed in uniform way. No significant change was observed in the pattern of lectins binding to the chorion of eggs taken from the brood pouch, thus demonstrating how, after the fertilization, no significant rearrangement in the distribution of saccharide residues present on the egg surface occurs in A. vulgare. The ways in which, therefore, the recognition, the binding and the entry of the peculiar sperm of oniscidean isopods into the egg occur, still remain all to be deciphered. Microsc. Res. Tech. 79:792-798, 2016. © 2016 Wiley Periodicals, Inc.

Redistribution of the intra-acrosomal EGFP before acrosomal exocytosis in mouse spermatozoa

Mammalian spermatozoa must undergo complex physiological and morphological alterations within the female reproductive tract before they become fertilization competent. Two important alterations are capacitation and the acrosome reaction (AR), by which spermatozoa become capable of penetrating the zona pellucida (ZP) of the oocyte. Although various biochemical stimulants have been reported to induce the AR, the true physiological inducer in vivo remains to be identified. Previously, it has been reported that most fertilizing spermatozoa undergo the AR before contacting the ZP and that only a small fraction of in vitro-capacitated spermatozoa can penetrate the ZP. Therefore, it is important to identify which capacitating spermatozoa undergo the AR in response to potential AR inducers such as progesterone. Here we show that spermatozoa undergo a dynamic rearrangement of the acrosome during in vitro capacitation. This involves the rapid movement of an artificially introduced soluble component of the acrosome, enhanced green fluorescent protein (EGFP), from the acrosomal cap region to the equatorial segment (EQ) of the sperm head. Spermatozoa exhibiting the EQ pattern were more sensitive to progesterone than were those without it. We suggest that spermatozoa that are ready to undergo acrosomal exocytosis can be detected by real-time EGFP imaging. This offers a promising new method for identifying where spermatozoa undergo the AR in the female reproductive tract in vivo.

Lipocalin 2 binds to membrane phosphatidylethanolamine to induce lipid raft movement in a PKA-dependent manner and modulates sperm maturation

Mammalian sperm undergo multiple maturation steps after leaving the testis in order to become competent for fertilization, but the molecular mechanisms underlying this process remain unclear. In terms of identifying factors crucial for these processes in vivo, we found that lipocalin 2 (Lcn2), which is known as an innate immune factor inhibiting bacterial and malarial growth, can modulate sperm maturation. Most sperm that migrated to the oviduct of wild-type females underwent lipid raft reorganization and glycosylphosphatidylinositol-anchored protein shedding, which are signatures of sperm maturation, but few did so in Lcn2 null mice. Furthermore, we found that LCN2 binds to membrane phosphatidylethanolamine to reinforce lipid raft reorganization via a PKA-dependent mechanism and promotes sperm to acquire fertility by facilitating cholesterol efflux. These observations imply that mammals possess a mode for sperm maturation in addition to the albumin-mediated pathway.

The identity of zona pellucida receptor on spermatozoa: an unresolved issue in developmental biology

Mammalian oocytes are surrounded by an acellular zona pellucida (ZP). Fertilization begins when a capacitated spermatozoon binds to the ZP. Defective sperm-ZP interaction is a cause of male infertility and reduced fertilization rates in clinical assisted reproduction treatment. Despite the importance of spermatozoa-ZP binding, the mechanisms and regulation of the interaction are unclear partly due to the failure in the identification of ZP receptor on spermatozoa. Most of the previous studies assumed that the sperm ZP receptor is a single molecular species, and a number of potential candidates had been suggested. Yet none of them can be considered as the sole sperm ZP receptor. Accumulated evidence suggested that the sperm ZP receptor is a dynamic multi-molecular structure requiring coordinated action of different proteins that are assembled into a functional complex during post-testicular maturation and capacitation. The complex components may include carbohydrate-binding, protein-binding and acrosomal matrix proteins which work as a suite to mediate spermatozoa-ZP interaction. This article aims to review the latest insights in the identification of the sperm ZP receptor. Continued investigation of the area will provide considerable understanding of the regulation of fertilization that will be useful for practical application in human contraception and reproductive medicine.

Posttranslational modifications of zona pellucida proteins

The zona pellucida (ZP), which surrounds the mammalian oocyte, functions in various aspects of fertilization. The ZP consists of three or four glycoproteins, which are derived from transmembrane proteins that lack the ability to self-assemble. Following posttranslational processing at specific sites, ectodomains of ZP precursor proteins are released from the membrane and begin to form a matrix. Glycosylational modification is thought to be involved in species-selective sperm recognition by ZP proteins. However, in mice, the supramolecular structure of the zona matrix is also important in sperm recognition. One ZP protein, ZP2, is processed at a specific site upon fertilization by ovastacin, which is released from cortical granules inside the oocyte. This phenomenon is involved in the block to polyspermy. The proteolysis of ubiquitinated ZP proteins by a sperm-associated proteasome is involved in penetration of the zona matrix by sperm, at least in the pigs. Thus, the posttranslational modification of ZP proteins is closely tied to ZP formation and the regulation of sperm-oocyte interactions.

Proteomic analysis of seminal fluid from men exhibiting oxidative stress

BACKGROUND

Seminal plasma serves as a natural reservoir of antioxidants. It helps to remove excessive formation of reactive oxygen species (ROS) and consequently, reduce oxidative stress. Proteomic profiling of seminal plasma proteins is important to understand the molecular mechanisms underlying oxidative stress and sperm dysfunction in infertile men.

METHODS

This prospective study consisted of 52 subjects: 32 infertile men and 20 healthy donors. Once semen and oxidative stress parameters were assessed (ROS, antioxidant concentration and DNA damage), the subjects were categorized into ROS positive (ROS+) or ROS negative (ROS-). Seminal plasma from each group was pooled and subjected to proteomics analysis. In-solution digestion and protein identification with liquid chromatography tandem mass spectrometry (LC-MS/MS), followed by bioinformatics analyses was used to identify and characterize potential biomarker proteins.

RESULTS

A total of 14 proteins were identified in this analysis with 7 of these common and unique proteins were identified in both the ROS+ and ROS- groups through MASCOT and SEQUEST analyses, respectively. Prolactin-induced protein was found to be more abundantly present in men with increased levels of ROS. Gene ontology annotations showed extracellular distribution of proteins with a major role in antioxidative activity and regulatory processes.

CONCLUSIONS

We have identified proteins that help protect against oxidative stress and are uniquely present in the seminal plasma of the ROS- men. Men exhibiting high levels of ROS in their seminal ejaculate are likely to exhibit proteins that are either downregulated or oxidatively modified, and these could potentially contribute to male infertility.

Glycobiology of reproductive processes in marine animals: the state of the art

Glycobiology is the study of complex carbohydrates in biological systems and represents a developing field of science that has made huge advances in the last half century. In fact, it combines all branches of biomedical research, revealing the vast and diverse forms of carbohydrate structures that exist in nature. Advances in structure determination have enabled scientists to study the function of complex carbohydrates in more depth and to determine the role that they play in a wide range of biological processes. Glycobiology research in marine systems has primarily focused on reproduction, in particular for what concern the chemical communication between the gametes. The current status of marine glycobiology is primarily descriptive, devoted to characterizing marine glycoconjugates with potential biomedical and biotechnological applications. In this review, we describe the current status of the glycobiology in the reproductive processes from gametogenesis to fertilization and embryo development of marine animals.

Identification and purification of an aspartic proteinase from human semen

To purify and evaluate the molecular changes associated with an aspartic protease (Cathepsin D) in human semen from infertile subjects. Cathepsin D was purified from normo-, oligo- and azoospermic semen, by a procedure involving detergent solubilisation, affinity chromatography and gel filtration chromatography. The enzyme from normo-, oligo- and azoospermic samples was purified 86, 60 and 44 fold respectively. The purified enzyme appeared as a single band on SDS as well as on native PAGE irrespective of the pathological conditions. The molecular weight of Cathepsin D from oligospermic and normospermic samples was 40 kDa while that of azoospermic sample was found to be 43 kDa. The enzyme was inhibited by pepstatin while other proteinase inhibitors and metal ions did not have any effect. Purified Cathepsin D from azoospermic sample differs from normospermia and oligospermia.

The biology and dynamics of mammalian cortical granules

Cortical granules are membrane bound organelles located in the cortex of unfertilized oocytes. Following fertilization, cortical granules undergo exocytosis to release their contents into the perivitelline space. This secretory process, which is calcium dependent and SNARE protein-mediated pathway, is known as the cortical reaction. After exocytosis, the released cortical granule proteins are responsible for blocking polyspermy by modifying the oocytes' extracellular matrices, such as the zona pellucida in mammals. Mammalian cortical granules range in size from 0.2 um to 0.6 um in diameter and different from most other regulatory secretory organelles in that they are not renewed once released. These granules are only synthesized in female germ cells and transform an egg upon sperm entry; therefore, this unique cellular structure has inherent interest for our understanding of the biology of fertilization. Cortical granules are long thought to be static and awaiting in the cortex of unfertilized oocytes to be stimulated undergoing exocytosis upon gamete fusion. Not till recently, the dynamic nature of cortical granules is appreciated and understood. The latest studies of mammalian cortical granules document that this organelle is not only biochemically heterogeneous, but also displays complex distribution during oocyte development. Interestingly, some cortical granules undergo exocytosis prior to fertilization; and a number of granule components function beyond the time of fertilization in regulating embryonic cleavage and preimplantation development, demonstrating their functional significance in fertilization as well as early embryonic development. The following review will present studies that investigate the biology of cortical granules and will also discuss new findings that uncover the dynamic aspect of this organelle in mammals.

Mouse sperm undergo GPI-anchored protein release associated with lipid raft reorganization and acrosome reaction to acquire fertility

Mammalian sperm undergo several maturation steps after leaving the testis to become competent for fertilization. Important changes occur in sperm within the female reproductive tract, although the molecular mechanisms underlying these processes remain unclear. To investigate sperm membrane remodeling upon sperm maturation, we developed transgenic mouse lines carrying glycosylphosphatidylinositol (GPI)-anchored enhanced green fluorescent protein (EGFP–GPI) and traced the fate of this fluorescent protein during the fertility-acquiring process in sperm in vitro and in vivo. When the GFP-labeled sperm were treated with compounds for promoting the acrosome reaction, EGFP–GPI was released from the sperm surface crosslinked with characteristic relocation of a lipid raft marker ganglioside GM1. Sperm ejaculated into the uterus strongly expressed EGFP–GPI in the head region, whereas a part of the oviductal sperm lost fluorescence in a manner that was dependent on the presence of angiotensin-converting enzyme (ACE). Moreover, sperm on the zona pellucida of eggs in the oviduct were all found to have low levels of GFP. These results suggest that sperm undergoing GPI-anchored protein release associated with reorganization of lipid rafts and the acrosome reaction acquire fertilization potential.

Evolution of 2 reproductive proteins, ZP3 and PKDREJ, in cetaceans

The rapid evolution of proteins involved in reproduction has been documented in several animal taxa. This is thought to be the result of forces involved in sexual selection and is expected to be particularly strong in promiscuous mating systems. In this study, a range of cetacean species were used to analyze the patterns of evolution in 2 reproductive proteins involved in fertilization: the zona pellucida 3 (ZP3), present in the egg coat, and PKDREJ, localized in the sperm head. We targeted exons 6 and 7 of ZP3 and a part of the REJ domain in PKDREJ for a total of 958 bp in 18 species. We found very low levels of amino acid sequence divergence in both proteins, a very weak signal of positive selection in ZP3 and no signal in PKDREJ. These results were consistent with previous reports of a slow rate of molecular evolution in cetaceans but unexpected due to the existence of promiscuous mating systems in these species. The results raise questions about the evolution of reproductive isolation and species recognition in whales and dolphins.

Syntaxin and VAMP association with lipid rafts depends on cholesterol depletion in capacitating sperm cells

Sperm cells represent a special exocytotic system since mature sperm cells contain only one large secretory vesicle, the acrosome, which fuses with the overlying plasma membrane during the fertilization process. Acrosomal exocytosis is believed to be regulated by activation of SNARE proteins. In this paper, we identified specific members of the SNARE protein family, i.e., the t-SNAREs syntaxin1 and 2, and the v-SNARE VAMP, present in boar sperm cells. Both syntaxins were predominantly found in the plasma membrane whereas v-SNAREs are mainly located in the outer acrosomal membrane of these cells. Under non-capacitating conditions both syntaxins and VAMP are scattered in well-defined punctate structures over the entire sperm head. Bicarbonate-induced in vitro activation in the presence of BSA causes a relocalization of these SNAREs to a more homogeneous distribution restricted to the apical ridge area of the sperm head, exactly matching the site of sperm zona binding and subsequent induced acrosomal exocytosis. This redistribution of syntaxin and VAMP depends on cholesterol depletion and closely resembles the previously reported redistribution of lipid raft marker proteins. Detergent-resistant membrane isolation and subsequent analysis shows that a significant proportion of syntaxin emerges in the detergent-resistant membrane (raft) fraction under such conditions, which is not the case under those conditions where cholesterol depletion is blocked. The v-SNARE VAMP displays a similar cholesterol depletion-dependent lateral and raft redistribution. Taken together, our results indicate that redistribution of syntaxin and VAMP during capacitation depends on association of these SNAREs with lipid rafts and that such a SNARE-raft association may be essential for spatial control of exocytosis and/or regulation of SNARE functioning.

Carbohydrate affinity chromatography indicates that arylsulfatase-A from capacitated boar sperm has mannose and N-acetylglucosamine/sialic acid residues

Carbohydrate residues on membrane proteins from sperm are important in gamete interaction. In recent years, Arylsulfatase A (AS-A) has been acquiring an important role from the various putative gamete interaction responsibles in sperm. The aim of this study was to determine if the capacitated boar sperm Arylsulfatase-A (AS-A), contains D-mannose, N-acetylglucosamine and/or sialic acid residues by its purification using affinity chromatography with Concanavalia ensiformis Agglutinin(Con-A) or Wheat Germ Agglutinin (WGA) as ligands. Sperm samples were capacitated in TALP-HEPES medium. Protein extract was added to the affinity columns. Sequencing of retained proteins was done after SDS-PAGE. Total capacitated sperm proteins electrophoresis showed molecular masses between 14 kDa and 102 kDa. A major band of 68 kDa, and 2 minor bands of 52 kDa and 47 kDa were observed. They were AS-A, hyaluronidase and lactadherin, respectively. The Con-A-retained proteins (RP) pattern showed bands from 14 to 98 kDa. After sequencing and BLAST analysis, the 62 kDa band corresponded to Arylsulfatase-A. The WGA RP fraction showed bands from 14 to 100 kDa. The 65 kDa band corresponded to AS-A. This study showed that AS-A has mannose, N-acetylglucosamine and/or sialic acid residues as part of its glycosilation. In this study AS-A was isolated from boar capacitated sperm by affinity chromatography using separately Con-A and WGA, indicating that there are mannose, N-acetylglucosamine and/or sialic acid residues in its glycosilation. AS-A is a membrane protein of capacitated sperm. Further investigation is needed to fully characterize the glycosidic residues bore by AS-A and to determine its function.

Adaptive evolution of fertilization proteins within a genus: variation in ZP2 and ZP3 in deer mice (Peromyscus)

Rapid evolution of reproductive proteins has been documented in a wide variety of taxa. In internally fertilized species, knowledge about the evolutionary dynamics of these proteins between closely related taxa is primarily limited to accessory gland proteins in the semen of Drosophila. Investigation of additional taxa and functional classes of proteins is necessary in order to determine if there is a general pattern of adaptive evolution of reproductive proteins between recently diverged species. We performed an evolutionary analysis of 2 egg coat proteins, ZP2 and ZP3, in 15 species of deer mice (genus Peromyscus). Both of these proteins are involved in egg-sperm binding, a critical step in maintaining species-specific fertilization. Here, we show that Zp2 and Zp3 gene trees are not consistent with trees based on nonreproductive genes, Mc1r and Lcat, where species formed monophyletic clades. In fact, for both of the reproductive genes, intraspecific amino acid variation was extensive and alleles were sometimes shared across species. We document positive selection acting on ZP2 and ZP3 and identify specific amino acid sites that are likely targets of selection using both maximum likelihood approaches and patterns of parallel amino acid change. In ZP3, positively selected sites are clustered in and around the region implicated in sperm binding in Mus, suggesting changes may impact egg-sperm binding and fertilization potential. Finally, we identify lineages with significantly elevated rates of amino acid substitution using a Bayesian mapping approach. These findings demonstrate that the pattern of adaptive reproductive protein evolution found at higher taxonomic levels can be documented between closely related mammalian species, where reproductive isolation has evolved recently.

Participation of the nonreducing terminal beta-galactosyl residues of the neutral N-linked carbohydrate chains of porcine zona pellucida glycoproteins in sperm-egg binding

The zona pellucida (ZP) surrounding the mammalian oocyte is composed of three glycoprotein components (ZPA, ZPB, and ZPC). Mammalian sperm bind to carbohydrate chains of a ZP glycoprotein in the initial phase of fertilization. Sperm-ligand carbohydrate chains have been characterized in mouse, cow, and pig. In pigs, triantennary/tetraantennary neutral complex-type chains from ZPB/ZPC mixture possess stronger sperm-binding activity than those of biantennary chains (Kudo et al., 1998: Eur J Biochem 252:492-499). Most of these oligosaccharides have beta-galactosyl residues at the nonreducing ends. This study used two in vitro competition assays to investigate the participation of the nonreducing terminal beta-galactosyl residues of the ligand active chains in porcine sperm binding. The removal of the nonreducing terminal beta-galactosyl residues from either the ligand active carbohydrate chains or endo-beta-galactosidase-digested glycoproteins significantly reduced their inhibition of sperm-egg binding, indicating that the beta-galactosyl residues at the nonreducing ends are involved in porcine sperm-egg binding. A correlation between the sperm-binding activity and in vitro fertilization rate is also presented.

Calcium Channels and Ca2+ Fluctuations in Sperm Physiology

Generating new life in animals by sexual reproduction depends on adequate communication between mature and competent male and female gametes. Ion channels are instrumental in the dialogue between sperm, its environment, and the egg. The ability of sperm to swim to the egg and fertilize it is modulated by ion permeability changes induced by environmental cues and components of the egg outer layer. Ca(2+) is probably the key messenger in this information exchange. It is therefore not surprising that different Ca(2+)-permeable channels are distinctly localized in these tiny specialized cells. New approaches to measure sperm currents, intracellular Ca(2+), membrane potential, and intracellular pH with fluorescent probes, patch-clamp recordings, sequence information, and heterologous expression are revealing how sperm channels participate in fertilization. Certain sperm ion channels are turning out to be unique, making them attractive targets for contraception and for the discovery of novel signaling complexes.

Zonadhesin assembly into the hamster sperm acrosomal matrix occurs by distinct targeting strategies during spermiogenesis and maturation in the epididymis

Zonadhesin is the only sperm protein known to bind in a species-specific manner to the zona pellucida. The zonadhesin precursor is a mosaic protein with a predicted transmembrane segment and large extracellular region composed of cell adhesion, mucin, and tandem von Willebrand D domains. Because the precursor possesses a predicted transmembrane segment and localizes to the anterior head, the mature protein was presumed to be a sperm surface zona pellucida-binding protein. In this study of hamster spermatozoa, we demonstrate that zonadhesin does not localize to the sperm surface but is instead a constituent of the acrosomal matrix. Immunoelectron microscopy revealed that distinct targeting pathways during spermiogenesis and sperm maturation in the epididymis result in trafficking of zonadhesin to the acrosomal matrix. In round spermatids, zonadhesin localized specifically to the acrosomal membrane, where it appeared to be evenly distributed between the outer and inner membrane domains. Subsequent redistribution of zonadhesin resulted in its elimination from the inner acrosomal membrane and restriction to the outer acrosomal membrane of the apical and principal segments and the contents of the posterior acrosome. During sperm maturation in the epididymis, zonadhesin dissociated from the outer acrosomal membrane and became incorporated into the forming acrosomal matrix. These data suggest an important structural role for zonadhesin in assembly of the acrosomal matrix and further support the view that the species specificity of zona pellucida adhesion is mediated by egg-binding proteins contained within the acrosome rather than on the periacrosomal plasma membrane.

Towards a more precise assay of sperm function in egg binding

Historically, the treatment of severe male factor infertility has relied on donor sperm insemination. A decade ago the option of treating severe male factor infertility with partner sperm became a viable alternative. With the introduction of intracytoplasmic sperm injection (ICSI) in conjunction with in vitro fertilization (IVF), only men who produce no sperm are denied the option of fathering their own children. The use of ICSI has been extended to couples with mild male factors. Despite the known genetic risks (both inherent and de novo) of ICSI to offspring, couples with male factors as part of their infertility problem often prefer ICSI to standard IVF, due to apprehension that their sperm might not otherwise succeed in fertilization. This apprehension would be alleviated if an assay for the egg binding capability of human sperm were available. We examine here the possibility that recombinant human zona pellucida 3 (rec hZP3), the primary sperm receptor sulfoglycoprotein of the egg zona pellucida (ZP), be used as a human ZP surrogate for assessing sperm ability to bind to the ZP. Unlike human eggs, which cannot be obtained for this purpose, rec hZP3 can be produced in quantity. An efficient assay can be established by incubating sperm with rec hZP3 coated to a microwell plate. Infertile men with sperm having ability to bind to rec hZP3 can be advised to select standard IVF or intrauterine insemination, which have fewer genetic and medical risks.

Alterations of branching and differential expression of sialic acid on alpha-1-acid glycoprotein in human seminal plasma

BACKGROUND

The degree of branching and types of fucosylation of glycans on alpha(1)-acid glycoprotein (AGP) have been found to be associated with alpha(1)-acid glycoprotein concentrations in human seminal plasma. The glycosylation pattern of alpha(1)-acid glycoprotein in seminal plasma obtained from men living in infertile couples can undergo alterations in relation to sperm analysis and/or alpha(1)-acid glycoprotein concentrations.

METHODS

The glycosylation of alpha(1)-acid glycoprotein was studied upon the reactivity with specific lectins by crossed affinity immunoelectrophoresis (concanavalin A), and by glycoprotein lectin immunosorbent assay (Maackia amurensis and Sambucus nigra lectins), as well as high pH anion-exchange chromatography with pulsed amperometric detection.

RESULTS

Nonsignificant differences in alpha(1)-acid glycoprotein glycan branching and degree of its sialylation were observed among the AGP derived from seminal plasmas in relation to spermiogram and sperm morphology. However, significant concentration-dependent differences were found in extent of branching and type of sialylation.

CONCLUSIONS

The presence in seminal plasma of high concentrations of aberrantly glycosylated AGP molecules might be indicative for a chronic inflammatory condition in the reproductive tract, and can be used as additional tool to subdivide the seminal plasmas of men living in infertile couples.

Purification and characterization of plasma membrane-associated human sperm alpha-L-fucosidase

Detergent and salt extraction studies, as well as cytochemical localization with fluorescein isothiocyanate-bovine serum albumin-L-fucose, have provided further evidence for the plasma membrane association of a novel human sperm, alpha-L-fucosidase. This alpha-L-fucosidase has been solubilized and purified 8600-fold to high specific activity (35 000 U/mg protein) by affinity chromatography on agarose-C(24)-fucosylamine. To our knowledge, this is the first report concerning the purification and characterization of a mammalian plasma membrane-associated alpha-L-fucosidase. Both SDS-PAGE and Western blot analysis indicated the alpha-L-fucosidase is highly purified and contains a single subunit with a molecular mass of 51 kDa. N-glycanase studies indicated the subunit contains N-glycans, and lectin blot analysis detected the presence of mannose, but no terminal galactose or sialic acid residues. Isoelectric focusing indicated the presence of two major alpha-L-fucosidase isoforms (pIs 6.5 and 6.7) and a possible minor isoform (pI 6.3). Treatment of alpha-L-fucosidase with neuraminidase did not change its isoform profile, providing further evidence for the enzyme's lack of sialic acid residues. Kinetic analysis with 4-methylumbelliferyl alpha-L-fucopyranoside indicated that sperm alpha-L-fucosidase has a pH optimum near 7, an apparent K(m) of 0.08 mM, and a V(max) of 6.8 micro mol/min/mg protein. The unusual properties of human sperm alpha-L-fucosidase argue in support of a potentially important, but presently unknown, role for this enzyme in human reproduction.

Immunoglobulins against gp273, the ligand for sperm-egg interaction in the mollusc bivalve Unio elongatulus, are directed against charged O-linked oligosaccharide chains bearing a Lewis-like structure and interact with epitopes of the human zona pellucida

In oocytes of the mollusc bivalve Unio elongatulus, gp273 is the ligand molecule for sperm-egg interaction and binding is mediated by its O-glycans. A serum raised against this protein enabled its localization in the crater region, the area of the vitelline coat where sperm recognition occurs, and showed that after cyanogen bromide fragmentation, the anti-gp273 epitope(s) was retained by a peptide where the O-glycans are localized. In this article, we utilized purified anti-gp273 immunoglobulins to characterize the corresponding epitope by: (i) immunoblotting analysis of the protein after removal of O- and N-glycans; (ii) solid phase binding analysis of anti-gp273 IgG to gp273 N- and O-glycans; and (iii) binding analysis of the same antibody to commercially available oligosaccharides. The results showed that the epitope consists of O-glycans and contains a Lewis-like structure with fucose as determinant. Anti-gp273 IgG were then used to investigate human zona pellucida by immunoelectronmicroscopy and immunoblotting. Epitopes recognized by the antibody were demonstrated on the outer surface of the zona pellucida and shown to belong to a zona pellucida protein having electrophoretic mobility similar to human ZP3. Since human sperm specifically bind to gp273, and anti-gp273 interferes with this binding a functional role for these epitopes is suggested.

Absence of the prion protein homologue Doppel causes male sterility

The agent that causes prion diseases is thought to be identical with PrP(Sc), a conformer of the normal prion protein PrP(C). PrP(C)-deficient mice do not exhibit major pathologies, perhaps because they express a protein termed Dpl, which shares significant biochemical and structural homology with PrP(C). To investigate the physiological function of Dpl, we generated mice harbouring a homozygous disruption of the Prnd gene that encodes Dpl. Dpl deficiency did not interfere with embryonic and postnatal development, but resulted in male sterility. Dpl protein was expressed at late stages of spermiogenesis, and spermatids of Dpl mutants were reduced in numbers, immobile, malformed and unable to fertilize oocytes in vitro. Mechanical dissection of the zona pellucida partially restored in vitro fertilization. We conclude that Dpl regulates male fertility by controlling several aspects of male gametogenesis and sperm-egg interaction.

Arylsulfatase a is present on the pig sperm surface and is involved in sperm-zona pellucida binding

We have previously described the affinity of a pig sperm surface protein, P68, to mammalian zonae pellucidae (ZP). In this report, we identified P68 as arylsulfatase A (AS-A) based on the presence of P68 tryptic peptide sequences in the pig testis AS-A cDNA sequence. Our objective was to demonstrate the presence of AS-A on the sperm surface and to elucidate its role in ZP binding. Immunogold electron microscopy revealed the presence of AS-A on the sperm surface. Furthermore, live pig sperm and the extract of peripheral sperm plasma membrane proteins exhibited AS-A's desulfation activity. Significantly, the role of pig sperm surface AS-A in ZP binding was demonstrated by dose-dependent decreases of sperm-ZP binding upon sperm pretreatment with anti-AS-A IgG/Fab, and by the binding of Alexa-430-conjugated sperm surface AS-A to homologous ZP. ZP pretreatment with anti-pig-ZP3 antibody abolished AS-A binding, suggesting that ZP3, recognized as the pig sperm receptor, was AS-A's binding ligand. This was further confirmed by the ability of exogenous ZP3 to competitively inhibit AS-A-ZP binding. Similarly, purified ZP3alpha, a major sperm receptor component of ZP3, exhibited great inhibitory effect on AS-A-ZP binding. All of these results designated a new function of AS-A in gamete interaction.

Role of sperm surface arylsulfatase A in mouse sperm-zona pellucida binding

We have previously described the zonae pellucidae (ZP) binding ability of a pig sperm surface protein, P68. Our recent results on peptide sequencing of 3 P68 tryptic peptides and molecular cloning of pig testis arylsulfatase A (AS-A) revealed the identity of P68 as AS-A. In this report, we demonstrate the presence of AS-A on the mouse sperm surface and its role in ZP binding. Using anti-AS-A antibody, we have shown by immunoblotting that AS-A was present in a Triton X-100 extract of mouse sperm. The presence of AS-A on the sperm plasma membrane was conclusively demonstrated by indirect immunofluorescence, immunogold electron microscopy, and AS-A's desulfation activity on live mouse sperm. The AS-A remained on the head surface of in vivo capacitated sperm, as revealed by positive immunofluorescent staining of oviductal/uterine sperm. Significantly, the role of mouse sperm surface AS-A on ZP binding was demonstrated by dose-dependent decreases of sperm-ZP binding on sperm pretreatment with anti-AS-A IgG/Fab. Furthermore, Alexa-430 conjugated AS-A bound to mouse ZP of unfertilized eggs but not to fertilized ones, and this level of binding increased and approached saturation with increasing Alexa-430 AS-A concentrations. Moreover, in vivo fertilization was markedly decreased when mouse sperm pretreated with anti-AS-A IgG were artificially inseminated into females. All of these results designated a new function for AS-A in mouse gamete interaction.

Sulfogalactosylglycerolipid is involved in human gamete interaction

Recent results from our laboratory have revealed the role of sulfogalactosylglycerolipid (SGG) in mouse sperm-zona pellucida (ZP) binding. In this report, we demonstrated the presence of SGG in Percoll-gradient centrifuged (PGC) human sperm by high performance thin layer chromatography with orcinol and Azure A staining, specific for glycolipids and sulfolipids, respectively. SGG in human PGC sperm was quantified by its affinity to Azure A to be 12-15 mol% of sperm lipids. Indirect immunofluorescence revealed that SGG existed on both live and aldehyde fixed human sperm in the head region. Pretreatment of human PGC sperm with affinity purified antiSGG Fab markedly inhibited sperm binding to the ZP in a concentration dependent manner, without any changes in the spontaneous acrosome rate or sperm motility parameters. Fluorescently labeled SGG liposomes also bound uniformly to isolated human ZP, while fluorescently labeled galactosylglycerolipid (GG, SGG's parental lipid) or phosphatidylserine (PS, negatively charged like SGG) liposomes did not. All of these results suggested the role of human sperm SGG in ZP binding.

Structural characterization of the N-glycans of gp273, the ligand for sperm-egg interaction in the mollusc bivalve Unio elongatulus

Gp273, a glycoprotein of the egg extracellular coats of the mollusc bivalve Unio elongatulus, is the ligand molecule for sperm-egg interaction during fertilization. In this study we have analyzed the N-glycans from gp273. N-glycans were enzymatically released by PNGase F digestion and their structures were elucidated by normal phase HPLC profiling of the 2-aminobenzamide-labeled N-glycans, MALDI-TOF mass spectrometry and 1H NMR spectroscopy. The combined data revealed that the N-glycans of gp273 consist of Glc1Man9GlcNAc2 and Man9GlcNAc2. In Unio, the presence of noncomplex-type N-glycans parallels the inefficacy of these glycans in the ligand function. Their role in the protection of the polypeptide chain from proteolytic attack is suggested by the electrophoretic patterns obtained after enzymatic digestion of the native and the N-deglycosylated protein. These results are discussed in the light of the evolution of the recognition and adhesion properties of oligosaccharide chains in the fertilization process.

The species recognition system: a new corollary for the human fetoembryonic defense system hypothesis

We have previously suggested that the human fetus is protected during human development by a system of both soluble and cell surface associated glycoconjugates that utilize their carbohydrate sequences as functional groups to enable them to evoke tolerance. The proposed model has been referred to as the human fetoembryonic defense system hypothesis (hu-FEDS). In this paradigm, it has previously been proposed that similar oligosaccharides are used to mediate crucial recognition events required during both human sperm-egg binding and immune-inflammatory cell interactions. This vertical integration suggested to us that the sperm-egg binding itself is related to universal recognition events that occur between immune and inflammatory cells, except that in this case recognition of 'species' rather than recognition of 'self' is being manifested. In this paper, we have designated this component of hu-FEDS as the species recognition system (SRS). We propose that the SRS is an integral component of the hu-FEDS used to enable sperm-egg recognition and protection of the gametes from potential immune responses. Recent structural data indicates that the glycan sequences implicated in mediating murine gamete recognition are also expressed on CD45 in activated murine T lymphocytes and cytotoxic T lymphocytes. This overlap supports our contention that there is an overlap between the immune and gamete recognition systems. Therefore the hu-FEDS paradigm may be a subset of a larger model that also applies to other placental mammals. We therefore propose that the hu-FEDS model for protection should in the future be referred to as the eutherian fetoembryonic defense system hypothesis (eu-FEDS) to account for this extension. The possibility exists that the SRS component of eu-FEDS could predate eutherians and extend to all sexually reproducing organisms. Future investigation of the interactions between the immune and gamete recognition system will be required to determine the degree of overlap.

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.

New insights into the origin, structure and role of CD52: a major component of the mammalian sperm glycocalyx

The sperm glycocalyx represents the primary interface between the male gamete and its environment, and gamete interaction inevitably involves interaction with this structure. Thus, it has potential significance as a target for antibodies that inhibit sperm function. Still, little is known about the components and biological role of the sperm glycocalyx. Despite the apparent complexity of the sperm membrane, surface carbohydrate labelling experiments show a high selectivity suggesting that carbohydrate side chains of CD52, an unusually short, bipolar glycopeptide of epididymal origin, form major components of the sperm glycocalyx in all mammalian species investigated. Acquisition of the highly sialylated, lipid-anchored CD52 antigen is one of the few well-defined modifications that occur to the sperm membrane during epididymal passage. It would explain changes in lectin-binding patterns and also the remarkable surface charge differences occurring during epididymal transit, most probably attributable to its terminal sialic acid residues. CD52 seems to be immunodominant on human spermatozoa, and antibodies directed against it can agglutinate and completely immobilize human sperm in the presence of complement. Expression of the same peptide backbone in lymphocytes had largely discounted its consideration as a candidate for contraceptive development. However, the recent proof of male-specific modifications indicates the feasibility of this approach.

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.

Sperm adhesion molecules: structure and function

Fertilisation is a unique event in which the morphologically disparate gametes recognise, bind and fuse with each other. This event follows a highly regulated schedule of biochemical interactions, in which molecules are involved that mediate cell adhesion, signal transduction and the initiation of metabolic pathways. A plethora of molecules has been found on the male gamete and with regard to the different protein structures it is almost impossible to overlook the structures involved. Even more, carbohydrate structures cause an additional diversity with regard to the generation of surface structures. In this communication we try to elucidate the structures of proteins that have been known so far. We have focussed on spermadhesins, the zonadhesin, proacrosin and the PH-20 antigen. The variety of structures and also the common features among them as well as the presence of redundant systems are attributable to the evolutionary force of intraspecific sperm competition. This evolutionary force is assumed to be also responsible for the species selectivity observed in these adhesion molecules, which explains the preferential binding of gametes in a homologous system.

Carbohydrate-mediated sperm-egg interaction and species specificity: a clue from the Unio elongatulus model

As a first step fertilization requires that sperm bind to the extracellular coat of the egg. It is generally accepted that binding is mediated by complex carbohydrates on the egg coat, recognized by carbohydrate-binding proteins on the sperm surface. In the mollusc bivalve Unio elongatulus, the main functional epitope of the carbohydrate ligand has been determined, and it shares several characteristics with other sugar structures involved in the fertilization process in different animal models. A polyclonal antibody against the Unio epitope reacts with the human ZP3 glycoprotein and the Unio ligand binds human spermatozoa in an in vitro assay. These findings are discussed in the light of the species specificity of sperm-egg binding at fertilization.

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.