Characterization of a novel ZP3-independent sperm-binding ligand that facilitates sperm adhesion to the egg coat

Sex and tissue specific gene expression patterns identified following de novo transcriptomic analysis of the Norway lobster, Nephrops norvegicus

BACKGROUND

The Norway lobster, Nephrops norvegicus, is economically important in European fisheries and is a key organism in local marine ecosystems. Despite multi-faceted scientific interest in this species, our current knowledge of genetic resources in this species remains very limited. Here, we generated a reference de novo transcriptome for N. norvegicus from multiple tissues in both sexes. Bioinformatic analyses were conducted to detect transcripts that were expressed exclusively in either males or females. Patterns were validated via RT-PCR.

RESULTS

Sixteen N. norvegicus libraries were sequenced from immature and mature ovary, testis and vas deferens (including the masculinizing androgenic gland). In addition, eyestalk, brain, thoracic ganglia and hepatopancreas tissues were screened in males and both immature and mature females. RNA-Sequencing resulted in >600 million reads. De novo assembly that combined the current dataset with two previously published libraries from eyestalk tissue, yielded a reference transcriptome of 333,225 transcripts with an average size of 708 base pairs (bp), with an N50 of 1272 bp. Sex-specific transcripts were detected primarily in gonads followed by hepatopancreas, brain, thoracic ganglia, and eyestalk, respectively. Candidate transcripts that were expressed exclusively either in males or females were highlighted and the 10 most abundant ones were validated via RT-PCR. Among the most highly expressed genes were Serine threonine protein kinase in testis and Vitellogenin in female hepatopancreas. These results align closely with gene annotation results. Moreover, a differential expression heatmap showed that the majority of differentially expressed transcripts were identified in gonad and eyestalk tissues. Results indicate that sex-specific gene expression patterns in Norway lobster are controlled by differences in gene regulation pattern between males and females in somatic tissues.

CONCLUSIONS

The current study presents the first multi-tissue reference transcriptome for the Norway lobster that can be applied to future biological, wild restocking and fisheries studies. Sex-specific markers were mainly expressed in males implying that males may experience stronger selection than females. It is apparent that differential expression is due to sex-specific gene regulatory pathways that are present in somatic tissues and not from effects of genes located on heterogametic sex chromosomes. The N. norvegicus data provide a foundation for future gene-based reproductive studies.

The chaperonin containing TCP1 complex (CCT/TRiC) is involved in mediating sperm-oocyte interaction

Sperm-oocyte interactions are among the most remarkable processes in cell biology. These cellular recognition events are initiated by an exquisitely specific adhesion of free-swimming spermatozoa to the zona pellucida, an acellular matrix that surrounds the ovulated oocyte. Decades of research focusing on this interaction have led to the establishment of a widely held paradigm that the zona pellucida receptor is a single molecular entity that is constitutively expressed on the sperm cell surface. In contrast, we have employed the techniques of blue native-polyacrylamide gel electrophoresis, far Western blotting, and proximity ligation to secure the first direct evidence in support of a novel hypothesis that zona binding is mediated by multimeric sperm receptor complex(es). Furthermore, we show that one such multimeric association, comprising the chaperonin-containing TCP1 complex (CCT/TRiC) and a zona-binding protein, zona pellucida-binding protein 2, is present on the surface of capacitated spermatozoa and could account for the zona binding activity of these cells. Collectively, these data provide an important biochemical insight into the molecular basis of sperm-zona pellucida interaction and a plausible explanation for how spermatozoa gain their ability to fertilize.

Cellular mechanisms regulating sperm-zona pellucida interaction

For mammalian spermatozoa to exhibit the ability to bind the zona pellucida (ZP) they must undergo three distinct phases of maturation, namely, spermatogenesis (testis), epididymal maturation (epididymis) and capacitation (female reproductive tract). An impressive array of spermatozoa surface remodeling events accompany these phases of maturation and appear critical for recognition and adhesion of the outer vestments of the oocyte, a structure known as the ZP. It is becoming increasingly apparent that species-specific zona adhesion is not mediated by a single receptor. Instead, compelling evidence now points toward models implicating a multiplicity of receptor-ligand interactions. This notion is in keeping with emerging research that has shown that there is a dynamic aggregation of proteins believed to be important in sperm-ZP recognition to the regions of sperm that mediate this binding event. Such remodeling may in turn facilitate the assembly of a multimeric zona recognition complex (MZRC). Though formation of MZRCs raises questions regarding the nature of the block to polyspermy, formation and assembly of such a structure would no doubt explain the strenuous maturation process that sperm endure on their sojourn to functional maturity.

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

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

Sperm-zona pellucida interaction: molecular mechanisms and the potential for contraceptive intervention

At the moment of insemination, millions of mammalian sperm cells are released into the female reproductive tract with the single goal of finding the oocyte. The spermatozoa subsequently ignore the thousands of cells they make contact with during their journey to the site of fertilization, until they reach the surface of the oocyte. At this point, they bind tenaciously to the acellular coat, known as the zona pellucida, which surrounds the oocyte and orchestrate a cascade of cellular interactions that culminate in fertilization. These exquisitely cell- and species- specific recognition events are among the most strategically important cellular interactions in biology. Understanding the cellular and molecular mechanisms that underpin them has implications for the etiology of human infertility and the development of novel targets for fertility regulation. Herein we describe our current understanding of the molecular basis of successful sperm-zona pellucida binding.

Mouse oviduct-specific glycoprotein is an egg-associated ZP3-independent sperm-adhesion ligand

Mouse sperm-egg binding requires a multiplicity of receptor-ligand interactions, including an oviduct-derived, high molecular weight, wheat germ agglutinin (WGA)-binding glycoprotein that associates with the egg coat at ovulation. Herein, we report the purification and identification of this sperm-binding ligand. WGA-binding, high molecular weight glycoproteins isolated from hormonally primed mouse oviduct lysates competitively inhibit sperm-egg binding in vitro. Within this heterogeneous glycoprotein preparation, a distinct 220 kDa protein selectively binds to sperm surfaces, and was identified by sequence analysis as oviduct-specific glycoprotein (OGP). The sperm-binding activity of OGP was confirmed by the loss of sperm-binding following immunodepletion of OGP from oviduct lysates, and by the ability of both immunoprecipitated OGP and natively purified OGP to competitively inhibit sperm-egg binding. As expected, OGP is expressed by the secretory cells of the fimbriae and infundibulum; however, in contrast to previous reports, OGP is also associated with both the zona pellucida and the perivitelline space of mouse oocytes. Western blot analysis and lectin affinity chromatography demonstrate that whereas the bulk of OGP remains soluble in the ampullar fluid, distinct glycoforms associate with the cumulus matrix, zona pellucida and perivitelline space. The sperm-binding activity of OGP is carbohydrate-dependent and restricted to a relatively minor peanut agglutinin (PNA)-binding glycoform that preferentially associates with the sperm surface, zona pellucida and perivitelline space, relative to other more abundant glycoforms. Finally, pretreatment of two-cell embryos, which do not normally bind sperm, with PNA-binding OGP stimulates sperm binding.

Reassessing the role of protein-carbohydrate complementarity during sperm-egg interactions in the mouse

Despite years of intense study by many investigators, it may appear that we have made little progress towards a molecular understanding of mammalian sperm binding to the egg zona pellucida. An abundance of evidence derived from in vitro assays suggests that sperm-zona pellucida binding is dependent upon sperm recognition of specific glycan moieties on the zona pellucida glycoproteins. However, there is considerable disagreement regarding the identity of the zona pellucida sugars thought to mediate sperm binding, as well as disagreement over the identity of the sperm receptors themselves. Moreover, results from in vivo gene-targeting strategies fail to support a role for many, if not all, of the sperm receptors and their zona pellucida ligands implicated from in vitro assays. Nevertheless, a retrospective view of the literature suggests that some common principles are emerging regarding the molecular basis of mammalian sperm-zona binding, both with respect to the nature of the components that mediate binding, as well as the involvement of distinct receptor-ligand interactions, that involve both protein- and carbohydrate-dependent mechanisms of binding.

Oviduct-specific glycoprotein and heparin modulate sperm-zona pellucida interaction during fertilization and contribute to the control of polyspermy

Polyspermy is an important anomaly of fertilization in placental mammals, causing premature death of the embryo. It is especially frequent under in vitro conditions, complicating the successful generation of viable embryos. A block to polyspermy develops as a result of changes after sperm entry (i.e., cortical granule exocytosis). However, additional factors may play an important role in regulating polyspermy by acting on gametes before sperm-oocyte interaction. Most studies have used rodents as models, but ungulates may differ in mechanisms preventing polyspermy. We hypothesize that zona pellucida (ZP) changes during transit of the oocyte along the oviductal ampulla modulate the interaction with spermatozoa, contributing to the regulation of polyspermy. We report here that periovulatory oviductal fluid (OF) from sows and heifers increases (both, con- and heterospecifically) ZP resistance to digestion with pronase (a parameter commonly used to measure the block to polyspermy), changing from digestion times of approximately 1 min (pig) or 2 min (cattle) to 45 min (pig) or several hours (cattle). Exposure of oocytes to OF increases monospermy after in vitro fertilization in both species, and in pigs, sperm-ZP binding decreases. The resistance of OF-exposed oocytes to pronase was abolished by exposure to heparin-depleted medium; in a medium with heparin it was not altered. Proteomic analysis of the content released in the heparin-depleted medium after removal of OF-exposed oocytes allowed the isolation and identification of oviduct-specific glycoprotein. Thus, an oviduct-specific glycoprotein-heparin protein complex seems to be responsible for ZP changes in the oviduct before fertilization, affecting sperm binding and contributing to the regulation of polyspermy.

Multiple proteins present in purified porcine sperm apical plasma membranes interact with the zona pellucida of the oocyte

An important step in fertilization is the recognition and primary binding of the sperm cell to the zona pellucida (ZP). Primary ZP binding proteins are located at the apical plasma membrane of the sperm head. In order to exclusively study primary zona binding proteins, plasma membranes of sperm heads were isolated, highly purified and subsequently solubilized with a mild or a strong solubilization procedure. Native, highly purified ZP ghosts were used as the binding substrate for solubilized sperm plasma membrane proteins, and a proteomic approach was employed to identify ZP binding proteins. Two-dimensional gel electrophoresis of ZP fragments with bound sperm proteins showed very reproducibly 24 sperm protein spots to be associated to the zona ghosts after mild plasma membrane solubilization whereas only three protein spots were detected after strong plasma membrane solubilization. This indicates the involvement of multiple sperm proteins in ZP binding. The three persistently bound proteins were identified by a tandem mass spectrometry as isoforms of AQN-3 and probably represent the main sperm protein involved in ZP binding. P47, fertilin beta and peroxiredoxin 5 were also conclusively identified. None of the identified proteins has a known acrosomal origin, which further indicated that there was no sample contamination with secondary ZP binding proteins from the acrosomal matrix. In this study, we showed and identified multiple zona binding proteins involved in primary sperm-zona binding. Although we were not able to identify all of the proteins involved, this is a first step in understanding the event of primary sperm-zona interactions and the relevance of this for fertilization is discussed.

Fertilization in mouse does not require terminal galactose or N-acetylglucosamine on the zona pellucida glycans

Fertilization in mammals requires sperm to bind to the zona pellucida (ZP) that surrounds the egg. Galactose (Gal) or N-acetylglucosamine (GlcNAc) residues on the glycans of ZP protein 3 (ZP3) have been implicated as mouse sperm receptors. However, Mgat1(-/-) eggs with modified N-glycans lacking terminal Gal and GlcNAc residues are fertilized. To determine if Gal and GlcNAc on O-glycans of the ZP are required for fertilization, a conditional allele of the T-synthase gene (T-syn(F)) was generated. T-syn encodes core 1 beta1,3-galactosyltransferase 1 (T-synthase), which initiates the synthesis of core-1-derived O-glycans, the only O-glycans on mouse ZP3. T-syn(F/F):ZP3Cre females in which T-syn(F) was deleted at the beginning of oogenesis generated eggs lacking core-1-derived O-glycans. Nevertheless, T-syn(F/F):ZP3Cre females were fertile and their eggs bound sperm similarly to controls. In addition, T-syn(-/-) embryos generated from T-syn null eggs developed until approximately E12.5. Thus, core-1-derived O-glycans are not required for blastogenesis, implantation, or development prior to midgestation. Moreover, T-syn(-/-)Mgat1(-/-) eggs lacking complex and hybrid N-glycans as well as core-1-derived O-glycans were fertilized. The combined data show that mouse ZP3 does not require terminal Gal or GlcNAc on either N- or O-glycans for fertilization.

Analysis of protein-linked glycosylation in a sperm-somatic cell adhesion system

Murine sperm initiate fertilization by binding to the specialized extracellular matrix of their complementary eggs, known as the zona pellucida. On the basis of data reported in this study, mouse sperm also bind to rabbit erythrocytes with higher affinity than they do to murine eggs. This unusual interaction between a germ cell and a somatic cell ("sperm-somatic cell adhesion system") is also carbohydrate dependent based on its sensitivity to mild periodate oxidation. To determine what types of carbohydrate sequences could be involved in this interaction, the protein-linked oligosaccharides of rabbit erythrocytes were sequenced using novel matrix-assisted laser desorption/ionization time-of-flight tandem mass spectrometry methods that enabled the analysis of individual components up to m/z 9000. The N-glycans are primarily complex biantennary and triantennary types terminated with Galalpha1-3Gal sequences. The majority of these oligosaccharides also possess one antenna consisting of a highly branched polylactosamine-type sequence that is also associated with many glycosphingolipids that coat rabbit erythrocytes. These erythrocytes also express Core 1 and Core 2 O-glycans terminated primarily with Galalpha1-3Gal sequences and to a lesser extent sialic acid. These results confirm that rabbit erythrocytes and mouse eggs present very different types of carbohydrate sequences on their surfaces. However, oligosaccharides terminated with beta1-6-linked N-acetyllactosamine or its alpha1-3 galactosylated analog are expressed on both the mouse zona pellucida and this somatic cell type. The far more abundant presentation of such sequences on rabbit erythrocytes compared with murine eggs could explain why mouse sperm display such exceptional affinity for this somatic cell type.

Role of sialic acid in bovine sperm–zona pellucida binding

Sperm binding activity has been detected in zona pellucida (ZP) glycoproteins and it is generally accepted that this activity resides in the carbohydrate moieties. In the present study we aim to identify some of the specific carbohydrate molecules involved in the bovine sperm-ZP interaction. We performed sperm binding competition assays, in vitro fecundation (IVF) in combination with different lectins, antibodies and neuraminidase digestion, and chemical and cytochemical analysis of the bovine ZP. Both MAA lectin recognising alpha-2,3-linked sialic acid and neuraminidase from Salmonella typhimurium with catalytic activity for alpha-2,3-linked sialic acid, demonstrated a high inhibitory effect on the sperm-ZP binding and oocyte penetration. These results suggest that bovine sperm-ZP binding is mediated by alpha-2,3-linked sialic acid. Experiments with trisaccharides (sialyllactose, 3'-sialyllactosamine and 6'-sialyllactosamine) and glycoproteins (fetuin and asialofetuin) corroborated this and suggest that at least the sequence Neu5Ac(alpha2-3)Gal(beta1-4)GlcNAc is involved in the sperm-ZP interaction. Moreover, these results indicate the presence of a sperm plasma membrane specific protein for the sialic acid. Chemical analysis revealed that bovine ZP glycoproteins contain mainly Neu5Ac (84.5%) and Neu5GC (15.5%). These two types of sialic acid residues are probably linked to Galbeta1,4GlcNAc and GalNAc by alpha-2,3- and alpha-2,6-linkages, respectively, as demonstrated by lectin cytochemical analysis. The use of a neuraminidase inhibitor resulted in an increased number of spermatozoa bound to the ZP and penetrating the oocyte. From this last result we hypothesize that a neuraminidase from cortical granules would probably participate in the block to polyspermy by removing sialic acid from the ZP.

Molecular cloning and characterization of a C-type lectin from Ancylostoma ceylanicum: evidence for a role in hookworm reproductive physiology

Lectins comprise a family of related proteins that mediate essential cell functions through binding to carbohydrates. Within this protein family, C-type lectins are defined by the requirement of calcium for optimal biologic activity. Using reverse transcription PCR, a cDNA corresponding to a putative C-type lectin has been amplified from the hookworm parasite Ancylostoma ceylanicum. The 550 nucleotide open reading frame of the A. ceylanicum C-type Lectin-1 (AceCTL-1) cDNA corresponds to a 167 amino acid mature protein (18,706 Da) preceded by a 17 amino acid secretory signal sequence. The recombinant protein (rAceCTL-1) was expressed in Drosophila S2 cells and purified using a combination of affinity chromatography and reverse phase HPLC. Using in vitro carbohydrate binding studies, it was determined that rAceCTL-1 binds N-acetyl-d-glucosamine, a common component of eukaryotic egg cell membranes. Using a polyclonal IgG raised against the recombinant protein, the native AceCTL-1 was identified in sperm and soluble protein extracts of adult male A. ceylanicum by immunoblot. Probing of adult hookworm sections with the polyclonal IgG demonstrated localization to the testes in males, as well as the spermatheca and developing embryos in females, consistent with its role as a sperm protein. Together, these data strongly suggest that AceCTL-1 is a male gender-specific C-type lectin with a function in hookworm reproductive physiology.

Synchronous modulation of cell surface lectin and its receptor in a homologous cell population: A novel mechanism of cellular regulation

Testicular immotile sperm undergo maturation during epididymal transit when these cells pass through caput, corpus, and cauda-epididymal regions. Maturing goat spermatozoa specifically at the distal corpus epididymal stage show head-to-head autoagglutination when incubated in vitro in a modified Ringer's solution. Here, we show the biochemical mechanism of autoagglutination event and its functional significance. A lectin-like molecule located on sperm surface specifically interacts with its receptor of the neighboring homologous cells to cause autoagglutination. Lectin is a Ca++-dependent galactose-specific protein. Failure of the pre- and post-distal corpus sperm to show autoagglutination is due to lack of lectin-like molecule and its receptors, respectively. Maturing sperm at distal corpus stage acquire lectin-like molecule followed by sharp disappearance of its receptor, and this event is synchronously associated with the initiation of sperm forward motility that is essential for fertilization in vivo. Lectin and its receptor isolated from sperm plasma membrane showed high efficacy for blocking autoagglutination phenomenon. The data are consistent with the view that synchronous modulation of homologous cell surface lectin and their receptors constitutes a novel mechanism for cellular regulation by generating waves of signals by manipulating lectin-sugar-dependent "self-talk" and cell-cell "cross-talk".

Identification of novel gamete receptors that mediate sperm adhesion to the egg coat

Mammalian fertilization is initiated by the species-specific binding of sperm to the zona pellucida, or egg coat. Earlier studies suggested that sperm-egg adhesion in mouse is mediated by the binding of beta1,4-galactosyltransferase-I (GalT) on the sperm surface to specific glycoside ligands on the egg coat glycoprotein, ZP3. Binding of multiple ZP3 oligosaccharides induces GalT aggregation, triggering a pertussis toxin-sensitive G-protein cascade leading to induction of the acrosome reaction. Consistent with this, sperm bearing targeted deletions in GalT are unable to bind ZP3 nor undergo ZP3-dependent acrosomal exocytosis; however, GalT-null sperm are still able to bind to the egg coat. This indicates that sperm-egg binding requires at least two independent binding mechanisms: a GalT-ZP3-independent event that mediates initial adhesion, followed by a GalT-ZP3 interaction that facilitates acrosomal exocytosis. During the past few years, novel GalT-ZP3-independent gamete receptors have been identified that appear to participate in initial gamete adhesion. On such receptor is SED1, an EGF repeat and discoidin domain protein that coats sperm as they traverse through the epididymis, and which is required for sperm to bind the egg coat. Similarly, a novel egg coat ligand is present on ovulated oocytes, but not on ovarian eggs, and which also appears to function in initial sperm binding. The identification of novel gamete receptors that are required for sperm-egg binding opens up new avenues for the development of specific contraceptive strategies.

Molecular Models for Murine Sperm-Egg Binding

Murine sperm initiate fertilization by binding to the specialized extracellular matrix of mouse eggs, known as the zona pellucida. Over the past decade, powerful genetic, biophysical, and biochemical techniques have been employed to gain new insights into this interaction. Evidence from these studies does not support either of two major models for binding first proposed over two decades ago. Two more recently established models suggest that protein-protein interactions predominate during this initial stage of fertilization. Another model proposes that about 75-80% of the murine sperm bound to zona pellucida under well defined in vitro conditions is carbohydrate dependent, with the remaining sperm bound via protein-protein interactions. Mounting evidence suggests that the carbohydrate sequences coating the murine egg could be employed as specific immune recognition markers. Continued investigation of this system may resolve many of these controversial findings and reveal novel functions for murine zona pellucida glycoproteins.

SED1 function during mammalian sperm-egg adhesion

A prerequisite for successful fertilization is the species-specific binding of sperm to the extracellular coat of the egg. Gamete binding triggers the release of sperm hydrolytic enzymes that digest a path through the egg coat, thus bringing sperm into proximity with the egg plasma membrane where gamete fusion occurs. Although some components of the sperm membrane and the egg coat that participate in sperm-egg interactions have been identified, results from targeted deletions and gene substitutions indicate that other, as yet unidentified, gamete receptors must contribute to sperm-egg binding. Recent studies implicate the bi-motif protein, SED1, as being required for successful sperm-egg adhesion in mouse. SED1 contains Notch-like EGF repeats as well as discoidin/F5/8 complement domains--motifs that mediate a variety of cell-cell and cell-matrix interactions. SED1's ability to promote gamete adhesion resides within its two discoidin/F5/8C domains, which are able to dock to substrates as diverse as phospholipid membranes and extracellular matrices. SED1 is also expressed in a wide range of tissues and epithelia, where it may function similarly as an adhesive protein facilitating cell-cell and/or cell-matrix interactions.

Molecular physiology and pathology of Ca2+-conducting channels in the plasma membrane of mammalian sperm

Current evidence indicates that mechanisms controlling the intracellular Ca2+concentration play pivotal roles in determining sperm fertilizing ability. Multiple Ca2+-permeable channels have been identified and characterized in the plasma membrane and in the acrosome membrane of mammalian sperm. This review summarizes the recent findings and assesses the evidence suggesting that these channels play roles in controlling a host of sperm functions ranging from motility to the acrosome reaction, and describes recent advances in the identification of the underlying gene defects of inherited sperm Ca2+channelopathies.

Spatial and temporal sequence of events in cell adhesion: from molecular recognition to focal adhesion assembly

A new concept that attributes a pivotal role to the pericellular coat in the regulation of the early stages of cell adhesion is presented. Quick, adaptable, and transient adhesion through multiple cooperative weak interactions provides the cell with an additional level of modulation in the decision-making process that precedes the commitment to adhesion at a particular site. Hyaluronan emerges as a modulator of cell adhesion in certain cells, mediating binding or repulsion through its polyelectrolyte character, in addition to its chirality and molecular-recognition properties. The biophysical properties of hyaluronan as well as its ultrastructural organization are analyzed in relation to this proposed function.

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.

Defending the zygote: search for the ancestral animal block to polyspermy

Fertilization is the union of a single sperm and an egg, an event that results in a diploid embryo. Animals use many mechanisms to achieve this ratio; the most prevalent involves physically blocking the fusion of subsequent sperm. Selective pressures to maintain monospermy have resulted in an elaboration of diverse egg and sperm structures. The processes employed for monospermy are as diverse as the animals that result from this process. Yet, the fundamental molecular requirements for successful monospermic fertilization are similar, implying that animals may have a common ancestral block to polyspermy. Here, we explore this hypothesis, reviewing biochemical, molecular, and genetic discoveries that lend support to a common ancestral mechanism. We also consider the evolution of alternative or radical techniques, including physiological polyspermy, with respect to our ability to describe a parsimonious guide to fertilization.

Inactivation of the Mgat1 gene in oocytes impairs oogenesis, but embryos lacking complex and hybrid N-glycans develop and implant

Complex and hybrid N-glycans contain sugar residues that have been implicated in fertilization, compaction of the embryo, and implantation. Inactivation of the Mgat1 gene responsible for their synthesis is embryonic lethal, but homozygous mutant blastocysts are phenotypically normal due to the presence of maternal Mgat1 gene transcripts. To identify roles for complex and hybrid N-glycans in oogenesis and preimplantation development, the Mgat1 gene in oocytes was deleted by using a ZP3Cre recombinase transgene. All mutant oocytes had an altered zona pellucida (ZP) that was thinner than the control ZP, and they did not possess complex N-glycans but contained ZP1, ZP2, and ZP3 glycoproteins. Mutant eggs were fertilized, all embryos implanted, and heterozygotes developed to birth. However, mutant females had decreased fertility, yielded fewer eggs after stimulation with gonadotropins, and produced a reduced number of preimplantation embryos and less progeny than controls. About 25% of embryonic day 3.5 (E3.5) embryos derived from mutant eggs were severely retarded in development, even when they were heterozygous and expressed complex N-glycans. Thus, a proportion of Mgat1(-)(/)(-) oocytes were developmentally compromised. Surprisingly, mutant eggs also gave rise to Mgat1(-)(/)(-) embryos that developed normally, implanted, and progressed to E9.5. Therefore, complex or hybrid N-glycans are required at some stage of oogenesis for the generation of a developmentally competent oocyte, but fertilization, blastogenesis, and implantation may proceed in their absence.

Lewis X-Containing Neoglycoproteins Mimic the Intrinsic Ability of Zona Pellucida Glycoprotein ZP3 to Induce the Acrosome Reaction in Capacitated Mouse Sperm1

The binding of zona pellucida (ZP) glycoprotein ZP3 to mouse sperm surface receptors is mediated by protein-carbohydrate interactions. Subsequently, ZP3 induces sperm to undergo the acrosome reaction, an obligatory step in fertilization. We have previously identified Lewis X (Le(x); Gal beta 4[Fuc alpha 3]GlcNAc) as a potent inhibitor of in vitro sperm-ZP binding (Johnston et al. J Biol Chem 1998; 273:1888-1895). This glycan is recognized by approximately 70% of the ZP3 binding sites on capacitated, acrosome-intact mouse sperm, whereas Lewis A (Le(a); Gal beta 3[Fuc alpha 4]GlcNAc) is recognized by most of the remaining sites (Kerr et al. Biol Reprod 2004; 71:770-777). Herein, we test the hypothesis that Le(x)- and Le(a)-containing glycans, when clustered on a neoglycoprotein, bind ZP3 receptors on sperm and induce sperm to undergo the acrosome reaction via the same signaling pathways as ZP3. Results show that a Le(x)-containing neoglycoprotein induced the acrosome reaction in a dose-dependent and capacitation-dependent manner. A Le(a)-containing neoglycoprotein also induced sperm to undergo the acrosome reaction but was less potent than Le(x)-containing neoglycoproteins. In contrast, neoglycoproteins containing beta4-lactosamine (Gal beta 4GlcNAc), Lewis B (Fuc alpha 2Gal beta 3[Fuc alpha 4]GlcNAc), and sialyl-Le(x) glycans were inactive, as were four other neoglycoproteins with different nonfucosylated glycans. Consistent with these results, unconjugated Le(x)- and Le(a)-capped glycans were dose-dependent inhibitors, which at saturation, reduced the ZP-induced acrosome reaction by about 60% and 30%, respectively. Experiments utilizing pharmacological inhibitors suggest that induction of the acrosome reaction by solubilized ZP and Le(x)- and Le(a)-containing neoglycoproteins require the same calcium-dependent pathway. However, only the ZP-induced acrosome reaction requires a functional G(i) protein. Thus, Le(x)-containing neoglycoproteins bind to a major class of ZP3 receptors on capacitated sperm. A Le(a)-containing neoglycoprotein binds a second ZP3 receptor but is a less-potent inducer of the acrosome reaction.