The functional roles of protein glycosylation in human maternal-fetal crosstalk

BACKGROUND

The establishment of maternal-fetal crosstalk is vital to a successful pregnancy. Glycosylation is a post-translational modification in which glycans (monosaccharide chains) are attached to an organic molecule. Glycans are involved in many physiological and pathological processes. Human endometrial epithelium, endometrial gland secretions, decidual immune cells, and trophoblasts are highly enriched with glycoconjugates and glycan-binding molecules important for a healthy pregnancy. Aberrant glycosylation in the placenta and uterus has been linked to repeated implantation failure and various pregnancy complications, but there is no recent review summarizing the functional roles of glycosylation at the maternal-fetal interface and their associations with pathological processes.

OBJECTIVE AND RATIONALE

This review aims to summarize recent findings on glycosylation, glycosyltransferases, and glycan-binding receptors at the maternal-fetal interface, and their involvement in regulating the biology and pathological conditions associated with endometrial receptivity, placentation and maternal-fetal immunotolerance. Current knowledge limitations and future insights into the study of glycobiology in reproduction are discussed.

SEARCH METHODS

A comprehensive PubMed search was conducted using the following keywords: glycosylation, glycosyltransferases, glycan-binding proteins, endometrium, trophoblasts, maternal-fetal immunotolerance, siglec, selectin, galectin, repeated implantation failure, early pregnancy loss, recurrent pregnancy loss, preeclampsia, and fetal growth restriction. Relevant reports published between 1980 and 2023 and studies related to these reports were retrieved and reviewed. Only publications written in English were included.

OUTCOMES

The application of ultrasensitive mass spectrometry tools and lectin-based glycan profiling has enabled characterization of glycans present at the maternal-fetal interface and in maternal serum. The endometrial luminal epithelium is covered with highly glycosylated mucin that regulates blastocyst adhesion during implantation. In the placenta, fucose and sialic acid residues are abundantly presented on the villous membrane and are essential for proper placentation and establishment of maternal-fetal immunotolerance. Glycan-binding receptors, including selectins, sialic-acid-binding immunoglobulin-like lectins (siglecs) and galectins, also modulate implantation, trophoblast functions and maternal-fetal immunotolerance. Aberrant glycosylation is associated with repeated implantation failure, early pregnancy loss and various pregnancy complications. The current limitation in the field is that most glycobiological research relies on association studies, with few studies revealing the specific functions of glycans. Technological advancements in analytic, synthetic and functional glycobiology have laid the groundwork for further exploration of glycans in reproductive biology under both physiological and pathological conditions.

WIDER IMPLICATIONS

A deep understanding of the functions of glycan structures would provide insights into the molecular mechanisms underlying their involvement in the physiological and pathological regulation of early pregnancy. Glycans may also potentially serve as novel early predictive markers and therapeutic targets for repeated implantation failure, pregnancy loss, and other pregnancy complications.

Antioxidant Nanoparticles Restore Cisplatin-Induced Male Fertility Defects by Promoting MDC1-53bp1-Associated Non-Homologous DNA Repair Mechanism and Sperm Intracellular Calcium Influx

Introduction

Cisplatin, a commonly used anticancer compound, exhibits severe off-target organ toxicity. Due to its wide application in cancer treatment, the reduction of its damage to normal tissue is an imminent clinical need. Cisplatin-induced testicular oxidative stress and damage lead to male sub- or infertility. Despite earlier studies showing that the natural polyphenol extracts honokiol serve as the free radical scavenger that reduces the accumulation of intracellular free radicals, whether honokiol exhibits direct effects on the testis and sperm is unclear. Thus, the aim of the current study is to investigate the direct effects of honokiol on testicular recovery and sperm physiology.

Methods

We encapsulated this polyphenol antioxidation compound into liposome-based nanoparticles (nHNK) and gave intraperitoneally to mice at a dosage of 5 mg/kg body mass every other day for consecutive 6 weeks.

Results

We showed that nHNK promotes MDC1-53bp1-associated non-homologous DNA double-strand break repair signaling pathway that minimizes cisplatin-induced DNA damage. This positive effect restores spermatogenesis and allows the restructuring of the multi-spermatogenic layers in the testis. By reducing mitochondrial oxidative damage, nHNK also protects sperm mitochondrial structure and maintains both testicular and sperm ATP production. By a yet-to-identify mechanism, nHNK restores sperm calcium influx at the sperm midpiece and tail, which is essential for sperm hypermotility and their interaction with the oocyte.

Discussion

Taken together, the nanoparticulated antioxidant counteracts cisplatin-induced male fertility defects and benefits patients undertaking cisplatin-based chemotherapy. These data may allow the reintroduction of cisplatin for systemic applications in patients at clinics with reduced testicular toxicity.

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.

Transcriptome sequencing reveals key potential long non-coding RNAs related to duration of fertility trait in the uterovaginal junction of egg-laying hens

Duration of fertility, (DF) is an important functional trait in poultry production and lncRNAs have emerged as important regulators of various process including fertility. In this study we applied a genome-guided strategy to reconstruct the uterovaginal junction (UVJ) transcriptome of 14 egg-laying birds with long- and short-DF (n = 7); and sought to uncover key lncRNAs related to duration of fertility traits by RNA-sequencing technology. Examination of RNA-seq data revealed a total of 9977 lncRNAs including 2576 novel lncRNAs. Differential expression (DE) analysis of lncRNA identified 223 lncRNAs differentially expressed between the two groups. DE-lncRNA target genes prediction uncovered over 200 lncRNA target genes and functional enrichment tests predict a potential function of DE-lncRNAs. Gene ontology classification and pathway analysis revealed 8 DE-lncRNAs, with the majority of their target genes enriched in biological functions such as reproductive structure development, developmental process involved in reproduction, response to cytokine, carbohydrate binding, chromatin organization, and immune pathways. Differential expression of lncRNAs and target genes were confirmed by qPCR. Together, these results significantly expand the utility of the UVJ transcriptome and our analysis identification of key lncRNAs and their target genes regulating DF will form the baseline for understanding the molecular functions of lncRNAs regulating DF.

Early ovariectomy reveals the germline encoding of natural anti-A- and Tn-cross-reactive immunoglobulin M (IgM) arising from developmental O-GalNAc glycosylations. (Germline-encoded natural anti-A/Tn cross-reactive IgM)

While native blood group A-like glycans have not been demonstrated in prokaryotic microorganisms as a source of human "natural" anti-A isoagglutinin production, and metazoan eukaryotic N-acetylgalactosamine O-glycosylation of serine or threonine residues (O-GalNAc-Ser/Thr-R) does not occur in bacteria, the O-GalNAc glycan-bearing ovarian glycolipids, discovered in C57BL/10 mice, are complementary to the syngeneic anti-A-reactive immunoglobulin M (IgM), which is not present in animals that have undergone ovariectomy prior to the onset of puberty. These mammalian ovarian glycolipids are complementary also to the anti-A/Tn cross-reactive Helix pomatia agglutinin (HPA), a molluscan defense protein, emerging from the coat proteins of fertilized eggs and reflecting the snail-intrinsic, reversible O-GalNAc glycosylations. The hexameric structure of this primitive invertebrate defense protein gives rise to speculation regarding an evolutionary relationship to the mammalian nonimmune, anti-A-reactive immunoglobulin M (IgM) molecule. Hypothetically, this molecule obtains its complementarity from the first step of protein glycosylations, initiated by GalNAc via reversible O-linkages to peptides displaying Ser/Thr motifs, whereas the subsequent transferase depletion completes germ cell maturation and cell renewal, associated with loss of glycosidic bonds and release of O-glycan-depleted proteins, such as complementary IgM revealing the structure of the volatilely expressed "lost" glycan carrier through germline Ser residues. Consequently, the evolutionary/developmental first glycosylations of proteins appear metabolically related or identical to that of the mucin-type, potentially "aberrant" monosaccharide GalNAcα1-O-Ser/Thr-R, also referred to as the Tn (T "nouvelle") antigen, and explain the anti-Tn cross-reactivity of human innate or "natural" anti-A-specific isoagglutinin and the pronounced occurrence of cross-reactive anti-Tn antibody in plasma from humans with histo-blood group O. In fact, A-allelic, phenotype-specific GalNAc glycosylation of plasma proteins does not occur in human blood group O, affecting anti-Tn antibody levels, which may function as a growth regulator that contributes to a potential survival advantage of this group in the overall risk of developing cancer when compared with non-O blood groups.

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.

A subunit of the oligosaccharyltransferase complex is required for interspecific gametophyte recognition in Arabidopsis

Species-specific gamete recognition is a key premise to ensure reproductive success and the maintenance of species boundaries. During plant pollen tube (PT) reception, gametophyte interactions likely allow the species-specific recognition of signals from the PT (male gametophyte) by the embryo sac (female gametophyte), resulting in PT rupture, sperm release, and double fertilization. This process is impaired in interspecific crosses between Arabidopsis thaliana and related species, leading to PT overgrowth and a failure to deliver the sperm cells. Here we show that ARTUMES (ARU) specifically regulates the recognition of interspecific PTs in A. thaliana. ARU, identified in a genome-wide association study (GWAS), exclusively influences interspecific--but not intraspecific--gametophyte interactions. ARU encodes the OST3/6 subunit of the oligosaccharyltransferase complex conferring protein N-glycosylation. Our results suggest that glycosylation patterns of cell surface proteins may represent an important mechanism of gametophyte recognition and thus speciation.

Functional glycosylation in the human and mammalian uterus

Background

Glycosylation is the most common and structurally diverse of all the post-translational modifications of proteins. Lipids and extracellular matrices are also often glycosylated. The mammalian uterus is highly enriched in glycoconjugates that are associated with the apical surfaces of epithelial cells and the secretions released by both epithelial and stromal cells. These glycoconjugates interact primarily with sperm, the implanting embryo, the fetus, and any pathogen that happens to gain entry into the uterus. Secretions of the endometrial glands increase substantially during the luteal phase of the menstrual cycle. These secretions are highly enriched in glycoproteins and mucins that promote specific uterine functions.

Findings

Lectins and antibodies have been employed in the majority of the studies focused on uterine glycosylation have employed to define the expression of carbohydrate sequences. However, while these studies provide insight about potential glycosylation, precise information about glycan structure is lacking. Direct sequencing studies that employ biochemical or mass spectrometric methods are far more definitive, but have rarely been employed with uterine glycoproteins. Both lectin/antibody binding and direct carbohydrate sequencing studies that have been focused on the mammalian uterus are reviewed. The primary functional role of the eutherian uterus is to facilitate fertilization and nurture the developing embryo/fetus. Trophoblasts are the primary cells that mediate the binding of the embryo and placenta to the uterine lining. In mammals that utilize hemochorial placentation, they invade the decidua, the specialized endometrial lining that forms during pregnancy. Trophoblasts have also been analyzed for their lectin/antibody binding as a complement to the analysis of the uterine cells and tissues. They will also be reviewed here.

Conclusions

The functional roles of the glycans linked to uterine and trophoblast glycoconjugates remain enigmatic. Another major question in the human is whether defects in placental or uterine glycosylation play a role in the development the Great Obstetrical Syndromes. More recent findings indicate that changes in glycosylation occur in trophoblasts obtained from patients that develop preeclampsia and preterm birth. The functional significance of these changes remain to be defined. Whether such shifts happen during the development of other types of obstetrical syndromes remains to be determined.

Protein-Carbohydrate Interaction between Sperm and the Egg-Coating Envelope and Its Regulation by Dicalcin, a Xenopus laevis Zona Pellucida Protein-Associated Protein

Protein-carbohydrate interaction regulates multiple important processes during fertilization, an essential biological event where individual gametes undergo intercellular recognition to fuse and generate a zygote. In the mammalian female reproductive tract, sperm temporarily adhere to the oviductal epithelium via the complementary interaction between carbohydrate-binding proteins on the sperm membrane and carbohydrates on the oviductal cells. After detachment from the oviductal epithelium at the appropriate time point following ovulation, sperm migrate and occasionally bind to the extracellular matrix, called the zona pellucida (ZP), which surrounds the egg, thereafter undergoing the exocytotic acrosomal reaction to penetrate the envelope and to reach the egg plasma membrane. This sperm-ZP interaction also involves the direct interaction between sperm carbohydrate-binding proteins and carbohydrates within the ZP, most of which have been conserved across divergent species from mammals to amphibians and echinoderms. This review focuses on the carbohydrate-mediated interaction of sperm with the female reproductive tract, mainly the interaction between sperm and the ZP, and introduces the fertilization-suppressive action of dicalcin, a Xenopus laevis ZP protein-associated protein. The action of dicalcin correlates significantly with a dicalcin-dependent change in the lectin-staining pattern within the ZP, suggesting a unique role of dicalcin as an inherent protein that is capable of regulating the affinity between the lectin and oligosaccharides attached on its target glycoprotein.

TURAN and EVAN mediate pollen tube reception in Arabidopsis Synergids through protein glycosylation

Pollen tube (PT) reception in flowering plants describes the crosstalk between the male and female gametophytes upon PT arrival at the synergid cells of the ovule. It leads to PT growth arrest, rupture, and sperm cell release, and is thus essential to ensure double fertilization. Here, we describe TURAN (TUN) and EVAN (EVN), two novel members of the PT reception pathway that is mediated by the FERONIA (FER) receptor-like kinase (RLK). Like fer, mutations in these two genes lead to PT overgrowth inside the female gametophyte (FG) without PT rupture. Mapping by next-generation sequencing, cytological analysis of reporter genes, and biochemical assays of glycoproteins in RNAi knockdown mutants revealed both genes to be involved in protein N-glycosylation in the endoplasmic reticulum (ER). TUN encodes a uridine diphosphate (UDP)-glycosyltransferase superfamily protein and EVN a dolichol kinase. In addition to their common role during PT reception in the synergids, both genes have distinct functions in the pollen: whereas EVN is essential for pollen development, TUN is required for PT growth and integrity by affecting the stability of the pollen-specific FER homologs ANXUR1 (ANX1) and ANX2. ANX1- and ANX2-YFP reporters are not expressed in tun pollen grains, but ANX1-YFP is degraded via the ER-associated degradation (ERAD) pathway, likely underlying the anx1/2-like premature PT rupture phenotype of tun mutants. Thus, as in animal sperm–egg interactions, protein glycosylation is essential for the interaction between the female and male gametophytes during PT reception to ensure fertilization and successful reproduction.

Protein glycosylation is essential for gametophyte interactions between the male pollen tube and the female ovule in plants, reminiscent of gamete interactions during fertilization in mammals.

In flowering plants, gametes are produced by the haploid, multicellular male (pollen), and female (embryo sac) gametophytes, which develop within the reproductive organs of the flower. Successful fertilization depends on delivery of the sperm cells to the embryo sac, which is embedded in the ovule, by the pollen tube. Upon arrival of the pollen tube at the opening of the ovule, crosstalk between male and female gametophytes, known as pollen tube reception, ensues; the pollen tube slows or stops its growth, then resumes rapid growth, and finally bursts to release the sperm cells and effect double fertilization. Although several members of the pollen tube reception pathway, including the receptor-like kinase FERONIA, have been identified, the molecular mechanisms underlying this communication process remain unclear. Here, we show that protein N-glycosylation is required for normal pollen tube reception. A mutant screen identified two genes, TURAN and EVAN, which are involved in protein N-glycosylation in the endoplasmic reticulum. Both genes act in the FERONIA-mediated pollen tube reception pathway, which is impaired in these mutants. Thus, in plants, a “dual recognition system,” involving interactions between both protein and glycosyl residues on the surface of male and female gametophytes, appears to be required for successful pollen tube reception, conceptually similar to sperm–egg interactions in mammals, for which N-glycosylation of cell surface proteins also plays an important role.

Oviductal secretion and gamete interaction

Experimental evidence from the last 30 years supports the fact that the oviduct is involved in the modulation of the reproductive process in eutherian mammals. Oviductal secretion contains molecules that contribute to regulation of gamete function, gamete interaction, and the early stages of embryo development. The oviductal environment would act as a sperm reservoir, maintaining sperm viability, and modulating the subpopulation of spermatozoa that initiates the capacitation process. It could also contribute to prevent the premature acrosome reaction and to reduce polyspermy. Many studies have reported the beneficial effects of the oviductal environment on fertilization and on the first stages of embryo development. Some oviductal factors have been identified in different mammalian species. The effects of oviductal secretion on the reproductive process could be thought to result from the dynamic combined action (inhibitory or stimulatory) of multiple factors present in the oviductal lumen at different stages of the ovulatory cycle and in the presence of gametes or embryos. It could be hypothesized that the absence of a given molecule would not affect fertility as its action could be compensated by another factor with similar functions. However, any alteration in this balance could affect certain events of the reproductive process and could perhaps impair fertility. Thus, the complexity of the reproductive process warrants a continuous research effort to unveil the mechanisms and factors behind its regulation in the oviductal microenvironment.

Identification and characterization of seminal fluid proteins in the Asian tiger mosquito, Aedes albopictus

The Asian tiger mosquito (Aedes albopictus) is an important vector for pathogens that affect human health, including the viruses that cause dengue and Chikungunya fevers. It is also one of the world's fastest-spreading invasive species. For these reasons, it is crucial to identify strategies for controlling the reproduction and spread of this mosquito. During mating, seminal fluid proteins (Sfps) are transferred from male mosquitoes to females, and these Sfps modulate female behavior and physiology in ways that influence reproduction. Despite the importance of Sfps on female reproductive behavior in mosquitoes and other insects, the identity of Sfps in Ae. albopictus has not previously been reported. We used transcriptomics and proteomics to identify 198 Sfps in Ae. albopictus. We discuss possible functions of these Sfps in relation to Ae. albopictus reproduction-related biology. We additionally compare the sequences of these Sfps with proteins (including reported Sfps) in several other species, including Ae. aegypti. While only 72 (36.4%) of Ae. albopictus Sfps have putative orthologs in Ae. aegypti, suggesting low conservation of the complement of Sfps in these species, we find no evidence for an elevated rate of evolution or positive selection in the Sfps that are shared between the two Aedes species, suggesting high sequence conservation of those shared Sfps. Our results provide a foundation for future studies to investigate the roles of individual Sfps on feeding and reproduction in this mosquito. Functional analysis of these Sfps could inform strategies for managing the rate of pathogen transmission by Ae. albopictus.

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.

Mammalian sperm fertility related proteins

Infertility is an important aspect of human and animal reproduction and still presents with much etiological ambiguity. As fifty percent of infertility is related to the male partner, molecular investigations on sperm and seminal plasma can lead to new knowledge on male infertility. Several comparisons between fertile and infertile human and other species sperm proteome have shown the existence of potential fertility markers. These proteins have been categorized into energy related, structural and other functional proteins which play a major role in sperm motility, capacitation and sperm-oocyte binding. The data from these studies show the impact of sperm proteome studies on identifying different valuable markers for fertility screening. In this article, we review recent development in unraveling sperm fertility related proteins.

Localization of cathepsin D in mouse reproductive tissues and its acquisition onto sperm surface during epididymal sperm maturation

Sperm maturation in the epididymis involves multiple complex events, that include the adsorption of epididymal secretory proteins, re-organization and removal of sperm surface ligands. In this study, we investigated the existence and distribution of cathepsin D (CAT-D) transcripts and proteins in mouse reproductive tissues and proposed a transfer mechanism of CAT-D to the sperm surface. CAT-D transcripts were highly expressed in cultured Sertoli cells, but not in germ cells. The transcriptional level was relatively higher in the caput epididymis (CP) than in the cauda epididymis (CD). At the translational level, CAT-D was detected in testicular somatic cells and in the principal and basal cells in the CP. The expression of CAT-D was fairly specific to the clear cells in the CD. All forms of CAT-D were detected in ultracentrifuged epididymosomes. In conjunction with the expression levels in epididymal epithelium and epididymosomes, CAT-D expression level on the sperm surface was relatively high in CP sperm, but gradually declined toward the CD. Overall, our results indicated that CAT-D was not inherent to sperm themselves, but rather of epididymal origin and was presumably transported to the sperm surface via epididymosomes.

Binding of Sperm to the Zona Pellucida Mediated by Sperm Carbohydrate-Binding Proteins is not Species-Specific in Vitro between Pigs and Cattle

Carbohydrates are candidates for the basis of species-selective interaction of gametes during mammalian fertilization. In this study, we sought to clarify the roles of sugar residues in the species-selective, sperm-oocyte interaction in pigs and cattle. Acrosome-intact porcine and bovine sperm exhibited their strongest binding affinities for β-Gal and α-Man residues, respectively. Porcine-sperm specificity changed from β-Gal to α-Man after the acrosome reaction, while bovine-sperm specificity did not. Binding of acrosome-intact and acrosome-reacted sperm decreased after trypsinization, indicating that the carbohydrate-binding components are proteins. While immature oocytes bound homologous sperm preferentially to heterologous sperm, oocytes matured in vitro bound similar numbers of homologous and heterologous sperm. Lectin staining revealed the aggregation of α-Man residues on the outer surface of the porcine zona during maturation. In both species, zona-free, mature oocytes bound homologous sperm preferentially to heterologous sperm. The lectin-staining patterns of the zona pellucida and zona-free oocytes coincided with the carbohydrate-binding specificities of acrosome-intact and acrosome-reacted sperm, respectively, supporting the involvement of carbohydrates in gamete recognition in pigs and cattle. These results also indicate that sperm-zona pellucida and sperm-oolemma bindings are not strictly species-specific in pigs and cattle, and further suggest that sperm penetration into the zona and/or fusion with oolemma may be species-specific between pigs and cattle.

Dynamic regulation of sperm interactions with the zona pellucida prior to and after fertilisation

Recent findings have refined our thinking on sperm interactions with the cumulus–oocyte complex (COC) and our understanding of how, at the molecular level, the sperm cell fertilises the oocyte. Proteomic analyses has identified a capacitation-dependent sperm surface reordering that leads to the formation of functional multiprotein complexes involved in zona–cumulus interactions in several mammalian species. During this process, multiple docking of the acrosomal membrane to the plasma membrane takes place. In contrast with the dogma that the acrosome reaction is initiated when spermatozoa bind to the zona pellucida (ZP), it has been established recently that, in mice, the fertilising spermatozoon initiates its acrosome reaction during its voyage through the cumulus before it reaches the ZP. In fact, even acrosome-reacted mouse spermatozoa collected from the perivitelline space can fertilise another ZP-intact oocyte. The oviduct appears to influence the extracellular matrix properties of the spermatozoa as well as the COC. This may influence sperm binding and penetration of the cumulus and ZP, and, in doing so, increase monospermic while decreasing polyspermic fertilisation rates. Structural analysis of the ZP has shed new light on how spermatozoa bind and penetrate this structure and how the cortical reaction blocks sperm–ZP interactions. The current understanding of sperm interactions with the cumulus and ZP layers surrounding the oocyte is reviewed with a special emphasis on the lack of comparative knowledge on this topic in humans, as well as in most farm mammals.

Rabbit zona pellucida composition: a molecular, proteomic and phylogenetic approach

The zona pellucida (ZP) participates in sperm-egg interactions during the first steps of fertilization. Recent studies have shown that the ZP matrix of oocytes in several species is composed of four glycoproteins, designated as ZP1, ZP2, ZP3 and ZP4, rather than the three described in mouse, pig and cow. In this study, investigations were carried out to unveil a fourth glycoprotein in the rabbit (Oryctolagus cuniculus) ZP. Using total RNA isolated from rabbit ovaries, the complementary deoxyribonucleic acid (cDNA) encoding rabbit ZP1 was amplified by reverse transcribed polymerase chain reaction (RT-PCR). The ZP1 cDNA contains an open reading frame of 1825 nucleotides encoding a polypeptide of 608 amino acid residues. The deduced amino acid sequence of rabbit ZP1 showed high identity with other species: 70% identity with human and horse ZP1, and 67% identity with mouse and rat ZP1. At the proteomic level, peptides corresponding to the four proteins were detected by mass spectrometry. In addition, a molecular phylogenetic analysis of ZP1 showed that pseudogenization of this gene has occurred at least four times during the evolution of mammals. The data presented in this manuscript provide evidence, for the first time, that the rabbit ZP is composed of four glycoproteins.

Growth of mouse oocytes to maturity from premeiotic germ cells in vitro

In the present study, we established an in vitro culture system suitable for generating fertilizable oocytes from premeiotic mouse female germ cells. These results were achieved after first establishing an in vitro culture system allowing immature oocytes from 12–14 day- old mice to reach meiotic maturation through culture onto preantral granulosa cell (PAGC) monolayers in the presence of Activin A (ActA). To generate mature oocytes from premeiotic germ cells, pieces of ovaries from 12.5 days post coitum (dpc) embryos were cultured in medium supplemented with ActA for 28 days and the oocytes formed within the explants were isolated and cocultured onto PAGC monolayers in the presence of ActA for 6–7 days. The oocytes were then subjected to a final meiotic maturation assay to evaluate their capability to undergo germinal vesicle break down (GVBD) and reach the metaphase II (MII) stage. We found that during the first 28 days of culture, a significant number of oocytes within the ovarian explants reached nearly full growth and formed preantral follicle-like structures with the surrounding somatic cells. GSH level and Cx37 expression in the oocytes within the explants were indicative of proper developmental conditions. Moreover, the imprinting of Igf2r and Peg3 genes in these oocytes was correctly established. Further culture onto PAGCs in the presence of ActA allowed about 16% of the oocytes to undergo GVBD, among which 17% reached the MII stage during the final 16–18 hr maturation culture. These MII oocytes showed normal spindle and chromosome assembly and a correct ERK1/2 activity. About 35% of the in vitro matured oocytes were fertilized and 53.44% of them were able to reach the 2-cell stage. Finally, around 7% of the 2-cell embryos developed to the morula/blastocyst stage.

Oocytes use the plasminogen-plasmin system to remove supernumerary spermatozoa

BACKGROUND

The role of the plasminogen-plasmin (PLG-PLA) system in fertilization is unknown, although its dysfunction has been associated with subfertility in humans. We have recently detected and quantified plasminogen in the oviductal fluid of two mammals and showed a reduction in sperm penetration during IVF when plasminogen is present. The objective of this study was to describe the mechanism by which PLG-PLA system regulates sperm entry into the oocyte.

METHODS AND RESULTS

By combining biochemical, functional, electron microscopic, immunocytochemical and live cell imaging methods, we show here that (i) plasminogen is activated into the protease plasmin, by gamete interaction; (ii) urokinase-type and tissue-type plasminogen activators are present in oocytes, but they are not of cortical granule origin; (iii) sperm binding to oocytes triggers the releasing of plasminogen activators and (iv) the generated plasmin causes sperm detachment from the zona pellucida.

CONCLUSIONS

Our results describe a novel mechanism for the success or failure of fertilization in mammals, by which molecules present in the oviductal environment are activated by molecules originating within the gametes. We anticipate that therapeutic up- or down-regulation of this physiological mechanism may be used to help in conception or as a contraceptive tool. Since components of the PLG-PLA system are already available as drugs for heart attacks or cancer therapies, basic research on this novel function would be rapidly transferable for clinical application.

Function and composition of male accessory gland secretions in Anopheles gambiae: a comparison with other insect vectors of infectious diseases

Human malaria, a major public health burden in tropical and subtropical countries, is transmitted exclusively by the bite of a female Anopheles mosquito. Malaria control strategies aimed at inducing sexual sterility in natural vector populations are an attractive alternative to the use of insecticides. However, despite their importance as disease vectors, limited information is available on the molecular mechanisms regulating fertility in Anopheles mosquitoes. In the major malaria vector, An. gambiae, the full complement of sperm and seminal fluid required for a female's lifelong egg production is obtained from a single mating event. This single mating has important consequences for the physiology and behavior of An. gambiae females: in particular, they become refractory to further insemination, and they start laying eggs. In other insects including Drosophila, similar post-copulatory changes are induced by seminal proteins secreted by the male accessory glands and transferred to the female during mating. In this review, we analyze the current state of knowledge on the function and characterization of male seminal proteins in An. gambiae, and provide a comparative assessment of the role of these male reproductive factors in other mosquito vectors of human disease in which female post-copulatory behavior has been studied. Knowledge of the factors and mechanisms regulating fertility in An. gambiae and other vectors can help the design of novel control strategies to fight the spread of disease.

Function of the acrosomal matrix: zona pellucida 3 receptor (ZP3R/sp56) is not essential for mouse fertilization

In mammalian fertilization, sperm-zona pellucida binding is considered to be a critical aspect of gamete interaction. In this study, we examine the mouse sperm acrosomal matrix protein zona pellucida 3 receptor (ZP3R; formerly called sp56) because of our interest in defining the function of the acrosomal matrix, the particulate compartment within the sperm secretory acrosome. Using targeted deletion of the Zp3r gene by homologous recombination, we examined the fertility of nullizygous animals. Our experiments showed that males and females homozygous for the affected gene exhibited no differences in litter sizes compared to wild-type and heterozygous animals. Testis weights of nullizygous males were equivalent to those of wild-type and heterozygous males, and no differences in the number of sperm produced by mice of three genotypes were found. In vitro fertilization rates using cumulus-intact and cumulus-free oocytes were also equivalent. Examination of sperm-binding zonae of unfertilized eggs and the ability of the sperm to undergo acrosomal exocytosis in response to calcium ionophore A23187 displayed no differences between wild-type, heterozygous, and nullizygous mouse sperm. These results provide further evidence that either ZP3R is not involved in sperm-zona pellucida binding or this process might be functionally redundant, involving multiple proteins for gamete interactions.

Porcine zona pellucida glycoprotein ZP4 is responsible for the sperm-binding activity of the ZP3/ZP4 complex

The zona pellucida (ZP) is a transparent envelope that surrounds the mammalian oocyte and mediates species-selective sperm-egg interactions. Porcine and bovine ZPs consist of glycoproteins ZP2, ZP3, and ZP4. In both pig and bovine a heterocomplex consisting of ZP3 and ZP4 binds to sperm, however it is not clarified whether ZP3 or ZP4 in the complex is responsible for the sperm binding. Previously, we have established a baculovirus-Sf9 cell expression system for porcine ZP glycoproteins. A mixture of recombinant ZP3 (rZP3) and rZP4 displayed sperm-binding activity toward bovine sperm but not porcine sperm, probably due to differences in carbohydrate structure between the native and recombinant ZP glycoproteins. In this study, a mixture of porcine rZP3 and native ZP4 (nZP4) inhibited the binding of porcine sperm to the ZP. In contrast, a mixture of porcine nZP3 and rZP4 did not inhibit the binding of porcine sperm, although the mixture inhibited the binding of bovine sperm. The porcine rZP3/nZP4 mixture bound to the acrosomal region of porcine sperm, in a manner similar to that of the nZP3/nZP4 mixture. nZP3 was precipitated with rZP4, and nZP4 was precipitated with rZP3 by utilising the N-terminal tags on the recombinant proteins. These results indicated that nZP4, but not rZP4, is necessary for binding activity of porcine ZP3/ZP4 complex towards porcine sperm and further suggested that the carbohydrate structures of ZP4 in the porcine ZP3/ZP4 complex are responsible for porcine sperm-binding activity of the complex.

The molecular basis of mouse sperm-zona pellucida binding: a still unresolved issue in developmental biology

During murine fertilization, sperm bind to the specialized extracellular matrix of the egg, known as the zona pellucida (ZP). This matrix is composed of three major glycoproteins designated ZP1, ZP2, and ZP3. Three models for sperm-ZP binding are now under consideration. The domain-specific model posits that adhesion relies primarily on interactions between N-glycans located within the C-terminal domain of ZP3 and a lectin-like egg-binding protein in the sperm plasma membrane. However, this model does not explain recent results obtained in studies with ZP2(mut) mice. In the supramolecular structure model, sperm bind to a three-dimensional zona matrix that depends on the cleavage status of ZP2. This paradigm does not explain the potent inhibitory effect of specific carbohydrate sequences or a C-terminal glycopeptide (gp55) derived from ZP3. Recently, O-glycans linked at Thr(155) and Thr(162) of ZP3 were implicated as potential ligands that mediate initial sperm-ZP binding. This novel model will be reviewed. A major challenge is to develop an alternate model for sperm-ZP binding that fits as much of the data as possible. Such a model is presented in this review. This paradigm could explain how the inability to cleave ZP2(mut) in ZP2(mut) mice could result in continued sperm binding to two-cell stage embryos without the formation of a supramolecular binding complex. These novel insights should guide future experiments that will eventually determine the molecular basis underlying gamete binding in the mouse and other eutherian mammals.

A structural view of egg coat architecture and function in fertilization

Species-restricted interaction between gametes at the beginning of fertilization is mediated by the extracellular coat of the egg, a matrix of cross-linked glycoprotein filaments called the zona pellucida (ZP) in mammals and the vitelline envelope in nonmammals. All egg coat subunits contain a conserved protein-protein interaction module-the "ZP domain"-that allows them to polymerize upon dissociation of a C-terminal propeptide containing an external hydrophobic patch (EHP). Recently, the first crystal structures of a ZP domain protein, sperm receptor ZP subunit zona pellucida glycoprotein 3 (ZP3), have been reported, giving a glimpse of the structural organization of the ZP at the atomic level and the molecular basis of gamete recognition in vertebrates. The ZP module is divided in two related immunoglobulin-like domains, ZP-N and ZP-C, that contain characteristic disulfide bond patterns and, in the case of ZP-C, also incorporate the EHP. This segment lies at the interface between the two domains, which are connected by a long loop carrying a conserved O-glycan important for binding to sperm in vitro. The structures explain several apparently contradictory observations by reconciling the variable disulfide bond patterns found in different homologues of ZP3 as well as the multiple ZP3 determinants alternatively involved in gamete interaction. These findings have implications for our understanding of ZP subunit biogenesis; egg coat assembly, architecture, and interaction with sperm; structural rearrangements leading to postfertilization hardening of the ZP and the block to sperm binding; and the evolutionary origin of egg coats.