Isolation and mapping the chicken zona pellucida genes: an insight into the evolution of orthologous genes in different species

Zona pellucida family genes in Chinese pond turtle: identification, expression profiles, and role in the spermatozoa acrosome reaction†

The zona pellucida (ZP) is an extracellular matrix that surrounds all vertebrate eggs, and it is involved in fertilization and species-specific recognition. Numerous in-depth studies of the ZP proteins of mammals, birds, amphibians, and fishes have been conducted, but systematic investigation of the ZP family genes and their role during fertilization in reptiles has not been reported to date. In this study, we identified six turtle ZP (Tu-ZP) gene subfamilies (Tu-ZP1, Tu-ZP2, Tu-ZP3, Tu-ZP4, Tu-ZPD, and Tu-ZPAX) based on whole genome sequence data from Mauremys reevesii. We found that Tu-ZP4 had large segmental duplication and was distributed on three chromosomes, and we also detected gene duplication in the other Tu-ZP genes. To evaluate the role of Tu-ZP proteins in sperm-egg binding, we assessed the expression pattern of these Tu-ZP proteins and their ability to induce the spermatozoa acrosome reaction in M. reevesii. In conclusion, this is the first report of the existence of gene duplication of Tu-ZP genes and that Tu-ZP2, Tu-ZP3, and Tu-ZPD can induce acrosome exocytosis of spermatogenesis in the reptile.

Egg envelope formation of medaka Oryzias latipes requires ZP proteins originating from both the liver and ovary

Teleost oocytes are surrounded by a structure, called chorion or egg envelopes, which is composed of zona pellucida (ZP) proteins. As a result of the gene duplication in teleost, the expression site of the zp genes, coding the major component protein of egg envelopes, changed from the ovary to the maternal liver. In Euteleostei, there are three liver-expressed zp genes, named choriogenin (chg) h, chg hm, and chg l, and the composition of the egg envelope is mostly made up of these Chgs. In addition, ovary-expressed zp genes are also conserved in the medaka genomes, and their proteins have also been found to be minor components of the egg envelopes. However, the specific role of liver-expressed versus ovary-expressed zp genes was unclear. In the present study, we showed that ovary-synthesized ZP proteins first form the base layer of the egg envelope, and then Chgs polymerize inwardly to thicken the egg envelope. To analyze the effects of dysfunction of the chg gene, we generated some chg knockout medaka. All knockout females failed to produce normally fertilized eggs by the natural spawning. The egg envelopes lacking Chgs were significantly thinner, but layers formed by ZP proteins synthesized in the ovary were found in the thin egg envelope of knockout as well as wild-type eggs. These results suggest that the ovary-expressed zp gene is well conserved in all teleosts, including those species in which liver-derived ZP proteins are the major component, because it is essential for the initiation of egg envelope formation.

Lineage-specific evolution of zona pellucida genes in fish

The zona pellucida (ZP) protein constitutes the egg envelope, which surrounds the vertebrate embryo. We performed a comprehensive study on the molecular evolution of ZP genes in Teleostei by cloning and analyzing the expression of ZP genes in fish of Anguilliformes in Elopomorpha, Osteoglossiformes in Osteoglossomorpha, and Clupeiformes in Otocephala to cover unsurveyed fish groups in Teleostei. The present results confirmed findings from our previous reports that the principal organ of ZP gene expression changed from ovary to liver in the common ancestors of Clupeocephala. Even fish species that synthesize egg envelopes in the liver carry the ovary-expressed ZP proteins as minor egg envelope components that were produced by gene duplication during the early stage of Teleostei evolution. The amino acid repeat sequences located at the N-terminal region of ZP proteins are known to be the substrates of transglutaminase responsible for egg envelope hardening and hatching. A repeat sequence was found in zona pellucida Cs of phylogenetically early diverged fish. After changing the synthesis organ, its role is inherited by the N-terminal Pro-Gln-Xaa repeat sequence in liver-expressed zona pellucida B genes of Clupeocephala. These results suggest that teleost ZP genes have independently evolved to maintain fish-specific functions, such as egg envelope hardening and egg envelope digestion, at hatching.

Zona Pellucida Genes and Proteins: Essential Players in Mammalian Oogenesis and Fertility

All mammalian oocytes and eggs are surrounded by a relatively thick extracellular matrix (ECM), the zona pellucida (ZP), that plays vital roles during oogenesis, fertilization, and preimplantation development. Unlike ECM surrounding somatic cells, the ZP is composed of only a few glycosylated proteins, ZP1–4, that are unique to oocytes and eggs. ZP1–4 have a large region of polypeptide, the ZP domain (ZPD), consisting of two subdomains, ZP-N and ZP-C, separated by a short linker region, that plays an essential role in polymerization of nascent ZP proteins into crosslinked fibrils. Both subdomains adopt immunoglobulin (Ig)-like folds for their 3-dimensional structure. Mouse and human ZP genes are encoded by single-copy genes located on different chromosomes and are highly expressed in the ovary by growing oocytes during late stages of oogenesis. Genes encoding ZP proteins are conserved among mammals, and their expression is regulated by cis-acting sequences located close to the transcription start-site and by the same/similar trans-acting factors. Nascent ZP proteins are synthesized, packaged into vesicles, secreted into the extracellular space, and assembled into long, crosslinked fibrils that have a structural repeat, a ZP2-ZP3 dimer, and constitute the ZP matrix. Fibrils are oriented differently with respect to the oolemma in the inner and outer layers of the ZP. Sequence elements in the ZPD and the carboxy-terminal propeptide of ZP1–4 regulate secretion and assembly of nascent ZP proteins. The presence of both ZP2 and ZP3 is required to assemble ZP fibrils and ZP1 and ZP4 are used to crosslink the fibrils. Inactivation of mouse ZP genes by gene targeting has a detrimental effect on ZP formation around growing oocytes and female fertility. Gene sequence variations in human ZP genes due to point, missense, or frameshift mutations also have a detrimental effect on ZP formation and female fertility. The latter mutations provide additional support for the role of ZPD subdomains and other regions of ZP polypeptide in polymerization of human ZP proteins into fibrils and matrix.

Proteomics support the threespine stickleback egg coat as a protective oocyte envelope

Ancestrally marine threespine stickleback fish (Gasterosteus aculeatus) have undergone an adaptive radiation into freshwater environments throughout the Northern Hemisphere, creating an excellent model system for studying molecular adaptation and speciation. Ecological and behavioral factors have been suggested to underlie stickleback reproductive isolation and incipient speciation, but reproductive proteins mediating gamete recognition during fertilization have so far remained unexplored. To begin to investigate the contribution of reproductive proteins to stickleback reproductive isolation, we have characterized the stickleback egg coat proteome. We find that stickleback egg coats are comprised of homologs to the zona pellucida (ZP) proteins ZP1 and ZP3, as in other teleost fish. Our molecular evolutionary analyses indicate that across teleosts, ZP3 but not ZP1 has experienced positive Darwinian selection. Mammalian ZP3 is also rapidly evolving, and surprisingly some residues under selection in stickleback and mammalian ZP3 directly align. Despite broad homology, however, we find differences between mammalian and stickleback ZP proteins with respect to glycosylation, disulfide bonding, and sites of synthesis. Taken together, the changes we observe in stickleback ZP protein architecture suggest that the egg coats of stickleback fish, and perhaps fish more generally, have evolved to fulfill a more protective functional role than their mammalian counterparts.

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.

Semen cryopreservation in golden-headed lion tamarin, Leontopithecus chrysomelas

Wild animal genetic resource banking (GRB) represents a valuable tool in conservation breeding programs, particularly in cases involving endangered species such as the golden-headed lion tamarin (Leontopithecus chrysomelas). Thus, we aimed to assess a sperm freezing protocol for golden-headed lion tamarins using two different exenders: BotuBOV® (BB) and Test Yolk Buffer® (TYB). Ejaculates were collected by penile vibrostimulation from animals housed at São Paulo Zoological Park Foundation, São Paulo, Brazil, and after immediate analysis, two aliquots were diluted in BB and TYB. Postthawing samples were evaluated for total and progressive motility, plasma membrane and acrosome integrities, mitochondrial activity, susceptibility to oxidative stress, and sperm-egg-binding. No differences between BB and TYB were found for most seminal parameters, except for acrosome integrity and susceptibility to oxidative stress (in both cases BB showed higher values). However, in spite of these differences and regardless of the extender used, postthaw sperm motility and viability with the described protocol were encouraging (on average >50% and >80%, respectively), indicating that sperm cryopreservation may be a short-term measure for the conservation of golden-headed lion tamarins.

Assessment of different sperm functional tests in golden-headed lion tamarins (Leontopithecus chrysomelas)

The golden-headed lion tamarin (Leontopithecus chrysomelas) is an endangered species endemic to Brazil's Atlantic Forest, a shrinking biodiversity hotspot. As in other Neotropical primates, its semen characteristics and freezability are poorly studied. Hence, reproductive technologies for callitrichids would greatly benefit from reliable methods of semen analysis. In a bid to promote reproductive research in tamarins, we validated simple and inexpensive sperm function tests that can be used to monitor sperm-egg binding, plasma membrane and acrosome integrity, mitochondrial activity, and DNA fragmentation. Ejaculates from adult males were individually diluted and divided into control and damage-induced aliquots, and then samples comprising assorted amounts of damaged spermatozoa were examined by organelle-specific tests. Our findings showed that sperm-binding in chicken egg perivitelline membrane (EPM) positively correlated with the number of spermatozoa injured by snap-freezing. Eosin-nigrosin (EN) and propidium iodide readings were correlated with each other, and both provided robust measurements of plasma membrane integrity. A high correlation between expected and measured amounts of acrosome-intact spermatozoa was found using Fast Green-Rose Bengal (FG-RB), Coomassie Blue (CB), and FITC-PSA stains, and all three methods exhibited comparable results. Likewise, different percentages of UV-irradiated spermatozoa were accurately assessed for DNA integrity by Toluidine Blue (TB) and sperm chromatin dispersion (SCD) tests. Comparisons between 3,3'-diaminobenzidine (DAB) and JC-1 stains also indicated the reliability of the former assay to ascertain gradual increases in spermatozoa with greater mitochondrial function. These data confirmed that different parts of the tamarin spermatozoa can be simply and consistently evaluated by EPM, EN, FG-RB, CB, TB, and DAB protocols.

Vitelline membrane proteins promote left-sided nodal expression after neurula rotation in the ascidian, Halocynthia roretzi

Stereotyped left-right asymmetry both in external and internal organization is found in various animals. Left-right symmetry is broken by the neurula rotation in the ascidian, Halocynthia roretzi. Neurula embryos rotate along the anterior-posterior axis in a counterclockwise direction, and the rotation stops when the left side of the embryo is oriented downwards, resulting in contact of the left-side epidermis with the vitelline membrane at the bottom of perivitelline space. Then, such contact induces the expression of nodal and its downstream Pitx2 gene in the left-side epidermis. Vitelline membrane is required for the promotion of nodal expression. Here, we showed that a chemical signal from the vitelline membrane promotes nodal gene expression, but mechanical stimulus at the point of contact is unnecessary since the treatment of devitellinated neurulae with an extract of the vitelline membrane promoted nodal expression on both sides. The signal molecules are already present in the vitelline membranes of unfertilized eggs. These signal molecules are proteins but not sugars. Specific fractions in gel filtration chromatography had the nodal promoting activity. By mass spectrometry, we selected 48 candidate proteins. Proteins that contain both a zona pellucida (ZP) domain and epidermal growth factor (EGF) repeats were enriched in the candidates of the nodal inducing molecules. Six of the ZP proteins had multiple EGF repeats that are only found in ascidian ZP proteins. These were considered to be the most viable candidates of the nodal-inducing molecules. Signal molecules are anchored to the entire vitelline membrane, and contact sites of signal-receiving cells are spatially and mechanically controlled by the neurula rotation. In this context, ascidians are unusual with respect to mechanisms for specification of the left-right axis. By suppressing formation of epidermis monocilia, we also showed that epidermal cilia drive the neurula rotation but are dispensable for sensing the signal from the vitelline membrane.

Evolution and multiple origins of zona pellucida genes in vertebrates

Animal egg coats are composed of different glycoproteins collectively named zona pellucida (ZP) proteins. The characterized vertebrate genes encoding ZP proteins have been classified into six subfamilies, and exhibit low similarity to the ZP genes characterized in certain invertebrates. The origin and evolution of the vertebrate ZP genes remain obscure. A search against 97 representative metazoan species revealed various numbers (ranging from three to 33) of different putative egg-coat ZP genes in all 47 vertebrates and several ZP genes in five invertebrate species, but no putative ZP gene was found in the other 45 species. Based on phylogenetic and synteny analyses, all vertebrate egg-coat ZP genes were classified into eight ZP gene subfamilies. Lineage- and species-specific gene duplications and gene losses occurred frequently and represented the main causes of the patchy distribution of the eight ZP gene subfamilies in vertebrates. Thorough phylogenetic analyses revealed that the vertebrate ZP genes could be traced to three independent origins but were not orthologues of the characterized invertebrate ZP genes. Our results suggested that vertebrate egg-coat ZP genes should be classified into eight subfamilies, and a putative evolutionary map is proposed. These findings would aid the functional and evolutionary analyses of these reproductive genes in vertebrates.

Summary: Phylogenetic and synteny analyses indicate that the vertebrate zona pellucida (ZP) genes encoding egg coat proteins can be classified into eight subfamilies, and the evolutionary origins of these genes are discussed.

Gonadal Transcriptome Analysis of Pacific Abalone Haliotis discus discus: Identification of Genes Involved in Germ Cell Development

Little is known about the molecular mechanisms governing gonadal developmental processes in abalones. Here, we conducted transcriptome analysis of Pacific abalone Haliotis discus discus for gene discovery in the brain, ovary, testis, and unfertilized eggs. Among the annotated unigenes, 48.6% of unigenes were identified by Venn diagram analysis as having universal or tissue-specific expression. Twenty-three genes with gonad-biased gene ontology (GO) terms were first obtained. Secondly, 36 genes were found by screening known gene names related to germ cell development. Finally, 17 genes were obtained by querying the annotated unigene database for zygotically expressed gonadal genes (ovary and testis) and maternally expressed gonadal genes (ovary, testis, and unfertilized eggs) using keywords related to reproduction. To further verify tissue distribution pattern and subcellular localization of these genes, RT-PCR and in situ hybridization were performed using a unigene encoding a germ cell marker, vasa, as control. The results showed that vasa was expressed mainly in the early developmental stages of germ cells in both sexes. One of the candidate genes, vitelline envelope zona pellucida domain protein 12 (ZP12), was expressed in the primordial germ cells of immature gonad and early developmental stages of germ cells of the adult female. The results obtained from the present study suggest that vasa and ZP12 are involved in germ cell development of Pacific abalone and that ZP12 is an especially useful germ cell-specific marker in immature adults. The current gonadal transcriptome profile is an extensive resource for future reproductive molecular biology studies of this species.

Zona pellucida protein B2 messenger ribonucleic acid expression varies with follicular development and granulosa cell location

The freshly ovulated ovum in avian species is surrounded by a protein layer called the inner perivitelline layer (IPVL). The IPVL contains zona pellucida proteins and 6 distinct zona pellucida genes have been identified (ZPA, ZPB1, ZPB2, ZPC, ZPD and ZPX1) in the chicken. In the present research, the expression of the mRNA for ZPA, ZPB2, and ZPX1 was investigated in 2 lines of turkey hens selected for either increased egg production (E line) or increased body weight (F line). Theca and granulosa cell expression of the mRNA for ZPA and ZPB2 was also investigated in hierarchical and prehierarchical follicles from broiler breeder hens. Granulosa tissue was collected from F1 through F4 and F1 through F10 follicles in E line and F line hens, respectively. A one cm2 section of the granulosa layer around the germinal disc (GD) and an equivalent sized nongerminal disc (NGD) area was also collected from the F1 and F2 follicles from other hens from each genetic line. Granulosa and theca tissue was collected from hierarchical and prehierarchical follicles of broiler breeder hens. Total RNA was extracted from the samples. Minor groove-binding probes and primers for detecting ZPA, ZPB2, and ZPX1, were made for real-time PCR analyses. Expression of ZPA, ZPB2, and ZPX1 was detected in all follicle sizes from both genetic lines of hens. No significant differences in ZPA and ZPX1 mRNA expression were detected between the GD and NGD granulosa cells. However, the expression of the mRNA for ZPB2 was significantly greater in the GD granulosa cells when compared to the NGD granulosa cells in F1 and F2 follicles from E line and F line hens. In broiler breeder hens, the mRNA expression of ZPA and ZPB2 was greatest in the smallest prehierarchical follicles. The results suggest that higher expression of ZPB2 in the germinal disc area may be important for the preferential binding of sperm to this region of the IPVL.

Egg Coat Proteins Across Metazoan Evolution

All animal oocytes are surrounded by a glycoproteinaceous egg coat, a specialized extracellular matrix that serves both structural and species-specific roles during fertilization. Egg coat glycoproteins polymerize into the extracellular matrix of the egg coat using a conserved protein-protein interaction module-the zona pellucida (ZP) domain-common to both vertebrates and invertebrates, suggesting that the basic structural features of egg coats have been conserved across hundreds of millions of years of evolution. Egg coat proteins, as with other proteins involved in reproduction, are frequently found to be rapidly evolving. Given that gamete compatibility must be maintained for the fitness of sexually reproducing organisms, this finding is somewhat paradoxical and suggests a role for adaptive diversification in reproductive protein evolution. Here we review the structure and function of metazoan egg coat proteins, with an emphasis on the potential role their evolution has played in the creation and maintenance of species boundaries.

Bioinformatic analyses of zona pellucida genes in vertebrates and their expression in Nile tilapia

Zona pellucida (ZP) genes encode ZP glycoproteins which constitute the coat surrounding oocytes and early embryos. Genome-wide identification of ZP genes is still lacking in vertebrates, especially in fish species. Herein, we conducted bioinformatic analyses of the ZP genes of the Nile tilapia and other vertebrates. Totally 16, 9, 17, 27, 21, 20, 26, 19, 14,11, 24, 17, 9, 18, 8, 11, 9, 8, 5, and 4 ZP genes belonging to 5 subfamilies (ZPA, ZPB, ZPC, ZPD, and ZPAX) were found in the sea lamprey, elephant shark, coelacanth, spotted gar, zebrafish, medaka, stickleback, Nile tilapia, Amazon molly, platyfish, seahorse, Northern snakehead, cavefish, tetraodon, clawed frog, turtle, chicken, platypus, kangaroo rat, and human genomes, respectively. The expansion of ZP genes in basal vertebrates was mainly achieved by gene duplication of ZPB, ZPC, and ZPAX subfamilies, while the shrink of ZP gene number in viviparous mammals was achieved by keeping only one copy of the ZP genes in each subfamily or even secondary loss of some subfamilies. The number of ZP gene is related to the environment where the eggs are fertilized and the embryos develop in vertebrates. Transcriptomic analysis showed that 14 ZP genes were expressed in the ovary of Nile tilapia, while two (ZPB2b and ZPC2) were highly expressed in the liver. On the other hand, ZPB1a and ZPB2c were not found to be expressed in any tissue or at any developmental stage of the gonads examined. In the ovary, the expression of ZP genes started from 30 dah (days after hatching), significantly upregulated at 90 dah and maintained this level at 180 dah. The expression of ZPC2 in the liver and ZPC5-2 and ZPAX1 in the ovary was confirmed by in situ hybridization. The ovary- and liver-expressed ZP genes are expressed coordinately with oocyte growth in tilapia.

The Mouse Egg's Zona Pellucida

All mammalian eggs are surrounded by a highly specialized extracellular matrix (ECM), called the zona pellucida (ZP), that functions before, during, and after fertilization. Unlike somatic cell ECM the mouse ZP is composed of three different proteins, ZP1-3, that are synthesized and secreted by growing oocytes and assembled into long interconnected fibrils. ECM or vitelline envelope (VE) that surrounds fish, reptilian, amphibian, and avian eggs also consists of a limited number of proteins all closely related to ZP1-3. Messenger RNAs encoding ZP1-3 are expressed only by growing oocytes at very high levels from single-copy genes present on different chromosomes. Processing at the amino- and carboxy-termini of nascent ZP1-3 permits secretion of mature proteins into the extracellular space and assembly into fibrils and matrix. Structural features of nascent ZP proteins prevent assembly within secretory vesicles of growing oocytes. Homozygous knockout female mice that fail to synthesize either ZP2 or ZP3 are unable to construct a ZP, ovulate few if any eggs, and are infertile. ZP1-3 have a common structural feature, the ZP domain (ZPD), that has been conserved through 600 million years of evolution and is essential for ZP protein assembly into fibrils. The ZPD consists of two subdomains, each with four conserved cysteine residues present as two intramolecular disulfides, and resembles an immunoglobulin (Ig) domain found in a wide variety of proteins that have diverse functions, from receptors to mechanical transducers. ZP2 and ZP3 function as receptors for acrosome-reacted and acrosome-intact sperm, respectively, during fertilization of ovulated eggs, but are inactivated as sperm receptors as a result of fertilization.

Egg Envelope Glycoproteins ZP1 and ZP3 Mediate Sperm-Egg Interaction in the Japanese Quail

Fertilization is indispensable for zygotic formation leading to the birth of animals and the species-specific sperm-egg binding thought to be the initial step in this important process. In birds, the oocyte, which encounters the spermatozoa at the time of fertilization, is enclosed in a perivitelline membrane (pvm) constructed of several zona pellucida glycoproteins (ZP proteins: ZP1, ZP2, ZP3, ZP4 and ZPD). The aim of this study was to determine the ZP protein in the pvm responsible for sperm-pvm binding in Japanese quail. We tested the effects of anti-ZP protein antibodies on in vitro sperm perforation in the pvm. The results showed that the anti-ZP1 and ZP3 antibody significantly blocked hole formation by sperm, whereas anti-ZP2, ZP4 and ZPD as well as normal rabbit serum had no such effect. When the sperm acrosome reaction was inhibited in the presence of pertussis toxin, sperm-pvm binding was observed. This sperm-pvm binding was significantly prevented when the purified ZP1 or ZP3 was included in the reaction mixture. Moreover, both digoxigenin-labeled ZP1 and ZP3 were found to interact with the sperm head by immunocytochemical observation. Our results indicate that sperm binding to the pvm is, at least in part, mediated by the interaction of ZP1 and ZP3 with the sperm head during fertilization in Japanese quail.

Gonadal transcriptomics elucidate patterns of adaptive evolution within marine rockfishes (Sebastes)

Background

The genetic mechanisms of speciation and adaptation in the marine environment are not well understood. The rockfish genus Sebastes provides a unique model system for studying adaptive evolution because of the extensive diversity found within this group, which includes morphology, ecology, and a broad range of life spans. Examples of adaptive radiations within marine ecosystems are considered an anomaly due to the absence of geographical barriers and the presence of gene flow. Using marine rockfishes, we identified signatures of natural selection from transcriptomes developed from gonadal tissue of two rockfish species (Sebastes goodei and S. saxicola). We predicted orthologous transcript pairs, and estimated their distributions of nonsynonymous (Ka) and synonymous (Ks) substitution rates.

Results

We identified 144 genes out of 1079 orthologous pairs under positive selection, of which 11 are functionally annotated to reproduction based on gene ontologies (GOs). One orthologous pair of the zona pellucida gene family, which is known for its role in the selection of sperm by oocytes, out of ten was identified to be evolving under positive selection. In addition to our results in the protein coding-regions of transcripts, we found substitution rates in 3’ and 5’ UTRs to be significantly lower than Ks substitution rates implying negative selection in these regions.

Conclusions

We were able to identify a series of candidate genes that are useful for the assessment of the critical genes that diverged and are responsible for the radiation within this genus. Genes associated with longevity hold potential for understanding the molecular mechanisms that have contributed to the radiation within this genus.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1870-0) contains supplementary material, which is available to authorized users.

Proteomic characterization and evolutionary analyses of zona pellucida domain-containing proteins in the egg coat of the cephalochordate, Branchiostoma belcheri

Background

Zona pellucida domain-containing proteins (ZP proteins) have been identified as the principle constituents of the egg coat (EC) of diverse metazoan taxa, including jawed vertebrates, urochordates and molluscs that span hundreds of millions of years of evolutionary divergence. Although ZP proteins generally contain the zona pellucida (ZP) structural modules to fulfill sperm recognition and EC polymerization functions during fertilization, the primary sequences of the ZP proteins from the above-mentioned animal classes are drastically different, which makes it difficult to assess the evolutionary relationships of ZP proteins. To understand the origin of vertebrate ZP proteins, we characterized the egg coat components of Branchiostoma belcheri, an invertebrate species that belongs to the chordate subphylum Cephalochordata.

Results

Five ZP proteins (BbZP1-5) were identified by mass spectrometry analyses using the egg coat extracts from both unfertilized and fertilized eggs. In addition to the C-terminal ZP module in each of the BbZPs, the majority contain a low-density lipoprotein receptor domain and a von Willebrand factor type A (vWFA) domain, but none possess an EGF-like domain that is frequently observed in the ZP proteins of urochordates. Fluorescence in situ hybridization and immuno-histochemical analyses of B. belcheri ovaries showed that the five BbZPs are synthesized predominantly in developing eggs and deposited around the extracellular space of the egg, which indicates that they are bona fide egg coat ZP proteins. BbZP1, BbZP3 and BbZP4 are significantly more abundant than BbZP2 and BbZP5 in terms of gene expression levels and the amount of mature proteins present on the egg coats. The major ZP proteins showed high polymorphism because multiple variants are present with different molecular weights. Sequence comparison and phylogenetic analysis between the ZP proteins from cephalochordates, urochordates and vertebrates showed that BbZP1-5 form a monophyletic group and share no significant sequence similarities with the ZP proteins of urochordates and the ZP3 subtype of jawed vertebrates. By contrast, small regions of homology were identifiable between the BbZP and ZP proteins of the non-jawed vertebrate, the sea lamprey Petromyzon marinus. The lamprey ZP proteins were highly similar to the ZP1 and ZP2 subtypes of the jawed vertebrates, which suggests that the ZP proteins of basal chordates most likely shared a recent common ancestor with vertebrate ZP1/2 subtypes and lamprey ZP proteins.

Conclusions

The results document the spectra of zona pellucida domain-containing proteins of the egg coat of basal chordates. Particularly, the study provides solid evidence for an invertebrate origin of vertebrate ZP proteins and indicates that there are diverse domain architectures in ZP proteins of various metazoan groups.

Evolution of genes involved in gamete interaction: evidence for positive selection, duplications and losses in vertebrates

Genes encoding proteins involved in sperm-egg interaction and fertilization exhibit a particularly fast evolution and may participate in prezygotic species isolation [1], [2]. Some of them (ZP3, ADAM1, ADAM2, ACR and CD9) have individually been shown to evolve under positive selection [3], [4], suggesting a role of positive Darwinian selection on sperm-egg interaction. However, the genes involved in this biological function have not been systematically and exhaustively studied with an evolutionary perspective, in particular across vertebrates with internal and external fertilization. Here we show that 33 genes among the 69 that have been experimentally shown to be involved in fertilization in at least one taxon in vertebrates are under positive selection. Moreover, we identified 17 pseudogenes and 39 genes that have at least one duplicate in one species. For 15 genes, we found neither positive selection, nor gene copies or pseudogenes. Genes of teleosts, especially genes involved in sperm-oolemma fusion, appear to be more frequently under positive selection than genes of birds and eutherians. In contrast, pseudogenization, gene loss and gene gain are more frequent in eutherians. Thus, each of the 19 studied vertebrate species exhibits a unique signature characterized by gene gain and loss, as well as position of amino acids under positive selection. Reflecting these clade-specific signatures, teleosts and eutherian mammals are recovered as clades in a parsimony analysis. Interestingly the same analysis places Xenopus apart from teleosts, with which it shares the primitive external fertilization, and locates it along with amniotes (which share internal fertilization), suggesting that external or internal environmental conditions of germ cell interaction may not be the unique factors that drive the evolution of fertilization genes. Our work should improve our understanding of the fertilization process and on the establishment of reproductive barriers, for example by offering new leads for experiments on genes identified as positively selected.

Expression of zona pellucida B proteins in juvenile rare minnow (Gobiocypris rarus) exposed to 17α-ethinylestradiol, 4-nonylphenol and bisphenol A

Zona pellucida (ZP) containing proteins are glycoproteins in teleost chorion and are encoded by several gene subfamilies, mainly including ZPA, ZPB, ZPC and ZPX genes. In teleost species, ZP genes are expressed either in liver under regulation of estrogen or in ovary. In the present study, five ZP gene isoforms were isolated and characterized in Gobiocypris rarus. The putative amino acid sequences of these ZP gene isoforms contain the typical trefoil motif and a ZP domain. These five G. rarus ZP gene isoforms were named as grZPB.1, grZPB.2, grZPB.3, grZPB.4 and grZPB.5. Real-time quantitative reverse transcription PCR (RT-qPCR) analysis indicated that all these ZP mRNA isoforms were exclusively expressed in ovary. G. rarus juveniles at the age of 21 days postfertilization were exposed to 17α-ethinylestradiol (EE2; 0.01, 0.1 and 1 nM), 4-nonylphenol (4-NP; 10, 100 and 1000 nM) or bisphenol A (BPA; 0.1, 1 and 10nM) for 3 days. mRNA expressions of ZPB isoforms following the exposure to xenoestrogen were detected by RT-qPCR. Data were analyzed by the 2(-△△Cq) method. The results indicate that induction by 0.1-1nM EE2 on mRNA expression of the grZPB isoforms is weaker than for vitellogenin. 4-NP exposures at three concentrations had differential effects on the grZPBs. BPA at three concentrations weakly induced mRNA expression of the grZPB isoforms.

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.

Oocytic expression of zona pellucida protein ZP4 in Japanese quail (Coturnix japonica)

The avian perivitelline layer, an extracellular matrix homologous to the zona pellucida (ZP) of mammalian oocytes, is composed mainly by zona pellucida gene family glycoproteins. Our previous studies in Japanese quail have demonstrated that the matrix's components, ZP3 and ZPD, are synthesized in ovarian granulosa cells. Another component, ZP1, is synthesized in the liver. Recently, we demonstrated that another minor constituent, ZP2 is produced in the oocytes of the immature follicles. In the present study, we report the isolation of complementary DNA encoding quail ZP4 and its expression and origin in the female birds. By ribonuclease protection assay and in situ hybridization, we demonstrated that ZP4 transcripts were transcribed in the oocytes of small white follicles. The expression level of ZP4 decreased dramatically during follicular development, and the highest expression was observed in the small white follicles. Western blot analysis using the specific antibody against ZP4 indicated that the immunoreactive 58.2 kDa protein was present in the lysates of the small white follicles. These results demonstrate for the first time that the avian ZP4 is expressed in the oocyte, and that the expression pattern of the gene is similar to that of ZP2.

Acrosome reaction: relevance of zona pellucida glycoproteins

During mammalian fertilisation, the zona pellucida (ZP) matrix surrounding the oocyte is responsible for the binding of the spermatozoa to the oocyte and induction of the acrosome reaction (AR) in the ZP-bound spermatozoon. The AR is crucial for the penetration of the ZP matrix by spermatozoa. The ZP matrix in mice is composed of three glycoproteins designated ZP1, ZP2 and ZP3, whereas in humans, it is composed of four (ZP1, ZP2, ZP3 and ZP4). ZP3 acts as the putative primary sperm receptor and is responsible for AR induction in mice, whereas in humans (in addition to ZP3), ZP1 and ZP4 also induce the AR. The ability of ZP3 to induce the AR resides in its C-terminal fragment. O-linked glycans are critical for the murine ZP3-mediated AR. However, N-linked glycans of human ZP1, ZP3 and ZP4 have important roles in the induction of the AR. Studies with pharmacological inhibitors showed that the ZP3-induced AR involves the activation of the G(i)-coupled receptor pathway, whereas ZP1- and ZP4-mediated ARs are independent of this pathway. The ZP3-induced AR involves the activation of T-type voltage-operated calcium channels (VOCCs), whereas ZP1- and ZP4-induced ARs involve both T- and L-type VOCCs. To conclude, in mice, ZP3 is primarily responsible for the binding of capacitated spermatozoa to the ZP matrix and induction of the AR, whereas in humans (in addition to ZP3), ZP1 and ZP4 also participate in these stages of fertilisation.

Evolution of the teleostean zona pellucida gene inferred from the egg envelope protein genes of the Japanese eel, Anguilla japonica

A fish egg envelope is composed of several glycoproteins, called zona pellucida (ZP) proteins, which are conserved among vertebrate species. Euteleost fishes synthesize ZP proteins in the liver, while otocephalans synthesize them in the growing oocyte. We investigated ZP proteins of the Japanese eel, Anguilla japonica, belonging to Elopomorpha, which diverged earlier than Euteleostei and Otocephala. Five major components of the egg envelope were purified and their partial amino acid sequences were determined by sequencing. cDNA cloning revealed that the eel egg envelope was composed of four ZPC homologues and one ZPB homologue. Four of the five eel ZP (eZP) proteins possessed a transmembrane domain, which is not found in the ZP proteins of Euteleostei and Otocephala that diverged later, but is found in most other vertebrate ZP proteins. This result suggests that fish ZP proteins originally possessed a transmembrane domain and lost it during evolution. Northern blotting and RT-PCR revealed that all of the eZP transcripts were present in the ovary, but not in the liver. Phylogenetic analyses of fish zp genes showed that ezps formed a group with other fish zp genes that are expressed in the ovary, and which are distinct from the group of genes expressed in the liver. Our results support the hypothesis that fish ZP proteins were originally synthesized in the ovary, and then the site of synthesis was switched to the liver during the evolutionary pathway to Euteleostei.

ZP domain proteins in the abalone egg coat include a paralog of VERL under positive selection that binds lysin and 18-kDa sperm proteins

Identifying fertilization molecules is key to our understanding of reproductive biology, yet only a few examples of interacting sperm and egg proteins are known. One of the best characterized comes from the invertebrate archeogastropod abalone (Haliotis spp.), where sperm lysin mediates passage through the protective egg vitelline envelope (VE) by binding to the VE protein vitelline envelope receptor for lysin (VERL). Rapid adaptive divergence of abalone lysin and VERL are an example of positive selection on interacting fertilization proteins contributing to reproductive isolation. Previously, we characterized a subset of the abalone VE proteins that share a structural feature, the zona pellucida (ZP) domain, which is common to VERL and the egg envelopes of vertebrates. Here, we use additional expressed sequence tag sequencing and shotgun proteomics to characterize this family of proteins in the abalone egg VE. We expand 3-fold the number of known ZP domain proteins present within the VE (now 30 in total) and identify a paralog of VERL (vitelline envelope zona pellucida domain protein [VEZP] 14) that contains a putative lysin-binding motif. We find that, like VERL, the divergence of VEZP14 among abalone species is driven by positive selection on the lysin-binding motif alone and that these paralogous egg VE proteins bind a similar set of sperm proteins including a rapidly evolving 18-kDa paralog of lysin, which may mediate sperm-egg fusion. This work identifies an egg coat paralog of VERL under positive selection and the candidate sperm proteins with which it may interact during abalone fertilization.

Zona pellucida protein ZP2 is expressed in the oocyte of Japanese quail (Coturnix japonica)

The avian perivitelline layer (PL), a vestment homologous to the zona pellucida (ZP) of mammalian oocytes, is composed of at least three glycoproteins. Our previous studies have demonstrated that the matrix's components, ZP3 and ZPD, are synthesized in ovarian granulosa cells. Another component, ZP1, is synthesized in the liver and is transported to the ovary by blood circulation. In this study, we report the isolation of cDNA encoding quailZP2and its expression in the female bird. By RNase protection assay andin situhybridization, we demonstrate that ZP2 transcripts are restricted to the oocytes of small white follicles (SWF). The expression level ofZP2decreased dramatically during follicular development, and the highest expression was observed in the SWF. Western blot and immunohistochemical analyses using the specific antibody against ZP2 indicate that the 80 kDa protein is the authentic ZP2, and the immunoreactive ZP2 protein is also present in the oocytes. Moreover, ultrastructural analysis demonstrated that the immunoreactive ZP2 localizes to the zona radiata, the perivitelline space, and the oocyte cytoplasm in the SWF. By means of western blot analysis and immunofluorescence microscopy, we detected a possible interaction of the recombinant ZP2 with ZP3 and that this interaction might lead to the formation of amorphous structure on the cell surface. These results demonstrate for the first time that the avian ZP gene is expressed in the oocyte, and that the ZP2 protein in the oocyte might play a role for the PL formation in the immature follicles of the ovary.

Receptor-mediated mechanisms in ovarian follicle and oocyte development

The normal development of the chicken oocyte within the ovarian follicle depends on the coordinated expression and function of several members of the low density lipoprotein receptor gene family. The human low density lipoprotein receptor (LDLR) is the prototype of the gene family; since its discovery and the elucidation of the medical significance of mutations in the ldlr gene, many additional family members have been discovered and characterized, and some important advances have resulted from studies in the chicken. I describe the analogies as well as the differences that exist between the molecular genetics of the mammalian and avian members of this important gene family, with emphasis on receptor-mediated oocyte growth. Recent progress in the molecular characterization of the chicken genes whose products mediate oocyte growth, follicle development, and accessory pathways is described in detail, and emerging information of preliminary nature is included. As the availability of chicken genome sequence data has enhanced the rate of progress in the field, our understanding of the physiological roles of members of this receptor family in general has already gained from studies in the avian model system.

Testicular descent, sperm maturation and capacitation. Lessons from our most distant relatives, the monotremes

The present review examines whether monotremes may help to resolve three questions relating to sperm production in mammals: why the testes descend into a scrotum in most mammals, why spermatozoa are infertile when they leave the testes and require a period of maturation in the specific milieu provided by the epididymides, and why ejaculated spermatozoa cannot immediately fertilise an ovum until they undergo capacitation within the female reproductive tract. Comparisons of monotremes with other mammals indicate that there is a need for considerable work on monotremes. It is hypothesised that testicular descent should be related to epididymal differentiation. Spermatozoa and ova from both groups share many of the proteins that are thought to be involved in gamete interaction, and although epididymal sperm maturation is significant it is probably less complex in monotremes than in other mammals. However, the monotreme epididymis is unique in forming spermatozoa into bundles of 100 with greatly enhanced motility compared with individual spermatozoa. Bundle formation involves a highly organised interaction with epididymal proteins, and the bundles persist during incubation in vitro, except in specialised medium, in which spermatozoa separate after 2–3 h incubation. It is suggested that this represents an early form of capacitation.

Molecular characterization of egg envelope glycoprotein ZPD in the ovary of Japanese quail (Coturnix japonica)

The egg envelope surrounding avian oocytes exhibits a three-dimensional network of coarse fibers between the granulosa cells and the oocyte. Our previous studies have demonstrated that one of the matrix's components, ZP3, is synthesized in the ovarian granulosa cells. Another component, ZP1, which is critically involved in triggering the sperm acrosome reaction, is synthesized in the liver. We have previously isolated cDNAs encoding quail ZP3 and ZP1, and we now report the isolation of cDNA encoding quail ZPD. By RNase protection assay and in situ hybridization, we have demonstrated that ZPD transcripts are restricted to the granulosa cells of preovulatory follicles. The expression level of ZPD increased progressively during follicular development, and the highest expression was observed in the largest follicles. Western blot analyses using the specific antibody against ZPD indicate that the 40 kDa protein is the authentic ZPD, and the contents of ZPD protein also increased during follicular development. Moreover, we found that the addition of FSH to the culture media enhances the ZPD secretion in the cultured granulosa cells. Two-dimensional gel electrophoresis revealed the presence of several ZPD isoforms with different pI values ranging from 5.5 to 7. Immunohistochemical analyses indicate that the materials recognized with anti-quail ZPD antibody were accumulated in the egg envelope of large yellow follicles. These results demonstrate the presence of ZPD protein in the egg envelope, and that the amount of ZPD in the egg envelope as well as the mRNA in the cells increases at the latter stages of folliculogenesis.

Proteomic analysis of the chicken egg vitelline membrane

The avian vitelline membrane (VM) is a multilayered proteinaceous structure separating egg white from yolk. The innermost layer of the VM, deposited onto the oocyte plasma membrane in the ovary, corresponds to the mammalian zona pellucida (ZP). The outer layer is produced in the infundibulum, the first section of the oviduct. Using high-throughput, high-end LC-MS(n) 137 proteins were identified, only 13 of which were known previously to be components of the VM. Depending on the washing protocol, two largely overlapping, but not identical, sets of identified proteins were produced from water-washed and salt-washed VMs. Most of the components of the VM were known previously from other egg compartments, such as, for instance, the egg white proteins lysozyme C, ovalbumin, ovotransferrin, and ovomucin. Specific components of the VM not identified previously in other egg compartments included eight ZP proteins, oviductin protease, and two ATPases. The vitelline outer membrane protein (VMO) VMO II was identified as beta-defensin-11. The list of VM proteins presented in this report is by far the most comprehensive dataset available at present and complements proteomic analyses of chicken egg compartments published previously.

The mRNA for zona pellucida proteins B1, C and D in two genetic lines of turkey hens that differ in fertility

The avian inner perivitelline layer (IPVL) contains zona pellucida protein-B1 (ZPB1), zona pellucida protein-C (ZPC) and zona pellucida protein-D (ZPD). These three proteins may be involved in sperm binding to the IPVL. ZPB1 is produced by the liver and transported to the developing preovulatory follicle, while ZPC and ZPD are synthesized and secreted by the granulosa cells of the preovulatory follicle. The mRNA of ZPB1, ZPC, and ZPD was investigated in two lines of turkey hens selected for over 40 generations for either increased egg production (E line) or increased body weight (F line). Total RNA was extracted from the liver and from 1cm(2) sections of the granulosa layer around the germinal disc and a nongerminal disc area of the F(1) and F(2) follicles of hens from each genetic line. Northern analysis was performed using chicken cDNA probes for all three ZP proteins. Hepatic mRNA for ZPB1 was greater (P<0.05) in turkey hens from the E line than the F line. Although, there was no difference in ZPC mRNA between the germinal disc and nongerminal disc region of the two largest follicles in E line hens, ZPC mRNA was greater in the nongerminal disc region compared to the germinal disc region in the two largest follicles obtained from the F line hens. There were no differences in ZPD mRNA between the germinal disc and nongerminal disc regions of the F(1) and F(2) follicles for either genetic line. The results suggest that the greater rates of fertility previously observed in eggs from the E line hens compared with the F line of hens may be related to differential amounts of the potential sperm binding proteins ZPB1 and ZPC.

Phylogenetic analysis and identification of pseudogenes reveal a progressive loss of zona pellucida genes during evolution of vertebrates

Vertebrate eggs are surrounded by an extracellular matrix with similar functions and conserved individual components: the zona pellucida (ZP) glycoproteins. In mammals, chickens, frogs, and some fish species, we established an updated list of the ZP genes, studied the relationships within the ZP gene family using phylogenetic analysis, and identified ZP pseudogenes. Our study confirmed the classification of ZP genes in six subfamilies: ZPA/ZP2, ZPB/ZP4, ZPC/ZP3, ZP1, ZPAX, and ZPD. The identification of a Zpb pseudogene in the mouse genome, Zp1 pseudogenes in the dog and bovine genomes, and Zpax pseudogenes in the human, chimpanzee, macaque, and bovine genomes showed that the evolution of ZP genes mainly occurs by death of genes. Our study revealed that the extracellular matrix surrounding vertebrate eggs contains three to at least six ZP glycoproteins. Mammals can be classified in three categories. In the mouse, the ZP is composed of three ZP proteins (ZPA/ZP2, ZPC/ZP3, and ZP1). In dog, cattle and, putatively, pig, cat, and rabbit, the zona is composed of three ZP proteins (ZPA/ZP2, ZPB/ZP4, and ZPC/ZP3). In human, chimpanzee, macaque, and rat, the ZP is composed of four ZP proteins (ZPA/ZP2, ZPB/ZP4, ZPC/ZP3, and ZP1). Our review provides new directions to investigate the molecular basis of sperm-egg recognition, a mechanism which is not yet elucidated.

ZP genes in avian species illustrate the dynamic evolution of the vertebrate egg envelope

The vertebrate egg envelope is composed of a set of related proteins, encoded by the ZP genes. The apparent simplicity of the egg envelope is in contrast to the number of ZP genes identified by conventional cloning and data mining of genome sequences from a number of vertebrates. The vertebrate ZP genes fall into five classes, ZP1, ZP2, ZP3, ZPD and ZPAX. Analysis of chicken genome and EST sequence data has revealed the presence of seven distinct ZP genes, falling into these classes that are expressed in the female reproductive system. Comparison with the repertoire of ZP genes in other vertebrates suggests a major source of diversity in the composition of the egg envelope is a continual process of amplification, diversification and attrition of ZP gene sequences.

Positive selection and gene conversion in SPP120, a fertilization-related gene, during the East African cichlid fish radiation

The ability to infer historical natural selection from sequence data aides in finding genes that might be important in adaptation and the formation of new species. As the fastest evolving and largest known vertebrate radiation, the cichlid fish of the African Great Lakes exhibit a wide range of recent morphological diversification. We used DNA databases, mostly of expressed sequence tags, to find candidate orthologous coding sequences from 2 tribes of cichlids and, using an automated procedure, scanned these sequence pairs for high dN/dS, the signal of positive selection and protein adaptation. The results included vertebrate genes commonly found to be under selection (e.g., major histocompatibility complex [MHC] loci) as well as genes known to be important specifically in the cichlid radiation (e.g., long-wave-sensitive opsins). Further investigation focused on a gene encoding a fertilization-related protein, SPP120, which was previously known only from cichlids. Using maximum likelihood analysis on novel SPP120 cDNA sequences from a range of African cichlids, we demonstrate the influence of positive selection in a specific subregion of the protein. We also show that SPP120 is a tandemly arranged, multicopy gene evolving with occasional interlocus gene conversion. A search of the Medaka genome database also revealed a tandem arrangement of multiple SPP120 copies and evolutionary rate differences between Medaka gene subregions mirroring those found for cichlids. Combined, these results suggest that SPP120 has been under repeated diversifying selection for over 100 Myr.

Low density lipoprotein receptor relatives in chicken ovarian follicle and oocyte development

The normal development of the chicken oocyte within the ovarian follicle depends on the coordinated expression and function of several members of the low density lipoprotein receptor gene family. The human low density lipoprotein receptor is the prototype of the gene family; since its discovery and the elucidation of the medical significance of mutations in the LDLR gene, many additional family members have been discovered and characterized, and some important advances have resulted from studies in the chicken. I describe the analogies as well as the differences that exist between the molecular genetics of the mammalian and avian members of this important gene family, with emphasis on receptor-mediated oocyte growth. Recent progress in the molecular characterization of the chicken genes whose products mediate oocyte growth, follicle development, and accessory pathways is described in detail, and emerging information of preliminary nature is included. As the availability of chicken genome sequence data has enhanced the rate of progress in the field, our understanding of the physiological roles of members of this receptor family in general has already gained from studies in the avian model system.

Loss of zona pellucida binding proteins in the acrosomal matrix disrupts acrosome biogenesis and sperm morphogenesis

Zona pellucida binding protein 1 (ZPBP1), a spermatid and spermatozoon protein that localizes to the acrosome, was originally identified in pigs and named for its binding to the oocyte zona pellucida. In an in silico search for germ cell-specific genes, Zpbp1 and its novel paralog, Zpbp2, were discovered and confirmed to be expressed only in the testes in both mice and humans. To study the in vivo functions of both ZPBP proteins, we disrupted Zpbp1 and Zpbp2 in mice. Males lacking ZPBP1 were sterile, with abnormal round-headed sperm morphology and no forward sperm motility. Ultrastructural studies demonstrated that absence of ZPBP1 prevents proper acrosome compaction, resulting in acrosome fragmentation and disruption of the Sertoli-spermatid junctions. Males null for ZPBP2 were subfertile, demonstrated aberrant acrosomal membrane invaginations, and produced dysmorphic sperm with reduced ability to penetrate zona pellucida. Molecular phylogenetic analysis of ZPBPs from amphibians, birds, and mammals suggests that these paralogous genes coevolved to play cooperative roles during spermiogenesis. Whereas ZPBP1 was discovered for an in vitro role in sperm-egg interactions, we have shown that both ZPBP proteins play an earlier structural role during spermiogenesis.

Isolated ZP-N domains constitute the N-terminal extensions of Zona Pellucida proteins

Zona Pellucida (ZP) domains have been found in a wide variety of extracellular proteins, in which they play essential role for polymerization. They are shared by the ZP proteins, which constitute the extracellular coat of animal eggs. Except from ZP3, constituting the primary sperm receptor, the ZP proteins possess, in addition to their C-terminal ZP domains, N-terminal extensions, which are thought to play an important role in the species-specific gamete recognition. Here, we show that these extensions are made of single or multiple copies of a small globular domain, which can be significantly related to the N-terminal region of ZP domains (ZP-N domains). This finding brings new insights into the molecular evolution of ZP proteins, which may have evolved around a common ZP-N architecture, and more generally into the noticeable sequence diversity of ZP-N domains, which can be found as isolated subunits or tightly associated with ZP-C domains to form complete, canonical ZP domains.

Induction of sperm acrosome reaction by perivitelline membrane glycoprotein ZP1 in Japanese quail (Coturnix japonica)

The extracellular matrix surrounding avian oocytes, called the perivitelline membrane (PL), consists of at least two major glycoproteins, ZP3 and ZP1. Our previous study using Japanese quail had demonstrated that the PL obtained from the preovulatory follicles was incubatedin vitrowith spermatozoa, and perforations were observed. This result indicated that the PL might contain a constituent that possesses activity to initiate the acrosome reaction (AR) in quail. In order to elaborate upon our previous findings, we evaluated the effects of ZP3 and ZP1 on the induction of sperm AR in Japanese quail. Ejaculated sperm were incubated with or without the purified PL glycoprotein, and their acrosome status was determined based on the presence or absence of the acrosome. Treatment of spermatozoa with increasing doses of the purified monomeric ZP1 led to a concentration-dependent stimulation of AR. The purified dimeric ZP1 had similar effect. Moreover, we found that the ZP1-induced AR was significantly blocked by the digestion of the PL protein with PNGaseF. In contrast, the addition of purified ZP3 failed to induce AR at any doses tested. These results indicate thatN-linked glycans on ZP1 play an important role in triggering the AR in Japanese quail.

Zona Pellucida Domain of ZPB1 controls specific binding of ZPB1 and ZPC in Japanese quail (Coturnix japonica)

The extracellular matrix surrounding avian oocytes, referred to as the perivitelline membrane (PL), exhibits a three-dimensional network of fibrils between granulosa cells and the oocyte. We previously reported that one of its components, ZPC, is synthesized in granulosa cells that are specifically incorporated into the PL; this incorporation might be mediated by a specific interaction with ZPB1, another PL constituent, which is synthesized in the liver. In order to extend our previous findings, we established an expression system for quail ZPB1 using a mammalian cell line, and several ZPB1 mutants lacking the zona pellucida (ZP) domain or the glutamine-rich repeat region were produced. Western blot analysis of the immunoprecipitated materials with anti-ZPC antiserum indicated that ZPB1 was coimmunoprecipitated with the antiserum in the presence of ZPC. Ligand blotting also revealed the specific binding of ZPC and ZPB1 and indicated that the binding of these two components might be mediated via an ionic interaction. An analysis using recombinant ZPB1 demonstrated that the ZPB1 lacking the ZP domain did not bind to ZPC, whereas the mutant missing the glutamine-rich repeat region retained its capacity for binding. Furthermore, although the ZPB1 lacking the N-terminal half of the ZP domain was able to bind to ZPC, the deletion of the C-terminal half completely abolished ZPB1 binding to ZPC. These results suggested that the C-terminal half of the ZP domain of ZPB1 contains a binding site for ZPC, and that it appears to be involved in insoluble PL fibril formation in the quail ovary.

Rapidly evolving zona pellucida domain proteins are a major component of the vitelline envelope of abalone eggs

Proteins harboring a zona pellucida (ZP) domain are prominent components of vertebrate egg coats. Although less well characterized, the egg coat of the non-vertebrate marine gastropod abalone (Haliotis spp.) is also known to contain a ZP domain protein, raising the possibility of a common molecular basis of metazoan egg coat structures. Egg coat proteins from vertebrate as well as non-vertebrate taxa have been shown to evolve under positive selection. Studied most extensively in the abalone system, coevolution between adaptively diverging egg coat and sperm proteins may contribute to the rapid development of reproductive isolation. Thus, identifying the pattern of evolution among egg coat proteins is important in understanding the role these genes may play in the speciation process. The purpose of the present study is to characterize the constituent proteins of the egg coat [vitelline envelope (VE)] of abalone eggs and to provide preliminary evidence regarding how selection has acted on VE proteins during abalone evolution. A proteomic approach is used to match tandem mass spectra of peptides from purified VE proteins with abalone ovary EST sequences, identifying 9 of 10 ZP domain proteins as components of the VE. Maximum likelihood models of codon evolution suggest positive selection has acted among a subset of amino acids for 6 of these genes. This work provides further evidence of the prominence of ZP proteins as constituents of the egg coat, as well as the prominent role of positive selection in diversification of these reproductive proteins.