Cross-fertilization and structural comparison of egg extracellular matrix glycoproteins from Xenopus laevis and Xenopus tropicalis

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.

Xenopus Hybrids Provide Insight Into Cell and Organism Size Control

Determining how size is controlled is a fundamental question in biology that is poorly understood at the organismal, cellular, and subcellular levels. The Xenopus species, X. laevis and X. tropicalis differ in size at all three of these levels. Despite these differences, fertilization of X. laevis eggs with X. tropicalis sperm gives rise to viable hybrid animals that are intermediate in size. We observed that although hybrid and X. laevis embryogenesis initiates from the same sized zygote and proceeds synchronously through development, hybrid animals were smaller by the tailbud stage, and a change in the ratio of nuclear size to cell size was observed shortly after zygotic genome activation (ZGA), suggesting that differential gene expression contributes to size differences. Transcriptome analysis at the onset of ZGA identified twelve transcription factors paternally expressed in hybrids. A screen of these X. tropicalis factors by expression in X. laevis embryos revealed that Hes7 and Ventx2 significantly reduced X. laevis body length size by the tailbud stage, although nuclear to cell size scaling relationships were not affected as in the hybrid. Together, these results suggest that transcriptional regulation contributes to biological size control in Xenopus.

Maternal mRNA knock-down studies: antisense experiments using the host-transfer technique in Xenopus laevis and Xenopus tropicalis

The ability to inhibit the activity of maternally stored gene products in Xenopus has led to numerous insights into early developmental mechanisms. Oocytes can be cultured and manipulated in vitro and then implanted into the body cavity of a host female to make them competent for fertilization. Here, we summarize the methods for obtaining, culturing, and fertilizing Xenopus oocytes, with the goal of inhibiting maternal gene function through antisense oligonucleotide-mediated mRNA knock-down. We describe a simplified technique for implanting donor oocytes into host females using intraperitoneal injection. Also, we present optimized methods for performing the host-transfer procedure with Xenopus tropicalis oocytes.

Deficient induction response in a Xenopus nucleocytoplasmic hybrid

Defects in induction signaling and response underlie the nucleocytoplasmic incompatibility between two evolutionarily distant frog species, while specific treatments partially restore this response in explants and whole embryos.

Incompatibilities between the nucleus and the cytoplasm of sufficiently distant species result in developmental arrest of hybrid and nucleocytoplasmic hybrid (cybrid) embryos. Several hypotheses have been proposed to explain their lethality, including problems in embryonic genome activation (EGA) and/or nucleo-mitochondrial interactions. However, conclusive identification of the causes underlying developmental defects of cybrid embryos is still lacking. We show here that while over 80% of both Xenopus laevis and Xenopus (Silurana) tropicalis same-species androgenetic haploids develop to the swimming tadpole stage, the androgenetic cybrids formed by the combination of X. laevis egg cytoplasm and X. tropicalis sperm nucleus invariably fail to gastrulate properly and never reach the swimming tadpole stage. In spite of this arrest, these cybrids show quantitatively normal EGA and energy levels at the stage where their initial gastrulation defects are manifested. The nucleocytoplasmic incompatibility between these two species instead results from a combination of factors, including a reduced emission of induction signal from the vegetal half, a decreased sensitivity of animal cells to induction signals, and differences in a key embryonic protein (Xbra) concentration between the two species, together leading to inefficient induction and defective convergence-extension during gastrulation. Indeed, increased exposure to induction signals and/or Xbra signalling partially rescues the induction response in animal explants and whole cybrid embryos. Altogether, our study demonstrates that the egg cytoplasm of one species may not support the development promoted by the nucleus of another species, even if this nucleus does not interfere with the cytoplasmic/maternal functions of the egg, while the egg cytoplasm is also capable of activating the genome of that nucleus. Instead, our results provide evidence that inefficient signalling and differences in the concentrations of key proteins between species lead to developmental defects in cybrids. Finally, they show that the incompatibilities of cybrids can be corrected by appropriate treatments.

When two species evolve separately for several million years, their respective genomes accumulate many small changes that together are responsible for the differences in their characters. Some of these affect the way eggs are prepared inside the germline, and/or how embryos develop, such that the egg cytoplasm of a given species can only support development promoted by its own genome or nucleus. Thus, developmental incompatibility arises between the cytoplasm and the nucleus of distant species during evolution and we don't know its mechanism. We have studied this phenomenon in an advantageous system using two evolutionarily distant frog species (Xenopus laevis and Xenopus tropicalis). We found that hybrid frog embryos with X. laevis cytoplasm and X. tropicalis nuclei are always defective in an important process that is necessary to generate morphogenetic cell movements during development. Through a series of experiments in which we dissect out and/or recombine parts of such hybrid embryos and observe their behaviour in culture, we show that this phenomenon occurs because of malfunctions in the signalling cascade that is responsible for generating these cell movements. Thus, we postulate that inefficient molecular signalling contributes to the death of such hybrids.

Sperm acrosin is responsible for the sperm binding to the egg envelope during fertilization in Japanese quail (Coturnix japonica)

An antibody library against quail sperm plasma membrane components was established and a mAb, which strongly inhibits sperm perforations of the perivitelline membrane (PVM) was obtained from the library. The antigen molecule of the mAb showed an apparent molecular weight of 45 kDa, and was distributed both on the surface and in the acrosomal matrix of the sperm head. Periodate oxidation revealed that the epitope of the antigen includes a sugar moiety. Tandem mass spectrometry analysis of the antigen revealed that the mAb recognizes sperm acrosin. When sodium dodecyl sulfate-solubilized PVM immobilized on a polyvinylidene difluoride membrane was incubated with sperm plasma membrane lysates, the sperm acrosin was detected on the PVM immobilized on the membrane, indicating that the sperm acrosin interacts with the components of PVM. Indeed, the mAb effectively inhibited the binding of acrosome-intact sperm to the PVM. These results indicate that the 45 kDa sperm acrosin is involved in the binding of sperm to the PVM in fertilization of Japanese quail.

Swimming of Xenopus laevis sperm exhibits multiple gears and its duration is extended by egg jelly constituents

The motility of Xenopus sperm is initiated by the osmotic shock experienced when these cells are ejaculated into low-salinity pond water. Motility is brief and is required for the sperm to penetrate the jelly layers and fertilize the egg. In this study we demonstrate that extracts of egg jelly contain factors that extend the period of sperm motility as well as providing a chemoattractant activity as previously reported. Both activities are partially dependent on extracellular calcium. Time-lapse and video microscopy show that after activation of motility the number of motile sperm decreases rapidly, with a half-time of about 2 min. Addition of 10% v/v egg jelly extract ("egg water") increased the number of motile sperm 2-fold over controls at 20 s and about 4- to 10-fold over controls at 10 min after initiation of motility. Extension of motility lifetime was not mediated by a nonspecific protein or by allurin, the egg-water protein that has chemoattractant activity. The helical path of Xenopus sperm exhibited tight coupling between rotational and forward velocities in egg jelly, but coupling changed rapidly from moment to moment in low-salinity buffer. Our observations suggest that jelly-derived factors regulate both the longevity and directionality of sperm propulsion.

Structure determination by MALDI-IRMPD mass spectrometry and exoglycosidase digestions of O-linked oligosaccharides from Xenopus borealis egg jelly

Differences in the fertilization behavior of Xenopus borealis from X. laevis and X. tropicalis suggest differences in the glycosylation of the egg jellies. To test this assumption, O-linked glycans were chemically released from the egg jelly coat glycoproteins of X. borealis. Over 50 major neutral glycans were observed, and no anionic glycans were detected from the released O-glycan pool. Preliminary structures of ∼30 neutral oligosaccharides were determined using matrix-assisted laser desorption/ionization (MALDI) infrared multiphoton dissociation tandem mass spectrometry (MS). The mass fingerprint of a group of peaks for the core-2 structure of O-glycans was conserved in the tandem mass spectra and was instrumental in rapid and efficient structure determination. Among the 29 O-glycans, 22 glycans contain the typical core-2 structure, 3 glycans have the core-1 structure and 2 glycans contained a previously unobserved core structure with hexose at the reducing end. There were seven pairs of structural isomers observed in the major O-linked oligosaccharides. To further elucidate the structures of a dozen O-linked glycans, specific and targeted exoglycosidase digestions were carried out and the products were monitored with MALDI-MS. Reported here are the elucidated structures of O-linked oligosaccharides from glycoproteins of X. borealis egg jelly coats. The structural differences in O-glycans from jelly coats of X. borealis and its close relatives may provide a better understanding of the structure-function relationships and the role of glycans in the fertilization process within Xenopodinae.

Identification, cloning and tissue localization of a rainbow trout (Oncorhynchus mykiss) intelectin-like protein that binds bacteria and chitin

Intelectins are a recently identified group of animal lectins involved in innate immune surveillance. This paper describes the primary structure, expression and immunohistochemical localization of a rainbow trout plasma intelectin (RTInt). RTInt exhibited calcium-dependent binding to N-acetylglucosamine (GlcNAc) and mannose conjugated Toyopearl Amino 650 M matrices. When GlcNAc eluates from chromatography matrices were analyzed by reducing 1D PAGE and Western blots, the lectin appeared as approximately 37 kDa and approximately 72 kDa bands. Similar analysis of plasma revealed a single 72 kDa band under reducing conditions. MALDI-TOF MS demonstrated five, approximately 37 kDa isoforms (pI 5.3-6.1) separated by 2D-PAGE. A 975 bp cDNA sequence obtained by RT-PCR from liver and spleen tissue encoded a 325 amino acid secretory protein with homology to human and murine intelectins, which bind bacterial components and are induced during parasitic infections. Gene expression and immunohistochemistry detected RTInt in gill, spleen, hepatic sinusoid, renal interstitium, intestine, skin, swim bladder and within leukocytes. Direct binding assays demonstrated the ability of RTInt to bind relevant bacterial and chitinous targets. These findings suggest that RTInt plays a role in innate immune defense against bacterial and chitinous microbial organisms.

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.

Analysis of terminal sugar moieties and species-specificities of acrosome reaction-inducing substance in Xenopus (ARISX)

The acrosome reaction of Xenopus sperm is triggered by the acrosome reaction-inducing substance in Xenopus (ARISX), an oviductal pars recta-derived, sugar-rich substance decorated on the entire surface of the vitelline envelope (VE) during ovulation. Here we addressed the functional importance of the sugar moiety in ARISX. Among various lectins examined, soybean agglutinin and Dolichos biflorus agglutinin were shown to abolish the acrosome reaction-inducing activity of ARISX present in pars recta extract or on the VE, indicating the importance of the terminal alpha-N-acetylgalactosamine residue for the function of ARISX. Consistently, the acrosome reaction-inducing activity was not affected by proteinase K digestion, in spite of the simultaneous shift of ARISX to a smaller molecular weight. Indirect immunofluorescence microscopic examinations showed that ARISX was distributed as two types of structures on VE; thick fiber-like materials and thin filamentous materials, and that a new structure appeared on the fertilization envelope instead of the thin filamentous materials. Sperm from several amphibian species were subjected to an in vitro assay during induction of the acrosome reaction with ARISX. The resulting limited population of sperm from a non-Xenopus species underwent acrosome reaction, implying a weak species-specificity of ARISX.

Purification, cloning and characterization of egg lectins from the teleost Tribolodon brandti

Three L-rhamnose-binding egg lectins, TBL1, TBL2 and TBL3, were isolated from the eggs of the Far East dace Tribolodon brandti by a combination of affinity chromatography on L-rhamnose-Sepharose 6B gel and reversed-phase HPLC. L-rhamnose is a common inhibitor of the purified lectins and strongly inhibited the hemagglutinating activity of TBL2 and TBL3, but less weakly that of TBL1. L-arabinose, which has the same hydroxyl group orientation at C2 and C4 as L-rhamnose, and D-galactose showed no inhibitory activity against TBL1 but showed weak inhibitory activity against TBL2 and TBL3. The open reading frames of the cDNAs of TBL1, TBL2 and TBL3 encoded for mature proteins of 207, 189, and 293 amino acid residues, respectively. A BLAST homology search showed that the TBLs have about 40% homology to the carbohydrate recognition domains of rhamnose-binding lectins in salmonid eggs. The tandem repeated domains present in TBL1, TBL2 and TBL3 were two, two and three, respectively. TBL2 was exclusively expressed in ovary, while TBL1 and TBL3 were expressed mainly in ovary and weakly in various tissues including gill, heart, kidney, liver, spleen and testis.

Glycoproteins of the vitelline envelope of Amphibian oocyte: Biological and molecular characterization of ZPC component (gp41) inBufo arenarum

The vitelline envelope (VE) participates in sperm-egg interactions during the first steps of fertilization. In Bufo arenarum, this envelope is composed of at least four glycoproteins, with molecular masses of 120, 75, 41, and 38 kDa and molar ratio of 1:1.3:7.4:4.8, respectively. These components were isolated and covalently coupled to silanized glass slides in order to study their sperm-binding capacity. When considering the molar ratio of the glycoproteins in the egg-envelope and assuming that each protein is monovalent for sperm, the assay showed that gp41 and gp38 possess 55 and 25% of total sperm-binding activity. We obtained a full-length cDNA of gp41 (ZPC), comprising a sequence for 486 amino acids, with 43.3% homology with Xenopus laevis ZPC. As in the case of mammalian ZP3 and Xenopus ZPC, Bufo ZPC presented a furin-like (convertase) and a C-terminal transmembrane domain (TMD) reflecting common biosynthetic and secretory pathways. As it was reported for some fishes, we obtained evidence that suggests the presence of more than one zpc gene in Bufo genome, based on different partial cDNA sequences of zpc, Southern blots and two-dimensional SDS-PAGE of deglycosylated egg-envelope components. As far as we are aware, this is the first observation of the presence of different zpc genes in an Amphibian species.

Characterization of Bufo arenarum oocyte plasma membrane proteins that interact with sperm

Sperm-oocyte plasma membrane interaction is an essential step in fertilization. In amphibians, the molecules involved have not been identified. Our aim was to detect and characterize oocyte molecules with binding affinity for sperm. We isolated plasma membranes free from vitelline envelope and yolk proteins from surface-biotinylated Bufo arenarum oocytes. Using binding assays we detected a biotinylated 100 kDa plasma membrane protein that consistently bound to sperm. Chromatographic studies confirmed the 100 kDa protein and detected two additional oocyte molecules of 30 and 70 kDa with affinity for sperm. Competition studies with an integrin-interacting peptide and cross-reaction with an anti-HSP70 antibody suggested that the 100 and 70 kDa proteins are members of the integrin family and HSP70, respectively. MS/MS analysis suggested extra candidates for a role in this step of fertilization. In conclusion, we provide evidence for the involvement of several proteins, including integrins and HSP70, in B. arenarum sperm-oocyte plasma membrane interactions.

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

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

Proteolysis of Xenopus laevis egg envelope ZPA triggers envelope hardening

The egg envelope of most animal eggs is modified following fertilization, resulting in the prevention of polyspermy and hardening of the egg envelope. In frogs and mammals a prominent feature of envelope modification is N-terminal proteolysis of the envelope glycoprotein ZPA. We have purified the ZPA protease from Xenopus laevis eggs and characterized it as a zinc metalloprotease. Proteolysis of isolated egg envelopes by the isolated protease resulted in envelope hardening. The N-terminal peptide fragment of ZPA remained disulfide bond linked to the ZPA glycoprotein moiety following proteolysis. We propose a mechanism for egg envelope hardening involving ZPA proteolysis by an egg metalloprotease as a triggering event followed by induction of global conformational changes in egg envelope glycoproteins.

Strategy for Profiling and Structure Elucidation of Mucin-Type Oligosaccharides by Mass Spectrometry

A strategy combining accurate mass determination, tandem mass spectrometry, structure homology, and exoglycosidases is described that allows the structural characterization of mucin-type O-linked oligosaccharides. The method is used to profile with quantitation the O-linked oligosaccharide (both neutral and anionic) components of the only diploid Xenopus frog, Xenopus tropicalis. Collision-induced dissociation was used to determine connectivity, to identify previously characterized oligosaccharides, and to determine the presence of structural motifs in unknown oligosaccharides. Exoglycosidase digestion was used to identify the individual residues along with the linkages. The enzymes were also used to cleave larger oligosaccharides to smaller units that are similar to previously elucidated components. By using CID, isomeric structures were compared to determine whether they were identical. In this way, the exoglycosidases were more effectively used, and their use was minimized. A total of 35 oligosaccharides including neutral, sialylated, and sulfated were characterized in this way. The relative abundances of all components were also determined based on HPLC.

The Xenopus laevis cortical granule lectin: cDNA cloning, developmental expression, and identification of the eglectin family of lectins

A Xenopus laevis egg cortical granule, calcium-dependent, galactosyl-specific lectin participates in forming the fertilization layer of the egg envelope and functions in establishing a block to polyspermy. We report the cDNA cloning of the lectin, expression of the cortical granule lectin gene during oogenesis and early development, and identification of a new family of lectins. The translated cDNA for the cortical granule lectin had a signal peptide, a structural sequence of 298 amino acids, a molecular weight of 32.7 K, contained consensus sequence sites for N-glycosylation and a fibrinogen domain. The lectin cDNA was expressed during early stages of oogenesis. Lectin glycoprotein levels were constant during development with 2/3 of the lectin associated with the extracellular perivitelline space and the egg/embryo fertilization envelope. Lectin mRNA levels were from 100- to 1000-fold greater in ovary than in other adult tissues. The lectin had no sequence homology to the previously identified lectin families. The lectin had 41-88% amino acid identity with nine translated cDNA sequences from an ascidian, lamprey, frog, mouse, and human. Based on the conserved carbohydrate binding and structural properties of these glycoproteins, we propose a new family of lectins, the eglectin family.