Vitelline envelope of Bufo arenarum: biochemical and biological characterization

NMDA-type glutamate receptors mediate the acrosome reaction and motility initiation in newt sperm

The N-methyl d-aspartate type glutamate receptor (NMDAR) is a ligand-gated cation channel that causes Ca²⁺ influx in nerve cells. An NMDAR agonist is effective to the sperm motility in fowls, although the actual role of NMDAR in sperm function is unknown. In the present study, RNA-seq of the spermatogenic testes suggested the presence of NMDAR in the sperm of the newt Cynops pyrrhogaster. Glutamate of at least 0.7 ± 0.5 mM was detected in the egg-jelly substances along with acrosome reaction-inducing substance (ARIS) and sperm motility-initiating substance (SMIS). In the egg-jelly extract (JE) that included the ARIS and SMIS, the acrosome reaction was inhibited by a NMDAR antagonists, memantine and MK801. MK801 also inhibited the spontaneous acrosome reaction in Steinberg's salt solution (ST). Furthermore, memantine and MK801 suppressed the progressive motility of the sperm in JE and spontaneous waving of the undulating membrane, which is the tail structure giving thrust for forward motility, in ST. The spontaneous waving of the undulating membrane was promoted when Mg²⁺ , which blocks Ca²⁺ influx through gated NMDARs, was removed from the ST. In addition, the ARIS-induced acrosome reaction was inhibited by a selective antagonist of the transient receptor potential vanilloid 4, whose activation might result in the membrane depolarization to release Mg²⁺ from the NMDAR. These results suggest that NMDAR acts together with other cation channels in the induction of the acrosome reaction and motility of the sperm during the fertilization process of C. pyrrhogaster.

Participation of the 39-kDa glycoprotein (gp39) of the vitelline envelope of Bufo arenarum eggs in sperm-egg interaction

The acquisition of egg fertilizability in Bufo arenarum takes place during the oviductal transit and during this process the extracellular coelomic envelope (CE) of the eggs is converted into the vitelline envelope (VE). It has been stated that one of the necessary events leading to a fertilizable state is the proteolytic cleavage of CE glycoproteins in the oviductal pars recta by oviductin, a serine protease. Consequently, there is a marked increase in the relative quantity of glycoproteins with 39 (gp39) and 42 kDa (gp42) in the VE. In the present study, sperm-VE binding assays using heat-solubilized biotin-conjugated VE glycoproteins revealed that both gp39 and gp42 have sperm binding capacity. According to this result, our study was focused on gp39, a glycoprotein that we have previously reported as a homologue of mammalian ZPC. For this purpose, rabbit polyclonal antibodies against gp39 were generated at our laboratory. The specificity of the antibodies was confirmed with western blot of VE glycoproteins separated on SDS-PAGE. Immunohistochemical and immunoelectron studies showed gp39 distributed throughout the width of the VE. In addition, immunofluorescence assays probed that gp39 bound to the sperm head. Finally, as an approach to elucidate the possible involvement of gp39 in fertilization, inhibition assays showed that pretreatment of eggs with antibodies against gp39 generated a significant decrease in the fertilization rate. Therefore, our findings suggest that gp39, which is modified by oviductal action, participates as a VE glycoprotein ligand for sperm in Bufo arenarum fertilization.

Vitellogenesis in Bufo arenarum: identification, characterization and immunolocalization of high molecular mass lipovitellin during oogenesis

Vitellogenin (Vtg), a large lipoglycophosphoprotein, is the most important precursor of the yolk proteins, and the major source of nutrients for the developing embryo in oviparous species. After its uptake by the oocytes, Vtg is converted into lipovitellins (high and light) and phosvitin, which are deposited into crystalline yolk platelets. We describe here the presence of two high molecular mass lipovitellin isoforms in Bufo arenarum mature oocytes with masses of 113 and 100 kDa, respectively. The amino acid sequence analysis of p113 and p100 peptides showed a high sequence homology between both polypeptides and the complete reported sequences of Xenopus laevis vitellogenin. Using specific antibodies, we determined that the Vtg uptake begins early during oogenesis, at the previtellogenic stage, and continues until oocytes have reached their mature status. In addition, we found that large endocytic vesicles mediate Vtg uptake in stage I oocytes, and that the size of the endocytic vesicles declines with oogenesis progression. In terms of the Vtg protein trafficking, we detected the Vtg precursor (190 kDa) in the liver of estradiol-injected females. Finally, we propose a subclassification of B. arenarum stage II oocytes into three physiologically and morphologically distinct periods (early, mid and late).

Egg water from the amphibian Bufo arenarum modulates the ability of homologous sperm to undergo the acrosome reaction in the presence of the vitelline envelope

Sperm from the toad Bufo arenarum must penetrate the egg jelly before reaching the vitelline envelope (VE), where the acrosome reaction is triggered. When the jelly coat is removed, sperm still bind to the VE, but acrosomal exocytosis is not promoted. Our previous work demonstrated that diffusible substances of the jelly coat, termed "egg water" (EW), triggered capacitation-like changes in B. arenarum sperm, promoting the acquisition of a transient fertilizing capacity. In the present work, we correlated this fertilizing capacity with the ability of the sperm to undergo the acrosome reaction, further substantiating the role of the jelly coat in fertilization. When sperm were exposed to the VE, only those preincubated in EW for 5 or 8 min underwent an increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)), which led to acrosomal exocytosis. Responsiveness to the VE was not acquired on preincubation in EW for 2 or 15 min or in Ringer solution regardless of the preincubation time. In contrast, depletion of intracellular Ca(2+) stores (induced by thapsigargin) promoted [Ca(2+)](i) rise and the acrosome reaction even in sperm that were not exposed to EW. Acrosomal exocytosis was blocked by the presence of Ca(2+) chelators independent of whether a physiological or pharmacological stimulus was used. However, Ni(2+) and mibefradil prevented [Ca(2+)](i) rise and the acrosome reaction of sperm exposed to the VE but not of sperm exposed to thapsigargin. These data suggest that the acrosomal responsiveness of B. arenarum sperm, present during a narrow period, is acquired during EW incubation and involves the modulation of a voltage-dependent Ca(2+) channel.

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.

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.

Egg jelly of the newt,Cynops pyrrhogaster contains a factor essential for sperm binding to the vitelline envelope

The acrosome reaction of newt sperm is induced at the surface of egg jelly and the acrosome-reacted sperm acquire the ability to bind to the vitelline envelope. However, because the substance that induces the acrosome reaction has not been identified, the mechanism by which the acrosome-reacted sperm bind to the vitelline envelope remains unclear. We found here that a Dolichos biforus agglutinin (DBA) specifically mimicked the acrosome reaction immediately upon its addition in the presence of milimolar level Ca(2+). Fluorescein isothiocyanate-labeled DBA bound specifically to the acrosomal cap of the intact sperm in the presence of a Ca(2+)-chelating agent, EDTA, suggesting that binding of DBA to the native receptor for the egg jelly substance on the acrosomal region took the place of the egg jelly substance-induced acrosome reaction. In contrast, the sperm that had been acrosome reacted by DBA treatment did not bind to the vitelline envelope of the egg whose jelly layers were removed. Subsequent addition of jelly extract caused the sperm binding to vitelline envelope, indicating that the egg jelly of the newt contains substances that are involved in not only inducing the acrosome reaction but also binding to the vitelline envelope. This is the first demonstration of the involvement of egg jelly substance in the binding of acrosome-reacted sperm to the vitelline envelope.

Characterization and biological properties of L-HGP, a glycoprotein from the amphibian oviduct with acrosome-stabilizing effects

BACKGROUND INFORMATION

The role of the jelly coat that surrounds the amphibian oocytes has been widely discussed, but is poorly understood. The presence of the jelly coat is essential for fertilization. However, the structure and function of the molecules that comprise the jelly coat have not been thoroughly documented. L-HGP (low-molecular-mass highly glycosylated protein) is a highly glycosylated protein that is present in the jelly coat of the toad, Bufo arenarum, oocytes and diffuses to the surrounding media. L-HGP, when purified from egg water, protects the sperm acrosome from breakdown induced by hypotonic solutions.

RESULTS

L-HGP is an acidic glycoprotein, formed by two different subunits, linked by disulphide bonds. We raised polyclonal antibodies in rabbits against the deglycosylated protein. We determined that L-HGP is secreted along the oviduct, being hence present in all the jelly layers. The molecular mass of L-HGP is higher in the most cephalic region of the oviduct. The lower-M(r) L-HGP isoform, produced in the caudal regions of the oviduct, presents an acrosome-protecting property. L-HGP is produced by secretory cells in the oviduct and is deposited on the cilia at the oviduct lumen.

CONCLUSIONS

Biochemical characterization of L-HGP has been carried out. It is synthesized by secretory cells in the oviduct and, when secreted, is deposited over the ciliated surface of the cells. The lower-M(r) isoform, secreted by the caudal region of the oviduct, protects acrosome integrity. This isoform diffuses into the medium. The role of the higher-M(r) L-HGP isoform in fertilization remains unknown.

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.

Characterization of the acrosome reaction-inducing substance in Xenopus (ARISX) secreted from the oviductal pars recta onto the vitelline envelope

We previously demonstrated that Xenopus sperm undergo an acrosome reaction on the vitelline envelope (VE) in response to the materials secreted from the oviductal pars recta [Dev. Biol. 243 (2002), 55]. A monoclonal antibody against the acrosome reaction-inducing substance in Xenopus (ARISX) was obtained by immunizing mice with pars recta extract (PRE). The acrosome reaction by PRE or on the VE was effectively inhibited by the intact anti-ARISX antibody as well as its Fab fragment, indicating that the antibody recognizes the epitopes localized on the acrosome reaction-inducing substance. On Western blots, the anti-ARISX antibody recognized a molecule with an apparent molecular mass of 300 kDa in PRE and in the VE, but this molecule was not detected in the coelomic envelope. The amount of ARISX in PRE was increased by the treatment of females with pregnant mare serum gonadotropin. Periodate oxidation of PRE completely abolished the acrosome reaction-inducing activity, indicating the involvement of the carbohydrate moieties of ARISX in inducing the acrosome reaction. On immunofluorescence observation, ARISX was localized in the epithelial cells in the posterior region of the pars recta and on the VE as fibrous structures.

One step purification and biochemical characterization of a spermatozoa-binding protein from porcine oviductal epithelial cells

In pigs the binding of sperm to oviductal epithelial cells to form a sperm reservoir involves carbohydrate interactions. In the present study, we purify a sperm binding glycoprotein (SBG) from cells from the isthmus of the oviduct using an affinity column. This protein conjugated with FITC is able to bind to the heads of pig sperm. SBG is shown to contain carbohydrates by PAS-silver staining and lectin binding assays. Enzymatic treatment and lectin affinity demonstrate that SBG exposes Galbeta1-3GalNAc disaccharide, which is bound to a serine or a threonin residue by an O-link. After enzymatic deglycosylation SBG shows an apparent molecular mass of 67.5 kDa, which changes to 85 kDa by reduction with 2-mercaptoethanol. Both SBG and enzymatically deglycosylated SBG show by isoelectrofocusing two forms of pI 3.6 and pI 3.8. SBG may be involved on sperm-oviduct interaction.

The envelopes of amphibian oocytes: physiological modifications in Bufo arenarum

A characterization of the Amphibian Bufo arenarum oocyte envelope is presented. It was made in different functional conditions of the oocyte: 1) when it has been released into the coelomic cavity during ovulation (surrounded by the coelomic envelope, (CE), 2) after it has passed through the oviduct and is deposed (surrounded by the viteline envelope, (VE), and 3) after oocyte activation (surrounded by the fertilization envelope, (FE). The characterization was made by SDS-PAGE followed by staining for protein and glycoproteins. Labeled lectins were used to identify glycosidic residues both in separated components on nitrocellulose membranes or in intact oocytes and embryos. Proteolytic properties of the content of the cortical granules were also analyzed. After SDS-PAGE of CE and VE, a different protein pattern was observed. This is probably due to the activity of a protease present in the pars recta of the oviduct. Comparison of the SDS-PAGE pattern of VE and FE showed a different mobility for one of the glycoproteins, gp75. VE and FE proved to have different sugar residues in their oligosaccharide chains. Mannose residues are only present in gp120 of the three envelopes. N-acetyl-galactosamine residues are present in all of the components, except for gp69 in the FE. Galactose residues are present mainly in gp120 of FE. Lectin-binding assays indicate the presence of glucosamine, galactose and N-acetyl galactosamine residues and the absence (or non-availability) of N-acetyl-glucosamine or fucose residues on the envelopes surface. The cortical granule product (CGP) shows proteolytic activity on gp75 of the VE.

The urodele egg-coat as the apparatus adapted for the internal fertilization

Fertilization is a significant event for reproducing offspring. It is achieved under a species-specific environment, which influences the conditions to assure the successful fertilization in some cases. Several studies about the basic mechanism of fertilization suggest that the fertilization mechanism is modified among species to be suited for the fertilization environment. In amphibians, many anurans undergo external fertilization while most urodeles do internal fertilization. An amphibian egg is surrounded by egg-coats, which are composed of vitelline envelope and layered egg-jelly. They are significant as fields for the sperm-egg interaction at fertilization. The fertilization processes that take place in the egg-coats are supposed to be easily influenced by the fertilization environment, because they, especially egg-jelly, are exposed to the surroundings at fertilization. In the present article, we describe the fertilization system equipped in newt egg-coats. Newt sperm are stored in spermatheca that exists in cloaca of a female and directly inseminated on the surface of egg-jelly. Sperm motility and acrosome reaction are induced in the outermost portion of the egg-jelly. Motion of the moving sperm becomes vigorous in the egg-jelly and sperm are guided to vitelline envelope by the aid of egg-jelly structure. Most of the sperm passing through the egg-jelly, as the result, has been induced acrosome reaction and those sperm can bind to the vitelline envelope to contribute to the successful fertilization. This fertilization system has a distinct feature from the known system in species undergoing external fertilization. The feature of the system in the newt egg-jelly is discussed with the view to achieving the successful fertilization in the internal environment.