Acrosome reaction in sperm of the frog, Xenopus laevis: its detection and induction by oviductal pars recta secretion

Generation of reproductively mature offspring from the endangered green and golden bell frog Litoria aurea using cryopreserved spermatozoa

Amphibians are becoming increasingly reliant on captive breeding programs for continued survival. Assisted reproductive technologies including gamete cryopreservation and IVF can help reduce costs of breeding programs, provide insurance against extinction and assist genetic rescue in wild populations. However, the use of these technologies to produce reproductively mature offspring has only been demonstrated in a few non-model species. We aimed to optimise sperm cryopreservation in the threatened frog Litoria aurea and generate mature offspring from frozen-thawed spermatozoa by IVF. We tested three concentrations (1.4, 2.1 and 2.8M) of the cryoprotectants dimethylsulfoxide (DMSO) and glycerol with 0.3M sucrose. Using DMSO was more likely to result in recovery of sperm motility, vitality and acrosome integrity than glycerol, regardless of concentration, with forward progressive motility being most sensitive to damage. The lowest concentrations of 1.4 and 2.1M provided the best protection regardless of cryoprotectant type. Spermatozoa cryopreserved in 2.1M DMSO outperformed spermatozoa cryopreserved in equivalent concentrations of glycerol in terms of their ability to fertilise ova, resulting in higher rates of embryos hatching and several individuals reaching sexual maturity. We have demonstrated that sperm cryopreservation and subsequent offspring generation via IVF is a feasible conservation tool for L. aurea and other threatened amphibians.

Species-specific mechanisms during fertilization

The perpetuation and preservation of distinct species rely on mechanisms that ensure that only interactions between gametes of the same species can give rise to viable and fertile offspring. Species-specificity can act at various stages, ranging from physical/behavioral pre-copulatory mechanisms, to pre-zygotic incompatibility during fertilization, to post-zygotic hybrid incompatibility. Herein, we focus on our current knowledge of the molecular mechanisms responsible for species-specificity during fertilization. While still poorly understood, decades of research have led to the discovery of molecules implicated in species-specific gamete interactions, starting from initial sperm-egg attraction to the binding of sperm and egg. While many of these molecules have been described as species-specific in their mode of action, relatively few have been demonstrated as such with definitive evidence. Thus, we also raise remaining questions that need to be addressed in order to characterize gamete interaction molecules as species-specific.

Sperm acrosome reaction: its site and role in fertilization

Manner and roles of sperm acrosome reaction in a variety of animals were compared.

Acrosomal alkalization triggers Ca2+ release and acrosome reaction in mammalian spermatozoa

The sperm acrosome reaction (AR), an essential event for mammalian fertilization, involves Ca²⁺ permeability changes leading to exocytosis of the acrosomal vesicle. The acrosome, an intracellular Ca²⁺ store whose luminal pH is acidic, contains hydrolytic enzymes. It is known that acrosomal pH (pH_acr ) increases during capacitation and this correlates with spontaneous AR. Some AR inducers increase intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) through Ca²⁺ release from internal stores, mainly the acrosome. Catsper, a sperm specific Ca²⁺ channel, has been suggested to participate in the AR. Curiously, Mibefradil and NNC55-0396, two CatSper blockers, themselves elevate [Ca²⁺ ]_i by unknown mechanisms. Here we show that these compounds, as other weak bases, can elevate pH_acr , trigger Ca²⁺ release from the acrosome, and induce the AR in both mouse and human sperm. To our surprise, μM concentrations of NNC55-0396 induced AR even in nominally Ca²⁺ free media. Our findings suggest that alkalization of the acrosome is critical step for Ca²⁺ release from the acrosome that leads to the acrosome reaction.

Extracellular Ca2+ Is Required for Fertilization in the African Clawed Frog, Xenopus laevis

Background

The necessity of extracellular Ca²⁺ for fertilization and early embryonic development in the African clawed frog, Xenopus laevis, is controversial. Ca²⁺ entry into X. laevis sperm is reportedly required for the acrosome reaction, yet fertilization and embryonic development have been documented to occur in high concentrations of the Ca²⁺ chelator BAPTA. Here we sought to resolve this controversy.

Methodology/principal finding

Using the appearance of cleavage furrows as an indicator of embryonic development, we found that X. laevis eggs inseminated in a solution lacking added divalent cations developed normally. By contrast, eggs inseminated in millimolar concentrations of BAPTA or EGTA failed to develop. Transferring embryos to varying solutions after sperm addition, we found that extracellular Ca²⁺ is specifically required for events occurring within the first 30 minutes after sperm addition, but not after. We found that the fluorescently stained sperm were not able to penetrate the envelope of eggs inseminated in high BAPTA, whereas several had penetrated the vitelline envelope of eggs inseminated without a Ca²⁺ chelator, or with BAPTA and saturating CaCl₂. Together these results indicate that fertilization does not occur in high concentrations of Ca²⁺ chelators. Finally, we found that the jelly coat includes >5 mM of readily diffusible Ca²⁺.

Conclusions/Significance

Taken together, these data are consistent with requirement of extracellular Ca²⁺ for fertilization. Based on our findings, we hypothesize that the jelly coat surrounding the egg acts as a reserve of readily available Ca²⁺ ions to foster fertilization in changing extracellular milieu.

Dicalcin, a zona pellucida protein that regulates fertilization competence of the egg coat in Xenopus laevis

Fertilization is a highly coordinated process whereby sperm interact with the egg-coating envelope (called the zona pellucida, ZP) in a taxon-restricted manner, Fertilization triggers the resumption of the cell cycle of the egg, ultimately leading to generation of a new organism that contains hereditary information of the parents. The complete sperm-ZP interaction comprises sperm recognition of the ZP, the acrosome reaction, penetration of the ZP, and fusion with the egg. Recent evidence suggests that these processes involve oligosaccharides associated with a ZP constituent (termed ZP protein), the polypeptide backbone of a ZP protein, and/or the proper three-dimensional filamentous structure of the ZP. However, a detailed description of the molecular mechanisms involved in sperm-ZP interaction remains elusive. Recently, I found that dicalcin, a novel ZP protein-associated protein, suppresses fertilization through its association with gp41, the frog counterpart of the mammalian ZPC protein. This review focuses on molecular aspects of sperm-ZP interaction and describes the fertilization-suppressive function of dicalcin and associated molecular mechanisms. The amount of dicalcin in the ZP significantly correlates with alteration of the lectin-staining pattern within the ZP and the orientation pattern of ZP filaments, which may assist in elucidating the complex molecular mechanisms that underlie sperm-ZP interaction.

The egg membrane microdomain-associated uroplakin III-Src system becomes functional during oocyte maturation and is required for bidirectional gamete signaling at fertilization in Xenopus laevis

In Xenopus laevis, sperm-egg interaction promotes partial proteolysis and/or tyrosine phosphorylation of uroplakin III (UPIII) and the tyrosine kinase Src, which both localize to the cholesterol-enriched egg membrane microdomains (MDs). Here we show that sperm promote proteolysis and/or tyrosine phosphorylation of UPIII and Src in MDs isolated from ovulated and unfertilized eggs (UF-MDs). An antibody against the extracellular domain of UPIII interferes with these events. Inhibition of fertilization by anti-UPIII antibody is rescued by co-incubation with UF-MDs. This suggests that, like MDs in intact eggs, the isolated UF-MDs are capable of interacting with sperm, an interaction that does not interfere with normal fertilization but rather augments the ability of sperm to fertilize eggs pretreated with anti-UPIII antibody. This unexpected effect of UF-MDs on sperm requires UPIII function in UF-MDs and protein kinase activity in sperm. MDs isolated from progesterone-treated mature oocytes, but not ovarian immature oocytes, are similarly functional as UF-MDs. The anti-UPIII extracellular domain antibody binds more effectively to the surface of mature than immature ovarian oocytes. We propose that the structural and functional competency of the UPIII-Src signaling system in MDs is strictly regulated during oocyte maturation and subsequently in sperm-mediated egg activation and fertilization. The fertilization-related signaling properties seen in UF-MDs can be partially reconstituted in MDs of human embryonic kidney 293 cells (293-MDs) expressing UPIII, Src and uroplakin Ib. However, 293-MDs expressing a proteolysis-resistant mutant of UPIII are less functional, suggesting that the availability of UPIII to protease action is important for MD function.

Effect of oviductal secretion components on the fertilizing capacity of amphibian sperm: biological and ultrastructural studies

The present study was carried out to analyze the fertilization-supporting activity of Rhinella arenarum egg-jelly components. Spermatozoa were treated with diffusible factor (DF) constituted by the components released from the jelly coat into deionized water or with full jelly (FJ) containing all the components secreted by the oviductal pars convoluta (PC) during the transit of the oocytes through the duct, or with washed jelly (WJ) constituted only by structural components. Both jellies were solubilized by ultraviolet irradiation. These gametes were used for ultrastructural and biological studies in order to determine the acrosome state and the fertilizing capacity. Additional experiments were performed by using Ca(2+), a diffusible cation present in R. arenarum jelly envelopes. Results demonstrated a marked increase in the acrosome reaction (AR) of sperm treated with FJ or DF compared to the controls (Ringer's solution), no significant differences being observed between both treatments, while WJ showed low AR percentages similar to the ones obtained with the controls. The addition of Ca(2+) induced an increase in this parameter in a dose-dependent manner, although the values reached with FJ or DF were not attained. The results of the "in vitro" fertilization show a strong inverse association to the acrosome reaction (AR) rate. Treatment with Ca(2+) at the concentration present in the jelly (6.3 ± 0.9 mM) inhibited the fertilizing capacity as a function of incubation time, showing that at 2 min there was a decrease in the fertilization percentages compared to 10% Ringer's. Data indicated that Ca(2+) present in jelly is involved in the AR induction but suggests the possible participation of other diffusible and/or structural components of the oviductal secretion in this process.

Analysis of a sperm surface molecule that binds to a vitelline envelope component of Xenopus laevis eggs

To analyze sperm surface molecules involved in sperm-egg envelope binding in Xenopus laevis, heat-solubilized vitelline envelope (VE) dot blotted onto a polyvinylidene difluoride (PVDF) sheet was incubated with a detergent extract of sperm plasma membrane (SP-ML). The membrane components bound to the VE were detected using an antibody library against sperm plasma membrane components, and a hybridoma clone producing a monoclonal antibody (mAb) 16A2A7 was identified. This mAb was used in a Far Western blotting experiment in which VE was separated by electrophoresis, and then transferred to a PVDF strip that was incubated with SP-ML. It was found that SP-ML binds to the VE component gp37 (Xenopus homolog of mammalian ZP1). The antigens reactive to mAb 16A2A7 showed apparent molecular weights of 65-130 and 20-30 kDa, and were distributed relatively evenly over the entire sperm surface. Periodate oxidation revealed that both the pertinent epitope on the sperm surface and the ligands of VE gp37 were sugar moieties. VE gp37 was exposed on the VE surface, and the mAb 16A2A7 dose-dependently inhibited sperm binding to VE. The sperm membrane molecules reactive with mAb 16A2A7 also reacted with mAb 2A3D9, which is known to recognize the glycoprotein SGP in the sperm plasma membrane and is involved in interactions with the egg plasma membrane, indicating that the sperm membrane glycoprotein has a bifunctional role in Xenopus fertilization.

The Sperm-surface glycoprotein, SGP, is necessary for fertilization in the frog, Xenopus laevis

To identify a molecule involved in sperm-egg plasma membrane binding at fertilization, a monoclonal antibody against a sperm-surface glycoprotein (SGP) was obtained by immunizing mice with a sperm membrane fraction of the frog, Xenopus laevis, followed by screening of the culture supernatants based on their inhibitory activity against fertilization. The fertilization of both jellied and denuded eggs was effectively inhibited by pretreatment of sperm with intact anti-SGP antibody as well as its Fab fragment, indicating that the antibody recognizes a molecule on the sperm's surface that is necessary for fertilization. On Western blots, the anti-SGP antibody recognized large molecules, with molecular masses of 65-150 kDa and minor smaller molecules with masses of 20-28 kDa in the sperm membrane vesicles. SGP was distributed over nearly the entire surface of the sperm, probably as an integral membrane protein in close association with microfilaments. More membrane vesicles containing SGP bound to the surface were found in the animal hemisphere compared with the vegetal hemisphere in unfertilized eggs, but the vesicle-binding was not observed in fertilized eggs. These results indicate that SGP mediates sperm-egg membrane binding and is responsible for the establishment of fertilization in Xenopus.

The acidic probe LysoSensor is not useful for acrosome evaluation of cryopreserved ram spermatozoa

To try new acrosomal probes for assessing ram spermatozoa, we compared the LysoSensor probe, which labels acidic organelles, with the frequently used peanut agglutinin acrosomal probe (PNA-PE; phycoerythrin as fluorescent moiety). The previous microscopic observations showed a lack of relationship of LysoSensor with acrosomal status. Semen was obtained from five rams and frozen in four pools. Each pool was analysed carrying out a triple staining propidium ioide/PNA-PE/LysoSensor Green DND-189 to test acrosome labelling, and a double staining SYBR-14/PI, to assess sperm viability. Stained samples were analysed by flow cytometry. All measurements were replicated. Data were processed using agreement and repeatability tests. LysoSensor labelling did not agree with PNA (mean of differences: 30.8%; coefficient of agreement: 22.6%), confirming microscopic observations. Nevertheless, when LysoSensor was compared with SYBR-14/PI, the agreement was high (mean of differences: -0.05%; coefficient of agreement: 5.07%). Repeatability of both methods was high and similar. LysoSensor did not seem to specifically stain the acrosome, but it may accumulate in the cytoplasm and label viable spermatozoa. Therefore, LysoSensor might not be used as an acrosomal probe in ram spermatozoa, but it could be used in other kind of studies, taking advantage of its pH sensitivity.

Applied reproductive technologies and genetic resource banking for amphibian conservation

As amphibian populations continue to decline, both government and non-government organisations are establishing captive assurance colonies to secure populations deemed at risk of extinction if left in the wild. For the most part, little is known about the nutritional ecology, reproductive biology or husbandry needs of the animals placed into captive breeding programs. Because of this lack of knowledge, conservation biologists are currently facing the difficult task of maintaining and reproducing these species. Academic and zoo scientists are beginning to examine different technologies for maintaining the genetic diversity of founder populations brought out of the wild before the animals become extinct from rapidly spreading epizootic diseases. One such technology is genetic resource banking and applied reproductive technologies for species that are difficult to reproduce reliably in captivity. Significant advances have been made in the last decade for amphibian assisted reproduction including the use of exogenous hormones for induction of spermiation and ovulation, in vitro fertilisation, short-term cold storage of gametes and long-term cryopreservation of spermatozoa. These scientific breakthroughs for a select few species will no doubt serve as models for future assisted breeding protocols and the increasing number of amphibians requiring conservation intervention. However, the development of specialised assisted breeding protocols that can be applied to many different families of amphibians will likely require species-specific modifications considering their wide range of reproductive modes. The purpose of this review is to summarise the current state of knowledge in the area of assisted reproduction technologies and gene banking for the conservation of amphibians.

Involvement of sperm proteases in the binding of sperm to the vitelline envelope in Xenopus laevis

Sperm binding to the vitelline envelope in dejellied Xenopus laevis eggs was effectively inhibited by inhibitors for trypsin (soybean trypsin inhibitor and p-toluenesulfonyl-L-lysine chloroethyl ketone) and aminopeptidase B (o-phenanthroline, bestatin, and arphamenine B). Likewise, synthetic 4-methylcoumaryl-7-amide (MCA) substrates (t-butoxycarbonyl-GlyArgArg-MCA, benzyloxycarbonyl-ArgArg-MCA, and Arg-MCA) inhibited binding. Consistently, when jellied eggs were inseminated in the presence of these substrates or inhibitors for proteases, fertilization was effectively blocked. The medium in which live sperm or the sperm membrane fraction were suspended exhibited hydrolyzing activities against the synthetic substrates mentioned above, and these activities were effectively inhibited by the protease inhibitors. Ultracentrifugal fractionation of the sperm suspension following induction of the acrosome reaction by a calcium ionophore, A23187, indicated that a considerable amount of the total tryptic and aminopeptidase B activity was released into the medium. On this occasion, part of the tryptic and aminopeptidase B activity was definitely estimated to be discharged in association with a vesiculated membrane, supporting the notion that the proteases involved in binding to the vitelline envelope are present on the sperm plasma membrane.

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.

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.

Oviductal protease and trypsin treatment enhance sperm–envelope interaction inBufo arenarum coelomic eggs

We describe the morphological and biochemical changes in Bufo arenarum coelomic egg envelopes (CE) following passage through the oviduct. In this species, the transformation of the CE into the vitelline envelope (VE) leads to the acquisition of fertilizability and involves the cleavage of a glycoprotein component. Electrophoretic patterns indicate that a pars recta oviductal protease selectively hydrolyzes in vitro the 84 and the 55 kDa glycoproteins of the CE. During the CE to VE transformation, the relative concentrations of gp48, 42 and 39 kDa also change. In in vitro tests, sperm binding to envelope glycoprotein occurs when they are exposed to VE but not when treated with CE, and VE labeled glycoproteins bind to the head and mid piece of the sperm. The gp39 VE component has 100% identity with internal domains of the sequence deduced from ovarian cDNA for the homologous zona pellucida glycoprotein type C (ZPC) protein precursor in B. arenarum. The effects of trypsin as a substitute for oviductal protease were also examined. Trypsin selectively attacks the 84 and the 55 kDa glycoproteins without hydrolyzing other components and renders coelomic eggs fertilizable in a jelly water preparation. Therefore, trypsin can mimic in vitro the biological action of the oviductal protease. However, it does not wholly mimic the biological action of the oviduct which, in B. arenarum at least, exceeds a mere proteolytic effect. This fact was verified by the lower fertility rates and the abnormal embryo development found when trypsin-treated coelomic eggs were fertilized in vitro.

The outermost layer of egg-jelly is crucial to successful fertilization in the newt,Cynops pyrrhogaster

The significance of egg-jelly layers in internal fertilization was evaluated in the newt, Cynops pyrrhogaster. In this species, six egg-jelly layers, J1, J2, J3, J4, J5 and the outermost J6 layers, are accumulated on the surface of the fertilizable eggs in pars convoluta of the oviduct. When a large number of sperm (about 6 x 10(5)) were placed on eggs having different numbers of jelly layers, all the eggs were fully fertilized, although many of the eggs developed abnormally. Upon insemination using about 600 sperm, only eggs with the full set of jelly layers were fertilized at a high rate with normal development. Since around 300 (the range of 48-1,192) sperm were observed on and in the egg-jelly in naturally spawned eggs, we conclude that the J6 layer must be present on the outermost surface of the egg-jelly for successful internal fertilization of the newt. Previous studies have suggested that the J6 layer is a prerequisite for the initiation of sperm motility and the acrosome reaction. In the present study, the fertilization rate decreased in eggs with a full set of jelly layers when inseminated using acrosome-reacted and motile sperm. However, the fertilization rate was high when motile sperm with intact acrosome was used. These results suggest that induction of the sperm acrosome reaction in the J6 layer is an important step in the internal fertilization of the newt.

Uroplakin III, a novel Src substrate in Xenopus egg rafts, is a target for sperm protease essential for fertilization

In a previous study, we identified Xenopus egg uroplakin III (xUPIII), a single-transmembrane protein that localized to lipid/membrane rafts and was tyrosine-phosphorylated upon fertilization. An antibody against the xUPIII extracellular domain abolishes fertilization, suggesting that xUPIII acts not only as tyrosine kinase substrate but also as a receptor for sperm. Previously, it has been shown that the protease cathepsin B can promote a transient Ca2+ release and egg activation as seen in fertilized eggs (Mizote, A., Okamoto, S., Iwao, Y., 1999. Activation of Xenopus eggs by proteases: possible involvement of a sperm protease in fertilization. Dev. Biol. 208, 79-92). Here, we show that activation of Xenopus eggs by cathepsin B is accompanied by tyrosine phosphorylation of egg-raft-associated Src, phospholipase Cgamma, and xUPIII. Cathepsin B also promotes a partial digestion of xUPIII both in vitro and in vivo. A synthetic xUPIII-GRR peptide, which contains a potential proteolytic site, inhibits the cathepsin-B-mediated proteolysis and tyrosine phosphorylation of xUPIII and egg activation. Importantly, this peptide also inhibits sperm-induced tyrosine phosphorylation of xUPIII and egg activation. Protease activity that digests xUPIII in an xUPIII-GRR peptide-sensitive manner is present in Xenopus sperm. Several protease inhibitors, which have been identified to be inhibitory toward Xenopus fertilization, are shown to inhibit sperm-induced tyrosine phosphorylation of xUPIII. Uroplakin Ib, a tetraspanin UP member, is found to be associated with xUPIII in egg rafts. Our results highlight novel mechanisms of fertilization signaling by which xUPIII serves as a potential target for sperm protease essential for fertilization.

In vitro fertilization and artificial activation of eggs of the direct-developing anuran Eleutherodactylus coqui

Although much is known about the reproductive biology of pond-breeding frogs, there is comparatively little information about terrestrial-breeding anurans, a highly successful and diverse group. This study investigates the activation and in vitro fertilization of eggs of the Puerto Rican coqui frog obtained by hormonally induced ovulation. We report that spontaneous activation occurs in 34% of eggs, probably in response to mechanical stress during oviposition. Artificial activation, as evidenced by the slow block to polyspermy and the onset of zygote division, was elicited both by mechanical stimulation and calcium ionophore exposure in 64% and 83% of the cases, respectively. Finally, one in vitro fertilization protocol showed a 27% success rate, despite the fact that about one third of all unfertilized eggs obtained by hormone injection auto-activate. We expect these findings to aid in the conservation effort of Eleutherodactylus frogs, the largest vertebrate genus.

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.

Jeltraxin, a Frog Egg Jelly Glycoprotein, Has Calcium-Dependent Lectin Properties and Is Related to Human Serum Pentraxins CRP and SAP

The egg jelly that encapsulates amphibian eggs is essential for fertilization, but its molecular composition and roles remain largely unknown. We identified a calcium-dependent lectin from the pentraxin superfamily in the egg jelly coat from the South American burrowing frog, Lepidobatrachus laevis. This lectin, jeltraxin, was related to the host-response acute phase serum proteins C-reactive P component (CRP) and serum amyloid P component (SAP). The amino acid sequence of jeltraxin is 44% identical to that of Xenopus laevis CRP, 31-35% identical to those of mammalian CRP and SAP, and 21-27% identical to those of the large fusion pentraxins. Expression of jeltraxin mRNA was restricted to the oviduct, which distinguishes it as the first serum-related pentraxin not expressed in the liver. Purified jeltraxin was previously shown to exist in an oligomeric complex of approximately 250 kDa comprised of self-associating subunits. We have demonstrated by MALDI-TOF that this configuration is due to a decameric complex of 27.7 kDa subunits. Biotinylated jeltraxin bound to the high-molecular mass components of the egg jelly in a calcium-dependent manner with specificity for beta-galactose residues. On the basis of homology modeling, we predict that jeltraxin will coordinate two calcium ions. The function of jeltraxin will likely be related to its calcium-dependent lectin properties.

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.

Oviductin, the oviductal protease that mediates gamete interaction by affecting the vitelline coat in Bufo japonicus: its molecular cloning and analyses of expression and posttranslational activation

Previous studies indicated that the acquisition of egg fertilizability during transit through the pars recta portion of the oviduct in Bufo japonicus is accompanied by hydrolytic conversion of the vitelline coat 40- to 52-kDa components to 39-kDa components induced by a 66-kDa serine protease, "oviductin." In this study, we cloned a 3028-bp cDNA that contained an open reading frame encoding 974 amino acids with a calculated molecular mass of 107.6 kDa, including two protease domains and three repeats of CUB domains. Sequence analysis indicated that the catalytically active 66-kDa protein comprised an N-terminally located oviductin protease and two CUB domains. The oviductin gene was transcribed as a part of 6-kb mRNA that was expressed specifically in the cells lining the bottom of epithelial folds in the oviductal pars recta, and this expression was highly accelerated when the pars recta fragments were cultured in the presence of hCG. Western blot analyses using antibodies against a protease domain revealed that the catalytically inactive 102-kDa proteins in the pars recta granules yield 66-kDa catalytically active and 82- and 59-kDa inactive molecules. We propose that the oviductin translated as 107.6-kDa precursors are processed both N- and C-terminally to give rise to a 66-kDa active form comprising a serine protease and two CUB domains.