Glycosidases,proteases and ascidian fertilization

Spermatozoa should bind to and then penetrate the vitelline coat for fertilization in ascidians and many other animals. There is substantial evidence that the binding of ascidian sperm is mediated by a sperm glycosidase and complementary saccharide chains of glycoproteins in the vitelline coat. Involvement of a sperm proteasome in the binding is also suggested. For the penetration, sperm proteases such as chymotrypsin-like enzyme, acrosin, spermosin and proteasome are suggested to play essential roles. Sperm glycosidase, that is translocated from the tip of sperm head to the surface overlying the mitochondrion, anchors the mitochondrion at the outer surface of vitelline coat. Therefore it assists sperm to penetrate the vitelline coat and traverse the perivitelline space. For fusion with egg plasma membrane, sperm metalloendoprotease seems to be involved. Egg glycosidases and proteases serve for some steps after fertilization, such as the prevention of polyspermy, expansion of perivitelline space and regulation of cell cycle.     

Specific inhibition of sperm β-N-acetylglucosaminidase by the synthetic inhibitor N-acetylglucosaminono-1,5-lactone O-(phenylcarbamoyl)oxime inhibits fertilization in the ascidian, Phallusia mammillata

Increasing evidence has evolved from studies in ascidians and mammals that sperm β- N -acetylglucosaminidase (GlcNAc'ase) plays a crucial role in fertilization. In the ascidian Phallusia mammillata , GlcNAc'ase is the predominant sperm-bound glycosidase and N-acetylglucosamine (GlcNAc) is the prevailing glycoside residue on the vitelline coat. We report here that the GlcNAc'ase inhibitor O -(2-acetamido-2-deoxy-D-glucopyrano-sylidene)-amino- N -phenylcarbamate (PUGNAC) is a potent competitive inhibitor of sperm-bound GlcNAc'ase in P. mammillata . The inhibitor constant K_i for the isolated enzyme is 47 nmol/L. Fertilization of eggs is inhibited by PUGNAC in a dose dependent competitive manner with 50% inhibition at an inhibitor concentration of 85 μmol/L. Further experiments, in which intact eggs possessing an egg coat were mixed with eggs from which the coat had been removed, showed that only fertilization of intact eggs was inhibited by PUGNAC. This finding suggests that PUGNAC prevents the binding of the sperm-associated GlcNAc'ase to terminal GlcNAc residues on the vitelline coat, thus inhibiting sperm binding and subsequently fertilization. Furthermore and most importantly, it shows that treatment with PUGNAC does not affect the viability of sperm and that the process of sperm-egg fusion is not affected.     

Ascidian sperm penetration and the translocation of a cell surface glycosidase

Sperm bind to vitelline coat (VC) glycosides of ascidian eggs by means of a sperm surface glycosidase (Hoshi et al.: Zool Sci 2:65, 1985). In the genus Ascidia, N-acetylglucosamine (NAG) is the VC ligand. After initial binding by the tip of the head, sperm pass through the VC and perivitelline space leaving the single mitochondrion outside. This process can also be followed in vitro on a coverslip. Analysis of recorded video images shows that the sperm moves away from the anchored mitochondrion. Our model for sperm penetration suggests that mitochondrial translocation is responsible for driving the sperm into the egg. In the work presented here, we have demonstrated that ascidian sperm have N-acetyl-beta-D-glucosaminidase (NAGase) activity with an acidic pH optimum. This enzyme, which can be removed from the sperm with Triton X-100, binds to concanavalin A, demonstrating that it is glycosylated. Histochemical methods disclose that the enzyme is originally located at the tip of the head but subsequently remains with the surface overlying the mitochondrion during translocation. Fluorescent Con A was used as a second label for localization of the enzyme on the cell surface during translocation. Colocalization of both probes of the enzyme support a crucial facet of our model; the sperm surface VC binding site remains over the mitochondrion during translocation. This would couple mitochondrial translocation with sperm penetration and drive the sperm into the egg.     

Participation of sperm proteasome in fertilization of the phlebobranch ascidian Ciona intestinalis

Sperm proteasomes are thought to be involved in sperm binding to and in sperm penetration through the vitelline coat of the eggs of the stolidobranch ascidian Halocynthia roretzi. However, it is not known whether they are involved in the fertilization of eggs of other ascidians. Therefore, we investigated whether sperm proteasomes are also involved in the fertilization of the eggs of the primitive phlebobranch ascidian Ciona intestinalis. Fertilization of the eggs of C. intestinalis was potently inhibited by the proteasome inhibitors MG115 and MG132 but not by the cysteine protease inhibitor E-64-d. On the other hand, neither fertilization of the vitelline coat-free eggs nor sperm binding to the vitelline coat was inhibited by the two proteasome inhibitors at a concentration sufficient to inhibit fertilization of intact eggs. These results indicate that the proteasome plays an essential role in sperm penetration through the vitelline coat rather than in sperm binding to the coat or in sperm-egg membrane fusion. The proteasome activity, which was detected in the sperm extract using Suc-Leu-Leu-Val-Tyr-MCA as a substrate, was strongly inhibited by both MG115 and MG132, and was weakly inhibited by chymostatin, whereas neither leupeptin nor E-64-d inhibited the activity. The molecular mass of the enzyme was estimated to be 600-kDa by Superose 12 gel filtration, and the activity in sperm extract was immunoprecipitated with an anti-proteasome antibody. These results indicate that the proteasome present in sperm of C. intestinalis is involved in fertilization, especially in the process of sperm penetration through the vitelline coat, probably functioning as a lysin. Mol. Reprod. Dev. 50:493–498, 1998. © 1998 Wiley-Liss, Inc.     

Ascidian sperm lysin system

Fertilization is a precisely controlled process involving many gamete molecules in sperm binding to and penetration through the extracellular matrix of the egg. After sperm bind to the extracellular matrix (vitelline coat), they undergo the acrosome reaction which exposes and partially releases a lytic agent called "lysin" to digest the vitelline coat for the sperm penetration. The vitelline coat sperm lysin is generally a protease in deuterostomes. The molecular mechanism of the actual degradation of the vitelline coat, however, remains poorly understood. In order to understand the lysin system, we have been studying the fertilization mechanism in ascidians (Urochordata) because we can obtain large quantities of gametes which are readily fertilized in the laboratory. Whereas ascidians are hermaphrodites, which release sperm and eggs simultaneously, many ascidians, including Halocynthia roretzi, are strictly self-sterile. Therefore, after sperm recognize the vitelline coat as nonself, the sperm lysin system is thought to be activated. We revealed that two sperm trypsin-like proteases, acrosin and spermosin, the latter of which is a novel sperm protease with thrombin-like substrate specificity, are essential for fertilization in H. roretzi. These molecules contain motifs involved in binding to the vitelline coat. We found that the proteasome rather than trypsin-like proteases has a direct lytic activity toward the vitelline coat. The target for the ascidian lysin was found to be a 70-kDa vitelline coat component called HrVC70, which is made up of 12 EGF-like repeats. In addition to the proteasome system, the ubiquitination system toward the HrVC70 was found to be necessary for ascidian fertilization. In this review, I describe recent progress on the structures and roles in fertilization of the two trypsin-like proteases, acrosin and spermosin, and also on the novel extracellular ubiquitin-proteasome system, which plays an essential role in the degradation of the ascidian vitelline coat.     

Identification of sperm plasma membrane proteins exhibiting binding affinity for the ascidian egg coat

In the initial stage of ascidian fertilization sequential sperm–egg coat interactions assure successful species-specific fertilization. Sperm recognize, bind to, and then penetrate the egg investment that consists of follicle cells (FC) and an acellular vitelline coat (VC). To identify plasma proteins that recognize the egg coat, a membrane fraction was prepared from Phallusia mammillata sperm using nitrogen cavitation followed by three centrifugation steps. The purity of the membrane fractions was assessed by transmission electron microscopy and marker enzymes. Comparison of the electrophoretic pattern of sperm extracellular membrane domains labeled by radio-iodination or biotinylation and recorded by autoradiography or enhanced chemiluminescence, respectively, showed the non-radioactive procedure to be a convenient and efficient method. Isolated sperm membrane components were found to inhibit fertilization in a concentration-dependent manner and to bind mainly to the FC. Eggs were used as an affinity matrix to determine which of the solubilized sperm membrane proteins possess egg-binding activity. Three biotinylated proteins (66kDa, 120kDa and 140kDa) were found to bind to the VC. Assays probing heterospecific binding to Ascidia mentula eggs revealed that the 120kDa protein possesses species-specific binding activity. Thus, the current data suggest the 120 kDa sperm membrane protein as a candidate adhesion molecule with a possible role in gamete binding and species-specific recognition in P. mammillata .     

Concanavalin A Modifies Allo-specific Sperm-Egg Interactions in the Ascidian, Ciona intestinalis

In the self sterile ascidian, Ciona intestinalis , the spermatozoa rarely bind to the vitelline coat of autologous eggs and never penetrate it. We report here that concanavalin A (ConA), a lectin recognizing mannose or glucose residues of carbohydrates, can modify these self- and nonself-specific sperm-egg interactions. When eggs were pretreated with 0.1–0.5 mg/ml of ConA, about two thousand spermatozoa became attached to the autologous vitelline coat within five minutes of insemination. The effect of ConA was not modified by the addition of D-mannose or pretreatment of spermatozoa with ConA, showing that ConA does not function merely as a ligand bridging the sperm and vitelline coat. In contrast to the marked enhancement of sperm-egg binding, ConA did not facilitate the penetration of spermatozoa through the autologous vitelline coat. Even in non-autologous insemination, it blocked the sperm penetration and, consequently, fertilization did not occur, as shown by Rosati et al. (1978). D-Mannose, when mixed with ConA in advance, completely abolished this inhibitory effect of ConA. Lotus agglutinin, a fucose-binding lectin, was less effective and wheat germ agglutinin and soy bean agglutinin had no effect on sperm entry in the perivitelline space. The results of this study are discussed in relation to the possible involvement of mannosyl and/or glucosyl glycoconjugates in allo-specific sperm-egg interactions.     

Two Extracellular Matrices From Oocytes of the Marine Shrimp Sicyonia ingentis that Independently Mediate Only Primary or Secondary Sperm Binding

During spawning, female Sicyonia ingentis simultaneously release ova and stored nonmotile sperm and mix them externally to initiate gamete interaction. Sperm bind to a thin vitelline envelope (VE) via their anterior appendage and within seconds are induced to undergo acrosomal exocytosis. The sperm penetrate the VE and become secondarily bound to the surface coat (SC), a glycocalyx on the oocyte surface. In this study, both extracellular matrices were isolated from S. ingentis oocytes. Isolated VEs mediated only primary sperm binding (i.e., before the acrosome reaction), while the isolated SCs mediated only secondary sperm binding (i.e., after acrosomal exocytosis). Isolated S. ingentis VEs were used to characterize primary sperm binding activity. The two extracellular matrices differ morphologically and possess different polypeptide profiles. Soluble fractions of isolated VEs inhibited primary sperm binding in a concentration dependent manner, and immunolocalization of VE components demonstrated highly localized VE binding sites at the tip of the sperm anterior appendage by which sperm bind eggs. Extensive Pronase digestion of VE components did not affect sperm binding activity of solubilized VE components, while complete deglycosylation with trifluoromethanesulfonic acid destroyed sperm binding activity. However, neither alkaline treatment nor enzyme digestion using glycosidases specific for asparagine and serine/ threonine linked oligosaccharides affected sperm binding activity.     

Phorbol Ester Induces Elevation of the Vitelline Coat of Eggs of the Ascidian Halocynthia roretzi

Elevation of the vitelline coat of eggs of the ascidian, Halocynthia roretzi , was induced by 12-O-tetradecanoylphorbol-13-acetate or phorbol-12, 13-didecanoate, but not by their 4-epimers. After elevation of the vitelline coat in this way, eggs were unable to undergo cleavage on addition of sperm. This elevation of the vitelline coat by phorbol esters was inhibited by specific inhibitors of trypsin-like enzyme, calmodulin, phospholipase A₂, and protein kinase C. In association with elevation of the vitelline coat, a trypsin-like enzyme was released from the eggs. The properties of this enzyme were similar to those of the enzyme released by calcium ionophore, which also induced elevation of the vitelline coat, suggesting that similar exocytosis of intracellular granules occurred in both cases. Calmodulin, possibly involved in the elevation process, was isolated from eggs and characterized.     

Pre-fusion events of sperm-oocyte interaction in the marine shrimp,Sicyonia ingentis

The non-motile sperm of Sicyonia ingentis, mixed with eggs by a spawning female, undergo a primary binding to the vitelline envelope (VE) of the oocyte. Once bound to the VE, sperm undergo exocytosis of the acrosomal vesicle, penetrate the VE, and secondarily bind to a surface coat that is closely associated with the oolemma. Unreacted sperm preincubated with solubilized VE components exhibit diminished binding to VEs in a concentration dependent manner. The ligand responsible for this binding is a carbohydrate moiety in the VE. The ligand preferentially binds to the anterior tip of unreacted sperm, as demonstrated with anti-VE polyclonal antibodies. Acrosome intact sperm will not bind to surface coats; however, acrosome reacted sperm do bind to surface coats via an externalized acrosomal granule.     

Helper Function of Follicle Cells in Sperm-Egg Interactions of the Ascidian, Ciona intestinalis

Studies were made on the involvement in sperm-egg interactions of follicle cells of Ciona intestinalis , which are tall, vacuolated cells attached to the outer surface of the egg vitelline coat. The basal surface of the follicle cells is polygonal. The borders between cells could easily be observed by the binding of fluorescent SBA (soy bean agglutinin), a lectin recognizing N-acetylgalactosamine (GaINAc) residues. At fertilization many spermatozoa aggregate along these polygonal borders of cells on the vitelline coat, through which they entered the perivitelline space. The removal of follicle cells was sometimes associated with loss of SBA-binding sites, and in such cases the sperm did not show a hexagonal pattern of aggregation, but became dispersed all over the vitelline coat. Removal of the follicle sometimes delayed fertilization. Examination of sections of gametes stained with DAPI, a fluorescent dye staining DNA, showed that removal of the follicle reduced the number of spermatozoa bound to the vitelline coat and, more especially, the number of spermatozoa penetrating through the vitelline coat. The blockage of GalNAc residues on the vitelline coat with SBA did not appreciably affect the time course of fertilization or the number of sperm associated with eggs. These findings are discussed in relation to the role of follicle cells in facilitating sperm aggregation on the vitelline coat and their penetration through it.     

Sperm-egg binding mediated by sperm alpha-L-fucosidase in the ascidian,Halocynthia roretzi

Spermatozoa bind to the vitelline coat in the ascidians and many other animals. The binding of sperm in Halocynthia roretzi is mediated by a sperm alpha-L-fucosidase and complementary-L-fucosyl residues of glycoproteins in the vitelline coat. cDNA clones for alpha-L-fucosidase were isolated from growing testis mRNA. It contained a 1398 bp full-length cDNA insert (HrFuc'ase) that encoded the 466 amino acid residues of H. roretzi sperm alpha-L-fucosidase. A putative signal peptide of 21 amino acid residues proceeded the sequence for the mature protein (M.W. 52.4 kDa). The coding sequence for HrFuc'ase showed 47.7% sequence identity to the human liver fucosidase sequence. The polyclonal antibody was prepared against a lacZ-HrFuc'ase fusion protein expressed in E. coli. The antibody crossed to a 54 kDa protein in sperm on western blotting and inhibited fertilization in a dose dependent manner. These data suggest that sperm-egg binding is mediated by the sperm alpha-L-fucosidase, HrFuc'ase in the ascidian, H. roretzi.     

Extracellular ubiquitin system implicated in fertilization of the ascidian, Halocynthia roretzi: isolation and characterization

The ubiquitin-proteasome system is essential for intracellular protein degradation, but there are few studies of this system in the extracellular milieu. Recently, we reported that a 70-kDa sperm receptor, HrVC70, on the vitelline coat is ubiquitinated and then degraded by the sperm proteasome during fertilization of the ascidian, Halocynthia roretzi. Here, we investigated the mechanism of extracellular ubiquitination. The HrVC70-ubiquitinating enzyme activity was found to be released from the activated sperm during the fertilization process. This enzyme was purified from an activated sperm exudate, by chromatography on DEAE-cellulose and ubiquitin-agarose columns, and by glycerol density gradient centrifugation. The molecular mass of the enzyme was estimated to be 700 kDa. The purified enzyme requires CaCl2 and MgATP for activity, and is active in seawater. The purified enzyme preparation, but not the crude enzyme preparation, showed narrow substrate specificity to HrVC70. Moreover, ATP and ubiquitin are released from the activated sperm to the surrounding seawater during fertilization. These results indicate that ascidian sperm release a novel extracellular ubiquitinating enzyme system together with ATP and ubiquitin during penetration of the vitelline coat of the egg, which catalyzes the ubiquitination of the HrVC70, an essential component of ascidian fertilization.     

Ascidian eggs release glycosidase activity which aids in the block against polyspermy

To ensure normal development, most animals have evolved a number of mechanisms to block polyspermy including prevention of binding to surface coats as well as sperm-egg fusion. Ascidian sperm bind to vitelline coat (VC) glycosides. In the genus Ascidia, N-acetylglucosamine (GlcNAc) is the ligand to which sperm bind. The number of sperm bound to the VC is biphasic following fertilization; sperm binding increases through the first minute or so, then abruptly declines. At fertilization, the eggs of Ascidia callosa, A. ceratodes, A. mentula, A. nigra and Phallusia mammillata release N-acetylglucosaminidase into the sea water (SW). This has been shown to inactivate VC GlcNAc groups, blocking the binding of supernumerary sperm and polyspermy in A. nigra. This block to polyspermy is inactivated by GlcNAc (2mM) or 150 mM-Na+ (choline substituted) SW. These treatments are not additive and therefore probably affect the same process. In A. callosa, fertilization in low Na+ SW causes a 60% decline in enzyme release and a similar increase in the number of sperm remaining on the VC at 4 min as well as a great increase in polyspermy. Thus the principal block to polyspermy in ascidian eggs involves the release of N-acetylglucosaminidase which appears to be Na+ dependent. Enzyme activity is found in the supernatant SW by 15 s after fertilization, suggesting that it is stored very near the egg surface. Histochemical staining of whole eggs and embryos shows loss of surface-associated enzyme activity following fertilization. Like other lysosomal enzymes this N-acetylglucosaminidase is mannosylated and has an acidic pH optimum.     

Effects of protease inhibitors on sperm-related events in sea urchin fertilization

Evidence for sperm-borne proteolytic enzymes exposed during the acrosome reaction in sea urchin sperm has been accumulating. To investigate the possible role(s) such enzymes have in fertilization, we studied the effects of several protease inhibitors on sperm-related events. Soybean trypsin inhibitor, Nα-p-tosyl-l-lysine, chloromethyl ketone, phenylmethylsulfonyl fluoride, and chymostatin neither reduced the number of acrosome reactions nor interfered with gamete binding. p-Nitrophenyl-p′-guanidinobenzoate caused sperm to fuse into irregular clumps, rendering them unable to fertilize eggs. However, l-1-tosylamide-2-phenylethyl chloromethyl ketone (TPCK), an inhibitor of chymotrypsin, prevented the acrosome reaction in Strongylocentrotus purpuratus, S. droebachiensis, and Lytechinus pictus. The effects of TPCK on sperm in subsequent steps of fertilization were also investigated. First, gamete binding assays were performed on fixed eggs. This precluded any effects TPCK might have had on egg-derived secretions (e.g., proteases). Binding of prereacted sperm occurred with both fixed and living eggs. However, fertilization of living eggs in the presence of TPCK was greatly reduced, even though sperm had been prereacted with egg jelly. Vitelline coats were then removed from eggs by trypsin treatment. Eggs in TPCK fertilized and developed normally after the above treatment. These observations are consistent with the hypothesis of a sperm protease participating in the acrosome reaction and the penetration of the egg vitelline coat in the sea urchin.     

The vitelline coat spikes: a new peculiar structure of Mytilus galloprovincialis eggs with a role in sperm-egg interaction

In the course of this study we found that in Mytilus galloprovincialis eggs long filamentous protrusions never described before, which we have termed "vitelline coat spikes," could be clearly detected using the lectin from Dolichos biflorus, which recognizes the GalNAc residues. The spikes could be also observed by transmission electron microscope but only in some fortuitous sections could their origin in the vitelline coat be clearly observed. The spikes were also clearly visible using the scanning electron microscope. Observations of the sperm-egg interaction very few seconds after insemination or using fixed eggs suggested that the spikes could play a role in a primary binding to the unreacted sperm. Experiments have been done to test the effect of GalNAc on the sperm-egg binding and on the fertilization process which seem to confirm this hypothesis.     

Purification and characterization of a vitelline coat lysin from Ciona intestinalis spermatozoa.

In Ciona intestinalis a chymotrypsin-like activity is involved in sperm penetration of the egg vitelline coat. A chymotrypsin-like enzyme has been purified from spermatozoa by a protocol including ion exchange chromatography, gel filtration, and native polyacrylamide gel electrophoresis. The purified enzyme resulted homogeneous when analyzed by SDS-PAGE. The molecular weight of the chymotrypsin-like enzyme was estimated to be 35 kDa by gel filtration and 24 KDa by SDS-PAGE in nonreducing conditions. The pH optimum of the enzyme is 8.4 and its activity is enhanced by Ca2+. It shows the highest activity towards the synthetic substrate Suc-Ala-Ala-Pro-Phe-AMC. Furthermore, by electron microscopy, the purified enzyme affects the structure of egg vitelline coat, and thus it fulfills one of the criteria of a lysin.     

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.     

Molecular Reproduction & Development Volume 78, Issue 7 cover image

Ascidians are hermaphrodites, releasing sperm and eggs nearly simultaneously, but several species including the ascidian (prochodate) Ciona intestinalis are self‐sterile (self‐incompatible). In this species, the self‐incompatibility system is mediated by the vitelline coat ligand v‐Themis‐A/B and the sperm‐side receptor s‐Themis‐A/B. In this issue, it is described that a sperm GPIanchored protein CiUrabin in lipid rafts may play a key role in the primary binding of sperm to the vitelline coat. This image of a C. intestinalis egg was taken by Takako Saito.     

Sperm binding to eggs of Ciona intestinalis. Role of Ca2+

In this paper we show that in the ascidia Ciona intestinalis extracellular Ca2+ is required for the binding of the spermatozoa to the vitelline coat (VC) glycerol-treated eggs and for fertilization to occur. Divalent cations, Mg2+ and Mn2+, cannot replace Ca2+. Once bound, the spermatozoa cannot be detached from the vitelline coat by adding of EGTA. Verapamil does not interfere with the binding of spermatozoa to the vitelline coat, whereas it blocks the Ca2+ ionophore A23187-induced sperm activation and acrosome reaction. Fertilization too was inhibited by the presence of this drug.