Zona-induced acrosome reaction of hamster spermatozoa

New insights into the role of microheterogeneity of ZP3 during structural maturation of the avian equivalent of mammalian zona pellucida

The egg coat including mammalian zona pellucida (ZP) and the avian equivalent, i.e., inner-perivitelline layer (IPVL), is a specialized extracellular matrix being composed of the ZP glycoproteins and surrounds both pre-ovulatory oocytes and ovulated egg cells in vertebrates. The egg coat is well known for its potential importance in both the reproduction and early development, although the underlying molecular mechanisms remain to be fully elucidated. Interestingly, ZP3, one of the ZP-glycoprotein family members forming scaffolds of the egg-coat matrices with other ZP glycoproteins, exhibits extreme but distinctive microheterogeneity to form a large number of isoelectric-point isoforms at least in the chicken IPVL. In the present study, we performed three-dimensional confocal imaging and two-dimensional polyacrylamide-gel electrophoresis (2D-PAGE) of chicken IPVLs that were isolated from the ovarian follicles at different growth stages before ovulation. The results suggest that the relative proportions of the ZP3 isoforms are differentially altered during the structural maturation of the egg-coat matrices. Furthermore, tandem mass spectrometry (MS/MS) analyses and ZP1 binding assays against separated ZP3 isoforms demonstrated that each ZP3 isoform contains characteristic modifications, and there are large differences among ZP3 isoforms in the ZP1 binding affinities. These results suggest that the microheterogeneity of chicken ZP3 might be regulated to be associated with the formation of egg-coat matrices during the structural maturation of chicken IPVL. Our findings may provide new insights into molecular mechanisms of egg-coat assembly processes.

Elevated and Sustained Intracellular Calcium Signalling Is Necessary for Efficacious Induction of the Human Sperm Acrosome Reaction

Progesterone and prostaglandin E1 are postulated to trigger the human sperm acrosome reaction (AR). However, their reported efficacy is very variable which likely, in part, reflects the plethora of experimental conditions and methodologies used to detect this physiologically relevant event. The purpose of this study was to develop an assay for the robust induction and objective measurement of the complete AR. Sperm from healthy volunteers or patients undertaking IVF were treated with a variety of ligands (progesterone, prostaglandin E1 or NH4Cl, alone or in combinations). AR, motility and intracellular calcium measurements were measured using flow cytometry, computer-assisted sperm analysis (CASA) and fluorimetry, respectively. The AR was significantly increased by the simultaneous application of progesterone, prostaglandin E1 and NH4Cl, following an elevated and sustained intracellular calcium concentration. However, we observed notable inter- and intra-donor sample heterogeneity of the AR induction. When studying the patient samples, we found no relationship between the IVF fertilization rate and the AR. We conclude that progesterone and prostaglandin E1 alone do not significantly increase the percentage of live acrosome-reacted sperm. This assay has utility for drug discovery and sperm toxicology studies but is not predictive for IVF success.

Molecular Mechanism Underlying the Action of Zona-pellucida Glycoproteins on Mouse Sperm

Mammalian oocytes are enveloped by the zona pellucida (ZP), an extracellular matrix of glycoproteins. In sperm, stimulation with ZP proteins evokes a rapid Ca²⁺ influx via the sperm-specific, pH-sensitive Ca²⁺ channel CatSper. However, the physiological role and molecular mechanisms underlying ZP-dependent activation of CatSper are unknown. Here, we delineate the sequence of ZP-signaling events in mouse sperm. We show that ZP proteins evoke a rapid intracellular pH_i increase that rests predominantly on Na⁺/H⁺ exchange by NHA1 and requires cAMP synthesis by the soluble adenylyl cyclase sAC as well as a sufficiently negative membrane potential set by the spem-specific K⁺ channel Slo3. The alkaline-activated CatSper channel translates the ZP-induced pH_i increase into a Ca²⁺ response. Our findings reveal the molecular components underlying ZP action on mouse sperm, opening up new avenues for understanding the basic principles of sperm function and, thereby, mammalian fertilization.

Sperm acrosome reaction: its site and role in fertilization

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

A Model for the Acrosome Reaction in Mammalian Sperm

The acrosome reaction is a complex, calcium-dependent reaction that results in an exocytotic event required for successful fertilization of the egg. It has long been thought that the acrosome reaction occurs upon sperm binding to the zona pellucida, a viscoelastic layer surrounding the oocyte. Recent studies have suggested that the reaction may even occur before the sperm encounters the zona, perhaps mediated by progesterone or some other agonist. It has been particularly difficult to understand differences between progesterone-induced and zona-induced reactions experimentally and whether one substance is the more biologically relevant trigger. Until this present work, there has been little effort to mathematically model the acrosome reaction in sperm as a whole. Instead, attention has been paid to modeling portions of the pathways involved in other cell types. Here we present a base model for the acrosome reaction which characterizes the known biochemical reactions and behaviors of the system. Our model allows us to analyze several pathways that may act as a stabilizing mechanism for avoiding sustained oscillatory calcium responses often observed in other cell types. Such an oscillatory regime might otherwise prevent acrosomal exocytosis and therefore inhibit fertilization. Results indicate that the acrosome reaction may rely upon multiple redundant mechanisms to avoid entering an oscillatory state and instead maintain a high resting level of calcium, known to be required for successful acrosomal exocytosis and, ultimately, fertilization of the oocyte.

Avian Egg and Egg Coat

An ovulated egg of vertebrates is surrounded by unique extracellular matrix, the egg coat or zona pellucida, playing important roles in fertilization and early development. The vertebrate egg coat is composed of two to six zona pellucida (ZP) glycoproteins that are characterized by the evolutionarily conserved ZP-domain module and classified into six subfamilies based on phylogenetic analyses. Interestingly, investigations of biochemical and functional features of the ZP glycoproteins show that the roles of each ZP-glycoprotein family member in the egg-coat formation and the egg-sperm interactions seemingly vary across vertebrates. This might be one reason why comprehensive understandings of the molecular basis of either architecture or physiological functions of egg coat still remain elusive despite more than 3 decades of intensive investigations. In this chapter, an overview of avian egg focusing on the oogenesis are provided in the first section, and unique features of avian egg coat, i.e., perivitelline layer, including the morphology, biogenesis pathway, and physiological functions are discussed mainly on chicken and quail in terms of the characteristics of ZP glycoproteins in the following sections. In addition, these features of avian egg coat are compared to mammalian zona pellucida, from the viewpoint that the structural and functional varieties of ZP glycoproteins might be associated with the evolutionary adaptation to their reproductive strategies. By comparing the egg coat of birds and mammals whose reproductive strategies are largely different, new insights into the molecular mechanisms of vertebrate egg-sperm interactions might be provided.

Seminal vesicle protein SVS2 is required for sperm survival in the uterus

In mammals, sperm migrate through the female reproductive tract to reach the egg; however, our understanding of this journey is highly limited. To shed light on this process, we focused on defining the functions of seminal vesicle secretion 2 (SVS2). SVS2(-/-) male mice produced sperm but were severely subfertile, and formation of a copulatory plug to cover the female genital opening did not occur. Surprisingly, even when artificial insemination was performed with silicon as a substitute for the plug, sperm fertility in the absence of SVS2 remained severely reduced because the sperm were already dead in the uterus. Thus, our results provide evidence that the uterus induces sperm cell death and that SVS2 protects sperm from uterine attack.

Replacement of calcium for strontium in hamster sperm incubation media: effect on sperm function

Calcium (Ca(2+)) is an absolute requirement for a decisive sperm function event: the acrosome reaction (AR). Physiologically, sperm capacitation is a prerequisite for this specialized exocytosis and both events are intimately related. In an effort to separate capacitation from AR, we have been using a modified sperm incubation medium where Ca(2+) is replaced by Strontium (Sr(2+)). The aim of this report is to analyze with more detail the difference between sperm incubated with Ca(2+) or Sr(2+) in several events. We found that sperm undergo the capacitation-related changes in the chlortetracycline (CTC) pattern and tyrosine phosphorylation, and also bind to the zona pellucida (ZP) when using Sr(2+)-instead of Ca(2+)-containing media. However, the spontaneous AR typical of hamster sperm does not take place in Sr(2+)-medium, even if sperm are previously capacitated with Ca(2+). Nevertheless, Sr(2+) was able to sustain AR when cells were treated with thapsigargin or depolarized with K(+) in Na(+)-depleted medium. Considering that the absence of Na(+) increased spontaneous AR in Sr(2+)-medium, we tested whether Na(+)-transport systems could be involved in the inability of Sr(2+)-incubated sperm to undergo AR. We found that when sperm incubated in Sr(2+)-medium are treated with amiloride to inhibit epithelial Na(+) channel (ENaC), they are able to undergo spontaneous AR. The same result was obtained when analyzing AR on the ZP. On the contrary, addition of ouabain (a Na(+)/K(+)-ATPase inhibitor) or DIDS (a Na(+)/HCO3(-) co-transporter inhibitor) showed no effect. These results suggest that, differing from what happens in Ca(2+)-incubated sperm, cells incubated in Sr(2+)-modified medium would have an active ENaC.

Gamete interaction: Is it species-specific?

Reproductive isolation is pivotal to maintain species separation and it can be achieved through a plethora of mechanisms. In addition, the development of barriers to gamete interaction may drive speciation. Such barriers to interspecific gamete interaction can be prezygotic or postzygotic. Considering the great diversity in animal species, it is easy to assume that regulation of the early steps of fertilization is critical to maintain species identity. One prezygotic mechanism that is often mentioned in the literature is that gamete interaction is limited to gametes of the same species. But do gametes of all animals interact in a species-specific way? Are gamete interactions completely species-specific or perhaps just species-restricted? In species in which species-restrictions have been described, is the interspecies barrier at one major step in the fertilization process or is it a combination of partially restricted steps that together lead to a block in interspecific fertilization? Are the mechanisms used to avoid interspecific crosses different between free-spawning organisms and those with internal fertilization? This review will address these questions, focusing on prezygotic barriers, and will describe what is known about the molecular biology that may account for species-limited gamete recognition and fertilization.

Analysis of the participation of N-acetylglucosamine in the different steps of sperm-zona pellucida interaction in hamster

Glycoproteins and lectin-like proteins mediate sperm-zona pellucida interaction. The present study analysed the participation of carbohydrates in the different stages of sperm interaction with the zona pellucida in hamster, by determining the effects of different monosaccharides. N-acetylglucosamine (GlcNAc, 1 mM) reduced sperm ability to bind to the zona pellucida. Surprisingly, spontaneous acrosome reaction (AR) was also inhibited by this sugar. In order to analyse the effect of GlcNAc on sperm-zona pellucida binding, independent of its effect on the AR, strontium (Sr) was used as a calcium (Ca) replacement in the sperm capacitation and co-incubation medium. Sr seemed to be able to replace Ca for sperm capacitation, at least when measured as the ability to bind to the zona pellucida, and undergo AR when Ca is provided. Moreover, sperm-zona pellucida binding could also take place in a Sr-modified medium. When binding assays were carried out in the Sr medium, GlcNAc also produced an inhibitory effect. This could be reproduced when sperm, but not oocytes, were pre-incubated with the monosaccharide. IVF assays were also carried out to analyse the participation of GlcNAc in the different steps of sperm-oocyte interaction. Taken together, the results support the involvement of the GlcNAc residues of the zona pellucida in the early steps of the interaction with sperm.

Acrosomal integrity and capacitation are not influenced by sperm cryopreservation in the giant panda

Sperm cryopreservation and artificial insemination are important management tools for giant panda breeding and the preservation of extant genetic diversity. This study examined the influence of freeze-thawing on sperm function, specifically capacitation. Sperm from nine giant pandas were assessed before and after rapid (- 40 and - 100 degrees C/min) cryopreservation by incubation in HEPES-buffered Ham's F10 medium with and without the capacitation accelerators, 3-isobutyl-1-methylxanthine (IBMX) and dibutyryl cyclic AMP (dbcAMP). At 0, 3 and 6 h of exposure, aliquots were assessed for sperm motility traits and capacitation, defined as the proportion of sperm with intact acrosomes following exposure to solubilised zonae pellucidae (ursid or felid) or calcium ionophore subtracted from the proportion of sperm with intact acrosomes before exposure. Although mean+/-S.E.M. sperm motility post-thaw (56.1 +/- 3.9% at 0 h) was less (P < 0.05) than pre-freeze (71.7 +/- 6.0%), there was no difference (P > 0.05) in the proportion of acrosome-intact sperm (fresh, 93.0 +/- 1.7% versus cryopreserved-thawed, 81.7 +/- 4.7% at 0 h). Incidence of capacitation was greater (P < 0.05) in fresh sperm incubated with capacitation accelerators IBMX and dbcAMP (9 h: 50.9 +/- 1.1) compared with fresh sperm incubated without accelerators (9 h: 41.2 +/- 1.1%). Frozen-thawed sperm preincubated without accelerators underwent capacitation (49.6 +/- 1.1%) to a greater extent (P < 0.05) compared with these fresh counterparts. Thawed samples with (9 h: 45.9 +/- 1.4%) and without accelerators (9 h: 41.2 +/- 1.1%) did not differ (P > 0.05) during the 9-h incubation. We conclude that giant panda spermatozoa (1) undergo capacitation in vitro with or without chemical accelerators and (2) withstand a rapid cryopreservation protocol, including retaining normal acrosomal integrity and functional capacitation ability.

Study of the acrosome reaction and the fertilizing ability of hamster epididymal cauda spermatozoa treated with antibodies against phospholipase A2 and/or lysophosphatidylcholine

The present report describes experiments in vitro that were designed to evaluate the involvement of phospholipase A2 (PLA2) in the acrosome reaction of mammalian sperm and the interaction of gametes. Hamster spermatozoa were incubated in a defined medium (TALP) to induce capacitation and the acrosome reaction. This medium was supplemented with antibodies against porcine pancreatic PLA2 and/or lysophosphatidylcholine (LPC). For in vitro fertilization, spermatozoa and/or oocytes were incubated in TALP medium that contained PLA2-specific antibodies, LPC, or antibodies plus LPC. The antibodies inhibited the acrosome reaction in a dose-dependent manner, without any effect on sperm motility or hyperactivation. These antibodies also inhibited fertilization in vitro. LPC, a product of the reaction catalysed by PLA2, speeds up and synchronizes the acrosome reaction and facilitates penetration of the zona pellucida by spermatozoa, the fusion process and polyspermy. The results of addition of the antibodies plus LPC showed that LPC is able to reverse the inhibitory effects of the antibodies on the acrosome reaction and fertilization. It is possible that endogenous PLA2 plays a role in the final stages of the acrosome reaction and the interaction of gametes, perhaps through one of its reaction products, LPC. The role of LPC might be to stimulate the fertilizing ability of spermatozoa, as well as to induce changes in the zona pellucida and the oolemma that allow sperm-egg fusion. Thus, it seems possible that PLA2 and one of its reaction products might contribute to membrane-fusion events during mammalian fertilization.

A sperm membrane protein that binds in a species-specific manner to the egg extracellular matrix is homologous to von Willebrand factor

We have purified a sperm membrane protein, designated zonadhesin, that binds in a species-specific manner to the extracellular matrix (zona pellucida) of the egg, and cloned its cDNA. The cDNA encodes a novel protein with a single transmembrane segment separating a 36 amino acid, highly basic intracellular C terminus from a 2418-amino acid extracellular region. The extracellular sequence specifies a mosaic protein comprising a unique N-terminal domain, a mucin-like domain, and five tandem domains proximal to the membrane that are homologous to prepro von Willebrand factor. The N-terminal and mucin-like domains were absent from zonadhesin that bound to the egg extracellular matrix, suggesting that processing occurs during sperm maturation and/or capacitation. By Northern blotting and in situ hybridization, zonadhesin mRNA was detected only within the testis, where it was expressed primarily in haploid spermatids. The unique domain structure of zonadhesin suggests multiple functions, one of which is to mediate sperm adhesion to the zona pellucida.

Comparison of the acrosome reaction-inducing ability of the outer and inner surfaces of the zona pellucida and oolemma: a study using the golden hamster

The outer surface of hamster zona pellucida has a strong ability to induce the sperm acrosome reaction, whereas neither the inner surface of the zona nor the oolemma has this ability. Therefore, the spermatozoa that fertilise oocytes after subzonal insemination of spermatozoa (SUZI) must have been either acrosome-reacted prior to injection or acrosome-reacted spontaneously while swimming within the perivitelline space. The outer surface of fresh mouse zona is not capable of inducing the acrosome reaction of hamster spermatozoa. When fixed with glutaraldehyde, however, it became capable of inducing the hamster sperm acrosome reaction rather efficiently. The reason for this is not clear.

Participation of glycosylated residues in the human sperm acrosome reaction: possible role of N-acetylglucosaminidase

Sperm binding to the egg zona pellucida is mediated by complementary protein-carbohydrate interaction. This binding results in the exocytosis of the sperm acrosome, or acrosome reaction (AR). We report the effect of different neoglycoproteins (sugar residues covalently bound to bovine serum albumin) on the human sperm AR. p-Aminophenyl-N-acetyl-beta-D-glucosaminide-BSA (BSA-GlcNAc) and p-aminophenyl-alpha-D-mannopyranoside-BSA (BSA-Man) at 1 micrograms/ml were capable of inducing the greatest percentages of AR (3-fold stimulation with respect to controls), while other NeoGPs had only a weak effect on this process. The BSA-GlcNAc-induced acrosome reaction was inhibited by N-acetylglucosamine (GlcNAc), p-nitrophenyl-GlcNAc, and purified soluble beta-N-acetylglucosaminidase (beta NAG). The induction of the AR with BSA-Man could be inhibited by mannose, while soluble alpha-mannosidase was only partially effective. These data suggest that binding sites for GlcNAc and mannose may be involved in the induction of the AR in human sperm. The characteristics of the BSA-GlcNAc induction suggest that the beta NAG molecule may be the mediator of this effect.

Induction of the acrosome reaction in human spermatozoa by human zona pellucida and effect of cervical mucus on zona-induced acrosome reaction

OBJECTIVES

To investigate the induction of the acrosome reaction in human spermatozoa by the zona pellucida (ZP), cumulus oophorus, and cervical mucus (CM), and to examine the effect of cumulus oophorus and CM on the zona-induced acrosome reaction. DESIGN, PATIENTS, MAIN OUTCOME MEASURES: The acrosome status of spermatozoa from healthy donors that were cultured with salt-stored human ZP, cumulus oophorus, and/or cervical mucus was assessed using fluoresceinated pisum sativum agglutinin.

RESULTS

The acrosome reaction rate after sperm attachment to the zona for 6 hours was 35.7% +/- 17.7%, which was higher than controls (2.8% +/- 1.9%). The acrosome reaction was not observed after passage through cumulus oophorus or CM; however, the acrosome reaction rate of spermatozoa passed through CM after the attachment to zona was 51.6% +/- 6.8%, a higher value in comparison with spermatozoa that had not passed through CM (25.6% +/- 9.4%).

CONCLUSION

These data indicate that human ZP are capable of inducing the acrosome reaction in human spermatozoa; however, cumulus oophorus and CM are unable to do so. The data further provide evidence that CM promotes the zona-induced acrosome reaction in human spermatozoa.

Monoclonal antibody AG7 inhibits fertilization post sperm-zona binding

Monoclonal antibodies (mAbs) against sperm cells are currently being used in an effort to define spermatozoal antigens involved in the fertilization process. We have produced a number of anti-human sperm mAbs by immunization of female mice with the 100,000 x g supernatant of octylglycoside-solubilized washed human sperm. From a panel of mAbs, 1 antibody, AG7, was selected and characterized due to its fertilization-inhibiting characteristics. MAb AG7 defines a sperm acrosome antigen-1 (SAA-1) located in the acrosomal region of human sperm as evaluated by indirect immunofluorescence. Staining of life sperm cells indicated that the antigen is present on the sperm surface. SAA-1 was also found on sperm of several other mammalian species, implying evolutionary conservation of the antigen. SAA-1 was first observed on testicular sperm and can be followed through epididymal transit, ejaculation, and capacitation. When applied in a mouse in vitro fertilization assay, mAb AG7 inhibits fertilization by greater than 95%, and inhibition is dose dependent, with half-maximal inhibition at 0.8 micrograms/ml. The block to fertilization could not be attributed to sperm agglutination, inhibition of motility, interference with adhesion to the zona pellucida, or inhibition of fusion with the oocyte membrane. MAb AG7 was demonstrated to inhibit calcium influx in spermatozoa in vitro (measured using the fluorescent indicator fura 2), a prerequisite for the acrosome reaction. Initial biochemical characterization of the antigen suggests it is proteinlike in nature, with a molecular weight of approximately 220 kD. The results suggest that SAA-1, identified by mAb AG7, is a sperm antigen crucially involved in the fertilization process, possibly an atypical steroid receptor or ion channel located within the sperm plasma membrane.

Sperm capacitation in the domestic cat (Felis catus) and leopard cat (Felis bengalensis) as studied with a salt-stored zona pellucida penetration assay

The ability of domestic cat or leopard cat spermatozoa to penetrate zonae pellucidae (ZP) of salt-stored, domestic cat oocytes was examined as an assay for sperm capacitation. Ovarian oocytes were recovered after ovariectomy and matured in vitro for 18-36 h. Following removal of cumulus cells, the oocytes were used fresh, or stored (4 degrees C, 0.5-24 weeks) in a HEPES-buffered hypertonic salt solution. Electroejaculated, washed sperm (2-4 x 10(6) sperm/ml) were preincubated for 1.0 h (38 degrees C, 5% CO2 in air) and then co-incubated (2 x 10(5) sperm/ml) with fresh or stored oocytes for 6.0 h. Gametes were incubated in a protein-free, modified Tyrode's solution (TLP-PVA) or in the same medium containing 4.0 mg/ml bovine serum albumin (BSA; TALP-PVA). Treatments were compared for percentage ZP penetration (defined as sperm heads reaching more than halfway through the ZP) as an index of sperm capacitation. In both the domestic cat and leopard cat, there was no difference (P greater than 0.05) in sperm penetration of fresh ZP (domestic cat, 42.5 +/- 5.4%; leopard cat, 38.6 +/- 2.8%) or stored ZP (domestic cat, 32.4 +/- 4.2%; leopard cat, 27.6 +/- 2.3%). Sperm incubated in protein-free medium (TLP-PVA) were less capable (P less than 0.05) of ZP penetration (domestic cat, 14.6 +/- 5.9%; leopard cat, 7.9 +/- 3.0%) than sperm incubated in medium TALP-PVA containing BSA (domestic cat, 60.3 +/- 5.9%; leopard cat, 58.4 +/- 3.0%).(ABSTRACT TRUNCATED AT 250 WORDS)

Hamster oocyte penetrability during preovulatory maturation

In vitro fertilization techniques were used to analyze the penetrability of preovulatory hamster oocytes. The zonas of granulosa cell-free primary (GV) oocytes were penetrated in vitro in 2-3 h as readily as those of ovulated secondary oocytes (80% vs. 88%), whether inseminated separately or as mixed oocyte groups. In fact, a significantly higher (P less than 0.05) mean number of perivitelline spermatozoa was present in immature (3.6) compared with secondary (1.9) oocytes, primarily reflecting a lack of the zona block to polyspermy in the immature population. By contrast, when granulosa cells remained around GV oocytes, zona penetration was low and more were penetrated, with more spermatozoa incorporated into the vitellus as a function of increasing time of oocyte recovery after hCG. We conclude, contrary to previous reports, that the zona pellucida of the hamster GV oocyte is readily penetrable by spermatozoa in vitro. However, the resumption of meiosis brings an increase in the penetrability of the granulosa cell vestment as well as the capacity for cortical granule exocytosis and the ability to decondense and transform the fertilizing sperm nucleus. The fact that the zona pellucida of the immature oocyte has proved to be penetrable in vitro and/or in vivo in all the mammals studied in this respect is discussed with particular reference to the situation in man.

Genomic organization and polypeptide primary structure of zona pellucida glycoprotein hZP3, the hamster sperm receptor

During the course of fertilization in mammals, free-swimming sperm bind tightly to receptors located in the egg extracellular coat, or zona pellucida. Recently, the hamster sperm receptor, a 56,000 Mr zona pellucida glycoprotein called hZP3, was identified and partially characterized (C. C. Moller et al., (1990). Dev. Biol. 137, 276-286). Here, we describe genomic cloning of hZP3, certain organizational features of the hZP3 gene, and primary structures of hZP3 mRNA and polypeptide. The findings are compared with reported results of comparable analyses of the mouse sperm receptor, an 83,000 Mr zona pellucida glycoprotein called mZP3. Such comparisons reveal a high degree of conservation of genomic organization and polypeptide structure for the two mammalian sperm receptors, despite the considerable difference in their Mrs. These findings are of interest in view of the extremely restricted expression of the ZP3 gene during development and the important role of ZP3 oligosaccharides in gamete adhesion.

Effect of follicle cells on the acrosome reaction, fertilization, and developmental competence of bovine oocytes matured in vitro

The role of follicle cells in the acrosome reaction of frozen-thawed bovine spermatozoa, in vitro fertilization, cleavage, and development in vitro was investigated. Cumulus-oocyte complexes were cocultured and matured in vitro with additional granulosa cells for 24 hr. Immediately before in vitro insemination, the oocytes were divided into three types with different follicle cells: denuded and corona- and cumulus-enclosed oocytes. The proportion of live, acrosome-reacted spermatozoa significantly increased at 3 and 6 hr after insemination in all types of oocytes. However, the mean proportion of live, acrosome-reacted spermatozoa that inseminated cumulus-enclosed oocytes at 6 hr after insemination was significantly higher than that of spermatozoa inseminating denuded oocytes (18.3% and 13.3%, respectively). The frequency of in vitro fertilization was significantly higher for cumulus-enclosed oocytes (65.4%) than for denuded and corona-enclosed oocytes (30.8% and 39.4%, respectively). Cumulus-enclosed oocytes when cocultured with oviduct epithelial cells also had significantly higher rates of cleavage (two- to eight-cell, 59.8%; eight-cell, 22.4%) and blastocyst formation (7.7%) than denuded and corona-enclosed oocytes. No eight-cell embryos or more advanced stages of embryonic development were observed in either denuded or corona-enclosed oocytes without the coculture. The present results indicate that cumulus cells at fertilization play an important role in inducing the acrosome reaction and promoting a high fertilization rate, cleavage, and development into blastocysts in vitro.

Structural and functional relationships between mouse and hamster zona pellucida glycoproteins

The hamster egg's extracellular coat, or zona pellucida, consists of three glycoproteins, designated hZP1, hZP2, and hZP3, that exhibit extensive heterogeneity on SDS-PAGE. hZP1 is a relatively minor component of hamster zonae pellucidae, as compared with hZP2 and hZP3. In the presence of reducing agents, hZP1, 200,000 apparent Mr, migrates on SDS-PAGE with an apparent Mr of 103,000. This suggests that hZP1, like mouse ZP1, is composed of two polypeptides held together by intermolecular disulfides. When purified hamster ZP glycoproteins were tested at relatively low concentrations in an in vitro competition assay, employing either hamster or mouse gametes, only hZP3 (56,000 apparent Mr) exhibited sperm receptor activity (i.e., inhibited binding of sperm to eggs). Thus, apparently hZP3 is the hamster counterpart of mouse ZP3, the mouse egg receptor for sperm. Furthermore, at relatively high concentrations, solubilized hamster egg ZP preparations induced both hamster and mouse sperm to undergo the acrosome reaction in vitro. hZP3 is encoded by a relatively abundant ovarian mRNA that is detected by a mouse ZP3 cDNA probe and is the same size, about 1.5 kb, as mRNA encoding the mouse sperm receptor, ZP3 (83,000 apparent Mr). Like mouse ZP2, hZP2 undergoes limited proteolysis following artificial activation of hamster eggs in vitro. Results of in vitro assays employing intact eggs and isolated zonae pellucidae demonstrate that hamster eggs possess a ZP2-proteinase which has a substrate specificity similar to that of the mouse enzyme. These observations are discussed in terms of structural and functional relationships that may exist between hamster and mouse zona pellucida glycoproteins.