The initial molecular interaction between mouse sperm and the zona pellucida is a complex binding event

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.

The Role of the Epididymis and the Contribution of Epididymosomes to Mammalian Reproduction

It is well-established that testicular spermatozoa are immature and acquire motility and fertilization capabilities during transit throughout the epididymis. The epididymis is a duct-like organ that connects the testis to the vas deferens and is comprised of four anatomical regions: the initial segment, caput, corpus, and cauda. Sperm maturation occurs during epididymal transit by the interaction of sperm cells with the unique luminal environment of each epididymal region. In this review we discuss the epididymis as an essential reproductive organ responsible for sperm concentration, maturation (including sperm motility acquisition and fertilizing ability), protection and storage. Importantly, we also discuss specific characteristics and roles of epididymal-derived exosomes (epididymosomes) in establishing sperm competency within the intricate process of reproduction. This review suggests that an increasing body of evidence is working to develop a complete picture of the role of the epididymis in male reproduction, offspring health, and disease susceptibility.

Functional Amyloids in Reproduction

Amyloids are traditionally considered pathological protein aggregates that play causative roles in neurodegenerative disease, diabetes and prionopathies. However, increasing evidence indicates that in many biological systems nonpathological amyloids are formed for functional purposes. In this review, we will specifically describe amyloids that carry out biological roles in sexual reproduction including the processes of gametogenesis, germline specification, sperm maturation and fertilization. Several of these functional amyloids are evolutionarily conserved across several taxa, including human, emphasizing the critical role amyloids perform in reproduction. Evidence will also be presented suggesting that, if altered, some functional amyloids may become pathological.

Network analyses of sperm-egg recognition and binding: ready to rethink fertility mechanisms?

The rapid growth of published literature makes biomedical text mining increasingly invaluable for unpacking implicit knowledge hidden in unstructured text. We employed biomedical text mining and biological networks analyses to research the process of sperm egg recognition and binding (SERB). We selected from the literature the molecules expressed either on spermatozoa or on oocytes thought to be involved in SERB and, using an automated literature search software (Agilent Literature Search), we realized a network, SERBN, characterized by a hierarchical scale free and a small world topology. We used an integrated approach, either based on selection of hubs or by a cluster analysis, to discern the key molecules of SERB. We found that in most cases some of them are not directly situated on spermatozoa and oocyte, but are dispersed in oviductal fluid or embedded in exosomes present in the perivitelline space. To confirm and validate our results, we performed further analyses using STRING and Reactome FI software. Our findings underscore that the fertility is not a property of gametes in isolation, but rather depends on the functional integrity of the entire reproductive system. These observations collectively underscore the importance of integrative biology in exploring biological systems and in rethinking of fertility mechanisms in the light of this innovative approach.

Differential expression and localization of glycosidic residues in in vitro- and in vivo-matured cumulus-oocyte complexes in equine and porcine species

Glycoprotein oligosaccharides play major roles during reproduction, yet their function in gamete interactions is not fully elucidated. Identification and comparison of the glycan pattern in cumulus-oocyte complexes (COCs) from species with different efficiencies of in vitro spermatozoa penetration through the zona pellucida (ZP) could help clarify how oligosaccharides affect gamete interactions. We compared the expression and localization of 12 glycosidic residues in equine and porcine in vitro-matured (IVM) and preovulatory COCs by means of lectin histochemistry. The COCs glycan pattern differed between animals and COC source (IVM versus preovulatory). Among the 12 carbohydrate residues investigated, the IVM COCs from these two species shared: (a) sialo- and βN-acetylgalactosamine (GalNAc)-terminating glycans in the ZP; (b) sialylated and fucosylated glycans in cumulus cells; and (c) GalNAc and N-acetylglucosamine (GlcNAc) glycans in the ooplasm. Differences in the preovulatory COCs of the two species included: (a) sialoglycans and GlcNAc terminating glycans in the equine ZP versus terminal GalNAc and internal GlcNAc in the porcine ZP; (b) terminal galactosides in equine cumulus cells versus terminal GlcNAc and fucose in porcine cohorts; and (c) fucose in the mare ooplasm versus lactosamine and internal GlcNAc in porcine oocyte cytoplasm. Furthermore, equine and porcine cumulus cells and oocytes contributed differently to the synthesis of ZP glycoproteins. These results could be attributed to the different in vitro fertilization efficiencies between these two divergent, large-animal models.

The identity of zona pellucida receptor on spermatozoa: an unresolved issue in developmental biology

Mammalian oocytes are surrounded by an acellular zona pellucida (ZP). Fertilization begins when a capacitated spermatozoon binds to the ZP. Defective sperm-ZP interaction is a cause of male infertility and reduced fertilization rates in clinical assisted reproduction treatment. Despite the importance of spermatozoa-ZP binding, the mechanisms and regulation of the interaction are unclear partly due to the failure in the identification of ZP receptor on spermatozoa. Most of the previous studies assumed that the sperm ZP receptor is a single molecular species, and a number of potential candidates had been suggested. Yet none of them can be considered as the sole sperm ZP receptor. Accumulated evidence suggested that the sperm ZP receptor is a dynamic multi-molecular structure requiring coordinated action of different proteins that are assembled into a functional complex during post-testicular maturation and capacitation. The complex components may include carbohydrate-binding, protein-binding and acrosomal matrix proteins which work as a suite to mediate spermatozoa-ZP interaction. This article aims to review the latest insights in the identification of the sperm ZP receptor. Continued investigation of the area will provide considerable understanding of the regulation of fertilization that will be useful for practical application in human contraception and reproductive medicine.

Posttranslational modifications of zona pellucida proteins

The zona pellucida (ZP), which surrounds the mammalian oocyte, functions in various aspects of fertilization. The ZP consists of three or four glycoproteins, which are derived from transmembrane proteins that lack the ability to self-assemble. Following posttranslational processing at specific sites, ectodomains of ZP precursor proteins are released from the membrane and begin to form a matrix. Glycosylational modification is thought to be involved in species-selective sperm recognition by ZP proteins. However, in mice, the supramolecular structure of the zona matrix is also important in sperm recognition. One ZP protein, ZP2, is processed at a specific site upon fertilization by ovastacin, which is released from cortical granules inside the oocyte. This phenomenon is involved in the block to polyspermy. The proteolysis of ubiquitinated ZP proteins by a sperm-associated proteasome is involved in penetration of the zona matrix by sperm, at least in the pigs. Thus, the posttranslational modification of ZP proteins is closely tied to ZP formation and the regulation of sperm-oocyte interactions.

Mammalian zona pellucida glycoproteins: structure and function during fertilization

Zona pellucida (ZP) is a glycoproteinaceous translucent matrix that surrounds the mammalian oocyte and plays a critical role in the accomplishment of fertilization. In humans, it is composed of 4 glycoproteins designated as ZP1, ZP2, ZP3 and ZP4, whereas mouse ZP is composed of ZP1, ZP2 and ZP3 (Zp4 being a pseudogene). In addition to a variable sequence identity of a given zona protein among various species, human ZP1 and ZP4 are paralogs and mature polypeptide chains share an identity of 47%. Employing either affinity purified native or recombinant human zona proteins, it has been demonstrated that ZP1, ZP3 and ZP4 bind to the capacitated human spermatozoa and induce an acrosome reaction, whereas in mice, ZP3 acts as the putative primary sperm receptor. Human ZP2 only binds to acrosome-reacted spermatozoa and thus may be acting as a secondary sperm receptor. In contrast to O-linked glycans of ZP3 in mice, N-linked glycans of human ZP3 and ZP4 are more relevant for induction of the acrosome reaction. Recent studies suggest that Sialyl-Lewis(x) sequence present on both N- and O-glycans of human ZP play an important role in human sperm-egg binding. There are subtle differences in the downstream signaling events associated with ZP3 versus ZP1/ZP4-mediated induction of the acrosome reaction. For example, ZP3 but not ZP1/ZP4-mediated induction of the acrosome reaction is dependent on the activation of the Gi protein-coupled receptor. Thus, various studies suggest that, in contrast to mice, in humans more than one zona protein binds to spermatozoa and induces an acrosome reaction.

Sperm penetration to the zona pellucida of an oocyte: a computational model incorporating acrosome reaction

For fertilisation to occur, a spermatozoon needs to cross the zona pellucida (ZP), which is a glycoprotein layer surrounding the oocyte. Crossing the ZP requires an acrosome reaction (AR) where enzymes released from the spermatozoon head locally digest and soften the ZP so that the spermatozoon can penetrate deeper. Here, a biomechanical sperm-oocyte interaction model that considers the AR using the finite element method was formulated. This modelling is used to determine which of the following factors directly contribute to the crossing of the ZP: local ZP softening by AR, sperm head shape, ZP hardening elsewhere than in the AR site, ZP thickness and sperm hyperactivation (more flagellar beating). It has been found that an AR softening the ZP to over one-tenth of its basal stiffness is important for successful sperm penetration, and that 'sharper' heads have a biomechanical advantage in penetrating deeper. The approach is promising for understanding this exciting stage of reproduction.

Defective protein kinase A and C pathways are common causes of disordered zona pellucida (ZP)--induced acrosome reaction in normozoospermic infertile men with normal sperm-ZP binding

OBJECTIVE

To determine association between defective protein kinases C (PKC) and A (PKA) and disordered zona pellucida (ZP)-induced acrosome reaction (DZPIAR) in normozoospermic infertile men with normal sperm-ZP binding.

DESIGN

Sperm from DZPIAR infertile men were treated without (control) or with (test) phorbol myristate acetate (PMA, PKC activator) or dibutyryl cyclic AMP (dbcAMP, PKA activator) under in vitro standard culture condition. The ZP-induced AR was assessed and compared between control and test.

SETTING

Public and private hospital-based clinical assisted reproduction technology (ART) centers.

PATIENT(S)

A total of 51 DZPIAR infertile men were involved in this study.

INTERVENTION(S)

None.

MAIN OUTCOMES MEASURE(S)

Sperm-ZP binding and the ZP-induced IAR.

RESULT(S)

Both PMA and dbcAMP enhanced ZP-induced AR up to a normal level (≥25%) in some subjects with DZPIAR: 29 (57%) with PMA and 27 (53%) with dbcAMP. Overall 35 (69%) had the ZP-induced AR enhanced to normal by PMA or dbcAMP but 16 (31%) had little or no response to either agent. Fourteen men responded to the two activators differently: 8 effective only with PMA and 6 effective only with dbcAMP.

CONCLUSION(S)

Defective upstream of PKC and PKA pathways are highly associated with disordered ZPIAR in normozoospermic infertile men with normal sperm-ZP binding.

The relative contributions of propulsive forces and receptor-ligand binding forces during early contact between spermatozoa and zona pellucida of oocytes

When reaching the zona pellucida (ZP) of the oocyte, spermatozoa apply propulsive forces produced by the motion of their flagella, which push against the ZP and theoretically should contribute to their penetration into the ZP. Additionally, specific receptors on the spermatozoon head bind to ZP3 ligands located on the surface of the ZP, which locks the sperm's head onto the oocyte. Both mechanisms are important cofactors in the initial sperm penetration into the ZP, which is required for successful fertilization of the oocyte, but it is unclear which forces-mechanical thrust or biochemical binding-are more influential at this stage. To address this question, we developed a biomechanical sperm-oocyte contact model, which is based on the Johnson-Kendall-Roberts model adopted from the contact mechanics theory. The modeling predicted that during the early stage of penetration into the ZP, biochemical binding forces acting on spermatozoa, which are swimming at a (normal) velocity of 100μm/s are ∼4.2-times to ∼16.7-times less than the mechanically-generated propulsive forces. In a simulated pathology of a low number of properly functioning receptors (50 out of 300receptors/μm(2)), the biochemical binding forces are ∼63-times less than the propulsive forces for the normally swimming sperm. It is suggested that such dominance of the propulsive forces over the biochemical binding forces can prevent efficient binding of spermatozoa to the ZP of the oocyte due to continuous movement of the sperm (which is not necessarily perpendicular to the ZP surface, and can cause sliding of sperm over the ZP). Thus, our theoretical analysis indicates that a sufficiently large density of receptors to ZP3 ligands on the sperm head is critical at the stage of early sperm-oocyte contact, in order to allow an efficient acrosome reaction to follow, so that the spermatozoon can start penetrating into the ZP.

The mammalian zona pellucida: a matrix that mediates both gamete binding and immune recognition?

The crucial cell adhesion events required for mammalian fertilization commence when spermatozoa bind to the specialized extracellular matrix of the oocyte, known as the zona pellucida (ZP). Bound gametes then undergo a signal transduction cascade known as acrosomal exocytosis that enables them to penetrate this matrix and fuse with the oocyte to create a new individual. The ZP is therefore the target of intense investigation in the mouse, pig, bovine, and human models. Major goals in such studies are to define the adhesion molecules, signal transduction pathways, and the molecular basis for the species-restricted binding of gametes. Evidence exists indicating that protein-carbohydrate and to a lesser extent protein-protein interactions play a role in the initial gamete binding. More recent findings in an unusual sperm-somatic cell adhesion system indicate that tri- and tetraantennary N-glycans mediate initial sperm-oocyte binding in both the murine and porcine models, but conflicting data exist. A novel paradigm designated the "domain specific model" will be presented that could explain these inconsistencies. Another potential functional role of the ZP is immune recognition. Both spermatozoa and oocytes lack major histocompatibility (MHC) class I molecules that mediate the recognition of self in the immune system. This absence makes gametes less susceptible to class I restricted cytotoxic T lymphocytes, but more vulnerable to natural killer (NK) cells. Therefore a "fail safe" system for NK cell recognition should exist on both types of gametes. Another issue is that oocytes could begin to express paternal major histocompatibility antigens during the blastocyst stage prior to hatching, and thus mechanisms could also be in place to block the development of maternal adaptive immune responses. An enhanced understanding of these issues could facilitate the development of superior infertility treatments and contraceptive strategies, and define central operating principles of immune recognition in the female reproductive system.

Acrosome reaction: relevance of zona pellucida glycoproteins

During mammalian fertilisation, the zona pellucida (ZP) matrix surrounding the oocyte is responsible for the binding of the spermatozoa to the oocyte and induction of the acrosome reaction (AR) in the ZP-bound spermatozoon. The AR is crucial for the penetration of the ZP matrix by spermatozoa. The ZP matrix in mice is composed of three glycoproteins designated ZP1, ZP2 and ZP3, whereas in humans, it is composed of four (ZP1, ZP2, ZP3 and ZP4). ZP3 acts as the putative primary sperm receptor and is responsible for AR induction in mice, whereas in humans (in addition to ZP3), ZP1 and ZP4 also induce the AR. The ability of ZP3 to induce the AR resides in its C-terminal fragment. O-linked glycans are critical for the murine ZP3-mediated AR. However, N-linked glycans of human ZP1, ZP3 and ZP4 have important roles in the induction of the AR. Studies with pharmacological inhibitors showed that the ZP3-induced AR involves the activation of the G(i)-coupled receptor pathway, whereas ZP1- and ZP4-mediated ARs are independent of this pathway. The ZP3-induced AR involves the activation of T-type voltage-operated calcium channels (VOCCs), whereas ZP1- and ZP4-induced ARs involve both T- and L-type VOCCs. To conclude, in mice, ZP3 is primarily responsible for the binding of capacitated spermatozoa to the ZP matrix and induction of the AR, whereas in humans (in addition to ZP3), ZP1 and ZP4 also participate in these stages of fertilisation.

Mechanisms of sperm-egg interactions: between sugars and broken bonds

A model of the early events of mammalian fertilization has emerged during the past 30 years. However, studies during the past decade have used newly available mouse models to readdress these processes. Here, we will consider these new data in light of the existing model and point to areas of reconciliation and of controversy.

Exclusive expression of a membrane-bound Spink3-interacting serine protease-like protein TESPL in mouse testis

We identified a testis-specific protease-like protein tentatively named TESPL and a pancreatic trypsinogen Prss2 from the clones of a yeast two-hybrid screen against a mouse testicular cDNA library using the trypsin inhibitor Spink3 from male accessory sexual glands as bait. The enzymatic motifs and the cysteine patterns in serine proteases are highly conserved in these two proteins. Based on the phylogenetic analysis, Prss2 duplicated recently and TESPL underwent distant evolution without gene duplication from the progenitor of trypsin-like and chymotrypsin-like proteases. We found that TESPL transcription was restricted to the testis and that the level of transcription was positively correlated with animal maturation. In contrast, Prss2 was constitutively expressed in many tissues including testis. Alignment of the cDNA-deduced sequences of serine proteases showed the replacement of an essential serine residue in the catalytic triad of serine proteases by a proline residue in TESPL, which was demonstrated to be a membrane-bound protein devoid of proteolytic activity. The immunohistochemical staining patterns of seminiferous tubules in the testis revealed TESPL mainly on postmeiotic cells such as spermatids and spermatozoa. On the mouse sperm from caudal epididymis, TESPL was localized mainly on the plasma membrane overlaying the acrosomal region. Further, orthology group for mouse TESPL was identified in the conserved gene family of eutherian testis serine protease 5.

Oviductal secretions: will they be key factors for the future ARTs?

A variety of evolutionary processes has led to the development of different organs to ensure that internal fertilization occur successfully. Fallopian tubes are a particularly interesting example of such organs. Some of the key events during fertilization and early embryo development occur in the oviduct. Knowledge of the different components described in the oviduct is extensive. Oviductal components include hormones, growth factors and their receptors that have important roles in the physiology of the oviduct and embryo development. Other oviductal factors protect the gamete and the embryos against oxidative stress and pathogens. Different proteins and enzymes are present in the oviductal fluid and have the ability to interact with the oocyte and the sperm before the fertilization occurs. Of special interest is the oviduct-specific glycoprotein (OVGP1), a glycoprotein that is conserved in different mammals, and its association with the zona pellucida (ZP). Interaction of the oocyte with oviductal secretions leads us to emphasize the concept of 'ZP maturation' within the oviduct. The ZP changes produced in the oviduct result in an increased efficiency of the in vitro fertilization technique in some animal models, contributing in particular to the control of polyspermy and suggesting that a similar role could be played by oviductal factors in human beings. Finally, attention should be given to the presence in the oviductal fluid of several embryotrophic factors and their importance in relation to the in vivo versus in vitro developmental ability of the embryos.

Glioma pathogenesis-related 1-like 1 is testis enriched, dynamically modified, and redistributed during male germ cell maturation and has a potential role in sperm-oocyte binding

The glioma pathogenesis-related 1 (GLIPR1) family consists of three genes [GLIPR1, GLIPR1-like 1 (GLIPR1L1), and GLIPR1-like 2 (GLIPR1L2)] and forms a distinct subgroup within the cysteine-rich secretory protein (CRISP), antigen 5, and pathogenesis-related 1 (CAP) superfamily. CAP superfamily proteins are found in phyla ranging from plants to humans and, based largely on expression and limited functional studies, are hypothesized to have roles in carcinogenesis, immunity, cell adhesion, and male fertility. Specifically data from a number of systems suggests that sequences within the C-terminal CAP domain of CAP proteins have the ability to promote cell-cell adhesion. Herein we cloned mouse Glipr1l1 and have shown it has a testis-enriched expression profile. GLIPR1L1 is posttranslationally modified by N-linked glycosylation during spermatogenesis and ultimately becomes localized to the connecting piece of elongated spermatids and sperm. After sperm capacitation, however, GLIPR1L1 is also localized to the anterior regions of the sperm head. Zona pellucida binding assays indicate that GLIPR1L1 has a role in the binding of sperm to the zona pellucida surrounding the oocyte. These data suggest that, along with other members of the CAP superfamily and several other proteins, GLIPR1L1 is involved in the binding of sperm to the oocyte complex. Collectively these data further strengthen the role of CAP domain-containing proteins in cellular adhesion and propose a mechanism whereby CAP proteins show overlapping functional significance during fertilization.

Sperm-zona pellucida interaction: molecular mechanisms and the potential for contraceptive intervention

At the moment of insemination, millions of mammalian sperm cells are released into the female reproductive tract with the single goal of finding the oocyte. The spermatozoa subsequently ignore the thousands of cells they make contact with during their journey to the site of fertilization, until they reach the surface of the oocyte. At this point, they bind tenaciously to the acellular coat, known as the zona pellucida, which surrounds the oocyte and orchestrate a cascade of cellular interactions that culminate in fertilization. These exquisitely cell- and species- specific recognition events are among the most strategically important cellular interactions in biology. Understanding the cellular and molecular mechanisms that underpin them has implications for the etiology of human infertility and the development of novel targets for fertility regulation. Herein we describe our current understanding of the molecular basis of successful sperm-zona pellucida binding.

Acrosomal exocytosis of mouse sperm progresses in a consistent direction in response to zona pellucida

Sperm acrosomal exocytosis is essential for successful fertilization, and the zona pellucida (ZP) has been classically considered as the primary initiator in vivo. At present, following what is referred to as primary binding of the sperm to the ZP, the acrosome reaction paradigm posits that the outer acrosomal membrane and plasma membrane fuse at random points, releasing the contents of the acrosome. It is then assumed that the inner acrosomal membrane mediates secondary binding of the sperm to the ZP. In the present work we used a live fluorescence imaging system and mouse sperm containing enhanced green fluorescent protein (EGFP) in their acrosomes. We compared the processes of acrosomal exocytosis stimulated by the calcium ionophore ionomycin or by solubilized ZP. As monitored by the loss of EGFP from the sperm, acrosomal exocytosis driven by these two agents occurred differently. When ionomycin was used, exocytosis started randomly (no preference for the anterior, middle or posterior acrosomal regions). In contrast, following treatment with solubilized ZP, the loss of acrosomal components always started at the posterior zone of the acrosome and progressed in an anterograde direction. The exocytosis was slower when stimulated with ZP and on the order of 10 sec, which is in accordance with other reports. These results demonstrate that ZP stimulates acrosomal exocytosis in an orderly manner and suggest that a receptor-mediated event controls this process of membrane fusion and release of acrosomal components. These findings are incorporated into a model.

Enhancement of sperm-zona pellucida (ZP) binding capacity by activation of protein kinase A and C pathways in certain infertile men with defective sperm-ZP binding

BACKGROUND

Defective sperm-zona pellucida (ZP) binding (DSZPB) is a common cause of failure of fertilization in vitro. This study was to determine if DSZPB is caused by defective pathways upstream of protein kinase A (PKA) and C (PKC), or reduced protein tyrosine phosphorylation (TP).

METHODS

Infertile men with DSZPB and either normal sperm morphology (NSM) > or = 14% (n = 15) or < or =5% (n = 15) were studied. Sperm-ZP binding test was performed by incubation of motile sperm with oocytes for 2 h with or without dibutyryl cyclic AMP (dbcAMP, PKA activator) or phorbol myristate acetate (PMA, PKC activator). TP of capacitated sperm in medium was assessed by immunofluorescence with an anti-phosphotyrosine monoclonal antibody.

RESULTS

For normal sperm with normal sperm-ZP binding, both PMA and dbcAMP significantly enhanced sperm-ZP binding in a dose-response manner. Only dbcAMP, but not PMA, significantly increased TP of capacitated sperm. In DSZPB men with severe teratozoospermia (NSM < or = 5%), neither PMA nor dbcAMP enhanced sperm-ZP binding, despite dbcAMP significantly increasing the TP of capacitated sperm for all samples. In contrast, for DSZPB with NSM > or = 14%, PMA caused significantly increased sperm binding up to normal levels (> or =40 sperm bound/ZP) in five men, and dbcAMP had a similar result in two men. Again TP was significantly enhanced only by dbcAMP, but not by PMA.

CONCLUSIONS

There is defective signalling in pathways upstream of PKC and PKA in some men with DSZPB and normal semen analysis. Stimulation of TP by dbcAMP does not enhance sperm-ZP binding capacity in DSZPB men with low TP, regardless of sperm morphology.

Relevance of glycosylation of human zona pellucida glycoproteins for their binding to capacitated human spermatozoa and subsequent induction of acrosomal exocytosis

To delineate the functional aspects of zona pellucida (ZP) glycoproteins during fertilization in human, in the present study, fluorochrome-conjugated Escherichia coli (E. coli)- and baculovirus-expressed recombinant human ZP glycoprotein-2 (ZP2), -3 (ZP3), and -4 (ZP4) were employed. In an immunofluorescence assay, capacitated human sperm exhibited binding of the baculovirus-expressed recombinant ZP3 as well as ZP4 to either acrosomal cap or equatorial region whereas acrosome-reacted sperm failed to show any binding to the acrosomal cap. Using double labeling experiments, simultaneous binding of ZP3 and ZP4 to the acrosomal cap was observed suggesting the possibility of different binding sites of these proteins on the sperm surface. No binding of ZP2 was observed to the capacitated sperm. However, acrosome-reacted sperm (20.00 +/- 1.93%) showed binding of ZP2 that was restricted to only equatorial region. Interestingly, E. coli-expressed recombinant human zona proteins also showed very similar binding profiles. Competitive inhibition studies with unlabeled recombinant human zona proteins revealed the specificity of the above binding characteristics. Binding characteristics have been further validated by an indirect immunofluorescence assay using native human heat solubilized isolated zona pellucida. Employing baculovirus-expressed recombinant ZP3 and ZP4 with reduced N-linked glycosylation and respective E. coli-expressed recombinant proteins, it was observed that glycosylation is required for induction of acrosomal exocytosis but its absence may not compromise on their binding ability. These studies have revealed the binding profile of individual human zona protein to spermatozoa and further strengthened the importance of glycosylation of zona proteins for acrosomal exocytosis in spermatozoa.

Purified mouse egg zona pellucida glycoproteins polymerize into homomeric fibrils under non-denaturing conditions

The mouse egg's zona pellucida (ZP) is composed of three glycoproteins, called ZP1, ZP2, and ZP3, that migrate as relatively broad, single bands on SDS-PAGE. The glycoproteins are organized within the ZP as a network of long interconnected fibrils that exhibit a structural periodicity. Here, ZP2 and ZP3 were purified by HPLC to homogeneity and analyzed by Blue Native- (BN-) PAGE and transmission electron microscopy (TEM), as well as by SDS-PAGE. As opposed to SDS-PAGE, BN-PAGE, and TEM permit analysis of ZP2 and ZP3 under non-denaturing conditions. ZP2 and ZP3 migrate on BN-PAGE, not as single bands, but as several discrete oligomers that give rise to larger structures which remain at the origin of the gel. Consistent with this, ZP2 and ZP3 are visualized by TEM as long interconnected fibrils that consist of contiguous beads. Therefore, under non-denaturing conditions both purified ZP2 and ZP3 polymerize into higher order structures. These findings are of interest since purified ZP3 inhibits binding of mouse sperm to eggs and induces sperm to undergo the acrosome reaction in vitro. Results presented here suggest that these biological effects of ZP3 are due to binding of homomeric fibrils of ZP3 to sperm.

Sex and flow: the consequences of fluid shear for sperm–egg interactions

Fertilization is a complex interaction among biological traits of gametes and physical properties of the fluid environment. At the scale of fertilization (0.01–1 mm), sperm encounter eggs while being transported within a laminar (or viscous) shear flow. Varying laminar-shear in a Taylor-Couette flow tank, our experiments simulated important aspects of small-scale turbulence within the natural habitats of red abalone(Haliotis rufescens), a large marine mollusk and external fertilizer. Behavioral interactions between individual cells, sperm–egg encounter rates, and fertilization success were quantified, simultaneously, using a custom-built infrared laser and computer-assisted video imaging system. Relative to still water, sperm swam faster and moved towards an egg surface,but only in comparatively slow flows. Encounter rate, swim speed and orientation, and fertilization success each peaked at the lowest shear tested(0.1 s–1), and then decayed as shear increased beyond 1.0 s–1. The decay did not result, however, from damage to either sperm or eggs. Analytical and numerical models were used to estimate the propulsive force generated by sperm swimming (Fswim) and the shear force produced by fluid motion within the vicinity of a rotating egg(Fshear). To first order, male gametes were modeled as prolate spheroids. The ratio Fswim/Fshear was useful in explaining sperm–egg interactions. At low shears where Fswim/Fshear&gt;1, sperm swam towards eggs, encounter rates were pronounced, and fertilization success was very high; behavior overpowered fluid motion. In contrast, sperm swimming,encounter rate and fertilization success all decayed rapidly when Fswim/Fshear&lt;1; fluid motion dominated behavior. The shears maximizing fertilization success in the lab typically characterized natural flow microenvironments of spawning red abalone. Gamete behavior thus emerges as a critical determinant of sexual reproduction in the turbulent sea.

Distribution of alpha-D-mannose residues on zona pellucida and their role(s) in fertilization in pigs

The present study aims to identify the distribution of alpha-D-mannose residues on zona pellucida (ZP) and their role(s) in fertilization in pigs. In experiment 1, in vitro matured pig oocytes were freed from cumulus cells and treated with fluorescein isothiocyanate-labelled Lens culinaris (FITC-LCA), a D-mannose specific binding lectin. After 30 min of treatment, LCA bound evenly throughout the ZP with strong fluorescence. In experiment 2, when LCA-treated oocytes were used for in vitro fertilization, the number of sperm bound to ZP was significantly decreased, and sperm penetration was almost completely blocked. In experiment 3, polysaccharide mannan was added to the in vitro fertilization medium as a competitive inhibitor. Both the number of sperm bound to ZP and the rate of fertilized oocytes were significantly reduced in the mannan-treated group compared with the control group. In experiment 4, spermatozoa were incubated with mannan in vitro. The number of acrosome-reacted spermatozoa was evidently increased in a time-dependent manner during the incubation. These results suggest that alpha-D-mannose residues presenting on pig ZP might be an important component of sperm receptor and might induce sperm acrosome reaction and thus facilitate the sperm penetration into the ZP.

Kinetic analysis of nanoparticulate polyelectrolyte complex interactions with endothelial cells

A non-toxic, nanoparticulate polyelectrolyte complex (PEC) drug delivery system was formulated to maintain suitable physicochemical properties at physiological pH. Toxicity, binding, and internalization were evaluated in relevant microvascular endothelial cells. PEC were non-toxic, as indicated by cell proliferation studies and propidium iodide staining. Inhibitor studies revealed that PEC were bound, in part, via heparan sulfate proteoglycans and internalized through macropinocytosis. A novel, flow cytometric, Scatchard protocol was established and showed that PEC, in the absence of surface modification, bind cells non-specifically with positive cooperativity, as seen by graphical transformations.

Multiple proteins present in purified porcine sperm apical plasma membranes interact with the zona pellucida of the oocyte

An important step in fertilization is the recognition and primary binding of the sperm cell to the zona pellucida (ZP). Primary ZP binding proteins are located at the apical plasma membrane of the sperm head. In order to exclusively study primary zona binding proteins, plasma membranes of sperm heads were isolated, highly purified and subsequently solubilized with a mild or a strong solubilization procedure. Native, highly purified ZP ghosts were used as the binding substrate for solubilized sperm plasma membrane proteins, and a proteomic approach was employed to identify ZP binding proteins. Two-dimensional gel electrophoresis of ZP fragments with bound sperm proteins showed very reproducibly 24 sperm protein spots to be associated to the zona ghosts after mild plasma membrane solubilization whereas only three protein spots were detected after strong plasma membrane solubilization. This indicates the involvement of multiple sperm proteins in ZP binding. The three persistently bound proteins were identified by a tandem mass spectrometry as isoforms of AQN-3 and probably represent the main sperm protein involved in ZP binding. P47, fertilin beta and peroxiredoxin 5 were also conclusively identified. None of the identified proteins has a known acrosomal origin, which further indicated that there was no sample contamination with secondary ZP binding proteins from the acrosomal matrix. In this study, we showed and identified multiple zona binding proteins involved in primary sperm-zona binding. Although we were not able to identify all of the proteins involved, this is a first step in understanding the event of primary sperm-zona interactions and the relevance of this for fertilization is discussed.

Glycodelin-A interacts with fucosyltransferase on human sperm plasma membrane to inhibit spermatozoa-zona pellucida binding

Fertilization depends on successful binding of the spermatozoa to the zona pellucida of the oocyte. Glycodelin-A inhibits spermatozoa-zona pellucida binding. Previous data showed that glycodelin-A receptor(s) and zona pellucida protein receptor(s) on human spermatozoa are closely related. Using a chemical cross-linking approach, the glycodelin-A-sperm receptor complex was isolated. The receptor was identified to be fucosyltransferase-5 (FUT5) by mass spectrometry and confirmed with the use of anti-FUT5 antibodies. Sperm FUT5 was an externally oriented integral membrane protein in the acrosomal region of human spermatozoa. Biologically active FUT5 was purified from spermatozoa. Co-immunoprecipitation confirmed the interaction between glycodelin-A and sperm FUT5. Solubilized zona pellucida reduced the binding of glycodelin-A to sperm FUT5. An anti-FUT5 antibody and FUT5 acceptor blocked the binding of glycodelin-A to spermatozoa and the zona binding inhibitory activity of glycodelin-A. Sperm FUT5 bound strongly to intact and solubilized human zona pellucida. The equilibrium dissociation constant of sperm FUT5 binding to solubilized zona pellucida was 42.82 pmol/ml. These observations suggest that human sperm FUT5 is a receptor of glycodelin-A and zona pellucida proteins, and that glycodelin-A inhibits spermatozoa-zona binding by blocking the binding of sperm FUT5 to the zona pellucida.

Role of sialic acid in bovine sperm–zona pellucida binding

Sperm binding activity has been detected in zona pellucida (ZP) glycoproteins and it is generally accepted that this activity resides in the carbohydrate moieties. In the present study we aim to identify some of the specific carbohydrate molecules involved in the bovine sperm-ZP interaction. We performed sperm binding competition assays, in vitro fecundation (IVF) in combination with different lectins, antibodies and neuraminidase digestion, and chemical and cytochemical analysis of the bovine ZP. Both MAA lectin recognising alpha-2,3-linked sialic acid and neuraminidase from Salmonella typhimurium with catalytic activity for alpha-2,3-linked sialic acid, demonstrated a high inhibitory effect on the sperm-ZP binding and oocyte penetration. These results suggest that bovine sperm-ZP binding is mediated by alpha-2,3-linked sialic acid. Experiments with trisaccharides (sialyllactose, 3'-sialyllactosamine and 6'-sialyllactosamine) and glycoproteins (fetuin and asialofetuin) corroborated this and suggest that at least the sequence Neu5Ac(alpha2-3)Gal(beta1-4)GlcNAc is involved in the sperm-ZP interaction. Moreover, these results indicate the presence of a sperm plasma membrane specific protein for the sialic acid. Chemical analysis revealed that bovine ZP glycoproteins contain mainly Neu5Ac (84.5%) and Neu5GC (15.5%). These two types of sialic acid residues are probably linked to Galbeta1,4GlcNAc and GalNAc by alpha-2,3- and alpha-2,6-linkages, respectively, as demonstrated by lectin cytochemical analysis. The use of a neuraminidase inhibitor resulted in an increased number of spermatozoa bound to the ZP and penetrating the oocyte. From this last result we hypothesize that a neuraminidase from cortical granules would probably participate in the block to polyspermy by removing sialic acid from the ZP.

Roles of glycodelin in modulating sperm function

Glycodelin is a glycoprotein with three well-defined isoforms. They are named as glycodelin-S, glycodelin-A and glycodelin-F. The three isoforms have similar protein core but different carbohydrate moieties. Glycodelin-S is abundant in the human seminal plasma. It suppresses sperm capacitation and in doing so, it maintains the spermatozoa in an uncapacitated state before they enter into the uterine cavity. Glycodelin-A is abundant in the amniotic fluid. It is also secreted from endometrial glands into uterine fluid and is produced by the fallopian tube. Glycodelin-A is the first endogenous glycoprotein that was found to inhibit the binding of spermatozoa to the zona pellucida. The immunosuppressive properties of glycodelin-A suggest that the molecule may protect the spermatozoa from immune attack in the maternal reproductive tract. Glycodelin-F was first found in the follicular fluid, hence its name. It also inhibits spermatozoa-zona pellucida binding. In addition, glycodelin-F suppresses progesterone-induced acrosome reaction, and may serve to prevent premature acrosome reaction. Preliminary findings suggest possible presence of yet another glycodelin isoform in the extracellular matrix of cumulus oophorus. Unlike glycodelin-A and -F, it stimulates spermatozoa-zona pellucida binding. In summary, different isoforms of glycodelin have different biological roles on sperm function, and they act in succession to contribute to the success of fertilization.

Polypeptide encoded by mouseZP3 exon-7 Is Necessary and Sufficient for binding of mouse sperm in vitro

Fertilization in mice is initiated by species-specific binding of sperm to mZP3, one of three mouse zona pellucida (ZP) glycoproteins. At nanomolar concentrations, purified egg mZP3 binds to acrosome-intact sperm heads and inhibits binding of sperm to eggs in vitro. Although several reports suggest that sperm recognize and bind to a region of mZP3 encoded by mZP3 exon-7 (so-called, sperm combining-site), this issue remains controversial. Here, exon-swapping and an IgG(Fc) fusion construct were used to further evaluate whether mZP3 exon-7 is essential for binding of sperm to mZP3. In one set of experiments, hamster ZP3 (hZP3) exon-6, -7, and -8 were individually replaced with the corresponding exon of mZP3. Stably transfected embryonal carcinoma (EC) cell lines carrying the recombinant genes were produced and secreted recombinant glycoprotein was purified and assayed for the ability to inhibit binding of sperm to eggs. While EC-hZP3, a recombinant form of hZP3 made by EC cells, is unable to inhibit binding of mouse sperm to eggs in vitro, the results suggest that substitution of mZP3 exon-7 for hZP3 exon-7, but not mZP3 exon-6 or -8, can impart inhibitory activity to EC-hZP3. In this context, a fusion construct consisting of human IgG(Fc) and mZP3 exon-7 and -8 was prepared, an EC cell line carrying the recombinant gene was produced, and secreted chimeric glycoprotein, called EC-huIgG(Fc)/mZP3(7), was purified and assayed. It was found that the chimeric glycoprotein binds specifically to plasma membrane overlying sperm heads to a similar extent as egg mZP3 and, at nanomolar concentrations, inhibits binding of mouse sperm to eggs in vitro. Collectively, these observations provide new evidence that sperm recognize and bind to a region of mZP3 polypeptide immediately downstream of its ZP domain that is encoded by mZP3 exon-7. The implications of these findings are discussed.

Capacitation-dependent concentration of lipid rafts in the apical ridge head area of porcine sperm cells

Lipid architecture of the plasma membrane plays an important role in the capacitation process of the sperm cell. During this process, an increase in membrane fluidity takes place, which coincides with a redistribution of cholesterol to the apical region of the head plasma membrane and subsequently an efflux of cholesterol. Cholesterol is also a major player in the formation of lipid rafts or microdomains in the membrane. Lipid rafts favour specific protein-protein interactions by concentrating certain proteins in these microdomains while excluding others. In this study, we investigated the organization of lipid rafts during in vitro capacitation of boar sperm cells. We report on the presence of the lipid raft-specific proteins caveolin-1 and flotillin-1 in sperm cells. Capacitation induced a change in membrane distribution of these proteins. Lipid analysis on detergent-resistant membranes (DRMs) of sperm cells indicated that capacitation induces a lipid raft concentration rather than a disintegration of lipid rafts, because the total amount of lipid in the DRM fraction remained unaltered. Using a proteomic approach, we identified several major DRM proteins, including proteins involved in capacitation-dependent processes and zona pellucida binding. Our data indicate that sperm raft reorganization may facilitate capacitation-specific signalling events and binding to the zona pellucida.

Recombinant Porcine Zona Pellucida Glycoproteins Expressed in Sf9 Cells Bind to Bovine Sperm but Not to Porcine Sperm

The zona pellucida, which surrounds the mammalian oocyte, consists of the ZPA, ZPB, and ZPC glycoproteins and plays roles in species-selective sperm-egg interactions via its carbohydrate moieties. In the pig, this activity is conferred by tri- and tetraantennary complex type chains; in cattle, it is conferred by a chain of 5 mannose residues. In this study, porcine zona glycoproteins were expressed as secreted forms, using the baculovirus-Sf9 insect cell system. The sperm binding activities of the recombinant proteins were examined in three different assays. The assays clearly demonstrated that recombinant ZPB bound bovine sperm weakly but did not bind porcine sperm; when recombinant ZPC was also present, bovine sperm binding activity was greatly increased, but porcine sperm still was not bound. The major sugar chains of ZPB were pauci and high mannose type chains that were similar in structure to the major neutral N-linked chain of the bovine zona. In fact, the nonreducing terminal alpha-mannose residues were necessary for the sperm binding activity. These results show that the carbohydrate moieties of zona glycoproteins, but not the polypeptide moieties, play an essential role in species-selective recognition of porcine and bovine sperm. Moreover, Asn to Asp mutations at either of two of the N-glycosylation sites of ZPB, residue 203 or 220, significantly reduced the sperm binding activity of the ZPB/ZPC mixture, whereas a similar mutation at the third N-glycosylation site, Asn-333, had no effect on binding. These results suggest that the N-glycans located in the N-terminal half of the ZP domain of porcine ZPB are involved in sperm-zona binding.

Localization and Significance of Molecular Chaperones, Heat Shock Protein 1, and Tumor Rejection Antigen gp96 in the Male Reproductive Tract and During Capacitation and Acrosome Reaction1

Although the molecular basis of sperm-oocyte interaction is unclear, recent studies have implicated two chaperone proteins, heat shock protein 1 (HSPD1; previously known as heat shock protein 60) and tumor rejection antigen gp96 (TRA1; previously known as endoplasmin), in the formation of a functional zona-receptor complex on the surface of mammalian spermatozoa. The current study was undertaken to investigate the expression of these chaperones during the ontogeny of male germ cells through spermatogenesis, epididymal sperm maturation, capacitation, and acrosomal exocytosis. In testicular sections, both HSPD1 and TRA1 were closely associated with the mitochondria of spermatogonia and primary spermatocytes. However, this labeling pattern disappeared from the male germ line during spermiogenesis to become undetectable in testicular spermatozoa. Subsequently, these chaperones could be detected in epididymal spermatozoa and in previously unreported "dense bodies" in the epididymal lumen. The latter appeared in the precise region of the epididymis (proximal corpus), where spermatozoa acquire the capacity to recognize and bind to the zona pellucida, implicating these structures in the functional remodeling of the sperm surface during epididymal maturation. Both HSPD1 and TRA1 were subsequently found to become coexpressed on the surface of live mouse spermatozoa following capacitation in vitro and were lost once these cells had undergone the acrosome reaction, as would be expected of cell surface molecules involved in sperm-egg interaction. These data reinforce the notion that these chaperones are intimately involved in the mechanisms by which mammalian spermatozoa both acquire and express their ability to recognize the zona pellucida.

Contribution of mouse egg zona pellucida glycoproteins to gamete recognition during fertilization

For sperm to fertilize eggs, they must first bind to the thick zona pellucida (ZP) that surrounds the plasma membrane of all unfertilized mammalian eggs. An extensive literature suggests that mouse sperm recognize and bind to a specific ZP glycoprotein called mZP3. However, the role of individual ZP glycoproteins in binding of mouse sperm to eggs has been called into question by recent transgenic experiments with null mice. Results of such experiments have been interpreted to mean that binding of sperm depends on the supramolecular structure of the ZP, not on an individual ZP glycoprotein. Here, it is argued that results of these transgenic experiments actually are consistent with the prevailing view of gamete recognition that implicates a specific ZP glycoprotein in both binding of mouse sperm to eggs and induction of the acrosome reaction.

The Contribution of d-Mannose, l-Fucose, N-Acetylglucosamine, and Selectin Residues on the Binding of Glycodelin Isoforms to Human Spermatozoa1

Previous data showed that glycodelin-A from amniotic fluid and glycodelin-F from follicular fluid inhibited sperm-zona pellucida binding. Solubilized zona pellucida reduced the binding of glycodelin-F to sperm extract dose dependently. This study demonstrated that the zona pellucida proteins also reduced the binding of glycodelin-A to sperm extract. Ionophore-induced acrosome reaction reduced the binding of iodinated glycodelin-A and -F to sperm, indicating that the glycodelin-binding sites are on the outer acrosomal membrane or on the sperm plasma membrane overlying the acrosome. While the binding of glycodelin-A to sperm was suppressed by mannose and fucose neoglycoproteins, that of glycodelin-F was also reduced by acetylglucosamine neoglycoprotein. Pretreatment of sperm with inhibitors of mannosidase and acetylglucosaminidase reduced the binding of glycodelin-F to sperm. On the other hand, inhibitor of mannosidase but not of acetylglucosaminidase inhibited the binding of glycodelin-A. In a competition binding assay, mannosidase reduced both glycodelin-A and -F binding whereas acetylglucosaminidase reduced only glycodelin-F binding. While fucosidase reduced the binding of both glycodelins, fucosidase inhibitor was marginally active in suppressing the binding of glycodelins to human sperm. Among the selectins tested, only E-selectin had a slight inhibitory effect on the binding of glycodelin-A to sperm. The binding of glycodelin-F was unaffected by selectins and their antibodies. In conclusion, the binding of glycodelin-A to sperm involves mannose, fucose, and possibly E- selectin residues, while that of glycodelin-F involves mannose, fucose, and N-acetylglucosamine but not the selectin residue.

In the beginning: lessons from fertilization in mice and worms

Sexual reproduction proceeds by fertilization; formation of new individuals by the union of haploid gametes. Recent reports in Cell and in Developmental Cell may provide new insights as to how this process begins and is regulated.

Capacitated acrosome-intact mouse spermatozoa bind to Sepharose beads coated with functional neoglycoproteins

Capacitated acrosome-intact mouse spermatozoa bind to the egg's extracellular coat, the zona pellucida (ZP), in a carbohydrate-mediated receptor-ligand manner. The tight irreversible binding of the opposite gametes triggers a signal transduction pathway resulting in the exocytosis of acrosomal contents (i.e., induction of the acrosome reaction [AR]). Previously, we demonstrated that a hexose (mannose) and two amino sugars (N-acetylglucosamine and N-acetylgalactosamine), when covalently conjugated to bovine serum albumin (BSA) (functional neoglycoproteins, ngps), mimicked mZP3 and induced the AR [Biol. Reprod. 60 (1999) 94-101]. To further elucidate the specificity of sperm-ngp interaction and the mZP3 mimicking role of the functional ngps, we have examined binding of the mouse spermatozoa to Sepharose 4B beads coated with the functional and non-functional ngps as well as BSA, ovalbumin (OVA), or asialofetuin (ASF). A significantly greater number of capacitated acrosome-intact spermatozoa bound to the beads coated with functional ngps than the beads coated with non-functional ngps, BSA, OVA, or ASF. The binding was temperature-sensitive and was highest when the sperm-bead assay was carried out at 37 degrees C. Blocking of in vitro capacitation, by including calmodulin antagonists in the incubation medium, prevented sperm from binding to the beads. Furthermore, inclusion of free sugars (mannose, N-acetylglucosamine, or N-acetylgalactosamine) in the binding assay, either individually or as a mixture, inhibited sperm-bead binding in a concentration-dependent manner. Taken together, our data provide evidence strongly suggesting that binding of capacitated spermatozoa to the ngp-coated Sepharose beads is specific. The beads that mimic zona-intact eggs provide an excellent tool for examining pharmacological effects of reagents that alter the sperm function. In addition, the immobilized ngp(s) will be useful as an affinity medium to isolate the sperm surface receptor(s) that recognize and bind to the sugar residues.

Real-time observations of individual macaque sperm undergoing tight binding and the acrosome reaction on the zona pellucida

Changes in binding affinity, acrosomal status, and motility of living sperm on the zona pellucida were for the first time in any mammalian species directly observed and analyzed with video microscopy. A single zona was air-dried and rehydrated on a microscope slide, and a coverslip supported by glass beads was added. Capacitated sperm were added together with Alexa-SBTI, a probe for acrosin that can detect the acrosome reaction. The heads of loosely attached sperm oscillated on the zona and the flagella beat symmetrically with a sigmoid-shaped waveform. Tight binding was observed after 16 sec as the sperm head became fixed in place on the zona. The shape of the flagellar beat simultaneously shifted to a more rigid, C-shaped waveform. The first signs of the acrosome reaction were detected within 11 sec of tight binding. Rapid flushing removed approximately 65% of sperm that were loosely attached but only 2% of those that were tightly bound. In the 2 min following the onset of tight binding, the lateral displacement of the flagellum increased by approximately 30% and the beat frequency decreased by 25%. Lignosulfonic acid (LSA) inhibited loose sperm attachment and the development of tight binding. LSA had no effect on the time of the acrosome reaction following tight binding or on changes in motility that followed tight binding. These data suggest that LSA affects the initial attachment or docking of sperm to the zona, a step that may align or recruit one or more specific zona receptors to be responsible for mediating the acrosome reaction.

Sperm surface hyaluronan binding protein (HABP1) interacts with zona pellucida of water buffalo (Bubalus bubalis) through its clustered mannose residues

Sperm-oocyte interaction during fertilization is multiphasic, with multicomponent events, taking place between zona pellucida (ZP) glycoproteins and sperm surface receptor. d-mannosylated glycoproteins, the major constituents of ZP are considered to serve as ligands for sperm binding. The presence of hyaluronan binding protein 1 (HABP1) on sperm surface of different mammals including cattle and its possible involvement in sperm function is already reported. Recently, we have demonstrated the specificity of clustered mannose as another ligand for HABP1 (Kumar et al., 2001: J Biosci 26:325-332). Here, we report that only N-linked mannosylated zona-glycoproteins bind to sperm surface HABP1. Labeled HABP1 interacts with ZP of intact oocyte of Bubalus bubalis, which can be competed with unlabeled HABP1 or excess d-mannosylated albumin (DMA). This data suggests the specific interaction of HABP1 with ZP, through clustered mannose residues. In order to examine the physiological significance of such an interaction, the capacity of sperm binding to oocytes under in vitro fertilization plates was examined either in presence of DMA alone or in combination with HABP1. The number of sperms, bound to oocytes was observed to reduce significantly in presence of DMA, which could be reversed by the addition of purified recombinant HABP1 (rHABP1) in the same plate. This suggests that sperm surface HABP1 may act as mannose binding sites for zona recognition.

Assessment of acrosomal status in rat spermatozoa: studies on carbohydrate and non-carbohydrate agonists

In the mouse and several other species, including man, capacitated acrosome-intact spermatozoa interact with natural [soluble zona pellucida (ZP) and progesterone (P4)] and synthetic [neoglycoproteins (ngps) and calcium (Ca(2+)) ionophore] agonists, prior to the initiation of a Ca(2+)-dependent signal transduction cascade. The net result is the fusion of the sperm plasma membrane overlying the outer acrosomal membrane at multiple sites and exocytosis of acrosomal contents [i.e., induction of the acrosome reaction (AR)]. This step is believed to be a prerequisite that enables the acrosome-reacted spermatozoon to penetrate the ZP and fertilize the egg. Although the rat is one of the most commonly used laboratory animals, very little is known about the chemical nature of agonists that induce the AR in this species. The lack of this information is primarily due to the fact that the rat sperm acrosome is a relatively thin structure. Thus, it is difficult to assess the status of the sperm acrosome in this species. In this report, we describe the use of a Coomassie brilliant blue dye staining procedure to assess the status of the rat sperm acrosome by light microscopy. The procedure is highly reproducible and has allowed us to determine the effects of carbohydrate (ngps and mouse ZP) and noncarbohydrate (P4 and Ca(2+) ionophore) agonists on capacitated spermatozoa. In addition, we have used a pharmacological approach to examine the functional significance of calmodulin (CaM), a Ca(2+)-binding protein, in induction of the AR in spermatozoa. Data presented in this report demonstrate that several ngps, solubilized mZP, P4, and Ca(2+) ionophores induce the AR in rat spermatozoa. Furthermore, we demonstrate that, whereas CaM antagonists blocked P4-induced AR, most of the inhibitors used had no significant effect on the Ca(2+) ionophore-induced (nonphysiological) AR.

Arylsulfatase a is present on the pig sperm surface and is involved in sperm-zona pellucida binding

We have previously described the affinity of a pig sperm surface protein, P68, to mammalian zonae pellucidae (ZP). In this report, we identified P68 as arylsulfatase A (AS-A) based on the presence of P68 tryptic peptide sequences in the pig testis AS-A cDNA sequence. Our objective was to demonstrate the presence of AS-A on the sperm surface and to elucidate its role in ZP binding. Immunogold electron microscopy revealed the presence of AS-A on the sperm surface. Furthermore, live pig sperm and the extract of peripheral sperm plasma membrane proteins exhibited AS-A's desulfation activity. Significantly, the role of pig sperm surface AS-A in ZP binding was demonstrated by dose-dependent decreases of sperm-ZP binding upon sperm pretreatment with anti-AS-A IgG/Fab, and by the binding of Alexa-430-conjugated sperm surface AS-A to homologous ZP. ZP pretreatment with anti-pig-ZP3 antibody abolished AS-A binding, suggesting that ZP3, recognized as the pig sperm receptor, was AS-A's binding ligand. This was further confirmed by the ability of exogenous ZP3 to competitively inhibit AS-A-ZP binding. Similarly, purified ZP3alpha, a major sperm receptor component of ZP3, exhibited great inhibitory effect on AS-A-ZP binding. All of these results designated a new function of AS-A in gamete interaction.

Characterization of zona pellucida glycoprotein 3 (ZP3) and ZP2 binding sites on acrosome-intact mouse sperm

There is considerable evidence that mouse fertilization requires the binding of sperm to two of the three glycoproteins that form the zona pellucida (ZP), ZP3 and ZP2. Despite the biologic importance of this binding, no one has demonstrated that sperm express separate, saturable, and specific binding sites for ZP3 and for ZP2. Such a demonstration is a prerequisite for defining the distribution, numbers, affinities, and regulation of function of ZP3 and ZP2 binding sites on sperm. The experiments reported herein used fluorochrome-labeled ZP3 and ZP2 and quantitative image analysis to characterize the saturable binding of ZP3 and ZP2 to distinct sites on living, capacitated, acrosome-intact mouse sperm. Approximately 20% of the ZP3 binding sites were found over the acrosomal cap, and the remaining sites were located over the postacrosomal region of the head. In contrast, ZP2 binding sites were detected only over the postacrosomal region. Saturation analysis estimated numbers and affinities of the binding sites for ZP3 (B(max) approximately 185 000 sites per sperm; K(d) approximately 67 nM) and ZP2 (B(max) approximately 500 000 sites per sperm; K(d) approximately 200 nM). Use of unlabeled ZP3, ZP2, and ZP1 as competitive inhibitors of the binding of fluorochrome-labeled ZP3 and ZP2 demonstrated that ZP3 and ZP2 bound specifically to their respective sites on sperm. Finally, we demonstrate that extracellular calcium as well as capacitation and maturation of sperm are required for these sites to bind their respective ligands.

Sonication of mouse sperm membranes reveals distinct protein domains

Molecular interactions between sperm and zona pellucida (ZP) during mammalian fertilization are not well characterized. To begin to characterize sperm components that are involved in sperm-ZP interactions, we isolated and density fractionated sperm membranes. The membrane fractions recovered from a density fractionation protocol were characterized, and sonication was compared with vortexing for preparation of sperm membranes by examining the distribution of proteins in the membrane fractions obtained from these 2 protocols. Biochemical and microscopic analyses were used to determine the composition of the sonicated membrane fractions, and immunoblotting was used to identify fractions containing some of the previously suggested ZP3 receptors. Transmission electron microscopy revealed that bands 1-3 contained membrane vesicles and band 4 contained axonemal and midpiece fragments. SDS-PAGE revealed that bands 1 and 2 shared many proteins, but band 3 contained a number of unique proteins. Surface labeling with 125I demonstrated that bands 1 and 2 contained the majority of the sperm surface protein markers, whereas band 3 contained minor amounts of surface markers. Lectin-binding characteristics of sperm membrane glycoproteins were used to compare the relative distribution of glycosylated proteins in vortexed or sonicated membrane preparations. These characterizations indicate that sonication enhanced the differential distribution of sperm membrane proteins among the density fractions and suggests that this method is preferable for preparation of membrane fractions to be used for identification of proteins that mediate sperm-egg interactions.

Distinct membrane fractions from mouse sperm bind different zona pellucida glycoproteins

Interactions between sperm and zona pellucida (ZP) during mammalian fertilization are not well characterized at the molecular level. To identify sperm proteins that recognize ligand ZP3, we used sonicated sperm membrane fractions as competitors in a quantitative binding assay. Sonicated membranes were density fractionated into 4 fractions. Bands 1-3 contained membrane vesicles, and band 4 contained axonemal and midpiece fragments. In competitive binding assays, bands 1, 2, and 3 but not band 4 were able to compete with live, capacitated, intact sperm for soluble 125I-ZP binding. Affinity-purified ZP fractions consisting of a ZP3-enriched fraction (125I-ZP3) and a fraction enriched for ligands ZP1 and ZP2 and depleted of ZP3 (125I-ZP1/2) were obtained by antibody affinity purification of ZP3. In competitive binding assays, bands 2 and 3 competed for 125I-ZP3 binding, but band 1 did not interact with enriched 125I-ZP3. None of the membrane fractions competed for 125I-ZP1/2 binding. These results demonstrate that band 2 and band 3 contain sperm components that interact with ZP3 alone and that components in band 1 interact with ZP3 in conjunction with either ZP1 or ZP2. These data indicate that there must be at least 2 unique sperm plasma membrane components that mediate intact sperm interactions with ZP glycoproteins in mouse. Bands 2 and 3 are likely to contain a primary ZP-binding protein because they interacted directly with ZP3, whereas band 1 may contain sperm proteins involved in later interactions with the ZP, perhaps transitional interactions to maintain sperm contact with the ZP during acrosomal exocytosis.

Calmodulin Signals Capacitation and Triggers the Agonist-Induced Acrosome Reaction in Mouse Spermatozoa

Capacitated acrosome-intact spermatozoa interact with specific sugar residues on neoglycoproteins (ngps) or solubilized zona pellucida (ZP), the egg's extracellular glycocalyx, prior to the initiation of a signal transduction cascade that results in the fenestration and fusion of the sperm plasma membrane and the outer acrosomal membrane at multiple sites and exocytosis of acrosomal contents (i.e., induction of the acrosome reaction (AR)). The AR releases acrosomal contents at the site of sperm-zona binding and is thought to be a prerequisite event that allows spermatozoa to penetrate the ZP and fertilize the egg. Since Ca(2+)/calmodulin (CaM) plays a significant role in several cell signaling pathways and membrane fusion events, we have used a pharmacological approach to examine the role of CaM, a calcium-binding protein, in sperm capacitation and agonist-induced AR. Inclusion of CaM antagonists (calmodulin binding domain, calmidazolium, compound 48/80, ophiobolin A, W5, W7, and W13), either in in vitro capacitation medium or after sperm capacitation blocked the npg-/ZP-induced AR. Purified CaM largely reversed the AR blocking effects of antagonists during capacitation. Our results demonstrate that CaM plays an important role in priming (i.e., capacitation) of mouse spermatozoa as well as in the agonist-induced AR. These data allow us to propose that CaM regulates these events by modulating sperm membrane component(s).