Human spermatozoal FA-1 binds with ZP3 of porcine zona pellucida

Ligands and Receptors Involved in the Sperm-Zona Pellucida Interactions in Mammals

Sperm-zona pellucida (ZP) interaction, involving the binding of sperm surface ligands to complementary carbohydrates of ZP, is the first direct gamete contact event crucial for subsequent gamete fusion and successful fertilization in mammals. It is a complex process mediated by the coordinated engagement of multiple ZP receptors forming high-molecular-weight (HMW) protein complexes at the acrosomal region of the sperm surface. The present article aims to review the current understanding of sperm-ZP binding in the four most studied mammalian models, i.e., murine, porcine, bovine, and human, and summarizes the candidate ZP receptors with established ZP affinity, including their origins and the mechanisms of ZP binding. Further, it compares and contrasts the ZP structure and carbohydrate composition in the aforementioned model organisms. The comprehensive understanding of sperm-ZP interaction mechanisms is critical for the diagnosis of infertility and thus becomes an integral part of assisted reproductive therapies/technologies.

Remodeling of the plasma membrane in preparation for sperm-egg recognition: roles of acrosomal proteins

The interaction of sperm with the egg's extracellular matrix, the zona pellucida (ZP) is the first step of the union between male and female gametes. The molecular mechanisms of this process have been studied for the past six decades with the results obtained being both interesting and confusing. In this article, we describe our recent work, which attempts to address two lines of questions from previous studies. First, because there are numerous ZP binding proteins reported by various researchers, how do these proteins act together in sperm–ZP interaction? Second, why do a number of acrosomal proteins have ZP affinity? Are they involved mainly in the initial sperm–ZP binding or rather in anchoring acrosome reacting/reacted spermatozoa to the ZP? Our studies reveal that a number of ZP binding proteins and chaperones, extracted from the anterior sperm head plasma membrane, coexist as high molecular weight (HMW) complexes, and that these complexes in capacitated spermatozoa have preferential ability to bind to the ZP. Zonadhesin (ZAN), known as an acrosomal protein with ZP affinity, is one of these proteins in the HMW complexes. Immunoprecipitation indicates that ZAN interacts with other acrosomal proteins, proacrosin/acrosin and sp32 (ACRBP), also present in the HMW complexes. Immunodetection of ZAN and proacrosin/acrosin on spermatozoa further indicates that both proteins traffic to the sperm head surface during capacitation where the sperm acrosomal matrix is still intact, and therefore they are likely involved in the initial sperm–ZP binding step.

Heterologous murine and bovine IVF using bottlenose dolphin (Tursiops truncatus) spermatozoa

Assisted reproductive technologies are of great importance for increasing the genetic diversity in captive animals. The use of bovine or murine oocytes in heterologous IVF provides advantages compared to homologous IVF in nondomestic animals, such as the accessibility to oocytes and the availability of well-developed in vitro maturation systems. The aim of this study was to determine the heterologous IVF parameters using cryopreserved dolphin spermatozoa and zona-intact bovine or murine oocytes and to examine the nuclear chromatin status of the dolphin spermatozoa. All the processes involved in the fertilization including embryo cleavage were observed by confocal microscopy and hybrid embryo formation was confirmed by polymerase chain reaction. Heterologous bovine IVF showed no polyspermy, lower percentages of pronuclear formation, and a lower cleavage rate compared to homologous IVF group (34.8% vs. 89.3%). Heterologous murine IVF showed a lower cleavage rate than homologous IVF (9.6% vs. 77.1%). With respect to dolphin sperm chromatin, it was more stable, i.e. more resistant to EDTA-SDS decondensation than the bovine sperm chromatin. This study revealed the stability of the dolphin sperm chromatin and the ability of the dolphin spermatozoa to penetrate zona-intact bovine and murine oocytes, leading to hybrid embryo formation.

Sperm surface antigens and the prospects for contraceptive vaccine development

It has been estimated that 500 million couples world-wide have no access to contraception and that approximately 20% of births between 1995 and 2000 will be unwanted. Such statistics have important implications for the rate of world population growth and the possibility of maintaining a sustainable population. Although political change and the empowerment of women across the world will help address these issues, it will also be important to increase the availability of contraceptives; not only the modalities that are in current use, but also novel methods that will satisfy needs that are presently unfulfilled. One such alternative could be a vaccine targeting the human spermatozoon.

Molecular and immunological characteristics of sperm antigens involved in egg binding

Development of a vaccine(s) based on sperm antigens represents a promising approach to contraception. The utility of an antigen in immunocontraception is contingent upon sperm/testis-specificity and its involvement in the fertilization process. Since, the sperm-zona pellucida (ZP) recognition and binding constitutes the most important event in the fertilization process, molecules involved at this site are attractive candidates for immunocontraception. Using hybridoma technology, subtractive hybridization, and differential display technology, our laboratory has delineated several sperm antigens. These antigens have testis-specific expression and have a role in the fertilization process. The cDNAs encoding for the antigens have been cloned and sequenced. Among these, the fertilization antigen (FA-1) is particularly interesting, because it is involved in immuno-infertility in humans. Using the phage peptide display technique, a novel dodecamer sequence of a approximately 72+/-5 kD antigen, designated as YLP(12), that is testis-specific and involved in human sperm-ZP recognition/binding, was identified. A synthetic 12-mer peptide was generated based on this sequence. In the hemizona assay, YLP(12) peptide and its monovalent Fab' antibodies specifically and significantly inhibited human sperm-ZP binding. Furthermore, the presence of specific antibodies reactive with YLP(12) peptide, were identified in the serum and seminal plasma of immuno-infertile men. Thus, FA-1 and YLP(12) are promising target antigens for the development of contraceptive vaccines as well as for specific diagnosis and treatment of male infertility.

Vaccine for contraception targeting sperm

Development of a vaccine(s) based on sperm antigens represents a promising approach to contraception. The utility of a sperm antigen in immunocontraception is contingent upon its tissue specificity, involvement in fertility and on raising high antibody titer, especially locally in the genital tract, that is capable of inducing reversible infertility. Several sperm antigens, such as lactate dehydrogenase C4, PH-20, sperm protein (SP)-10, fertilization antigen (FA)-1, FA-2, cleavage signal (CS)-1, NZ-1, and NZ-2 have been proposed as potential candidates for the vaccine development. Spermzona pellucida (ZP) binding is a pivotal tissue- and mostly species-specific event in the fertilization process, and the molecules involved in this site constitute the most exciting candidates for immuno-contraception. FA-1 is a sperm-specific glycoprotein having receptor activity for ZP recognition and binding. Complementary DNA encoding for FA-1 antigen has been cloned and sequenced. Active immunization of animals with recombinant FA-1 antigen causes a long-lasting reversible inhibition in fertility by raising a sperm-specific immune response. This antigen is also involved in human immunoinfertility. The exciting findings from the recent trial in immunoinfertile couples indicate that the FA-1 antigen may have clinical application in the treatment of male infertility. A vaccine having most appropriate tissue-specific and effective recombinant and/or synthetic epitopes of various sperm antigens, such as the FA-1 antigen, in a single formulation may provide a highly immunogenic and efficacious antisperm vaccine for contraception. The advances made during the last 5 years suggest that it may be a realistic proposition.

Fertilization antigen-1 removes antisperm autoantibodies from spermatozoa of infertile men and results in increased rates of acrosome reaction

OBJECTIVE

To determine if fertilization antigen (FA)-1 will remove autoantibodies from the surface of sperm cells of immunoinfertile men by immune adsorption and permit an increased acrosome reaction (AR).

DESIGN

Prospective analytic study.

SETTING

University medical center.

PATIENT(S)

Men from 18 infertile couples with autoantibodies present on their spermatozoa.

INTERVENTION(S)

Sperm samples after processing were examined for antibody binding and AR before and after adsorption with control medium or FA-1.

MAIN OUTCOME MEASURE(S)

Sperm-bound antibody was assessed by the immunobead assay (immunoglobulin [Ig] A and IgG) and the AR by induction with ionophore A23187.

RESULT(S)

Adsorption with FA-1 compared with control medium increased immunobead-free swimming sperm an average of 50% and 76% for IgA and IgG antisperm antibodies, respectively, with 78% and 100% of the 18 semen specimens increasing significantly. The AR rate increased an average of 10.3% compared with control medium and showed improvement in 78% of the sperm samples after FA-1 adsorption.

CONCLUSION(S)

The FA-1 sperm antigen appears to significantly free sperm cells coated with autoantibodies in the semen of most infertile men examined. Reducing sperm-bound antibodies that inhibited the AR allowed the sperm cells to undergo successful AR induction by calcium ionophore.

Fertilization antigen-1: cDNA cloning, testis-specific expression, and immunocontraceptive effects

cDNA encoding for a sperm antigen, designated fertilization antigen (FA-1), was cloned and sequenced from murine testis cDNA-lambdagt11 expression library using FA-1 mAb. Computer-generated translation analysis of 649-bp cDNA yielded an ORF of 164 amino acids with the first ATG Met start codon at nucleotide 81 and the stop codon TAA at nucleotide 577 and a polyadenylylation tail following the stop codon. The translated protein has a calculated molecular mass of 18.2 kDa and a potential N-linked glycosylation site at amino acids 158-160, besides at least two O-linked glycosylation sites. The hydropathy plot generated from the deduced amino acid sequence indicated it to be a membrane-anchored peptide. Extensive computer search in the GenBank, National Biomedical Research Foundation, and Swiss sequence banks did not identify any known nucleotide/amino acid sequence having homology with FA-1 cDNA or deduced amino acids, indicating it to be a novel protein. Northern blot analysis and reverse transcription-PCR indicated testis-specific expression of FA-1 antigen. The FA-1 cDNA was subcloned into pGEX-2T vector and expressed in glutathione S-transferase gene fusion system to obtain the recombinant protein. The recombinant protein specifically reacted with ZP3 of oocyte zona pellucida and its affinity-purified antibodies completely blocked sperm-zona pellucida interaction in mice. These findings suggest that the sperm-specific recombinant FA-1 antigen is an attractive candidate for the development of a contraceptive vaccine.

Protein tyrosine phosphorylation and signal transduction during capacitation-acrosome reaction and zona pellucida binding in human sperm

During capacitation and acrosome reaction of human sperm, 7-14 proteins are phosphorylated, and two of these proteins (95 and 51 kD) are phosphorylated at tyrosine resides. The sperm proteins that bind ZP3 in humans have molecular identities of 95, 63, 51 (FA-1 antigen), and 14-18 kD, respectively. Three of these molecules, 95-, 51-, and 14-18-kD proteins, undergo tyrosine phosphorylation, and 51 kD (FA-1 antigen) also undergoes autophosphorylation. Many of the sperm proteins that participate in ZP binding are also involved in capacitation/acrosome reaction. These findings indicate a vital role of protein tyrosine phosphorylation and tyrosine receptor kinases in sperm capacitation, acrosome reaction, and ZP binding. Since tyrosine phosphorylation is the primary, or even exclusive, indication of signal transduction, it would appear that a signal transduction pathway is involved in these processes. However, the exact mechanism requires further study.

Contraceptive vaccines

The basic principle of a contraceptive (or anti-fertility) vaccine is to use the body's own immune defence mechanisms to provide protection against an unplanned pregnancy. Factors such as: economic production, convenience of use, relatively long-lasting but reversible action, low failure rate, and the avoidance of mechanical devices or exogenous hormones make this approach a potentially attractive option for family planning programmes in both developing and developed countries. The major efforts in research and development have involved the prospect of active immunization against specific antigens of sperm, ovum, zygote and early embryo, and the pregnancy hormone hCG. Several anti-hCG vaccines have entered clinical trials and show promising results. These vaccines operate by preventing or interrupting pregnancy at the peri-implantation stage probably by neutralizing the luteotrophic action of hCG. The most refined vaccine is one directed against the unique C-terminal peptide on the beta-subunit of hCG. This vaccine provokes antibodies that are specific for hCG and do not cross react with hLH. Future research efforts aim to optimize the anti hCG approach, utilize new vaccine delivery systems, and broaden the spectrum of target antigens of potential utility for contraceptive vaccines.

LL95 monoclonal antibody mimics functional effects of ZP3 on mouse sperm: evidence that the antigen recognized is not hexokinase

In the mouse, a 95 kD sperm protein has been identified as a putative receptor for the zona pellucida glycoprotein ZP3. The 95 kD sperm protein is a tyrosine kinase substrate, with phosphorylation on tyrosine stimulated upon zona pellucida binding. The latter finding is observed not only in live cells but also in isolated sperm membranes and in an electroeluted 95 kD protein. Stimulation of 95 kD protein tyrosine phosphorylation by zona pellucida is completely abolished by tyrosine kinase inhibitors, which effectively inhibit the sperm acrosome reaction. Since receptor oligomerization by ZP3 is essential for acrosome reaction triggering, we hypothesized that application of an external crosslinking agent will lead to the acrosome reaction, even in the absence of natural ligand ZP3. Here, we report the generation of a mouse monoclonal antibody (mAb) raised against the 95 kD protein. This antibody, termed LL95, mimics the bioactivities of ZP3 in inhibiting sperm-zona binding and inducing the acrosome reaction. The latter depends on receptor oligomerization. Immunolocalization revealed that the LL95 antigen is restricted to the head surface in the acrosomal region of live sperm. Thus, LL95 fulfills several criteria predicted for an antibody that recognizes a sperm receptor for the zona pellucida. Recently, it was reported that the amino acid sequence of the 95 kD protein we described corresponds to a mouse hepatoma hexokinase (Kalab et al., 1994: J Biol Chem 269:3810-3817). Although both hexokinase and LL95 antigen migrate at 95 kD in nonreducing gels, we show here that LL95 does not recognize hexokinase. Identification of different proteins is clear where hexokinase is a 116 kD protein and LL95 recognizes sperm proteins of 110 and 130 kD. Moreover, mAb anti-phosphotyrosine immunoprecipitates LL95 antigen under conditions where hexokinase is absent. Use of anti-hexokinase antibodies in gamete interaction assays failed to demonstrate any effect on either sperm-zona binding or acrosome reaction triggering. Finally, antihexokinase antibodies bind to a sperm tail antigen, thus direct involvement of hexokinase in gamete interaction seems improbable.

Fertilization antigen (FA-1) completely blocks human sperm binding to human zona pellucida: FA-1 antigen may be a sperm receptor for zona pellucida in humans

The effects of purified human sperm fertilization antigen-1 (FA-1), affinity-purified monoclonal Fab' antibody to FA-1, and monoclonal Fab' antibody to phosphotyrosine residues on human sperm-zona interaction were investigated. The purified FA-1 antigen completely blocked sperm binding to zona pellucida (P < 0.0001). Also, the monoclonal Fab' antibodies to FA-1 antigen and phosphotyrosine residues significantly (P < 0.05) reduced sperm-zona pellucidae and the antibodies were preincubated with sperm before insemination and not vice versa. These results suggest that the tyrosine phosphorylation especially of FA-1 antigen has an important role in zona pellucida receptor recognition and binding. These findings also suggest that FA-1 antigen may be the sperm receptor involved in zona pellucida binding in humans.

Molecular identities of human sperm proteins that bind human zona pellucida: nature of sperm-zona interaction, tyrosine kinase activity, and involvement of FA-1

The present study was conducted to investigate the molecular identities, nature of interaction, and tyrosine phosphorylation activity of the sperm-zona pellucida binding proteins in humans. Sperm proteins belonging to four major molecular regions, namely 95, 63, 51, and 14-18 kDa, reacted with zona pellucida proteins in the Western blot and immunoprecipitation procedures. In these procedures, zona pellucida protein that reacted strongest with the sperm proteins belonged to the molecular region of 55 kDa (ZP3), besides weakly reacting proteins in the 110-kDa (ZP1/ZP2) and 14-18-kDa molecular regions. The major forces involved in the sperm-zona protein interactions were of hydrophobic and ionic in nature. Three (95, 51, and 14-18 kDa) of the four molecular regions of sperm proteins that bound to the zona pellucida proteins also seem to involve o-phospho-L-tyrosine residues in their interaction, and these proteins demonstrated the presence of phosphotyrosine residues, and the 51-kDa protein also showed autophosphorylating activity in the in vitro kinase assay. The sperm binding zona protein of 55 kDa also demonstrated autophosphorylating activity. Using specific monoclonal antibody to the well characterized sperm-specific glycoprotein, designated FA-1, and the competitive inhibition in the immunoprecipitation procedure, it was found that the 51 kDa protein is indeed FA-1 antigen. Besides elucidating the molecular nature of the sperm-zona interaction, these antigens will find application in the development of a multivalent contraceptive vaccine, and may also help in specific diagnosis and treatment of infertility mediated through defective gamete (sperm or oocyte) function.

Antibodies to sperm-specific human FA-1 inhibit in vitro fertilization in rhesus monkeys: development of a simian model for testing of anti-FA-1 contraceptive vaccine

The feasibility of using the rhesus monkey as a non-human primate model for testing the efficacy of a contraceptive vaccine based on FA-1 antigen was evaluated. Affinity-purified anti-FA-1 polyclonal antibodies (Fab' fragments) and anti-FA-1 monoclonal antibody were used as probes in these studies. Anti-FA-1 antibodies (polyclonal Fab' as well as monoclonal IgG) predominantly reacted with the postacrosomal, mid-piece and tail regions of rhesus monkey sperm, as with human sperm, by an indirect immunofluorescence technique (IFT). These antibodies also specifically recognized a single protein band of 51 +/- 2 kDa, corresponding to the dimeric form of FA-1 antigen, on a Western blot of lithium diiodosalicylate (LIS)-solubilized monkey sperm. Anti-FA-1 antibodies, when present in the insemination mixture, inhibited the in vitro fertilization (IVF) of monkey oocytes. These results indicate that FA-1 antigen in rhesus monkey sperm is similar in subcellular localization, molecular identity and function to that in human sperm, and that the rhesus monkey represents a permissible non-human primate model in which the efficacy of a contraceptive vaccine based on FA-1 antigen can be tested.

Significance of antibodies against human sperm FA-1 antigen in immunoinfertility

A fertilization antigen (FA-1) has been identified on human sperm cells that is involved in human fertilization. To better explain the participation of FA-1 in human infertility, the relationship of anti-FA-1 antibodies with the presence of general antisperm antibodies and with sperm function was examined in the sera of 30 men and women. None of the sera from fertile men and women (n = 10) reacted with FA-1. In contrast, 55% of the immunoinfertile sera (n = 20) that were positive for antisperm antibodies detected by OPTI-BEAD (immune-labeled microsphere) tested positive with FA-1 (p < .05). In these sera, penetration of hamster oocytes was reduced and was inversely related to monospecific anti-FA-1 absorbance values determined by an ELISA and with heterospecific general antisperm antibody binding to sperm in the OPTI-BEAD test (p < .05). Anti-FA-1 activity approached significance with antisperm antibody binding to the sperm head (p = .052). It is concluded that immunoinfertile sera have antibodies reacting with the FA-1 sperm antigen and their presence is significantly related to results of other functional tests, such as the SPA and the OPTI-BEAD test. The association of anti-FA-1 antibodies with antibody binding site (sperm head) may be relevant to the diagnosis and treatment of immunoinfertility.

Immunoglobulin (Ig) G, IgA, and IgA subclass antibodies against fertilization antigen-1 in cervical secretions and sera of women of infertile couples

OBJECTIVES

To assess the occurrence of immunoglobulin (Ig) G, IgA, and IgA subclass antibodies against human sperm fertilization antigen-1 (FA-1) in cervical mucus (CM) and serum of women of infertile couples.

DESIGN

Enzyme-linked immunosorbent assay methodology was used to detect anti-FA-1 antibodies. Antisperm antibodies were detected by agglutinating, immobilizing, and indirect immunobead (IB) methods. Control samples for the ELISA were from 10 women negative in the antisperm antibody assays.

PARTICIPANTS

Samples were from women of 32 infertile couples undergoing antisperm antibody analysis.

RESULTS

One of 10 control CM samples was slightly positive for IgG anti-FA-1 and none for IgA. Of the 22 CM samples from antisperm antibody-positive women, 9 were positive for IgG antibodies, 9 for IgA, 7 for IgA1, and 6 for IgA2. Cervical mucus samples from eight women were positive for both IgA and IgG antibodies. Assay of 19 serum samples, including 8 controls, by ELISA, indicated 9 of 11 from antisperm antibody-positive women and none from controls were positive for IgA and IgG (7 of 9 identical women). In addition, of the nine IgA-positive sera, seven were of the A1 subclass and five were of the A2 subclass. Positive IB assays occurred more frequently in CM and serum samples positive for anti-FA-1 antibodies than in negative samples.

CONCLUSION

The results suggest that cervical secretions and sera of antisperm antibody-positive women contain IgA and IgG antibodies against sperm antigen FA-1 that may be involved in antifertility effects.

Vaccines against fertility

The first evidence for the efficacy of a birth control vaccine in humans is now available from the Phase II trials on the human chorionic gonadotrophin vaccine in India. Several sperm antigens have been identified as potential contraceptive immunogens and zona pellucida antigens have been reported that reversibly control fertility.

The fertilization antigen-1 does not have proteolytic/acrosin activity, but its monoclonal antibody inhibits sperm capacitation and acrosome reaction

OBJECTIVE

To determine if human sperm surface fertilization antigen exhibits proteolytic or acrosin activity and to investigate the mechanism(s) whereby monoclonal antibody (mAb) to fertilization antigen inhibits human sperm penetration of zona-free hamster ova.

DESIGN

Proteolytic and acrosin activities of human fertilization antigen were determined. Acrosomal status, acrosin activity, and motion characteristics were evaluated after incubation of human sperm with immunoaffinity-purified mAb to fertilization antigen.

SETTING

Academic research environment.

PARTICIPANTS

Fertile donors used as controls for infertile patients for fertility evaluation.

INTERVENTIONS

Human spermatozoa were treated with mAb to fertilization antigen and induced to undergo acrosome reaction using calcium ionophore A23187.

MAIN OUTCOME MEASURES

Proteolytic and acrosin activities of fertilization antigen. Sperm penetration assay, acrosomal status, and motion parameters.

RESULTS

Fertilization antigen does not exhibit proteolytic or acrosin activity; however, its mAb completely blocks human sperm penetration of zona-free hamster ova. The mAb to fertilization antigen inhibits ionophore-induced acrosome reaction and blocks development of the hyperactivated state of human sperm cells.

CONCLUSIONS

Monoclonal antibody to fertilization antigen blocks fertilization by inhibiting capacitation and acrosome reaction.

Effects of antibodies to sperm surface fertilization antigen-1 on human sperm-zona pellucida interaction

OBJECTIVE

To investigate the effects of antibodies to well-defined sperm surface antigens (the fertilization antigen [FA-1] and germ-cell antigen [GA-1]) and nuclear antigen (protamine) on human sperm-zona interaction.

DESIGN

Number of total and acrosome-reacted human sperm bound to the human zona pellucida and the sperm movement characteristics assessed by computer-aided sperm analysis were evaluated after incubation of sperm with the antibodies.

SETTING

Academic research environment approved by the Institute Review Board.

PATIENTS

Human oocytes were obtained from ovaries removed at surgery. Semen from fertile donors was used in all assays.

INTERVENTIONS

Human oocytes were stored in salt solution at -80 degrees C until used. Spermatozoa were treated with the antibodies to various sperm antigens.

MAIN OUTCOME MEASURES

Total and acrosome-reacted sperm bound to zona pellucida and sperm movement characteristics were evaluated after 3 to 5 hours of incubation of the antibodies with human sperm.

RESULTS

Anti-FA-1 antibodies significantly reduced human sperm fusion with zona-free hamster oocytes and sperm binding to the human zona pellucida but did not affect binding of acrosome-reacted sperm and sperm movement characteristics. Anti-GA-1 and antiprotamine antibodies did not affect sperm-oocyte interaction, acrosomal reaction, or sperm motility.

CONCLUSIONS

Antibodies to FA-1 but not to GA-1 and protamine inhibit human sperm-zona interaction.