Testase 1 (ADAM 24) a plasma membrane-anchored sperm protease implicated in sperm function during epididymal maturation or fertilization

Plasma membrane-anchored proteases have key roles in cell signaling, migration and refashioning the cell surface and its surroundings. We report the first example of a plasma membrane-anchored protease on mature sperm, testase 1 (ADAM 24). Unlike other studied sperm ADAMs (fertilin (α) and (β), cyritestin) whose metalloprotease domains are removed during sperm development, we found testase 1 retains an active metalloprotease domain, suggesting it acts as a protease on mature sperm. Testase 1 is a glycoprotein (molecular mass 88 kDa), localized to the equatorial region of the plasma membrane of cauda epididymal sperm. Typically, proteolytic removal of the pro-domain is an initial activation step for ADAM proteases. The pro-domain of the testase 1 precursor (108 kDa) is proteolytically removed as sperm transit the caput epididymis to produce processed (mature) testase 1 (88 kDa). Testase 1 is unique among all studied ADAMs in that its proteolytic processing occurs on the sperm plasma membrane instead of at an intracellular site (the Golgi). Using GST-fusion proteins and a synthetic testase 1 C-terminal peptide, we found that the cytoplasmic tail of testase 1 could be phosphorylated in vitro by protein kinase C (PKC). Thus testase 1 apparently has a cytoplasmic PKC phosphorylation site(s). Protein kinase C is known to stimulate other ADAMs' protease activity. Because events of the acrosome reaction include PKC activation, we speculate that testase 1 protease function could be important in sperm penetration of the zona pellucida after sperm PKC is activated during the acrosome reaction.

Analysis of loss of adhesive function in sperm lacking cyritestin or fertilin beta

We produced mice lacking the sperm surface protein cyritestin (ADAM 3) and found mutant males are infertile. Similar to fertilin beta (ADAM 2) null sperm (C. Cho et al., 1998, Science 281, 1857-1859), cyritestin null sperm are drastically deficient in adhesion to the egg zona pellucida (0.3% of wild type) and to the egg plasma membrane (9% of wild type). Thus sperm from male mice with a gene deletion of either ADAM have a loss of adhesive function in at least two steps of fertilization. We found deletion of either ADAM gene resulted in the loss of multiple gene products. This loss of multiple gene products (sperm membrane proteins) appears to result from a novel, developmental mechanism during sperm differentiation. Because the altered sperm protein expression must be responsible for the fertilization defects, our data suggest new models for the molecular basis of the affected steps in fertilization.

A role for a TIMP-3-sensitive, Zn(2+)-dependent metalloprotease in mammalian gamete membrane fusion

During fertilization, sperm and egg plasma membranes adhere and then fuse by a mechanism that is not well understood. Zinc metalloproteases are necessary for some intercellular fusion events, for instance, cell-cell fusion in yeast. In this study we tested the effects of class-specific and family-specific protease inhibitors on mouse gamete fusion. Capacitated, acrosome-reacted sperm and zona-free eggs were used in assays designed to define the effects of inhibitors on sperm-egg plasma membrane binding or fusion. Inhibitors of the aspartic, cysteine, and serine protease classes had no effect on sperm-egg binding or fusion. Both a synthetic metalloprotease substrate (succinyl-Ala-Ala-Phe-amidomethylcoumarin) and the zinc chelator 1,10-phenanthroline inhibited sperm-egg fusion but did not decrease sperm-egg binding. The fusion-inhibition effect of phenanthroline was reversible and activity of the inhibitable zinc metalloprotease was shown to be required during a short time window, the first 15 min after insemination. Tissue inhibitor of metalloprotease-3 and Ro 31-9790, specific inhibitors of zinc metalloproteases in the matrixin and adamalysin families, also inhibited sperm-egg fusion but not sperm-egg binding. These data indicate a role in gamete fusion for one or more zinc metalloproteases of the matrixin and/or adamalysin families that act after plasma membrane binding and before sperm-egg membrane fusion.

Genomic organization of the mouse fertilin beta gene that encodes an ADAM family protein active in sperm-egg fusion

The fertilin alpha and beta genes (Ftna and Ftnb, respectively) encode a sperm surface heterodimer that functions in sperm-egg fusion. They are the first identified members of a large gene family coding for multidomain membrane proteins (ADAMs) that include A Disintegrin And Metalloprotease domain. In this study, we report the isolation and structural organization of the mouse fertilin beta gene. The gene is present as a single copy and covers a region of approximately 55 kilobases in the genome. The fertilin beta gene is composed of at least 20 exons interrupted by 19 introns. The sizes of the exons are relatively small and vary from 56 to 193 bases; the sizes of introns vary from 350 bases to 9.4 kilobases. The exon-intron boundaries conform to the GT/AG rule with one exception: GC replaces GT at the 5' splice site in intron 13. Comparison of genomic organization between mouse fertilin beta and the previously sequenced ADAM family gene, human MDC [Katagiri et al. (1995): Cytogenet Cell Genet 68:39-44] showed 12 conserved exon-intron boundaries. In addition, we analyzed the fertilin alpha gene, demonstrating that more than one gene is present in the mouse genome.

Analysis of mouse fertilin in wild-type and fertilin beta(-/-) sperm: evidence for C-terminal modification, alpha/beta dimerization, and lack of essential role of fertilin alpha in sperm-egg fusion

The sperm surface protein fertilin functions in sperm-egg interaction. On guinea pig and bovine sperm, fertilin is a heterodimer of alpha and beta subunits. Both subunits are initially synthesized as precursors and then proteolytically processed by removing N-terminal domains. Since the mouse is currently the main mammalian species in which fertilization is studied, in the present report, we analyzed the structure, processing, and expression of fertilin in mouse. We found that the processing of mouse fertilin beta occurs during epididymal maturation and involves changes in the cytoplasmic tail domain as well as the N-terminal domains. Although we (R. Yuan et al., 1997, J. Cell Biol. 137, 105-112) and others (M. S. Chen et al., 1999, J. Cell Biol. 144, 549-561) have previously reported that mature fertilin beta is 55-57 kDa, here we show that 55 kDa is an unrelated protein in the sperm extract which cross-reacts with an antibody that recognizes precursor, but not mature, fertilin beta. Comparison of Western blots of wild-type and fertilin beta knockout sperm revealed that authentic, mature fertilin beta is 45 kDa. We also obtained direct evidence that mouse fertilin alpha and beta exist as a heterodimer. In addition, we found that in mice lacking the fertilin beta subunit, fertilin alpha is absent from mature sperm. A widely proposed model for sperm-egg fusion suggests that fertilin alpha is the sperm component that promotes membrane fusion by undergoing a conformational change that exposes a virus-like, hydrophobic fusion peptide. Because sperm lacking fertilin alpha and fertilin beta can fuse with eggs at 50% the wild-type rate, this model is called into question. The results suggest instead that other gamete surface molecules act to promote membrane fusion and that fertilin's role in gamete fusion is in sperm-egg plasma membrane adhesion.

The ADAM gene family: surface proteins with adhesion and protease activity

An ADAM is a transmembrane protein that contains a disintegrin and metalloprotease domain and, therefore, it potentially has both cell adhesion and protease activities. Currently, the ADAM gene family has 29 members, although the function of most ADAM gene products is unknown. We discuss the ADAM gene products with known functions that act in a highly diverse set of biological processes, including fertilization, neurogenesis, myogenesis, embryonic TGF-alpha release and the inflammatory response.

The frequency of calcium oscillations in mouse eggs at fertilization is modulated by the number of fused sperm

In a variety of calcium signaling systems, the frequency of intracellular calcium oscillations is physiologically important. Probably multiple factors control the frequency of calcium oscillations in the egg after fertilization and many of these remain to be identified. In this study, we present the first rigorous set of data showing that monospermic fertilization is important for setting the physiological calcium oscillation frequency. Recordings in 152 zona-free eggs show that the general pattern of the calcium oscillations is identical in monospermic and polyspermic eggs; however, the oscillation frequency is higher in polyspermic eggs (P < 10(-6)). The frequency of the late oscillations increases with the number of sperm heads incorporated: 5.2 +/- 0.3 spikes per hour (mean +/- SEM; n = 55) in monospermic eggs, 6.6 +/- 0.3 (n = 62) in dispermic eggs, 8.7 +/- 0.7 (n = 23) in trispermic eggs, and 8.9 +/- 0.9 (n = 12) in eggs with four or more sperm heads. The frequency of the early oscillations is also increased in polyspermic eggs. Seventy-eight additional eggs were divided into two groups and inseminated with two different sperm concentrations ("low" and "high") to obtain one group mainly monospermic and the other mainly polyspermic. The two groups of eggs oscillated at different frequencies (P < 10(-5)). These data rule out the possibility of an egg effect in which some eggs would have the dual properties of oscillating faster and of being able to fuse with several sperm cells. These data instead suggest that the sperm modulates the frequency of the oscillations in a dose-dependent manner.

Identification of four novel ADAMs with potential roles in spermatogenesis and fertilization

The ADAM (A Disintegrin And Metalloprotease) family is known to have important roles in various developmental systems, e.g., myogenesis and neurogenesis. In this study, we searched for ADAMs that may function in spermatogenesis or fertilization, and have cloned and sequenced four new mouse ADAM cDNAs: ADAM 24, ADAM 25, ADAM 26 and ADAM 27. The deduced amino acid sequences show that all four contain the complete domain organization common to ADAM family members. Messenger RNA for each of the four ADAMs was found only in the testis. The conserved zinc-dependent metalloprotease active site HEXGHXXGXXHD was found in the metalloprotease domain of three of the novel ADAMs, suggesting that they are testis-specific proteases, to which we give the alternative names: testase 1, ADAM 24; testase 2, ADAM 25; and testase 3, ADAM 26. Using RNA extracted from testes of pre-pubertal males of increasing age (8-40days), we found that adult levels of transcription, assessed in Northern blots, are reached by day 20 (ADAM 27), day 25 (ADAMs 24 and 25) and in the range day 25-50 (ADAM 26). These results suggest that each ADAM is transcribed in spermatogenic cells in a regulated pattern at a specific developmental stage.

Fertilization defects in sperm from mice lacking fertilin beta

Fertilin, a member of the ADAM family, is found on the plasma membrane of mammalian sperm. Sperm from mice lacking fertilin beta were shown to be deficient in sperm-egg membrane adhesion, sperm-egg fusion, migration from the uterus into the oviduct, and binding to the egg zona pellucida. Egg activation was unaffected. The results are consistent with a direct role of fertilin in sperm-egg plasma membrane interaction. Fertilin could also have a direct role in sperm-zona binding or oviduct migration; alternatively, the effects on these functions could result from the absence of fertilin activity during spermatogenesis.

Inhibition of monkey sperm hyaluronidase activity and heterologous cumulus penetration by flavonoids

A microplate assay for hyaluronidase and a heterologous cumulus penetration assay were used to determine the effects of four flavonoids (tannic acid, kaempferol, quercetin, and apigenin) on the function of cynomolgus monkey sperm. All four flavonoids inhibited the activity of hyaluronidase extracted from monkey sperm in a concentration-dependent manner over the range of 50-200 microM. Tannic acid and apigenin had lower inhibitory effects than kaempferol and quercetin. Kaempferol, quercetin, and apigenin at 100 microM were shown to significantly inhibit monkey sperm penetration into hamster cumulus. There was a significant linear relationship between the capacity of the flavonoids to inhibit monkey sperm hyaluronidase activity and their inhibitory effects on hamster cumulus penetration (r = 0.97). Tannic acid was observed to reduce sperm motility, and it was not used in the cumulus penetration assay. The other three flavonoids tested in the cumulus penetration assay did not affect sperm motility, nor did they induce acrosome reactions. The results demonstrate that the flavonoids are useful tools for assessing the involvement of hyaluronidase in the functions of monkey sperm that are involved in fertilization.

Reversible contraceptive effect of PH-20 immunization in male guinea pigs

Sperm proteins are currently being studied as antigens on which to base a contraceptive vaccine. Sperm plasma membrane proteins offer the theoretical possibility of immunizing either males or females and achieving a contraceptive effect. In this study, we investigated the sperm plasma membrane protein PH-20 as an antigen for inducing infertility in males. We found that infertility can reproducibly be induced in male guinea pigs immunized with purified PH-20: 100% (29 of 29) of PH-20-immunized males became infertile, whereas all 22 controls were fertile. The males were extremely responsive to PH-20 immunization: infertility could be induced with a single injection of only 5 microg PH-20. Among males that received their initial injection when they were approximately 300 g (body weight), 14 of 15 had regained fertility at about 1 yr after initial injection. Surprisingly, in another group of males that received their first injection when they were approximately 650 g (body weight), only 1 of 5 had regained fertility about 1 yr after initial injection. Anti-PH-20 titers in antisera (2 mo after initial injection) were generally in the range 1.1-4.2 x 10(4) in twice-injected males and the range 1.8-9.4 x 10(3) in once-injected males. Over the next 6-11 mo, twice-injected males' titers decreased > or = 4-fold, whereas once-injected males' titers decreased slightly (1.1 - to 1.8-fold). After 6-11 mo, anti-PH-20 titers were in the range 1.0-4.8 x 10(3), and the precise residual titer did not correlate with fertility/infertility. The results show that immunization of males with PH-20, even at low doses, results in a reproducible, completely effective contraceptive action.

Mechanism of infertility in male guinea pigs immunized with sperm PH-20

PH-20, a testis-specific protein first expressed in haploid germ cells, is present on the posterior head plasma membrane and inner acrosomal membrane of mature guinea pig sperm. PH-20 is bifunctional, having a hyaluronidase activity that allows sperm to penetrate the cumulus layer and a separate activity required for binding of acrosome-reacted sperm to the zona pellucida. The immunization of male guinea pigs with PH-20 reproducibly results in infertility with a duration of 6-12 mo or longer. In this study, we analyzed the immunopathology in the reproductive tract of PH-20-immunized males to probe the mechanism(s) responsible for the induced infertility and found two separate effects. Remarkably, in almost all infertile, PH-20-immunized males, the caudae epididymides were empty (contained no sperm) or contained only abnormal sperm. The complete loss of normal sperm in the epididymis apparently results in infertility. A second effect was the induction of experimental autoimmune orchitis (EAO), representing the first report of EAO induced by a purified testis/sperm molecule of known functions. PH-20-induced EAO differed from EAO induced by crude testis antigens in two respects: 1) an absence of epididymitis with abscess and granuloma and 2) the presence of antibody on germ cells within seminiferous tubules and inside the cauda epididymidis. The former suggests that crude testis antigens other than PH-20 are responsible for epididymitis, and the latter suggests a possible role of antibody in EAO pathogenesis and infertility induction. Return to fertility, after 6-12 mo, was accompanied by regression of EAO and reappearance of spermatozoa in the caudae epididymides.

A role for the disintegrin domain of cyritestin, a sperm surface protein belonging to the ADAM family, in mouse sperm-egg plasma membrane adhesion and fusion

Sperm-egg plasma membrane fusion is preceded by sperm adhesion to the egg plasma membrane. Cell-cell adhesion frequently involves multiple adhesion molecules on the adhering cells. One sperm surface protein with a role in sperm-egg plasma membrane adhesion is fertilin, a transmembrane heterodimer (alpha and beta subunits). Fertilin alpha and beta are the first identified members of a new family of membrane proteins that each has the following domains: pro-, metalloprotease, disintegrin, cysteine-rich, EGF-like, transmembrane, and cytoplasmic domain. This protein family has been named ADAM because all members contain a disintegrin and metalloprotease domain. Previous studies indicate that the disintegrin domain of fertilin beta functions in sperm-egg adhesion leading to fusion. Full length cDNA clones have been isolated for five ADAMs expressed in mouse testis: fertilin alpha, fertilin beta, cyritestin, ADAM 4, and ADAM 5. The presence of the disintegrin domain, a known integrin ligand, suggests that like fertilin beta, other testis ADAMs could be involved in sperm adhesion to the egg membrane. We tested peptide mimetics from the predicted binding sites in the disintegrin domains of the five testis-expressed ADAMs in a sperm-egg plasma membrane adhesion and fusion assay. The active site peptide from cyritestin strongly inhibited (80-90%) sperm adhesion and fusion and was a more potent inhibitor than the fertilin beta active site peptide. Antibodies generated against the active site region of either cyritestin or fertilin beta also strongly inhibited (80-90%) both sperm-egg adhesion and fusion. Characterization of these two ADAM family members showed that they are both processed during sperm maturation and present on mature sperm. Indirect immunofluorescence on live, acrosome-reacted sperm using antibodies against either cyritestin or fertilin beta showed staining of the equatorial region, a region of the sperm membrane that participates in the early steps of membrane fusion. Collectively, these data indicate that a second ADAM family member, cyritestin, functions with fertilin beta in sperm-egg plasma membrane adhesion leading to fusion.

Hyaluronidase activity of macaque sperm assessed by an in vitro cumulus penetration assay

A model system consisting of cynomolgus macaque sperm and ovulated hamster ova-cumulus complexes (OCCs) was utilized to study the role of the sperm protein PH-20 in cumulus penetration. The hyaluronidase activity of solubilized macaque sperm PH-20 was evaluated using an ELISA-like microplate assay prior to and following the addition of the hyaluronidase inhibitors heparin (0-100 microg/ml) and apigenin (250 microM), as well as the Ig fraction of a polyclonal antibody raised against purified recombinant macaque PH-20 (R10; 10-400 microg/ml). Sperm motility following exposure to enzyme inhibitors was evaluated using computer-aided sperm motility analysis. Macaque sperm were labeled with the permeant fluorescent nuclear dye, Hoechst 33342, and were coincubated with ovulated hamster OCCs for 30 min at 37 degrees C. The addition of heparin, apigenin, or R10 antibody to solubilized sperm extracts resulted in a linear dose-dependent decrease in hyaluronidase activity (P < .01). In the heterologous cumulus penetration assay, fluorescently labeled macaque sperm that were pretreated with heparin (1-100 microg/ml), apigenin (250 microM), or R10 antibody (Ig fraction, 10-400 microg/ml) demonstrated a dose-dependent decrease in the ability to penetrate hamster OCCs (P < 0.01), in the absence of effects on sperm motility. In the homologous assay, experiments using macaque OCCs and fluorescently labeled macaque sperm confirmed that the same concentrations of heparin and R10 antibody similarly suppressed spermatozoal cumulus penetration (P < .01). These results suggest that macaque sperm PH-20-derived hyaluronidase participates in cumulus penetration in this species, and that this model system is useful for further studies into primate gamete interaction.

The PH-20 protein in human spermatozoa

PH-20 is a sperm plasma-membrane protein that has been shown to have hyaluronidase activity in several mammalian species including nonhuman primates. In this investigation, the PH-20 protein was characterized in noncapacitated human sperm and in capacitated human sperm. Two forms of PH-20 were observed in immunoblots of sodium dodecylsulfate polyacrylamide-gel electrophoresis (SDS PAGE) using a polyclonal antibody to recombinant PH-20: a major band of 64 kDa appeared in noncapacitated and capacitated sperm extracts and a 53-kDa band that appeared only in the acrosome-reaction supernatant of acrosome-reacted sperm. Using hyaluronic acid substrate gel analysis, we demonstrated that noncapacitated sperm extracts, capacitated sperm extracts, and the acrosome-reaction supernatant had hyaluronidase activity at neutral pH (pH 7) and acid pH (pH 4). The 64-kDa form in all samples had hyaluronidase activity at both neutral and acid pH, but the 53-kDa form was only active at acid pH. Total hyaluronidase activity, as measured by a microplate assay, was higher at pH 7 than at pH 4. Very low hyaluronidase activity was detected in the acrosome-reaction supernatant. Transmission electron microscopy and immunogold labeling showed that PH-20 of acrosome-intact human sperm was located on the plasma membrane over the entire head but not on the sperm midpiece and tail. After the acrosome reaction, PH-20 was also located on the inner acrosomal membrane. The biochemical characteristics and the ultrastructural localization of PH-20 in human sperm suggest that this protein is the human sperm hyaluronidase and, therefore, has an important function during fertilization.

Why did the sperm cross the cumulus? To get to the oocyte. Functions of the sperm surface proteins PH-20 and fertilin in arriving at, and fusing with, the egg

The sperm surface has an active role in the events of fertilization. The definition of the sperm surface in both its composition and domain organization begins during spermatogenesis and continues until the moment of sperm-egg fusion. Alterations of the surface proceed as a result of internal programming and environmental cues from both the male and female reproductive tracts, including interactions with the egg itself. We have investigated the sperm surface to understand its domain organization and the ongoing changes in this organization as well as the role of specific surface proteins in fertilization. Much of our research has concentrated on two surface proteins: PH-20 and fertilin. PH-20 is a single-chain protein, anchored in the membrane via a glycosyl phosphatidylinositol (GPI) anchor. The N-terminal domain of the molecule has a hyaluronidase activity. The hyaluronidase activity of PH-20 on the sperm plasma membrane enables sperm to penetrate the layer of cumulus cells surrounding the oocyte. PH-20 has a second function, unrelated to its hyaluronidase activity, in the binding of acrosome-reacted sperm to the zona pellucida (secondary sperm-zona binding). The fertilin molecule is an alpha,beta heterodimer whose two subunits are closely related transmembrane proteins. Fertilin beta has a disintegrin domain that has high sequence homology with the snake disintegrins, a known class of soluble integrin ligands. The binding site of the beta disintegrin domain functions to bind sperm to the egg plasma membrane via a mechanism that leads to sperm-egg fusion. The precursor of fertilin alpha, made in the testis, has an active metalloprotease site that could function in spermatogenesis. This metalloprotease domain is removed by proteolytic processing in the testis. Mature fertilin alpha on sperm also has a hydrophobic, putative "fusion peptide" that may promote the process of lipid bilayer fusion between sperm and egg plasma membranes. Fertilin alpha and beta are the first identified members of a new gene family of transmembrane proteins, the ADAM family, so called because they contain A Disintegrin And Metalloprotease domain. Many distinct ADAMs have now been found in diverse tissues and species (Drosophila to human) and are proposed to have a variety of functions in development and the adult. In addition to fertilin, other ADAMs are also present on the sperm plasma membrane and may participate with fertilin in sperm-egg fusion.

The mouse Spam1 maps to proximal chromosome 6 and is a candidate for the sperm dysfunction in Rb(6.16)24Lub and Rb(6.15)1Ald heterozygotes

We have determined the chromosomal localization of the murine gene encoding the 68-kDa sperm adhesion molecule 1, Spam1 or Ph-20. Using two independent approaches, fluorescence in situ hybridization (FISH) and interspecific backcross analysis we show the Spam1 maps to proximal mouse Chromosome (Chr) 6. This map position is within the conserved linkage group corresponding to human Chr 7q, where the human homolog, SPAM 1, has been shown to map previously. Genetic mapping shows the gene to be very closely linked to Met, one of the most proximal loci on MMU 6. It thus places the gene near the centromere and the junction of the Rb(6.16)24Lub and Rb(6.15)1Ald translocations. The essential role of the Spam1 sperm antigen in mouse sperm-egg interactions and its gene location provide strong support for its candidacy as the gene involved in the dysfunction of mouse sperm bearing the Rb(6.16)24Lub or Rb(6.15)1Ald translocation.

Sperm surface protein PH-20 is bifunctional: one activity is a hyaluronidase and a second, distinct activity is required in secondary sperm-zona binding

In previous studies, we have found that the sperm membrane protein PH-20 acts during two different stages of fertilization. On acrosome-intact sperm, PH-20 has a hyaluronidase activity that is required for sperm penetration through the cumulus cell layer that surrounds the oocyte. On acrosome-reacted sperm, PH-20 has a required function in sperm-zona binding (secondary binding). Because hyaluronic acid (HA) has been detected in the zona pellucida, secondary sperm-zona adhesion could depend on repetitive binding and hydrolysis of HA by PH-20 acting as a hyaluronidase. Alternatively, PH-20 may be bifunctional and have a second, different activity required for secondary binding. To distinguish between these two possibilities, in this study we used reagents that inhibit either PH-20's function in sperm-zona binding or its hyaluronidase activity. We found that an anti-PH-20 monoclonal antibody that inhibited sperm-zona binding (approximately 90%) had no effect on hyaluronidase activity. Conversely, apigenin, a hyaluronidase inhibitor, blocked PH-20 hyaluronidase activity 93% without inhibiting sperm-zona binding. Similarly, another anti-PH-20 monoclonal antibody that inhibited hyaluronidase activity 95% only partially inhibited sperm-zona binding (approximately 45%). We also extensively pretreated oocytes with hyaluronidase to remove all accessible HA on or in the zona pellucida and found little or no effect on secondary sperm-zona binding. Our results suggest that PH-20 is bifunctional and has two activities: a hyaluronidase activity and a second, separate activity required for secondary sperm-zona binding.

Chromosomal assignment of four testis-expressed mouse genes from a new family of transmembrane proteins (ADAMs) involved in cell-cell adhesion and fusion

A new gene family of multidomain membrane proteins (ADAMs) that include A Disintegrin And Metalloprotease domain comprises an increasing number of identified members. Two members of this family, fertilin alpha and fertilin beta, form a heterodimeric protein that is required for sperm-egg fusion. Most recently, it has been shown that a third family member, meltrin alpha, is involved in myoblast fusion (Yagami-Hiromasa et al., 1995, Nature 377: 652-656). Using restriction fragment length polymorphism analysis of a DNA panel from an interspecific backcross, we have determined the chromosomal locations of four mouse genes of this family that are expressed in testis: fertilin alpha, fertilin beta, ADAM 4, and ADAM 5. These genes have been given the locus symbols Ftna (fertilin alpha), Ftnb (fertilin beta), Adam4 (ADAM 4), and Adam5 (ADAM 5). They were mapped to chromosomes 5, 14, 9, and 8, respectively, revealing a dispersed localization. Human chromosome locations of these genes are predicted on the basis of the mapping results using the information provided by comparative linkage maps. Because all four of these ADAM genes are expressed in testis and fertilin alpha and beta have been found to be important for fertilization, we compared their chromosomal locations with known mouse mutations affecting spermatogenesis and fertility.

Structural relationship of sperm soluble hyaluronidase to the sperm membrane protein PH-20

The sperm plasma membrane protein PH-20 has a hyaluronidase activity that enables acrosome-intact sperm to pass through the cumulus cell layer of the egg. In this study we analyzed the relationship of guinea pig PH-20 and the "classical" soluble hyaluronidase released at the time of the acrosome reaction of guinea pig sperm. PH-20 is a membrane protein, anchored in the plasma and inner acrosomal membranes by a glycosyl phosphatidyl inositol anchor. Several types of experiments indicate a structural relationship of PH-20 and the soluble hyaluronidase released during the acrosome reaction. First, an antiserum raised against purified PH-20 is positive in an immunoblot of the soluble protein fraction released during the acrosome reaction. In the released, soluble protein fraction, the anti-PH-20 antiserum recognizes a protein of approximately 64 kDa, i.e., identical in molecular mass to PH-20 (approximately 64 kDa). Second, the enzymatic activity of the released hyaluronidase is completely inhibited (100%) by the anti-PH-20 antiserum. Third, almost all (97%) of the soluble hyaluronidase is removed from the released protein fraction by a single pass through an affinity column made with an anti-PH-20 monoclonal antibody. These findings suggest that the released, soluble hyaluronidase is a soluble form of PH-20 (sPH-20). During the acrosome reaction, PH-20 undergoes endoproteolytic cleavage into two disulfide-linked fragments whereas the released sPH-20 is not cleaved, suggesting the possible activity of a membrane-bound endoprotease on PH-20. We searched for a cDNA encoding sPH-20 but none was found. This result suggests that sPH-20 may arise from the enzymatic release of PH-20 from its membrane anchor, possibly at the time of acrosome reaction.

The PH-20 protein in cynomolgus macaque spermatozoa: identification of two different forms exhibiting hyaluronidase activity

In these experiments, we have characterized the bifunctional sperm protein PH-20 in macaque sperm and studied its hyaluronidase activity. Intact sperm were evaluated before the acrosome reaction (AR), and a soluble form of PH-20 released during acrosomal exocytosis was also investigated. Western blots of SDS-PAGE of acrosome-intact sperm extracts revealed a 64-kDa form of PH-20 was recognized by a polyclonal antibody (R-10) raised in rabbits against purified, recombinant cynomolgus macaque sperm PH-20. The soluble components released during the AR which were recognized by the R-10 antibody included both the 64-kDa form and a 53-kDa form of PH-20. An ELISA-like procedure for determining PH-20 hyaluronidase activity indicated that acrosome-intact sperm exhibited two peaks of hyaluronidase activity near pH 4 and > or = pH 7. The majority of enzyme activity in acrosome-intact sperm extracts occurred at neutral pH, while the soluble hyaluronidase activity released at the AR was predominantly acid-active. Hyaluronidase activity of PH-20 at different pH optima was investigated using hyaluronic acid substrate gel electrophoresis, and results indicated that the 64-kDa polypeptide had a broad range, with the majority of activity at neutral pH (pH 7). The 53-kDa polypeptide in sperm extracts only exhibited activity at acid pH (pH 4). The hyaluronidase activities of both enzymes could be inhibited by apigenin. The soluble PH-20 hyaluronidase activity released during the AR was primarily of the acid-active 53-kDa form. Fine structural localization of PH-20 using Fab fragments of R-10 IgG demonstrated that PH-20 was associated not only with sperm membranes, but also with the dispersing acrosomal contents. These data suggest that the more neutral-active form of PH-20 (64 kDa) is present on the plasma and inner acrosomal membranes and gives rise to the soluble acid-active form at the time of the AR. The generation of the soluble form of PH-20 may result from the action of acrosomal enzymes, which could include proteases, glycosidases, and phospholipases.

Initial evaluation of fertilin as an immunocontraceptive antigen and molecular cloning of the cynomolgus monkey fertilin beta subunit

Fertilin (PH-30) is a sperm surface protein that functions in sperm adhesion and fusion with the egg plasma membrane. Because of its essential function in fertilization, fertilin is a potential target for novel contraceptive approaches. In a pilot fertility trial, immunization of male guinea pigs with purified guinea pig fertilin resulted in complete infertility. The contraceptive effect was partial (two out of six animals were infertile) when female guinea pigs were immunized with the antigen. These results suggest that fertilin or domains of fertilin may be effective as immunocontraceptive antigens. As a step toward achieving this goal, we communicate the cDNA and deduced amino acid sequence of the monkey fertilin beta subunit.

Mouse egg integrin alpha 6 beta 1 functions as a sperm receptor

Binding between sperm and egg plasma membranes is an essential step in fertilization. Whereas fertilin, a mammalian sperm surface protein, is involved in this crucial interaction, sperm receptors on the egg plasma membrane have not been identified. Because fertilin contains a predicted integrin ligand domain, we investigated the expression and function of integrin subunits in unfertilized mouse eggs. Polymerase chain reactions detected mRNAs for alpha 5, alpha 6, alpha v, beta 1, beta 3, and beta 5. Immunofluorescence revealed alpha 6 beta 1 and alpha v beta 3 on the plasma membrane. GoH3, a function-blocking anti-alpha 6 monoclonal antibody, abolished sperm binding, but a nonfunction-blocking anti-alpha 6 monoclonal antibody, a function-blocking anti-alpha v beta 3 polyclonal antibody, and an RGD peptide had no effect. Somatic cells bound sperm avidly, but only if they expressed alpha 6 beta 1. A peptide analog of the fertilin integrin ligand domain inhibited sperm binding to eggs and alpha 6 beta 1+ cells and diminished GoH3 staining of eggs. Our results indicate a novel role for the integrin alpha 6 beta 1 as a cell-cell adhesion receptor that mediates sperm-egg binding.

ADAM, a widely distributed and developmentally regulated gene family encoding membrane proteins with a disintegrin and metalloprotease domain

Fertilin alpha and beta, previously known as PH-30 alpha and beta, are two subunits of a guinea pig sperm integral membrane protein implicated in sperm-egg binding and fusion. They are derived from sequence-similar precursors which contain a metalloprotease-like and a disintegrin-like domain and which are related to a family of metalloprotease and disintegrin domain-containing snake venom proteins. We report here the cloning, sequencing, and characterization of mouse fertilin alpha and beta as well as five additional sequence-similar cDNAs from guinea pig and mouse testis. We name this gene family ADAM, for proteins containing A Disintegrin And Metalloprotease domain, and in honor of its dual origins in the fields of snakes and fertility. In situ hybridization demonstrated that, in testis, RNA encoding these ADAMs is expressed only in spermatogenic cells and that this expression is developmentally regulated. PCR analysis of mouse tissue cDNA showed that these ADAMs display different patterns of tissue distribution. Some ADAMs (e.g., fertilin alpha) have the consensus active-site sequence for a zinc-dependent metalloprotease in their metalloprotease-like domain. All have a disintegrin-like domain, which could bind integrins or other receptors. Some have sequences which may be active in membrane fusion. All encode potential membrane-spanning domains. Searches of sequence databases revealed that additional mammalian members of the ADAM gene family have been cloned from a variety of tissues. Thus, the ADAMs are a large, widely expressed, and developmentally regulated family of proteins with multiple potential functions in cell-cell and cell-matrix interactions.

Location of the PH-20 protein on acrosome-intact and acrosome-reacted spermatozoa of cynomolgus macaques

Fluorescence microscopy and transmission electron microscopy (TEM) were used to determine the location of the membrane protein PH-20 on spermatozoa of cynomolgus macaques. Rabbit antiserum raised against recombinant cynomolgus macaque sperm PH-20 was used as the primary antibody, and the second antibody was goat anti-rabbit IgG conjugated with either fluorescein isothiocyanate or 15 nm gold particles. Spermatozoa were evaluated before capacitation and after capacitation and induction of acrosome reactions with calcium ionophore A23187. In sperm suspensions with a high percentage of intact acrosomes, fluorescence labeling was observed uniformly over most of the sperm head. The sperm midpiece and tail were not labeled. In sperm suspensions with a high percentage of acrosome reactions, most spermatozoa labeled intensely over the anterior sperm head, but labeling of the posterior sperm head was greatly reduced. TEM of acrosome-intact spermatozoa revealed gold particles distributed uniformly on the plasma membrane overlying the acrosome, the equatorial segment, and most of the post-acrosomal region. After the acrosome reaction, gold label was present on the inner acrosomal membrane and on the plasma membrane overlying the equatorial segment. Very little label was present on the plasma membrane in the post-acrosomal region of acrosome-reacted spermatozoa. The location of PH-20 on the surface of macaque spermatozoa suggests a function for this protein in primary and/or secondary binding to the zona pellucida. The apparent decrease in amount of PH-20 on the posterior head of macaque spermatozoa following the acrosome reaction is consistent with the migration of this protein to the inner acrosomal membrane, as demonstrated previously for the homologous PH-20 protein of guinea pig spermatozoa.

A hyaluronidase activity of the sperm plasma membrane protein PH-20 enables sperm to penetrate the cumulus cell layer surrounding the egg

A typical mammalian egg is surrounded by an outer layer of about 3,000 cumulus cells embedded in an extracellular matrix rich in hyaluronic acid. A current, widely proposed model is that the fertilizing sperm, while it is acrosome intact, passes through the cumulus cell layer and binds to the egg zona pellucida. This current model lacks a well-supported explanation for how sperm penetrate the cumulus layer. We report that the sperm protein PH-20 has a hyaluronidase activity and is present on the plasma membrane of mouse and human sperm. Brief treatment with purified, recombinant PH-20 can release all the cumulus cells surrounding mouse eggs. Acrosome intact mouse sperm incubated with anti-PH-20 antibodies can not pass through the cumulus layer and thus can not reach the zona pellucida. These results, indicating that PH-20 enables acrosome intact sperm to penetrate the cumulus barrier, reveal a mechanism for cumulus penetration, and thus provide the missing element in the current model.

Identification of a binding site in the disintegrin domain of fertilin required for sperm-egg fusion

Fertilization and certain later stages in mammalian embryonic development require fusion between membranes of individual cells. The mechanism of eukaryotic cell-cell fusion is unknown, and no surface molecules required for this process have been unequivocally identified. The role of the sperm surface protein fertilin in sperm-egg fusion was tested by using peptide analogues of a potential integrin binding site in the fertilin beta subunit. Peptide analogues that include a TDE sequence from the disintegrin region of fertilin beta are able to bind to the egg plasma membrane and strongly inhibit sperm-egg fusion. These results show that the disintegrin domain of fertilin beta binds to the egg plasma membrane and that this binding is required for membrane fusion.

The precursor region of a protein active in sperm-egg fusion contains a metalloprotease and a disintegrin domain: structural, functional, and evolutionary implications

PH-30, a sperm surface protein involved in sperm-egg fusion, is composed of two subunits, alpha and beta, which are synthesized as precursors and processed, during sperm development, to yield the mature forms. The mature PH-30 alpha/beta complex resembles certain viral fusion proteins in membrane topology and predicted binding and fusion functions. Furthermore, the mature subunits are similar in sequence to each other and to a family of disintegrin domain-containing snake venom proteins. We report here the sequences of the PH-30 alpha and beta precursor regions. Their domain organizations are similar to each other and to precursors of snake venom metalloproteases and disintegrins. The alpha precursor region contains, from amino to carboxyl terminus, pro, metalloprotease, and disintegrin domains. The beta precursor region contains pro and metalloprotease domains. Residues diagnostic of a catalytically active metalloprotease are present in the alpha, but not the beta, precursor region. We propose that the active sites of the PH-30 alpha and snake venom metalloproteases are structurally similar to that of astacin. PH-30, acting through its metalloprotease and/or disintegrin domains, could be involved in sperm development as well as sperm-egg binding and fusion. Phylogenetic analysis indicates that PH-30 stems from a multidomain ancestral protein.

Molecular cloning of the human and monkey sperm surface protein PH-20

The guinea pig sperm surface protein PH-20 has an essential function in sperm adhesion to the zona pellucida of guinea pig eggs. Fully effective contraception has been achieved by immunizing either male or female guinea pigs with purified guinea pig PH-20. Here we report the isolation of human and cynomolgus monkey PH-20 cDNAs as a key step toward testing the function of primate PH-20 and the contraceptive efficacy of PH-20 immunization in primates. The deduced amino acid sequence of human PH-20 has 509 residues and is 59% identical with guinea pig PH-20, suggesting they may have a conserved function and immunogenicity. Southern blots show that there is a single PH-20 gene in the human genome and Northern blots of human testis poly(A)+ RNA show a 2.4-kb message. Northern blots of tissues other than testis are negative for PH-20, indicating that human PH-20 is testis-specific.

A role for the WH-30 protein in sperm-sperm adhesion during rouleaux formation in the guinea pig

Mammalian spermatozoa participate in specific cell adhesion phenomena during their development and functional lifespan; this includes interaction with Sertoli cells, the zona pellucida, and the oolemma. In some species such as the guinea pig, an additional sperm-sperm adhesion occurs during epididymal maturation which results in the formation of rouleaux in which the sperm heads are stacked one upon the other and the periacrosomal plasma membranes of adjacent sperm are linked by periodic cross-bridges. In this study, we have used a monoclonal antibody to investigate the role of the WH-30 protein on the sperm surface in the formation of the junctional zones between adjacent guinea pig sperm in rouleaux. WH-30 monoclonal antibodies caused a dose- and time-dependent dissociation of rouleaux and an increase in the percentage of single, acrosome-intact sperm; there were no effects on sperm motility (maintained at 80-90%) or ultrastructure during the 120-min incubations. The maximal effect of about 80% single sperm was obtained with a 1:4 dilution of the WH-30 hybridoma supernatant or 5-50 micrograms/ml of purified WH-30 IgG. In contrast, incubation of sperm in AH-20 IgG, myeloma cell supernatants, or purified, nonspecific mouse IgG1 had no effect on rouleaux. Treatment of sperm with a WH-30 Fab fragment resulted in almost complete dissociation of rouleaux without any observed effect on sperm motility or acrosomal status. Surface labeling of sperm followed by immunoprecipitation and SDS-PAGE revealed that the WH-30 antibody recognizes a single polypeptide of 43-45 kDa. Using immunofluorescence, the WH-30 protein was localized over the entire surface of the sperm head (whole-head pattern), and immunogold labeling showed that WH-30 is localized in the glycocalyx on both the dorsal and ventral surfaces of the periacrosomal and postacrosomal plasma membranes. These results indicate that the WH-30 protein on the sperm surface is a cell adhesion protein which is involved in the molecular interactions that maintain guinea pig sperm in rouleaux.

A potential fusion peptide and an integrin ligand domain in a protein active in sperm-egg fusion

The union of sperm and egg is a special membrane fusion event that gives a signal to begin development. We have hypothesized that proteins mediating cell-cell fusion events resemble viral fusion proteins and have shown that PH-30, a sperm surface protein involved in sperm-egg fusion, shares biochemical characteristics with viral fusion proteins. We report here the complementary DNA and deduced amino-acid sequences of the mature alpha and beta subunits of PH-30. Both are type-I integral membrane glycoproteins. The alpha subunit contains a putative fusion peptide typical of viral fusion proteins and the beta subunit contains a domain related to a family of soluble integrin ligands found in snake venoms. Thus, the PH-30 alpha/beta complex resembles many viral fusion proteins in both its membrane topology and its predicted binding and fusion functions.

Sperm proteins that serve as receptors for the zona pellucida and their post-testicular modification

The search for molecules on the surface of mammalian sperm that are responsible for the binding of sperm to the zona pellucida has led to the identification of a number of different surface proteins. Different experimental approaches have been used to identify these proteins and the strength of the evidence for their putative role in zona binding is therefore quite variable. In this paper we have discussed some of the approaches that are used to identify cell adhesion molecules and criteria that might be applied in future research. Further research will be required to answer questions as to whether multiple surface antigens are involved in zona binding and if the sperm receptors for the zona are conserved among species. Both of these results would be expected based on what is known about cell adhesion in other systems. We have also discussed the modifications that occur to surface proteins after the sperm leave the testis and how these modifications can potentially activate or increase the efficiency of the function of a protein in zona binding.

cDNA cloning reveals the molecular structure of a sperm surface protein, PH-20, involved in sperm-egg adhesion and the wide distribution of its gene among mammals

Sperm binding to the egg zona pellucida in mammals is a cell-cell adhesion process that is generally species specific. The guinea pig sperm protein PH-20 has a required function in sperm adhesion to the zona pellucida of guinea pig eggs. PH-20 is located on both the sperm plasma membrane and acrosomal membrane. We report here the isolation and sequence of a full-length cDNA for PH-20 (available from EMBL/GenBank/DDBJ under accession number X56332). The derived amino acid sequence shows a mature protein of 468 amino acids containing six N-linked glycosylation sites and twelve cysteines, eight of which are tightly clustered near the COOH terminus. The sequence indicates PH-20 is a novel protein with no relationship to the mouse sperm adhesion protein galactosyl transferase and no significant homology with other known proteins. The two PH-20 populations, plasma membrane and acrosomal membrane, could arise because one form of PH-20 is encoded and differentially targeted at different spermatogenic stages. Alternatively, two different forms of PH-20 could be encoded. Our evidence thus far reveals only one sequence coding for PH-20: Southern blots of guinea pig genomic DNA indicated there is a single PH-20 gene, Northern blots showed a single size PH-20 message (approximately 2.2 kb), and no sequence variants were found among the sequenced cDNA clones. Cross-species Southern blots reveal the presence of a homologue of the PH-20 gene in mouse, rat, hamster, rabbit, bovine, monkey, and human genomic DNA, showing the PH-20 gene is conserved among mammals. Since genes for zona glycoproteins are also conserved among mammals, the general features of sperm and zona proteins involved in mammalian sperm-egg adhesion may have been evolutionarily maintained. Species specificity may result from limited changes in these molecules, either in their binding domains or in other regions that affect the ability of the binding domains to interact.

Evidence that proteolysis of the surface is an initial step in the mechanism of formation of sperm cell surface domains

On terminally differentiated sperm cells, surface proteins are segregated into distinct surface domains that include the anterior and posterior head domains. We have analyzed the formation of the anterior and posterior head domains of guinea pig sperm in terms of both the timing of protein localization and the mechanism(s) responsible. On testicular sperm, the surface proteins PH-20, PH-30 and AH-50 were found to be present on the whole cell (PH-20) or whole head surface (PH-30, AH-50). On sperm that have completed differentiation (cauda epididymal sperm), PH-20 and PH-30 proteins were restricted to the posterior head domain and AH-50 was restricted to the anterior head domain. Thus these proteins become restricted in their distribution late in sperm differentiation, after sperm leave the testis. We discovered that the differentiation process that localizes these proteins can be mimicked in vitro by treating testicular sperm with trypsin. After testicular sperm were treated with 20 micrograms/ml trypsin for 5 min at room temperature, PH-20, PH-30, and AH-50 were found localized to the same domains to which they are restricted during in vivo differentiation. The in vitro trypsin-induced localization of PH-20 to the posterior head mimicked the in vivo differentiation process quantitatively as well as qualitatively. The quantitative analysis showed the process of PH-20 localization involves the migration of surface PH-20 from other regions to the posterior head domain. Immunoprecipitation experiments confirmed that there is protease action in vivo on the sperm surface during the late stages of sperm differentiation. Both the PH-20 and PH-30 proteins were shown to be proteolytically cleaved late in sperm differentiation. These findings strongly implicate proteolysis of surface molecules as an initial step in the mechanism of formation of sperm head surface domains.

Proteolytic processing of a protein involved in sperm-egg fusion correlates with acquisition of fertilization competence

A protein located on the surface of guinea pig sperm (PH-30) has been implicated in the process of sperm-egg fusion (Primakoff, P., H. Hyatt, and J. Tredick-Kline. 1987. J. Cell Biol. 104:141-149). In this paper we have assessed basic biochemical properties of PH-30 and have analyzed the molecular forms of PH-30 present at different stages of sperm maturation. We show the following: (a) PH-30 is an integral membrane glycoprotein; (b) it is composed of two tightly associated and immunologically distinct subunits; (c) both subunits are made as larger precursors; (d) processing of the two subunits occurs at different developmental stages; (e) the final processing step occurs in the region of the epididymis where sperm become fertilization competent; (f) processing can be mimicked in vitro; (g) processing exposes at least two new epitopes on PH-30-one of the newly exposed epitopes is recognized by a fusion-inhibitory monoclonal antibody. These results are discussed in terms of the possible role of PH-30 in mediating fusion with the egg plasma membrane.

Identification of human sperm surface glycoproteins recognized by autoantisera from immune infertile men, women, and vasectomized men

To identify the surface antigens of human sperm recognized by antisera from immune infertility patients and vasectomized men, we labeled sperm surface proteins with 125I- and used patient antisera for immunoprecipitation. Sera were studied from 27 infertile males, 18 infertile females, and 4 vasectomized males, each possessing anti-sperm antibodies detected by immunobead binding. Sera from different infertile males, different infertile females, and vasectomized males were remarkably similar in their surface antigen recognition. The different sera specifically immunoprecipitated the same small group of 125I-labeled surface proteins, which included polypeptides in the region 90 kDa, 40-45 kDa, and 26 kDa. Treatment with N-glycanase showed that the proteins of 90 kDa, 40-45 kDa, and 26 kDa were glycoproteins with N-linked carbohydrate. The immunoprecipitated 125I-labeled proteins and the total extract of 125I-labeled surface proteins were compared on two-dimensional (2D) gels. The results show the 90 kDa polypeptide is a major sperm surface component, whereas 40-45 kDa and 26 kDa polypeptides are minor components. The 2D gel comparison also indicates that 90 kDa, 40-45 kDa, and 26 kDa are a small subset of the total ensemble of sperm surface proteins. Clinical data suggest antibodies to these few proteins interfere with sperm function.

Fully effective contraception in male and female guinea pigs immunized with the sperm protein PH-20

Immunization of male and female animals with extracts of whole sperm cells is known to cause infertility. Also, men and women who spontaneously produce antisperm antibodies are infertile but otherwise healthy. Although the critical sperm antigens are unknown, these observations have led to the proposal that sperm proteins might be useful in the development of a contraceptive vaccine. The guinea pig sperm surface protein PH-20 is essential in sperm adhesion to the extracellular coat (zona pellucida) of the egg, a necessary initial step in fertilization. Here, we report that 100% effective contraception was obtained in male and female guinea pigs immunized with PH-20. Antisera from immunized females had high titres, specifically recognized PH-20 in sperm extracts, and blocked sperm adhesion to the egg zona pellucida in vitro. The contraceptive effect was long-lasting and reversible: immunized females, mated at intervals of six to fifteen months after immunization, progressively regained fertility.

Restricted lateral diffusion of PH-20, a PI-anchored sperm membrane protein

The rate of lateral diffusion of integral membrane proteins is constrained in cells, but the constraining factors for most membrane proteins have not been defined. PH-20, a sperm surface protein involved in sperm-egg adhesion, was shown to be anchored in the plasma membrane by attachment to the lipid phosphatidylinositol and to have a diffusion rate that is highly restricted on testicular sperm, being more than a thousand times slower than lipid diffusion. These results support the hypothesis that lateral mobility of a membrane protein can be regulated exclusively by interactions of its ectodomain.

Purification of the guinea pig sperm PH-20 antigen and detection of a site-specific endoproteolytic activity in sperm preparations that cleaves PH-20 into two disulfide-linked fragments

Previous work has indicated that the guinea pig sperm membrane protein, PH-20, functions in sperm-egg adhesion and that its surface expression is regulated by the acrosome reaction. The PH-20 protein was purified by monoclonal antibody affinity chromatography. Sixty-seven to one hundred percent of the PH-20 antigenic activity present in an octylglucoside (OG) extract of sperm was recovered in the purified protein. From 10(10) sperm, approximately 0.4 mg of PH-20 protein was obtained, which was about 0.24% of the total protein in the OG extract. The purified protein retained the ability to bind the three anti-PH-20 monoclonal antibodies we have isolated. Silver staining of purified PH-20 on overloaded sodium dodecyl sulfate (SDS) gels allowed the estimate that silver-stainable contaminants were present at a level of one part in 2000. The purified PH-20 protein exists in three forms separable on SDS-polyacrylamide gel electrophoresis: a major form with a molecular mass of 64 kDa, a minor form of 56 kDa, and an endoproteolytically cleaved form composed of two disulfide-linked fragments of 41-48 kDa and 27 kDa. Cleveland digests of the 64 kDa and 56 kDa polypeptides indicated that they were structurally related. A proportion of the 64 kDa polypeptide in each purified preparation had undergone endoproteolysis at a specific site, so that it was cleaved into the two disulfide-linked fragments, 41-48 kDa and 27 kDa. It is speculated that the site-specific endoproteolysis of PH-20 may occur during the acrosome reaction and have biological significance.

Rearrangement of sperm surface antigens prior to fertilization

During spermiogenesis and epididymal transit, proteins on the sperm surface become localized to specific domains. In at least one case (PH-20), the protein is initially inserted throughout the membrane and subsequently becomes restricted to a domain by some mechanism that has not yet been determined. Other proteins could become localized through localized insertion. The sperm surface is a dynamic structure that is altered even after the spermatozoon leaves the male. In the female reproductive tract the spermatozoa undergo capacitation and the acrosome reaction that enables them to fertilize the egg. Both of these processes are accompanied by alterations in protein localization: the PT-1 protein migrates during capacitation, and the PH-20 protein migrates after the acrosome reaction. In addition, an upregulation of the surface expression of PH-20 occurs during the acrosome reaction. This additional PH-20 is incorporated into the plasma membrane by the irreversible fusion of the acrosomal membrane with the plasma membrane. The acrosomal membrane contains PH-20 protein that has been stored there since the formation of the acrosome at the spermatid stage of spermiogenesis. Proteins that are freely diffusing must be maintained in a domain by a mechanism that does not involve immobilization or slowing of protein diffusion. We have suggested that barriers to membrane protein diffusion exist at the equatorial region, the posterior ring, and the annulus and that they are responsible for maintaining a localized distribution of at least some of the surface proteins. The migration of surface proteins could result from an alteration of these barriers, a change in the protein structure so that it can pass through the barrier, or active transport across the barrier. These observed changes in surface expression (localization and the level of expression) may be acting to control surface function post-testicularly.

Binding of both acrosome-intact and acrosome-reacted guinea pig sperm to the zona pellucida during in vitro fertilization

Mammalian sperm-egg adhesion occurs when sperm bind to the zona pellucida of the egg. In this study with guinea pig gametes, we have asked if sperm can initiate binding to the zona before and after the acrosome reaction and if the sperm surface protein PH-20 is involved in the binding at these two stages. Sperm binding to the zona was examined under a variety of conditions. Sperm were suspended in 0.9% NaCl or capacitated by two different methods. Eggs were immobilized on lectin-coated coverslips, compressed between a coverslip and a glass slide, or free in tissue culture dishes. The sperm-egg interaction was recorded on videotape or assessed after fixation of the eggs with bound sperm. Under all conditions studied, both acrosome-intact and acrosome-reacted sperm initiated binding to the zona. The binding was persistent and not transitory. In particular, acrosome-intact sperm that bound the zona were observed to remain bound for up to 80 min. One acrosome-intact sperm, bound to the zona, was videotaped while it acrosome-reacted. When mixed sperm populations (on the average 24% acrosome-intact and 76% acrosome-reacted) were incubated with eggs for 30 min, an average of 10% of the bound sperm were acrosome-intact. The PH-20 monoclonal antibody has previously been shown to inhibit zona binding by guinea pig sperm of undetermined acrosomal status (P. Primakoff, H. Hyatt, and D. G. Myles (1985), J. Cell Biol. 101, 2239-2244). In this study, when the two populations of sperm were counted separately, PH-20 inhibited the binding of acrosome-reacted but not acrosome-intact sperm. Our results show that both acrosome-intact and acrosome-reacted guinea pig sperm can initiate binding to the zona; however, the binding in the two cases may not occur by the same mechanism.

A cholera toxin-sensitive G-protein stimulates exocytosis in sea urchin eggs

To identify guanine nucleotide binding proteins (G-proteins) in sea urchin eggs and to investigate their role in signal transduction at fertilization, we used cholera toxin (CTX) and pertussis toxin (PTX), which catalyze the specific ADP-ribosylation of G-proteins. Cell surface complex, consisting of plasma membranes and adhering cortical vesicles, was prepared from eggs of Lytechinus variegatus and incubated with 32P-labeled NAD in the presence of CTX or PTX. CTX catalyzed the ADP-ribosylation of a 47-kDa polypeptide, whereas PTX catalyzed the ADP-ribosylation of a 40-kDa polypeptide. Microinjection of approximately 30 micrograms/ml whole CTX or approximately 20 micrograms/ml CTX subunit A into intact eggs caused exocytosis of cortical vesicles. However, if the eggs were first injected with EGTA (0.6-1.4 mM), injection of CTX did not cause exocytosis. Eggs injected with 0.8-2.8 mM cAMP or 1.0-4.0 mM adenosine 3':5'-monophosphotioate cyclic Sp-isomer (cAMP-S), a hydrolysis-resistant analog of cAMP, did not undergo exocytosis. These results suggest that a CTX-sensitive G-protein is involved in regulating Ca2+ release and exocytosis of cortical vesicles in sea urchin eggs.

Actin filaments, localized to the region of the developing acrosome during early stages, are lost during later stages of guinea pig spermiogenesis

The presence and localization of actin was investigated in guinea pig spermatogenic cells and cauda epididymal sperm (CauE). Staining with rhodamine-phalloidin demonstrated the presence of actin filaments in the region of the developing acrosome in guinea pig spermatids. The actin filaments were visualized predominantly in the region of the inner acrosomal membrane in both round and elongating spermatids. As development progressed, the intensity of the staining diminished. No rhodamine-phalloidin staining was found in testicular sperm lacking a residual body or in CauE sperm. Analysis of actin levels by immunoblotting with an anti-actin monoclonal antibody showed that the disappearance of actin filaments is accompanied by a decrease in the level of actin per cell. By using immunoblotting techniques, actin was readily detected in preparations of purified spermatogenic cells, but not in preparations of purified CauE sperm. Actin was also not detected in cauda sperm by indirect immunofluorescence (IIF) with anti-actin antibodies or examination of whole cell extracts by two-dimensional gel electrophoresis.

Identification and purification of a sperm surface protein with a potential role in sperm-egg membrane fusion

Sperm-egg plasma membrane fusion during fertilization was studied using guinea pig gametes and mAbs to sperm surface antigens. The mAb, PH-30, strongly inhibited sperm-egg fusion in a concentration-dependent fashion. When zona-free eggs were inseminated with acrosome-reacted sperm preincubated in saturating (140 micrograms/ml) PH-30 mAb, the percent of eggs showing fusion was reduced 75%. The average number of sperm fused per egg was also reduced by 75%. In contrast a control mAb, PH-1, preincubated with sperm at 400 micrograms/ml, caused no inhibition. The PH-30 and PH-1 mAbs apparently recognize the same antigen but bind to two different determinants. Both mAbs immunoprecipitated the same two 125I-labeled polypeptides with Mr 60,000 (60 kD) and Mr 44,000 (44 kD). Boiling a detergent extract of sperm severely reduced the binding of PH-30 but had essentially no effect on the binding of PH-1, indicating that the two mAbs recognize different epitopes. Immunoelectron microscopy revealed that PH-30 mAb binding was restricted to the sperm posterior head surface and was absent from the equatorial region. The PH-30 and PH-1 mAbs did not bind to sperm from the testis, the caput, or the corpus epididymis. PH-30 mAb binding was first detectable on sperm from the proximal cauda epididymis, i.e., sperm at the developmental stage where fertilization competence appears. After purification by mAb affinity chromatography, the PH-30 protein retained antigenic activity, binding both the PH-30 and PH-1 mAbs. The purified protein showed two polypeptide bands of 60 and 44 kD on reducing SDS PAGE. The two polypeptides migrated further (to approximately 49 kD and approximately 33 kD) on nonreducing SDS PAGE, showing that they do not contain interchain disulfide bonds, but probably have intrachain disulfides. 44 kD appears not to be a proteolytic fragment of 60 kD because V8 protease digestion patterns did not reveal related peptide patterns from the 44- and 60-kD bands. In the absence of detergent, the purified protein precipitates, suggesting that either 60 or 44 kD could be an integral membrane polypeptide.

Sperm exocytosis increases the amount of PH-20 antigen on the surface of guinea pig sperm

Evidence has been presented that the PH-20 protein functions in sperm adhesion to the egg zona pellucida (Primakoff, P., H. Hyatt, and D. G. Myles, 1985, J. Cell Biol., 101:2239-2244). The PH-20 protein migrates from its original surface domain to a new surface domain after the acrosome reaction (Myles, D. G., and P. Primakoff, 1984, J. Cell Biol., 99:1634-1641). The acrosome reaction is an exocytotic event that results in insertion of a region of the secretory granule membrane, the inner acrosomal membrane (IAM), into the plasma membrane. After the acrosome reaction, PH-20 protein migrates to the IAM from its initial domain on the posterior head surface. We have now found a new dynamic feature of the regulation of PH-20 protein on the sperm surface; exocytosis increases the surface expression of PH-20 protein. After the acrosome reaction there is an approximately threefold increase in the number of PH-20 antigenic sites on the sperm surface. These new antigenic sites are revealed on the surface by insertion of the IAM into the plasma membrane. Our evidence indicates that before the acrosome reaction an intracellular population of PH-20 antigen is localized to the IAM. When migration of the surface population of the PH-20 protein is prevented, PH-20 protein can still be detected on the IAM of acrosome-reacted sperm. Also, PH-20 protein can be detected on the IAM of permeabilized acrosome-intact sperm by indirect immunofluorescence. Thus, the sperm cell regulates the amount of PH-20 protein on its surface by sequestering about two-thirds of the protein on an intracellular membrane and subsequently exposing this population on the cell surface by an exocytotic event. This may be a general mechanism for regulating cell surface composition where a rapid increase in the amount of a cell surface protein is required.

An antisperm monoclonal antibody inhibits sperm fusion with zona-free hamster eggs but not homologous eggs

The zona-free hamster egg penetration assay (HEPA) was evaluated as a test for identifying fertilization-blocking antibodies. A monoclonal antibody, AH-20, that binds to the surface of guinea pig sperm was used to test antibody inhibition of sperm-egg fusion. AH-20 strongly inhibited guinea pig sperm fusion with zona-free hamster eggs but had no effect on guinea pig sperm fusion with zona-free guinea pig eggs. No inhibition by AH-20 was found in the homologous fusion assay over a wide range of sperm concentration, fertilization rate, and fertilization index. The results suggest that although guinea pig sperm can fuse with both hamster and guinea pig eggs, some aspect of the fusion mechanism is different in the two cases. The findings also indicate that HEPA, which is frequently used to assess the fertility potential of human sperm, can identify as blockers of sperm-egg fusion antibodies that have no effect on homologous sperm-egg fusion.

A role for the migrating sperm surface antigen PH-20 in guinea pig sperm binding to the egg zona pellucida

After the acrosome reaction, the PH-20 surface antigen of guinea pig sperm migrates from its original location on the posterior head surface to a new location on the inner acrosomal membrane (Myles, D.G., and P. Primakoff, 1984, J. Cell Biol., 99:1634-1641). We have isolated three monoclonal antibodies (MAbs) of the IgG1 subclass, PH-20, PH-21, and PH-22, that bind to the PH-20 antigen. The PH-20 MAb strongly inhibited (approximately 90%) sperm binding to the guinea pig egg zona pellucida at saturating antibody concentrations (greater than 20 micrograms/ml). Half-maximal inhibition of sperm binding to the zona was obtained with approximately 2 micrograms/ml PH-20 MAb. The PH-21 MAb at saturating concentration (50 micrograms/ml) partially inhibited (approximately 45%) sperm-zona binding, and the PH-22 MAb (50 micrograms/ml) did not inhibit (0%) sperm-zona binding. Essentially the same amounts of the three MAbs were bound to sperm under the conditions where inhibition (PH-20, PH-21) or no inhibition (PH-22) of sperm-zona binding was observed, which indicates that the different levels of inhibition did not arise from different levels of MAb binding. Competition binding assays with 125I-labeled MAbs showed that PH-21 binding to sperm was not affected by the binding of PH-20 or PH-22. However, that PH-20 and PH-22 blocked each other's binding to sperm suggests that their recognized determinants may be relatively close to one another. The results indicate that the migrating PH-20 antigen has a required function in sperm binding to the zona pellucida and that the PH-20 MAb affects is active site.

Localized surface antigens of guinea pig sperm migrate to new regions prior to fertilization

We have previously defined distinct localizations of antigens on the surface of the guinea pig sperm using monoclonal antibodies. In the present study we have demonstrated that these antigen localizations are dynamic and can be altered during changes in the functional state of the sperm. Before the sperm is capable of fertilizing the egg, it must undergo capacitation and an exocytic event, the acrosome reaction. Prior to capacitation, the antigen recognized by the monoclonal antibody, PT-1, was restricted to the posterior tail region (principle piece and end piece). After incubation in capacitating media at 37 degrees C for 1 h, 100% of the sperm population showed migration of the PT-1 antigen onto the anterior tail. This redistribution of surface antigen resulted from a migration of the surface molecules originally present on the posterior tail. It did not occur in the presence of metabolic poisons or when tail-beating was prevented. It was temperature-dependent, and did not require exogenous Ca2+. Since the PT-1 antigen is freely diffusing on the posterior tail before migration, the mechanism of redistribution could involve the alteration of a presumptive membrane barrier. In addition, we observed the redistribution of a second surface antigen after the acrosome reaction. The antigen recognized by the monoclonal antibody, PH-20, was localized exclusively in the posterior head region of acrosome-intact sperm. Within 7-10 min of induction of the acrosome reaction with Ca2+ and A23187, 90-100% of the acrosome-reacted sperm population no longer demonstrated binding of the PH-20 antibody on the posterior head, but showed binding instead on the inner acrosomal membrane. This redistribution of the PH-20 antigen also resulted from the migration of pre-existing surface molecules, but did not appear to require energy. The migration of PH-20 antigen was a selective process; other antigens localized to the posterior head region did not leave the posterior head after the acrosome reaction. These rearrangements of cell surface molecules may act to regulate cell surface function during fertilization.