Characterization of mouse sperm TMEM190, a small transmembrane protein with the trefoil domain: evidence for co-localization with IZUMO1 and complex formation with other sperm proteins

TMEM190, a small transmembrane protein containing the trefoil domain, was previously identified by our proteomic analysis of mouse sperm. Two structural features of TMEM190, ‘trefoil domain’ and ‘small transmembrane protein’, led us to hypothesize that this protein forms a protein–protein complex required during fertilization, and we characterized TMEM190 by biochemical, cytological, and genetic approaches. We showed in this study that the mouse Tmem190 gene exhibits testis-specific mRNA expression and that the encoded RNA is translated into a 19-kDa protein found in both testicular germ cells and cauda epididymal sperm. Treatment of the cell surface with proteinase K, subcellular fractionation, and immunofluorescence assay all revealed that mouse TMEM190 is an inner-acrosomal membrane protein of cauda epididymal sperm. During the acrosome reaction, TMEM190 partly relocated onto the surface of the equatorial segment, on which sperm–oocyte fusion occurs. Moreover, TMEM190 and IZUMO1, which is an immunoglobulin-like protein required for gamete fusion, co-localized in mouse sperm both before and after the acrosome reaction. However, immunoprecipitates of TMEM190 contained several sperm proteins, but did not include IZUMO1. These findings suggest that a mouse sperm protein complex(es) including TMEM190 plays an indirect role(s) in sperm–oocyte fusion. The role(s), if any, is probably dispensable since Tmem190-null male mice were normally fertile.

Izumo is part of a multiprotein family whose members form large complexes on mammalian sperm

Izumo, a sperm membrane protein, is essential for gamete fusion in the mouse. It has an Immunoglobulin (Ig) domain and an N-terminal domain for which neither the functions nor homologous sequences are known. In the present work we identified three novel proteins showing an N-terminal domain with significant homology to the N-terminal domain of Izumo. We named this region "Izumo domain," and the novel proteins "Izumo 2," "Izumo 3," and "Izumo 4," retaining "Izumo 1" for the first described member of the family. Izumo 1-3 are transmembrane proteins expressed specifically in the testis, and Izumo 4 is a soluble protein expressed in the testis and in other tissues. Electrophoresis under mildly denaturing conditions, followed by Western blot analysis, showed that Izumo 1, 3, and 4 formed protein complexes on sperm, Izumo 1 forming several larger complexes and Izumo 3 and 4 forming a single larger complex. Studies using different recombinant Izumo constructs suggested the Izumo domain possesses the ability to form dimers, whereas the transmembrane domain or the cytoplasmic domain or both of Izumo 1 are required for the formation of multimers of higher order. Co-immunoprecipitation studies showed the presence of other sperm proteins associated with Izumo 1, suggesting Izumo 1 forms a multiprotein membrane complex. Our results raise the possibility that Izumo 1 might be involved in organizing or stabilizing a multiprotein complex essential for the function of the membrane fusion machinery.

Izumo is part of a multiprotein family whose members form large complexes on mammalian sperm

Izumo, a sperm membrane protein, is essential for gamete fusion in the mouse. It has an Immunoglobulin (Ig) domain and an N-terminal domain for which neither the functions nor homologous sequences are known. In the present work we identified three novel proteins showing an N-terminal domain with significant homology to the N-terminal domain of Izumo. We named this region "Izumo domain," and the novel proteins "Izumo 2," "Izumo 3," and "Izumo 4," retaining "Izumo 1" for the first described member of the family. Izumo 1-3 are transmembrane proteins expressed specifically in the testis, and Izumo 4 is a soluble protein expressed in the testis and in other tissues. Electrophoresis under mildly denaturing conditions, followed by Western blot analysis, showed that Izumo 1, 3, and 4 formed protein complexes on sperm, Izumo 1 forming several larger complexes and Izumo 3 and 4 forming a single larger complex. Studies using different recombinant Izumo constructs suggested the Izumo domain possesses the ability to form dimers, whereas the transmembrane domain or the cytoplasmic domain or both of Izumo 1 are required for the formation of multimers of higher order. Co-immunoprecipitation studies showed the presence of other sperm proteins associated with Izumo 1, suggesting Izumo 1 forms a multiprotein membrane complex. Our results raise the possibility that Izumo 1 might be involved in organizing or stabilizing a multiprotein complex essential for the function of the membrane fusion machinery.

Testase 1 (ADAM 24) a sperm surface metalloprotease is required for normal fertility in mice

ADAM family members play important roles in various physiological and pathological processes, for example, fertilization, embryogenesis, neurogenesis, and development of asthma and arthritis (Primakoff and Myles, 2000. Trends Genet 16: 83-87; Edwards et al., 2008. Mol Aspects Med 29: 258-289). We previously reported that testase 1 (ADAM 24) is the first identified metalloprotease present on the surface of mature sperm. To investigate a potential role of testase 1 in fertilization, we generated testase 1 deficient mice. Testase 1 null male mice showed reduced fertility, producing only half the number of offspring when compared to wild-type littermates. In a standard in vitro fertilization assay, we found that sperm lacking testase 1 gave rise to polyspermic fertilization, a phenotypic feature that might contribute to failure of normal embryo development due to polyaneuploidy. Furthermore, in vivo, we found that testase 1 null males produced a higher number of polyspermic embryos at the pronuclear stage. These findings suggest that testase 1 is a sperm plasma membrane component which contributes to the prevention of polyspermy at the level of the oocyte plasma membrane.

Identification of an ADAM2-ADAM3 complex on the surface of mouse testicular germ cells and cauda epididymal sperm

Male mice lacking ADAM2 (fertilin beta) or ADAM3 (cyritestin) are infertile; cauda epididymal sperm (mature sperm) from these mutant mice cannot bind to the egg zona pellucida. ADAM3 is barely present in Adam2-null sperm, despite normal levels of this protein in Adam2-null testicular germ cells (TGCs; sperm precursor cells). Here, we have explored the molecular basis for the loss of ADAM3 in Adam2-null TGCs to clarify the biosynthetic and functional linkage of ADAM2 and ADAM3. A small portion of total ADAM3 was found present on the surface of wild-type and Adam2(-/-) TGCs at similar levels. In the Adam2-null TGCs, however, surface-localized ADAM3 exhibited an increased amount of an endoglycosidase H-resistant form that may be related to instability of ADAM3. Moreover, we found a complex between ADAM2 and ADAM3 on the surface of TGCs and sperm. The intracellular chaperone calnexin was a component of the testicular ADAM2-ADAM3 complex. Our findings suggest that the association with ADAM2 is a key element for stability of ADAM3 in epididymal sperm. The presence of the ADAM2-ADAM3 complex in sperm also suggests a potential role of ADAM2 with ADAM3 in sperm binding to the egg zona pellucida.

Cell-cell membrane fusion during mammalian fertilization

The mechanism of sperm-egg fusion in mammals is a research area that has greatly benefited from the use of gene deletion technology. Because fertilization is internal in mammals and the gametes (particularly the eggs) are sparse in number, in vitro studies have considerable limitations. Using gene deletions, a few cell surface proteins in both gametes have been identified as essential for gamete fusion. Ongoing studies are directed at analysis of the function of these proteins and the search for additional proteins that may be involved in this process. So far, no mammalian proteins have been found that also function in sperm-egg fusion of non-mammalian species or in other types of cell-cell fusion.

Oocyte CD9 is enriched on the microvillar membrane and required for normal microvillar shape and distribution

Microvilli are found on the surface of many cell types, including the mammalian oocyte, where they are thought to act in initial contact of sperm and oocyte plasma membranes. CD9 is currently the only oocyte protein known to be required for sperm-oocyte fusion. We found CD9 is localized to the oocyte microvillar membrane using transmission electron microscopy (TEM). Scanning electron microscopy (SEM) showed that CD9 null oocytes, which are unable to fuse with sperm, have an altered length, thickness and density of their microvilli. One aspect of this change in morphology was quantified using TEM by measuring the radius of curvature at the microvillar tips. A small radius of curvature is thought to promote fusibility and the radius of curvature of microvillar tips on CD9 wild-type oocytes was found to be half that of the CD9 null oocytes. We found that oocyte CD9 co-immunoprecipitates with two Ig superfamily cis partners, EWI-2 and EWI-F, which could have a role in linking CD9 to the oocyte microvillar actin core. We also examined latrunculin B-treated oocytes, which are known to have reduced fusion ability, and found altered microvillar morphology by SEM and TEM. Our data suggest that microvilli may participate in sperm-oocyte fusion. Microvilli could act as a platform to concentrate adhesion/fusion proteins and/or provide a membrane protrusion with a low radius of curvature. They may also have a dynamic interaction with the sperm that serves to capture the sperm cell and bring it into close contact with the oocyte plasma membrane.

A Role for Sperm Surface Protein Disulfide Isomerase Activity in Gamete Fusion: Evidence for the Participation of ERp57

In mammals, sperm-egg interaction is based on molecular events either unique to gametes or also present in somatic cells. In gamete fusion, it is unknown which features are gamete specific and which are shared with other systems. Conformational changes mediated by thiol-disulfide exchange are involved in the activation of some virus membrane fusion proteins. Here we asked whether that mechanism is also operative in sperm-egg fusion. Different inhibitors of protein disulfide isomerase (PDI) activity were able to inhibit sperm-egg fusion in vitro. While pretreatment of oocytes had no effect, pretreatment of sperm reduced their fusion ability. Some members of the PDI family were detected on the sperm head, and use of specific antibodies and substrates suggested that the oxidoreductase ERp57 has a role in gamete fusion. The results support the idea that thiol-disulfide exchange is a mechanism that may act in gamete fusion to produce conformational changes in fusion-active proteins.

Proteomic analysis of sperm regions that mediate sperm-egg interactions

The sperm interacts with three oocyte-associated structures during fertilization: the cumulus cell layer surrounding the oocyte, the egg extracellular matrix (the zona pellucida), and the oocyte plasma membrane. Each of these interactions is mediated by the sperm head, probably through proteins both on the sperm surface and within the acrosome, a specialized secretory granule. In this study, we have used subcellular fractionation in order to generate a proteome of the sperm head subcellular compartments that interact with oocytes. Of the proteins we identified for which a gene knockout has been tested, a third have been shown to be essential for efficient reproduction in vivo. Many of the other presently untested proteins are likely to have a similarly important role. Twenty-five percent of the cell surface fraction proteins are previously uncharacterized. We have shown that at least two of these novel proteins are localized to the sperm head. In summary, we have identified over 100 proteins that are expressed on mature sperm at the site of sperm-oocyte interactions.

Defects in secretory pathway trafficking during sperm development in Adam2 knockout mice

Adam2-null and Adam3-null male mice exhibit reduced levels of one or more ADAM proteins on mature sperm, in addition to the loss of the genetically targeted protein. ADAM protein loss was believed to occur posttranslationally, although the timing of loss and the mechanism by which the loss occurred were not explored. In this study we have found that in Adam3-null mice, fertilin beta (also known as ADAM2) is lost during the formation of testicular sperm. In Adam2-null males, most cyritestin (ADAM3) protein is also lost at this stage, but 25% of cyritestin is lost later, during sperm passage through the epididymis. Although normal levels of cyritestin are synthesized and acquire Endoglycosidase H resistance, indicating transit through the Golgi, the protein does not reach the cell surface. We also discovered that the majority of both fertilin beta and cyritestin are found in a Triton X-100 insoluble compartment on testicular sperm, when most of the cyritestin was observed on the cell surface. This insoluble compartment may represent a sorting platform, because in Adam2-knockout cells, only a small fraction of the cyritestin becomes Triton X-100 insoluble. Thus, it appears that cyritestin loss in Adam2-knockout mice may result, at least in part, from a disruption in protein trafficking.

Sperm-egg fusion: events at the plasma membrane

Sperm-egg fusion is a cell-cell membrane fusion event essential for the propagation of sexually reproducing organisms. In gamete fusion, as in other fusion events, such as virus-cell and intracellular vesicle fusion, membrane fusion is a two-step process. Attachment of two membranes through cell-surface molecules is followed by the physical merger of the plasma membrane lipids. Recent progress has demonstrated an essential role for an oocyte tetraspanin, CD9, in mouse sperm-egg fusion, and a specific molecular site crucial for CD9 function has been identified. Absence of glycosylphosphatidylinositol-anchored proteins on the oocyte surface also results in loss of oocyte fusion competence in this gamete. These discoveries provide a strong starting point for the identification of additional proteins that have roles in sperm-egg fusion.

Direct binding of the ligand PSG17 to CD9 requires a CD9 site essential for sperm-egg fusion

The function currently attributed to tetraspanins is to organize molecular complexes in the plasma membrane by using multiple cis-interactions. Additionally, the tetraspanin CD9 may be a receptor that binds the soluble ligand PSG17, a member of the immunoglobulin superfamily (IgSF)/CEA subfamily. However, previous data are also consistent with the PSG17 receptor being a CD9 cis-associated protein. In the current study, CD9 extracellular loop (EC2) specifically bound to PSG17-coated beads, indicating a direct interaction between the two proteins. However, CD9-EC2 did not bind to PSG17-coated beads if the CD9-EC2 had the mutation SFQ (173-175) to AAA, a previously studied mutation in egg CD9 that abolishes sperm-egg fusion. Also, PSG17 bound to 293 T cells transfected with wild-type CD9 but not the mutant CD9. By immunofluorescence, PSG17 bound to wild-type eggs but not to CD9 null eggs. The presence of approximately 2 microM recombinant PSG17 produced a significant and reversible inhibition (60-80%) of sperm-egg fusion. Thus, we conclude that CD9 is a receptor for PSG17 and when the PSG17 binding site is mutated or occupied, sperm-egg fusion is impaired. These findings suggest that egg CD9 may function in gamete fusion by binding to a sperm IgSF/CEA subfamily member and such proteins have previously been identified on sperm.

Infertility in female mice with an oocyte-specific knockout of GPI-anchored proteins

Glycosylphosphatidylinositol-anchored proteins on the egg surface have been proposed to play a role in gamete fusion on the basis of in vitro experiments. We tested this hypothesis by asking if oocyte GPI-anchored proteins are required for fertilization in vivo. Oocyte-specific knockout mice were created using the Cre/loxP system to delete a portion of the Pig-a gene, which encodes an enzyme involved in GPI anchor biosynthesis. Conditional Pig-a-knockout females are infertile, and eggs recovered from the females after mating are unfertilized. In in vitro assays, the knockout eggs are severely deficient in their ability to fuse with sperm. These results demonstrate that GPI-anchored proteins are required for gamete fusion. Loss of the GPI-anchored complement of plasma membrane proteins could prevent fusion by altering the organization and function of GPI-anchored protein-containing lipid domains. Alternatively, a single GPI-anchored protein may be required in the fusion process. To distinguish between these possibilities, we have begun to identify the GPI-anchored proteins on the egg surface. We have identified one egg GPI-anchored protein as CD55, an approximately 70 kDa complement regulatory protein. It has previously been found that CD55-knockout mice are fertile, demonstrating that CD55 is not essential for fertilization. This finding also means that the presence of the full complement of egg GPI-anchored proteins is not necessary for gamete fusion. Other egg GPI-anchored proteins acting in the fusion process can now be investigated, with the goal of understanding the mechanism of their function in sperm-egg fusion.

None of the integrins known to be present on the mouse egg or to be ADAM receptors are essential for sperm-egg binding and fusion

Antibody inhibition and alpha6beta1 ligand binding experiments indicate that the egg integrin alpha6beta1 functions as a receptor for sperm during gamete fusion; yet, eggs null for the alpha6 integrin exhibit normal fertilization. Alternative integrins may be involved in sperm-egg binding and fusion and could compensate for the absence of alpha6beta1. Various beta1 integrins and alphav integrins are present on mouse eggs. Some of these integrins are also reported to be receptors for ADAMs, which are expressed on sperm. Using alpha3 integrin null eggs, we found that the alpha3beta1 integrin was not essential for sperm-egg binding and fusion. Oocyte-specific, beta1 integrin conditional knockout mice allowed us to obtain mature eggs lacking all beta1 integrins. We found that the beta1 integrin null eggs were fully functional in fertilization both in vivo and in vitro. Furthermore, neither anti-mouse beta3 integrin function-blocking monoclonal antibody (mAb) nor alphav integrin function-blocking mAb inhibited sperm binding to or fusion with beta1 integrin null eggs. Thus, function of beta3 or alphav integrins does not seem to be involved in compensating for the absence of beta1 integrins. These results indicate that none of the integrins known to be present on mouse eggs or to be ADAM receptors are essential for sperm-egg binding/fusion, and thus, egg integrins may not play the role in gamete fusion previously attributed to them.

Penetration, adhesion, and fusion in mammalian sperm-egg interaction

Fertilization is the sum of the cellular mechanisms that pass the genome from one generation to the next and initiate development of a new organism. A typical, ovulated mammalian egg is enclosed by two layers: an outer layer of approximately 5000 cumulus cells and an inner, thick extracellular matrix, the zona pellucida. To reach the egg plasma membrane, sperm must penetrate both layers in steps requiring sperm motility, sperm surface enzymes, and probably sperm-secreted enzymes. Sperm also bind transiently to the egg zona pellucida and the egg plasma membrane and then fuse. Signaling in the sperm is induced by sperm adhesion to the zona pellucida, and signaling in the egg by gamete fusion. The gamete molecules and molecular interactions with essential roles in these events are gradually being discovered.

Residues SFQ (173-175) in the large extracellular loop of CD9 are required for gamete fusion

Gamete fusion is the fundamental first step initiating development of a new organism. Female mice with a gene knockout for the tetraspanin CD9 (CD9 KO mice) produce mature eggs that cannot fuse with sperm. However, nothing is known about how egg surface CD9 functions in the membrane fusion process. We found that constructs including CD9’s large extracellular loop significantly inhibited gamete fusion when incubated with eggs but not when incubated with sperm, suggesting that CD9 acts by interaction with other proteins in the egg membrane. We also found that injecting developing CD9 KO oocytes with CD9 mRNA restored fusion competence to the resulting CD9 KO eggs. Injecting mRNA for either mouse CD9 or human CD9, whose large extracellular loops differ in 18 residues, rescued fusion ability of the injected CD9 KO eggs. However, when the injected mouse CD9 mRNA contained a point mutation (F174 to A) the gamete fusion level was reduced fourfold, and a change of three residues (173-175, SFQ to AAA) abolished CD9’s activity in gamete fusion. These results suggest that SFQ in the CD9 large extracellular loop may be an active site which associates with and regulates the egg fusion machinery.

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.

Normal fertilization occurs with eggs lacking the integrin alpha6beta1 and is CD9-dependent

Previous results, based on inhibition of fertilization by an anti-alpha6 integrin mAb (GoH3), suggest that the alpha6beta1 integrin on mouse eggs functions as the receptor for sperm (Almeida, E.A., A.P. Huovila, A.E. Sutherland, L.E. Stephens, P.G. Calarco, L. M. Shaw, A.M. Mercurio, A. Sonnenberg, P. Primakoff, D.G. Myles, and J.M. White. 1995. Cell. 81:1095-1104). Because the egg surface tetraspanin CD9 is essential for gamete fusion (Kaji, K., S. Oda, T. Shikano, T. Ohnuki, Y. Uematsu, J. Sakagami, N. Tada, S. Miyazaki, and A. Kudo. 2000. Nat. Genet. 24:279-282; Le Naour, F., E. Rubinstein, C. Jasmin, M. Prenant, and C. Boucheix. 2000. Science. 287:319-321; Miyado, K., G. Yamada, S. Yamada, H. Hasuwa, Y. Nakamura, F. Ryu, K. Suzuki, K. Kosai, K. Inoue, A. Ogura, M. Okabe, and E. Mekada. 2000. Science. 287:321-324) and CD9 is known to associate with integrins, recent models of gamete fusion have posited that egg CD9 acts in association with alpha6beta1 in fusion (Chen, M.S., K.S. Tung, S.A. Coonrod, Y. Takahashi, D. Bigler, A. Chang, Y. Yamashita, P.W. Kincade, J.C. Herr, and J.M. White. 1999. Proc. Natl. Acad. Sci. USA. 96:11830-11835; Kaji, K., S. Oda, T. Shikano, T. Ohnuki, Y. Uematsu, J. Sakagami, N. Tada, S. Miyazaki, and A. Kudo. 2000. Nat. Genet. 24:279-282; Le Naour, F., E. Rubinstein, C. Jasmin, M. Prenant, and C. Boucheix. 2000. Science. 287:319-321; Miyado, K., G. Yamada, S. Yamada, H. Hasuwa, Y. Nakamura, F. Ryu, K. Su- zuki, K. Kosai, K. Inoue, A. Ogura, M. Okabe, and E. Mekada. 2000. Science. 287:321-324). Using eggs from cultured ovaries of mice lacking the alpha6 integrin subunit, we found that the fertilization rate, fertilization index, and sperm binding were not impaired compared with wild-type or heterozygous controls. Furthermore, a reexamination of antibody inhibition, using an assay that better simulates in vivo fertilization conditions, revealed no inhibition of fusion by the GoH3 mAb. We also found that an anti-CD9 mAb completely blocks sperm fusion with either wild-type eggs or eggs lacking alpha6beta1. Based on these results, we conclude that the alpha6beta1 integrin is not essential for sperm-egg fusion, and we suggest a new model in which CD9 acts by itself, or interacts with egg protein(s) other than alpha6beta1, to function in sperm-egg fusion.

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.