Isolation and biochemical characterization of two isoforms of a boar sperm zona pellucida-binding protein

Boar spermadhesin AWN: Novel insights in its binding behavior and localization on sperm

As a major spermadhesin first found in the seminal plasma of boars, AWN is described to fulfil a variety of reproduction related tasks. Although being the best investigated boar spermadhesin, information about its interaction with membranes is inconsistent. In this regard, previous reports locate AWN either inside or on the surface of sperm cells and at different regions, depending on the method and antibody used. Here, we localize native AWN in/on epididymal, ejaculated, capacitated and acrosome-reacted boar sperm using epifluorescence and electron microscopy, as well as an analysis of potential lipid binding partners of native and recombinant AWN. By applying a custom-made anti-AWN antibody, localization of AWN in the equatorial segment of ejaculated, capacitated and acrosome-reacted boar sperm was discovered. Electron microscopy showed that AWN is localized both on the sperm surface and on the cytoplasmic side of the plasma membrane, and in close vicinity to the nuclear and both acrosomal membranes of sperm. Analysis of epididymal sperm indicated migration of AWN from the retral postacrosomal part to the equatorial segment during the epididymal passage. In contrast to hypotheses claiming a specific association of AWN to phosphatidylethanolamine and in line with our previous study describing an interaction with phosphatidic acid, the current results show a rather electrostatically-driven binding mechanism of AWN to negative lipids. In conclusion, this work provides new insights into the arrangement of AWN in the equatorial segment that suggest a possible role in sperm-oocyte fusion.

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

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

Recombinant expression of porcine spermadhesin AWN and its phospholipid interaction: Indication for a novel lipid binding property

AWN is a porcine (Sus scrofa domestica) seminal plasma protein and has been linked to a variety of processes related to fertilization. To acquire the protein in sufficient amount and purity for functional studies, we established its recombinant expression in E. coli and a three-step purification protocol based on different chromatographies. The test for AWN-phospholipid interaction revealed phosphatidic acid and cardiolipin as potential binding partners. As phosphatidic acid is surmised to play a role in cation-induced membrane destabilization and fusion events, we propose a membrane protective function of the presented binding affinity. Further studies with recombinant AWN will allow new insights into the mechanism of sperm-spermadhesin interaction and might provide new approaches for artificial reproduction techniques.

Sperm surface antigens and the prospects for contraceptive vaccine development

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

Spermadhesin AQN1 is a candidate receptor molecule involved in the formation of the oviductal sperm reservoir in the pig

Sperm are stored in the isthmic region of the oviduct under conditions that maintain viability and suppress early capacitation steps until ovulation occurs. The initial contact between sperm and oviductal epithelium is mediated by carbohydrate-protein interactions. In the pig, the carbohydrate recognition system has been shown to involve oligomannosyl structures. The spermadhesins AWN and AQN1 are the dominant porcine carbohydrate-binding sperm proteins. The objective of this study was to demonstrate that AQN1 contributes to sperm binding to the oviductal epithelium. AQN1 showed a broad carbohydrate-binding pattern as it recognizes both alpha- and beta-linked galactose as well as Manalpha1-3(Manalpha1-6)Man structures, whereas AWN bound only the galactose species. Binding of ejaculated sperm to oviductal epithelium was inhibited by addition of AQN1 but not by AWN. Mannose-binding sites were localized over the rostral region of the sperm head. Flow cytometry showed that, under capacitating conditions, the population of live sperm was shifted within 30 min toward an increase in the proportion of cells with low mannose- and high galactose-binding. The loss of mannose-binding sites was accompanied by the loss of AQN1 in sperm extracts and the significant reduction in the sperm-oviduct binding. The oviductal epithelium was shown by GNA-lectin histochemistry and by SDS-PAGE and lectin blotting of the apical membrane fraction to express mannose components that could be recognized by AQN1. These results demonstrate that the sperm lectin AQN1 fulfils the criteria for an oviduct receptor in the pig and may play a role in the formation of the oviductal sperm reservoir.

Expression of spermadhesin genes in porcine male and female reproductive tracts

Spermadhesins, secretory proteins of the male genital tract, constitute the bulk of seminal plasma proteins in pig. The nucleotide sequences of spermadhesins AWN, AQN-1, and AQN-3 from the cDNAs, derived from seminal vesicles, were determined. The coding sequences of spermadhesins revealed an overall sequence similarity of 40% at nucleotide level. Expression of spermadhesins (AWN, AQN-1, AQN-3, PSP-I, and PSP-II) in porcine male and female reproductive tracts were studied by means of RT-PCR and immunological approaches. All spermadhesins are transcribed and translated in seminal vesicles and prostate. In caudal epididymis mRNA transcripts of all spermadhesins have been detected by RT-PCR. PSP-I showed additional signals in caput epididymis and rete testis. Translation can be detected only for AWN in tissue extracts by SDS-PAGE and Western blotting. Remarkably, AWN is the only spermadhesin that is also expressed in the uterus, the uterotubal junction, and the oviduct of the female genital tract as shown by RT-PCR, cDNA-sequencing, and immunological analysis. In sows at estrus and interestrus, in gilts and gilts 12 hr after insemination no obvious differences were noticed in the pattern of AWN-immunoreactivity in epithelial cells either of the uterotubal junction, isthmus, or ampulla. While strong staining was observed in the superficial uterine glands and in the glands of the uterotubal junction during estrus, in diestrus this declined distinctly. The role of spermadhesins in relation to their expression is discussed.

Effects of semen plasma from different fractions of individual ejaculates on IVF in pigs

We applied IVM/IVF techniques to investigate effects of preincubation of sperm with different fractions of semen plasma harvested from fresh ejaculates on in vitro penetration and fertilization of in vitro matured oocytes. Three fractions of semen plasma were separated from the complete ejaculate of three Landrace boars and used to coincubate sperm obtained from the first sperm-rich fraction of the same ejaculates. After 14 to 16 h coincubation at room temperature, sperm were preincubated in capacitation medium and then inseminated into fertilization medium containing porcine oocytes matured in vitro. The semen plasma used for coincubation affected penetration rate (P < 0.001); Sperm coincubated with Fraction 1 semen plasma had a higher penetration rate compared with sperm coincubated with Fraction 2 (P < 0.05), but not with Fraction 3. Boar affected male pronucleus formation rates after insemination (P < 0.05), but no difference among boars was found in monospermy rate, average number of sperm penetrating into each fertilized oocyte, or the average number of sperm attached. No boar by fraction interaction was found for any parameters studied.

Immunohistochemical localization of the spermadhesin AWN-1 in the equine male genital tract

Spermadhesins are proteins with various functions in sperm capacitation and zona pellucida binding. In this study the cellular localization of the spermadhesin AWN-1 has been examined in the equine male genital tract. Results obtained by immunohistochemical methods reveal that in the horse AWN-1 is synthesized in spermatogonia, in the rete testis, the ductus epididymidis and the seminal vesicles. These findings indicate that the cellular origin of spermadhesins is species-specific.

TSG-6: an IL-1/TNF-inducible protein with anti-inflammatory activity

The pro-inflammatory cytokines IL-1 and TNF-alpha are primary mediators of the acute phase response, the complex reaction of the mammalian organism to infection and injury. Among the genes activated by TNF-alpha and IL-1 in a variety of cells is TNF-stimulated gene 6 (TSG-6). The TSG-6 cDNA encodes a secreted 35 kDa glycoprotein which is abundant in synovial fluids of patients with various forms of arthritis and detectable in serum of patients with different inflammatory or autoimmune disorders. TSG-6 protein consists of two structural domains: a hyaluronan-binding link module, the characteristic domain of the hyaladherin family of proteins, and a C-terminal CUB domain, present in a variety of diverse proteins. TSG-6 forms a stable complex with components of the plasma protein inter-alpha-inhibitor (I[alpha]I), a Kunitz-type serine protease inhibitor. TSG-6 and I(alpha)I synergize to inhibit plasmin, a serine protease involved in the activation of matrix metalloproteinases which are part of the proteolytic cascade associated with inflammation. Recombinant human TSG-6 protein exerts a potent anti-inflammatory effect in a murine model of acute inflammation. Modulation of the proteolytic network associated with inflammatory processes may be a mechanism whereby TSG-6, in cooperation with I(alpha)I, inhibits inflammation. Activation of the TSG-6 gene by pro-inflammatory cytokines, presence of TSG-6 protein in inflammatory lesions and its anti-inflammatory effect suggest a role for TSG-6 in a negative feed-back control of the inflammatory response.

Immunohistochemical localization of spermadhesin AWN in the porcine male genital tract

Boar spermadhesin (AWN) is a 14-kDa multifunctional protein, attached to the surface of the spermatozoa and involved in sperm capacitation and zona pellucida binding. The cellular origin of AWN was previously unknown. Moreover, the region of the male genital tract in which AWN becomes attached to the surface of spermatozoa was also uncertain. By using monospecific polyclonal antibodies against AWN, the immunohistochemical distribution pattern of AWN epitopes has been investigated in tissue sections of the porcine male genital tract. Our study has revealed that AWN is synthesized in the rete testis and in the epithelium of the seminal vesicles. The latter are also the major contributors of seminal plasma AWN. In addition, immunoblotting analysis has shown that AWN is present on epididymal spermatozoa. Our results indicate that the cellular origin of spermadhesins is species-specific. The attachment of AWN to epididymal spermatozoa is probably important in developing the capacity for fertilization.

Cloning of boar SPMI gene which is expressed specifically in seminal vesicle and codes for a sperm motility inhibitor protein

Boar semen contains a seminal plasma motility inhibitor (SPMI) that blocks the motility of demembranated-reactivated spermatozoa as well as of intact spermatozoa. In this paper, we describe the primary structure of SPMI, the coding of boar SPMI cDNA gene and its expression in various porcine tissues. Nucleotide sequence analysis of the 645-bp SPMI cDNA predicts a coded polypeptide of 137 amino acid residues which includes a 21-residue signal peptide and a 116-residue secreted protein. The amino acid sequence of SPMI was found to be highly homologous to AQN-3, a member of spermadhesin family proteins of boar that bind to spermatozoa. Expression of the boar SPMI gene detected by Northern blot analysis revealed that its expression is very abundant in seminal vesicles and specific to this tissue.

Dynamics of the seminal vesicle epithelium

In newborn rodents, seminal vesicle epithelium (SVEP) cells display a poorly developed rough endoplasmic reticulum (RER) and Golgi complex, and they show no signs of secretion. From puberty onward, secretory material starts to appear, and the RER and Golgi complex progressively develop and reorganize until the adult ultrastructure is established around 40-60 days of age. Multivesicular bodies and lysosomes follow in this development but lysosomes evolve to lipofucsin granules with aging. The duration of the secretory cycle in SVEP cells is shorter than in other exocrine cells and the secretory protein pattern depends on the animal species, androgen status, and sexual activity. SVEP cells are also involved in endocytosis, which is coupled to exocytosis, and their endocytic pathway intersects the exocytic pathway in Golgi cisterns. The structure and function of SVEP cells depends mainly on testosterone, but other hormones and factors, such as the neuropeptide VIP, also influence their activity. Castration leads to programmed death and regression of SVEP cells to an extent that depends on the animal species. In addition, castration induces changes in the secretory protein pattern and delays its intracellular transport. Endocytic kinetics is also delayed following castration. Primary cultures of SVEP cells in a bicameral system are proposed as a model to investigate the activities of SVEP cells further.

Immunolocalization and quantitation of acidic seminal fluid protein (aSFP) in ejaculated, swim-up, and capacitated bull spermatozoa

Acidic seminal fluid protein (aSFP, 12.9 kDa), a major protein of bull seminal plasma, belongs to the spermadhesin protein family. Boar spermadhesins become bound to the sperm head's surface at ejaculation and are thought to play a role as capacitation factors and/or in gamete recognition and binding. Here, we have investigated the topographical distribution and fate of bovine spermadhesin aSFP during sperm capacitation in order to assess whether aSFP could be involved in similar aspects of the fertilization process as its boar homologous proteins. 5.7 +/- 2.1 x 10(6) molecules/spermatozoa were quantitated on the surface of fresh ejaculated and washed sperm. The binding site of aSFP was restricted to a thin coat at the apical part of the acrosomal cap. The amount of aSFP in swim-up sperm was 1.8 +/- 1.0 x 10(6) molecules/spermatozoa, but decreased dramatically to 22 +/- 10 x 10(3) and to undetectable levels after incubation of sperm for 1.5 h and 18 h, respectively, in capacitation medium. This indicates that the bull spermatozoa surface may be completely depleted of spermadhesin aSFP before spermatozoa reach the surroundings of the investing egg. Therefore, our results suggest that aSFP may act as a decapacitation factor on bull spermatozoa rather than as a zona pellucida binding molecule.

Quantitation of boar spermadhesins in accessory sex gland fluids and on the surface of epididymal, ejaculated and capacitated spermatozoa

Spermadhesins are multifunctional proteins involved in boar sperm capacitation and gamete recognition. Using anti-AWN antibodies, we have followed the fate of spermadhesin AWN along the maturation and capacitation stages of boar spermatozoa. In addition, the amount of spermadhesins AQN-1, AQN-2, and AQN-3 relative to that of AWN was determined by amino acid analysis after reverse-phase HPLC isolation. Our data show that AWN-1 is the only spermadhesin on the surface of epididymal sperm and that a large amount of AQN-1, AQN-2, AQN-3, AWN-1 and AWN-2 become coated on ejaculated spermatozoa. The number of spermadhesin molecules on ejaculated sperm (12-60 x 10(6)/spermatozoa) is sufficient to form a many-molecules-thick coat over the sperm head. However, 50-75% of the AQN-1, AQN-2, and AQN-3 population, and around 90% of coated AWN (1 + 2) are released from ejaculated sperm during capacitation. This indicates that a large subpopulation of each boar spermadhesin is loosely associated to the sperm surface and may function as decapacitation factors. The remaining spermadhesin molecules, which are tightly bound to the sperm head's surface may play a role as either positive capacitation factors and/or in gamete recognition and binding.

Characterization of AWN-1 glycosylated isoforms helps define the zona pellucida and serine proteinase inhibitor-binding region on boar spermadhesins

Spermadhesin AWN-1 (14 kDa) belongs to a recently described family of boar sperm surface-associated proteins. AWN-1 is a multifunctional protein which possesses heparin-, serine proteinase inhibitor-, and zona pellucida glycoprotein-binding capability. Therefore it has been implicated in sperm capacitation and sperm-oocyte attachment. Here, we report the characterization of 22-25 kDa isoforms of AWN-1 isolated by heparin-affinity chromatography, which fail to bind to zona pellucida glycoproteins or serine proteinase inhibitors. Our results show that the structure of the high and low molecular mass AWN-1 forms differ in that the former is N-glycosylated at Asp50 and truncated at the C-terminus. The inability of the glycosylated AWN-1 molecules to bind ligands is due solely to the presence of the oligosaccharide moieties, however. This indicates that glycosylation of AWN-1 may modulate its ligand-binding capabilities. On the other hand, the effect of glycosylation on ligand-binding suggests that both the zona pellucida- and the serine proteinase inhibitor binding domain(s) may be located around the glycosylation point.

Porcine zona pellucida ZP3 alpha glycoprotein mediates binding of the biotin-labeled M(r) 55,000 family (ZP3) to boar sperm membrane vesicles

The two M(r) 55,000 glycoproteins, ZP3 alpha and ZP3 beta, of porcine zona pellucida copurify as a preparation designated ZP3. Gamete binding assays have implicated ZP3 alpha, but not ZP3 beta, as participating in sperm-zona recognition events. We now report that boar sperm contain membrane-associated binding sites with specificity for ZP3 alpha. Biotin-labeled (b-) preparations of ZP3 bind to intact boar sperm in a saturable manner, with localization on the anterior head region. Membrane vesicles obtained from capacitated sperm by nitrogen cavitation retain b-ZP3 binding sites as determined by an enzyme-linked method employing alkaline phosphatase-conjugated strepavidin. In competitive binding assays using b-ZP3 (0.1 microgram/ml) as probe, heat-solubilized zonae and ZP3 were effective competitors, whereas the nonzona molecules fetuin and fucoidin were not. Digestion of ZP3 with endo-beta-galactosidase, an enzyme that trims polylactosamines, enhanced its affinity for membrane receptors. In contrast treatments such as chemical deglycosylation, pronase digestion, or disruption of disulfide bonds abolished the ligand activity of ZP3. Finally, purified ZP3 alpha was an at least 100-fold better antagonist than purified ZP3 beta. The results demonstrate that binding of b-ZP3 to isolated boar sperm membranes is mediated by sperm receptors with specificity for the ZP3 alpha macromolecular component and reveal a complex contribution of both carbohydrate and protein moieties toward the ligand activity of this sperm adhesive zona molecule.

Molecular cloning and sequence analysis of two porcine seminal proteins, PSP-I and PSP-II: new members of the spermadhesin family

Two full-length cDNAs encoding porcine seminal proteins, PSP-I and PSP-II, have been isolated from a porcine seminal vesicle cDNA library. Nucleotide sequence analysis of the 706-bp PSP-I cDNA predicts a precursor protein of 133 amino acid residues, which includes a 21-residue signal peptide and a 112-residue secreted protein. On the other hand, the complete sequence of the 686-bp PSP-II cDNA reveals a coding sequence for a 21-residue signal peptide and a 116-residue secreted protein. The predicted amino acid sequences agree very well with those determined by conventional amino acid sequence analysis. Alignment of the two cDNA sequences shows an overall 66% sequence homology throughout their entire length. However, the sequence homology is much higher in the 3' untranslated region (72%) than in the coding region (61%). This suggests that these two genes evolved by duplication and divergence from a common ancestral gene. They share about 50% amino acid sequence homology and a similar overall structure with three members of the spermadhesin family.

Binding characteristics and immunolocalization of porcine seminal protein, PSP-I

PSP-I, a 13 kDa protein purified from boar seminal plasma, was found to have about 50% amino acid sequence homology with a family of zona pellucida-binding proteins known as spermadhesins. These proteins are produced by the accessory gland(s) of the male reproductive tract and coat the spermatozoa during ejaculation. In this study, we have investigated the possible biological functions of PSP-I using a solid-phase protein binding assay and its site of synthesis using both Western blot and immunocytochemical analyses. PSP-I was found to bind a number of proteins including endo-beta-galactosidase digested ZP3, soybean trypsin inhibitor, IgA, IgG and alpha-casein, indicating that it may have multiple functions. The protein or carbohydrate structures were not critical in the binding, since polyvinyl sulfate could effectively inhibit the binding of PSP-I to these proteins. Western blot analysis using specific antiserum to PSP-I showed that the protein was present in the seminal vesicle but not in the testes, epididymis or prostate. The protein was revealed by immunocytochemical analysis in the epithelium of seminal vesicles but not in the testes or the epididymis. It is concluded that PSP-I is synthesized by the epithelium of the seminal vesicles, secreted into the semen during ejaculation, and may be involved in various reproductive functions, such as preventing premature acrosome reaction and immunosuppression.

Isolation and biochemical characterization of heparin-binding proteins from boar seminal plasma: a dual role for spermadhesins in fertilization

Sperm surface-coated heparin-binding proteins originating from secretions of the male sexual accessory glands, are known to play a pivotal role as extrinsic regulatory factors during sperm capacitation in many mammalian species. They interact with glycosaminoglycans present in the female genital tract and enhance the subsequent zona pellucida-induced acrosome reaction. We have isolated heparin-binding proteins from boar seminal plasma by affinity chromatography on heparin-Sepharose and reverse-phase HPLC. N-Terminal sequence analysis of these proteins identified a boar counterpart of the bovine capacitation factors BSP-A1/2 (also called PDC-109) and BSP-A3. Several carbohydrate- and zona pellucida-binding proteins, which belong to the newly described spermadhesin family, were also identified as heparin-binding proteins. Our results imply that, besides other capacitation factors, members of the spermadhesin family may play a dual role in sperm capacitation and fertilization in the pig.