Sequence and localization of the mouse sperm autoantigenic protein, Sp17

Spermadhesins: a new protein family. Facts, hypotheses and perspectives

Spermadhesins are a novel family of secretory proteins expressed in the male genital tract of pig, horse and bull. They are major products of the seminal plasma and have been found to be peripherally associated to the sperm surface. The structure and function of spermadhesins have been thoroughly investigated in the pig, which exhibits the greatest diversity of members: AWN, AQN-1, AQN-2, PSP-I and PSP-II and its glycosylated isoforms. They are multifunctional proteins showing a range of ligand-binding abilities, e.g. carbohydrates, sulfated glycosaminoglycans, phospholipids and protease inhibitors, suggesting that they may be involved in different steps of fertilization. Isolated porcine spermadhesins bind the zona pellucida glycoproteins in a cation-dependent manner with a Kd in a low micromolar range, and AWN, AQN-1 and AQN-3 display similar binding affinity for glycoproteins containing Gal beta(1-3)-GalNAc and Gal beta(1-4)-GlcNAc sequences in O-linked and N-linked oligosaccharides, respectively. During sperm passage through the epididymis AQN-3 and AWN have been shown to bind tightly to the sperm surface by interaction with the phospholipids of the membrane bilayer. At ejaculation the spermadhesins form a protective coat around the sensitive acrosomal region of the sperm head, thus possibly preventing premature acrosome reaction. During in vitro capacitation most of these aggregated sperm adhesins are lost, with the exception of phospholipid-bound spermadhesins. AWN and AQN-3 may now serve as a primary receptor for the oocyte zona pellucida, thus contributing to initial binding and recognition between sperm and egg. The amino acid sequence of spermadhesins does not show any discernible similarity with known carbohydrate recognition domains (CRD). However, they belong to the superfamily of proteins with a CUB domain with a predicted all-beta structure. The crystal structure of the heterodimeric complex of the spermadhesins PSP-I/PSP-II has been solved, showing that the overall structure of both spermadhesins consists of a beta-sandwich with five (parallel and antiparallel) beta-strands. It is the first three-dimensional structure of a zona pellucida-binding protein and reveals the architecture of the CUB domain. The spermadhesins represent a novel class of lectins that may be involved in sequential steps of fertilization, at least in the pig.

The 2.4 A resolution crystal structure of boar seminal plasma PSP-I/PSP-II: a zona pellucida-binding glycoprotein heterodimer of the spermadhesin family built by a CUB domain architecture

The crystal structure of porcine seminal plasma spermadhesin PSP-I/PSP-II heterodimer has been determined in two crystal forms by multiple isomorphous replacement in an hexagonal crystal (space group P6(1)22) and molecular replacement in a trigonal crystal of space group P3(2)21. The crystal structure has been refined at 2.4 A resolution to an R-factor of 20.0% (Rfree = 25.9%) for 14,809 independent reflections with intensities greater than 2 sigma (I), with root-mean-square deviations of 0.009 A and 1.657 degrees from ideal bond lengths and bond angles, respectively. The final model includes 1688 non-hydrogen protein atoms of 221 amino acids and 79 water molecules. PSP-I/PSP-II represents the first crystal structure of a mammalian zona pellucida-binding protein. PSP-II displays a putative carbohydrate-recognition site located around its Asn50. This region shares structural features with sugar binding sites of known lectin structures of the leguminous and galectin families. PSP-I and PSP-II are N-glycosylated at asparagine residues 50 and 98, respectively, and show site heterogeneity. Only the innermost N-acetylglucosamine of PSP-I is defined in the crystal structure. Both subunits of the PSP-I/PSP-II heterodimer are built by a single CUB domain architecture. The CUB domain displays a novel fold, which consists of a compact ellipsoidal beta-sandwich structure (42 A x 27 A x 23 A) organized into two 5-stranded beta-sheets. Each sheet contains parallel and antiparallel beta-strands. Two disulphide bridges, which are conserved in all spermadhesin molecules and many CUB domains, crosslink loop LA and strand beta 4 and loops LE and LG, respectively, at opposite edges of the same face of the domain. The four highly conserved aromatic residues and 15 out of 17 invariant hydrophobic residues, which define the CUB domain signature, display an interior location, suggesting that this hydrophobic core may be essential for maintaining the overall folding of the domain. Most of the hydrophobic core residue characteristics are conserved in the jellyroll topology of certain icosahedral virus capsid proteins, indicating that the CUB domain and the viral proteins share a minimal structural core.

Designing an effective immunocontraceptive

The immunological inhibition of fertilization is the goal of gamete immunocontraception. To achieve this goal a gamete-specific antigen target must be defined and the presentation of the immunogen to the immune system must be clearly understood in order to elicit a defined immune response which will target the native gamete molecule. Almost 20 years ago C.B. Metz suggested six studies which would answer the questions necessary for the development of a successful immunocontraceptive, and although much work has gone into answering each of these questions, none has been completed. Hyaluronidase is an example of a well studied sperm antigen whose native, membrane bound form (PH-20) is a successful immunocontraceptive in female guinea pigs. However, it remains to be demonstrated that a successful native antigen can be a successful synthetic or recombinant gamete immunocontraceptive (GAMICON). The problem of converting a successful native contraceptive antigen into an effective synthetic or recombinant GAMICON is at the heart of the problem of GAMICON design. If the epitopes of native and synthetic immunogens are not the same, then can a conversion ever be made? One approach to understanding how to make this conversion is to use defined, synthetic B- T-cell epitopes as specific B-cell epitopes on native antigens affect fertility.

Molecules involved in mammalian sperm-egg interaction

To achieve fertilization, sperm and egg are equipped with specific molecules which mediate the steps of gamete interaction. In mammals, the first interaction between sperm and egg occurs at an egg-specific extracellular matrix, the zona pellucida (zp). The three glycoproteins, ZP1, ZP2, and ZP3, that comprise the zp have been characterized from many species and assigned different roles in gamete interaction. A large number of candidate-binding partners for the zp proteins have been described; a subset of these have been characterized structurally and functionally. Galactosyltransferase, sp56, zona receptor kinase, and spermadhesins are thought to participate in the primary binding between sperm and zp and may initiate the exocytotic release of hydrolytic enzymes in the sperm head, the acrosome reaction. Digestion of the zp by these enzymes enables sperm to traverse the zp, at which time the proteins PH20, proacrosin, sp38, and Sp17 are thought to participate in secondary binding between the acrosome-reacted sperm and zp. Once through the zp, sperm and egg plasma membranes meet and fuse in a process reported to involve the egg integrin alpha 6 beta 1 and the sperm proteins DE and fertilin. These molecules and the processes involved in gamete interaction are reviewed in this chapter within a physiological context.

Sequence and characterization of the sperm protein Sp17 from the baboon

In this study, cDNAs encoding the sperm protein Sp17 from the baboon (Papio papio) have been cloned and sequenced. Three clones, differing in the lengths of their 3' untranslated regions, were identified, which were encoded by mRNA transcripts of 0.8-1.35 kb. The open reading frame encodes 163 amino acids with a predicted molecular mass of 18.8 kDa. The baboon Sp17 protein sequence is 97% identical to human Sp17 but differs significantly by the addition of 12 amino acids at the C-terminal, providing an additional potential protein kinase C phosphorylation site. Northern blot analysis demonstrated that the baboon Sp17 mRNA was specific to the baboon testes and was not detected in the ovary, placenta, or any of the other somatic tissues tested. Western blot analysis using anti-Sp17 antibodies demonstrated that the native baboon sperm Sp17 protein consists of a doublet with an apparent M(r) of 26.5 and 27.2 kDa. Immunocytochemical staining of baboon testis with anti-Sp17 antibodies demonstrated Sp17 in spermatocytes, spermatids, and spermatozoa within the seminiferous epithelium. No specific staining was observed on spermatogonia, Sertoli cells, Leydig cells, or other somatic cell types.

Autoimmunogenicity of the human sperm protein Sp17 in vasectomized men and identification of linear B cell epitopes

OBJECTIVE

To assess whether human sera positive for antisperm antibodies have detectable levels of Sp17 autoantibodies and to determine the linear B cell epitopes to which these are raised for both native and recombinant Sp17.

DESIGN

Enzyme-linked immunoaborbent assays were performed against recombinant HSp17 on 15 serum samples from prevasovasostomy and postvasovasostomy patients. Positive sera then were used in mimotope analyses to determine HSp17 immunodominant linear B cell epitopes. These were compared with the linear B cell epitopes of recombinant HSp17.

SETTING

University research laboratory.

PATIENT(S)

Fifteen vasectomized or vasovasostomized men.

MAIN OUTCOME MEASURE(S)

Serum antibody reactivity to human Sp17.

RESULT(S)

Sera from vasectomized and vasovasostomized men exhibit Sp17 antibodies raised predominantly to two immunodominant linear B cell epitopes (amino acids 4 to 19 and amino acids 118 to 127), which differed from those of recombinant HSp17 (amino acids 52 to 79 and amino acids 124 to 136).

CONCLUSION(S)

The results show that Sp17 is an antigen to which vasectomized men raise autoantibodies. Two linear B cell epitopes predominate in native Sp17 and these differ from (but overlap with) those of the bacterially expressed recombinant protein.

Sperm antigens and their use in the development of an immunocontraceptive

PROBLEM

Overpopulation is a global problem of significant magnitude, with grave implications for the future. World population is predicted to reach 10 billion people by the year 2050, an increase of more than 75% over the current population. Development of new contraceptives is necessary, since current forms of birth control are unavailable to many individuals due to sociological, financial, or educational limitations.

METHODS

In an effort to identify new contraceptive alternatives, sperm-specific antigens are under the investigation as the basis for immunological regulation of fertility through contraceptive immunization. Anti-sperm antibodies (ASA) inhibit sperm function in vitro, induce infertility in experimental models, and have been implicated in some cases of clinical infertility. Furthermore, the development of ASA in vasectomized men does not lead to physiological complications despite the persistence of these antibodies for years; thus, ASA induced by immunization of men and women may be similarly harmless. Immunization has several conceptual advantages as a method of fertility regulation. Employment of non-pharmacologically active agents, convenience of administration, low cost, and the potential for relatively long-lasting yet reversible effects.

RESULTS

Various approaches have been taken to identify candidate sperm antigens for immunocontraceptive development. Studies that utilized monoclonal antibodies and polyclonal antisera generated against sperm preparations have identified such promising candidates as PH-20, fertilin, SP-17, and SP-10. In animal model, 100% contraceptive effects were induced following PH-20 immunization and the effects were reversible. More recently, ASA from the sera of infertile patients were employed to identify several candidates as expressed by testis cDNA libraries. Perhaps the most extensively characterized and effective immunogen candidate identified thus far is lactate dehydrogenase-C4 (LDH-C4). Active immunization with LDH-C4 suppressed fertility in a variety of mammalian species, including primates, and the reversibility of these effects was demonstrated.

CONCLUSION

The successful results obtained thus far support the feasibility of an effective immunocontraceptive and indicate the importance for continued investigation of additional sperm antigens as contraceptive immunogen candidates.

Cloning and sequencing of cDNAs encoding the human sperm protein, Sp17

In the present study we have cloned and sequenced two testis-specific cDNAs (1.3 kb and 1.6 kb) encoding a human sperm protein, designated HSp17. Each cDNA gave rise to identical protein sequences and differed only in the 5' untranslated region. The predicted amino-acid sequence revealed a protein of 17.5 kDa which exhibited a high degree of homology with both rabbit and mouse Sp17. Analysis of native and recombinant Sp17 by SDS-PAGE has shown the apparent molecular weight of the protein to be 24.5 kDa.