Localization and role of sulfoglycolipid immobilizing protein 1 on the mouse sperm head

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.

The specific localization of seminolipid molecular species on mouse testis during testicular maturation revealed by imaging mass spectrometry

More than 90% of the glycolipid in mammalian testis consists of a unique sulfated glyceroglycolipid called seminolipid. The galactosylation of the molecule is catalyzed by UDP-galactose:ceramide galactosyltransferase (CGT). Disruption of the CGT gene in mice results in male infertility due to the arrest of spermatogenesis, indicating that seminolipid plays an important role in reproductive function. Seminolipid molecules can be assigned to different molecular species based on the fatty acid composition. In this report, we investigated the localizations of the molecular species of seminolipid by imaging mass spectrometry and demonstrated that major molecule (C16:0-alkyl-C16:0-acyl) was expressed throughout the tubules: some (C16:0-alkyl-C14:0-acyl and C14:0-alkyl-C16:0-acyl) were predominantly expressed in spermatocytes and the other (C17:0-alkyl-C16:0-acyl) was specifically expressed in spermatids and spermatozoa. This is the first report to show the cell-specific localization of each molecular species of seminolipid during testicular maturation.

Anti-SLIP1-reactive proteins exist on human spermatozoa and are involved in zona pellucida binding

Sulpholipid immobilizing protein 1 (SLIP1) is an evolutionarily conserved 68 kDa plasma membrane protein, present selectively in germ cells. We have previously shown that mouse sperm SLIP1 is involved in sperm-zona pellucida (ZP) binding. In this report, we extended our study to the human system. Immunoblotting demonstrated that anti-SLIP1-reactive proteins (mol. wt 68 and 48 kDa) could be extracted from human spermatozoa by an ATP-containing solution, a result that is consistent with observations in other species. Direct immunofluorescence, using Cy3-conjugated anti-SLIP1 IgG, revealed SLIP1 staining over the acrosomal region, with higher intensity at the posterior area. Using the human sperm-ZP binding assay, we demonstrated that pretreatment of human spermatozoa from three donors with anti-SLIP1 IgG revealed lower numbers of zona-bound spermatozoa, as compared to the corresponding control spermatozoa treated with normal rabbit serum IgG. This decrease in zona pellucida binding was not from an antibody-induced decline in sperm motility or an increase in the premature acrosome reaction. The results strongly suggest that anti-SLIP-reactive proteins on human spermatozoa play an important role in ZP binding.

Isolation of antiSLIP1-reactive boar sperm P68/62 and its binding to mammalian zona pellucida

Single-step purification of boar sperm P68/62 that is cross-reactive with a polyclonal antibody against sulfolipidimmobilizing protein 1 (SLIP1) was achieved by chromatofocusing. This method is useful for obtaining P68/62 in quantity. The two proteins, P68 and P62, were antigenically related, since the antibody generated specifically against the 68-kDa band reacted with both the 68- and 62-kDa bands. Like rat testis SLIP1, purified boar sperm P68/62 bound to sulfogalactosylglycerolipid (SGG) and inhibited sperm-egg binding in a dose-dependent manner when added exogenously to sperm-egg coincubates. This inhibitory effect occurred at the level of the zona pellucida (ZP), and further studies showed that biotinylated boar sperm P68/62 bound to the ZP of unfertilized mouse eggs. Furthermore, biotinylated boar sperm P68/62 bound to isolated ZP of unfertilized eggs from other species, including pig, rat, cat, dog, and human, as well as to ZP of intact fertilized mouse eggs and preimplantation embryos of various developmental stages, although the degree of its binding to the ZP of intact eight-cell embryos, morulae, and blastocysts was much lower than that of fertilized eggs and two-cell embryos. These results suggest that P68/62 of capacitated sperm must act together with other sperm surface proteins/molecules that regulate zona binding specificity within homologous species and in unfertilized eggs. Together with our previous findings, we suggest that rather than being a true ZP receptor, sperm P68/62 may be involved in the initial step of sperm-ZP binding that is adhesive in nature.