Current concepts in gamete receptors for fertilization in mammals

Cellular and molecular biology of capacitation and acrosome reaction in spermatozoa

A comparative account is given of advances in cellular and molecular biology of capacitation and acrosome reaction in spermatozoa by comparing and contrasting their biochemical and physiological changes in response to various factors in vivo and in vitro. It can now be stated that phenomena of sperm capacitation and acrosome reaction are endogenous molecular events occurring at the membrane level which can be modulated by external environmental factors. The molecular mechanisms and the signal transduction pathways mediating the process of capacitation and acrosome reaction are only partially defined and appear to involve modification of intracellular Ca2+ and other ions, lipid transfer, and phospholipid remodeling in the sperm plasma membrane as well as changes in protein phosphorylation. Evidences for the involvement of cAMP-dependent kinase pathway in the acrosome reaction are discussed. The mediation of one or more external signals by the sperm plasma membrane appears to activate this pathway after or simultaneously with the influx of Ca2+. Concurrent with or following entry of Ca2+, adenylate cyclase is activated, leading to increased concentrations of cAMP-activation of cAMP-dependent kinase and protein phosphorylation; the identity of such proteins and their role in the acrosome reaction must be determined. The roles of biological effectors of the acrosome reaction, such as ZP3 and follicular fluid are still to be defined at the molecular level. The gaps in our knowledge about the cellular and molecular aspects of capacitation and acrosome reaction are emphasized.

Induction of thumbtack sperm during coculture with oviduct epithelial cell monolayers in a marsupial, the brushtail possum (Trichosurus vulpecula)

A reorientation of the sperm head so that it is perpendicular to the sperm tail (i.e., T-shape or thumbtack) is considered an indicator of sperm capacitation in the Australian marsupial the brushtail possum (Trichosurus vulpecula). This study describes a method of oviduct epithelial cell monolayer and sperm coculture in the brushtail possum to obtain a high percentage of thumbtack sperm. The oviduct epithelial cell (OEC) monolayers were prepared in vitro from the isthmal and ampullary segments of eCG- and LH-primed brushtail possum oviducts. Coculture experiments demonstrated that cauda epididymidal sperm from the brushtail possum attached equally to the OEC monolayers derived from the isthmal and ampullary segments of the oviduct. After 2 h of coculture, a large number of sperm attached to OEC monolayers (ampulla, 60.1+/-4.7% and isthmus, 63.1+/-5.7%) as well as to controls (tracheal epithelial cell monolayer, 46.2+/-3.7%; Matrigel, 57.4+/-7.7%; plastic, 29.2+/-3.2%). After 6 h, fewer sperm were attached to tracheal epithelial cell monolayers (1.2+/-0.2%; P<0.01) and Matrigel (10.2+/-2.5%; P<0.01), compared to those attached to ampullary and isthmal OEC monolayers (37.9+/-7.2% and 44.6+/-2.2%, respectively), and none were attached to the plastic surface. Fewer sperm were released from the ampullary and isthmal OEC monolayers compared to those from controls (P<0.05). At 6 h of coculture with ampullary and isthmal OEC, the percentage motility of both attached and unattached spermatozoa was maintained at 40-50%, which was higher (P<0.05) than in controls. Progressive motility of unattached sperm was maintained at about 2 (on an arbitrary scale of 1-5) and was not different among treatments until 6 h. More than 60-70% sperm were viable at 6 h of coculture in all the treatments. Coculture of brushtail possum epididymal sperm with OEC monolayers transformed 60% of motile streamlined spermatozoa to thumbtack orientation at 2 h compared to approximately 25% in controls. No acrosomal modifications were induced in spermatozoa in any of the treatments. This study has demonstrated a role of the oviduct in transforming a large number of sperm from a streamlined to thumbtack orientation, which may have relevance in sperm capacitation and fertilization in this species.

Analysis of zona pellucida modifications due to cortical granule exocytosis in single porcine oocytes, using enhanced chemiluminescence

The present study was carried out to determine whether modification of zona pellucida (ZP) of a single oocyte following the cortical granule (CG) exocytosis induced by electrical stimulation could be analyzed using enhanced chemiluminescent (ECL) detection of the biotinylated ZP in a porcine oocyte. When a biotinylated ZP derived from a single oocyte matured in vitro was subjected to SDS-PAGE, 3 major bands (ZP1, ZP2 and ZP3) were observed following ECL detection. In these oocytes, CGs staining with fluorescein isothiocyanate (FITC)-labeled peanut agglutinin (FITC-PNA) had formed a monolayer underlying the plasma membrane. Electrical stimulation to induce artificial activation caused a decline in the fluorescent intensity of the CGs with a concomitant decrease in the amounts of ZP1 and ZP2 bands. However, the mobility changes of ZP1 and ZP2 on SDS-PAGE were not found under the inhibitory condition of the CG exocytosis in which oocytes were treated with ethylene glycol-bis(beta-aminoethyl ether) N, N, N',N'-tetraacetic acid (EGTA) or 1, 2-bis(2-aminophenoxy)ethane-N, N, N', N'-tetraacetic acid tetrakis(acetoxymethyl) ester (BAPTA/AM). In addition, when a time-dependent decrease in amounts of ZP1 and ZP2 bands on SDS-PAGE was observed in a single oocyte during activation, a maximum decrease in these bands was detected in oocytes incubated for at least 3.5 h after electrical stimulation. These results show that the method employed, ECL detection of the biotinylated ZP of a single oocyte, is a valuable tool for the analysis of ZP modification resulting from a decrease in amounts of ZP1 and ZP2 glycoproteins in combination with exocytosis of CGs, and that the prolonged period after activation is required for complete ZP modification in porcine oocytes.

Identification of low density lipoprotein receptor-related protein-2/megalin as an endocytic receptor for seminal vesicle secretory protein II

The low density lipoprotein receptor-related protein-2/megalin (LRP-2) is an endocytic receptor that is expressed on the apical surfaces of epithelial cells lining specific regions of the male and female reproductive tracts. In the present study, immunohistochemical staining revealed that LRP-2 is also expressed by epithelial cells lining the ductal region and the ampulla of the rat seminal vesicle. To identify LRP-2 ligands in the seminal vesicle, we probed seminal vesicle fluid with 125I-labeled LRP-2 in a gel-blot overlay assay. A 100-kDa protein (under non-reducing conditions) was found to bind the radiolabeled receptor. The protein was isolated and subjected to protease digestion, and the proteolytic fragments were subjected to mass spectroscopic sequence analysis. As a result, the 100-kDa protein was identified as the seminal vesicle secretory protein II (SVS-II), a major constituent of the seminal coagulum. Using purified preparations of SVS-II and LRP-2, solid-phase binding assays were used to show that the SVS-II bound to the receptor with high affinity (Kd = 5.6 nM). The binding of SVS-II to LRP-2 was inhibited using a known antagonist of LRP-2 function, the 39-kDa receptor-associated protein RAP. Using a series of recombinant subfragments of SVS-II, the LRP-2 binding site was mapped to a stretch of repeated 13-residue modules located in the central portion of the SVS-II polypeptide. To evaluate the ability of LRP-2 to mediate 125I-SVS-II endocytosis and lysosomal degradation, ligand clearance assays were performed using differentiated mouse F9 cells, which express high levels of LRP-2. Radiolabeled SVS-II was internalized and degraded by the cells, and both processes were inhibited by antibodies to LRP-2 or by RAP. The results indicate that LRP-2 binds SVS-II and can mediate its endocytosis leading to lysosomal degradation.

Composition of human zona pellucida as revealed by SDS-PAGE after silver staining

The macromolecular composition of zona pellucida (ZP) isolated from human oocytes and embryos was characterized by one- and two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (1-D- and 2-D-SDS-PAGE) under reducing conditions after silver staining. ZP specimens obtained after in vitro fertilization were removed from pools of heavily fragmented embryos and inseminated oocytes that failed to fertilize. For unfertilized oocytes, two major bands with an apparent molecular weight of, respectively, 96 and 76-54 kDa were observed after 1-D-SDS-PAGE and silver staining. When ZP were isolated from fragmented embryos, the electrophoretic pattern showed a marked attenuation of the 96-kDa band. Silver-stained 2-D-SDS-PAGE analysis of ZP components from unfertilized oocytes revealed the presence of four protein trains: ZP1 (Mr = 92-80 kDa, pl = 4.9-5.9), ZP2 (Mr = 66-58 kDa, pl = 5.0-6.0), ZP3H (high) (Mr = 72-58 kDa, pl = 3.5-5.1), and ZP3L (low) (Mr = 62-54 kDa, pl = 3.5-5.1). The human ZP3 family (ZP3H and ZP3L) showed marked heterogeneity. Fertilization-associated changes were apparent in the electrophoretic pattern. ZP1 (Mr = 92-86 kDa, pl = 5.0-5.8) displayed a dramatic decrease in intensity, and a new component had migrated to a position similar to that of ZP2. This modification may have been responsible for one aspect of the zona reaction, and could have contributed to a zona block to polysperma.

Comparative immunoreactivity of mouse and hamster sperm proteins recognized by an anti-P26h hamster sperm protein

The binding of the spermatozoon to the zona pellucida is a species-specific phenomenon. We have previously shown that the binding of hamster sperm to the homologous zona pellucida involves a sperm 26-kDa glycoprotein, the P26h, originating in the epididymis. In order to establish to what extent this sperm protein is involved in the species-specific recognition of the egg's extracellular coat, we have compared the inhibitory properties of anti-P26h antibodies in a sperm-zona pellucida assay using hamster and mouse gametes. Anti-P26h IgGs inhibit, in a dose-dependent manner, gamete interactions in both species, although in a less efficient manner in the mouse than in the hamster. While anti-26kDa Fab fragments are as efficient as the intact IgG to inhibit hamster sperm-zona pellucida binding, they have no effect on mouse gamete interaction. ELISA, Western blot, and immunohistochemical experiments have been performed in order to characterize the mouse antigen(s) recognized by the anti-P26h antiserum. ELISA and Western blots showed that this antiserum recognized two proteins on mouse spermatozoa that are less reactive than the hamster P26h. These antigens are localized in the acrosomal region of epididymal spermatozoa of both species. These results indicate that the hamster P26H involved in zona pellucida interaction has certain unique epitopes, while others are common to the sperm of both species.

Monoclonal antibody against a 52 K sperm surface protein inhibits sperm-zona pellucida interactions in the rat

A major regulatory site for species specificity of fertilization in mammals lies at the level of sperm binding to the zona pellucida. This implies a high degree of complementarity between gamete and receptor molecules. These molecules support species-specific interactions between sperm and oocyte that lead to gamete fusion (fertilization). We identified a rat sperm head surface antigen using an IgG1 monoclonal antibody (HD1) against rat sperm of epididymis cauda. By electron microscopy the antigen was shown to be present on the plasma membrane surface of the sperm dorsal head. One- and two-dimensional immunoblotting analysis of sperm proteins demonstrated that HD1 reacted only with a 52 K molecule with a pI ranging from 6.6 to 7.2. The 52 K protein was first detected in situ by indirect immunofluorescence and showed to be underlining few elongated spermatids in testis. However, when the sperm reached the epididymis caput, the antigen was seen to be expressed on the dorsal surface of spermatozoa head. A similar fluorescence reaction was detected on sperms in the epididymis corpus and cauda. The specific spermatozoa-zona pellucida interaction was inhibited in the presence of monoclonal antibody HD1 in a sperm binding assay of in vitro fertilization.

Sperm-binding proteins

Gamete recognition and binding are mediated by specific proteins on the surface of the sperm and egg. Identification and characterization of some of these proteins from several model systems, particularly mouse and sea urchin, have focused interest on the general properties and functions of gamete recognition proteins. Sperm-binding proteins located in egg extracellular coats as well as sperm-binding proteins that are localized to the egg plasma membrane are presented in the context of their structure and function in gamete binding. Unifying and disparate characteristics are discussed in light of the diverse biology of fertilization among species. Outstanding questions, alternative mechanisms and models, and strategies for future work are presented.

Effect of calmodulin-like protein from buffalo (Bubalus bubalis) seminal plasma on Ca2+, Mg(2+)-ATPase of purified plasma membrane of buffalo spermatozoa

Buffalo sperm heads and tails were cleaved by sonication and isolated in relatively pure proportions i.e. 95% and 98% respectively, by discontinuous sucrose density-gradient centrifugation. Purified plasma membranes from the isolated sperm heads and tails were obtained by hypotonic treatment and brief sonication followed by discontinuous sucrose density-gradient centrifugation. Ca2+, Mg(2+)-ATPase activity was evident in plasma membrane from sperm heads and tails, although activity was greater in the latter. A calmodulin-like protein isolated from buffalo seminal plasma increased the Ca2+, Mg(2+)-ATPase of plasma membrane from the sperm heads and tails by 128 and 136% respectively. Based upon the data obtained here and elsewhere (Sidhu & Guraya, 1989a) a model is proposed which explains regulation of Ca2+ in buffalo spermatozoa and implicates calmodulin-like protein and Ca2+, Mg(2+)-ATPase in sperm acrosome reaction.