Boar sperm membranes antigens. II. Reorganization of an integral membrane antigen during capacitation and acrosome reaction

Surface mapping of binding of oviductin to the plasma membrane of golden hamster spermatozoa during in vitro capacitation and acrosome reaction

Oviductins are high-molecular-weight glycoproteins synthesized and secreted by nonciliated oviductal epithelial cells and have been shown to play a role in fertilization and early embryo development. The present study was carried out to examine the in vitro binding capacity of hamster oviductin to homologous sperm and to determine the sites of its localization in untreated, capacitated, and acrosome-reacted spermatozoa. Freshly prepared epididymal and capacitated sperm as well as acrosome-reacted sperm were incubated with oviductal fluid prepared from isolated hamster oviducts, fixed and then probed with a monoclonal antibody against hamster oviductin. Results obtained with pre-embedding immunolabeling experiments revealed binding of oviductin to the acrosomal cap and the apical aspect of the postacrosomal region. Immunolabeling of both regions appeared to be more intense in capacitated spermatozoa. Acrosome-reacted sperm showed an immunoreaction of moderate intensity over the postacrosomal region. The plasma membrane overlying the equatorial segment also exhibited a weak labeling. Quantitative analysis obtained with the surface replica technique indicated that oviductin had a higher binding affinity for the acrosomal cap than the postacrosomal region and that the binding of oviductin to the latter plasma membrane domain was enhanced during capacitation. Binding of oviductin to the postacrosomal region, however, was attenuated after acrosome reaction. Immunolabeling for oviductin was found to be the weakest over the equatorial segment regardless of the experimental conditions. The binding of hamster oviductin to specific membrane domains of the homologous sperm and the changes in its distribution during capacitation and acrosome reaction may be important for the function of hamster oviductin preceding and during fertilization.

Dynamic quantification of the tyrosine phosphorylation of the sperm surface proteins during capacitation

BACKGROUND

Spermatozoa acquire active fertilizing competence only after deposition in the female tract and subsequent capacitation. Recent studies on the cellular location of major sperm phosphoproteins suggest that capacitation is associated with tyrosine phosphorylation of proteins exposed on the sperm surface. However, these changes have not yet been quantified objectively. A calcium influx seems to be required for the completion of tyrosine phosphorylation in some species; however, the exact temporal coordination between these processes is still poorly understood.

METHODS

Flow cytometry was used to quantify the degree of phosphorylation of the sperm surface proteins by probing with fluorescein isothiocyanate-conjugated anti-phosphotyrosine (pY) antibody raised in mouse. Dynamic changes in other sperm parameters (calcium influx, membrane integrity, and spontaneous acrosome reaction) were assessed to analyze their temporal coordination.

RESULTS

: The changes in specific phosphotyrosine (pY) fluorescence signal detected in live, nonpermeabilized boar cell suspensions were biphasic during incubation under capacitating conditions. After 120 min of incubation, the degree of pY fluorescence increased threefold, indicating the changes in proteins exposed on sperm surface. At the same time there was a gradual increase in cytosolic calcium ion levels with the maximal rate at 60 min of incubation. This rate slowed immediately before the onset of the massive rise in tyrosine phosphorylation and decreased by 90% after its completion. The integrity of plasma and acrosome membranes decreased only slowly, illustrating that the changes observed were not due to the process of spontaneous acrosome reaction.

CONCLUSIONS

These data provide quantitative evidence for the appearance of tyrosine-phosphorylated proteins on the surface of live boar spermatozoa during capacitation. An exact temporal coordination exists between cytosolic calcium ion content and protein tyrosine phosphorylation under these conditions. This novel approach has the advantage of making possible a precise quantification and kinetic comparison of molecular processes in different cell subpopulations.

Immunolocalization of heat shock protein 70 (Hsp 70) in boar spermatozoa and its role during fertilization

The presence and cellular distribution of heat protein 70 (Hsp70) in ejaculated, capacitated, and acrosome-reacted boar spermatozoa was evaluated by immunofluorescence and Western blot; the role of Hsp70 during fertilization was also studied. In freshly ejaculated spermatozoa, Hsp70 immunoreactivity is present in a well-defined triangular-shaped area in the equatorial segment that seems to correspond to the equatorial sub-segment. The distribution of the fluorescent signal changes in capacitated sperm, that exhibit different patterns probably in relation to the stage of capacitation of individual cells; after acrosome reaction Hsp70 immunoreactivity is localized on both a thick sub-equatorial band and a triangle in the equatorial segment. In reacted spermatozoa, Hsp70 seems to be not only relocalized but also translocated from the inner to the outer leaflet of the sperm plasma membrane, as a significant (P < 0.05) increase in the proportion of unfixed cells showing the fluorescent signal has been recorded. No differences in Hsp70 amount between fresh, capacitated, and reacted semen were observed by Western blot. The presence of anti-Hsp70 antibody in the fertilization medium significantly reduced, in a concentration-dependent manner, the fertilization rate of both zona-intact and zona-free oocytes. The overall data demonstrate that Hsp70 is present on boar sperm with a dynamic redistribution as the sperm undergoes capacitation and acrosome reaction and suggest an important role of this protein during porcine gamete interaction.

Functional significance of responsiveness to capacitating conditions in boar spermatozoa

New methods are needed for rapid and sensitive assessment of sperm function. As the ability to fertilize an oocyte is acquired during the capacitation process, assessments of sperm function have to be performed under fertilizing conditions. In this study, we monitored the dynamics of the temporal response of sperm from ejaculates of both fertile and subfertile boars to capacitating conditions in vitro (responsiveness) by following the changes in the response to calcium ionophore treatment and in [Ca(2+)](i). The differences between individual males were also investigated. Ionophore-induced changes and increased intracellular calcium ion content in boar spermatozoa were found to progress as a function of time during incubation under capacitating conditions. After primary kinetic analysis, 120 min was chosen as the point in time for assessment of responsiveness. Intra-boar variability in responsiveness parameters was relatively high (variation coefficient CV>30%), especially in the response to ionophore treatment, indicating that an isolated test may be inadequate for the evaluation of sperm function. Despite this high variability, there were markedly significant individual differences with respect to changes during capacitation, and there were significant correlations between conventional and responsiveness sperm parameters. The population of samples from subfertile boars, was found to be heterogeneous in regard to sperm responsiveness to capacitating conditions. There were two significantly different classes of subfertile boars ("low" and "high" responders), indicating that fertility may be associated with suboptimal rather than maximal response (both too rapid and too slow membrane changes). Therefore, criteria for quality judgement should include both the low and upper limits of responsiveness. The use of responsiveness parameters together with conventional spermatological parameters improved the prediction level of multiple regression models for farrowing rate and litter size. It can be concluded that the combination of sperm responsiveness parameters applied here is a suitable tool for the evaluation of sperm function.

Detection of cooling-induced membrane changes in the response of boar sperm to capacitating conditions

There is a need for methods of rapid and sensitive sperm function assessment. As spermatozoa are not able to fertilize an oocyte before having undergone a series of complex physiological changes collectively called capacitation, it is logical to assess sperm function under fertilizing conditions in vitro. In this study, the responsiveness of sperm to capacitating conditions in vitro was monitored by changes in sperm response to ionophore and by changes in the amount of intracellular calcium ions in stored boar semen. Boar semen was diluted at 32 and 20 degrees C and stored for 24 and 72 h at 16 and 10 degrees C. Ionophore-induced changes and increased intracellular calcium ion content in boar spermatozoa were recorded by flow cytometry and found to progress as a function of time during incubation under capacitating conditions. All responsiveness parameters (increases in proportions of membrane-defective spermatozoa, acrosome-reacted spermatozoa, and cells with high intracellular calcium levels) were shown to be sensitive to subtle physiological changes occurring at low storage temperatures. The initial levels of sperm with a high calcium content were higher in semen stored at 10 degrees C, but the accumulation of internal calcium was lower than in semen stored at 16 degrees C. The loss of membrane integrity and increase in the proportion of acrosome-reacted cells were higher in semen stored at 10 degrees C. Dilution at 20 degrees C had no negative effect on membrane integrity or responsiveness to capacitating conditions. There was no significant difference between semen stored for 24 and 72 h in terms of membrane integrity, acrosome reaction, and intracellular calcium after capacitation treatment. However, dynamics of cell death and acrosome reaction in response to capacitating conditions were somewhat accelerated after 72 h storage, especially in semen stored at 10 degrees C. It can be concluded that the simultaneous use of the sperm membrane responsiveness and kinetic parameters is a sensitive tool for the detection of storage-related membrane changes in boar semen.

Identification of human sperm proteins involved in the interaction with homologous zona pellucida

OBJECTIVE

To identify human sperm proteins involved in homologous zona pellucida (ZP) interaction.

DESIGN

Prospective study.

SETTINGS

Basic research laboratory.

PATIENT(S)

Semen samples from normozoospermic donors, tissue sections from surgical pieces, and ZP from nonfertilized oocytes.

INTERVENTION(S)

Antibodies for sperm proteins (HSE; high salt extract) were developed (anti-HSE) and partially characterized. Participation of sperm proteins on ZP-interaction was tested with the hemizona assay (HZA). Antigens were immunolocalized in sperm and tissues.

MAIN OUTCOME MEASURE(S)

Sperm and tissue immunostaining; Western blotting; and number of sperm bound to the ZP.

RESULT(S)

Anti-HSE antibodies recognized several polypeptides in HSE (9 to 200 kd). Specific antibodies for 49 and 66 kd proteins (p49, p66) were obtained. Both (anti-p49 and anti-p66) stained the head of ejaculated and capacitated sperm. In the HZA, sperm preincubation with a mixture of anti-p49 and anti-p66 (100 micro g/mL) resulted in a decrease in the number of spermatozoa bound to the ZP. Presence of p66 (10 micro g/mL) inhibited sperm-ZP interaction. In contrast, p49 did not alter sperm binding to the ZP. Immunohistochemical analysis showed that p66 is present in the epididymis. No staining was observed in testicular sections.

CONCLUSION(S)

We found that p66 is an epididymal protein that participates in human sperm interaction with homologous ZP.

Function of the mammalian oviductal sperm reservoir

Sperm are stored in the isthmic region of the oviduct under conditions that maintain sperm viability and suppress motility. This region is also the site in which essential steps of the capacitation process are coordinated with the appearance of the ovulated egg. The influx of Ca(2+) and phosphorylation of sperm proteins are features of the ongoing capacitation process. Using a cell-culture system of oviductal epithelial cells, it was found that sperm bound to the epithelial cells showed a reduced Ca(2+) uptake and almost no tyrosine phosphorylation as shown by indirect immunofluorescence. Furthermore, sperm viability, measured as membrane integrity with propidium iodide, is significantly prolonged as compared to sperm in suspension. The formation of the sperm reservoir appears to be mediated by carbohydrate-protein interaction. In the pig, it has been found that mannosyl-oligosaccharides exposed by the epithelial cells are high-affinity ligands for sperm-associated lectins. Ovalbumin and mannopentaose are effective inhibitors of sperm binding to explants of oviductal epithelium. Spermadhesins, a new class of animal lectins and the major secretory products of the porcine seminal vesicle, associate with the sperm surface at ejaculation and are candidate molecules for the receptors of the epithelial carbohydrates.

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

Sulfoglycolipid immobilizing protein 1 (SLIP 1) is an evolutionally conserved sperm head plasma membrane protein (M(r) = 68 kDa) that binds to sulfogalactosylglycerolipid (SGG), the major sulfoglycolipid present in mammalian sperm. The purpose of this study was to characterize the initial localization and the immunoaggregated relocalization of SLIP1 on the mouse sperm head. Direct immunofluorescence (DF) of live sperm using FITC-antiSLIP1 Fab fragments and FITC-antiSLIP1 IgG indicated that SLIP1 was present in the postacrosomal region of the sperm head, although the intensity of immunostaining by FITC-antiSLIP1 IgG was greatest at the border between the postacrosomal region and the acrosome. Unlike that observed with FITC-antiSLIP1 Fab, DF using FITC-antiSLIP1 IgG indicated that SLIP1 was also present in the anterior tip of the sperm head convex ridge. Results from electron microscopic studies, using antiSLIP1 IgG followed by protein A-gold on live mouse sperm, were similar to the DF findings. In contrast, indirect immunofluorescence (IIF) of live mouse sperm using antiSLIP1 IgG and FITC-secondary antibody IgG detected SLIP1 in the sperm head convex ridge only. The IIF and DF results strongly suggest that these bivalent antibodies could induce the sperm antigen relocalization on live sperm heads. SLIP1 redistribution may be dependent on availability of excess SGG, the SLIP1 binding ligand, based on the observation that purified exogenous biotinylated SLIP1 bound to live mouse sperm at both the postacrosomal and convex ridge regions of the mouse sperm head. Immunoaggregation induced by the primary antiSLIP1 IgG or antiSLIP1 Fab with secondary antibody IgG did not cause the acrosome reaction, suggesting that SLIP1 is not involved in sperm signal transduction. Furthermore, postacrosomal SLIP1 was shown to be involved in zona binding, since sperm pretreated with antiSLIP1 Fab fragments (100 micrograms/ml) bound to the egg zona pellucida in vitro at approximately 35% of control levels.

ConA-binding proteins of the sperm surface are conserved through evolution and in sperm maturation

Lectin-binding glycoconjugates present on the surface of spermatozoa are believed to play a crucial role in sperm maturation, capacitation, acrosome reaction, or sperm-egg interaction. We have studied ConA-binding surface proteins on spermatozoa from different mammalian species. First, ConA-binding proteins were isolated from boar spermatozoa by affinity chromatography. ConA-binding ability was confirmed by Enzyme-linked Lectin assay (ELLA). Monoclonal (MAb436/10) and polyclonal antibodies were raised against chromatography fractions containing purified ConA-binding proteins of boar spermatozoa. MAb436/10 (IgG2a) recognizes a 40 kD ConA-binding antigen. Indirect immunofluorescence on fixed and unfixed boar spermatozoa with MAb436/10 indicated a plasma membrane localization of antigen 436/10 in the acrosomal macrodomain. Interspecies cross-reactivity with MAb436/10 was found by whole cell ELISA and immunocytochemistry. MAb436/10 cross-reacted with human, horse, guinea-pig, bull, and ram spermatozoa in both assays. Expression of ConA-binding antigen 436/10 on guinea pig sperm surface was detectable during spermiogenesis and in early stages of sperm maturation. Change of regionalization of the antigen did not occur during the epididymal passage. ConA-binding antigen 436/10 was also detectable in testis and caudal segments of the epididymis. These findings suggest that ConA-binding proteins located in the acrosomal region are highly conserved through evolution as well as in sperm maturation indicating an important role for the physiology of spermatozoa.

Redistribution of a rat sperm epididymal glycoprotein after in vitro and in vivo capacitation

Rat epididymal glycoprotein DE (37 kDa) associates with the sperm surface during maturation and is localized over the dorsal region of the acrosome. In the present study we examine, by indirect immunofluorescence, the localization of DE after in vitro and in vivo capacitation. While 49% of sperm capacitated in vitro for 5 hr still presented fluorescence over the dorsal region, 51% showed labeling distributed over a domain that corresponds to the equatorial segment of the sperm head. This change in the localization of fluorescence was not associated with sperm deterioration or death and increased gradually as a function of capacitation time, reaching the maximum at 5 hr. The presence of labeling over the equatorial segment results from protein migration and cannot be induced by permeabilization, proteinase, or high ionic strength treatments. The omission of Ca2+ from the standard capacitation medium inhibited the relocalization of DE, and incubation with Ca2+ ionophore A23187 for induction of the acrosome reaction (AR) significantly raised the percentage of cells with DE localized over the equatorial region. Finally, while free and cumulus-associated spermatozoa recovered from the oviducts of in vivo inseminated females presented 15% and 21% of cells with redistribution respectively, all perivitelline (acrosome reacted) spermatozoa showed DE over the equatorial segment. These results indicate that epididymal protein DE migrates to the equatorial segment under in vitro and in vivo capacitating conditions and suggest a possible association between the redistribution of DE and the occurrence of the AR.