Cholesterol efflux-mediated signal transduction in mammalian sperm: cholesterol release signals an increase in protein tyrosine phosphorylation during mouse sperm capacitation

Current concepts of molecular events during bovine and porcine spermatozoa capacitation

Spermatozoa are required to undergo the processes of capacitation before they obtain fertilizing ability. The molecular changes of capacitation are still not fully understood. However, it is accepted that capacitation is a sequential process involving numerous physiological changes including destabilization of the plasma membrane, alterations of intracellular ion concentrations and membrane potential, and protein phosphorylation. There are no known morphological changes that occur to the spermatozoon during capacitation. The purpose of this review is to summarize current evidence on the molecular aspects of capacitation both in vivo and in vitro in bovine and porcine spermatozoa. For the purpose of this review, the process of sperm capacitation will encompass maturational events that occur following ejaculation up to binding to the zona pellucida, that triggers acrosomal exocytosis and initiates fertilization.

Evaluation of Chromosomal Risk Following Intracytoplasmic Sperm Injection in the Mouse

To investigate whether cytogenetic risks occur using the mouse intracytoplasmic sperm injection (ICSI) technique, the incidence of chromosome aberrations was compared in one-cell embryos produced by ICSI technique and those by conventional in vitro fertilization (IVF) technique. Spermatozoa were incubated in TYH medium for 1.5-2 h before IVF insemination. For the ICSI technique, spermatozoa were incubated in five different media: TYH, Hepes-buffered TYH (H-TYH), modified CZB (mCZB), Hepes-buffered mCZB (H-mCZB), and PB1 for 0.5 h, 2-2.5 h, and 6 h before injection into metaphase II oocytes. The incidence of IVF embryos with structural chromosome aberrations was 2%, whereas the occurrence of structural chromosome aberrations in ICSI embryos was dependent on the kind of medium and sperm incubation time. When spermatozoa were incubated in TYH medium for 2 h or more, the aberration rates in the resultant ICSI embryos (4%) were not significantly different from that of IVF embryos. However, there was a significant increase in aberration rates in ICSI embryos derived from spermatozoa that were incubated in other culture conditions (6%-28%). In addition, a time-dependent increase in aberration rates was found in ICSI embryos when H-TYH, H-mCZB, and PB1 were used for sperm incubation. There was no significant difference in incidence of aneuploidy between IVF and ICSI embryos. The chromosome analysis results of one-cell embryos were reflected by the performance of postimplantation embryo development. The causal mechanism of chromosome damage in ICSI embryos was discussed in relation to the plasma membrane cholesterol, the acrosome, and in vitro aging of spermatozoa.

Egg water from the amphibian Bufo arenarum induces capacitation-like changes in homologous spermatozoa

Mammalian sperm acquire fertilizing capacity after residing in the female tract, where physiological changes named capacitation take place. In animals with external fertilization as amphibians, gamete interactions are first established between sperm and molecules of the egg jelly coat released into the medium. Since dejellied oocytes are not normally fertilized, the aim of this study was to determine if the jelly coat of the toad Bufo arenarum promotes a "capacitating" activity on homologous sperm. We found that sperm incubation in diffusible substances of the jelly coat (egg water) for 90-180 s is sufficient to render sperm transiently capable of fertilizing dejellied oocytes. The fertilizing state was correlated with an increase of protein tyrosine phosphorylation and a decrease of sperm cholesterol content. Inhibition of either the increase in tyrosine phosphorylation or cholesterol efflux affected the acquisition of fertilizing capacity. Phosphorylation and fertilization could be promoted with NaHCO(3) and also by addition of beta cyclodextrin. Moreover, sperm could gain the ability to fertilize dejellied oocytes in the presence of these compounds. These data indicate that sperm should undergo a series of molecular changes to gain fertilizing capacity; these changes are reminiscent of mammalian sperm capacitation and take place before the acrosome reaction.

Signal transduction mechanisms involved in in vitro ram sperm capacitation

We validate the chlortetracycline (CTC) technique for the evaluation of capacitation and acrosome reaction-like changes in ram sperm, carrying out a double estimation of the acrosome status after treatment with lysophosphatidylcholine, using fluorescein isocyanate (FITC)-RCA/ethidium homodimer 1 (EthD-1) and CTC/EthD-1. Highly consistent results and a positive correlation between the results of acrosome-reacted sperm evaluated with both techniques were obtained. In this study, we evaluate the effects of ram sperm capacitation of BSA, Ca(2+), NaHCO(3) and cAMP agonists and their influence on the associated protein tyrosine phosphorylation. We found a time-dependent increase in capacitation related to protein tyrosine phosphorylation, either in the absence or the presence of BSA. The addition of an increasing concentration of cholesterol to samples containing BSA did not influence results. The effect of bicarbonate was concentration-dependent, with a significantly lowered value of non-capacitated sperm in the presence 18 and 25 mM. The addition of extracellular calcium did not significantly increase either the proportion of capacitated sperm or the protein tyrosine phosphorylation signalling, although a significantly higher value of acrosome-reacted sperm was found in samples containing 4 mM Ca(2+). cAMP agonists increased capacitated sperm and protein tyrosine phosphorylation signalling. The inhibition of protein kinase A by H-89 caused a decrease in sperm capacitation. Addition of a calcium-entry blocker (Verapamil; Sigma) did not influence results, which suggests that the calcium entry blocker was unable to inhibit the calcium influx associated with capacitation in ram sperm. Our findings might benefit our understanding of the biochemical mechanisms involved in mammalian sperm capacitation and ultimately, fertility.

Sperm abnormalities in heterozygous acid sphingomyelinase knockout mice reveal a novel approach for the prevention of genetic diseases

Acid sphingomyelinase knockout mice are a model of the inherited human disorder types A and B Niemann-Pick disease. Herein, we show that heterozygous (ASMKO(+/-)) mice have two distinct sperm populations resembling those found in normal and mutant animals, respectively, and that these two populations could be distinguished by their morphology, ability to undergo capacitation or the acrosome reaction, and/or mitochondrial membrane potential (MMP). The abnormal morphology of the mutant sperm could be normalized by demembranation with detergents or by the addition of recombinant acid sphingomyelinase to the culture media, and the corrected sperm also had an enhanced fertilization capacity. Methods were then explored to enrich for normal sperm from the mixed ASMKO(+/-) population, and flow cytometric sorting based on MMP provided the best results. In vitro fertilization was performed using ASMKO(+/-) oocytes and sperm before and after MMP sorting, and it was found that the sorted sperm produced significantly more wild-type pups than nonsorted sperm. Sperm sorting is much less invasive and more cost-effective than egg isolation, and offers several advantages over the existing assisted reproduction options for Niemann-Pick disease carrier couples. It therefore could have a major impact on the prevention of this and perhaps other genetic diseases.

Relationships between sterol/phospholipid composition and xenobiotic transport in nematodes

Therapeutic failure limits prophylaxis of nematode diseases and has been mainly attributed to mutations in cellular targets of anthelmintics. Besides these specific mechanisms, alterations of drug transport also occur in parasites resistant to anthelmintics and depend on both the presence of membrane pumps such as P-glycoprotein (Pgp) and on the lipid composition of membranes. We recently showed in the nematode Haemonchus contortus, using eggs as a model, that the total cholesterol (TC) concentration alters the transport of lipophilic molecules due to membrane pumps such as P-glycoprotein and the resistance to anthelmintics. The effect of TC may depend on the presence of other lipids interacting with TC. Therefore, we analysed the lipid composition and its relationship with Pgp and resistance to anthelmintics. Better correlations were found between Pgp and free cholesterol (FC) than with TC. We also showed that the relationships between lipid composition and resistance to anthelmintics or Pgp depended on the equilibrium between FC and phospholipids (PLs), mainly PLs known to be present primarily in either the external leaflets of cell membranes or the internal leaflets. The PLs phosphatidylcholine and phosphatidylethanolamine played the most significant role, but phosphatidic acid also influenced drug resistance.

Semen-coagulating protein, SVS2, in mouse seminal plasma controls sperm fertility

Mammalian seminal plasma is known to contain a decapacitation factor(s) that prevents capacitation and thus, the fertility of sperm. This phenomenon has been observed in experiments conducted in vitro that assessed the inhibition of epididymal sperm fertility by seminal plasma or by the purified decapacitation factor. However, the phenomenon of decapacitation has not yet been characterized in vivo. In the present study, we demonstrate that seminal vesicle protein secretion 2 (SVS2), which is a 40-kDa basic protein and a major component of the copulatory plug, enters the uterus and interacts with ejaculated sperm heads after copulation. The SVS2-binding region of sperm changed from the postacrosomal region to the equatorial segment, while the sperm migrated through the uterus and finally disappeared in the oviduct. Furthermore, SVS2 reduced the fertility of epididymal sperm. The sperm treated with SVS2 decreased the percentage of fertilized oocytes from 60% to 10%. The capacitation state was assessed by protein tyrosine phosphorylation and the comprehensiveness of the acrosome reaction. SVS2 functioned to maintain sperm in the uncapacitated state and to reverse capacitated sperm to the uncapacitated state. We found that the fertility of ejaculated sperm is associated with SVS2 distribution in the female reproductive tract. These results indicate that SVS2 functions as a decapacitation factor for mouse sperm.

Reorganization of mouse sperm lipid rafts by capacitation

One of the hallmarks of mammalian sperm capacitation is the loss of cholesterol from the plasma membrane. Cholesterol has been associated with the formation of detergent insoluble membrane microdomains in many cell types, and sperm from several mammalian species have been shown to contain detergent-resistant membranes (DRMs). The change in cholesterol composition of the sperm plasma membrane during capacitation raises the question of whether the contents of DRMs are altered during this process. In this study, we investigated changes in protein composition of DRMs isolated from uncapacitated or capacitated mouse sperm. TX-100 insoluble membranes were fractionated by sucrose flotation gradient centrifugation and analyzed by Western and lectin blotting, and capacitation-related differences in protein composition were identified. Following capacitation, the detergent insoluble fractions moved to lighter positions on the sucrose gradients, reflecting a global change in density or composition. We identified several individual proteins that either became enriched or depleted in DRM fractions following capacitation. These data suggest that the physiological changes in sperm motility, ability to penetrate the zona pellucida (ZP), ZP responsiveness, and other capacitation-dependent changes, may be due in part to a functional reorganization of plasma membrane microdomains.

Segregation of micron-scale membrane sub-domains in live murine sperm

Lipid rafts, membrane sub-domains enriched in sterols and sphingolipids, are controversial because demonstrations of rafts have often utilized fixed cells. We showed in living sperm that the ganglioside G(M1) localized to a micron-scale membrane sub-domain in the plasma membrane overlying the acrosome. We investigated four models proposed for membrane sub-domain maintenance. G(M1) segregation was maintained in live sperm incubated under non-capacitating conditions, and after sterol efflux, a membrane alteration necessary for capacitation. The complete lack of G(M1) diffusion to the post-acrosomal plasma membrane (PAPM) in live cells argued against the transient confinement zone model. However, within seconds after cessation of sperm motility, G(M1) dramatically redistributed several microns from the acrosomal sub-domain to the post-acrosomal, non-raft sub-domain. This redistribution was not accompanied by movement of sterols, and was induced by the pentameric cholera toxin subunit B (CTB). These data argued against a lipid-lipid interaction model for sub-domain maintenance. Although impossible to rule out a lipid shell model definitively, mice lacking caveolin-1 maintained segregation of both sterols and G(M1), arguing against a role for lipid shells surrounding caveolin-1 in sub-domain maintenance. Scanning electron microscopy of sperm freeze-dried without fixation identified cytoskeletal structures at the sub-domain boundary. Although drugs used to disrupt actin and intermediate filaments had no effect on the segregation of G(M1), we found that disulfide-bonded proteins played a significant role in sub-domain segregation. Together, these data provide an example of membrane sub-domains extreme in terms of size and stability of lipid segregation, and implicate a protein-based membrane compartmentation mechanism.

Visualization of GM1 with cholera toxin B in live epididymal versus ejaculated bull, mouse, and human spermatozoa

The organization of membrane subdomains in mammalian sperm has recently generated controversy, with several reports describing widely differing localization patterns for the ganglioside GM1. Using the pentameric B subunit of cholera toxin (CTB), we found GM1 to be restricted to the plasma membrane overlying the acrosome in the heads of live murine sperm. Interestingly, CTB had minimal binding to live bovine and human sperm. To investigate whether this difference in GM1 localization was because of species differences or differences between collection from the epididymis (mouse) or an ejaculate (bull, human), we examined epididymal bovine and human sperm. We found that GM1 localized to the plasma membrane overlying the acrosome in sperm from these species. To determine whether some component of seminal plasma was interfering with the ability of CTB to access GM1, we incubated epididymal mouse sperm with fluid from murine seminal vesicles and epididymal bull sperm with bovine seminal plasma. This treatment largely abolished the ability of the CTB to bind to GM1, producing a fluorescence pattern similar to that reported for the human. The most abundant seminal plasma protein, PDC-109, was not responsible for this loss. As demonstration that the seminal plasma was not removing GM1, sperm exposed to seminal plasma were fixed before CTB addition, and again displayed fluorescence over the acrosome. These observations reconcile inconsistencies reported for the localization of GM1 in sperm of different species, and provide evidence for the segregation of GM1 to a stable subdomain in the plasma membrane overlying the acrosome.

Effects of 2-hydroxypropyl-β-cyclodextrin and cholesterol on porcine sperm viability and capacitation status following cold shock or incubation

Porcine sperm are extremely sensitive to the damaging effects of cold shock. It has been shown that cholesterol-binding molecules, such as 2-hydroxypropyl-beta-cyclodextrin (HBCD), improve post-cooling porcine sperm viability when added to an egg yolk-based extender, but also enhance sperm capacitation in other species. The objective of this study was to determine the effects of HBCD and cholesterol 3-sulfate (ChS) on porcine sperm viability and capacitation following cold shock or incubation under conditions that support capacitation using a defined medium. We report here that porcine sperm incubated in medium containing both HBCD and ChS have significantly improved viability following cold shock (10 min at 10 degrees C) when compared to sperm incubated without HBCD or ChS, or with either component alone. Treatment with HBCD plus ChS also completely inhibited the increase in protein tyrosine phosphorylation induced by the cold shock treatment or by incubation for 3 hr under conditions that support capacitation. Two assays of sperm capacitation, the rate of calcium ionophore-induced acrosome reactions and chlortetracycline (CTC) staining, were not significantly altered by HBCD and ChS following cold shock. However, 3-hr incubation with HBCD plus ChS or with 1 mM ChS alone decreased the percentage of sperm undergoing the induced acrosome reaction without significantly affecting viability when compared to the control. These results indicate that the manipulation of sperm plasma membrane cholesterol content affects porcine sperm viability and capacitation status and could therefore be useful to protect sperm from cold shock during cryopreservation by improving viability without promoting premature capacitation.

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.

Sodium and epithelial sodium channels participate in the regulation of the capacitation-associated hyperpolarization in mouse sperm

In a process called capacitation, mammalian sperm gain the ability to fertilize after residing in the female tract. During capacitation the mouse sperm plasma membrane potential (E(m)) hyperpolarizes. However, the mechanisms that regulate sperm E(m) are not well understood. Here we show that sperm hyperpolarize when external Na(+) is replaced by N-methyl-glucamine. Readdition of external Na(+) restores a more depolarized E(m) by a process that is inhibited by amiloride or by its more potent derivative 5-(N-ethyl-N-isopropyl)-amiloride hydrochloride. These findings indicate that under resting conditions an electrogenic Na(+) transporter, possibly involving an amiloride sensitive Na(+) channel, may contribute to the sperm resting E(m). Consistent with this proposal, patch clamp recordings from spermatogenic cells reveal an amiloride-sensitive inward Na(+) current whose characteristics match those of the epithelial Na(+) channel (ENaC) family of epithelial Na(+) channels. Indeed, ENaC-alpha and -delta mRNAs were detected by reverse transcription-PCR in extracts of isolated elongated spermatids, and ENaC-alpha and -delta proteins were found on immunoblots of sperm membrane preparations. Immunostaining indicated localization of ENaC-alpha to the flagellar midpiece and of ENaC-delta to the acrosome. Incubations known to produce capacitation in vitro or induction of capacitation by cell-permeant cAMP analogs decreased the depolarizing response to the addition of external Na(+). These results suggest that increases in cAMP content occurring during capacitation may inhibit ENaCs to produce a required hyperpolarization of the sperm membrane.

Inhibition of the vacuolar H(+)-pump with bafilomycin A1 does not induce acrosome reaction or activate proacrosin in mouse spermatozoa

Acrosomal protease activation is regarded as an important event triggered by acrosomal reaction and leading to sperm passage through zona pellucida. Mammalian acrosome has an internal acid pH that probably helps to maintain inactive proenzymes that otherwise could be precociously activated and prevent normal fertilization. In this work, we have studied the effect of bafilomycin A1, a potent and specific inhibitor of vacuolar H(+)-pump (V-ATPase), on acrosome reaction and proacrosin activation. We used the pH-sensitive probe Lysotracker Green DND-26 to monitor qualitatively intra-acrosomal pH in cauda epididymal mouse spermatozoa. Our results showed that loss of Lysotracker label induced by bafilomycin A1 (acrosome alkalinization) did not induce acrosome reaction or proacrosin activation. We also developed a new technique for imaging the acrosome, and for evaluating the acrosome reaction, in live mouse spermatozoa using Lysotracker DND-26. These results showed that the V-ATPase is a key regulator of mammalian acrosome pH, and that acrosome alkalinization is not the only prerequisite to activate proacrosin under in vivo conditions. Our results suggest that acrosome alkalinization and acrosome reaction are two processes that could be independently regulated during mammalian sperm capacitation.

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.

Cholesterol-depleting compounds modulate K+-currents in Drosophila Kenyon cells

Sterol-enriched lipid rafts have been involved in Drosophila membrane signalling such as Hedgehog targeting and glutamate receptor ligand-affinity regulation. Here, we show that the voltage-dependent K(+) currents expressed by the intrinsic neurons of the Mushroom bodies are upward-modulated by compounds that remove sterols from the plasma membrane. Modulation seems to rely on a fast-exchanging sterol-pool, which more strongly affects the slowly inactivating current. Our results provide the first evidence that sterols influence the operation of voltage-gated ion channels in Drosophila neurons and strengthen the importance of lipid rafts in this biological model.

Bicarbonate and bovine serum albumin reversibly ‘switch’ capacitation-induced events in human spermatozoa

We have investigated the reversibility of biochemical and physiological changes that occur upon suspension of ejaculated human spermatozoa during in vitro capacitation. Cells were swum up in a simple HEPES-based saline [lacking bicarbonate and bovine serum albumin (BSA)], then resuspended either in supplemented Earle's balanced salt solution (sEBSS) (25 mM bicarbonate) with 0.3% BSA (for in vitro capacitation) or in medium-lacking bicarbonate and/or BSA. Progesterone-induced acrosome reaction (AR) developed during in vitro capacitation (6 h). A progesterone-induced [Ca2+]i signal was detectable in cells maintained in the simple HEPES-based saline, but upon transfer to sEBSS, the response increased three- to four-fold, saturating within <30 min. Serine/threonine phosphorylation saturated within minutes of resuspension, but tyrosine phosphorylation developed over 3 h. Return of cells to non-capacitating conditions caused reversal of all capacitation-dependent changes. The [Ca2+]i signal reverted to its 'uncapacitated' size within <30 min. Protein phosphorylation reversed gradually and could be reinduced (kinetics resembling the first response) upon resuspension in sEBSS. The ability of cells to undergo progesterone-induced AR fell to levels similar to those in uncapacitated cells within 1 h of resuspension in medium not supporting capacitation. Loss of protein phosphorylation occurred only in the absence of both bicarbonate and BSA, but effects on [Ca2+]i signalling and AR could be seen after removal of only one of these factors. We conclude that key events in the capacitation of human spermatozoa are both reversible and repeatable.

Capacitation-associated protein tyrosine phosphorylation and membrane fluidity changes are impaired in the spermatozoa of asthenozoospermic patients

Sperm protein tyrosine phosphorylation has been associated with capacitation, motility changes, zona binding, and fertilizing ability. We previously demonstrated that gradient-isolated human sperm subpopulations differ in their plasma membrane composition, their ability to phosphorylate proteins in tyrosine residues, and their capacity to undergo hyperactivation. In this study, we have characterized capacitation-associated changes in protein tyrosine phosphorylation and membrane fluidity in spermatozoa of asthenozoospermic and normozoospermic patients consulting for infertility. Semen samples were studied at baseline and after a capacitating incubation with or without the addition of a permeable cAMP analog and a phosphodiesterase inhibitor. Basic sperm and computer-assisted motion parameters, hyperactivation, protein tyrosine phosphorylation (immunofluorescence and Western blot), and membrane fluidity (fluorescent Laurdan probe) were the main study parameters. In comparison with normozoospermic and proven-fertile donor semen, asthenozoospermic samples showed lower motility, velocity, and amplitude of lateral head displacement, both originally and after a 6-h capacitating incubation. Unlike those in normal samples, asthenozoospermic spermatozoa were unable to increase protein tyrosine phosphorylation during capacitation. Such impairment, however, was overcome when they were incubated with a membrane-permeable cAMP analog and a phosphodiesterase inhibitor, indicating a possible membrane defect. Confirming this hypothesis, plasma membranes of asthenozoospermic sperm showed decreased fluidity (increased Laurdan polarization), even after a capacitating incubation. In conclusion, spermatozoa from functional asthenozoospermic samples may owe their poor motility, and their inability to properly capacitate and develop hyperactivation, to an impairment in the tyrosine phosphorylation of critical proteins caused by decreased membrane fluidity. These findings suggest a molecular pathogenetic mechanism for a common seminal pathology associated with male infertility.

Cholesterol content regulates acrosomal exocytosis by enhancing Rab3A plasma membrane association

The acrosome is an exocytic granule that overlies the spermatozoan nucleus. In response to different stimuli, it undergoes calcium-regulated exocytosis. Freshly ejaculated mammalian sperm are not immediately capable of undergoing acrosome reaction. The acquisition of this ability is called capacitation and involves a series of still not well-characterized changes in the sperm physiology. Plasma membrane cholesterol removal is one of the sperm modifications that are associated with capacitation. However, how sterols affect acrosomal exocytosis is unknown. Here, we show that short incubations with cyclodextrin, a cholesterol removal agent, just before stimulation promote acrosomal exocytosis. Moreover, the effect was also observed in permeabilized cells stimulated with calcium, indicating that cholesterol plays a direct role in the calcium-dependent exocytosis associated with acrosome reaction. Using a photo-inhibitable calcium chelator, we show that cholesterol affects an early event of the exocytic cascade rather than the lipid bilayers mixing. Functional data indicate that one target for the cholesterol effect is Rab3A. The sterol content does not affect the Rab3A activation-deactivation cycle but regulates its membrane anchoring. Western blot analysis and immunoelectron microscopy confirmed that cholesterol efflux facilitates Rab3A association to sperm plasma membrane. Our data indicate that the cholesterol efflux occurring during capacitation optimizes the conditions for the productive assembly of the fusion machinery required for acrosome reaction.

Expression of progesterone receptor(s) during capacitation and incidence of acrosome reaction induced by progesterone and zona proteins in boar spermatozoa

Sperm acrosome reaction (AR) is a prerequisite step for in vivo fertilization. In the vicinity of the oocyte, zona protein(s) (ZP) and progesterone (P4), a component of follicular fluid, are proven to be responsible for physiological AR induction. In the present study, a thorough analysis of the role of the progesterone receptor (PR) in this processing including in vitro physiological studies and biochemical isolation and characterization of the receptor protein was conducted. Following capacitation for 0, 2, 4 and 6h, pooled fertile boar semen samples (n=6) with >70% sperm motility were labeled with P4-BSA-FITC (100 microg/ml) to detect the activation of PR. Parallel sperm samples were treated with P4 (10 microg/ml) for 20 min to test AR inducing efficiency at different time points. To compare the ability of ZP and P4 to induce AR, spermatozoa capacitated in a modified medium supplemented with 1mg/ml heparin for 4h, were then treated with heat solubilized ZP (150 microg/ml), P4 (10 microg/ml) or ZP+P4 for 20 min. FITC-peanut agglutinin staining was applied to observe the disrupt acrosomal morphology. A purification protocol for crude boar sperm membrane proteins was developed based on ligand-receptor affinity chromatography procedures. The PR proteins were then identified by using mAb C262 raised against intracellular PR, combined with second antibody (SDS-PAGE, Western blotting). Their N-terminal amino acid sequence was determined. The amount of PR-activated spermatozoa was enhanced with time (onset: 27+/-5%, 2h: 41+/-4%, 4h: 49+/-3% and 6h: 52+/-4%, mean+/-S.E., n=6) as evidenced by increasing percentage of spermatozoa with completed cap fluorescent staining. In parallel sperm samples, percentages of AR induced by P4 were 9+/-2, 14+/-2, 18+/-2, and 24+/-2%, respectively. In solvent control at all time points, less than 10% spermatozoa had undergone AR. Capacitation for 4h or greater time periods resulted in optimal percentage of PR-activated and acrosome-reacted spermatozoa. After sperm incubation in heparin-medium, ZP+P4 treatment induced greater amounts of AR than either P4 or ZP alone (13+/-1% compared with 8+/-1 and 10+/-1%, P<0.01). Inducing capacity of P4 was comparable to that of ZP. The molecule weights of two apparent PR molecular masses were detected to be at Mr 74 kDa and Mr 63 kDa. N-terminal amino acid sequence of 74 kDa protein was XPXNIVLIFADXLXY, which had 78% homology to arylsulfatase A and 88% homology to 72 kDa protein from boar spermatozoa. The activation of PR is associated with the capacitating process and that appears to be required for P4-induced AR. P4 and ZP appear to be equally capable of independently inducing the AR but lack synergetic or additive effects in this induction process. Both might represent alternative pathways thus resulting in alternative systems for induction of the prerequisite acrosomal exocytosis (supported by NSC 90-2313-B-005-114; 91-2313-B-005-131).

Fertility evaluation of frozen/thawed semen

In vitro semen analyses have been used for more than half a century to estimate the fertilizing potential of a semen sample. Unfortunately, none of the assays developed provide results that consistently correlate well with fertility. The reasons for this lack of consistency, due in part to the complexity of the spermatozoon itself, the collection of fertility data, and factors beyond control of the semen analyses themselves, are discussed. Different spermatozoal attributes that are necessary for a spermatozoon to fertilize an oocyte are presented and assays used to evaluate each attribute described. Although laboratory assay results do not correlate well with semen fertility, the importance of conducting laboratory assays on every semen sample used for artificial insemination or to attempt to determine causes for infertility, is discussed.

Capacitation-dependent concentration of lipid rafts in the apical ridge head area of porcine sperm cells

Lipid architecture of the plasma membrane plays an important role in the capacitation process of the sperm cell. During this process, an increase in membrane fluidity takes place, which coincides with a redistribution of cholesterol to the apical region of the head plasma membrane and subsequently an efflux of cholesterol. Cholesterol is also a major player in the formation of lipid rafts or microdomains in the membrane. Lipid rafts favour specific protein-protein interactions by concentrating certain proteins in these microdomains while excluding others. In this study, we investigated the organization of lipid rafts during in vitro capacitation of boar sperm cells. We report on the presence of the lipid raft-specific proteins caveolin-1 and flotillin-1 in sperm cells. Capacitation induced a change in membrane distribution of these proteins. Lipid analysis on detergent-resistant membranes (DRMs) of sperm cells indicated that capacitation induces a lipid raft concentration rather than a disintegration of lipid rafts, because the total amount of lipid in the DRM fraction remained unaltered. Using a proteomic approach, we identified several major DRM proteins, including proteins involved in capacitation-dependent processes and zona pellucida binding. Our data indicate that sperm raft reorganization may facilitate capacitation-specific signalling events and binding to the zona pellucida.

Isolation and proteomic analysis of mouse sperm detergent-resistant membrane fractions: evidence for dissociation of lipid rafts during capacitation

Mammalian sperm acquire fertilization capacity after residing in the female tract during a process known as capacitation. The present study examined whether cholesterol efflux during capacitation alters the biophysical properties of the sperm plasma membrane by potentially reducing the extent of lipid raft domains as analyzed by the isolation of detergent-resistant membrane fractions using sucrose gradients. In addition, this work investigated whether dissociation of the detergent-resistant membrane fraction during capacitation alters resident sperm raft proteins. Mouse sperm proteins associated with such fractions were studied by silver staining, tandem mass spectrometry, and Western blot analysis. Caveolin 1 was identified in sperm lipid rafts in multimeric states, including a high-molecular-weight oligomer that is sensitive to degradation under reducing conditions at high pH. Capacitation resulted in reduction of the light buoyant-density, detergent-resistant membrane fraction and decreased the array of proteins isolated within this fraction, including loss of the high-molecular-weight caveolin 1 oligomers. Proteomic analysis of sperm proteins isolated in the light buoyant-density fraction identified several proteins, including hexokinase 1, testis serine proteases 1 and 2, TEX101, hyaluronidase (PH20, SPAM1), facilitated glucose transporter 3, lactate dehydrogenase A, carbonic anhydrase IV, IZUMO, pantophysin, basigin, and cysteine-rich inhibitory secretory protein 1. Capacitation also resulted in a significant reduction of sperm labeling by the fluorescent lipid-analog DiIC16, indicating that capacitation alters the liquid-ordered domains in the sperm plasma membrane. The observations that capacitation alters the protein composition of the detergent-resistant membrane fractions is consistent with the hypothesis that cholesterol efflux during capacitation dissociates lipid raft constituents, initiating signaling events that lead to sperm capacitation.

Cyclic beta-1,2-glucan is a Brucella virulence factor required for intracellular survival

Pathogenic brucella bacteria have developed strategies to persist for prolonged periods of time in host cells, avoiding innate immune responses. Here we show that the cyclic beta-1,2-glucans (CbetaG) synthesized by brucella is important for circumventing host cell defenses. CbetaG acted in lipid rafts found on host cell membranes. CbetaG-deficient mutants failed to prevent phagosome-lysosome fusion and could not replicate. However, when treated with purified CbetaG or synthetic methyl-beta-cyclodextrin, the mutants were able to control vacuole maturation by avoiding lysosome fusion, and this allowed intracellular brucella to survive and reach the endoplasmic reticulum. Fusion between the endoplasmic reticulum and the brucella-containing vacuole depended on the brucella virulence type IV secretion system but not on CbetaG. Brucella CbetaG is thus a virulence factor that interacts with lipid rafts and contributes to pathogen survival.

Macaque sperm release ESP13.2 and PSP94 during capacitation: the absence of ESP13.2 is linked to sperm-zona recognition and binding

ESP13.2 coats the entire surface of macaque sperm and remains until sperm become capacitated (Yudin et al., 2003: Biol Reprod 69: 1118-1128). Capacitation of macaque sperm is synchronized by treatment with dibutyrl cAMP (dbcAMP) and caffeine. ESP13.2 and PSP94 constituted approximately 95% of the proteins released from the sperm surface following treatment with caffeine + dbcAMP. Caffeine and dbcAMP alone induce different patterns of ESP13.2 release. As determined by ELISAs of supernatants and immuno-fluorescent labeling of sperm heads, caffeine alone and caffeine + dbcAMP induced comparable release of ESP13.2, while dbcAMP-treated sperm did not differ from controls. Sperm treated with caffeine + dbcAMP showed a reduction of ESP13.2 from the entire surface, while caffeine treatment alone induced removal of ESP13.2 from the sperm head and midpiece. As confirmed with immunofluorescence, ESP13.2 could be added back to the surfaces of sperm that had been previously exposed to caffeine. Treatment with caffeine significantly increased the number of sperm that bound tightly to the zona pellucida as compared with controls (42 +/- 9 and 13 +/- 3 sperm/zona, respectively; P < or = 0.01). This increase in binding was inhibited by "adding back" ESP13.2 to the sperm surface (12.8 +/- 3; P < or = 0.01). Alexa-conjugated anti-ESP13.2 Ig labeling of live sperm showed that only sperm lacking ESP13.2 over the head were capable of tight binding to the zona. Our results suggest that ESP13.2 masks zona pellucida ligands on the sperm surface and its release, as part of capacitation, is required for sperm-zona interaction.

Percoll Gradient-Centrifuged Capacitated Mouse Sperm Have Increased Fertilizing Ability and Higher Contents of Sulfogalactosylglycerolipid and Docosahexaenoic Acid-Containing Phosphatidylcholine Compared to Washed Capacitated Mouse Sperm1

Although Percoll gradient centrifugation has been used routinely to prepare motile human sperm, its use in preparing motile mouse sperm has been limited. Here, we showed that Percoll gradient-centrifuged (PGC) capacitated mouse sperm had markedly higher fertilizing ability (sperm-zona pellucida [ZP] binding and in vitro fertilization) than washed capacitated mouse sperm. We also showed that the lipid profiles of PGC capacitated sperm and washed capacitated sperm differed significantly. The PGC sperm had much lower contents of cholesterol and phospholipids. This resulted in relative enrichment of male germ cell-specific sulfogalactosylglycerolipid (SGG), a ZP-binding ligand, in PGC capacitated sperm, and this would explain, in part, their increased ZP-binding ability compared with that of washed capacitated sperm. Analyses of phospholipid fatty acyl chains revealed that PGC capacitated sperm were enriched in phosphatidylcholine (PC) molecular species containing highly unsaturated fatty acids (HUFAs), with docosahexaenoic acid (DHA; C22: 6n-3) being the predominant HUFA (42% of total hydrocarbon chains of PC). In contrast, the level of PC-HUFAs comprising arachidonic acid (20:4n-6), docosapentaenoic acid (C22:5n-6), and DHA in washed capacitated sperm was only 27%. Having the highest unsaturation degree among all HUFAs in PC, DHA would enhance membrane fluidity to the uppermost. Therefore, membranes of PGC capacitated sperm would undergo fertilization-related fusion events at higher rates than washed capacitated sperm. These results suggested that PGC mouse sperm should be used in fertilization experiments and that SGG and DHA should be considered to be important biomarkers for sperm fertilizing ability.

Effects of methyl-β-cyclodextrin-mediated cholesterol depletion in porcine sperm compared to somatic cells

In this study, the use of methyl-beta-cyclodextrin (MBCD) to support capacitation of sperm cells was studied. Sperm were incubated with MBCD or alternatively capacitated in an in vitro fertilization medium. The effects of these incubations on phospholipid scrambling (using merocyanin), cholesterol depletion, GM-1 localization (using cholera-toxin B (CTX)), and membrane deterioration were assessed. For comparison, this was also tested in MBCD-treated MDCK cells. In MDCK cells, upto 71% of cholesterol was depleted, which coincided with a more diffuse CTX staining without any obvious effects on cell viability. In sperm, a similar depletion of 53% cholesterol was found after a 10 mM MBCD treatment. However, no merocyanin response was observed in viable sperm after MBCD treatments (indicating a lack of membrane changes associated with sperm capacitation). In contrast to MDCK, cells >1 mM MBCD caused plasma membrane disintegration and rendered sperm immotile. At higher concentrations also acrosome disruption was noted. CTX staining was absent at < 0.1 mM MBCD incubations but appeared at higher MBCD levels and was found to be specific for deteriorated cells that showed morphological signs of acrosome disruption. No significant plasma membrane deterioration, acrosome disruption, and sperm immotility nor CTX staining and only a modest (< 15%) cholesterol depletion were observed in conventionally capacitated sperm, where 40% of the intact sperm showed merocyanin staining. Taken together, the results indicate that membranes of sperm are more sensitive to MBCD-mediated cholesterol depletion than MDCK cells and that the use of MBCD to support sperm capacitation cannot be recommended due to its spermicidal effects.

Characterisation of the progesterone receptor on canine spermatozoa

The present study was conducted to characterise and localise the progesterone receptor (PR) on canine spermatozoa. Using a progesterone–bovine serum albumin–fluorescein isothiocyanate conjugate (PBF) and different monoclonal antibodies (C262 and NCL-PGR against the steroid binding domain and N-terminus of intracellular PR, respectively, and h151 against the hinge domain of the intracellular oestrogen receptor), the PR was identified on the plasma membrane over the acrosomal region. Two proteins (54 kDa and 65 kDa) were detected by recognition of the three monoclonal antibodies using Western blotting. PBF labelling was observed in the majority of cauda epididymal spermatozoa (63 ± 4%), but this labelling was markedly reduced (33 ± 17%) after the addition of canine seminal plasma. Over a 7-h capacitation, the proportion of ejaculated spermatozoa exhibiting PBF labelling (indicating the presence of the PR) increased from 18 ± 10% (onset) to 59 ± 7% by 5 h, where it plateaued. Progesterone (P4) induced the acrosome reaction (AR) in a dose-dependent manner (0, 0.1, 1 and 10 µg/mL P4 corresponding to 10 ± 5%, 16 ± 9%, 23 ± 7% and 30 ± 7%). Pre-treatment of capacitated spermatozoa with canine seminal plasma reduced the incidence of the P4-induced AR (12 ± 5%). In addition, treatment with the monoclonal antibodies significantly reduced the incidence of the P4-induced AR (10 µg/mL) in capacitated ejaculated spermatozoa from 19 ± 6% to 11 ± 4% (h151, 1 : 10) and 12 ± 6% (C262, 1 : 10), respectively. A typical Scatchard plot revealed one binding with high affinity and low capacity, and another binding with low affinity and high capacity, suggesting at least two different characteristic PR. Taken together, these results demonstrate that P4 induced the AR in a dose-dependent manner via functional transmembranal receptors in the acrosomal region of the canine sperm plasma membrane. The characteristics of this membrane receptor seem similar to those of other mammalian spermatozoa, and it shows structural homology to the intracellular PR.

Changes in sperm quality and lipid composition during cryopreservation of boar semen

Egg yolks are commonly used in diluents in order to improve the freezability of semen. Two aspects of the role of lipids in boar semen freezability are reported in this article. The first one concerns the eventual exchanges of lipid components between the spermatozoa and the yolk-based diluent during cryopreservation. Two types of yolk have been considered as ingredients in diluents for cryopreservation: yolks with a standard fatty acid composition and yolks enriched in docosahexaenoic acid (DHA). The relation between lipid exchanges and the quality of fresh semen is considered. The other aspect concerns the possibility to enhance the freezability of boar spermatozoa by altering the plasma membranes under the influence of dietary fatty acids. Spermatozoa were damaged significantly by the cryopreservation cycle in all experiments. Spermatozoa with the best fresh quality had accumulated the largest quantity of lipids upon thawing. A general decrease in the proportion of polyunsaturated fatty acids was observed after thawing. The yolks enriched in n-3 fatty acids failed to improve the quality of sperm following cryopreservation. The proportion of DHA was significantly higher in spermatozoan phospholipids from thawed cells that had been in contact with n-3 yolks. A significant reduction in cholesterol was observed in spermatozoa after the cryopreservation cycle, which correlated with an increased number of acrosome-reacted cells and changes in the parameters of motility. The addition of 3% fish oil to the daily boar ration significantly increased the content of DHA (from 33 to 45% of the total fatty acids) in the spermatozoa. Ejaculate concentrations were significantly increased in the experimental group. DHA-enriched semen did not show improved freezability, at least not as assessed by in vitro parameters.

Bicarbonate-induced membrane processing in sperm capacitation

During capacitation, major changes take place in the sperm plasma membrane so as to render it fusogenic and responsive to zona pellucida glycoproteins. However, the mechanisms involved have not been defined. As bicarbonate is known to be the key component that induces capacitation, we have investigated the bicarbonate-dependent changes in the boar sperm's plasma membrane architecture. We have discovered that bicarbonate induces a rapid collapse of phospholipid transverse asymmetry, exposing phosphatidylethanolamine and phosphatidylserine at the outer surface of the lipid bilayer. The collapse, which is reversible, is brought about as a result of activation of the phospholipid scramblase that exchanges phospholipids in a non-specific fashion between the two leaflets of the lipid bilayer. The activation takes place via a cyclic AMP-protein kinase A-dependent pathway and is initiated via stimulation of the so-called 'soluble' adenylyl cyclase in the sperm cell by bicarbonate. As a result of the collapse and the concurrent increase in phospholipid exchange, removal of cholesterol by albumin is facilitated (perhaps due to increased lipid packing disorder). This finding is in conflict with earlier surmises that cholesterol loss precedes activation of the cyclic AMP-protein kinase A axis. We have noted that not all cells in a given sperm population show rapid changes in response to bicarbonate stimulation; samples from individual boars also differ in their response. Maturation differences between cells have been found to play an important role in such functional heterogeneity.

Prostasome antigens as targets for sperm agglutinating antibodies demonstrated by 1-D gel electrophoresis and immunoblottings

Many immunoinfertile men have sperm agglutinating antibodies that are directed against prostasome-derived antigens, but these antigens have not been defined so far. We selected serum samples with high ELISA titres against prostasomes from a group of immunoinfertile patients with sperm agglutinating antibodies and used the sera for immunoblottings on 1-D SDS-PAGE of prostasomes and sperm cells. The immunoblottings with individual antiprostasome antisera on 1-D SDS-PAGE of prostasomes, revealed three to 10 bands for each serum. Eighty-five per cent of the serum samples contained bands in the 70-75 kDa region and 80% of the samples contained bands in the 50-55 kDa region. Immunoblottings of extracted sperm cells, revealed one to six bands in the molecular weight range 25-82 kDa and two of the samples recognized two bands with molecular weights (50 and 43 kDa) similar to immunoblottings of prostasomes. The prostasomal antigens recognized by the high titre-antisera of immunoinfertile men were generally different from the sperm antigens recognized by the same sera. This suggests that prostasomes offer a new set of antigens available for research on male immunoinfertility and immunocontraception.

Regulation of T-cell receptor signalling by membrane microdomains

There is now considerable evidence suggesting that the plasma membrane of mammalian cells is compartmentalized by functional lipid raft microdomains. These structures are assemblies of specialized lipids and proteins and have been implicated in diverse biological functions. Analysis of their protein content using proteomics and other methods revealed enrichment of signalling proteins, suggesting a role for these domains in intracellular signalling. In T lymphocytes, structure/function experiments and complementary pharmacological studies have shown that raft microdomains control the localization and function of proteins which are components of signalling pathways regulated by the T-cell antigen receptor (TCR). Based on these studies, a model for TCR phosphorylation in lipid rafts is presented. However, despite substantial progress in the field, critical questions remain. For example, it is unclear if membrane rafts represent a homogeneous population and if their structure is modified upon TCR stimulation. In the future, proteomics and the parallel development of complementary analytical methods will undoubtedly contribute in further delineating the role of lipid rafts in signal transduction mechanisms.

Reorganization of Lipid Rafts During Capacitation of Human Sperm1

Ejaculated mammalian sperm must complete a final maturation, termed capacitation, before they can undergo acrosomal exocytosis and fertilize an egg. In human sperm, loss of sperm sterol is an obligatory, early event in capacitation. How sterol loss leads to acrosomal responsiveness is unknown. These experiments tested the hypothesis that loss of sperm sterol affects the organization of cold detergent-resistant membrane microdomains (lipid "rafts"). The GPI-linked protein CD59, the ganglioside GM1, and the protein flotillin-2 were used as markers for lipid rafts. In uncapacitated sperm, 51% of the CD59, 41% of the GM1, and 90% of the flotillin-2 were found in the raft fraction. During capacitation, sperm lost 67% of their 3beta-hydroxysterols, and the percentages of CD59 and GM1 in the raft fraction decreased to 34% and 31%, respectively. The distribution of flotillin-2 did not change. Preventing a net loss of sperm sterol prevented the loss of CD59 and GM1 from the raft fraction. Fluorescence microscopy showed CD59 and GM1 to be distributed over the entire sperm surface. Flotillin-2 was located mainly in the posterior head and midpiece. Patching using bivalent antibodies indicated that little of the GM1 and CD59 was stably associated in the same membrane rafts. Likewise, GM1 and flotillin-2 were not associated in the same membrane rafts. In summary, lipid rafts of heterogeneous composition were identified in human sperm and the two raft components, GM1 and CD59, showed a partial sterol loss-dependent shift to the nonraft domain during capacitation.

Immunolocalization of bovine sperm protease BSp120 by light and electron microscopy during capacitation and the acrosome reaction: Its role in in vitro fertilization

Mammalian fertilization involves various steps in which the participation of specific enzymes has been demonstrated by numerous studies. Acrosin is one of the most widely acrosomal protease in mammalian spermatozoa studied, including bovine; however, other proteases have also been described. A new trypsin-like serine protease named bovine serine protease of 120 kDa (BSp120) and its pre-cursor BSp66 (66 kDa) were identified in bovine spermatozoa. Cytological and ultrastructural immunolocalization studies on BSp120 were performed in live and fixed cells. Immunoflorescence assays with specific polyclonal antibodies revealed localization of BSp120 on the sperm head, with a signal homogeneously distributed over the acrosome resembling a horseshoe. After the acrosome reaction, sperm showed a patchy pattern in the acrosomal cap. Immune electron microscopy analysis indicated that BSp120 is located over the head plasma membrane of capacitated spermatozoa and acrosome reacting spermatozoa. To assess BSp120 function in sperm-oocyte interaction, in vitro fertilization studies were conducted. Oocytes were incubated with spermatozoa pre-treated with anti-BSp120, anti-guinea pig acrosin, and anti-BSp120 plus anti-guinea pig acrosin. Pre-treatment of bovine spermatozoa with antibodies towards each protein did not significantly modify fertilization rates. However, when both anti-acrosin and anti-BSp120 antibodies were simultaneously added, there was a significant decrease in the fertilization rate, suggesting that both enzymes may be required for fertilization. Altogether, the results from the present study described the localization of BSp120 over the acrosome of bovine sperm, and suggest its involvement in fertilization.

Effects of PDC-109 on bovine sperm functional activity in presence or absence of heparin

PDC-109, a heparin-binding protein (from the seminal vesicles) that binds to sperm surface phospholipids at ejaculation, may modulate several aspects of sperm activity. The objectives of the present study were to determine: (1) in the presence or absence of heparin, the effects of exogenous PDC-109 on sperm motility (Makler chamber), viability (Hoechst 33258) and membrane functional integrity (hypoosmotic swelling test) of bovine spermatozoa; (2) the role of PDC-109 as a capacitation-inducing factor; and (3) its ability to induce the acrosome reaction (fluorescein staining). After 4-h capacitation in the presence of heparin, the addition of PDC-109 (0.5, 1.5 or 3.0mg/ml) significantly decreased the percentages of motile, progressive, and viable cells; these effects were also detected in the absence of heparin. However, PDC-109 elicited a twofold increase (from 14 to 28%) in the proportion of acrosome-reacted spermatozoa, but only in the presence of heparin. Progesterone (10 microM) or angiotensin II (100 or 1000 nM) stimulated the acrosome reaction after capacitation in the presence of PDC-109 without heparin (from 10 to 17, 23 and 22%, respectively). In conclusion, PDC-109 appears to modulate sperm functional activity, with some effects manifest in the absence of heparin.

Bicarbonate and its role in mammalian sperm function

This paper deals with the effects of bicarbonate induced signaling pathways on plasma membrane lipid organization and downstream protein signaling, and their role in sperm-egg interactions. It also provides an overview of results that indicate that bicarbonate responses are not related to cell death or apoptosis. The information presented shows that only those sperm cells that have functionally completed maturation in the epididymis are sensitive to bicarbonate whereas immature sperm fail to respond to this physiological challenge. Therefore, it is important to selectively analyze the responsive sperm subpopulations when studying sperm capacitation. Moreover, bicarbonate induced signaling responses differ within the diverse sperm structures (e.g. the tail versus the head). Consequently, dissecting sperm structures and signaling areas from each other deserves more attention in sperm capacitation research. The information discussed was obtained from a variety of mammalian species but the basic bicarbonate-mediated sperm responses are similar in most Eutherian species despite some species to species variations (most notably in kinetics rather than the sequence of events). The objective of the paper is to provide a comparative experimental overview of bicarbonate mediated sperm capacitation in the hope that this information will lead to a better understanding of the complex biochemical nature of the involvement of bicarbonate in mammalian sperm capacitation.

A flow cytometric assay for global estimation of tyrosine phosphorylation associated with capacitation of spermatozoa from two marsupial species, the tammar wallaby (Macropus eugenii) and the brushtail possum (Trichosurus vulpecula)

The phosphorylation of tyrosine residues in cellular proteins is a major signal transduction event during sperm capacitation. In this study protein phosphorylation was monitored using a fluorescein isothiocyanate (FITC)-labeled antiphosphotyrosine monoclonal antibody and a flow cytometric procedure optimized for sperm. Using this technique, the correlation between tyrosine phosphorylation and sperm capacitation was examined in two marsupial species, the brushtail possum and the tammar wallaby and compared with that of ram spermatozoa. The levels of tyrosine phosphorylation in sperm from all three species were increased by the addition of cyclic AMP (cAMP) and vandate, a phosphotyrosine phosphatase inhibitor and were decreased by the addition of the phosphotyrosine kinase inhibitor, staurosporine. Oviductal conditioned media (CM) induced a progressive increase in tyrosine phosphorylation in both marsupial species and also induced morphological transition from a streamlined to a 'T'-shape configuration in brushtail possum spermatozoa but not in tammar wallaby spermatozoa. Transition to the 'T'-shape orientation associated with capacitation in marsupial spermatozoa was observed by 2 h of incubation in both species when tyrosine phosphorylation was increased by higher levels of cAMP i.e. 5 mM dibutyryl cAMP plus 3 mM pentoxyphylline. Thus the tyrosine phosphorylation trigger with CM may differ in these two marsupial species. Ram sperm tyrosine phosphorylation could be increased by addition of lower levels of cAMP (1 mM). These results support the finding that tyrosine phosphorylation is associated with sperm capacitation in marsupials. Similar results were obtained by using SDS PAGE/Western blot analysis of tyrosine phosphorylation in the brushtail possum spermatozoa. The specificity, efficiency and sensitivity of the procedure described here make it applicable for routine assessment of capacitation in large numbers of samples and in other species.

Effect of adding cholesterol to bull sperm membranes on sperm capacitation, the acrosome reaction, and fertility

When cholesterol is added to sperm membranes before cryopreservation, higher percentages of motile and viable cells are recovered after thawing. However, because one of the first steps in sperm capacitation is cholesterol efflux from the sperm plasma membrane, adding cholesterol to enhance cryosurvival may retard sperm capacitation. These studies evaluated the ability of sperm treated with cholesterol-loaded cyclodextrins (CLC) to capacitate, acrosome react, and fertilize oocytes. Control (non-CLC-treated) and CLC-treated sperm were treated with heparin, dilauroylphosphatidylcholine (PC12), or calcium ionophore A23187 (A23187) to capacitate and induce the acrosome reaction. Sperm capacitation, assessed by an increase in intracellular calcium level, and acrosome-reacted sperm were measured using flow cytometry. Fresh CLC-treated sperm cells underwent capacitation and/or the acrosome reaction at rates different from control samples, and the differences detected were dependent on the method used to induce sperm capacitation and the acrosome reaction. After cryopreservation, however, CLC-treated and control sperm underwent capacitation and the acrosome reaction at similar rates regardless of the method used to induce capacitation and the acrosome reaction. The primary concern for CLC-treated sperm, however, is whether this treatment would affect in vitro or in vivo fertility. Adding either control or CLC-treated cryopreserved sperm to bovine oocytes in vitro resulted in similar oocyte cleavage rates and blastocyst formation rates. In addition, when inseminated into heifers, pregnancy rates for control and CLC-treated sperm were also similar. Therefore, treating bull sperm with CLC permits greater numbers of sperm to survive cryopreservation while preserving the fertilizing potential of each individual sperm.

Cholesterol efflux alters lipid raft stability and distribution during capacitation of boar spermatozoa

A reduction in plasma membrane cholesterol is one of the early events that either triggers or is closely associated with capacitation of mammalian spermatozoa. In this investigation, we have examined the effects of cholesterol efflux on tyrosine phosphorylation, lipid diffusion, and raft organization in boar spermatozoa. Results show that a low level of cholesterol efflux, mediated by 5 mM methyl-beta-cyclodextrin (MBCD), enhances capacitation and induces phosphorylation of two proteins at 26 and 15 kDa without affecting sperm viability. Lipid diffusion rates under these conditions are largely unaffected except when cholesterol efflux is excessive. Low-density Triton X100-insoluble complexes (lipid rafts) were isolated from spermatozoa and found to have a restricted profile of proteins. Capacitation-associated cholesterol efflux has no effect on raft composition, but cholesterol depletion destabilizes them completely and phosphorylation is suppressed. During MBCD-mediated capacitation, the distribution of GM1 gangliosides on spermatozoa changes in a sequential manner from overlying the sperm tail to clustering on the sperm head. It is concluded that there is a safe window for removal of plasma membrane cholesterol from spermatozoa within which protein phosphorylation and polarized migration of lipid rafts take place. A preferential loss of cholesterol from the nonraft pool may be the stimulus that promotes raft clustering over the anterior sperm head.

Cholesterol depletion induces PKA-mediated basolateral-to-apical transcytosis of the scavenger receptor class B type I in MDCK cells

Cholesterol-based membrane microdomains, or lipid rafts, are believed to play important, yet poorly defined, roles in protein trafficking and signal transduction. In polarized epithelial cells, the current view is that rafts are involved in apical but not in basolateral protein transport from the trans-Golgi network (TGN). We report here that cholesterol is required in a post-TGN mechanism of basolateral regionalization. Permanently transfected Madin-Darby canine kidney cells segregated the caveolae/raft-associated high-density lipoprotein scavenger receptor class B type I (SR-BI) predominantly to the basolateral domain where it was constitutively internalized and recycled basolaterally. Acute cholesterol depletion did not significantly alter SR-BI internalization, implying a cholesterol depletion-insensitive endocytic process but instead induced its transcytosis through a protein kinase A (PKA)- and microtubule-dependent mechanism. Forskolin also elicited SR-BI transcytosis. The basolateral distribution of endogenous epidermal growth factor receptor remained unaffected. Strikingly, cholesterol depletion induced PKA activity without increasing the cAMP levels. Thus, our results are consistent with a scenario in which cholesterol-based rafts promote internalization and basolateral recycling of internalized SR-BI whereas a PKA pool sensitive to cholesterol depletion mediates SR-BI transcytosis. Regulated transcytosis of SR-BI may provide an additional mechanism to control cholesterol homeostasis. These results disclose relationships between cholesterol-based rafts and PKA activity operating in a post-TGN mechanism of regulated apical-to-basolateral cell surface protein distribution.

Regulation of capacitation of canine spermatozoa during co-culture with heterologous oviductal epithelial cells

Progress of essential steps of the capacitation is coordinated in the oviductal isthmus, where sperm are stored in close contact with the epithelium. A crucial capacitational event is the phosphorylation of sperm membrane proteins. Regulation of the tyrosine phosphorylation by the oviduct has not been examined in dog sperm yet. The aim of this work was to study the effect of dog sperm binding to porcine oviductal epithelium on capacitation-induced cellular and molecular changes. Epithelial cells were stripped from the oviducts of post-puberal sows and cultured for 5-7 days at 39 degrees C and 5% CO2 on Biomatrix-covered Chamber slides. Sperm washed through Percoll was co-incubated with the oviductal epithelium cell cultures in a bicarbonate Tyrode's medium. During co-incubation, sperm membrane changes, the state of tyrosine phosphorylation and motility were determined after 3, 30, 90, 180, 240 and 360 min. Significant increases in the percentage of capacitated and dead cells were observed in unbound sperm, while bound sperm remained uncapacitated, live and motile. An increasing tyrosine phosphorylation of tail proteins in bound, unbound and control sperm suspensions and a subsequent phosphorylation of head proteins in unbound and control sperm suspensions were observed. A significant difference regarding head phosphorylation (p < 0.05) was found between sperm bound to oviductal epithelium and unbound sperm. Binding occurred mainly in sperm with non- phosphorylated heads, while higher proportions of phosphorylated cells were found in unbound populations. The head phosphorylation progressed significantly during incubation in unbound spermatozoa (p < 0.05); however, it was suppressed in population of sperm attached to oviductal epithelium. Significant correlations between motility parameters related to hyperactivation and tail phosphorylation were found in unbound sperm. These observations support the hypothesis that spermatozoa with non-phosphorylated heads preferentially attach to epithelial cells. It can be concluded that tyrosine phosphorylation of head membrane proteins and capacitation are delayed in canine spermatozoa being in closed contact with oviductal epithelium.

Rapid PKA-catalysed phosphorylation of boar sperm proteins induced by the capacitating agent bicarbonate

In boar spermatozoa, the capacitating agent bicarbonate has been shown to induce rapid changes both in plasma membrane lipid architecture and in motility; in each case, a PKA-dependent pathway is involved. Early bicarbonate-induced changes in protein phosphorylation were probed using a commercial antibody against the phosphorylated form of the consensus substrate site for cyclic AMP-dependent protein kinase. The antibody detected relatively few bands in sperm extracts, of which only a small number showed incubation-dependent changes. While the quantitative response varied between boar ejaculates, in general terms bicarbonate induced phosphorylation increases in bands of 96, 64, and 59 kDa within 80 sec. The changes reached a maximum after about 160 sec, declined somewhat thereafter, and then increased again slowly as incubation progressed further (up to 21 min). The bicarbonate-induced increases were strongly dependent on the presence of BSA in the incubation medium. They were inhibited by H89 (PKA inhibitor) but not by GF (PKC inhibitor), and were enhanced by papaverine (phosphodiesterase inhibitor) and by calyculin (protein phosphatase inhibitor). The cyclic AMP analogue cBIMPS was able to mimic bicarbonate action though its effect was less dramatic. Stearated Ht31, a permeable inhibitor of PKA's binding to A-kinase anchoring protein, did not affect either the intensity or the specificity of the bicarbonate-induced phosphorylation changes, though it blocked motility entirely. Immunocytochemical studies revealed marked bicarbonate-dependent phosphorylation changes in the post-acrosomal region of the head and in the neck, midpiece, and anterior regions of the tail. Fractionation of stimulated spermatozoa showed that all bands detectable with the antibody were bound to heads and to midpieces and associated large tail fragments; no bands were detected in either small tail or membrane fragments or in the cytoplasmic fraction. Differential extraction of the midpiece/large tail fraction revealed two protein bands with closely similar electrophoretic mobilities to the 96- and 59-kDa phosphorylated bands; MALDI-TOF analyses of these bands revealed both to be members of the Odf2 family.

Expression of gp20, a human sperm antigen of epididymal origin, is reduced in spermatozoa from subfertile men

gp20, a sialylglycoprotein of human sperm homologous to CD52, is present everywhere on the surface of the freshly ejaculated sperm but is prevalently localized in the equatorial region of the head of capacitated sperm. In the present study, we confirmed this feature on large scale and correlated equatorial exposure of the antigen to the presence of serum albumin (SA) in the capacitation medium. Furthermore, we analyzed the relationship between the presence of the antigen and its equatorial exposure after capacitation and fertility, by comparing immunostaining for gp20 in the motile fraction of spermatozoa from fertile and subfertile men. A significantly higher percentage of nonimmunostained spermatozoa before capacitation (38.5% +/- 23 vs. 12% +/- 7, P < 0.0001) and a lower increase in the percentage of sperm with equatorial localization after capacitation (19.3% +/- 25 vs. 34.6% +/- 22, P = 0.039) were observed in subfertile men (n = 60) compared to fertile men (n = 15). In the whole study group, a positive correlation was also found between the percentage of spermatozoa exhibiting equatorial localization in capacitated samples and normal head forms (R = 0.50; P < 0.0001).

Sperm from beta1,4-galactosyltransferase I-null mice exhibit precocious capacitation

Mammalian sperm must undergo a physiological maturation, termed capacitation, before they are able to fertilize eggs. Despite its importance, the molecular mechanisms underlying capacitation are poorly understood. In this paper, we describe the capacitation phenotype of sperm lacking the long isoform of beta1,4-galactosyltransferase I (GalT I), a sperm surface protein that functions as a receptor for the zona pellucida glycoprotein, ZP3, and as an inducer of the acrosome reaction following ZP3-dependent aggregation. As expected, wild-type sperm must undergo capacitation in order to bind the zona pellucida and undergo a Ca(2+) ionophore-induced acrosome reaction. By contrast, GalT I-null sperm behave as though they are precociously capacitated, in that they demonstrate maximal binding to the zona pellucida and greatly increased sensitivity to ionophore-induced acrosome reactions without undergoing capacitation in vitro. The loss of GalT I from sperm results in an inability to bind epididymal glycoconjugates that normally maintain sperm in an 'uncapacitated' state; removing these decapacitating factors from wild-type sperm phenocopies the capacitation behavior of GalT I-null sperm. Interestingly, capacitation of GalT I-null sperm is independent of the presence of albumin, Ca(2+) and HCO(3)(-); three co-factors normally required by wild-type sperm to achieve capacitation. This implies that intracellular targets of albumin, Ca(2+) and/or HCO(3)(-) may be constitutively active in GalT I-null sperm. Consistent with this, GalT I-null sperm have increased levels of cAMP that correlate closely with both the accelerated kinetics and co-factor-independence of GalT I-null sperm capacitation. By contrast, the kinetics of protein tyrosine phosphorylation and sperm motility are unaltered in mutant sperm relative to wild-type. These data suggest that GalT I may function as a negative regulator of capacitation in the sperm head by suppressing intracellular signaling pathways that promote this process.

Decrease in order of human sperm lipids during capacitation

Ejaculated mammalian sperm must undergo a final maturation (capacitation) before they can acrosome-react and fertilize eggs. Loss of the sperm sterols, cholesterol and desmosterol, is an obligatory step in the capacitation of human sperm. Because sterols can increase the order of membrane phospholipids, it has been suggested that the importance of sterol loss is that it decreases membrane lipid order. The present study tested the hypotheses that sterol loss decreases sperm membrane lipid order during capacitation and that lipid disorder is a sufficient stimulus for capacitation. Steady-state fluorescence anisotropy of the membrane probe, 1,6-diphenyl-1,3,5-hexatriene, decreased during capacitation, indicating a decrease in lipid order. The decrease was dependent on the loss of sperm sterols, suggesting that it reflected diminished sterol-mediated phospholipid ordering. However, the lipid-fluidizing agents, benzyl alcohol and 2-(2-methoxyethoxy)ethyl 8-(cis-2-n-octylcyclopropyl) octanoate, did not cause sperm capacitation or overcome inhibition by cholesterol. In summary, loss of sperm sterols caused a significant decline in lipid order during capacitation; however, decreased bulk lipid order was not sufficient to trigger the subsequent events that complete capacitation.

Progesterone receptor availability in mouse spermatozoa during epididymal transit and capacitation: ligand blot detection of progesterone-binding protein

The goals of the present study were to determine the availability of progesterone (P4) receptor (P4r) in mouse sperm during maturation and capacitation and to make the first steps toward a characterization of P4r. It has been proposed that P4 is able to induce an acrosomal reaction (AR) by using a membrane P4r. This induction was verified in sperm isolated from the cauda epididymis (fully mature) when incubated in specific conditions that capacitate sperm. First, we set up the conditions in our laboratory to induce an AR in mature and capacitated sperm triggered by P4 that was detected by a chlortetracycline (CTC) assay. Then, we examined sperm isolated from the caput epididymis (immature) incubated under conditions that support cauda sperm capacitation and found that the AR could not be detected. Moreover, P4 was unable to induce the AR when it was applied to sperm isolated from either region and incubated under conditions that did not support capacitation. These results can be explained by changes in P4r availability. A suitable marker for P4r is the gold (Au)-P4 complex. This marker shows a binding capacity that can be visualized directly by electron microscopy (EM) and indirectly by silver-enhanced methods with light microscopy. The Au-P4 complex was localized in capacitated cauda sperm at the dorsal edge of the head. Using these techniques, we observed a significant decrease in both noncapacitated cauda sperm and caput sperm (whether incubated in capacitating media or not). Genomic P4r could be responsible for the signal detected, but antibodies against the P4 nuclear receptor did not recognize any sites in the sperm by immunostaining methodology. Instead, a 44-kd protein band was detected in the sperm by a ligand blot assay. In conclusion, P4 promotes the AR in capacitated cauda sperm but is unable to do so in noncapacitated or immature sperm because the availability of P4r increases during epididymal transit and after capacitation. The P4r responsible for this behavior is different from a classical nuclear receptor-on the basis of the immunostaining results-and is probably a protein close to 44 kd-on the basis of the ligand assay results.

Endogenous redox activity in mouse spermatozoa and its role in regulating the tyrosine phosphorylation events associated with sperm capacitation

We investigated the role of endogenous redox activity in regulating the signal transduction pathway leading to tyrosine phosphorylation in mouse spermatozoa. Endogenous redox activity was monitored using a luminol-peroxidase chemiluminescent probe. Chemiluminescence increased in spermatozoa that were actively undergoing cAMP-mediated tyrosine phosphorylation events associated with capacitation and was inhibited in a dose-dependent manner by addition of catalase or diphenylene iodonium, both of which also inhibited tyrosine phosphorylation within the cell at points downstream of cAMP. Excluding bicarbonate from the incubation medium reduced the redox activity of sperm by 80-90% and dramatically reduced tyrosine phosphorylation. This study provides the first evidence that tyrosine phosphorylation associated with capacitation in mouse spermatozoa is redox regulated by a flavinoid-containing enzyme involving mediation by hydrogen peroxide. Bicarbonate regulated the redox activity of mouse spermatozoa, and this regulation may contribute to the impact of this anion on tyrosine phosphorylation during capacitation of mouse spermatozoa.

Caspase-independent exposure of aminophospholipids and tyrosine phosphorylation in bicarbonate responsive human sperm cells

Only capacitated sperm cells are able to fertilize egg cells, and this process is triggered by high levels of bicarbonate. Bicarbonate renders the plasma membrane more fluid, which is caused by protein kinase A (PKA)-mediated alterations in the phospholipid (PL) bilayer. We studied exposure of phosphatidylserine (PS) and phosphatidylethanolamine (PE) in human sperm cells. Surface exposure of PS and PE on sperm cell activation in vitro was found to be bicarbonate dependent and restricted to the apical area of the head plasma membrane. The PL scrambling in bicarbonate-triggered human sperm was not related to apoptosis, because the incubated cells did not show any signs of caspases or degeneration of mitochondria or DNA. The PL scramblase (PLSCR) gene family has been implicated in this nonspecific, bidirectional PL movement. A 25-kDa isoform of PLSCR was identified that was homogeneously distributed in human sperm cells. We propose that compartment-dependent activation of PKA is required for the surface exposure of aminophospholipids at the apical plasma membrane of sperm cells. Bicarbonate-induced PL scrambling appears to be an important event in the capacitation process, because the entire intact scrambling sperm subpopulation showed extensive tyrosine phosphorylation, which was absent in the nonscrambling subpopulation. The proportion of live cells with PL scrambling corresponded with that showing capacitation-specific chlortetracyclin staining.

Specific order in the appearance of protein tyrosine phosphorylation patterns is functionally coordinated with dog sperm hyperactivation and capacitation

The aims of the present study were to characterize a slow capacitation system that records initial changes in the sperm membrane state, and, using a canine model, to order the specific protein tyrosine phosphorylation signaling in the sequence of capacitational events and to associate them with hyperactivated motility. Dog sperm washed through Percoll were incubated in complete bicarbonate Tyrode medium for 6 hours in 5% CO(2). Capacitation was evaluated using chlortetracycline staining. Tyrosine phosphorylation patterns were assessed by immunocytochemistry. Parallel to this, a computer-assisted motility analysis was performed. Significant changes in the percentage of capacitated and acrosome-reacted cells were first observed after 90 minutes, increasing in a linear manner during further incubation (P <.05). Changes in the percentage of capacitated cells were accompanied by motility changes. During incubation, a strictly sequential phosphorylation of sperm tail (midpiece, principal piece, and end piece) and head proteins was observed. According to an analysis of kinetics, phosphorylation of head proteins occurred after the tail became completely phosphorylated. Changes in head phosphorylation progressed at the same rates as capacitation and acrosome reaction. Sperm motility, curvilinear velocity, average path velocity, straight line velocity, and lateral head displacement were correlated positively or negatively with phosphorylation of midpiece or end piece proteins, respectively. The bicarbonate-stimulated increases in cyclic adenosine monophosphate levels and changes in protein phosphatase activity may be involved in the signaling system that controls membrane changes and motility in dog sperm. Phosphorylation kinetics of sperm proteins are potentially useful for diagnostic purposes to characterize the response of individual males to fertilizing conditions.

Cyclic AMP signalling during mammalian sperm capacitation--still largely terra incognita

Cyclic AMP is known to play a major role in intracellular signalling during mammalian sperm capacitation. However, despite much research, many of the molecular details of cyclic AMP's involvement remain obscure. In this review, I discuss the following aspects, presenting some original data as illustration where relevant. With respect to cyclic AMP synthesis, uncertainties exist as to the number of forms of adenylyl cyclase that are present in the spermatozoon, whether they are cytosolic or bound to subcellular structures, and which physiological effectors they respond to (e.g. bicarbonate, Ca2+, or receptor-coupled G-proteins). While net intracellular levels of cyclic AMP in spermatozoa depend upon the relative activities of adenylyl cyclase and phosphodiesterase, there are wide between-sample variations within species, both in basal levels and in levels attained after activation of the cyclase (e.g. after sperm treatment with bicarbonate). Moreover, minor changes in bulk cyclic AMP levels can result in large changes in cyclic AMP-dependent functions. Finally, while cyclic AMP levels respond very rapidly to sperm treatment by effectors such as bicarbonate and Ca2+ (key components of capacitating media), there are big discrepancies between the rates of functional response. For example, enhancement of motility and collapse of phospholipid asymmetry take place within a few minutes, whereas more than 1 h of exposure to capacitating conditions is needed for cyclic AMP-dependent protein tyrosine phosphorylation to become detectable or for the sperm population to attain a capacitated state.