The extragenomic action of progesterone on human spermatozoa is influenced by redox regulated changes in tyrosine phosphorylation during capacitation

Role of hydrogen peroxide in sperm capacitation and acrosome reaction

The generation of reactive oxygen species (ROS) has been implicated in the regulation of sperm capacitation and acrosome reaction; however, the mechanisms underlying this regulation remain unclear. To examine the cellular processes involved, we studied the effect of different concentrations of hydrogen peroxide (H(2)O(2)) on protein tyrosine phosphorylation under various conditions. Treatment of spermatozoa with H(2)O(2) in medium without heparin caused a time- and dose-dependent increase in protein tyrosine phosphorylation of at least six proteins in which maximal effect was seen after 2 h of incubation with 50 microM H(2)O(2). At much higher concentrations of H(2)O(2) (0.5 mM), there is significant reduction in the phosphorylation level, and no protein tyrosine phosphorylation is observed at 5 mM H(2)O(2) after 4 h of incubation. Exogenous NADPH enhanced protein tyrosine phosphorylation similarly to H(2)O(2). These two agents, but not heparin, induced Ca(2+)-dependent tyrosine phosphorylation of an 80-kDa protein. Treatment with H(2)O(2) (50 microM) caused approximately a twofold increase in cAMP, which is comparable to the effect of bicarbonate, a known activator of soluble adenylyl cyclase in sperm. This report suggests that relatively low concentrations of H(2)O(2) are beneficial for sperm capacitation, but that too high a concentration inhibits this process. We also conclude that H(2)O(2) activates adenylyl cyclase to produce cAMP, leading to protein kinase A-dependent protein tyrosine phosphorylation.

Transplasma membrane electron transport: enzymes involved and biological function

The notion of transmembrane electron transport is usually associated with mitochondria and chloroplasts. However, since the early 1970s, it has been known that this phenomenon also occurs at the level of the plasma membrane. Ever since, evidence has accumulated for the existence of a plethora of transplasma membrane electron transport enzymes. In this review, we discuss the various enzymes known, their molecular characteristics and their biological functions.

Development of the signalling pathways associated with sperm capacitation during epididymal maturation

As spermatozoa mature within the epididymis they acquire the potential for capacitation and ultimately fertilization. In biochemical terms, the former is reflected in the progressive activation of a signal transduction pathway characterized by cAMP-mediated induction of phosphotyrosine expression on the sperm tail. In this study, we have examined the cellular mechanisms controlling this maturational event. Caput epididymal spermatozoa exhibited tyrosine phosphorylation on the sperm head that was largely unresponsive to cAMP and not significantly impaired by removal of extracellular HCO(3) (-). In contrast, caudal epididymal spermatozoa exhibited low levels of phosphorylation on the sperm head, yet responded dramatically to cAMP by phosphorylating a new set of proteins on the sperm tail via mechanisms that were highly dependent on extracellular HCO(3) (-). The impact of extracellular HCO(3) (-) depletion on caudal cells was not associated with a significant change in the redox regulation of cAMP but could be fully reversed by buffering the intracellular pH with N-Tris[Hydroxymethyl]methyl-3-amino-propanesulfonic acid (TAPS). The pattern of tyrosine phosphorylation was also profoundly influenced by the presence or absence of added extracellular calcium. In the presence of this cation, only caudal spermatozoa could respond to increased extracellular cAMP with tyrosine phosphorylation of the sperm tail. However, in calcium-depleted medium, this difference completely disappeared. Under these conditions, caput and caudal spermatozoa were equally competent to exhibit phosphotyrosine expression on the sperm tail in response to cAMP. These results emphasize the pivotal role played by calcium and HCO(3) (-) in modulating the changes in tyrosine phosphorylation observed during epididymal maturation.

Remodeling of the actin cytoskeleton during mammalian sperm capacitation and acrosome reaction

The sperm acrosome reaction and penetration of the egg follow zona pellucida binding only if the sperm has previously undergone the poorly understood maturation process known as capacitation. We demonstrate here that in vitro capacitation of bull, ram, mouse, and human sperm was accompanied by a time-dependent increase in actin polymerization. Induction of the acrosome reaction in capacitated cells initiated fast F-actin breakdown. Incubation of sperm in media lacking BSA or methyl-beta-cyclodextrin, Ca(2+), or NaHCO(3), components that are all required for capacitation, prevented actin polymerization as well as capacitation, as assessed by the ability of the cells to undergo the acrosome reaction. Inhibition of F-actin formation by cytochalasin D blocked sperm capacitation and reduced the in vitro fertilization rate of metaphase II-arrested mouse eggs. It has been suggested that protein tyrosine phosphorylation may represent an important regulatory pathway that is associated with sperm capacitation. We show here that factors known to stimulate sperm protein tyrosine phosphorylation (i.e., NaHCO(3), cAMP, epidermal growth factor, H(2)O(2), and sodium vanadate) were able to enhance actin polymerization, whereas inhibition of tyrosine kinases prevented F-actin formation. These data suggest that actin polymerization may represent an important regulatory pathway in with sperm capacitation, whereas F-actin breakdown occurs before the acrosome reaction.

Regulation of the human sperm tyrosine kinase c-yes. Activation by cyclic adenosine 3',5'-monophosphate and inhibition by Ca(2+)

During the process of capacitation, spermatozoa go through a whole set of signaling cascade events in order to become fully competent at fertilizing the egg. An increase in sperm protein tyrosine phosphorylation has been described during this final maturational event in different animal species as well as in humans. Although the phosphotyrosine content of sperm protein is modulated by cAMP, Ca(2+), BSA, oxygen derivatives, and cholesterol, no protein tyrosine kinase (PTK) nor the phosphotyrosine protein phosphatase (PTPase) directly involved in the control of the phosphotyrosine content of sperm protein has been identified. Therefore, the goal of the present study was to identify the tyrosine kinases putatively responsible for the increases in sperm protein phosphotyrosine content. In the present study, we show that the src-related tyrosine kinase c-yes is present in the head of human spermatozoa in both membranes and Triton X-100-insoluble extracts. Our hypothesis was that c-yes is a tyrosine kinase responsible for at least some of the capacitation-induced increase in protein tyrosine phosphorylation. When spermatozoa were previously incubated in the presence of 3-isobutyl-1-methylxanthine or 1,2-bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, treatments known to increase the phosphotyrosine content of human sperm proteins, an increase in the kinase activity of immunoprecipitated yes was measured using enolase as a substrate. These results suggest that cAMP activates while Ca(2+) inhibits human sperm c-yes kinase activity.

The zona pellucida-induced acrosome reaction of human spermatozoa involves extracellular signal-regulated kinase activation

Extracellular signal-regulated kinases (ERKs), belonging to the family of mitogen-activated protein kinases (MAPKs), are cytoplasmic and nuclear serine/threonine kinases involved in the signal transduction of several extracellular effectors. Recent evidence indicates the presence of p21 Ras and the phosphorylation of ERK1 and ERK2, suggesting the occurrence of the Ras/ERK cascade in mammalian spermatozoa. The present article describes the biological role of ERK during the acrosome reaction of human spermatozoa on stimulation with zona pellucida (ZP). The mitogen-activated protein-kinase inhibitor PD098059 was used as a pharmacological tool to study the involvement of extracellular signal-regulated kinases in the induction of the acrosome reaction in human spermatozoa. This compound significantly inhibited the acrosome reaction induced by both ZP and the calcium ionophore A23187. These results suggest that ERKs are involved in the signal transduction pathway through which ZP stimulation works during the process of fertilization.

Analysis of reactive oxygen species generating systems in rat epididymal spermatozoa

Epididymal sperm maturation culminates in the acquisition of functional competence by testicular spermatozoa. The expression of this functional state is dependent upon a redox-regulated, cAMP-mediated signal transduction cascade that controls the tyrosine phosphorylation status of the spermatozoa during capacitation. Analysis of superoxide anion (O2(-.)) generation by rat epididymal spermatozoa has revealed a two-component process involving electron leakage from the sperm mitochondria at complexes I and II and a plasma membrane NAD(P)H oxidoreductase. Following incubation in a glucose-, lactate-, and pyruvate-free medium (-GLP), O2(-.) generation was suppressed by 86% and 96% in caput and cauda spermatozoa, respectively. The addition of lactate, malate, or succinate to spermatozoa incubated in medium -GLP stimulated O2(-.) generation. This increase could be blocked by rotenone and oligomycin (R/O) in the presence of malate or lactate but not succinate. Stimulation with all three substrates, as well as spontaneous O2(-.) production in +GLP medium, was blocked by the flavoprotein inhibitor, diphenylene iodonium. Diphenylene iodonium, but not R/O, suppressed NAD(P)H-induced lucigenin-dependent chemiluminescence. This NAD(P)H-dependent enzyme resided in the sperm plasma membrane and its activity was regulated by zinc and uncharacterized cytosolic factors. Reverse transcription-polymerase chain reaction analysis indicated that the sperm NAD(P)H oxidoreductase complex is quite distinct from the equivalent leukocyte system.

Impact of epididymal maturation on the tyrosine phosphorylation patterns exhibited by rat spermatozoa

As mammalian spermatozoa migrate through the epididymis, they acquire functionality characterized by the potential to express coordinated movement and the competence to undergo capacitation. The mechanisms by which spermatozoa gain the ability to capacitate during epididymal transit are poorly understood. The purpose of this study was to investigate the impact of epididymal maturation on the signal transduction pathways regulating tyrosine phosphorylation, because this process is thought to be central to the attainment of a capacitated state and expression of hyperactivated motility. Western blot and immunocytochemical analyses demonstrated that epididymal maturation in vivo is associated with a progressive loss of phosphotyrosine residues from the sperm head. As cells pass from the caput to the cauda epididymis, tyrosine phosphorylation becomes confined to a narrow band at the posterior margin of the acrosomal vesicle. Epididymal maturation of rat spermatozoa was also associated with an acquired competence to respond to high levels of intracellular cAMP by phosphorylating tyrosine residues on the sperm tail. Immature caput spermatozoa were incapable of exhibiting this response, despite the apparent availability of cAMP and protein kinase A. These findings help to clarify the biochemical changes associated with the functional maturation of spermatozoa during epididymal transit.

Progesterone accelerates the onset of capacitation in mouse sperm via T-type calcium channels

This study was undertaken to evaluate whether progesterone induces capacitation of mouse spermatozoa. When sperm were evaluated by chlortetracycline staining, addition of progesterone significantly increased the proportion of spermatozoa exhibiting the B pattern at 60 minutes of incubation, compared with that before incubation (23 +/- 6.2% vs. 13 +/- 2.9%, p < 0.01) and that in hTF medium without progesterone (23 +/- 6.2% vs. 13 +/- 4.2%, p < 0.01). If the redistribution of proteins in sperm plasma membrane such as protein binding calcium ion were defined as capacitation, it could be said that progesterone promoted capacitation of mouse sperm. This progesterone-induced capacitation was prevented by depletion of extracellular calcium ion and addition of NiCl2, a T-type calcium channel blocker, although thapsigargin, an inhibitor of Ca2+-ATPase, did not increase the number of capacitated sperm (B pattern; progesterone vs. progesterone + depletion of calcium ion, 18 +/- 3.5% vs. 8 +/- 2.5%, p < 0.05, progesterone vs. progesterone + NiCl2, 20 +/- 3.8% vs. 6 +/- 5.2%, p < 01). Furthermore, genistein, a protein tyrosine phosphorylation inhibitor, inhibited progesterone-induced capacitation (B pattern; progesterone vs. progesterone + genistein, 20 +/- 3.8% vs. 11 +/- 2.4%, p < 01). In conclusion, progesterone induces capacitation in mouse sperm and this capacitation may be associated with calcium influx and tyrosine phosphorylation.

Novel sperm-binding proteins of epididymal origin contain four fibronectin type II-modules

Novel fibronectin type II (Fn2)-module proteins were cloned from human and canine epididymal cDNA libraries. cDNA sequences predicted a highly conserved protein family, related but not homologous to ungulate seminal plasma proteins (approximately 50% sequence identity), and the first known examples of proteins with four tandemly arranged Fn2-domains. By Northern blot and in situ hybridization analyses the encoding mRNAs were shown to be abundant products of the epididymal duct epithelium, but not detectable in other tissues. Homologous mRNAs were identified in the epididymides of various mammals, representing members of this novel protein family of epididymal origin. Within the Fn2-module-encoding stretches, species homologues displayed >85% sequence identity, but showed high variability at their predicted N-termini. An antipeptide antiserum in Western blot analyses detected 30-35 kDa immunoreactive protein bands in epididymal tissue, cauda epididymidal fluid, and sperm membrane protein preparations. The tandem arrangement of increasing numbers of Fn2-modules might functionally correspond to the tendency to form oligomers that has been described for lipid-binding proteins.

Oocyte-sperm interactions

The penetration of the zona pellucida is a crucial step during fertilization. Spermatozoa that are unable to recognize and bind to the zona pellucida glycoproteins or respond to the zona pellucida by undergoing the acrosome reaction fail to fertilize the egg. In most mammalian species, after entering the fallopian tube sperm are stored in the isthmic part of the oviduct under conditions that maintain sperm viability and synchronize both sperm transport and the process of acquisition of fertilizing ability, called capacitation. Only capacitated sperm are enabled to recognize the oocyte and respond to the oocyte signals in an appropriate manner. Close to time of ovulation sperm are released from the oviductal epithelium and swim to site of fertilization. The oviduct and the oocyte itself appear to coordinate sperm function and gamete interaction. The gamete recognition and the next levels of interaction are probably granted by the carbohydrate-protein interactions. Upon binding the signal cascade leading to acrosomal exocytosis is activated, eventually initiated by aggregation of zona pellucida receptor molecules. These signal transducing mechanisms are primed during the capacitation process. Tyrosine phosphorylation, tightly connected to the cholesterol efflux from the plasma membrane, and hyperpolarization seem be involved in this priming by activation of Ca(2+) pathways. Further preparational steps of the acrosome reaction may be mediated by osmosensitive signal transducing mechanisms. The current perspective focuses on the molecules involved in the complex hierarchy of sperm-egg interactions and regulative events priming sperm cell during capacitation for the acrosome reaction.

Protein tyrosine phosphorylation during prolonged in vitro incubation of ejaculated bovine spermatozoa is regulated by the oxidative state of the medium

Protein tyrosine phosphorylation plays a regulatory role in a multitude of physiological processes in sperm. Changes in protein tyrosine phosphorylation, viability, and motility were studied as a function of extended incubation of bovine sperm in vitro at ambient temperature (18-20 degrees C). Fresh ejaculates were incubated after dilution for 8 days. On Days 0, 2, 5, and 8, an aliquot of sperm was incubated with or without theophylline at 37 degrees C for 30 min prior to assessing sperm viability, motility, and tyrosine phosphorylation of soluble and whole-cell proteins. There was a time-dependent decline in sperm motility, which was to some extent reversed by incubation with theophylline. The sum of the phosphotyrosine signal from two soluble proteins (M(r) 67 000 and 36 000) declined with incubation time in both theophylline-treated and untreated sperm. There were major differences in the pattern of tyrosine phosphorylation during incubation between ejaculates from different bulls. Tyrosine phosphorylation of a number of proteins from whole-cell extracts increased in a time-dependent manner during in vitro incubation and was unaffected by the presence of theophylline in the medium. The oxygenation state of the incubation medium had profound effects on sperm motility, viability, and tyrosine phosphorylation of proteins from whole-cell extracts. Sperm motility and viability declined more rapidly under aerobic compared with anaerobic conditions. Tyrosine phosphorylation of proteins from whole-cell extracts increased considerably during anaerobic incubation, while there was no significant change during aerobic incubation. This increase in phosphorylation due to anaerobic incubation was reversed when sperm were transferred from an anaerobic to an aerobic environment, indicating that the oxygenation state of the medium regulates both protein tyrosine kinases and phosphatases. In addition, sperm incubated under aerobic conditions for 5 days retained the ability to phosphorylate proteins when transferred to an anaerobic environment. The increase in protein tyrosine phosphorylation during in vitro incubation took place in a medium that did not contain capacitating substances such as heparin, sodium bicarbonate, or BSA. It transpired over a time scale of days and was not augmented by an increase in intracellular cAMP concentration through phosphodiesterase inhibition. Protein tyrosine phosphorylation during extended in vitro incubation at ambient temperature was significantly inhibited by the presence of oxygen in the medium.

Responses of monkey epididymal sperm of different maturational status to second messengers mediating protein tyrosine phosphorylation, acrosome reaction, and motility

The maturation of various aspects of sperm function have been demonstrated in monkey and human epididymal sperm, including the ability to undergo the acrosome reaction. The present study aimed to investigate the maturational changes in non-human primate sperm in the signal transduction mechanisms leading to the acrosome reaction involving cyclic AMP, Ca(2+) influx, protein kinase C, and protein tyrosine phosphorylation. Sperm from the caput, corpus, and cauda epididymidis of cynomolgus monkeys were incubated in a complete medium for 2.5 hr, followed by 30 min stimulation with 1 mM dibutyryl cAMP and 1 mM caffeine, 50 microM 1, 2-dioctanoyl-sn-glycerol (DOG), and 50 microM Ca(2+)-ionophore A23187. Quantitative Western blotting revealed little difference in tyrosine phosphorylated proteins among the caput, corpus, and cauda sperm without stimulation. Incubation with cAMP increased the amount of tyrosine phosphorylated proteins up to 10-fold in the corpus and cauda sperm, but to a lower extent in the caput sperm. Ca(2+)-ionophore attenuated the cAMP stimulation but had no effect on its own. Such responses in tyrosine phosphorylated proteins were in great contrast to the responses in the acrosome reaction, where A23187 was the strongest stimulant, resulting in induction of the reaction in 50 +/- 5%, 11 +/- 5%, and 8 +/- 4% cauda, corpus and caput sperm, respectively (mean +/- sem, n = 6). DOG and cAMP in combination induced acrosome reactions in about 10% of viable cells in the cauda and corpus but not caput sperm. Caput sperm responded to cAMP with increases in percentage motility without forward progression whereas cauda sperm displayed marked kinematic changes expected of hyperactivation. Comparisons of responses suggest that the major tyrosine phosphorylated proteins detected are unlikely to be involved immediately in the precipitation of the acrosome reaction, but more related to flagellar motion. Development of signal transduction pathways is part of the epididymal maturational process.

Reactive oxygen species requirements for bovine sperm capacitation and acrosome reaction

Sperm capacitation is necessary for the fertilization of oocytes. During capacitation intracellular and membrane changes occur, that culminate with an exocytotic event called the acrosome reaction. The aim of this work was to study the participation of the superoxide anion (O2-.) and of hydrogen peroxide (H2O2) in the capacitation process and acrosome reaction in spermatozoa from cryopreserved bovine semen. Samples were capacitated with heparin or treated with the xanthine-xanthine oxidase-catalase system (X-XO-C) for the production of O2-. The percentage of capacitated spermatozoa was determined using the chlortetracycline (CTC) technique, by means of epifluorescence microscopy. Addition of X-XO-C to the incubation medium significantly induced capacitation (P < 0.05), but there were no differences with samples incubated with heparin. When the medium contained heparin or the X-XO-C, addition of superoxide dismutase (SOD, 0.5 mg/mL) significantly inhibited capacitation (P < 0.05). In samples treated with heparin and with diverse concentrations of H2O2 (10, 25, 50 and 250 microM) in the incubation medium, the percentage of capacitated spermatozoa was significantly reduced (P < 0.05); however, acrosome reaction was produced at concentrations of 10 and 25 microM H2O2. At concentrations greater than 25 microM H2O2 a deleterious effect was observed on sperm motility. From these results it may be inferred that O2-. is required in the capacitation process and that H2O2 may participate as an inductor of the acrosome reaction in spermatozoa from cryopreserved bovine semen.

Gene expression in the epididymis

The epididymis is a tubular organ exhibiting vectorial functions of sperm concentration, maturation, transport, and storage. The molecular basis for these functions is poorly understood. However, it has become increasingly clear that regional differences along the length of the duct play a role in epididymal physiology and that region-specific gene expression is involved in the formation of these differences. Although not an overtly segmented organ, the epididymis consists of a series of highly coiled "zones," separated by connective tissue septulae and distinct by cell morphology and their pattern of gene expression. Thus, it constitutes an interesting mammalian model to study how pattern formation is achieved by differential gene activity. A large number of epididymis-expressed genes have been cloned and analyzed at the molecular level, most of them have been characterized by a distinct temporal and spatial expression pattern within the organ. Only recently have theories been developed about how and when during ontogenesis this pattern formation takes place and what its significance might be. This review summarizes the current knowledge on regionalized gene expression in the epididymis and presents hypotheses concerning its ontogenetic origin and regulation in the adult.

Cholesterol efflux-mediated signal transduction in mammalian sperm. beta-cyclodextrins initiate transmembrane signaling leading to an increase in protein tyrosine phosphorylation and capacitation

Sperm capacitation in vitro is highly correlated with an increase in protein tyrosine phosphorylation that is regulated by cAMP through a unique mode of signal transduction cross-talk. The activation of this signaling pathway, as well as capacitation, requires bovine serum albumin (BSA) in the incubation medium. BSA is hypothesized to modulate capacitation through its ability to remove cholesterol from the sperm plasma membrane. Here we demonstrate that the cholesterol-binding heptasaccharides, methyl-beta-cyclodextrin and OH-propyl-beta-cyclodextrin, promote the release of cholesterol from the mouse sperm plasma membrane in media devoid of BSA. Both of these beta-cyclodextrins were also demonstrated to increase protein tyrosine phosphorylation in the absence of BSA in both mouse and bull sperm, and the patterns of phosphorylation were similar to those induced by media containing BSA. The potency of the different beta-cyclodextrins to increase protein tyrosine phosphorylation in sperm was correlated with their cholesterol binding efficiencies, and preincubation of the beta-cyclodextrins with cholesterol-SO4- to saturate their cholesterol-binding sites blocked the ability of these compounds to stimulate protein tyrosine phosphorylation. The beta-cyclodextrin effect on protein tyrosine phosphorylation was both NaHCO3 and protein kinase A-dependent. The beta-cyclodextrins were also able to capacitate mouse sperm in the absence of BSA, as measured by the ability of the zona pellucida to induce the acrosome reaction and by successful fertilization in vitro. In summary, beta-cyclodextrins can completely replace BSA in media to support signal transduction leading to capacitation. These data further support the coupling of cholesterol efflux to the activation of membrane and transmembrane signaling events leading to the activation of a unique signaling pathway involving the cross-talk between cAMP and tyrosine kinase second messenger systems, thus defining a new mode of cellular signal transduction initiated by cholesterol release.

Progesterone-mediated calcium influx and acrosome reaction of human spermatozoa: pharmacological investigation of T-type calcium channels

The mechanisms of the progesterone (P4)-activated Ca2+ influx and the relationship between the intracellular free Ca2+ concentration ([Ca2+]i) and the acrosome reaction (AR) were investigated in this study. We compared the [Ca2+]i of uncapacitated and capacitated human sperm populations in response to P4 stimulation; characterized the effects of the pharmacological agents pimozide and mibefradil, inhibitors of T-type voltage-operated calcium channels (VOCCT), on the P4-activated Ca2+ influx; and determined the effects of these drugs on the P4-initiated AR. Since pimozide can also inhibit calmodulin-dependent enzymes, we examined the effects of the calmodulin antagonist, calmidazolium, on the above-mentioned events. The basal [Ca2+]i and the amplitude of the P4-activated Ca2+ influx were significantly (p < 0.05) higher in capacitated sperm populations. Also, in capacitated sperm populations, all three pharmacological agents significantly (p < 0.05) inhibited the P4-activated Ca2+ influx (IC50): calmidazolium (0.7 microM) > pimozide (8 microM) > mibefradil (11 microM). By contrast, the effects of these drugs on the P4-initiated AR were varied: pimozide (10 and 20 microM) significantly (p < 0.05) increased the percentage of AR spermatozoa, calmidazolium was without effect, and mibefradil (20 microM) significantly (p < 0.05) inhibited the AR. These disparate results do not allow us to reach any definitive conclusion concerning the role of a sperm VOCCT in the mechanism of the P4-initiated AR. However, the differences between the [Ca2+]i and AR effects, in particular the inverse relationship in the case of pimozide, suggest a dissociation between the amplitude of the P4-stimulated Ca2+ signal and the downstream biological effect of that signal, the AR.

GPx3: the plasma-type glutathione peroxidase is expressed under androgenic control in the mouse epididymis and vas deferens

We report here-using northern experiments, western blotting, and immunohistochemistry-on the findings that the plasma type glutathione peroxidase, GPx3, a major enzyme in reducing lipid hydroperoxides and hydrogen peroxide in plasma, is also expressed at significant levels in tissues of the male genital tract including epididymis and vas deferens. Within the epididymis and the kidney, the accumulation of the GPx3 mRNA and protein were investigated during postnatal development and found to be temporally regulated in a tissue-specific manner. Furthermore, we show here that androgen withdrawal by castration down regulates the expression of the GPx3 gene both in the epididymis and vas deferens while GPx3 expression in the kidney was found to be androgen-independent. Finally, immunohistochemistry data reveals that within the epididymis GPx3 distribution is quite peculiar suggesting the existence in this organ of complex traductional and/or transcriptional regulatory processes.

Paradoxical effect of reagents for sulfhydryl and disulfide groups on human sperm capacitation and superoxide production

Spermatozoa must undergo capacitation prior to fertilization. In humans, this process appears regulated by oxidoreduction reactions. We investigated the possibility that these reactions involved the sulfhydryl-disulfide pair, which offers a reversible regulation of cellular processes. The effects of reagents targeted for sulfhydryl and disulfide groups on human sperm capacitation, superoxide (O2-.) generation and protein tyrosine phosphorylation were evaluated. The sulfhydryl targeted agents, phenylarsine oxide (PAO), diamide, dithiopyridine (DTP), N-ethylmaleimide (NEM), maleimidylpropionyl biocytin (MPB), p-chloromercuribenzoic acid (PCMB), and bromobimane analogs (mBBr and qBBr) triggered sperm capacitation to levels comparable to those observed with a biological inducer, fetal cord serum ultrafiltrate (FCSu). Capacitation induced by NEM, MPB, PCMB, and PAO was prevented by superoxide dismutase (SOD) and associated with an increased sperm production of O2-.. However, SOD did not affect the increase in protein tyrosine phosphorylation of spermatozoa treated with NEM, PAO, or MPB. Disulfide reductants, dithiothreitol (DTT), thioredoxin (TRX), glutathione (GSH), tris-(2-carboxyethyl) phosphine (TCEP), and tris-(2-cyanoethyl) phosphine (TCP) partially to totally inhibited FCSu-induced sperm capacitation and O2-. production. TCEP, DTT, and TRX decreased the capacitation-associated tyrosine phosphorylation of sperm proteins. The strong time-dependent increase of sperm membrane sulfhydryl groups exposed to the extracellular space occurring during the first hour of capacitation could indicate an important rearrangement of sulfhydryl carrying proteins during the initiation of capacitation. Therefore, protein sulfhydryl-disulfide status may be important for the regulation of human sperm capacitation and the mechanisms involved may be complex and multifactorial.

Relative impact of oxidative stress on the functional competence and genomic integrity of human spermatozoa

Reactive oxygen metabolites are known to disrupt sperm-oocyte fusion, sperm movement, and DNA integrity; however, the relative sensitivities of these elements to oxidative stress are unknown. In this study these factors were assessed in human spermatozoa exposed to increasing levels of oxidative stress achieved through the stimulation of endogenous oxidant generation with NADPH or direct exposure to hydrogen peroxide. At low levels of oxidative stress, DNA fragmentation was significantly reduced while the rates of sperm-oocyte fusion were significantly enhanced. As the level of oxidative stress increased, the spermatozoa exhibited significantly elevated levels of DNA damage (p < 0.001) and yet continued to express an enhanced capacity for sperm-oocyte fusion. At the highest levels of oxidative stress, extremely high rates of DNA fragmentation were observed but the spermatozoa exhibited a parallel loss in their capacities for movement and oocyte fusion. These studies emphasize how redox mechanisms can either enhance or disrupt the functional and genomic integrity of human spermatozoa depending on the intensity of the oxidative stimulus. Because these qualities are affected at different rates, spermatozoa exhibiting significant DNA damage are still capable of fertilizing the oocyte. These results may have long-term implications for the safety of assisted conception procedures in cases associated with oxidative stress.

A novel signal transduction cascade in capacitating human spermatozoa characterised by a redox-regulated, cAMP-mediated induction of tyrosine phosphorylation

Capacitation is a priming event that renders mammalian spermatozoa responsive to signals originating from the cumulus-oocyte complex. The attainment of a capacitated state is dependent upon an increase in tyrosine phosphorylation and results in the acquisition of responsiveness to physiological agonists such as progesterone and ZP3. In this study we have shown that this capacitation-dependent increase in tyrosine phosphorylation is controlled by a unique redox-regulated, cAMP-mediated, signal transduction cascade. Either stimulation of reactive oxygen species generation or elevation of intracellular cAMP induced increases in phosphotyrosine expression by human spermatozoa and enhanced their responsiveness to progesterone. Ultimate convergence of the redox- and cAMP-regulated pathways was indicated by the ability of the protein kinase A inhibitor, H89, to block both modes of signal transduction. Furthermore, the fact that the redox-regulated pathway could be silenced by catalase, while this enzyme had no effect on the cAMP-mediated response, indicated that oxidant generation must lie upstream from cAMP in the reaction sequence. In keeping with this conclusion, a functional association was demonstrated between the redox status of human spermatozoa and their cAMP content. The continuous production of reactive oxygen species was also shown to be necessary for the protein kinase A-tyrosine phosphorylation axis to remain functional. If the generation of oxidising conditions during capacitation was prevented with 2-mercaptoethanol, 2-deoxyglucose or the flavoprotein inhibitor, diphenylene iodonium, then cAMP could no longer trigger tyrosine phosphorylation. These data support a model for human sperm capacitation as a redox-regulated process, involving a unique sequence of interactive events including reactive oxygen species production, elevation of intracellular cAMP, stimulation of protein kinase A and the induction of tyrosine phosphorylation. This is the first report of such a signal transduction cascade and may have implications for the functional significance of reactive oxygen metabolites in other cell types.

The cell biology of fertilization

Research into the cell biology of mammalian fertilization has been stimulated by the desire to provide a theoretical framework for the development of novel approaches to contraception and the need to understand the cellular basis of human infertility. The results of such studies have revealed a complex cascade of interactions initiated by the contact between capacitated spermatozoa on the oocyte-cumulus complex and culminating in sperm-oocyte fusion. In this review we shall examine our current understanding of the fertilization process, highlighting the strategic importance of recent findings and key areas where information is lacking.

Reactive oxygen species generation by human spermatozoa is induced by exogenous NADPH and inhibited by the flavoprotein inhibitors diphenylene iodonium and quinacrine

Human spermatozoa possess a specialized capacity to generate reactive oxygen species (ROS) that is thought to be of significance in the redox regulation of sperm capacitation (De Lamirande and Gagnon, 1993; Aitken et al., 1995). However, the mechanisms by which ROS are generated by these cells are not understood. In this study we have examined the possible significance of NADPH as a substrate for ROS production by human spermatozoa. Addition of NADPH to viable populations of motile spermatozoa induced a sudden dose-dependent increase in the rate of superoxide generation via mechanisms that could not be disrupted by inhibitors of the mitochondrial electron transport chain (antimycin A, rotenone, carbonyl cyanide m-chlorophenylhydrazone [CCCP], and sodium azide), diaphorase (dicoumarol) xanthine oxidase (allopurinol), or lactic acid dehydrogenase (sodium oxamate). However, NADPH-induced ROS generation could be stimulated by permeabilization and was negatively correlated with sperm function. Both NADH and NADPH were active electron donors in this system, while NAD+ and NADP+ exhibited little activity. Stereo-specificity was evident in the response in that only the beta-isomer of NADPH supported superoxide production. The involvement of a flavoprotein in the electron transfer process was indicated by the high sensitivity of the oxidase to inhibition by diphenylene iodonium and quinacrine. These results indicate that NAD(P)H can serve as an electron donor for superoxide generation by human spermatozoa and present a simple strategy for the production of motile populations of free radical generating cells with which to study the significance of these molecules in the control of normal and pathological sperm function.

Comparative analysis of the ability of precursor germ cells and epididymal spermatozoa to generate reactive oxygen metabolites

Male germ cells at various stages of differentiation from pachytene spermatocytes to mature caudal epididymal spermatozoa were examined for their ability to generate reactive oxygen species (ROS) using sensitive chemiluminescence techniques. In general, spermatozoa were found to spontaneously generate hydrogen peroxide as they progressed through the epididymis, maximal activity being observed on the release of mature cells from the caudal region into a modified Krebs-Ringer's solution. The spontaneous production of hydrogen peroxide rose rapidly during the first 10 min after the spermatozoa had been diluted into culture medium and thereafter stabilized, neither phorbol esters nor A23187 subsequently influencing this activity. Low levels of superoxide generation were also detected in suspensions of epididymal spermatozoa, but did not correlate with maturation status. However, superoxide production could be dramatically enhanced by the addition of exogenous NADPH, in a manner that was closely correlated with the stage of epididymal development being maximal for immature cells recovered from the caput epididymis in all species. Precursor germ cells (pachytene spermatocytes, round and elongate spermatids) similarly generated chemiluminescent signals compatible with the low level generation of ROS. Superoxide generation in these cells could again be stimulated by NADPH, via mechanisms that were inversely related to the stage of germ cell differentiation, the greatest activity being observed in pachytene spermatocytes. These results demonstrate that differentiating male germ cells have the potential to generate ROS, and have implications for the redox regulation of gonadal function and the development of reproductive pathologies involving oxidative stress.

Superoxide dismutase in human sperm suspensions: relationship with cellular composition, oxidative stress, and sperm function

Sensitive techniques have been developed for monitoring superoxide dismutase (SOD) activities in human sperm preparations. In contradiction to the protective role normally assigned to SOD, populations of defective spermatozoa recovered from the low density region of Percoll gradients were found to have three times more SOD than functionally competent preparations pelleting in high density Percoll. SOD activity was negatively correlated with the movement characteristics of human spermatozoa and their capacity for oocyte fusion, and positively associated with the induction of peroxidative damage. SOD activity was also highly correlated with other markers of the cytoplasmic space, creatine kinase (CK), and glucose-6-phosphate dehydrogenase (G-6-PDH). We conclude that while SOD may play a physiological role in maintaining a balance between O2.- and H2O2, high levels of this enzyme are associated with impaired sperm function because (a) the human spermatozoon is highly susceptible to the cytotoxic effects of H2O2, (b) O2.- is an important mediator of normal sperm function, and (c) high SOD activities reflect errors in spermatogenesis associated with germ cell exfoliation and the retention of excess residual cytoplasm by the spermatozoa.