Redox regulation of tyrosine phosphorylation in human spermatozoa and its role in the control of human sperm function

Active oxygen species in blue light mediated signal transduction in coleoptile tips

Coleoptile tip is a blue-light sensitive tissue possessing a "blue light receptor" which, upon activation, elicits a signal cascade resulting in phototropic curvature of the coleoptile. In this context, the nature of the photoreceptors and the exact mechanism through which the photoreceptors transduces the signal across the membrane are not clear. In this study, we attempted to examine whether the blue light receptor perturbs redox status of the coleoptile tip and sensitizes molecular oxygen as part of the signal reactions. Coleoptile tips of Sorghum bicolor and wheat (Triticum vulgare) grown in the dark showed pronounced ascorbate free radical signal, which diminished upon illumination with weak blue light for one minute. Concomitantly, the generation of superoxide radical by the coleoptile tip was augmented upon illumination with blue light. Various thiol blockers tested in this study caused powerful inhibition of blue light induced superoxide anion radical generation. Treatment with these thiol blockers, with the exception of NEM, resulted in marked increase in the levels of ascorbic acid free radical in the blue light irradiated coleoptiles. The blue light stimulated O*-2-generation by the coleoptile tip homogenate is also inhibited by the inhibitors of blue light responses viz phenylacetic acid, potassium iodide, and sodium azide. Based on our observations, we postulate that the activated blue light receptor present in the coleoptile tip sensitizes molecular oxygen to superoxide anion radical in the tip initializing the blue light signal cascade reactions.

A possible role for the pentose phosphate pathway of spermatozoa in gamete fusion in the mouse

Glucose metabolism is essential for successful gamete fusion in the mouse. Although the metabolic activity of the oocyte does not appear to play a significant role in the fusion step, the metabolic role of the spermatozoon is not known. The aim of this study was therefore to characterize the role of glucose metabolism in mouse spermatozoa. Initially, the high-affinity glucose transporter GLUT3 was identified in mouse sperm. In characterizing the glucose metabolism of mouse sperm, we have shown 1) that mouse epididymal spermatozoa have a functional pentose phosphate pathway (PPP), implying that they produce NADPH, which is required for reducing reactions, and ribose 5-phosphate, which is required for nucleic acid synthesis; and 2) that sperm are able to fuse with the oocyte when NADPH is substituted for glucose, suggesting that sperm need to produce NADPH via the PPP in order to be able to achieve fertilization. The existence of an NADPH-regulated event that influences the ability of the sperm to fuse with the oocyte is envisaged.

Localisation of enzymes in live spermatozoa by CellProbe reagents (preliminary results)

As a new approach, various synthetic fluorogenic substrates, the CellProbe reagents, were applied to examine the topography of their cleavage in vital human spermatozoa. These substrates are able to enter the cells without requiring previous cell permeabilization and can produce a fluorescent dye after cleavage, depending on enzyme activity. Vital spermatozoa from samples with normal spermiogram parameters showed fluorescence in different areas and intensity after incubation with a variety of substrates for aminopeptidase A, peroxides, subtilisin, dipeptidylpeptidase IV (DPP IV), cathepsin D, glucosidase and glucuronidase, but not with the substrate for galactosidase. Fluorescence was mainly located in the acrosomal cap (substrates for DPP IV, subtilisin, cathepsin D, glucosidase and glucuronidase) in the middle piece and head (substrates for peroxides, glucosidase), in the sperm head (substrates for aminopeptidase A) and occasionally in the tail (substrate for glucosidase). The substrate for subtilisin may play a role in andrology, because subtilisin is a serine protease like acrosin. This substrate may possibly be used to determine the acrosin activity in vital spermatozoa. The CellProbe reagents for fluorescence cytoenzymology may serve advanced methods in both clinical andrology and spermiological research, presuming that the characteristics and qualities of the synthetic substrates are correct. Therefore, more extended studies will be necessary to determine their clinical utility and significance under physiological and pathological conditions.

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.

Charge-based interactions of mammalian sperm with oocytes: inhibition of fertilization of mouse oocytes by ligands of macrophage scavenger receptor(s)

OBJECTIVE

To determine whether anionic ligands for the macrophage scavenger receptor inhibit the fertilization of mouse oocytes by mouse spermatozoa.

DESIGN

In vitro study of sperm binding and two-cell embryo formation in the presence of scavenger receptor ligands. Sperm-oocyte interaction may be mediated by sulfated sugars. In this study, we tested other nonsulfated anionic ligands for the scavenger receptor for their ability to affect fertilization. The only common feature of these ligands is their anionic nature.

SETTING

Oocytes and sperm from mice were used.

MAIN OUTCOME MEASURE(S)

Binding of sperm to oocytes and subsequent formation of two-cell embryos were determined.

RESULT(S)

Fucoidin, polyinosinic acid, oxidized low-density lipoprotein, acetyl low-density lipoprotein, and malondialdehyde-modified LDL inhibited the binding and fertilization of mouse sperm to mouse oocytes. Addition of fresh sperm to oocytes previously treated with sperm in the presence of these agents restored the binding and fertilization.

CONCLUSION(S)

These results show that charge-based interactions analogous to the interactions of the scavenger receptor with its ligands may play an important role in mammalian fertilization.

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.

Differentiation of ejaculates showing reactive oxygen species production by spermatozoa or leukocytes

Differences between subgroups and correlations between reactive oxygen species (ROS), sperm motility, concentration of leukocytes and viability in semen samples from 143 men were investigated. Patients with azoospermia or leukocytospermia were excluded from the study. Spermatozoa were separated by means of glass wool filtration. Reactive oxygen species were determined by means of luminol chemiluminescence before and after sperm separation; thereafter, normozoospermic and oligozoospermic patients were divided into three subgroups using the mean of all patients investigated (17462 count 10(-7) viable spermatozoa) as cut-off value as follows: G1--high reactive oxygen species production in native semen and after glass wool filtration; G2--high production of reactive oxygen species only in native semen; G3--low levels of reactive oxygen species before and after glass wool filtration. In general, reactive oxygen species were significantly higher in oligozoospermic samples than in normozoospermic samples. In men with normal sperm count, ROS production correlated significantly with the number of leukocytes in the ejaculate before glass wool filtration, but not thereafter. Glass wool filtration is useful to distinguish between sperm samples in which reactive oxygen species are generated by leukocytes and those in which reactive oxygen species are excessively generated by the spermatozoa themselves.

Monoclonal antibodies to canine intra-acrosomal sperm proteins recognizing acrosomal status during capacitation and acrosome reaction

Monoclonal antibodies Ds-1 and Ds-2 specifically labelling dog sperm acrosome were prepared by immunization of mice with acetic acid extracts of dog spermatozoa. Electron microscopy and indirect immunofluorescence localized the site of Ds-1 and Ds-2 proteins inside the acrosomal vesicle. Ds-1 antibody detected 55, 76, 115, 120 and 190 kDa proteins under non-reducing conditions, and 73 kDa and 54 kDa proteins after reduction (p73/Ds-1 and p54/Ds-1). 92 kDa and 40 kDa proteins recognized by Ds-2 (p92/Ds-2 and p40/Ds-2) migrated at > 200 kDa in the absence of reducing agent. In vivo, p73/Ds-1 and p54/Ds-1 are therefore likely to be present both in free and complexed form, while all of p92/Ds-2 and p40/Ds-2 form disulfide-bonded complexes. Decrease in the rate of acrosomes stained with Ds-1 and Ds-2 was correlated with the progress of capacitation resulting in the increased rate of spontaneous acrosome reactions, as suggested by a dramatic effect of A23187. Monoclonal antibody to boar acrosin (ACR-2) recognized dog sperm acrosin homologue. A higher rate of ACR-2-negative spermatozoa was observed after capacitation and A23187 treatment compared to Ds-1 and Ds-2, indicating that proteins recognized by Ds-1 and Ds-2 are localized in a specific compartment of acrosome, distinct from acrosin and possibly representing fraction of acrosomal matrix.

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.

Heme oxygenase-2 is a hemoprotein and binds heme through heme regulatory motifs that are not involved in heme catalysis

The heme oxygenase (HO) system degrades heme to biliverdin and CO and releases chelated iron. In the primary sequence of the constitutive form, HO-2, there are three potential heme binding sites: two heme regulatory motifs (HRMs) with the absolutely conserved Cys-Pro pair, and a conserved 24-residue heme catalytic pocket with a histidine residue, His151 in rat HO-2. The visible and pyridine hemochromogen spectra suggest that the Escherichia coli expressed purified HO-2 is a hemoprotein. The absorption spectrum, heme fluorescence quenching, and heme titration analysis of the wild-type protein versus those of purified double cysteine mutant (Cys264/Cys281 --> Ala/Ala) suggest a role of the HRMs in heme binding. While the His151 --> Ala mutation inactivates HO-2, Cys264 --> Ala and Cys281 --> Ala mutations individually or together (HO-2 mut) do not decrease HO activity. Also, Pro265 --> Ala or Pro282 --> Ala mutation does not alter HO-2 activity. Northern blot analysis of ptk cells indicates that HO-2 mRNA is not regulated by heme. The findings, together with other salient features of HO-2 and the ability of heme-protein complexes to generate oxygen radicals, are consistent with HO-2, like five other HRM-containing proteins, having a regulatory function in the cell.

Scavenging effect of N-acetyl-L-cysteine against reactive oxygen species in human semen: a possible therapeutic modality for male factor infertility?

A new approach to reduce the level of reactive oxygen species (ROS) in human semen by using N-acetyl-L-cysteine (NAC) was evaluated. Semen samples were incubated with or without NAC (1.0 mg ml-1) at room temperature. The chemiluminescent signal of the oxidation of luminol was detected by means of an MTP reader after 0, 20, 40, 60 and 120 min, respectively, using 200 microM luminol. In addition, the dose-dependent action of NAC (0.1, 1.0 and 5.0 mg ml-1) and the influence of NAC on functional sperm parameters (motility and acrosome reaction) were studied. ROS levels decreased significantly after 20 min incubation with NAC. This reduction was greater in the high ROS group (> 30000 counts/10(7) viable sperm at t = 0) than in the low ROS group (< 30000). In addition, a marked dose-dependence of NAC was observed. Concerning sperm function, total sperm motility improved after incubation with NAC, but no significant change was observed with respect to the acrosome reaction. NAC (at concentrations of 1.0 mg ml-1) significantly reduced ROS in human semen and showed the possibility of improving impaired sperm function. After further testing NAC might be useful for the treatment of male infertility patients.

Erythrocyte thiol status regulates band 3 phosphotyrosine level via oxidation/reduction of band 3-associated phosphotyrosine phosphatase

Oxidative stress-induced tyrosine phosphorylation has been ascribed to activation of phosphotyrosine kinase or to inhibition of phosphotyrosine phosphatase (PTP). We have previously identified a PTP associated with band 3 in the human erythrocyte membrane, a PTP that is normally highly active and prevents the appearance of band 3 phosphotyrosine. Here we show that treatment of erythrocytes with the thiol-oxidizing agent diamide leads to the formation of PTP disulfides (PTP-band 3 mixed disulfides) and inhibition of dephosphorylation, allowing the accumulation of band 3 phosphotyrosine. Upon reduction of the disulfides, the band 3 phosphotyrosine is dephosphorylated. Erythrocyte thiol alkylation by N-ethylmaleimide results in irreversible PTP inhibition and irreversible phosphorylation. The results are consistent with the notion that alterations in cellular thiol status affect the cell phosphotyrosine status and that oxidative stress-induced tyrosine phosphorylation involves inhibition of PTP.

Regulation of protein-tyrosine phosphorylation and human sperm capacitation by reactive oxygen derivatives

Spermatozoa undergoing capacitation, a necessary prerequisite event to successful fertilization that can be induced in vitro by reactive oxygen species (ROS), generate superoxide anion (O2.-). Because, in neutrophils, the generation of O2.- is associated with tyrosine phosphorylation of several proteins, the aim of the present study was to investigate the association between protein-tyrosine phosphorylation and ROS-induced human sperm capacitation. Human spermatozoa express two major phosphotyrosine-containing proteins of 105 and 81 kDa, the phosphotyrosine content of which is increased when spermatozoa are incubated under capacitating conditions. Superoxide dismutase and catalase abolish both sperm capacitation and tyrosine phosphorylation of p105 and p81, suggesting the involvement of O2.- and hydrogen peroxide in these two processes. Inhibitors of NADPH oxidase, the enzyme responsible for the neutrophil's respiratory burst, decrease both p105 and p81 tyrosine phosphorylation and sperm capacitation while hydrogen peroxide stimulates these two processes. Tyrosine phosphorylation of p105 and p81 occurs through a herbimycin A-sensitive tyrosine kinase, and sperm incubation with phosphotyrosine-protein phosphatase inhibitors results in an increase in phosphotyrosine content of these two proteins. Indirect immunocytochemical studies reveal phosphotyrosine-containing proteins mostly in the principal piece of the flagellum, in agreement with the localization of p105 and p81 in the human sperm fibrous sheath. Although tyrosine phosphorylation of p105 and p81 and sperm capacitation are related in a time-dependent fashion, some discrepancies are observed in the regulation of these two processes according to the redox status of the spermatozoa.

Effect of catalase and thioredoxin addition to sperm incubation medium before in vitro fertilization on sperm capacity to support embryo development

OBJECTIVE

To determine whether addition of catalase and thioredoxin to sperm incubation medium before IVF improves sperm potential to support embryo development.

DESIGN

CD-1 mouse spermatozoa were preincubated without or with catalase or thioredoxin for 1 hour before IVF, and sperm motility parameters, fertilization rate, and embryo development were determined.

SETTING

A conventional laboratory setting.

INTERVENTION(S)

Mice were superovulated with pregnant mare serum gonadotropin and hCG. Eggs in cumulus oophorus were collected and used for fertilization with epididymal spermatozoa.

MAIN OUTCOME MEASURE(S)

Sperm motility parameters, fertilization rate, and embryo development.

RESULT(S)

Sperm motility parameters and fertilization rates were not affected by catalase treatment. However, the rates of blastocyst and hatching blastocyst formation when catalase-treated (16 micrograms/mL) spermatozoa were used were significantly higher than those observed with nontreated spermatozoa (44% versus 24%, 31% versus 2%, respectively). Addition of thioredoxin to preincubation media did not affect the percentage of motility and the fertilization rate but increased the rate of blastocyst formation to an extent similar to that triggered by catalase (2.7- and 3.3-fold, respectively).

CONCLUSION(S)

These results suggest that H2O2 produced in sperm suspensions before IVF reduces their potential to promote embryo development, that these toxic effects of H2O2 are latent, appearing mainly 3 to 5 days after IVF, and that catalase and thioredoxin are efficient agents to protect sperm potential to support embryo development.

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

Capacitation had no effect on the ability of progesterone to elicit a rapid calcium transient in the acrosomal domain of human spermatozoa but had a marked influence of the ability of this steroid to induce a biological response. The development of this responsiveness to progesterone appeared to be redox regulated in that it was promoted by the stimulation of reactive oxygen species generation and inhibited by the presence of antioxidants, including catalase and membrane permeant thiols. The ability of redox conditions to influence the biological responsiveness of human spermatozoa did not involve changes in the dynamics of the calcium transients induced by progesterone but was causally linked with clear differences in tyrosine phosphorylation. We conclude that the ability of human spermatozoa to respond to the calcium transients induced by progesterone depends on a background of phosphotyrosine expression that can be profoundly influenced by the redox status of the spermatozoa during capacitation.

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.