Analysis of the mechanism by which calcium negatively regulates the tyrosine phosphorylation cascade associated with sperm capacitation

Phenotyping male infertility in the mouse: how to get the most out of a 'non-performer'

BACKGROUND

Functional male gametes are produced through complex processes that take place within the testis, epididymis and female reproductive tract. A breakdown at any of these phases can result in male infertility. The production of mutant mouse models often yields an unexpected male infertility phenotype. It is with this in mind that the current review has been written. The review aims to act as a guide to the ‘non-reproductive biologist’ to facilitate a systematic analysis of sterile or subfertile mice and to assist in extracting the maximum amount of information from each model.

METHODS

This is a review of the original literature on defects in the processes that take a mouse spermatogonial stem cell through to a fully functional spermatozoon, which result in male infertility. Based on literature searches and personal experience, we have outlined a step-by-step strategy for the analysis of an infertile male mouse line.

RESULTS

A wide range of methods can be used to define the phenotype of an infertile male mouse. These methods range from histological methods such as electron microscopy and immunohistochemistry, to hormone analyses and methods to assess sperm maturation status and functional competence.

CONCLUSION

With the increased rate of genetically modified mouse production, the generation of mouse models with unexpected male infertility is increasing. This manuscript will help to ensure that the maximum amount of information is obtained from each mouse model and, by extension, will facilitate the knowledge of both normal fertility processes and the causes of human infertility.

Release of DEFB126 from macaque sperm and completion of capacitation are triggered by conditions that simulate periovulatory oviductal fluid

Capacitation of macaque sperm in vitro has been achieved efficiently only with the addition of both cyclic nucleotides and methylxanthines. The use of these exogenous sperm activators clouds an understanding of the normal mechanisms underlying capacitation and may slow early embryo development following in vitro fertilization (IVF). We demonstrate that culture medium which simulates periovulatory oviductal fluid with respect to bicarbonate (HCO(3)(-)) and glucose concentration induces capacitation in a high percentage of macaque sperm as determined by the ability of sperm to undergo both the release of coating protein DEFB126 and the zona pellucida-induced acrosome reaction (AR). Few sperm were able to undergo the AR following 6 hr incubation in medium containing either 35 mM HCO(3)(-) (approximately 7.2 pH) or 90 mM HCO(3)(-) (approximately pH 7.8) with 5 mM glucose. When glucose concentration was lowered to 0.5 mM to match levels reported for women at midcycle, the AR rate increased significantly in sperm incubated in both levels of HCO(3)(-), indicating that glucose interferes with sperm responsiveness to increasing HCO(3)(-) concentration observed in the primate oviduct during ovulation. Even greater synchronization of capacitation could be achieved with nonphysiologic extremes of alkalinity or energy substrate deprivation. In the latter case, sperm achieved high rates of IVF. A shift in pH from 7.2 to 7.8 in a HEPES-buffered medium was sufficient to remove DEFB126 from the surface of most sperm after only 3 hr. The loss of DEFB126 from sperm under periovulaory fluid conditions has implications for the timing of release of sperm from the oviductal reservoir.

Phosphorylation and consequent stimulation of the tyrosine kinase c-Abl by PKA in mouse spermatozoa; its implications during capacitation

Upon ejaculation, spermatozoa undergo a series of post-translational modifications in a process known as capacitation in order to prepare for fertilization. In the absence of capacitation, fertilization cannot occur. Spermatozoa are unusual in that one of the hallmarks of capacitation is a global up-regulation in phosphotyrosine expression, which is known to be mediated upstream by PKA. Little is known about the signaling events downstream of PKA apart from the involvement of SRC, as a key mediator of PKA-induced tyrosine phosphorylation in the sperm tail. Here we describe the presence of c-Abl in mouse spermatozoa. In vitro analysis confirmed that PKA can up-regulate c-Abl kinase activity. In vivo, this tyrosine kinase was found to associate, and become threonine phosphorylated by PKA in the sperm flagellum. By treating spermatozoa with hemolysin we could demonstrate that a significant proportion of the tyrosine phosphorylation associated with capacitation could be suppressed by the c-Abl inhibitor, Gleevac. This is the first report of c-Abl being up-regulated by PKA for any cell type. We present a model, whereby these kinases may operate together with SRC to ensure optimal levels of tyrosine phosphorylation in the sperm flagellum during the attainment of a capacitated state.

Changes in content and localization of proteins phosphorylated at tyrosine, serine and threonine residues during ram sperm capacitation and acrosome reaction

Previously, we reported the involvement of tyrosine phosphorylation in events that lead to ram sperm capacitation. In this study, we carried out a comparative analysis of the localization of tyrosine, serine and threonine phosphoproteins in different functional stages of ram spermatozoa (after the swim-up procedure,in vitrocapacitation, and ionophore-induced acrosome reaction) by immunofluorescence, immunocytochemistry and confocal microscopy. Capacitation increased protein tyrosine, serine and threonine phosphorylation whereas the induction of the acrosome reaction resulted in significantly decreased phosphorylation, mainly in those proteins that increased following capacitation. Control samples showed tyrosine-phosphorylated proteins restricted to the head, mainly distributed at the equatorial region with some cells also displaying an acrosomal and/or post-acrosomal localization.In vitrocapacitation promoted both tail and acrosome phosphorylation, and the acrosome reaction induced the loss of labeling on the acrosome and the subsequent increase in the post-acrosomal region and flagellum. The preferential localization of serine- and threonine-phosphorylated proteins in the equatorial and acrosomal regions found in control samples changed during capacitation, which induced tail phosphorylation in a sequential manner. After the acrosome reaction, the labeling of both phosphoamino acids decreased in the acrosome and increased in the post-acrosome. The obtained results were proved by two immunodetection techniques and strengthened by confocal microscopy, and indicate that changes in phosphorylated proteins during capacitation and acrosome reaction of ram spermatozoa may have physiological significance in consolidating certain phosphorylated proteins to specific sperm regions involved in acrosomal exocytosis and zona pellucida recognition, binding and penetration.

Sperm from hyh mice carrying a point mutation in alphaSNAP have a defect in acrosome reaction

Hydrocephalus with hop gait (hyh) is a recessive inheritable disease that arose spontaneously in a mouse strain. A missense mutation in the Napa gene that results in the substitution of a methionine for isoleucine at position 105 (M105I) of αSNAP has been detected in these animals. αSNAP is a ubiquitous protein that plays a key role in membrane fusion and exocytosis. In this study, we found that male hyh mice with a mild phenotype produced morphologically normal and motile sperm, but had a strongly reduced fertility. When stimulated with progesterone or A23187 (a calcium ionophore), sperm from these animals had a defective acrosome reaction. It has been reported that the M105I mutation affects the expression but not the function of the protein. Consistent with an hypomorphic phenotype, the testes and epididymides of hyh mice had low amounts of the mutated protein. In contrast, sperm had αSNAP levels indistinguishable from those found in wild type cells, suggesting that the mutated protein is not fully functional for acrosomal exocytosis. Corroborating this possibility, addition of recombinant wild type αSNAP rescued exocytosis in streptolysin O-permeabilized sperm, while the mutant protein was ineffective. Moreover, addition of recombinant αSNAP. M105I inhibited acrosomal exocytosis in permeabilized human and wild type mouse sperm. We conclude that the M105I mutation affects the expression and also the function of αSNAP, and that a fully functional αSNAP is necessary for acrosomal exocytosis, a key event in fertilization.

The presence and function of dopamine type 2 receptors in boar sperm: a possible role for dopamine in viability, capacitation, and modulation of sperm motility

Several studies have shown that dopamine and other catecholamines are present in oviduct luminal fluid. We recently reported that dopamine type 2 receptors (DRD2) are present in a wide range of mammalian sperm, suggesting a role for dopaminergic signaling in events such as fertilization, capacitation, and sperm motility. In the present study, we used Western blot analysis to show that boar sperm express DRD2 and that their activation with dopamine (100 nM) has a positive effect on cell viability that can be correlated with AKT/PKB phosphorylation. Bromocriptine (100 nM) and dopamine (100 nM and 10 muM) increased tyrosine phosphorylation during the capacitation period. Immunofluorescence analysis indicated that DRD2 localization is dynamic and depends on the capacitation stage, colocalizing with tyrosine phosphorylated proteins in the acrosome and midpiece region of capacitated boar sperm. This association was confirmed by coimmunoprecipitation analysis. We also showed that bromocriptine (100 nM) and low-concentration dopamine (100 nM and 10 muM) increased total and progressive motility of sperm. However, high concentrations of dopamine (1 mM) decreased tyrosine phosphorylation and motility in in vitro sperm capacitation assays. This can be explained by the presence of the dopamine transporters (DAT, official symbol SLC6A3) in sperm, as demonstrated by Western blot analysis and immunocytochemistry. Taken together, our results support the idea that dopamine may have a fundamental role during sperm capacitation and motility in situ in the female upper reproductive tract.

Changes in calmodulin immunocytochemical localization associated with capacitation and acrosomal exocytosis of ram spermatozoa

The aim of this study was to determine the localization of calmodulin (CaM) in ram sperm and the possible changes during in vitro capacitation (CA) and the ionophore-induced acrosome reaction (AR). Likewise, changes in intracellular calcium levels ([Ca(2+)](i)) were also analysed by using flow cytometry. CA was induced in vitro in a medium containing BSA, CaCl(2), NaHCO(3), and AR by the addition of the calcium ionophore A23187. The acrosomal status was assessed by the chlortetracycline-fluorescence (CTC) assay. Flow cytometry (FC) analyses were performed by loading samples with Fluo-3 AM, that emits fluorescence at a high [Ca(2+)](i), combined with propidium iodide (PI) that allowed us to discriminate sperm with/without an integral plasma membrane both with high/low [Ca(2+)](i). Immunocytochemistry localized CaM to the flagellum, and some sperm also contained CaM in the head (equatorial and post-acrosomal regions). CA and AR resulted in a slight increase in the post-acrosomal labelling. The treatment of sperm with increasing concentrations of two CaM antagonists, W7 and calmidazolium (CZ), accounted for an increase in capacitated and acrosome-reacted CTC-sperm patterns. CZ induced a significant reduction in the content of three protein tyrosine-phosphorylated bands of approximately of 30, 40 and 45kDa. However, W7 showed no significant effect at any of the studied concentrations. Neither of them significantly influenced protein serine and threonine phosphorylation. FC analysis revealed that the main subpopulation in the control samples contained 70% of the total sperm with integral plasma membrane and a medium [Ca(2+)](i). After CA, 67.1% of the sperm preserved an integral membrane with a higher [Ca(2+)](i). After AR, only 7.2% of the total sperm preserved intact membranes with a very high [Ca(2+)](i). These results imply that CaM appears to be involved in ram sperm capacitation, and both treatments increased its localization in the post-acrosomal region.

Generation of live rats produced by in vitro fertilization using cryopreserved spermatozoa

In rats, the success of in vitro fertilization (IVF) was reported 40 years ago. Although it has been demonstrated in papers that these IVF oocytes using sperm freshly collected from cauda epididymides can be developed to term via embryo transfer, successful IVF with cryopreserved rat sperm has never been reported to date. Here, we report establishment of a successful IVF system using frozen/thawed rat spermatozoa. Our data showed that intracellular cAMP and free cholesterol levels in frozen/thawed rat sperm were maintained low, suppressing capacitation-associated tyrosine phosphorylation. The treatment of methyl-beta-cyclodextrin improved removal of free cholesterol from the membrane in frozen/thawed sperm but not induction of capacitation-associated tyrosine phosphorylation in the sperm. Treatment with a phosphodiesterase inhibitor, 3-isobutyl-1-methyl-xanthin (IBMX), dramatically increased cAMP and tyrosine phosphorylation levels in frozen/thawed rat sperm. When the IBMX-treated frozen/thawed sperm were used for IVF, the proportions of pronuclear formation and blastocyst formation were significantly higher than those of frozen/thawed sperm treated without IBMX (P < 0.05). The embryos were developed to term at a high success rate equivalent to the rate obtained with IVF using fresh sperm. Thus, we established for the first time a successful IVF system in rats using cryopreserved spermatozoa.

Increased activity of the human sperm tyrosine kinase SRC by the cAMP-dependent pathway in the presence of calcium

SRC-related tyrosine kinases are suggested to play a role in the increase of sperm protein phosphotyrosine content that occurs during capacitation. In our laboratory, we previously demonstrated that the SRC-related tyrosine kinase YES1 (also known as c-YES) is present in human spermatozoa. However, since it is negatively regulated by Ca(2+), whose intracellular concentration increases during capacitation, another kinase would most likely be involved in the capacitation-related increase in sperm protein tyrosine phosphorylation. The present study represents the first direct assessment of SRC tyrosine kinase activity in ejaculated mammalian sperm. By immunohistochemistry on human testis sections, it is clearly shown that SRC is expressed during spermatogenesis, mainly in round and elongating spermatids. Using an indirect immunofluorescence approach, SRC is detected in the acrosomal region of the head and in the sperm flagellum of ejaculated sperm. This tyrosine kinase is associated with the plasma membrane and with cytoskeletal elements, as suggested by its partial solubility in nonionic detergents. Despite its partial solubility, SRC kinase activity was assayed after immunoprecipitation using acid-denatured enolase as a substrate. It is clearly demonstrated that SRC activity is inhibited by SU6656 and PP1, selective SRC family tyrosine kinase inhibitors, and activated in a Ca(2+)-dependent manner. Furthermore, it is shown that SRC is activated in a cAMP/PRKA-dependent manner; SRC coimmunoprecipitates with the catalytic subunit of the cAMP-dependent protein kinase (PRKAC) and is phosphorylated by this latter kinase, resulting in an increase in enolase phosphorylation. All these results support the involvement of the tyrosine kinase SRC in the increase in sperm protein phosphotyrosine content observed during capacitation.

Dynamics of the global tyrosine phosphorylation during capacitation and acquisition of the ability to fuse with oocytes in human spermatozoa

Phosphorylation of tyrosine residues in cellular proteins represents a major event during sperm capacitaton, but its relationship with the acquisition of sperm-fertilizing ability is still unclear. In this study we explored the relationship between the kinetics of the global tyrosine phosphorylation, monitored with a flow cytometric assay, and the acquisition of the human sperm ability to fuse with oocytes, evaluated with the progesterone-enhanced hamster egg penetration test. Sperm tyrosine phosphorylation appeared to be an early event in the capacitation process, with a 3.6-fold mean increase within 1 h of capacitation, but at this time sperm-oocyte fusion was extremely poor compared with that observed at 5 h of capacitation. Capacitation in calcium-free medium produced a 2-fold mean increase in tyrosine phosphorylation compared with that seen in complete capacitation medium both at 1 h and 5 h of capacitation, whereas sperm-oocyte fusion significantly increased only at 1 h, remaining unchanged at 5 h of capacitation. The cAMP analog, N,2-O-dibutyryladenosine 3',5'-cyclic monophosphate (dbcAMP), prevented the inhibitory effect of seminal plasma on tyrosine phosphorylation but not on sperm-oocyte fusion. In conclusion, these results suggest that the acquisition of sperm-fertilizing ability is always associated with an increase of the global tyrosine phosphorylation, but tyrosine phosphorylation does not necessarily reflect the acquisition of the sperm-fertilizing ability. Flow cytometry assay, a reliable technique to quickly quantify the global levels of the human sperm tyrosine phosphorylation, could be useful for a further elucidation of the biological meaning of this process, with the perspective of its clinical use as a measure of the sperm-fertilizing potential.

The mouse sperm proteome characterized via IPG strip prefractionation and LC-MS/MS identification

Proteomic profiling of the mouse spermatozoon has generated a unique and valuable inventory of candidates that can be mined for potential contraceptive targets and to further our understanding of the PTMs that regulate the functionality of this highly specialized cell. Here we report the identification of 858 proteins derived from mouse spermatozoa, 23 of which demonstrated testis only expression. The list contained many proteins that are known constituents of murine spermatozoa including Izumo, Spaca 1, 3, and 5, Spam 1, Zonadhesin, Spesp1, Smcp, Spata 6, 18, and 19, Zp3r, Zpbp 1 and 2, Spa17, Spag 6, 16, and 17, CatSper4, Acr, Cylc2, Odf1 and 2, Acrbp, and Acrv1. Certain protein families were highly represented in the proteome. For example, of the 42 gene products classified as proteases, 26 belonged to the 26S-proteasome. Of the many chaperones identified in this proteome, eight proteins with a TCP-1 domain were found, as were seven Rab guanosine triphosphatases. Finally, our list yielded three putative seven-transmembrane proteins, two of which have no known tissue distribution, an extragenomic progesterone receptor and three unique testis-specific kinases all of which may have some potential in the future regulation of male fertility.

In spermatozoa, Stat1 is activated during capacitation and the acrosomal reaction

A role for sperm-specific proteins during the early embryonic development has been suggested by a number of recent studies. However, little is known about the participation of transcription factors in that stage. Here, we show that the signal transducer and activator of transcription 1 (Stat1), but not Stat4, was phosphorylated in response to capacitation and the acrosomal reaction (AR). Moreover, Stat1 phosphorylation correlated with changes in its localization: during capacitation, Stat1 moved from the cytoplasm to the theca/flagellum fraction. During AR, Stat1 phosphorylation increased again. In addition, blocking protein kinase A (PKA) and PKC during capacitation abolished both phosphorylation and migration of Stat1. Our results show tight spatio-temporal rearrangements of Stat1, suggesting that after fertilization Stat1 participates in the first rounds of transcription within the male pronucleus.

FSCB, a Novel Protein Kinase A-phosphorylated Calcium-binding Protein, Is a CABYR-binding Partner Involved in Late Steps of Fibrous Sheath Biogenesis

We report characterization of a novel testis- and sperm-specific protein, FSCB (fibrous sheath CABYR binding), that is expressed post-meiotically and localized in mouse sperm flagella. FSCB was identified as a binding partner of CABYR, a calcium-binding protein that is tyrosine-phosphorylated during capacitation. Orthologous genes of FSCB are present in other mammals, including rat and human, and conserved motifs in FSCB include PXXP, proline-rich and extensin-like regions. FSCB is phosphorylated by protein kinase A as shown by in vitro phosphorylation assay and also by determining phosphorylation sites in native FSCB from mouse sperm. Calcium overlay assay showed that FSCB is a calcium-binding protein from sperm. FSCB is a post meiotic protein first expressed at step 11 of mouse spermatogenesis in the elongating spermatids, and it subsequently incorporates into the flagellar principal piece of the sperm. Ultrastructurally, FSCB localized to a cortical layer of intermediate electron density at the surface of the ribs and longitudinal columns of the fibrous sheath. Due to its temporal appearance during spermiogenesis and location at the cortex of the fibrous sheath, FSCB is postulated to be involved in the later stages of fibrous sheath assembly.

The influence of fluorides on mouse sperm capacitation

Increasing infertility, due to pathological changes on sperm, has become a serious issue. Eco-toxicological effect of rising concentration of fluorides can be enhanced in the presence of aluminium ions by forming fluorometallic complexes, analogues of phosphate groups that interfere with the activity of G-proteins and P-type ATPases, which are part of several signalling pathways during sperm maturation. In order for sperm to gain fertilizing ability, they must undergo in the female reproductive tract, capacitation that includes tyrosine phosphorylation and consequent actin polymerization. The present paper reports the findings of 3-month oral toxicity in mice of fluorides at the concentrations 0, 1, 10, and 100ppm and their synergic action with aluminium at dose of 10ppm. There were no mortalities, clinical signs of discomfort or body weight loss during the experiment. The analysis revealed, for the concentrations of 10 and 100ppm, abnormalities of spermatogenesis and ability of epididymal spermatozoa to capacitate in vitro, as the result of decreased sperm head tyrosine phosphorylation and actin polymerization. The enhancing overload caused by fluorides represents a potential factor, having an impact on function of sperm, hence contributing to a growing infertility in the human population.

Proteomic changes in mammalian spermatozoa during epididymal maturation

Epididymal maturation is associated with the activation of a cAMP-induced tyrosine phosphorylation cascade, which is ultimately associated with the expression of capacitation-dependent sperm functions, such as hyperactivated movement and acrosomal exocytosis. As spermatozoa progress through the epididymis they first acquire the capacity to phosphorylate tyrosine on targets on the principal piece, followed by the midpiece. By the time these cells have reached the cauda epididymidis they can phosphorylate the entire tail from neck to endpiece. This particular pattern of phosphorylation is associated with the ontogeny of fully functional spermatozoa that are capable of fertilizing the oocyte. Proteomic analyses indicate that this change is associated with the phosphorylation of several mitochondrial proteins, creation of a mitochondrial membrane potential and activation of mitochondrial free radical generation. At least in rodent species, activation of sperm mitochondria appears to be a particularly important part of epididymal maturation.

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.

Identification of gene products present in Triton X-100 soluble and insoluble fractions of human spermatozoa lysates using LC-MS/MS analysis

A comprehensive analysis of the proteins found in human spermatozoa is essential for understanding the events leading up to, and including, fertilization and development. Proteomics offers a platform for investigating this process, provided that the dynamic range is relatively low. In this report, spermatozoa from a number of human sperm ejaculates were isolated in a pure state using discontinuous Percoll gradient centrifugation. Triton X-100 soluble and insoluble proteins were recovered and separated by SDS-PAGE. The separation lanes were dissected into 96 fractions and analyzed individually by LC-MS(n) . A comprehensive protocol, involving LC-MS/MS analysis eventually down to the ninth most intense peak found in the MS-survey scan, was performed. Analysis of purified human sperm populations resulted in the identification of 1056 gene products, of which approximately 8% have not previously been characterized. The data were supported by the large number of proteins represented by expressed sequence tags in the testis. Bioinformatic analysis demonstrated that 437 of the gene products were involved in various metabolic pathways including glycolysis and oxidative phosphorylation. The inventory of proteins present in the human sperm proteome includes a number of notable discoveries including the first description of a nicotinamide adenine dinucleotide phosphate oxidase, dual-oxidase 2, finally laying to rest any doubts about the presence of such enzymes in spermatozoa. Furthermore, a number of different classes of receptor have also been detected in these cells and are potential regulators of sperm function. This list includes at least six seven-pass transmembrane receptors, six tyrosine kinase receptors, a tyrosine phosphatase receptor, glutamate-gated ion channel receptors, transient receptor potential cation channels, and a non-genomic progesterone receptor. This is the first published list of identified proteins in human spermatozoa using LC-MS/MS analysis.

Ontogeny of Tyrosine Phosphorylation-Signaling Pathways During Spermatogenesis and Epididymal Maturation in the Mouse1

The objectives of this study were to map the ontogeny of tyrosine phosphorylation signal transduction pathways during germ cell development and to determine their association with the differentiation of a functional gamete. Until testicular germ cells differentiate into spermatozoa, cAMP-induced tyrosine phosphorylation is not detectable. Entry of these cells into the epididymis is accompanied by sudden activation of the tyrosine phosphorylation pathway, initially in the principal piece of the cell and subsequently in the midpiece. In the caput and corpus epididymides, the potential to express this pathway is inhibited by the presence of calcium in the extracellular medium. However, calcium has no effect on the expression of this pathway in caudal epididymal sperm. The competence of these cells to phosphorylate the entire sperm tail, from the neck to the tail-end piece, is accompanied by a capacity to exhibit hyperactivated motility on stimulation with cAMP. A distinctly different pattern of tyrosine phosphorylation, involving the acrosomal domain of the sperm head, is invoked as spermatozoa enter the caput epididymis, and phosphorylation remains high until these cells enter the distal corpus and cauda. The proportion of cells exhibiting this form of tyrosine phosphorylation is not affected by extracellular calcium or cAMP but is negatively correlated (R2 = 0.99) with their ability to acrosome-react. However, this relationship is not causative. Our findings indicate that the development of functional spermatozoa is accompanied by carefully orchestrated changes in tyrosine phosphorylation, controlled by independent regulatory mechanisms in distinct subcellular compartments of these highly specialized cells.

Characterization of cyclic adenine dinucleotide phosphate ribose levels in human spermatozoa

OBJECTIVE

To determine the presence of the Ca2+-releasing pyridine nucleotide derivative, cyclic adenine dinucleotide phosphate ribose (cADPR), in human spermatozoa and to investigate its role in progesterone-induced Ca2+ oscillations in spermatozoa.

DESIGN

Biochemical investigation on human spermatozoa from healthy volunteers.

SETTING

Healthy volunteers in an academic research environment.

PATIENT(S)

Ten volunteers.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

The cADPR levels.

RESULT(S)

Human spermatozoa contain micromolar concentrations of cADPR that do not change significantly during sperm capacitation. An active synthetic machinery for cADPR is present in human spermatozoa, whereas degradation activity is minimal. Although progesterone-induced Ca2+ oscillations are dependent on the ryanodine receptor, they are unaffected by cADPR antagonists.

CONCLUSION(S)

It appears that cADPR does not to play a role in Ca2+ oscillations in spermatozoa, but the presence of high concentrations of cADPR suggests that, instead, it may be introduced into the egg at fertilization and play a role in the Ca2+ transient immediately following sperm-egg fusion.

Identification of SRC as a key PKA-stimulated tyrosine kinase involved in the capacitation-associated hyperactivation of murine spermatozoa

Fertilization of the mammalian oocyte depends on the ability of spermatozoa to undergo a process known as capacitation as they ascend the female reproductive tract. A fundamental feature of this process is a marked increase in tyrosine phosphorylation by an unusual protein kinase A (PKA)-mediated pathway. To date, the identity of the intermediate PKA-activated tyrosine kinase driving capacitation is still unresolved. In this study, we have identified SRC as a candidate intermediate kinase centrally involved in the control of sperm capacitation. Consistent with this conclusion, the SRC kinase inhibitor SU6656 was shown to suppress both tyrosine phosphorylation and hyperactivation in murine spermatozoa. Moreover, SRC co-immunoprecipitated with PKA and this interaction was found to lead to an activating phosphorylation of SRC at position Y416. We have also used difference-in-2D-gel-electrophoresis (DIGE) in combination with mass spectrometry to identify a number of SRC substrates that become phosphorylated during capacitation including enolase, HSP90 and tubulin. Our data further suggest that the activation of SRC during capacitation is negatively controlled by C-terminal SRC kinase. The latter was localized to the acrosome and flagellum of murine spermatozoa by immunocytochemistry, whereas capacitation was associated with an inactivating serine phosphosphorylation of this inhibitory kinase.

KATP channels in mouse spermatogenic cells and sperm, and their role in capacitation

Mammalian sperm must undergo a series of physiological changes after leaving the testis to become competent for fertilization. These changes, collectively known as capacitation, occur in the female reproductive tract where the sperm plasma membrane is modified in terms of its components and ionic permeability. Among other events, mouse sperm capacitation leads to an increase in the intracellular Ca(2+) and pH as well as to a hyperpolarization of the membrane potential. It is well known that ion channels play a crucial role in these events, though the molecular identity of the particular channels involved in capacitation is poorly defined. In the present work, we report the identification and potential functional role of K(ATP) channels in mouse spermatogenic cells and sperm. By using whole-cell patch clamp recordings in mouse spermatogenic cells, we found K(+) inwardly rectifying (K(ir)) currents that are sensitive to Ba(2+), glucose and the sulfonylureas (tolbutamide and glibenclamide) that block K(ATP) channels. The presence of these channels was confirmed using inhibitors of the ATP synthesis and K(ATP) channel activators. Furthermore, RT-PCR assays allowed us to detect transcripts for the K(ATP) subunits SUR1, SUR2, K(ir)6.1 and K(ir)6.2 in total RNA from elongated spermatids. In addition, immunoconfocal microscopy revealed the presence of these K(ATP) subunits in mouse spermatogenic cells and sperm. Notably, incubation of sperm with tolbutamide during capacitation abolished hyperpolarization and significantly decreased the percentage of AR in a dose-dependent fashion. Together, our results provide evidence for the presence of K(ATP) channels in mouse spermatogenic cells and sperm and disclose the contribution of these channels to the capacitation-associated hyperpolarization.

Exploiting common targets in human fertilization and HIV infection: development of novel contraceptive microbicides

The continued high rates of unintended pregnancies and the unrelentless expansion of the acquired immune deficiency syndrome (AIDS) epidemic, especially in less developed countries, warrant the development of novel strategies to help individuals avoid these risks. Dually active compounds displaying contraceptive and microbicidal anti-human immunodeficiency virus (anti-HIV) properties constitute one such strategy. Sharing the same anatomical and functional context, sperm fertilization and genital infection by HIV offer an opportunity for simultaneous intervention. Some of the molecules and mechanisms used by sperm to fertilize the oocyte are similar, if not identical, to those used by HIV while infecting host cells. An example of common structures is the lipid membrane surrounding the spermatozoon and the HIV core. Disruption of its architecture by surface-active compounds exerts both spermicidal and virucidal activity. A more specific alteration of lipid rafts [membrane microdomains enriched in cholesterol and glycosylphosphatidylinositol (GPI)-anchored proteins] by beta-cyclodextrins also results in similar effects. During fertilization and infection, both sperm and HIV interact with their target cell receptors through chemical charges, hydrophobic forces and carbohydrate recognition. Anionic polymers such as cellulose sulphate and polystyrene sulphonate (PSS) inhibit sperm and HIV cell binding. Because some of the molecules involved in this interaction, e.g. heparin sulphate proteoglycan, are also used by other pathogens to infect their target tissues, polyanions exert broad antimicrobial activity as well. During fertilization and infection, sperm and HIV, as well as other microbes, use signal transduction molecules and mechanisms such as adenyl cyclase/cyclic adenosine monophosphate (cAMP)-dependent kinase, calcium and tyrosine phosphorylation, whose inhibition has been shown to impair sperm function and HIV replication. These commonalities at the level of sperm and HIV structure, cell binding and fusion processes, and signalling pathways therefore provide the biological framework to develop bifunctional inhibitors with both antimicrobial and contraceptive properties.

A cyclic adenosine 3',5'-monophosphate-induced tyrosine phosphorylation of Syk protein tyrosine kinase in the flagella of boar spermatozoa

A cyclic adenosine 3',5'-monophosphate (cAMP)-dependent protein tyrosine phosphorylation is involved in the expression of fertilizing ability in mammalian spermatozoa. However, there are only limited data concerning the identification of protein tyrosine kinase (PTK) that is activated by the cAMP signaling. In this study, we have shown data supporting that boar sperm flagellum possesses a unique cAMP-protein kinase A (PKA) signaling cascade leading to phosphorylation of Syk PTK at the tyrosine residues of the activation loop. Ejaculated spermatozoa were washed and then incubated in a modified Krebs-Ringer HEPES medium (mKRH) containing polyvinyl alcohol (PVA) plus 0.1 mM cBiMPS (a cell-permeable cAMP analog), 0.25 mM sodium orthovanadate (Na3VO4) (a protein tyrosine phosphatase (PTP) inhibitor) or both at 38.5 degrees C for 180 min. Aliquots of the sperm suspensions were recovered before and after incubation and then used to detect sperm tyrosine-phosphorylated proteins by Western blotting and indirect immunofluorescence. In the Western blotting, the anti-phosphotyrosine monoclonal antibody (4G10) recognized several bands including 72-kDa protein in the protein extracts from spermatozoa that were incubated solely with cBiMPS. The tyrosine phosphorylation in these sperm proteins was dependent on cBiMPS and enhanced by the addition of Na3VO4. The 72-kDa tyrosine-phosphorylated protein was apparently reacted with the anti-phospho-Syk antibody (Tyr525/526). Indirect immunofluorescence revealed that the connecting and principal pieces of spermatozoa incubated with cBiMPS and Na3VO4 were stained with the anti-phospho-Syk antibody. However, the reactivity of the 72-kDa protein with the anti-phospho-Syk antibody was reduced by the addition of H-89 (a PKA inhibitor, 0.01-0.1 mM) to the sperm suspensions but not affected by the pretreatment of spermatozoa with BAPTA-AM (an intracellular Ca2+ chelator, 0.1 mM). Fractionation of phosphorylated proteins from the spermatozoa with a detergent Nonidet P-40 suggested that the 72-kDa tyrosine-phosphorylated protein might be a cytoskeletal component. Based on these findings, we have concluded that the cAMP-PKA signaling is linked to the Ca2+-independent tyrosine phosphorylation of Syk in the connecting and principal pieces of boar spermatozoa.

Identification of cytochrome-b5 reductase as the enzyme responsible for NADH-dependent lucigenin chemiluminescence in human spermatozoa

Lucigenin-dependent chemiluminescence together with 2-[4-iodophenyl]-3-[4-nitrophenyl]-5-[2,4-disulfophenyl]-2H tetrazolium monosodium salt (WST-1) reduction can be detected following addition of NADH to many cell types, including human sperm suspensions. Although many reports suggest that such a phenomenon is due to reactive oxygen species production, other oxygen detecting metabolite probes, such as MCLA and luminol, do not produce a chemiluminescent signal in this model system. The enzyme responsible for NADH-dependent lucigenin chemiluminescence was purified and identified as cytochrome-b5 reductase. In support of this concept, COS-7 cells overexpressing cytochrome-b5 reductase displayed at least a 3-fold increase in the previously mentioned activity compared with mock-transfected cells. Fractions containing cytochrome-b5 reductase were capable of inducing both lucigenin-dependent chemiluminescence and WST-1 reduction. Oxygen radicals clearly did not mediate the cytochrome b5-mediated activation of these probes in vitro since neither luminol nor MCLA gave a chemiluminescence response in the presence of the enzyme and the cofactor NADH. These results emphasize the importance of the direct NADH-dependent reduction of these putative superoxide-sensitive probes by cytochrome-b5 reductase even though this enzyme does not, on its own accord, produce reactive oxygen species.

Evidence of the presence of calcium/calmodulin-dependent protein kinase IV in human sperm and its involvement in motility regulation

The mechanisms involved in the regulation of mammalian sperm motility are not well understood. Calcium ions (Ca(2+)) have been suggested to play a key role in the maintenance of motility; nevertheless, how Ca(2+) modulates this process has not yet been completely characterized. Ca(2+) can bind to calmodulin and this complex regulates the activity of multiple enzymes, including Ca(2+)/calmodulin-dependent protein kinases (CaM kinases). Results from this study confirmed that the presence of Ca(2+) in the incubation medium is essential for maintaining human sperm motility. The involvement of CaM kinases in Ca(2+) regulation of human sperm motility was evaluated using specific inhibitors (KN62 and KN93) or their inactive analogues (KN04 and KN92 respectively). Sperm incubation in the presence of KN62 or KN93 led to a progressive decrease in the percentage of motile cells; in particular, incubation with KN62 also reduced sperm motility parameters. These inhibitors did not alter sperm viability, protein tyrosine phosphorylation or the follicular fluid-induced acrosome reaction; however, KN62 decreased the total amount of ATP in human sperm. Immunological studies showed that Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) is present and localizes to the human sperm flagellum. Moreover, CaMKIV activity increases during capacitation and is inhibited in the presence of KN62. This report is the first to demonstrate the presence of CaMKIV in mammalian sperm and suggests the involvement of this kinase in the regulation of human sperm motility.

Identification of post-translational modifications that occur during sperm maturation using difference in two-dimensional gel electrophoresis

Difference in two-dimensional (2-D) gel electrophoresis (DIGE) is a novel method for analyzing up to three samples in one 2-D gel and using the information gained to study post-translational modifications of proteins. We describe the use of DIGE to isolate and characterize those proteins that undergo processing in spermatozoa as they transit the epididymal tract. We find up to 60 protein spots are significantly modified as sperm traverse the epididymis. In this article, we report eight unambiguous protein identifications and demonstrate that one protein, the beta-subunit of the mitochondrial F1-ATPase, is serine-phosphorylated as sperm undergo epididymal maturation. We suggest that phosphorylation of this particular protein in a cAMP-dependent manner may contribute to the mechanisms by which motility is conferred upon spermatozoa.

Tyrosine Phosphorylation Generates Multiple Isoforms of the Mitochondrial Capsule Protein, Phospholipid Hydroperoxide Glutathione Peroxidase (PHGPx), During Hamster Sperm Capacitation1

Sperm capacitation is a maturation process, occurring in the female reproductive tract, that produces fertilization-competent spermatozoa. Protein tyrosine phosphorylation represents an important event in capacitation. The present study demonstrates the capacitation-dependent tyrosine-phosphorylation of phospholipid hydroperoxide glutathione peroxidase (PHGPx), the disulfide cross-linked, major structural protein of the sperm mitochondrial capsule. Immunofluorescence microscopy using an antiphosphotyrosine monoclonal antibody (anti-pY20) demonstrated the presence of capacitation-associated tyrosine phosphorylated proteins in the flagellum of hamster spermatozoa. Among the tyrosine-phosphorylated polypeptides (M(r) 19,000- 99,000), a 19-kDa polypeptide was the only one that can be solubilized completely by Triton X-100-dithiothreitol (DTT). The 19-kDa polypeptide was purified by anion-exchange chromatography and by immunoaffinity chromatography. Proteomic identification of the 19-kDa polypeptide by nano-electrospray tandem mass spectrometry yielded six peptides that matched the National Center for Biotechnology Information (NCBI) database sequences of bovine PHGPx. Indirect immunofluorescence localized PHGPx to the midpiece of the flagellum and the immunoblot analysis demonstrated its DTT-dependent release from purified flagella. DTT extracts of noncapacitated spermatozoa exhibited a charge train of four major PHGPx isoforms (pIs 7.5- 9.0) by two-dimensional PAGE, whereas capacitated spermatozoa revealed the generation of new acidic PHGPx isoforms with isoelectric points ranging between pH 6.0-7.0 and 4.0-5.0, indicating that it is posttranslationally modified during capacitation. These data suggest that the tyrosine-phosphorylation of PHGPx may represent an important event in the signaling pathway(s) associated with capacitation and could potentially affect mitochondrial function.

Role of tyrosine phosphorylation in sperm capacitation / acrosome reaction

Capacitation is an important physiological pre-requisite before the sperm cell can acrosome react and fertilize the oocyte. Recent reports from several laboratories have amply documented that the protein phosphorylation especially at tyrosine residues is one of the most important events that occur during capacitation. In this article, we have reviewed the data from our and other laboratories, and have constructed a heuristic model for the mechanisms and molecules involved in capacitation/acrosome reaction.