A cyclic adenosine 3',5'-monophosphate-dependent protein kinase C activation is involved in the hyperactivation of boar spermatozoa

Involvement of Ca2+-ATPase in suppressing the appearance of bovine helically motile spermatozoa with intense force prior to cryopreservation

In cattle, cryopreserved spermatozoa are generally used for artificial insemination (AI). Many of these specimens exhibit helical movement, although the molecular mechanisms underlying this phenomenon remain unclear. This study aimed to characterize helically motile spermatozoa, investigate the involvement of Ca²⁺-ATPase in suppressing the appearance of these spermatozoa prior to cryopreservation, and examine the potential of helical movement as an index of sperm quality. In the cryopreserved semen, approximately 50% of spermatozoa were helically motile, whereas approximately 25% were planarly motile. The helically motile samples swam significantly faster than those with planar movement, in both non-viscous medium and viscous medium containing polyvinylpyrrolidone. In contrast, in non-cryopreserved semen, planarly motile spermatozoa outnumbered those that were helically motile. Fluorescence microscopy with Fluo-3/AM and propidium iodide showed that flagellar [Ca²⁺]_i was significantly higher in cryopreserved live spermatozoa than in non-cryopreserved live ones. The percentage of non-cryopreserved helically motile spermatozoa was approximately 25% after washing, and this increased significantly to approximately 50% after treatment with an inhibitor of sarcoplasmic reticulum Ca²⁺-ATPases (SERCAs), “thapsigargin.” Immunostaining showed the presence of SERCAs in sperm necks. Additionally, the percentages of cryopreserved helically motile spermatozoa showed large inter-bull differences and a significantly positive correlation with post-AI conception rates, indicating that helical movement has the potential to serve as a predictor of the fertilizing ability of these spermatozoa. These results suggest that SERCAs in the neck suppress the cytoplasmic Ca²⁺-dependent appearance of helically motile spermatozoa with intense force in semen prior to cryopreservation.

Calmodulin is involved in the occurrence of extracellular Ca2+ -dependent full-type hyperactivation in boar ejaculated spermatozoa incubated with cyclic AMP analogs

In mammals, hyperactivation is essential for sperm fertilization with oocytes in vivo. Two types of hyperactivation "full-type and nonfull-type patterns" can be observed in the spermatozoa from boars, bulls, and mice. We have a hypothesis that the full-type hyperactivation is a physiological (in vivo) pattern and are elucidating its molecular bases. The aims of this study were to detect calmodulin in boar sperm flagella by Western blotting and indirect immunofluorescence and to investigate effects of extracellular Ca²⁺ and calmodulin antagonists "W-7 and W-5 (W-5; a less potent antagonist)" on the occurrence of full-type hyperactivation in boar spermatozoa. Calmodulin was specifically detected as the 17-kDa antigen in the flagella and postacrosomal region of the heads. Full-type hyperactivation could be induced effectively in the samples incubated with 3.42 mM CaCl₂ for 120-180 min, and it was significantly reduced in the concentration-dependent manners of W-7 and W-5. Suppressing effects of W-7 on the full-type hyperactivation were stronger than those of W-5. These observations indicate that flagellar calmodulin is involved in the occurrence of extracellular Ca²⁺ -dependent full-type hyperactivation in boar spermatozoa. This is the first indication of the intracellular Ca²⁺ -sensing molecule which can function in the full-type hyperactivation.

Effects of digoxin on full-type hyperactivation in bovine ejaculated spermatozoa with relatively lower survivability for incubation with stimulators of cAMP signaling cascades

Previous researches of our laboratory reported that addition of cAMP analog cBiMPS and protein phosphatase inhibitor calyculin A (stimulators of cAMP signaling cascades) improved the capacity of incubation medium to induce full-type hyperactivation in bovine ejaculated spermatozoa. However, this modified medium was valid only for samples with relatively good survivability for incubation with stimulators of cAMP signaling cascades. Thus, it is necessary to make further modified medium for evaluation of potentials to exhibit full-type hyperactivation in bovine sperm samples with relatively lower survivability. Na⁺/K⁺-ATPase is an integral membrane protein and involved with the regulation of rodent sperm motility. To make further modification of the medium, we examined effects of Na⁺/K⁺-ATPase inhibition with digoxin on motility, full-type hyperactivation and protein tyrosine phosphorylation in bovine ejaculated spermatozoa with relatively lower survivability for incubation with stimulators of cAMP signaling cascades and also performed the immunodetection of bovine sperm Na⁺/K⁺-ATPase. The addition of Na⁺/K⁺-ATPase inhibitor digoxin to the incubation medium containing cBiMPS and calyculin A had the tendency to lessen the decreases in the percentages of motile spermatozoa in all of 12 samples after the incubation for 1-3 h and significantly increased the percentages of full-type hyperactivation in one group of 4 samples (Sample-A1) and another group of 4 samples (Sample-A2) after 1 and 2 h respectively, though it had no significant effects on full-type hyperactivation in the other group of 4 samples (Sample-B). In addition, incubation time-related changes in the sperm protein tyrosine phosphorylation (a good marker for sperm capacitation) were correlated with those in the percentages of full-type hyperactivation in Sample-A1 containing digoxin. Immunodetection showed that Na⁺/K⁺-ATPase is present in the middle and principal pieces of the flagella, indicating that Na⁺/K⁺-ATPase has possible relations with sperm motility. These results obtained with bull ejaculated spermatozoa with relatively lower survivability indicate that incubation method using digoxin is useful to evaluate potentials of sperm samples to exhibit full-type hyperactivation, that digoxin has effects on suppressing reduction of sperm motility, and that prolonged incubation with digoxin induces reduction of capacitation state which may suppress the maintenance of full-type hyperactivation.

Identification of isoforms of calyculin A-sensitive protein phosphatases which suppress full-type hyperactivation in bull ejaculated spermatozoa

In bull spermatozoa, extracellular Ca²⁺-dependent full-type hyperactivation, which is characterized by the asymmetrical beating in whole parts of the middle/principal pieces, is suppressed by calyculin A-sensitive protein phosphatases. The aim of this study was to identify isoforms of these protein phosphatases. Ejaculated spermatozoa were used for the investigation on effects of protein phosphatase inhibitors (calyculin A with high specificity for both of protein phosphatases 1 and 2A, and okadaic acid with relatively higher specificity for protein phosphatase 2A than protein phosphatase 1) on the induction of extracellular Ca²⁺-dependent full-type hyperactivation by incubation with CaCl₂ and cAMP analog (cBiMPS). They were also used for the immunodetection of protein phosphatases 1α, 1β, 1γ, 2Aα and 2Aβ. Percentages of full-type hyperactivated spermatozoa significantly increased after incubation with calyculin A (10 nM) in a concentration-dependent manner of CaCl₂ (0-3.42 mM), though only minor increases in the percentages of full-type hyperactivated spermatozoa were observed after incubation with okadaic acid (10 nM). Moreover, the immunodetection of protein phosphatase isoforms showed sperm connecting piece and flagellum included protein phosphatases 1α and 1γ, but did not do the other isoforms. These results suggest that calyculin A-sensitive and okadaic acid-less sensitive protein phosphatases (1α and 1γ) are suppressors for the extracellular Ca²⁺-dependent full-type hyperactivation in bull ejaculated spermatozoa.

AMPK Function in Mammalian Spermatozoa

AMP-activated protein kinase AMPK regulates cellular energy by controlling metabolism through the inhibition of anabolic pathways and the simultaneous stimulation of catabolic pathways. Given its central regulator role in cell metabolism, AMPK activity and its regulation have been the focus of relevant investigations, although only a few studies have focused on the AMPK function in the control of spermatozoa's ability to fertilize. This review summarizes the known cellular roles of AMPK that have been identified in mammalian spermatozoa. The involvement of AMPK activity is described in terms of the main physiological functions of mature spermatozoa, particularly in the regulation of suitable sperm motility adapted to the fluctuating extracellular medium, maintenance of the integrity of sperm membranes, and the mitochondrial membrane potential. In addition, the intracellular signaling pathways leading to AMPK activation in mammalian spermatozoa are reviewed. We also discuss the role of AMPK in assisted reproduction techniques, particularly during semen cryopreservation and preservation (at 17 °C). Finally, we reinforce the idea of AMPK as a key signaling kinase in spermatozoa that acts as an essential linker/bridge between metabolism energy and sperm's ability to fertilize.

Flagellar hyperactivation of bull and boar spermatozoa

BACKGROUND

In mammals, flagellar hyperactivation is indispensable to sperm fertilization with oocytes in vivo, although there are species differences in regulatory mechanisms for this event. In this study, I reviewed researches regarding hyperactivation of bull and boar spermatozoa, in comparison with those of spermatozoa from other species.

METHODS

Recent publications regarding sperm hyperactivation were collected and summarized.

RESULTS MAIN FINDINGS

In bull and boar spermatozoa, there are two types of hyperactivation "full-type hyperactivation and nonfull-type hyperactivation" which are equivalent to anti-hock hyperactivation and pro-hock hyperactivation of mouse spermatozoa, respectively, on the basis of the flagellar parts exhibiting asymmetrical beating. Full-type hyperactivation is initiated in response to a rapid increase of cytoplasmic Ca2+ in the connecting/middle and principal pieces by the mobilization of this divalent ion from extracellular space and internal store through cation channels. Regulatory molecules for the increase of cytoplasmic Ca2+ in the connecting/middle pieces are probably different from those in the principal pieces.

CONCLUSION

I have proposed a hypothesis on the regulation of full-type hyperactivation by the distinct signaling cascades leading to the increase in cytoplasmic Ca2+ between the connecting/middle and principal pieces of bull and boar spermatozoa.

4-tert-octylphenol injures motility and viability of human sperm by affecting cAMP-PKA/PKC-tyrosine phosphorylation signals

4-tert-octylphenol (4t-OP) is a well-known xenoestrogen. Our objective was to explore the effects and molecular mechanisms of 4t-OP on human sperm. Sperm samples were exposed to 0, 0.1, or 0.3 mM 4t-OP for two hours. Results showed that both sperm viability and motility were significantly injured by 0.3 mM 4t-OP. We applied comparative proteomics to explore the molecular targets affected by 4t-OP. 81 differentially expressed (DE) proteins were identified. Bioinformatic analysis showed that these proteins were highly associated with motility and apoptosis, and were mostly enriched in cAMP-PKA/PKC-phosphorylation-associated pathway. We further verified that 0.1 mM and 0.3 mM 4t-OP significantly decreased cAMP activity of sperm. Expression of RACK1 and PRDX6 were detected by western blot (WB) to verify their tendencies in gels; antiapoptotic factor BCL2 was also detected by WB. The data indicated that 4-tert-octylphenol injures the motility and viability of human sperm probably by affecting cAMP-PKA/PKC-tyrosine phosphorylation signals.

Extracellular cAMP activates molecular signalling pathways associated with sperm capacitation in bovines

STUDY QUESTION

Is extracellular cAMP involved in the regulation of signalling pathways in bovine sperm capacitation?

SUMMARY ANSWER

Extracellular cAMP induces sperm capacitation through the activation of different signalling pathways that involve phospholipase C (PLC), PKC/ERK1-2 signalling and an increase in sperm Ca2+ levels, as well as soluble AC and cAMP/protein kinase A (PKA) signalling.

WHAT IS KNOWN ALREADY

In order to fertilize the oocyte, ejaculated spermatozoa must undergo a series of changes in the female reproductive tract, known as capacitation. This correlates with a number of membrane and metabolic modifications that include an increased influx of bicarbonate and Ca2+, activation of a soluble adenylyl cyclase (sAC) to produce cAMP, PKA activation, protein tyrosine phosphorylation and the development of hyperactivated motility. We previously reported that cAMP efflux by Multidrug Resistance Protein 4 (MRP4) occurs during sperm capacitation and the pharmacological blockade of this inhibits the process. Moreover, the supplementation of incubation media with cAMP abolishes the inhibition and leads to sperm capacitation, suggesting that extracellular cAMP regulates crucial signalling cascades involved in this process.

STUDY DESIGN, SIZE, DURATION

Bovine sperm were selected by the wool glass column method, and washed by centrifugation in BSA-Free Tyrode's Albumin Lactate Pyruvate (sp-TALP). Pellets were resuspended then diluted for each treatment. For in vitro capacitation, 10 to 15 × 106 SPZ/ml were incubated in 0.3% BSA sp-TALP at 38.5°C for 45 min under different experimental conditions. To evaluate the role of extracellular cAMP on different events associated with sperm capacitation, 10 nM cAMP was added to the incubation medium as well as different inhibitors of enzymes associated with signalling transduction pathways: U73122 (PLC inhibitor, 10 μM), Gö6983 (PKC inhibitor, 10 μM), PD98059 (ERK-1/2 inhibitor, 30 μM), H89 and KT (PKA inhibitors, 50 μM and 100 nM, respectively), KH7 (sAC inhibitor, 10 μM), BAPTA-AM (intracellular Ca2+ chelator, 50 μM), EGTA (10 μM) and Probenecid (MRPs general inhibitor, 500 μM). In addition, assays for binding to oviductal epithelial cells and IVF were carried out to test the effect of cAMP compared with other known capacitant agents such as heparin (60 μg/ml) and bicarbonate (40 mM).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Straws of frozen bovine semen (20-25 × 106 spermatozoa/ml) were kindly provided by Las Lilas, CIALE and CIAVT Artificial Insemination Centers. The methods used in this work include western blot, immunohistochemistry, flow cytometry, computer-assisted semen analysis, live imaging of Ca2+ and fluorescence scanning. At least three independent assays with bull samples of proven fertility were carried.

MAIN RESULTS AND THE ROLE OF CHANCE

In the present study, we elucidate the molecular events induced by extracellular cAMP. Our results showed that external cAMP induces sperm capacitation, depending upon the action of PLC. Downstream, this enzyme increased ERK1-2 activation through PKC and elicited a rise in sperm Ca2+ levels (P < 0.01). Moreover, extracellular cAMP-induced capacitation also depended on the activity of sAC and PKA, and increased tyrosine phosphorylation, indicating that the nucleotide exerts a broad range of responses. In addition, extracellular cAMP-induced sperm hyperactivation and concomitantly increased the proportion of spermatozoa with high mitochondrial activity (P < 0.01). Finally, cAMP increased the in vitro fertilization rate compared to control conditions (P < 0.001).

LARGE SCALE DATA

None.

LIMITATIONS, REASONS FOR CAUTION

This is an in vitro study performed with bovine cryopreserved spermatozoa. Studies in other species and with fresh samples are needed to extrapolate these data.

WIDER IMPLICATIONS OF THE FINDINGS

These findings strongly suggest an important role of extracellular cAMP in the regulation of the signalling pathways involved in the acquisition of bull sperm fertilizing capability. The data presented here indicate that not only a rise, but also a regulation of cAMP levels is necessary to ensure sperm fertilizing ability. Thus, exclusion of the nucleotide to the extracellular space might be essential to guarantee the achievement of a cAMP tone, needed for all capacitation-associated events to take place. Moreover, the ability of cAMP to trigger such broad and complex signalling events allows us to hypothesize that cAMP is a self-produced autocrine/paracrine factor, and supports the emerging paradigm that spermatozoa do not compete but, in fact, communicate with each other. A precise understanding of the functional competence of mammalian spermatozoa is essential to generate clinical advances in the treatment of infertility and the development of novel contraceptive strategies.

STUDY FUNDING AND COMPETING INTEREST(S)

This work was supported by Consejo Nacional de Investigaciones Científicas y Técnicas [PIP0 496 to S.P.-M.], Agencia Nacional de Promoción Científica y Tecológica [PICT 2012-1195 and PICT2014-2325 to S.P.-M., and PICT 2013-2050 to C.D.], Boehringer Ingelheim Funds, and the Swedish Farmers Foundation [SLF-H13300339 to J.M.]. The authors declare there are no conflicts of interests.

Characterization of extracellular Ca2+ -dependent full-type hyperactivation in ejaculated boar spermatozoa preincubated with a cAMP analog

Ejaculated boar spermatozoa exhibit two types of hyperactivation: full and non-full. Full-type hyperactivation is characterized by the asymmetrical bending of the entire middle piece-principal piece and a twisting/figure-eight-like trajectory, and can be induced by simple incubation with CaCl₂ after preincubation with a cAMP analog (Sp-5,6-dichloro-1-β-D-ribofuranosyl-benzimidazole-3',5'-cyclic monophosphorothioate [cBiMPS]). Here, we compared the sperm flagellar motility after treatments with elevators of [Ca²⁺ ]_i (cBiMPS/CaCl₂ , thimerosal, procaine, and 4-aminopyridine) to characterize the regulatory mechanism of extracellular Ca²⁺ -dependent, full-type hyperactivation in ejaculated boar spermatozoa, and examined the possible involvement of Transient receptor potential cation channel subfamily C member 3 (TRPC3) in this event using the specific inhibitor Pyr3. Full-type hyperactivation was induced by a 60-min incubation with CaCl₂ following a 180-min preincubation with cBiMPS but without Ca²⁺ . Thimerosal-treated spermatozoa exhibited full-type hyperactivation in a manner independent of extracellular Ca²⁺ ; conversely, this was not observed in procaine- or 4-aminopyridine-treated spermatozoa. A 20-min treatment with Pyr3 between preincubation with cBiMPS and incubation with CaCl₂ , significantly suppressed the normal phenotype. These observations indicated that mechanisms underlying full-type hyperactivation in spermatozoa incubated with CaCl₂ after preincubation with cBiMPS are different from those in the thimerosal-treated spermatozoa. Furthermore, indirect immunofluorescence localized TRPC3 in the upper segment of the middle piece, which bends asymmetrically during full-type hyperactivation but not in non-full-type hyperactivation, suggesting that TRPC3 may be involved in the extracellular Ca²⁺ -dependent full-type hyperactivation in ejaculated boar spermatozoa.

Effects of a Protein Kinase Inhibitor on Sperm Motility in the Japanese Quail

Sperm motility is an essential trait for successful fertilization in animals. In birds, ejaculated sperm migrate into sperm storage tubules before fertilization and are stored in a quiescent state. We previously reported that this type of sperm's flagellar quiescence was induced by lactic acid through flagellar dynein ATPase inactivation following cytoplasmic acidification (<pH 6.0). However, signal transduction in the sperm cells leading to motility inactivation is not well understood. The aim of the present study was to investigate the role of protein kinases in putative signal transduction in quail spermatozoa motility in vitro. Following incubation with bisindolylmaleimide II (BisII), a potent-competitive protein kinase C (PKC) inhibitor, sperm motility decreased in a dose related-manner. However, no such inhibitory effect was found in sperm exposed to bisindolylmaleimide V, H-89, or LY294002, a weak inhibitor of PKC, a potent inhibitor of protein kinase A (PKA) and a selective inhibitor of phosphatidylinositol 3-kinase, respectively. BisII-treated sperm exhibited no significant differences in pH _i , [Ca²⁺] _i , mitochondrial activity, intracellular cAMP or ATP concentration, as well as dynein ATPase activity, compared to the control sperm. However, when the phosphorylated substrate proteins by PKC were detected by Western blot analysis, the intensity of the band in sperm incubated in the presence of BisII decreased. Moreover, immunoreactive PKCι and µ isoforms in the sperm lysates were also detected. These results indicated that the PKC signaling pathway may be involved in sperm motility regulation, and protein phosphorylation by PKC may be required to maintain flagellar movement in the Japanese quail.

Translocation of Tektin 3 to the equatorial segment of heads in bull spermatozoa exposed to dibutyryl cAMP and calyculin A

Tektins (TEKTs) are filamentous proteins associated with microtubules in cilia, flagella, basal bodies, and centrioles. Five TEKTs (TEKT1, -2, -3, -4, and -5) have been identified as components of mammalian sperm flagella. We previously reported that TKET1 and -3 are also present in the heads of rodent spermatozoa. The present study clearly demonstrates that TEKT2 is present at the acrosome cap whereas TEKT3 resides just beneath the plasma membrane of the post-acrosomal region of sperm heads in unactivated bull spermatozoa, and builds on the distributional differences of TEKT1, -2, and -3 on sperm heads. We also discovered that hyperactivation of bull spermatozoa by cell-permeable cAMP and calyculin A, a protein phosphatase inhibitor, promoted translocation of TEKT3 from the post-acrosomal region to the equatorial segment in sperm heads, and that TEKT3 accumulated at the equatorial segment is lost upon acrosome reaction. Thus, translocation of TEKT3 to the equatorial segment may be a capacitation- or hyperactivation-associated phenomenon in bull spermatozoa. Mol. Reprod. Dev. 84: 30-43, 2017. © 2016 Wiley Periodicals, Inc.

Involvement of cAMP-dependent unique signaling cascades in the decrease of serine/threonine-phosphorylated proteins in boar sperm head

We previously suggested that protein phosphatase-dependent decrease of postacrosomal phosphorylated proteins may be necessary for the occurrence of acrosome reaction in livestock spermatozoa (Adachi et al., J Reprod Dev 54, 171-176, 2008; Mizuno et al., Mol Reprod Dev 82, 232-250, 2015). The aim of this study was to examine the involvement of the intracellular cAMP signaling cascades in the regulation of the decrease of postacrosomal phosphorylated proteins in boar spermatozoa. Boar ejaculated spermatozoa were incubated with cAMP analogs and then used for the immunodetection of serine/threonine-phosphorylated proteins and assessment of acrosome morphology. The protein phosphatase-dependent decrease of postacrosomal phosphorylated proteins was greatly promoted by the incubation with a cAMP analog Sp-5,6-dichloro-1-β-D-ribofuranosyl-benzimidazole-3',5'-monophosphorothioate (cBiMPS). This decrease was induced before the initiation of acrosome reaction and did not require the millimolar concentration of extracellular Ca(2+) which was necessary for the initiation of acrosome reaction. Moreover, suppression of protein kinase A activity with an inhibitor (H89) had almost no influence on both decrease of phosphorylated proteins and occurrence of acrosome reaction in the spermatozoa incubated with cBiMPS. In addition, the prolonged incubation with a potentially exchange protein directly activated by cAMP-selective cAMP analog (8pM) could only partially mimic effects of cBiMPS on these events. These results indicate that the cAMP-dependent signaling cascades which are less dependent on protein kinase A may regulate the decrease of postacrosomal phosphorylated proteins in boar spermatozoa before the extracellular Ca(2+)-triggered initiation of acrosome reaction.

Second messengers, steroids and signaling cascades: Crosstalk in sperm development and function

Signaling cascades control numerous aspects of sperm physiology, ranging from creation to fertilization. Novel aspects of several kinases and their influence on sperm development will be discussed in the first section and cover proliferation, chromatin remodeling and morphology. In the second section, protein kinases (A, B and C) that affect sperm function and their regulation by second messengers, cyclic-AMP and phosphoinositides, as well as steroids will be featured. Key areas of integration will be presented on the topics of sperm motility, capacitation, acrosome reaction and fertilization.

Distinct segment-specific functions of calyculin A-sensitive protein phosphatases in the regulation of cAMP-triggered events in ejaculated bull spermatozoa

Livestock spermatozoa possess more tenacious suppressors of cAMP-triggered events-including capacitation-associated changes-than laboratory animal spermatozoa, leading to flagellar hyperactivation. In order to identify the suppressors, we examined effects of an inhibitor of serine/threonine protein phosphatases (calyculin A) on cAMP-triggered changes in the protein phosphorylation state, and subsequent occurrence of hyperactivation and acrosome reaction in ejaculated bull spermatozoa. Ejaculated spermatozoa were incubated in cAMP-supplemented medium, then assessed for motility, acrosome morphology, and phosphorylated protein localization. The addition of calyculin A greatly enhanced cAMP-triggered protein phosphorylation at serine/threonine and tyrosine residues in the connecting piece and induction of flagellar hyperactivation. Most hyperactivated spermatozoa exhibited extremely asymmetrical bends at the middle piece, which produced intensive twisting or figure-eight movements. In the sperm head, however, cAMP-triggered dephosphorylation of serine/threonine-phosphorylated proteins and subsequent acrosome reaction were abolished by the addition of calyculin A. Based on these results, we suggest that calyculin A-sensitive protein phosphatases in the connecting piece are suppressors of cAMP-triggered events leading to hyperactivation. By contrast, similar protein phosphatases in the sperm head accelerate cAMP-triggered events leading to the acrosome reaction. These findings are consistent with the indication that calyculin A-sensitive protein phosphatases have distinct functions in the regulation of cAMP-triggered events in different regions of ejaculated bull spermatozoa.

Roles of extracellular Ca(2+) in the occurrence of full-type hyperactivation in boar ejaculated spermatozoa pre-incubated to induce the cAMP-triggered events

There are species differences in the regulatory system for sperm capacitation and subsequent hyperactivation between livestock and laboratory animals. In livestock spermatozoa, it is poorly understood when and how extracellular Ca(2+) is necessary for hyperactivation, although it has been demonstrated that the [Ca(2+) ]i increase is indispensable to occurrence of hyperactivation. In this study, we examined necessity of extracellular Ca(2+) for the initiation and maintenance of hyperactivation and then sought possible target molecule of Ca(2+) that was involved in hyperactivation of boar spermatozoa. Boar ejaculated spermatozoa were pre-incubated with a cell-permeable cyclic adenosine monophosphate (cAMP) analog 'cBiMPS' and without CaCl2 to induce the cAMP-triggered events including capacitation-associated changes. Subsequently, they were incubated with CaCl2 to induce hyperactivation and then used for motility assessment. Many of the spermatozoa after the incubation exhibited full-type hyperactivation which was characterized by high-amplitude and extremely asymmetrical beating of whole middle piece and principal piece. The initiation of full-type hyperactivation required the millimolar concentration of CaCl2 in the medium. However, CaCl2 of the medium was less necessary for maintenance than initiation of full-type hyperactivation, as hyperactivated spermatozoa were barely affected by the incubation with the Ca(2+) -chelating reagent. On the other hand, the pre-treatment with the inhibitor for Ca(2+) -dependent protease 'calpain 1 and 2' clearly suppressed the occurrence of CaCl2 -induced hyperactivation without influences on the percentages of motile spermatozoa. Western blotting and indirect immunofluorescence showed distribution of calpain 2 in the middle and principal pieces in which full-type hyperactivated spermatozoa exhibited extremely asymmetrical beating. On the basis of these results, we conclude that the millimolar concentration of extracellular Ca(2+) is necessary for the initiation, but not for the maintenance of full-type hyperactivation in boar spermatozoa that beforehand undergo the cAMP-triggered events including capacitation-associated changes. Moreover, we suggest possible involvement of calpain 2 in the intracellular Ca(2+) signal transduction leading to full-type hyperactivation.

Roles of intracellular cyclic AMP signal transduction in the capacitation and subsequent hyperactivation of mouse and boar spermatozoa

It is not until accomplishment of a variety of molecular changes during the transit through the female reproductive tract that mammalian spermatozoa are capable of exhibiting highly activated motility with asymmetric whiplash beating of the flagella (hyperactivation) and undergoing acrosomal exocytosis in the head (acrosome reaction). These molecular changes of the spermatozoa are collectively termed capacitation and promoted by bicarbonate, calcium and cholesterol acceptors. Such capacitation-promoting factors can stimulate intracellular cyclic AMP (cAMP) signal transduction in the spermatozoa. Meanwhile, hyperactivation and the acrosome reaction are essential to sperm fertilization with oocytes and are apparently triggered by a sufficient increase of intracellular Ca²⁺ in the sperm flagellum and head, respectively. Thus, it is necessary to investigate the relationship between cAMP signal transduction and calcium signaling cascades in the spermatozoa for the purpose of understanding the molecular basis of capacitation. In this review, I cover updated insights regarding intracellular cAMP signal transduction, the acrosome reaction and flagellar motility in mammalian spermatozoa and then account for possible roles of intracellular cAMP signal transduction in the capacitation and subsequent hyperactivation of mouse and boar spermatozoa.

The calcium/CaMKKalpha/beta and the cAMP/PKA pathways are essential upstream regulators of AMPK activity in boar spermatozoa

Spermatozoa successfully fertilize oocytes depending on cell energy-sensitive processes. We recently showed that the cell energy sensor, the AMP-activated protein kinase (AMPK), plays a relevant role in spermatozoa by regulating motility as well as plasma membrane organization and acrosomal integrity, and contributes to the maintenance of mitochondrial membrane potential. As the signaling pathways that control AMPK activity have been studied exclusively in somatic cells, our aim is to investigate the intracellular pathways that regulate AMPK phosphorylation at Thr(172) (activity) in male germ cells. Boar spermatozoa were incubated under different conditions in the presence or absence of Ca(2+), 8Br-cAMP, IBMX, PMA, the AMPK activator A769662, or inhibitors of PKA, PKC, or CaMKKalpha/beta. AMPK phosphorylation was evaluated by Western blot using anti-phospho-Thr(172)-AMPK antibody. Data show that AMPK phosphorylation in spermatozoa is potently stimulated by an elevation of cAMP levels through the activation of PKA, as the PKA inhibitor H89 blocks phospho-Thr(172)-AMPK. Another mechanism to potently activate AMPK is Ca(2+) that acts through two pathways, PKA (blocked by H89) and CaMKKalpha/beta (blocked by STO-609). Moreover, phospho-Thr(172)-AMPK levels greatly increased upon PKC activation induced by PMA, and the PKC inhibitor Ro-32-0432 inhibits TCM-induced AMPK activation. Different stimuli considered as cell stresses (rotenone, cyanide, sorbitol, and complete absence of intracellular Ca(2+) by BAPTA-AM) also cause AMPK phosphorylation in spermatozoa. In summary, AMPK activity in boar spermatozoa is regulated upstream by different kinases, such as PKA, CaMKKalpha/beta, and PKC, as well as by the essential intracellular messengers for spermatozoan function, Ca(2+) and cAMP levels.

Relationship between cyclic AMP-dependent protein tyrosine phosphorylation and extracellular calcium during hyperactivation of boar spermatozoa

In mammalian spermatozoa, the state of protein tyrosine phosphorylation is modulated by protein tyrosine kinases and protein tyrosine phosphatases that are controlled via cyclic AMP (cAMP)-protein kinase A (PKA) signaling cascades. The aims of this study were to examine the involvement of cAMP-induced protein tyrosine phosphorylation in response to extracellular calcium and to characterize effects of pharmacological modulation of the cAMP-induced protein phosphorylation state and calmodulin activity during hyperactivation in boar spermatozoa. Ejaculated spermatozoa were incubated with cBiMPS (a cell-permeable cAMP analog) and CaCl(2) at 38.5°C to induce hyperactivation, and then used for Western blotting and indirect immunofluorescence of phosphorylated proteins and for the assessment of motility. Both cBiMPS and CaCl(2) were necessary for hyperactivation. The increase in hyperactivated spermatozoa exhibited a dependence on the state of cBiMPS-induced protein tyrosine phosphorylation in the connecting and principal pieces. The addition of calyculin A (an inhibitor for protein phosphatases 1/2A (PP1/PP2A), 50-100 nM) coincidently promoted hyperactivation and cAMP-induced protein tyrosine phosphorylation in the presence of cBiMPS and CaCl(2). Moreover, the addition of W-7 (a calmodulin antagonist, 2-4 µM) enhanced the percentages of hyperactivated spermatozoa after incubation with cBiMPS and CaCl(2), independently of protein tyrosine phosphorylation. These findings indicate that cAMP-induced protein tyrosine phosphorylation in the connecting and principal pieces is involved in hyperactivation in response to extracellular calcium, and that calmodulin may suppress hyperactivation via the signaling cascades that are independent of cAMP-induced protein tyrosine phosphorylation.

Relationship of protein tyrosine phosphorylation state with tolerance to frozen storage and the potential to undergo cyclic AMP-dependent hyperactivation in the spermatozoa of Japanese Black bulls

The aim of this study was to elucidate the relationship between protein tyrosine phosphorylation state and sperm characteristics in frozen-stored spermatozoa of Japanese Black bulls. The spermatozoa were washed with PBS containing polyvinyl alcohol and then incubated with cell-permeable cAMP analog cBiMPS to induce flagellar hyperactivation. Before and after incubation, the spermatozoa were used for immunodetection of tyrosine-phosphorylated proteins, assessment of morphological acrosome condition and evaluation of motility. In bulls whose frozen-stored spermatozoa were classified as having a high-grade acrosome condition before incubation, sperm tyrosine-phosphorylated proteins, including the 33-kDa tyrosine-phosphorylated SPACA1 protein, were localized in the anterior region of the acrosome and equatorial subsegment. The immunodetection level of the 41- and 33-kDa sperm tyrosine-phosphorylated proteins in the Western blots and the immunofluorescence of tyrosine-phosphorylated proteins and SPACA1 proteins in the anterior region of the sperm acrosome were lower in bulls whose frozen-stored sperm were classified as having a low-grade acrosome condition. On the other hand, after incubation with cBiMPS, immunodetection levels of at least 10 tyrosine-phosphorylated proteins increased in the connecting and principal pieces of spermatozoa, coincident with the induction of flagellar hyperactivation. Many of the spermatozoa also exhibited detection patterns similar to those of boar hyperactivated spermatozoa. These results are consistent with the suggestion that immunodetection levels of tyrosine-phosphorylated proteins are valid markers that can predict the level of tolerance to frozen storage and the potential to undergo cAMP-dependent hyperactivation for the spermatozoa of individual Japanese Black bulls.

Protein kinases A and C and phosphatidylinositol 3 kinase regulate glycogen synthase kinase-3A serine 21 phosphorylation in boar spermatozoa

The cAMP-dependent protein kinase (PKA), protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) pathways control most relevant functions in male germ cells including motility. Recently we demonstrated that phosphorylation state of glycogen synthase kinase-3alpha (GSK3A) is also a key event in the control of boar spermatozoa motility. However, the upstream regulators of GSK3A serine phosphorylation (inhibition) in male germ cells remain largely unknown. This work investigates the involvement of PKA, PKC and PI3K pathways in GSK3A phosphorylation in boar spermatozoa. A capacitating medium (TCM) or the phosphodiesterase-resistant cell permeable cAMP analogue 8Br-cAMP cause a significant increase in Ser21 GSK3A phosphorylation associated with a simultaneous significant increase in boar spermatozoa motility. These effects are blocked after preincubation of spermatozoa with PKA inhibitor H89 or PKC inhibitor Ro-32-0432. The PI3K inhibitor LY294002 increases both spermatozoa motility parameters and the basal GSK3A phosphorylation, but does not affect either TCM- or 8Br-cAMP-stimulated GSK3A phosphorylation. PI3K inhibition effects are mediated by an increase in intracellular cAMP levels in boar spermatozoa and are suppressed by PKA inhibitor H89. In summary, we demonstrate that PKA, PKC and PI3K pathways crosstalk in porcine male germ cells to crucially regulate GSK3A phosphorylation which subsequently controls cell motility. In addition, our results suggest that PI3K is upstream of PKA which lies upstream of PKC in this regulatory cascade(s). Our findings contribute to elucidate the molecular mechanisms underlying the regulation of one of the most relevant male germ cell functions, motility.

Evidence of the existence of adenylyl cyclase 10 (ADCY10) ortholog proteins in the heads and connecting pieces of boar spermatozoa

The aim of this study is to provide evidence of the existence of the adenylyl cyclase 10 (ADCY10) ortholog proteins in boar spermatozoa. Experiments with RT-PCR techniques, nucleotide sequence analyses and Northern blot analyses revealed that boar testes exclusively express approximately 5.1-kbp RNA, the nucleotide sequence of which is highly similar to that of human ADCY10. Database analyses with CDART suggested that pig ADCY10 ortholog proteins conserve two catalytic domains of adenylyl cyclase. Western blot techniques and indirect immunofluorescence with a specific antiserum to pig recombinant ADCY10 ortholog proteins showed that 48-kDa and 70-kDa truncated forms of pig ADCY10 ortholog proteins are localized in the equatorial segments and connecting pieces of boar ejaculated spermatozoa. Finally, cell imaging techniques with fluo-3/AM indicated that incubation with sodium bicarbonate (an ADCY10 activator) can initiate the calcium influx in the boar sperm heads that is controlled via the cyclic AMP signaling cascades. These results are consistent with the suggestion that functional ADCY10 ortholog proteins exist in the heads of boar spermatozoa. This is the first direct evidence of the existence of ADCY10 proteins in the heads of mammalian spermatozoa.

Metabolism of exogenous fatty acids, fatty acid-mediated cholesterol efflux, PKA and PKC pathways in boar sperm acrosome reaction

Purpose

For understanding the roles of fatty acids on the induction of acrosome reaction which occurs under association of cholesterol efflux and PKA or PKC pathways in boar spermatozoa, metabolic fate of alone and combined radiolabeled ¹⁴C-oleic acid and ³H-linoleic acid incorporated in the sperm was compared, and behavior of cholesterol and effects of PKA and PKC inhibitors upon fatty acid-induced acrosome reaction were examined.

Methods

Semen was collected from a Duroc boar, and the metabolic activities of fatty acids in the spermatozoa were measured using radioactive compounds and thin layer chromatography. Cholesterol efflux was measured with a cholesterol determination assay kit. Participation of fatty acids on the AR through PKA and PKC pathways was evaluated using a specific inhibitor of these enzymes.

Results

Incorporation rate of ¹⁴C-oleic acid into the sperm lipids was significantly higher than that of ³H-linoleic acid (P < 0.05). The oxidation of ¹⁴C-oleic acid was higher in combined radiolabeling rather than in one. The highest amounts of ³H-linoleic acid and ¹⁴C-oleic acid were recovered mainly in the triglycerides and phospholipids fraction, and ¹⁴C-oleic acid distribution was higher than the ³H-linoleic acid in both labeled (P < 0.05) sperm lipids. In the ³H-linoleic and ¹⁴C-oleic acid combined radiolabeling, the incorporation rate of the radioactive fatty acids in all the lipid fractions increased 15 times more than the alone radiolabeling. Boar sperm utilize oleic acid to generate energy for hyperactivation (P < 0.05). Supplementation of arachidonic acid significantly increased (P < 0.05) cholesterol efflux in sperm. When spermatozoa were incubated with PKA or PKC inhibitors, there was a significant reduction of arachidonic acid-induced acrosome reaction (AR) (P < 0.05), and inhibition by PKA inhibitor is stronger than that by PKC inhibitor.

Conclusions

Incorporation of unsaturated fatty acids, especially oleic acid, into triglycerides and phospholipids provides prerequisite energy for AR. Cholesterol efflux by arachidonic acid triggers AR. Arachidonic acid activated PKA and PKC pathway participate in induction of the AR.

Hyperactivated motility of frozen-thawed spermatozoa from fertile and subfertile Japanese black bulls induced by cyclic adenosine 3',5'-monophosphate analogue, cBiMPS

This study investigated whether a cyclic adenosine 3',5'-monophosphate (cAMP) analogue, cBiMPS, could induce hyperactivated motility in frozen-thawed Japanese Black bull spermatozoa and compared the ability of spermatozoa to undergo hyperactivation between fertile and subfertile bulls. Frozen-thawed spermatozoa from 3 fertile and 2 subfertile bulls were washed, suspended in BO-Hepes medium and incubated in the presence of 0.1 mM cBiMPS for up to 4 h. At 1-h intervals, the spermatozoa were examined for hyperactivated motility. The proportions of spermatozoa showing a circular swimming pattern with asymmetrical flagellar beating and those showing whiplash beating of flagella to the total number of motile spermatozoa were expressed as C% and W%, respectively. The motile spermatozoa % was barely affected by treatment with cBiMPS or the fertility status of the sperm donor, although it gradually decreased in all sperm samples during the 4-h incubation. In the fertile bulls, the C% was 0% at 0 h of incubation but rapidly increased during the 1-h incubation with cBiMPS. It then decreased slightly towards 4 h concomitantly with a gradual increase in W% towards 4 h. In the subfertile bulls, however, the cBiMPS-induced increase of C% was delayed for 1-3 h, although the incubation time-related changes in mean W% were similar between the fertile and subfertile bulls. In the vehicle controls for cBiMPS, the C% and W% were 0% throughout incubation for all the samples examined. The results suggest that hyperactivation of the flagellum can be induced by the cAMP analogue, cBiMPS, in frozen-thawed Japanese Black bull spermatozoa and that induction of hyperactivation may serve as a useful tool for detection of functional abnormality of spermatozoa from subfertile Japanese Black bulls.

Calyculin A-sensitive protein phosphatases are involved in maintenance of progressive movement in mouse spermatozoa in vitro by suppression of autophosphorylation of protein kinase A

Protein serine/threonine phosphorylation in mammalian sperm flagella has been considered to play important roles in regulation of motility. Protein phosphorylation state reflects balance of enzymatic activities between protein phosphatases and protein kinases [predominantly protein kinase A (PKA)]. The aims of this study were to disclose roles of protein phosphatases in the regulation of sperm motility and to provide evidence for suppression of PKA full activation by protein phosphatases in sperm flagella. Mouse epididymal spermatozoa were incubated with a cell-permeable protein phosphatase 1 (PP1)/protein phosphatase 2A (PP2A) inhibitor (calyculin A: 25-125 nM) at 37.5 C. After incubation, they were used for immunodetection of phosphorylated proteins, PKA and PP1 gamma2, assessment for motility and co-immunoprecipitation of PP1gamma2 with PKA. Incubation with calyculin A enhanced the phosphorylation states of several proteins (>250 kDa, 170 kDa, 155 kDa, 140 kDa and 42 kDa for serine/threonine phosphorylation and 70 kDa for tyrosine phosphorylation) and PKA catalytic subunits [at the autophosphorylation residue (Thr-197) for its full enzymatic activation] in the flagella. Coincidently, this incubation induced changes of sperm flagellar movement from the progressive type to the hyperactivation-like type. Indirect immunofluorescence and co-immunoprecipitation showed that PKA was co-localized with PP1 gamma2 in the principal pieces of sperm flagella. These findings suggest that calyculin A-sensitive protein phosphatases (PP1/PP2A) suppress full activation of PKA as well as enhancement of the phosphorylation states of other flagellar proteins in sperm flagella in order to prevent precocious changes of flagellar movement from the progressive type to hyperactivation.

Behavioral response of human spermatozoa to a concentration jump of chemoattractants or intracellular cyclic nucleotides

BACKGROUND

A major question in mammalian sperm chemotaxis is whether the cells sense a chemoattractant gradient by comparing the chemoattractant concentration between time points or between spatial points.

METHODS

To resolve this question, we exposed human spermatozoa to a temporal chemoattractant gradient under conditions of no spatial gradient by rapidly mixing the cells with progesterone or bourgeonal on a microscope slide and analyzing their swimming with motion analysis software.

RESULTS

The cells responded within seconds with an increase in velocity and lateral head displacement, and with a decrease in the linearity of swimming, becoming hyperactivated at the peak of the response. All the responses were transient, lasting for a number of seconds. Essentially similar results were obtained upon intracellular photorelease of cyclic adenosine monophosphate or cyclic guanosine monophosphate, which are thought to be involved in mediating the chemotactic response.

CONCLUSION

These results suggest that human spermatozoa sense and respond to a temporal chemoattractant gradient. On the basis of these observations, we propose a potential model for the chemotactic response of spermatozoa in a spatial chemoattractant gradient.

Changes of PKA and PDK1 in the principal piece of boar spermatozoa treated with a cell-permeable cAMP analog to induce flagellar hyperactivation

A cAMP-induced protein tyrosine phosphorylation and flagellar hyperactivation are controlled via complicated signaling cascades in mammalian spermatozoa. For instance, these events seem to be regulated positively by the PKA-mediated signaling and negatively by the PI3K/PDK1-mediated signaling. In this article, we have shown molecular changes of PKA and PDK1 in cAMP analog (cBiMPS)-treated boar spermatozoa in order to disclose possible roles of these kinases in protein tyrosine phosphorylation and hyperactivation. Ejaculated spermatozoa were incubated with cBiMPS, and then they were used for biochemical analyses of sperm kinases by Western blotting and indirect immunofluorescence and for assessment of flagellar movement. The first 30-min incubation with cBiMPS highly activated PKA of the principal piece to the accompaniment of autophosphorylation on Thr-197 of catalytic subunits. However, protein tyrosine phosphorylation and hyperactivation were fully induced in the sperm samples after the 180-min incubation. A potentially active form of PDK1 (54/55-kDa phospho-PDK1) was detected in the principal piece of the spermatozoa during the 90-min incubation. Another potentially active form (59-kDa phospho-PDK1) gradually increased during the same incubation period. However, the PDK1 suddenly became inactive by the dephosphorylation after the 180-min incubation, namely coincidently with full induction of protein tyrosine phosphorylation and hyperactivation. Additionally, existence of PI3K-dependently suppressing mechanisms for protein tyrosine phosphorylation was confirmed in the principal piece by pharmacological experiments with LY294002 and biochemical analyses with anti-PI3K p85 antibodies. These findings suggest that dephosphorylation of PDK1 may be a molecular switch for enhancement of protein tyrosine phosphorylation and flagellar hyperactivation in boar spermatozoa.

K+ channels and the cAMP-PKA pathway modulate TGF-beta1-induced migration of rat vascular myofibroblasts

Our previous studies have indicated that TGF-beta1 exerts its effect on the expression of A-type potassium channels (I(A)) in rat vascular myofibroblasts by activation of protein kinase C during the phenotypic transformation of vascular fibroblasts to myofibroblasts. In the present study, patch-clamp whole-cell recording and transwell-migration assays were used to examine the effects of TGF-beta1- and phorbol 12-myristate 13-acetate (PMA)-induced expression of I(A) channels on myofibroblast migration and its modulation by the protein kinase A (PKA) pathway. Our results reveal that incubation of fibroblasts with TGF-beta1 or PMA up-regulates the expression of I(A) channels and increases myofibroblast migration. Blocking I(A) channel expression by 4-aminopyridine (4-AP) significantly inhibits TGF-beta1- and PMA-induced myofibroblast migration. Incubation of fibroblasts with forskolin does not result in increased expression of I(A) channels but does cause a slight increase in fibroblast migration at higher concentrations. In addition, forskolin increases the TGF-beta1- and PMA-induced myofibroblast migration but inhibits TGF-beta1- and PMA-induced the expression of I(A) channels. Whole-cell current recordings showed that forskolin augments the delayed rectifier outward K(+) (I(K)) current amplitude of fibroblasts, but not the I(A) of myofibroblasts. Our results also indicate that TGF-beta1- and PMA-induced expression of I(A) channels might be related to increase TGF-beta1- or PMA-induced myofibroblast migration. Promoting fibroblast and myofibroblast migration via the PKA pathway does not seem to involve the expression of I(A) channels, but the modulation of I(K) and I(A) channels might be implicated.

Compliance in the neck structures of the guinea pig spermatozoon, as indicated by rapid freezing and electron microscopy

Electron microscopy has been used to investigate whether the transversely striated columns of the connecting piece in the neck region of guinea pig spermatozoa, undergo lengthening and shortening as a result of the forces generated during motility. Motile spermatozoa were subjected to near-instantaneous rapid freezing, followed by freeze-substitution fixation and epoxy embedment. Thin sections passing longitudinally through the striated columns revealed that the periodicity was indeed variable. The repeat period, taken to have an unstressed width of 60 nm, could be found extended to 75 nm in some specimens, and reduced to 54 nm in others. The estimates of the coefficients of variation were 6.6% for the width of the 'dense' band and 33.5% for the 'pale' band. The 'pale' band in the extended state showed longitudinal striae. Such variations in length, which - it is suggested - are physiological, and passively induced, would have functional implications for the flagellum - for both bend initiation and bend growth. Also, hypothetically, any mechanism that could increase the degree of compliance in these columns, such as perhaps phosphorylation of the constituent proteins, could permit the flagellum to develop the exaggerated bend angles and asymmetries of the 'hyperactivated' state.

Effects of protein phosphatase inhibitor calyculin a on the postacrosomal protein serine/threonine phosphorylation state and acrosome reaction in boar spermatozoa incubated with a cAMP analog

In order to reveal the involvement of the sperm postacrosomal region in the acrosome reaction, we examined the effects of the protein phosphatase inhibitor calyculin A on the postacrosomal protein serine/threonine phosphorylation state and acrosome morphology in boar spermatozoa incubated with a cAMP analog. Proteins were highly phosphorylated on the serine/threonine residues only in the postacrosomal region before incubation. After 90-min incubation without calyculin A, the protein phosphorylation state declined in the postacrosomal region irrespective of the capacitation state while it remained under the detectable level in the other regions of the sperm head. However, addition of calyculin A effectively suppressed the decline in protein phosphorylation state and increased an inactive form of protein phosphatase 1 in the postacrosomal region. On the other hand, this inhibitor had no influence on the protein phosphorylation state in the acrosome and equatorial segment. After incubation without calyculin A for 180 or 360 min, many spermatozoa exhibited acrosomal changes and loss that indicated occurrence of the acrosome reaction. However, addition of calyculin A significantly blocked these events. These results are consistent with our suggestion that postacrosomal serine/threonine-phosphorylated proteins are involved in suppression of the acrosome reaction in boar spermatozoa in vitro.