Adenylate cyclase activity in porcine sperm in response to female reproductive tract secretions

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 effect of oestradiol, progesterone and heparin on bovine spermatozoa function after thawing

The present experiment was designed to determine the effects of various biologically active substances, such as oestradiol (OE), progesterone (P4) and heparin (Hep) alone or in combination on sperm plasma membrane scrambling, capacitation and acrosome reaction (AR) of post-thaw bovine spermatozoa. Spermatozoa were incubated for 180 min in capacitation medium supplemented with (i) 1 mug/ml OE; (ii) 1 mug/ml P4; (iii) 1 mug/ml OE and 1 mug/ml P4; (iv) 1 mug/ml OE and 5 mug/ml Hep; (v) 1 mug/ml P4 and 5 mug/ml Hep; (vi) 1 mug/ml OE, 1 mug/ml P4 and 5 mug/ml Hep. At predetermined time intervals aliquots were taken to assess sperm plasma membrane scrambling, or capacitation (AR induced by lysophosphatidylcholine) in spermatozoa. The second experiment was aimed to study the effects of OE, P4 and OE/P4 as potential inducers of AR in Hep-capacitated spermatozoa. Plasma membrane scrambling was assessed by a flow cytometer, using Merocyanine staining. Acrosomal status and viability of spermatozoa were evaluated under epifluorescence microscope with Ethidium homodimer-1/peanut agglutinin fluorescein isothiocyanate staining method (EthD-1/PNA-FITC). The results show that OE, P4 and a combination of OE/P4 at concentrations used did not affect sperm viability. Heparin significantly (p < 0.001) increased sperm plasma membrane scrambling of OE and P4-treated spermatozoa. P4 significantly affected the rate of sperm capacitation (p < 0.001) and AR (p < 0.05), but OE expressed membrane-stabilizing properties (p < 0.05). It can be concluded that in frozen-thawed bovine spermatozoa OE presents plasma membrane stabilizing properties that can be abolished by Hep, but not by P4. Progesterone possesses capacitating and AR-inducing properties in frozen-thawed bovine spermatozoa that can be alleviated by OE.

Heparin and high-density lipoprotein mediate bovine sperm capacitation by different mechanisms

Capacitation is an important process in bovine sperm maturation and is an obligatory step prior to fertilization. Two capacitating agents, namely heparin and high-density lipoprotein (HDL), have been shown to induce sperm capacitation. A family of major proteins of bovine seminal plasma designated BSP-A1/A2, BSP-A3, and BSP-30 kDa (collectively called BSP proteins) bind to the sperm surface upon ejaculation via their membrane choline phospholipids. Our previous studies with bovine epididymal sperm showed that BSP proteins potentiate sperm capacitation induced by heparin and HDL. This study was undertaken to clarify the mechanism of capacitation induced by heparin and HDL in the presence of BSP proteins. Washed bovine ejaculated sperm were incubated with heparin (12 microg/ml) or HDL (10-160 microg/ml) in the presence of polyclonal antibodies against purified BSP proteins (anti-BSP proteins). The percentage of capacitated sperm was evaluated after the induction of the acrosome reaction (AR) with lysophosphatidylcholine. When sperm were incubated for 5 h with heparin and anti-BSP proteins (40 microg/ml), the AR level was not significantly different from control levels (16. 8 +/- 0.9% vs. 12.9 +/- 0.9%). In contrast, incubation of sperm for 8 h with HDL and anti-BSP proteins did not inhibit the AR (42.4 +/- 1.1% vs. 17.1 +/- 1.6 for the control samples). We also investigated the effect of heparin and HDL on protein tyrosine phosphorylation associated with capacitation. The tyrosine phosphorylation of a group of proteins was increased in the presence of heparin. However, HDL did not significantly stimulate protein phosphorylation. The increase in phosphorylation was correlated with an increase in the AR after the incubation with heparin but not with HDL. These results indicate that heparin and HDL mediate capacitation via different mechanisms.

Modulation of postthaw motility, survival, calcium uptake, and fertility of bovine sperm by magnesium and manganese

Because Mg2+ and Mn2+ are potent stimulators of motility through the stimulation of adenylate cyclase activity, the current study was undertaken to modulate the fertilizing ability of bovine semen by incorporation of various concentrations of those two salts in extenders before freezing. Motility analysis at 6 h in vitro showed a positive effect of MgCl2 in a dose-dependent manner from 0.5 to 5 mM (31 to 50%). Manganese at the concentration of 0.1 mM also supported good sperm motility (53%) compared with that of the control (28%). Although survival was increased, no detrimental effects were seen on the number of sperm that penetrated mucus of cows in estrus. The intracellular Ca2+ concentration of sperm was very different across treatments after thawing; spermatozoa that were extended with 2 mM MgCl2 and 0.5 mM MnCl2 possessed the highest concentrations at thawing. Four hours later, in the presence of Ca, spermatozoa that were extended in 0.1 mM MnCl2 showed the highest uptake. In the presence of Ca and heparin, spermatozoa that were extended in different amounts of Mg showed Ca2+ concentrations that increased in a dose-dependent manner. This effect was negated by glucose. Functional fertilizing capacity was also evaluated by in vitro fertilization, and the different treatments did not show any detrimental effects. In summary, 5 mM MgCl2 and 0.1 mM MnCl2 both have beneficial effects for the maintenance of sperm motility without detrimental effects on mucus penetration and fertilizing ability. Furthermore, these treatments do not prevent subsequent Ca2+ uptake in response to heparin. These in vitro studies are potentially a good sorting system to predict the benefits of extender modifications.

Capacitation of mouse spermatozoa. II. Protein tyrosine phosphorylation and capacitation are regulated by a cAMP-dependent pathway

In the accompanying report (Visconti, P.E., Bailey, J.L., Moore, G.D., Pan, D., Olds-Clarke, P. and Kopf, G.S. (1995) Development, 121, 1129–1137) we demonstrated that the tyrosine phosphorylation of a subset of mouse sperm proteins of M(r) 40,000-120,000 was correlated with the capacitation state of the sperm. The mechanism by which protein tyrosine phosphorylation is regulated in sperm during this process is the subject of this report. Cauda epididymal sperm, when incubated in media devoid of NaHCO3, CaCl2 or bovine serum albumin do not display the capacitation-associated increases in protein tyrosine phosphorylation of this subset of proteins. This NaHCO3, CaCl2 or bovine serum albumin requirement for protein tyrosine phosphorylation can be completely overcome by the addition of biologically active, but not inactive, cAMP analogues. Addition of the active cAMP analogues to sperm incubated in media devoid of NaHCO3, CaCl2 or bovine serum albumin overcomes the inability of these media to support capacitation, as assessed by the ability of the cells to acquire the pattern B chlortetracycline fluorescence, to undergo the zona pellucida-induced acrosome reaction and, in some cases, to fertilize metaphase II-arrested eggs in vitro. The effects of the cAMP analogues to enhance protein tyrosine phosphorylation and to promote capacitation appears to be at the level of the cAMP-dependent protein kinase (PKA), since two specific inhibitors of this enzyme (H-89 and Rp-cAMPS) block the capacitation-dependent increases in protein tyrosine phosphorylation in sperm incubated in media supporting capacitation. Capacitation, as assessed by the aforementioned endpoints, also appears to be inhibited by H-89 in a concentration-dependent manner. These results provide further evidence for the interrelationship between protein tyrosine phosphorylation and the appearance of the capacitated state in mouse sperm. They also demonstrate that both protein tyrosine phosphorylation and capacitation appear to be regulated by cAMP/PKA. Up-regulation of protein tyrosine phosphorylation by cAMP/PKA in sperm is, to our knowledge, the first demonstration of such an interrelationship between tyrosine kinase/phosphatase and PKA signaling pathways.

Capacitation of mouse spermatozoa. I. Correlation between the capacitation state and protein tyrosine phosphorylation

The molecular basis of mammalian sperm capacitation, defined functionally as those processes that confer on the sperm the acquisition of fertilization-competence either in vivo in the female reproductive tract or in vitro, is poorly understood. We demonstrate here that capacitation of caudal epididymal mouse sperm in vitro is accompanied by a time-dependent increase in the protein tyrosine phosphorylation of a subset of proteins of M(r) 40,000-120,000. Incubation of sperm in media devoid of bovine serum albumin, CaCl2 or NaHCO3, components which individually are required for capacitation, prevent the sperm from undergoing capacitation as assessed by the ability of the cells to acquire the pattern B chlortetracycline fluorescence, to undergo the zona pellucida-induced acrosome reaction and, in some cases, to fertilize metaphase II-arrested eggs in vitro. In each of these cases the protein tyrosine phosphorylation of the subset of capacitation-associated proteins does not occur. Protein tyrosine phosphorylation of these particular proteins, as well as sperm capacitation, can be recovered in media devoid of each of these three constituents (bovine serum albumin, CaCl2 or NaHCO3) by adding back the appropriate component in a concentration-dependent manner. The requirement of NaHCO3 for these phosphorylations is not due to an alkalinization of intracellular sperm pH or to an increase in media pH. Caput epididymal sperm, which lack the ability to undergo capacitation in vitro, do not display this capacitation-dependent subset of tyrosine phosphorylated proteins in complete media even after extended incubation periods, and do not fertilize metaphase II-arrested eggs in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Potential assays for sperm capacitation in mammals

Sperm capacitation is an essential process in fertilization. It apparently involves a large number of processes, the common denominator of which is that they donate to sperm the potential to undergo the acrosome reaction, i.e., to release proteolytic enzymes enabling sperm penetration through the egg coat. Although the phenomenon of capacitation has been known for more than 40 years, it is far from understood, and, consequently, there is, as yet, no operational definition of it. The lack of an assay to identify capacitated spermatozoa is both the cause and the effect of this situation. Here we critically review the major changes that are thought to occur during sperm capacitation, and assess their potential use as markers for the identification of capacitated spermatozoa.

Biochemical changes in bull spermatozoa during capacitation in vitro

To evaluate the metabolic changes of bull spermatozoa (SPZ) during capacitation in vitro, SPZ were incubated for 0, 5 or 10 hours in the presence (co-culture) and absence (control) of monolayers of bovine oviduct epithelial cells, which promote capacitation-like changes in vitro. There was little change in the oxygen uptake of the SPZ after 5 hours, but after 10 hours there was a decrease, particularly in the co-cultured sample. After 5 hours there was little change in the cyclic adenosine monophosphate (cAMP) concentration of the co-culture or control SPZ, but by 10 hours the levels of cAMP decreased in both the co-cultured and control SPZ (P=0.06). The concentration of adenosine triphosphate (ATP) was somewhat decreased after 5 hours in both the co-cultured and control SPZ and the percentage of decline was much higher after 10 hours. Overall, there was no significant change in oxygen uptake or cAMP and ATP levels specifically associated with capacitation of bull SPZ.

Stimulation of partially purified adenylate cyclase from bull sperm by bicarbonate

Solubilized and partially purified adenylate cyclase from bull sperm was found to be specifically activated (up to 6-fold) by sodium bicarbonate (NaHCO3) and to a lesser extent by NaNO3. Other sodium salts were either ineffective (e.g. NaCOOH) or inhibitory (e.g. NaHSO3, NaHSO4 and Na2B4O7). Stimulation by NaHCO3 was dose-dependent in the range of 0-40 mM and was greater when enzyme activity was assayed in the presence of magnesium as compared with manganese ions. Bicarbonate seems to affect maximal enzyme velocity (Vmax) and has no effect on the Km of adenylate cyclase for Mn-ATP. Stimulation of adenylate cyclase by NaHCO3 coincided with the elution pattern of the enzyme as recorded following chromatography on DEAE-cellulose or gel filtration on BioGel P-100. These results suggest that in the course of stimulation of sperm adenylate cyclase, bicarbonate is likely to interact directly with the enzyme. Furthermore, this intrinsic and unique property of sperm adenylate cyclase may explain results reported by others on the stimulation of cAMP production by bicarbonate in intact and broken sperm preparations and suggest a biochemical basis for enhanced sperm motility associated with high bicarbonate concentrations.

Adenylate cyclase activity of mouse sperm during capacitation in vitro: effect of calcium and a GTP analogue

Earlier studies have shown increased adenylate cyclase (AC) activity in epididymal mouse sperm incubated under capacitating conditions in vitro. The present study investigated the effect on AC activity of excluding calcium and/or glucose from the sperm incubation medium, which would modulate expression of fertilizing potential. AC activity was higher in sperm incubated for 120 than 30 min, and was higher in sperm incubated in calcium-containing than calcium-free media for all except acrosome-reacted populations. Calcium added at the time of assay stimulated AC activity, the degree of this response being independent of the functional state of the sperm population. The guanine nucleotide analogue Gpp(NH)p slightly enhanced AC activity, but did not alter the stimulatory effect of calcium. Since calcium can increase AC activity, possibly by interaction with a divalent cation allosteric site on the catalytic subunit of the enzyme, a rise in intracellular calcium levels during capacitation may mediate the increased activity of AC, allowing expression of cAMP dependent events which are a prerequisite for fertilization.

Lowered levels of bicarbonate in seminal plasma cause the poor sperm motility in human infertile patients

Both the adenylate cyclase activity and the motility of human sperm were stimulated by bicarbonate with the same concentration dependency. The correlation between bicarbonate levels in semen and the motility of sperm from the patients with male infertility was investigated. Bicarbonate in semen was found to originate mainly from the seminal vesicles, and a significant positive correlation was observed between bicarbonate levels and volume of semen. The motility of infertile sperm was also found to correlate positively to the seminal levels of bicarbonate. These results suggest that the lowered levels of bicarbonate in semen are at least in part responsible for the poor sperm motility in infertile patients, as a result of the failure in the activation of sperm adenylate cyclase.

Vaginal fluid adenosine 3',5'-cyclic monophosphate (cAMP) in the rat: interaction with sperm cAMP-dependent protein kinase regulatory subunits

Cyclic AMP (adenosine 3',5'-cyclic monophosphate) concentrations were determined in rat vaginal fluids throughout the estrous cycle. Radioimmunoassay results demonstrated that estrus and early metestrus vaginal fluids had significantly (p less than 0.01) elevated cAMP concentrations compared to proestrus, late metestrus, and early and late diestrus. Ovariectomy reduced RIA-detectable cAMP in vaginal fluid. When cauda sperm were preincubated for 5 min with vaginal fluids from each stage of the estrous cycle, results demonstrated that only estrus- and early metestrus-stage vaginal fluids caused a decrease in [32P]-8N3cAMP (8-azido photoaffinity analogue of cAMP) photolabeling of sperm cAMP-dependent protein kinase regulatory subunits RI and RII. To examine if this reduction in [32P]-8N3cAMP photoincorporation by sperm RI and RII could be due to endogenous cAMP, vaginal fluids were boiled, trypsinized, and/or incubated with EGTA or phosphodiesterase. Only phosphodiesterase-treated vaginal fluids restored sperm regulatory subunit photoincorporation of [32P]-8N3cAMP. It is suggested that cAMP is present in rat vaginal fluids during the estrous cycle in a concentration sufficient to bind the regulatory subunits of rat sperm cAMP-dependent protein kinase.