Acetylcholine causes an increase of intracellular calcium in human sperm

Sperm nicotinic acetylcholine receptors (nAChRs) can influence motility and the initiation of acrosome reaction (AR). We report that AR initiation by acetylcholine (ACh) in capacitated human sperm requires both Na+ and Ca2+ in the external medium. Pre-incubation with 50 microM 3-quinuclidinyl benzilate (QNB) or 50 nM strychnine failed to inhibit the ACh-initiated AR, demonstrating that muscarinic AChRs and nAChRs containing alpha9 subunits do not mediate this event. Choline (2.5, 5 and 10 mM), a highly specific but low potency agonist of the alpha7 nAChR initiated AR, with its effect blocked by the nAChR antagonist methyllycaconitine (MLA). ACh (50-400 microM) stimulated a small transient rise in the intracellular Ca2+ in sperm populations loaded with FURA-2, with 200 microM ACh being maximal (146 nM +/- 23 SEM). The nAChR antagonists, alpha-bungarotoxin (alpha-BTX) and MLA, reduced the ACh-initiated Ca2+ rise by 75 and 78%, respectively, demonstrating the majority of the rise is mediated through nAChRs containing alpha7 or alpha9 subunits. Single cell imaging studies using FLUO-3 resolved two patterns of ACh-stimulated Ca2+ increase in the sperm head: 94% of responding sperm displayed a rise (59.6% +/- 5.7 SEM increase from resting fluorescence intensity), returning to resting levels over a period of 2-3 min. The remaining sperm (6%) displayed a sharp spike of Ca2+ ( approximately 1 min; 86% +/- 4.3 SEM change in fluorescence intensity), followed by abrupt loss of fluorescence, a pattern suggestive of AR. A Ca2+ influx in the sperm midpiece appeared to accompany the Ca2+ influx seen in the head. These observations confirm an ionotropic role for nAChRs in sperm function.

Hamster sperm glycine receptor: evidence for its presence and involvement in the acrosome reaction

Recent reports have provided evidence for the presence of amino acid neurotransmitter receptor/chloride channels in human and porcine spermatozoa and their involvement in the acrosome reaction (AR). In this work we investigated whether a glycine receptor (GlyR) was present in golden hamster sperm, and whether it had a role in the hamster AR. The neuronal GlyR agonist glycine, stimulated in a dose-dependent manner, the AR of hamster spermatozoa previously capacitated for at least 3 hr. This stimulation was completely inhibited by 50 microM (+)-bicuculline and by concentrations of strychnine as low as 10-50 nM; both agents are antagonists of neuronal GlyR when used at the concentrations reported in this study. beta-Alanine, another agonist of the neuronal GlyR, also stimulated the AR. The AR-stimulatory effect of this compound was completely abolished by 50 nM strychnine. The inhibitory effect of strychnine on the glycine-induced hamster sperm AR was completely overcome by subsequent treatment with the calcium ionophore ionomycin, demonstrating that the strychnine effect was specific for GlyR. Additional binding studies with (3)[H]-strychnine, the typical radioligand used to detect GlyR in several cells, demonstrated for the first time the presence of specific binding sites for strychnine in the hamster spermatozoa. Interestingly, binding increased during in vitro capacitation, particularly in those sperm suspensions showing high percentages of AR. Taken together these results strongly suggest the presence of a GlyR in the hamster spermatozoa, with a role in the AR when activated.

The zona pellucida-initiated acrosome reaction: defect due to mutations in the sperm glycine receptor/Cl(-) channel

The mammalian sperm acrosome reaction (AR) is essential to fertilization, and the egg zona pellucida (ZP) is generally believed to be an in vivo initiator of the fertilizing sperm AR. Previously a neuronal glycine receptor/Cl(-) channel (GlyR) was detected on the plasma membrane of mammalian sperm and earlier pharmacological studies suggested that this receptor/channel is important to the ZP-initiated AR. Here, sperm from mice with mutations in the neuronal GlyR alpha or beta subunits (spasmodic and spastic) were shown to be deficient in their ability to undergo the AR initiated in vitro by glycine or by solubilized ZP from mouse eggs. However, both spontaneous and calcium ionophore (A23187)-initiated AR were unaffected. The ZP-initiated AR in wild-type sperm was maximal after 2 h of capacitation, but capacitation of sperm from spasmodic mice for up to 3 h did not result in significant ZP-initiated AR. Similar results were observed when sperm from wild-type and spastic mice were compared. Testis from mice with the beta subunit mutation contained truncated beta subunit mRNAs. Moreover, a monoclonal antibody against GlyR completely blocked ZP initiation of AR in normal mouse sperm. Our results are consistent with an essential role for the sperm GlyR in the ZP-initiated AR.

Involvement of protein kinase A and A kinase anchoring protein in the progesterone-initiated human sperm acrosome reaction

The signal transduction pathways involved in the progesterone (P(4))-initiated mammalian sperm acrosome reaction (AR) are not fully understood. To investigate the role of the protein kinase A (PKA) pathway in the P(4)-initiated AR, we probed this pathway by pretreating capacitated human sperm with reagents designed to either inhibit PKA activation or disrupt PKA/A kinase anchoring protein (AKAP) interactions. Preincubation with the stearated (membrane permeable) PKA inhibitor, PKI alpha 5-24 (S-PKI alpha 5-24), significantly inhibited the P(4)-initiated AR at 10 microM as compared to stearated control peptide. In contrast, preincubation with 100 microM nonstearated PKI alpha 5-24 did not significantly inhibit versus solvent control. Preincubation with the PKA inhibitor Rp-8-Br-cAMP at 500 microM and 150 microM significantly inhibited the P(4)-initiated AR versus 8-Br-cAMP and versus solvent. Preincubation with the anchoring inhibitory peptide S-Ht-31 significantly stimulated the P(4)-initiated AR at 10, 3, and 1 microM versus inactive control peptide. The stimulation of the P(4)-initiated AR by 3 microM S-Ht31 was significantly inhibited by the addition of 30 microM S-PKI alpha 5-24 prior to the addition of S-Ht31. Preincubation with S-PKI alpha 5-24 (30 microM) partially inhibited the ionomycin (50 microM)-initiated AR. A role for PKA in the P(4)-initiated AR may exist both upstream and downstream of Ca(2+) entry. Our studies present the first evidence for the participation of PKA in the P(4)-initiated AR and also suggest that AKAPs are involved in the PKA-mediated events.

The mouse sperm glycine receptor/chloride channel: cellular localization and involvement in the acrosome reaction initiated by glycine

Previously, we reported that glycine initiates the in vitro acrosome reaction (AR) of porcine and human sperm by a mechanism that includes the glycine receptor/Cl- channel (GlyR) and that this receptor/channel is required for the zona-pellucida-initiated AR. Because mouse sperm are important tools in the study of fertilization, we investigated whether glycine initiated the mouse sperm AR and whether the sperm GlyR was involved in that initiation. Glycine (250 microM to 1 mM) initiated the AR of capacitated but not noncapacitated mouse sperm. The glycine-initiated AR was significantly inhibited by 50 nM strychnine, a neuronal GlyR antagonist. The neuronal GlyR agonists taurine and beta-alanine did not initiate the AR at 1 mM or 5 mM. A monoclonal antibody against the rat spinal cord GlyR significantly inhibited the glycine-initiated AR but not the spontaneous AR. Indirect immunofluorescence localization studies with that monoclonal antibody and postfixed live sperm detected 3 patterns of immunoreactivity involving 2 sites in the periacrosomal plasma membrane. These patterns were as follows: type A localization on the plasma membrane overlying the tip of the anterior acrosomal region; type B localization on the plasma membrane overlying the posterior part of the acrosomal equatorial segment and/or, in acrosome-reacted sperm, the posterior part of the modified equatorial segment; and type C localization that included both type A and type B. Type A and type C localization were only observed on the acrosome-intact sperm. During capacitation, the number of the sperm showing type A localization increased. Our results demonstrate that mouse sperm provide an excellent model for studying the role of the GlyR in the acrosome reaction.

Recognition of a human sperm surface protein involved in the progesterone-initiated acrosome reaction by antisera against an endomembrane progesterone binding protein from porcine liver

Antisera against a porcine liver endomembrane progesterone (P4)-binding protein inhibited the P4-initiated acrosome reaction (AR) but not the ionomycin-initiated AR of human sperm. Indirect immunofluorescence studies detected antigen in the sperm head that moved during capacitation from a posterior head region to a midhead region. Moreover, the antisera detected a 44.6 kDa protein in western blots of sperm digitonin extracts. These results suggest that a sperm protein with at least partial homology to the liver endomembrane P4-binding protein, is a putative P4-receptor on the sperm plasma membrane involved in the P4-initiated AR.

Specific progesterone binding to a membrane protein and related nongenomic effects on Ca2+-fluxes in sperm

Rapid, nongenomic effects of steroids are supposed to be transmitted by membrane receptors unrelated to the classic intracellular steroid receptors. In this context, a putative progesterone membrane binding protein (mPR) has been identified, recently. Here we show that expression of mPR-cDNA in CHO cells leads to increased microsomal progesterone binding. This result is mirrored by effects of an antibody raised against the recombinant E. coli mPR which suppressed the rapid progesterone-initiated Ca2+ increase in sperm. Our results support the assumption that mPR represents the first steroid membrane receptor or a part of it involved in rapid, nongenomic steroid signalling.

Regulation of intracellular pH in capacitated human spermatozoa by a Na+/H+ exchanger

We previously demonstrated that the progesterone-(P) initiated human sperm acrosome reaction (AR) was dependent on the presence of extracellular Na+ (Na(-)0). Moreover, Na(-)0 depletion resulted in a decreased cytosolic pH (pHi), suggesting involvement of a Na(+)-dependent pHi regulatory mechanism during the P-initiated AR. We now report that the decreased pHi resulting from Na(+)0 depletion is reversible and mediated by a Na+/H+ exchange (NHE) mechanism. To determine the role of an NHE in the regulation of pHi, capacitated spermatozoa were incubated in Na(+)-deficient, bicarbonate/CO2-buffered (ONaB) medium for 15-30 min, which resulted in an intracellular acidification as previously reported. These spermatozoa were then transferred to Na(+)-containing, bicarbonate/CO2-buffered (NaB) medium; Na(+)-containing, Hepes-buffered (NaH) medium; or maintained in the ONaB medium. Included in the NaH medium was the NHE inhibitor 5-(N-ethyl-N-isopropyl) amiloride (EIPA). The steady-state pHi was then determined by spectrofluorometric measurement of bis(carboxyethyl)5(6)-carboxyfluoroscein (BCECF) fluorescence. EIPA (0.1 microM) significantly (P < 0.05) inhibited the pHi recovery produced by NaH medium. Moreover, the pHi in NaH medium was not significantly (P < 0.05) different than NaB medium. These results indicate that a Na(+)-dependent, bicarbonate-independent pHi regulatory mechanism, with a pharmacological characteristic consistent with an NHE, is present in capacitated spermatozoa. In support of the involvement of a sperm NHE, we also demonstrated specific immunoreactivity for a 100 kDa porcine sperm protein using an NHE-1 specific monoclonal antibody. Interestingly, no significant (P = 0.79) effect was seen on the P-initiated AR when EIPA was included in either the NaH or NaB medium. While these findings suggest that inhibition of NHE-dependent pHi regulation in capacitated spermatozoa is not sufficient to block initiation of the AR by P, they do not preclude the possibility that an NHE mediates the regulation of capacitation or sperm motility.

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

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

Determination of the steady-state intracellular chloride concentration in capacitated human spermatozoa

Chloride channels participate in the mammalian sperm acrosome reaction (AR). However, the mechanism by which sperm regulate intracellular Cl- concentration ([Cl-]i) is not known. The steady-state [Cl-]i has also never been reported for mammalian spermatozoa. Therefore, using chloride-sensitive fluorescent dyes, we sought to determine the steady-state [Cl-]i of capacitated human spermatozoa by a null-point measurement technique. We found that the [Cl-]i was sufficiently elevated (a conservative estimate of > or = 41 mM) such that the opening of chloride channels should result in a Cl- efflux and, hence, in depolarization. Moreover, the [Cl-]i does not remain constant under varying extracellular Cl- concentrations ([Cl-]o).

The human acrosome reaction is highly sensitive to inhibition by cyclodiene insecticides

The mammalian sperm acrosome reaction (AR) is essential to fertilization. It can be initiated in vitro by progesterone, a putative physiological initiator that helps to activate sperm GABA(A) receptor/chloride channels and by glycine, a substitute for the egg zona pellucida, which activates sperm glycine receptor/chloride channels. Even at 1 nM (0.41 ng/ml or 0.41 ppb), chlordane and endosulfan, chlorinated cyclodiene blockers of insect neuronal GABA(A) receptor/chloride channels, strongly inhibited the AR initiated by progesterone or glycine. Inhibition of the latter was also seen at 0.1 nM chlordane and endosulfan, but neither cyclodiene inhibited either AR initiator at 0.01 nM. Inhibitory concentrations of these cyclodienes are well within the range detected in human and wildlife tissue and fluids as a result of environmental contamination.

Amino acid neurotransmitter receptor/chloride channels of mammalian sperm and the acrosome reaction

Two different amino acid neurotransmitter receptor/Cl-channels, a gamma-aminobutyric acid (GABA) receptor-like/Cl- channel and a glycine receptor/Cl- channel, have recently been found in mammalian sperm and shown to be involved in the sperm acrosome reaction (AR), an exocytotic event essential to mammalian fertilization. The identity of the receptor/channel involved in the AR depends upon the stimulatory ligand. The AR initiated by the egg zona pellucida requires the participation of the sperm glycine receptor/Cl- channel, whereas the AR initiated by another putative physiological AR initiator, progesterone, involves the sperm GABA receptor-like/Cl- channel. Cl- efflux through the latter has been shown to occur during the progesterone-initiated AR. Activation of sperm amino acid neurotransmitter receptor/Cl- channels may result in plasma membrane depolarization and thereby exert at least a partial control over voltage-sensitive sperm Ca2+ channels and Ca2+ influx important to the AR.

Progesterone triggers a wave of increased free calcium during the human sperm acrosome reaction

The mammalian sperm acrosome reaction (AR), an essential fertilization event, requires an influx of Ca2+. The Ca2+ increase occurring in the human sperm head during the AR initiated by progesterone, a putative in vivo AR initiator, was investigated using video-image analysis with fura-2, a fluorescent Ca2+ probe. Progesterone treatment of capacitated human sperm resulted in a wave-like increase in sperm head cytosolic [Ca2+]i that appears to increase fastest in a region near the equatorial segment and then spreads throughout the rest of the head. The progesterone-mediated Ca2+ increase in the sperm head was strongly inhibited and the wave eliminated by picrotoxin, a blocker of GABAA receptor/Cl- channels and an inhibitor of the progesterone-mediated Cl- efflux and progesterone-initiated AR of human sperm. These results are the first to detect a ligand-mediated Ca2+ wave in sperm and to suggest that Cl- efflux influences Ca2+ influx during the AR.

Importance of sodium ion to the progesterone-initiated acrosome reaction in human sperm

Progesterone (P) has previously been shown to rapidly increase free intracellular calcium concentration ([Ca2+]i), and subsequently to initiate the acrosome reaction (AR) in capacitated human sperm. The present study used cytochemical analysis of the AR, and spectrofluorometric determination of sperm [Ca2+]i and intracellular pH (pHi) in Na(+)-containing and Na(+)-deficient bicarbonate/CO2-buffered media to investigate the role of Na+ in these P-initiated changes. We found that P failed to initiate the AR in Na(+)-deficient medium, and that the initial rise in [Ca2+]i following P (1 microgram/ml) stimulation was similar for both media; however, the [Ca2+]i in the Na(+)-deficient medium regressed more rapidly and plateaued at a significantly lower [Ca2+]i. Moreover, the differences in plateau [Ca2+]i were directly related to the percentage of acrosome reactions, suggesting that the plateau phase is not due to [Ca2+]i, but rather to the release of intracellular fura-2 into the medium during the AR. These [Ca2+]i, and AR results are in contrast to those reported previously by others for human sperm and suggest that a Na(+)-dependent mechanism is important in the P-initiated human sperm AR. Such a Na+ requirement may reflect the involvement of this ion in pHi regulation, as capacitated sperm that were incubated in a Na(+)-deficient medium for > or = 30 min displayed a significantly lower pHi.

Chloride efflux during the progesterone-initiated human sperm acrosome reaction is inhibited by lavendustin A, a tyrosine kinase inhibitor

Previous studies showed that progesterone (P) can initiate the mammalian sperm acrosome reaction (AR) in vitro and that a sperm GABAA-like receptor/Cl- channel is involved in an essential Cl- efflux mediated by P during the AR. Here, we show that lavendustin A, a potent, specific inhibitor of tyrosine kinase activity, strongly inhibits the P-initiated human AR and the essential P-mediated Cl- efflux. Lavendustin B, a weak tyrosine kinase inhibitor, had no significant effect. These results suggest that, as part of AR initiation, P mediates tyrosine phosphorylation of the sperm GABAA- like receptor/Cl- channel.

Immunochemical identification of the glycine receptor/Cl-channel in porcine sperm

Cl- flux is essential for the mammalian sperm acrosome reaction (AR), a required fertilization event involving fusion of sperm head membranes. Our previous inhibitor studies suggested the involvement of a glycine receptor/Cl- channel (GlyR) in the zona pellucida-initiated mammalian sperm AR. Here, using a monoclonal antibody specific for the alpha (48-kDa) and beta (58-kDa) subunits of the rat spinal cord GlyR (mAb GlyR4a), we provide the first direct evidence for GlyR in mammalian sperm. Immunofluorescence studies with mAb GlyR4a detected immunoreactivity on the porcine sperm periacrosomal plasma membrane, a site supporting GlyR involvement in the AR. In Western immunoblotting studies, mAb GlyR4a bound specifically to porcine sperm proteins of 49.2 +/- 2.2 kDa and 58.0 +/- 2.7 kDa. This is the first direct demonstration of both alpha and beta subunits of GlyR in a nonnervous system cell.

Human sperm plasma membrane progesterone receptor(s) and the acrosome reaction

Progesterone initiation of te human sperm acrosome reaction (AR) includes stimulation of a transient Ca2+ influx and a transient Cl- efflux. A role for one or more plasma membrane receptors has been suggested, but, except for evidence supporting the involvement of a sperm GABAA-like receptor/Cl- channel, there is little information about possible receptor identity. Here, we attempt to identify plasma membrane progesterone receptors in human sperm using a mouse monoclonal antibody (mAb-C262) raised against the C-terminal steroid-binding domain of the human intracellular progesterone receptor. C-262 inhibited the progesterone-initiated AR in a dose-dependent manner. Maximum inhibition was 77% as detected by fluorescein isothiocyanate (FITC)-concanavalin A (conA). Motility was unaffected. A control mouse mAb (h-151) raised against the human estrogen receptor did not inhibit the progesterone-initiated AR. Western blotting with C-262, but not with h-151, detected a major sperm protein band of 50-52 kDa. In indirect immunofluorescence localization studies, live and ethanol-fixed uncapacitated sperm and fixed capacitated sperm incubated with C-262, but not with h-151, displayed fluorescence at the equatorial segment region of the sperm head plasma membrane. In spectrofluorometric studies using capacitated sperm loaded with the Ca2+ probe Fura-2 or the Cl- probe MEQ, C-262 but not h-151 inhibited both Ca2+ influx and Cl- efflux. These ion fluxes could be due to the binding of progesterone to two different receptor/channels or to its binding to one and cross talk with the other. Our results strongly support the involvement of sperm plasma membrane receptors in the progesterone-initiated AR and provide a candidate for one such receptor.

Studies of porcine and human sperm suggesting a role for a sperm glycine receptor/Cl- channel in the zona pellucida-initiated acrosome reaction

It is known that Cl- is required for the zona pellucida (zona)- and progesterone-initiated mammalian sperm acrosome reaction (AR). Recent evidence indicates the involvement of a unique progesterone receptor resembling a GABAA receptor/Cl- channel in the latter event. In the study reported here, we investigated whether the in vitro porcine sperm AR initiated by heat-solubilized porcine zonae involves a different receptor/Cl- channel than does the progesterone-initiated AR. Capacitated porcine sperm failed to undergo the zona- and progesterone-initiated AR when suspended in a medium deficient in Cl-. We then employed 1) strychnine, an antagonist of neuronal glycine receptor/Cl- channels at nanomolar concentrations and of GABAA receptor/Cl- channels at micromolar concentrations and 2) (+)-bicuculline, an antagonist of neuronal GABAA receptor/Cl- channels that inhibits glycine receptor/Cl- channels only at higher concentrations. We report here that the zona-initiated AR was inhibited by 50 nM strychnine, but the progesterone-initiated AR required 1 microM for inhibition. Moreover, 1 microM (+)-bicuculline inhibited the progesterone-initiated AR, but the zona-initiated AR required 50 microM for inhibition. In addition, glycine initiated the porcine sperm AR, and that event was inhibited by strychnine and (+)-bicuculline with the same order of potency observed for the zona-initiated AR. Finally, glycine initiated the AR in human sperm, and that initiation was inhibited by 50 nM strychnine. This work provides the first evidence suggesting that a glycine receptor/Cl- channel is involved in the zona-initiated mammalian sperm AR.

Progesterone-mediated efflux of cytosolic chloride during the human sperm acrosome reaction

Progesterone is capable of initiating the mammalian sperm acrosome reaction in vitro and is a putative initiator of this essential fertilization event in vivo. Our previous work has suggested that progesterone initiates the human sperm acrosome reaction, at least in part, by activating a unique steroid receptor/Cl- channel resembling a gamma-aminobutyric acidA receptor/Cl- channel (gamma-aminobutyric acidA-like receptor/Cl- channel). Here, the fluorescent intracellular Cl- probe, 6-methoxy-N-ethylquinolinium, was used to detect qualitative changes in sperm cytosolic Cl-. We demonstrate that progesterone can mediate a rapid transient decrease of human sperm cytosolic Cl- inhibitable by the gamma-aminobutyric acidA receptor/Cl- channel antagonists picrotoxin and (+)-bicuculline (which also inhibit the acrosome reaction). These results support the involvement of a gamma-aminobutyric acidA-like receptor/Cl- channel in the P-mediated human acrosome reaction and are the first to demonstrate that a rapid Cl- efflux plays a role in that event.

Importance of bicarbonate to the progesterone-initiated human sperm acrosome reaction

Progesterone, a putative in vivo initiator of the human sperm acrosome reaction (AR), has previously been shown to act at the sperm plasma membrane to initiate the AR in vitro. Here, we have investigated whether bicarbonate (HCO3-) was required for the progesterone-initiated human AR and whether HCO3(-)-dependent cAMP activation might be involved. Capacitated human sperm were suspended in the presence of high (25 mM) or low (1 mM) HCO3- media. The AR was assayed using fluorescein isothiocyanate (FITC)-conconavalin A with sperm fixed 5 minutes after progesterone or solvent control addition. Progesterone initiated the AR in both high and low HCO3- media, but the percentage of AR was significantly lower in the latter medium. In the presence of high HCO3-, 20-minute preincubation with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), a blocker of HCO3- transport, inhibited the progesterone-initiated AR in a dose-dependent manner. The maximum inhibition (85%) was obtained with 18 microM DIDS. Inhibition by DIDS was reversed by washing sperm after treatment. Preincubation of sperm with dibutyryl cAMP (0.1 microM-1 mM) plus DIDS almost completely eliminated the inhibition of the progesterone-initiated AR by DIDS. Dibutyryl cAMP alone did not have a stimulatory effect on the progesterone-initiated AR, when high HCO3- was present, but it was able to partially overcome the reduction of AR by low HCO3-. These results are the first to demonstrate the importance of HCO3- to: 1) mammalian AR initiation by the putative physiological initiator progesterone, and 2) the human sperm AR.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the ability of progesterone and heat solubilized porcine zona pellucida to initiate the porcine sperm acrosome reaction in vitro

It has been previously shown that progesterone can initiate the acrosome reaction (AR) of capacitated human and hamster sperm in vivo. We report here that progesterone can initiate a morphologically normal AR in porcine sperm that have undergone capacitation in a Hepes-buffered medium in vitro. In addition, we have compared the abilities of progesterone and heat-solubilized porcine zona pellucida (zona) to initiate the porcine sperm AR. Capacitated porcine sperm were treated with 1 micrograms/ml progesterone, 150 micrograms/ml porcine zona, or solvent control for 10 min. After treatment, sperm were incubated with the supravital dye Hoechst 33258, fixed and the acrosomal status determined in the previously viable sperm by fluorescence microscopy with fluorescein isothiocyanate-labeled Pisum sativum agglutinin (FITC-PSA). There was no significant difference between the percentage of AR initiated by zona compared to that initiated by progesterone. In order to determine whether there was a synergistic interaction between the two AR initiators, both were added simultaneously to capacitated porcine sperm at optimal (1 microgram/ml progesterone, 150 micrograms/ml zona) and suboptimal (75 ng/ml progesterone and 75 micrograms/ml zona) concentrations. Simultaneous addition of the two AR-initiators at the two concentrations stimulated an additive AR-initiating response, rather than a synergistic one. Several possible explanations for the additive results are discussed.

Progesterone initiation of the human sperm acrosome reaction: the obligatory increase in intracellular calcium is independent of the chloride requirement

The progesterone-initiated human sperm acrosome reaction (AR) requires a rise in intracellular Ca2+ ([Ca2+]i), extracellular Cl- and apparently increased Cl- flux through a unique steroid receptor/Cl- channel resembling but not identical to a GABA(A)/Cl- channel complex. The present study uses fura-2 loaded human sperm, GABA(A)/Cl- channel blockers (picrotoxin and pregnenolone sulfate) and Cl(-)-containing and Cl(-)-deficient media to determine whether the progesterone-mediated increase in [Ca2+]i is dependent on the Cl- requirement. There was no significant difference between the progesterone-mediated increases of [Ca2+]i obtained in Cl(-)-containing and Cl(-)-deficient media. Picrotoxin did not significantly inhibit the progesterone-mediated increase in [Ca2+]i, and pregnenolone sulfate increased [Ca2+]i to the same extent as progesterone. These results strongly suggest that the increase in [Ca2+]i essential to the AR is independent of the AR Cl- requirement and could be explained by the existence of two different sperm plasma membrane progesterone receptors.

Initiation of the human sperm acrosome reaction by thapsigargin

Previous studies have established that the mammalian sperm acrosome reaction (AR) is dependent upon an influx of extracellular Ca2+, but the involvement of a mobilizable store of intracellular Ca2+ has not been shown. In many other cells, the endoplasmic reticulum is the site of such a Ca(2+)-store. Here, we show that thapsigargin, a highly specific inhibitor of the endoplasmic reticulum Ca(2+)-ATPase Ca(2+)-pump (and thus a mobilizer of intracellular Ca2+) in other cells, can initiate the AR in capacitated human sperm. Thapsigargin at concentrations from 50-500 nM significantly increased the AR to the same extent when incubated with capacitated sperm for 1 min (assayed by indirect immunofluorescence). Transmission electron microscopy confirmed the occurrence of normal morphology in the AR initiated by thapsigargin. Thapsigargin (200 nM) did not initiate the AR in noncapacitated sperm. Initiation of the AR by thapsigargin apparently requires an influx of Ca2+ since 1 min preincubation with the calcium channel blockers La3+ (250 microM) or Ni2+ (250 microM) prior to addition of thapsigargin completely inhibits AR-initiation. Mobilization of an intracellular Ca(2+)-store by thapsigargin in capacitated human sperm may lead to an influx of extracellular Ca2+ and subsequently the AR. Putative sites for thapsigargin-sensitive intracellular Ca(2+)-stores in human sperm include the cytoplasmic droplet, the sperm nucleus and the acrosome.

Evidence suggesting involvement of a unique human sperm steroid receptor/Cl- channel complex in the progesterone-initiated acrosome reaction

Progesterone (4-pregnen-3-20-dione) initiates the acrosome reaction (AR) of human sperm in vitro. Anesthetic progestins (e.g., progesterone) potentiate gamma-aminobutyric acid (GABA) activation of neuronal GABAA receptor/Cl- channel complexes (GBRCs), and some can activate neuronal GBRCs in the absence of GABA. Here, we study whether human sperm acrosome reaction initiation by progesterone and several other progestins involves steroid interaction with a sperm GBRC. Anesthetically active 3 alpha-OH isomers of two progesterone metabolites, as well as the synthetic 3 alpha-OH anesthetic steroid Alphaxalone, their 3 beta-OH isomers (inactive as anesthetics), or progesterone, were added to capacitated human sperm. The 3 alpha-OH isomers initiated the AR, and the 3 beta-OH isomers were without effect. Moreover, the 3 alpha-OH isomers were less effective than progesterone, a progestin that does not activate neuronal GBRCs in the absence of GABA. Also, alphaxalone, a very potent activator of neuronal GBRCs, was the least active of the progestins tested. GABA was not detected in human sperm. Preincubation of capacitated sperm with GBRC Cl- channel blockers dramatically reduced the progesterone-initiated AR, and sperm were unable to undergo the progesterone-initiated AR in Cl(-)-deficient medium. AR initiation by the calcium ionophore ionomycin was not inhibited in Cl(-)-deficient medium or by GBRC Cl- channel blockers. Indirect immunofluorescence, using a monoclonal antibody to the bovine cerebral cortex GABAA receptor alpha-subunit, localized immunoreactivity in live and fixed sperm as a fluorescent band in the sperm plasma membrane overlying or near the narrow equatorial segment region of the acrosome. Immunoblotting using this antibody detected two major bands with apparent molecular weights of 50 kDa (as reported in other cell types) and 75 kDa (not reported in other cells). Our data suggest that (1) progesterone exerts its effect on capacitated human sperm at least partially by interaction with a unique sperm steroid receptor/Cl- channel complex resembling a neuronal GBRC but apparently possessing a different sensitivity to progestins; and that (2) this interaction results in an increased Cl- flux essential to AR initiation.

Effects of polyamine biosynthesis inhibitors on the progesterone-initiated increase in intracellular free Ca2+ and acrosome reactions in human sperm

This laboratory has previously reported that progesterone can initiate a rapid transient increase in the concentration of intracellular free Ca2+ ([Ca2+]i) and an increase in a Ca(2+)-requiring exocytotic event, the acrosome reaction (AR) in human sperm. Rapid increases in Ca2+ fluxes of some mammalian cells caused by another steroid, testosterone, require polyamine biosynthesis. Herein, we tested two polyamine biosynthesis suicide inhibitors for their effects on the progesterone-initiated increase in [Ca2+]i and AR in capacitated human sperm in vitro: DL-alpha-(difluoromethyl)ornithine hydrochloride (DFMO), an inhibitor of putrescine synthesis by ornithine decarboxylase and (5'[[(Z)-4-amino-2-butenyl]methylamino]-5'-deoxyadenosine (MDL 73811), an inhibitor of S-adenosylmethionine decarboxylase (required for spermidine and spermine synthesis). Sperm were capacitated in vitro and preincubated 10 min with 4.9 mM DFMO or 9.8 microM MDL 73811 with or without various polyamines (245 microM). Progesterone (3.09 microM final concentration) or progesterone solvent (ethanol, 0.1% final concentration) was then added, sperm fixed 1 min after additions and AR assayed by indirect immunofluorescence or with fluorescein-labeled Con A lectin. DFMO strongly inhibited the AR, but putrescine (product of ornithine decarboxylase and precursor of spermidine and spermine) reversed that inhibition. Preincubation for 25 min with DMFO + spermidine also reversed DFMO inhibition. MDL 73811 inhibited the progesterone-initiated AR, and a 10 min preincubation with spermidine, but not putrescine or spermine, reversed that inhibition. Preincubations with putrescine alone or with spermidine alone followed by addition of the progesterone solvent did not initiate the AR, and such preincubations followed by progesterone addition did not increase the AR more than progesterone alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Importance of mammalian sperm metalloendoprotease activity during the acrosome reaction to subsequent sperm-egg fusion: inhibitor studies with human sperm and zona-free hamster eggs

We have previously shown that each of the metalloendoprotease (MEP) inhibitors phosphoramidon, diethylenetriaminepentaacetic acid, and carbobenzoxy-L-phenylalanine, when present only during the human sperm acrosome reaction (AR), will not inhibit the AR or sperm motility but will decrease the number of sperm that penetrate zona-free hamster eggs. The present study was designed to investigate whether this inhibition of penetration is due to an effect on sperm binding to the egg plasma membrane and/or to an effect on the actual membrane fusion event. In these studies we used ionomycin to initiate the AR and assayed binding in a Ca(2+)-free medium and fusion in Ca(2+)-containing medium in the same experiment. Eggs were loaded with the fluorescent dye Hoechst 33342, and the appearance of fluorescence in a sperm head indicated that fusion had occurred. The three MEP inhibitors reduced binding only slightly but inhibited the actual fusion step by 50-60% (determined with an equation that corrected for any inhibition of fusion due to inhibition of binding). MEP inhibitors present only during gamete interactions had little or no effect on fusion. We also found that phosphoramidon-inhibitable MEP activity was released during the ionomycin-initiated AR. Incubation of AR supernatant containing MEP activity with previously acrosome-reacted, phosphoramidon-treated sperm resulted in a large reversal of the phosphoramidon-inhibitory effect on sperm-egg fusion. These results support the hypothesis that the acrosomal phosphoramidon-inhibitable MEP released during the AR acts directly or indirectly during that event to increase the fusibility of the sperm plasma membrane region required for subsequent sperm-egg fusion.

Trypsin inhibitors prevent the progesterone-initiated increase in intracellular calcium required for the human sperm acrosome reaction

Inhibitors of trypsin-like enzymes, benzamidine hydrochloride and 4'-acetamidophenyl 4-guanidinobenzoate (also an inhibitor of other serine proteases), were tested for their effects on the acrosome reaction (AR) of human sperm initiated by progesterone or the calcium ionophore ionomycin. The AR was assayed by indirect immunofluorescence and transmission electron microscopy. The trypsin inhibitors, when added 10 min prior to stimulation by progesterone, significantly inhibited the AR in comparison with progesterone treatment alone. Transmission electron microscopic examination of the sperm after progesterone treatment indicated that the inhibitors blocked the membrane fusion events of the AR. By contrast, when ionomycin (at final concentrations of 3 microM) was added to sperm preincubated in inhibitors, sperm underwent morphologically normal AR, acrosomal matrix loss was not inhibited, and the percentage of acrosome-reacted sperm was the same as that obtained in the absence of inhibitors. Using the cell calcium indicator fura-2, we further demonstrated that both trypsin inhibitors prevented the progesterone-stimulated rise in intracellular Ca2+ ([Ca2+]int) required for the AR, but did not affect [Ca2+]int in unstimulated sperm. These results suggest that sperm trypsin-like activity may be directly or indirectly involved in increasing sperm [Ca2+]int during stimulation by progesterone.

Progesterone acts at the plasma membrane of human sperm

There has been increasing interest in the relationship between rapid effects of steroids and steroid-plasma membrane interaction. This laboratory has previously reported that progesterone increases human sperm cytosolic free calcium ([Ca2+]i) and thereby initiates the human sperm acrosome reaction (AR) in less than 1 min. Herein, to test whether progesterone acts at the sperm plasma membrane, progesterone 3-(O-carboxymethyl)oxime: bovine serum albumin (BSA) conjugate (free of unconjugated progesterone) was added to capacitated human sperm. Fura-2 assays were used to detect less than 1 min changes in [Ca2+]i, and indirect immunofluorescence was used to assay the AR occurring 1 min after stimulus addition. The conjugate increased [Ca2+]i and the AR (though less than did unconjugated progesterone). Enzyme immunoassays demonstrated that the concentrations of unconjugated progesterone in conjugate-treated sperm suspensions did not increase over those of control suspensions. Since the progesterone: BSA conjugate presumably does not cross the sperm plasma membrane, progesterone must act at that membrane to increase [Ca2+]i and the AR.

Phosphatidylinositol 4,5-bisphosphate hydrolysis in human sperm stimulated with follicular fluid or progesterone is dependent upon Ca2+ influx

Hydrolysis of the phospholipid phosphatidylinositol 4,5-bisphosphate is thought to be intimately involved in agonist-induced changes in intracellular Ca2+ levels. Recently we have shown that human preovulatory follicular fluid, which induces exocytosis in human sperm, can stimulate a rapid, transient increase in sperm cytosolic [Ca2+] [Thomas & Meizel (1988) Gamete Res. 20, 397-411]. We report here that both a Sephadex G-75 column fraction, derived from follicular fluid, and progesterone (a component of both the G-75 fraction and whole follicular fluid) stimulate rapid hydrolysis of PtdIns(4,5)P2 and PtdIns4P in human sperm. We also report that progesterone stimulates a rapid influx of Ca2+ in human sperm. Human spermatozoa were labelled for 24 h with myo-[3H]inositol and then treated with either the G-75 fraction or progesterone. A 30-65% loss of label was detected in PtdIns(4,5)P2 and PtdIns4P within 15 s of stimulus addition; no changes were observed in PtdIns during 2 min of treatment. The loss of label from both lipids was accompanied by an increase in water-soluble inositol phosphates. Production of both InsP3 and InsP2 was seen within 10 s; however, InsP3 was rapidly removed and had reached control levels by 1 min. Similarly, formation of InsP2 reached a peak by 30 s and then began a decline accompanied by a corresponding increase in InsP. No increases in InsP4 were seen in sperm treated in this fashion. Stimulated hydrolysis of the phosphoinositides and release of inositol phosphates were both blocked by the Ca2+ antagonist La3+. Likewise, the progesterone-induced increase in intracellular Ca2+ was inhibited by La3+, and phosphoinositide hydrolysis stimulated by this hormone was dependent upon the presence of extracellular Ca2+.

Methods for evaluating the acrosomal status of mammalian sperm

A full understanding of the acrosome reaction is central to understanding sperm function. Acrosomal status can be determined on living, motile sperm in only a few mammalian species. For other species, many light microscopic methods have been developed, including colored stains for bright-field microscopy, and probes for fluorescence microscopy. We review the existing methods and the criteria that should be considered in the choice of an assay.

Steroid induced exocytosis: the human sperm acrosome reaction

By a combination of organic precipitation and high pressure liquid chromatography, human sperm acrosome reaction inducing activity has been purified from the fluid aspirated from preovulatory human ovarian follicles and identified as 4-pregnen-3,20-dione (progesterone) and 4-pregnen-17 alpha-ol-3,20-dione (17 alpha-hydroxyprogesterone). It is argued that progesterone is present at the site of fertilization of placental mammals in concentrations sufficient for activity, and hence provides a mechanism of inducing the acrosome reaction, an exocytotic event, in vivo.

Zona pellucida-induced acrosome reaction in boar sperm

Induction of the acrosome reaction in boar sperm by the zona pellucida (ZP) was investigated. A modified cytochemical staining method for measuring the acrosome reaction in boar sperm gave equivalent results to those obtained with transmission electron microscopy. Isolated heat-solubilized ZP effectively induced the acrosome reaction in boar sperm at a concentration of 25 micrograms/ml. Electrophoretically purified ZP components were also tested for acrosome reaction-inducing activity; both the 55,000 and 90,000 components of the ZP were effective. The carbohydrate moiety of the 55,000 component was necessary for activity because the polypeptides derived by chemical deglycosylation of the two glycoproteins did not induce the acrosome reaction.

Effects of metalloendoprotease substrates on the human sperm acrosome reaction

The substrates carbobenzyloxyserylleucylamide, carbobenzyloxyglycylleucylamide and carbobenzyloxyglycylphenylalanylamide were used as potential competitive inhibitors of endogenous metalloendoprotease activity. When the acrosome reaction was elicited by a potential physiological stimulus, human follicular fluid, each of the substrates (1-1.5 mM) inhibited exocytosis. Carbobenzyloxyserylleucylamide also inhibited the acrosome reaction when exocytosis was stimulated using the calcium ionophore ionomycin, but carbobenzyloxyglycylleucylamide was not inhibitory and carbobenzyloxyglycylphenylalanylamide actually enhanced exocytosis under these conditions. Experiments using the fluorescent indicator fura-2 revealed that the increase in intracellular, free calcium stimulated by follicular fluid in human spermatozoa was depressed by carbobenzyloxyglycylphenylalanylamide but not by carbobenzyloxyserylleucylamide. The peptide carbobenzyloxyglycylglycylamide, which is not a substrate for metalloendoproteases, had no effect on the acrosome reaction, whether stimulated by follicular fluid or ionomycin. While the results with carbobenzyloxyserylleucylamide suggest a possible involvement of a metalloendoprotease in the human sperm acrosome reaction, our other results demonstrate that these carbobenzyloxy peptides have complex effects on the process of exocytosis in human spermatozoa, and suggest caution in interpretation of data obtained using such peptides on intact cells.

Evidence suggesting a role for sperm metalloendoprotease activity in penetration of zona-free hamster eggs by human sperm

It has been reported that metalloendoprotease (MEP) activity is involved in somatic cell membrane fusion events and in the sea urchin sperm acrosome reaction (AR). MEP activity also has been demonstrated in human and other mammalian sperm. The present study was concerned with investigating whether a human sperm MEP is important in membrane events necessary for sperm egg fusion. Ejaculated human sperm were washed, capacitated in vitro, and preincubated with the competitive MEP inhibitors phosphoramidon (50 microM) or CBZ-L-phenylalanine (1 mM), with 100 microM diethylenetriaminepentaacetic acid (DTPA), a heavy metal chelator, or as controls, with the appropriate solvents. The AR was initiated in vitro with preovulatory human follicular fluid and the sperm washed to dilute inhibitors and then coincubated with zona-free golden hamster eggs (zonae and cumuli removed with trypsin and hyaluronidase, respectively). Eggs were washed after 0.5 h, and the number of sperm remaining bound was counted. After 2.5 h further incubation, the eggs were stained with acetolacmoid or acetoorcein and penetration was assayed by counting the number of decondensed sperm heads per egg (penetration index) and the percent of penetrated eggs. The inhibitor treatments did not decrease the percentage of penetrated eggs (range 80-90%), but a significant reduction in the penetration index was observed. Phosphoramidon reduced the penetration index by 45%, CBZ-L-phenylalanine by 57%, and DTPA by 56%. None of the inhibitors decreased the penetration index or the percentage of penetrated eggs when added directly to suspensions of acrosome-reacted sperm and zona-free eggs at the diluted levels that would have been present after washing inhibitor-treated sperm.(ABSTRACT TRUNCATED AT 250 WORDS)

An influx of extracellular calcium is required for initiation of the human sperm acrosome reaction induced by human follicular fluid

The role of Ca2+ in the human sperm acrosome reaction was investigated using the fluorescent calcium indicator fura-2. Previous experiments have shown that a Sephadex G-75 column fraction of human follicular fluid can stimulate the human sperm acrosome reaction [Suarez SS, Wolf DP, Meizel S (1986): Gamete Res 14:107-121]. Using fura-2, we demonstrated that this Sephadex G-75 fraction also stimulates a rapid, transient increase in intracellular free Ca2+. This Ca2+ transient is blocked either by chelation of extracellular calcium or by addition of the Ca2+ antagonist La3+. We have also been able to stimulate the acrosome reaction in human sperm without significant loss of motility, using the divalent cation ionophore ionomycin. Acrosome reactions stimulated by whole follicular fluid, the G-75 fraction, or ionomycin are all blocked by removal of extracellular Ca2+. These results strongly suggest that an influx of extracellular Ca2+ is responsible for initiating the acrosome reaction in human sperm treated with human follicular fluid. This is the first demonstration in mammalian sperm that a potentially physiological stimulus can cause an increase in intracellular Ca2+ concomitant with the acrosome reaction.

Ultrastructural studies of the early events of the human sperm acrosome reaction as initiated by human follicular fluid

It has been suggested that the morphology of the human sperm acrosome reaction (AR) is markedly different from that of other mammalian sperm. The present study examines the fine structural events of the early stages of the human sperm AR as initiated by preovulatory human follicular fluid. Human sperm, capacitated in vitro for 6 hr at 40 degrees C (90% motility) were diluted with equal volumes of follicular fluid before fixing in cacodylate-buffered glutaraldehyde at 5, 10, 15, 20, and 180 sec. Fixed sperm were treated with either tannic acid or thiocarbohydrazine and OsO4. Over 2,000 sperm were viewed. By 5 sec, 2% of the sperm had initiated the AR. By 15 sec, 8% of the sperm were in some stage of the reaction, and after 180 sec 40% of the sperm had completed the acrosome reaction. The initial stages of the human AR are characterized by a swelling or decondensation of the acrosomal matrix. The fusion between the plasma membrane and outer acrosomal membrane begins at the most anterior tip of the head and progresses toward the equatorial segment. Fusion and fenestration ended at the equatorial segment. With thiocarbohydrazine + OsO4 fixation the fused membranes are distinct hybrid vesicles with the outer acrosomal membrane being far more electron dense. The acrosomal matrix is retained by 20 sec, but by 180 sec the matrix is completely dispersed, even when viewed after tannic acid fixation. Also by 180 sec, vesicles were being progressively lost. It is therefore concluded that the morphology of the human sperm AR, as initiated by human follicular fluid, is not unique, but is similar to that described for other mammalian sperm.

Partial characterization of a fraction from human follicular fluid that initiates the human sperm acrosome reaction in vitro

A human follicular fluid (HFF) fraction prepared by Sephadex G-75 column chromatography has been previously shown by this laboratory to initiate the human sperm acrosome reaction (AR) in vitro. In the present report, the apparent molecular weight (MW) of this AR activity determined by a longer G-75 column than was used in the previous work was 50,000 +/- 5,106. The G-75 Sephadex void volume fractions of some but not all HFF samples were also found to contain some AR-initiating activity. The occasional void volume activity was less potent than that of the 50,000 MW fraction and was not studied further. Further characterization of the 50,000 MW fraction was carried out. A time-course study demonstrated that maximum AR were obtained within 5 min following the addition of the 50,000 MW fraction. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by silver staining revealed that the 50,000 MW fraction was still a relatively crude preparation. Treatment of the 50,000 MW fraction with chloroform:methanol did not extract the AR-initiating activity into the lipid phase. The AR-initiating activity of the untreated 50,000 MW fraction was precipitated when it was boiled, but the activity was partially resistant to boiling after overnight incubation. Treatment of the 50,000 MW fraction with pronase E or with several glycosaminoglycan hydrolases did not destroy the activity. Pronase treatment resulted in a higher amount of boiling-resistant AR-initiating activity. The AR-initiating activity of the untreated 50,000 MW fraction was partially dialyzable, but the activity of an undialyzed fraction did not pass through an ultrafiltration membrane with a 10,000 MW cut-off. However, treatment of the 50,000 MW fraction with protease, peptide:N-glycosidase F, and to a lesser extent chondroitinase ABC yielded an active lower MW activity which could pass through such an ultrafiltration membrane. The lower MW activity released by peptide:N-glycosidase F eluted in the included volume (5,000-1,000) of a Sephadex G-25 column. Neutral hexose but not protein or peptide was detected in the G-25 peak of AR-initiating activity. These results suggest that the AR-initiating activity present in the 50,000 MW fraction of HFF: 1) is present either as two different AR factors (a high-MW factor and a low-MW, noncovalently bound factor) or as a single factor responsible for both the nondialyzable and dialyzable AR-initiating activities (the latter being enzymatically released from the former), and 2) may be at least partially associated with N-linked oligosaccharides of a glycoprotein or proteoglycan.

Human sperm acrosome reaction-initiating activity associated with the human cumulus oophorus and mural granulosa cells

This report describes the detection and partial characterization of preovulatory human cumulus oophorus and mural granulosa cell-associated activity capable of initiating the human sperm acrosome reaction (AR) in vitro. Fragments of preovulatory human cumulus (cells plus extracellular matrix) were washed 3 times, incubated for 24 hr and the spent media and washes assayed for their ability to initiate the human sperm acrosome reaction (AR) in vitro. AR activity was present in the first two washes but not the third wash; however, AR activity was recovered in the spent medium after 3 X-washed fragments were incubated for 24 hr under conditions which maintained the viability of the cumulus cells. The spent media of preovulatory human mural granulosa cells contained AR-initiating activity after 1-3, 3-6, and 6-9 days of culture. The properties of the AR activity present in spent media of human cumulus fragments included resistance to loss of activity during treatment with pronase; resistance to loss of activity during treatment with chondroitinase ABC or bacterial hyaluronidase; heat stability after overnight incubation; lack of extraction by chloroform-methanol; an apparent molecular weight (MW) of 50,000, as determined by Sephadex G-75 column chromatography; conversion to a lower apparent MW activity by incubation with pronase. These properties are also characteristic of a fraction derived by Sephadex G-75 chromatography of preovulatory human follicular fluid which also has been shown to stimulate the human sperm acrosome reaction in vitro. The AR activity from spent media of human mural granulosa cells is also found in a 50,000 MW Sephadex G-75 fraction. We propose that the sources of the 50,000 MW human follicular fluid AR activity are the cumulus oophorus and the mural granulosa cells.

Biochemical studies of metalloendoprotease activity in the spermatozoa of three mammalian species

Ejaculated porcine and human spermatozoa, hamster spermatozoa from the cauda epididymidis, isolated hamster sperm heads and hamster cytoplasmic droplets contained activity that hydrolyzed the metalloendoprotease substrate ABZ-Ala-Gly-Leu-Ala-NBA (AAGLAN). Hamster sperm heads were isolated by treating spermatozoa with proteinase K and removing sperm tails with Dowex-50W beads. Hamster sperm activity was characterized using spermatozoa from which cytoplasmic droplets were removed by sonication and centrifugation. Porcine sperm preparations were essentially free of cytoplasmic droplets, while human sperm preparations retained somewhat more droplet material. Activity from all of these sources was inhibited by the metalloendoprotease inhibitors phosphoramidon, 1,10-phenanthroline, CBZ-D-Phe and CBZ-L-Phe but was not competitively inhibited by the metalloendoprotease substrate CBZ-Ser-Leu-amide. The AAGLAN hydrolyzing activity found in intact spermatozoa of all three species had a pH optimum of 6.2, while the optimum of the hamster sperm cytoplasmic droplet activity was 7.0. In addition, hamster sperm preparations were inhibited by ZnCl2 and dithiothreitol, but were not affected by toluene, benzamidine or chymostatin. The AAGLAN hydrolyzing activity of hamster sperm preparations was reduced, but not eliminated, by dialysis. It is concluded that spermatozoa from all three species, hamster sperm heads and hamster cytoplasmic droplets contain metalloendoprotease activity. Furthermore, metalloendoprotease activity found in hamster cytoplasmic droplets is different from that found in spermatozoa.

In vitro studies of the golden hamster sperm acrosome reaction: completion on the zona pellucida and induction by homologous soluble zonae pellucidae

We have studied the occurrence of the golden hamster sperm acrosome reaction (AR) in vitro during interaction with the oocyte investments: the cumulus cell matrix and the zona pellucida. Hamster sperm were capacitated in a defined medium that does not induce the AR. These spermatozoa were allowed to interact with the ovum vestments, the events of which were recorded using high-speed videomicrography. Frame-by-frame analysis revealed that sperm did not complete the AR in the cumulus cell matrix, but did so on the zona pellucida. Furthermore, a higher percentage of sperm completed the AR on the zona pellucida of cumulus-invested than on cumulus-free eggs. We also investigated the effect of solubilized hamster and mouse zonae pellucidae on the hamster sperm AR. Addition of solubilized hamster zonae to capacitated sperm elicited the AR within 15 min. Solubilized mouse zonae were significantly less effective, indicating that the zona-induced AR in hamster sperm may be species specific. These results suggest that the hamster zona pellucida is an inducer of the AR in the intact or soluble form, and that the majority of spermatozoa traverse the cumulus cell matrix without completing the AR in our in vitro system.

Glycosaminoglycans stimulate the acrosome reaction of previously capacitated hamster sperm

The glycosaminoglycans (GAGs) hyaluronic acid and heparin were added (10 micrograms and 100 micrograms/ml to golden hamster sperm suspensions previously incubated for 4.5 h under capacitating conditions. After additions, sperm were incubated for 5-15 min and acrosome reactions (AR) assayed in motile sperm by phase contrast microscopy. Hyaluronic acid and heparin significantly stimulated AR over control levels. Hyaluronic acid did not stimulate AR 15 min after addition to sperm previously incubated for only 2.5 h. Pre-incubation of hyaluronic acid with streptomyces hyaluronidase destroyed the ability of that GAG to stimulate the AR. These results indicate that GAGs (at least one of which, hyaluronic acid, is present in the oocyte cumulus oophorous) can rapidly stimulate the acrosome reaction in motile previously capacitated hamster sperm.

Molecules that initiate or help stimulate the acrosome reaction by their interaction with the mammalian sperm surface

This review deals with exogenous molecules that stimulate the acrosome reaction (AR) of mammalian sperm in vitro, presumably by acting at the sperm surface. Such molecules may exert their effect(s) by stimulating capacitation and/or by stimulation or initiation of the AR, and they are probably present at one of three putative in vivo sites (also discussed here) for the AR of a fertilizing sperm: the oviductal fluid, the cumulus oophorus matrix, and the zona pellucida. The molecules discussed include serum albumin, hydrolytic enzymes (particularly proteases); hormones including biogenic amines, estradiol, and arachidonic acid metabolites; sulfur-containing beta-amino acids; glycosaminoglycans such as hyaluronic acid; and a zona pellucida glycoprotein. Possible mechanisms to explain the effects of these molecules are also discussed. Several conclusions and suggestions are offered in this review: There is more than one site for the AR of a fertilizing sperm in vitro, depending on experimental conditions and species, but the site(s) at which the AR of a fertilizing sperm occur(s) in vivo is/are still a matter of disagreement; there are a number of molecules that can stimulate or initiate the AR in vitro, and such molecular duplication may also exist in vivo to ensure fertility; and synergistic interaction between some of those exogenous molecules may occur in the stimulation of capacitation and the stimulation or initiation of the AR.

Release of hyaluronidase and beta-N-acetylhexosaminidase during in vitro incubation of hamster sperm

Previous studies have shown that hamster sperm release a significant amount of hyaluronidase before and independently of the normal acrosome reaction. In this study, we have used improved methods for in vitro incubation to investigate the time course of the release of hyaluronidase and hexosaminidase from hamster sperm. When hamster sperm are incubated in medium which allows capacitation, 34 to 47% of the total mechanically extractable hyaluronidase and 34 to 51% of beta-N-acetylhexosaminidase are released into solution prior to and independently of the normal acrosome reaction (ARx). An additional 40 to 50% of the hyaluronidase and 34 to 51% of the hexosaminidase are released at the time of the normal ARx. Control experiments indicate that the early release is not due to the presence of dead sperm in culture and that the normal ARx is required for the second release. Increasing amounts of TCA-precipitated bovine serum albumin in the culture medium stimulated the early (1 hr) release of both enzymes. The data are consistent with the ideas that a significant amount of both enzymes is released from the sperm surface by 1 hr of incubation and that about the same amount of each enzyme is released during the normal ARx. Hyaluronidase and hexosaminidase release at the time of the acrosome reaction was measured for the first time using hamster sperm. The biphasic release of these enzymes may indicate that they have a dual function in fertilization and may help explain how sperm can penetrate the cumulus and corona radiata without undergoing an acrosome reaction.

Inhibition of hamster sperm Na+, K+-ATPase activity by taurine and hypotaurine

Taurine, hypotaurine and the structural analogue, beta-alanine, were tested for their effects on Na+, K+-ATPase activity of crude homogenates prepared from washed cauda epididymal hamster sperm. Preincubation with 0.1-10 mM taurine or hypotaurine inhibited Na+, K+-ATPase in a dose-dependent manner, while beta-alanine had an inhibitory effect only at 10 mM. The results of this study are the first evidence to demonstrate inhibition of Na+, K+-ATPase activity by taurine and hypotaurine and are discussed in relation to the ability of these compounds to sustain hamster sperm motility and fertility.

Evidence for the involvement of a sperm trypsinlike enzyme in the membrane events of the hamster sperm acrosome reaction

Previous studies from this laboratory have suggested a role for sperm trypsinlike activity in the membrane events of the hamster sperm acrosome reaction, but these conclusions have been disputed by studies done with guinea pig, mouse, and ram sperm. This study was designed to further investigate the role of such activity in the hamster sperm acrosome reaction. Washed hamster epididymal sperm were examined by electron microscopy after incubation in a defined medium. Fewer sperm incubated in the presence of the trypsin inhibitor benzamidine underwent the membrane events of the acrosome reaction compared to controls. Benzamidine did not interfere with the loss of the acrosomal contents in the sperm which did undergo acrosome reactions. The trypsin inhibitor NPGB (previously shown to inhibit the hamster acrosome reaction under other conditions) did not inhibit the membrane events or the loss of acrosomal contents when the acrosome reaction was induced by the fusogen lysophosphatidyl choline (a method of induction which may have bypassed the need for a trypsinlike enzyme). In a bioassay designed to measure acrosome reactions, fewer sperm preincubated with benzamidine subsequently penetrated zona-pellucida-free hamster eggs. The present results together with those of an earlier study by this laboratory strongly suggest that a sperm trypsinlike enzyme has a direct or indirect role in the membrane events of hamster sperm acrosome reaction. The relationship of these results to studies with other species is discussed.

The effects of products and inhibitors of arachidonic acid metabolism on the hamster sperm acrosome reaction

The mammalian sperm acrosome reaction (AR) is a fusion and fenestration of sperm head membranes which is essential for fertilization. Our earlier work demonstrated that arachidonic acid could stimulate the AR 15 min after addition to hamster sperm capacitated by incubation for 4.5 h. The present study was undertaken to determine whether inhibitors of arachidonic acid metabolism could affect the stimulation of the AR by arachidonic acid and whether products of its metabolism could stimulate the AR. Phenidone or nordihydroguaiaretic acid, inhibitors of both the cyclo-oxygenase and lipoxygenase pathways of arachidonic acid metabolism, and docosahexaenoic acid, a cyclo-oxygenase pathway inhibitor, inhibited the AR induced by arachidonic acid. PGE2, a product of the cyclo-oxygenase pathway of arachidonic acid metabolism and 5- or 12-hydroxyeicosatetraenoic acid (HETEs) products of the lipoxygenase pathway, stimulated the AR when added to sperm capacitated by incubation for 4.5 h. Prostaglandins not derived from arachidonic were also tested: PGE1 stimulated the AR, but PGF1 alpha and PGA2 did not. We suggest that arachidonic acid metabolites produced by the sperm and by the female reproductive tract are important for the mammalian sperm AR.

Hamster sperm Na+, K+-adenosine triphosphatase: increased activity during capacitation in vitro and its relationship to cyclic nucleotides

These in vitro studies of golden hamster sperm were undertaken to determine whether: Na+, K+-adenosine triphosphatase (ATPase) activity is required for capacitation; Na+, K+-ATPase activity is altered during capacitation; and cyclic nucleotides can control this enzyme activity. Hamster sperm were incubated in a medium in which capacitation occurred in an asynchronous manner and in which acrosome reactions began to occur after approximately 3.5 h of incubation. Inhibition of the hamster sperm acrosome reaction by the Na+, K+-ATPase inhibitor ouabain (1 microM) added at Time (T) = 2 or T = 3 h could be fully reversed by the addition of the ionophore nigericin (0.1 microM) at T = 3.5 h. However, when ouabain was added at T = 0 or T = 1 h, similar nigericin addition could not completely reverse the inhibition. Na+, K+-ATPase activity of hamster sperm increased by 2 h of incubation (compared to that measured initially after 15 min) and this activity remained elevated at 3.5 h. Addition of either monobutyryl cyclic adenosine 3':5'-monophosphate ( BtcAMP ) (12.9 microM) or monobutyryl cyclic guanosine monophosphate ( BtcGMP ) (10.5 microM), or the phosphodiesterase inhibitor SQ20009 (10 microM) at 2 h produced a stimulation of acrosome reactions at 4 and 5 h. However, while BtcGMP and SQ 20009 also induced a further increase in Na+, K+-ATPase activity measured at 3.5 h, BtcAMP had no effect. Intracellular cAMP and cGMP levels measured showed cAMP increased by 2 h and remained elevated when measured at 3.5 h, while cGMP could not be consistently detected at 15 min, 2 h or 3.5 h. However, assays of high numbers of uncapacitated sperm did detect a low level of cGMP. These results suggest that Na+, K+-ATPase activity increases in and is essential for early capacitation [and thereby eventually for the acrosome reaction (AR)] of hamster sperm and that the increase in Na+, K+-ATPase activity occurring during capacitation is probably mediated by intracellular cGMP but not cAMP, although both cyclic nucleotides stimulate the hamster sperm AR.