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)     

Artificial induction of exocytosis in bull sperm.

We have investigated an exocytotic event, the acrosome reaction (AR), induced by treatment of bovine sperm with vesicles composed of dilauroyl phosphatidylcholine (PC12). Cell membrane permeability barriers (dye exclusion), acrosomal status (pisum sativum (PSA) lectin binding), and intracellular Ca2+ (Fluo3 fluorescence) were evaluated utilizing flow cytometry and fluorescence microscopy. By these methods the AR is resolved into four kinetically distinct steps: (a) PC12 transfer to the sperm plasma membrane (PM); (b) increased permeability of the PM to extracellular Ca2+; (c) localized leakage of acrosomal contents at the anterior tip of the sperm; and (d) vesiculation of sperm membranes and complete exposure of acrosomal contents. Evidence for PC12 transfer to sperm includes transfer of a fluorescent PC12 analogue from vesicles to cells and the absence of detectable vesicle--cell fusion. The fusion inducing properties of PC12 appear to reside in the lipid head group as neither dilauroylphosphatidylethanolamine nor dilauroylphosphatidylglycerol stimulated the AR. The effect of PC chain length on AR induction closely parallels the aqueous phase solubility of the lipid tested. The rate and extent of the AR depend on the extracellular calcium concentration. Cells treated in the absence of calcium do not undergo the AR, but do so rapidly (less than 1 min) upon subsequent addition of calcium. This role of Ca2+ is partially filled by Sr2+, but not by Ba2+ or Mg2+. The rate of the AR decreases with decreasing temperature and the AR occurs very slowly below 27 degrees C. Simultaneous evaluation of intracellular calcium and acrosomal status reveals the kinetic relationship between Ca2+ influx and the exposure of acrosomal contents. N-Ethylmaleimide preincubation arrests PC12-treated sperm at an intermediate stage in the AR, characterized by punctate PSA binding over the tip of the sperm head. The AR, a developmentally regulated, receptor-mediated fusion event, synchronously induced here in vitro, provides a useful model for mechanistic studies of exocytosis.     

Calmodulin antagonists inhibit T-type Ca(2+) currents in mouse spermatogenic cells and the zona pellucida-induced sperm acrosome reaction.

The sperm acrosome reaction (AR) is a regulated exocytotic process required for gamete fusion. It depends on an increase in [Ca(2+)](i) mediated by Ca(2+) channels. Although calmodulin (CaM) has been reported to regulate several events during the AR, it is not known whether it modulates sperm Ca(2+) channels. In the present study we analyzed the effects of CaM antagonists W7 and trifluoroperazine on voltage-dependent T-type Ca(2+) currents in mouse spermatogenic cells and on the zona pellucida-induced AR in sperm. We found that these CaM antagonists decreased T-currents in a concentration-dependent manner with IC(50) values of approximately 10 and approximately 12 microM, respectively. W7 altered the channels' voltage dependence of activation and slowed both activation and inactivation kinetics. It also induced inactivation at voltages at which T-channels are not activated, suggesting a promotion of inactivation from the closed state. Consistent with this, W7 inhibited the ZP-induced [Ca(2+)](i) transients in capacitated sperm. Likewise, W7 and TFP inhibited the AR with an IC(50) of approximately 10 microM. In contrast, inhibitors of CaM-dependent kinase II and protein kinase A, as well as a CaM-activated phosphatase, had no effect either on T-currents in spermatogenic cells or on the sperm AR. Together these results suggest a functional interaction between CaM and the sperm T-type Ca(2+) channel. They are also consistent with the involvement of T-channels in the AR.     

SNAREs in mammalian sperm: possible implications for fertilization

Soluble N-ethylmalameide-sensitive factor attachment protein receptor (SNARE) proteins are present in mammalian sperm and could be involved in critical membrane fusion events during fertilization, namely the acrosome reaction. Vesicle-associated membrane protein/synaptobrevin, a SNARE on the membrane of a vesicular carrier, and syntaxin 1, a SNARE on the target membrane, as well as the calcium sensor synaptotagmin I, are present in the acrosome of mammalian sperm (human, rhesus monkey, bull, hamster, mouse). Sperm SNAREs are sloughed off during the acrosome reaction, paralleling the release of sperm membrane vesicles and acrosomal contents, and SNARE antibodies inhibit both the acrosome reaction and fertilization, without inhibiting sperm-egg binding. In addition, sperm SNAREs may be responsible, together with other sperm components, for the asynchronous male DNA decondensation that occurs following intracytoplasmic sperm injection, an assisted reproduction technique that bypasses normal sperm-egg surface interactions. The results suggest the participation of sperm SNAREs during membrane fusion events at fertilization in mammals.     

Autoradiographic visualization of the mouse egg's sperm receptor bound to sperm

The extracellular coat, or zona pellucida, of mammalian eggs contains species-specific receptors to which sperm bind as a prelude to fertilization. In mice, ZP3, one of only three zona pellucida glycoproteins, serves as sperm receptor. Acrosome-intact, but not acrosome-reacted, mouse sperm recognize and interact with specific O- linked oligosaccharides of ZP3 resulting in sperm-egg binding. Binding, in turn, causes sperm to undergo the acrosome reaction; a membrane fusion event that results in loss of plasma membrane at the anterior region of the head and exposure of inner acrosomal membrane with its associated acrosomal contents. Bound, acrosome-reacted sperm are able to penetrate the zona pellucida and fuse with the egg's plasma membrane (fertilization). In the present report, we examined binding of radioiodinated, purified, egg ZP3 to both acrosome intact and acrosome reacted sperm by whole-mount autoradiography. Silver grains due to bound 125I-ZP3 were found localized to the acrosomal cap region of heads of acrosome-reacted sperm. Under the same conditions, 125I-fetuin bound at only bacKground levels to heads of both acrosome-intact and - reacted sperm, and 125I-ZP2, another zona pellucida glycoprotein, bound preferentially to acrosome-reacted sperm. These results provide visual evidence that ZP3 binds preferentially and specifically to heads of acrosome intact sperm; properties expected of the mouse egg's sperm receptor.     

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.     

The Ca2+ increase by the sperm factor in physiologically polyspermic newt fertilization: its signaling mechanism in egg cytoplasm and the species-specificity

The newt, Cynops pyrrhogaster, exhibits physiological polyspermic fertilization, in which several sperm enter an egg before egg activation. An intracellular Ca(2+) increase occurs as a Ca(2+) wave at each sperm entry site in the polyspermic egg. Some Ca(2+) waves are preceded by a transient spike-like Ca(2+) increase, probably caused by a tryptic protease in the sperm acrosome at the contact of sperm on the egg surface. The following Ca(2+) wave was induced by a sperm factor derived from sperm cytoplasm after sperm-egg membrane fusion. The Ca(2+) increase in the isolated, cell-free cytoplasm indicates that the endoplasmic reticulum is the major Ca(2+) store for the Ca(2+) wave. We previously demonstrated that citrate synthase in the sperm cytoplasm is a major sperm factor for egg activation in newt fertilization. In the present study, we found that the activation by the sperm factor as well as by fertilizing sperm was prevented by an inhibitor of citrate synthase, palmitoyl CoA, and that an injection of acetyl-CoA or oxaloacetate caused egg activation, indicating that the citrate synthase activity is necessary for egg activation at fertilization. In the frog, Xenopus laevis, which exhibits monospermic fertilization, we were unable to activate the eggs with either the homologous sperm extract or the Cynops sperm extract, indicating that Xenopus sperm lack the sperm factor for egg activation and that their eggs are insensitive to the newt sperm factor. The mechanism of egg activation in the monospermy of frog eggs is quite different from that in the physiological polyspermy of newt eggs.     

A role for sperm surface protein disulfide isomerase activity in gamete fusion: evidence for the participation of ERp57

In mammals, sperm-egg interaction is based on molecular events either unique to gametes or also present in somatic cells. In gamete fusion, it is unknown which features are gamete specific and which are shared with other systems. Conformational changes mediated by thiol-disulfide exchange are involved in the activation of some virus membrane fusion proteins. Here we asked whether that mechanism is also operative in sperm-egg fusion. Different inhibitors of protein disulfide isomerase (PDI) activity were able to inhibit sperm-egg fusion in vitro. While pretreatment of oocytes had no effect, pretreatment of sperm reduced their fusion ability. Some members of the PDI family were detected on the sperm head, and use of specific antibodies and substrates suggested that the oxidoreductase ERp57 has a role in gamete fusion. The results support the idea that thiol-disulfide exchange is a mechanism that may act in gamete fusion to produce conformational changes in fusion-active proteins.     

Inhibition of the mouse sperm surface alpha-D-mannosidase inhibits sperm-egg binding in vitro

In previous reports from this laboratory, we identified the presence of a novel alpha-D-mannosidase on the surface of rat, mouse, hamster, and human spermatozoa [J Cell Biol 1989; 109:1257-1267 and Biol Reprod 1990; 42:843-858]. Since it has been suggested that mannosyl residues on the egg zona pellucida may be important for sperm-egg binding, studies were undertaken to examine the potential role of the sperm alpha-D-mannosidase during fertilization. Incubation of mouse spermatozoa in the presence of increasing concentrations of the inhibitory sugars, alpha-methyl mannoside, alpha-methyl glucoside, D-mannose, or D-mannitol, resulted in a dose-dependent decrease in the number of spermatozoa bound per egg without a deleterious effect on sperm motility or on the sperm acrosome, and a dose-dependent inhibition of the sperm mannosidase activity. Galactose, however had no effect on sperm-egg binding or on sperm mannosidase activity. Two nucleotide sugars (UDP-GlcNAc and UDP-gal) were also tested and shown to reduce sperm-egg binding but with only a minimal effect on sperm mannosidase activity. In additional studies, spermatozoa incubated in the presence of a mannose-containing oligosaccharide exhibited a dramatic reduction in sperm-egg binding that correlated with a similar inhibition of sperm mannosidase activity. The oligosaccharide substrate did not affect sperm motility or the sperm acrosome. These studies suggest that the sperm alpha-D-mannosidase may play an important role during fertilization.     

Strontium supports human sperm capacitation but not follicular fluid-induced acrosome reaction.

The ability of strontium (Sr(2+)) to replace calcium (Ca(2+)) in maintaining human sperm function has still not been completely characterized. In the present study, acrosome reaction (AR) inducibility in response to human follicular fluid (hFF) was compared in spermatozoa incubated in either Ca(2+)- or Sr(2+)-containing media. Other events related to sperm capacitation, such as protein tyrosine phosphorylation and hyperactivation as well as zona pellucida (ZP) recognition under both conditions, were also analyzed. Spermatozoa incubated overnight in the presence of Sr(2+) were unable to undergo the AR when exposed to hFF. Nevertheless, when spermatozoa were incubated under this condition and then transferred to medium with Ca(2+), sperm response to hFF was similar to that of cells incubated throughout in the presence of Ca(2+). The sperm protein tyrosine phosphorylation patterns and the percentages of sperm motility and hyperactivation were similar after incubation in Ca(2+)- or Sr(2+)-containing media. Under both conditions, the same binding capacity to homologous ZP was observed. Similar results were obtained when EGTA was added in order to chelate traces of Ca(2+) present in Sr(2+) medium. From these results, it can be concluded that Sr(2+) can replace Ca(2+) in supporting capacitation-related events and ZP binding, but not hFF-induced AR of human spermatozoa.     

Evidence that phospholipase C from the sperm is not responsible for initiating Ca(2+) release at fertilization in mouse eggs

Release of Ca(2+) from intracellular stores at fertilization of mammalian eggs is mediated by inositol 1,4,5-trisphosphate (IP3), but the mechanism by which the sperm initiates IP3 production is not yet understood. We tested the hypothesis that phospholipase C (PLC) activity introduced into the mouse egg as a consequence of sperm-egg fusion is responsible for causing Ca(2+) release. We demonstrated that microinjecting purified, recombinant PLCgamma1 protein into mouse eggs caused Ca(2+) oscillations like those seen at fertilization. However, the PLC activity in the minimum amount of purified PLCgamma1 protein needed to elicit Ca(2+) release when injected into eggs was approximately 500-900 times the PLC activity contained in a single sperm. This indicates that a single mouse sperm does not contain enough PLC activity to be responsible for causing Ca(2+) release at fertilization. We also examined whether phosphatidylinositol 3-kinase (PI3K) could have a role in this process, and found that several inhibitors of PI3K-mediated signaling had no effect on Ca(2+) release at fertilization.     

Mammalian sperm acrosome: formation, contents, and function

Sperm-egg interaction is a carbohydrate-mediated species-specific event which initiates a signal transduction cascade resulting in the exocytosis of sperm acrosomal contents (i.e., the acrosome reaction). This step is believed to be a prerequisite which enables the acrosome-reacted spermatozoa to penetrate the zona pellucida (ZP) and fertilize the egg. Successful fertilization in the mouse and several other species, including man, involves several sequential steps. These are (1) sperm capacitation in the female genital tract; (2) binding of capacitated spermatozoa to the egg's extracellular coat, the ZP; (3) induction of acrosome reaction (i.e., sperm activation); (4) penetration of the ZP; and (5) fusion of spermatozoon with the egg vitelline membrane. This minireview focuses on the most important aspects of the sperm acrosome, from its formation during sperm development in the testis (spermatogenesis) to its modification in the epididymis and function following sperm-egg interaction. Special emphasis has been given to spermatogenesis, a complex process involving multiple molecular events during mitotic cell division, meiosis, and the process of spermiogenesis. The last event is the final phase when a nondividing round spermatid is transformed into the complex structure of the spermatozoon containing a well-developed acrosome. Our intention is also to briefly discuss the functional significance of the contents of the sperm acrosome during fertilization. It is important to mention that only the carbohydrate-recognizing receptor molecules (glycohydrolases, glycosyltransferases, and/or lectin-like molecules) present on the surface of capacitated spermatozoa are capable of binding to their complementary glycan chains on the ZP. The species-specific binding event starts a calcium-dependent signal transduction pathway resulting in sperm activation. The hydrolytic and proteolytic enzymes released at the site of sperm-zona interaction along with the enhanced thrust of the hyperactivated beat pattern of the bound spermatozoon, are important factors in regulating the penetration of the zona-intact egg.     

Evidence for the involvement of calmodulin in mouse sperm capacitation

Although Ca(2+) is of fundamental importance in mammalian sperm capacitation, its downstream targets have not been definitively demonstrated. The purpose of this study was to use the calmodulin (CaM) antagonists W7 and calmidazolium (CZ) to investigate the possible role of CaM, a Ca(2+)-specific binding protein, in capacitation. Sperm membrane changes associated with capacitation were assessed by the B pattern after chlortetracycline staining and by the ability to undergo the acrosome reaction (AR) in response to lysophosphatidylcholine (LPC). The percentage of B pattern sperm was significantly inhibited by W7 or CZ in a concentration-dependent manner. At 100 microM W7 or 10 microM CZ, these inhibitors also significantly reduced the sperm's ability to undergo the LPC-induced AR. Inhibition of the B pattern and the LPC-induced AR was overcome by exogenous cAMP analogues. Treatment of the sperm with 100 microM W7 also resulted in a significant decrease in their ability to fertilize eggs in vitro. At 100 microM, W5, a less potent dechlorinated W7 analogue, had no effect on the B pattern, LPC-induced AR, or fertilization competence. Sperm viability and protein tyrosine phosphorylation were not substantially affected by 100 microM W7 (relative to 100 microM W5) or 10 microM CZ; however, the percentages of motile and hyperactivated sperm were significantly reduced. The antagonist-inhibited sperm motility was restored by dilution in control medium, but not by cAMP analogues. These results suggest that CaM participates in the regulation of membrane changes important for mouse sperm capacitation, at a point upstream from cAMP, and that this pathway is at least partially separable from pathways controlling tyrosine phosphorylation and hyperactivation.     

Mammalian sperm-egg fusion: evidence that epididymal protein DE plays a role in mouse gamete fusion

Rat epididymal protein DE associates with the sperm surface during epididymal maturation and is a candidate molecule for mediating gamete membrane fusion in the rat. Here, we provide evidence supporting a role for DE in mouse sperm-egg fusion. Western blot studies indicated that the antibody against rat protein DE can recognize the mouse homologue in both epididymal tissue and sperm extracts. Indirect immunofluorescence studies using this antibody localized the protein on the dorsal region of the acrosome. Experiments in which zona-free mouse eggs were coincubated with mouse capacitated sperm in the presence of DE showed a significant and concentration-dependent inhibition in the percentage of penetrated eggs, with no effect on either the percentage of oocytes with bound sperm or the number of sperm bound per egg. Immunofluorescence experiments revealed specific DE-binding sites on the fusogenic region of mouse eggs. Because mouse sperm can penetrate zona-free rat eggs, the participation of DE in this interaction was also investigated. The presence of the protein during gamete coincubation produced a significant reduction in the percentage of penetrated eggs, without affecting the binding of sperm to the oolemma. These observations support the involvement of DE in an event subsequent to sperm-egg binding and leading to fusion in both homologous (mouse-mouse) and heterologous (mouse-rat) sperm-egg interaction. The lack of disintegrin domains in DE indicates that the protein interacts with its egg-binding sites through a novel mechanism that does not involve the reported disintegrin-integrin interaction.     

Mammalian sperm-egg fusion: the rat egg has complementary sites for a sperm protein that mediates gamete fusion.

Rat epididymal protein DE is localized on the fusogenic region of the acrosome-reacted spermatozoa and has a potential role in sperm-egg fusion. We investigated the presence of DE binding sites on the egg surface by co-incubating zona-free eggs and capacitated sperm in different concentrations of pure DE. Results indicate that DE produced a concentration-dependent decrease in egg penetration by sperm (fusion), with almost complete inhibition at 200 micrograms/ml. This inhibition was not due to an effect of DE on initial sperm binding to the egg membrane, since the presence of this protein did not affect the percentage of oocytes with bound sperm nor the number of bound sperm per egg. Those sperm that failed to penetrate the egg in the presence of DE became able to do so after transfer of the eggs to protein- and sperm-free medium, indicating a role for DE in an event subsequent to binding and leading to fusion. Indirect immunofluorescence using a polyclonal antibody against DE revealed a patchy labeling over the entire egg surface, with the exception of the area overlying the second metaphase spindle. This conclusion was supported by the disappearance of the DE-negative area on the fertilized egg. Zona-free eggs, incubated with DE at 4 degrees C or fixed before exposure to DE, displayed a uniform staining, suggesting that the patchy labeling resulted from aggregation of DE binding sites by the purified protein. The aggregation of these egg components may represent a necessary step of the fusion process. To our knowledge, this is the first study reporting the existence and localization of complementary sites to a specific sperm protein on the plasma membrane of the mammalian egg.     

Mouse SLLP1, a sperm lysozyme-like protein involved in sperm-egg binding and fertilization

This study demonstrates the retention of mouse sperm lysozyme-like protein (mSLLP1) in the equatorial segment of spermatozoa following the acrosome reaction and a role for mSLLP1 in sperm-egg binding and fertilization. Treatment of cumulus intact oocytes with either recmSLLP1 or its antiserum resulted in a significant (P < or = 0.05) inhibition of fertilization. Co-incubation of zona-free mouse oocytes with capacitated mouse spermatozoa in the presence of varying concentrations of anti-recmSLLP1 serum or recmSLLP1 also inhibited sperm-oolemma binding. A complete inhibition of binding and fusion of spermatozoa to the oocyte occurred at 12.5 muM concentration of recmSLLP1, while conventional chicken and human lysozymes did not block sperm-egg binding. mSLLP1 showed receptor sites in the perivitelline space as well as on the microvillar region of the egg plasma membrane. The retention of mSLLP1 in the equatorial segment of acrosome-reacted sperm, the inhibitory effects of both recmSLLP1 and antibodies to SLLP1 on in vitro fertilization with both cumulus intact and zona-free eggs, and the definition of complementary SLLP1-binding sites on the egg plasma membrane together support the hypothesis that a c lysozyme-like protein is involved in the binding of spermatozoa to the egg plasma membrane during fertilization.     

A sustained increase in intracellular Ca(2+) is required for the acrosome reaction in sea urchin sperm.

The acrosome reaction (AR), necessary for fertilization in many species, requires an increase in intracellular Ca(2+) ([Ca(2+)](i)). In sea urchin sperm, the AR is triggered by an egg-jelly factor: the associated [Ca(2+)](i) elevation lasts minutes and involves two Ca(2+) permeable channels. Both the opening of the second channel and the onset of the AR occur approximately 5 s after treatment with egg factor, suggesting that these events are linked. In agreement, removal of Ca(2+) from sea water or addition of Ca(2+) channel blockers at the time when opening of the second channel is first detected inhibits AR and causes a "rapid" (t(1/2) = 3--15 s) decrease in [Ca(2+)](i) and partial inhibition of the intracellular pH change associated with the AR. Simultaneous addition of NH(4)Cl and either EGTA, Co(2+), or Ni(2+) 5 s after egg factor prevents the partial inhibition of the evoked pH(i) change observed but does not reverse AR inhibition. Therefore, the sustained increase in [Ca(2+)](i) caused by the second Ca(2+) channel is needed for the sperm AR. Experiments with agents that induce capacitative Ca(2+) uptake (thapsigargin and cyclopiazonic acid) suggest that the second channel opened during the AR could be a store-operated Ca(2+) channel.     

Carbohydrate involvement in sperm-egg fusion in mice

The potential involvement of cell-surface carbohydrates in sperm-egg fusion in mice was evaluated in this study. Zona-free mouse eggs were inseminated in the presence of a variety of simple saccharides to determine if certain sugars would act as competitive inhibitors of sperm-egg fusion. Of the sugars tested, L-fucose, galactose, and N-acetylglucosamine caused the greatest inhibition of sperm penetration levels relative to controls. A number of complex saccharides or glycoproteins with differing carbohydrate structures, including fucoidan, ascophyllan, ovomucoid, ovalbumin, fetuin, asialofetuin, and chondroitin sulfate, were also tested as competitive inhibitors of fusion. Only the L-fucose containing saccharides fucoidan and ascophyllan caused significant inhibition of fusion at concentrations of 0.05-1.0 mg/ml and 0.1-5.0 mg/ml, respectively. None of the other compounds tested had any inhibitory effect on fusion when tested at concentrations up to 5 mg/ml. The effect of the inhibitory saccharides was not due to the presence of residual zona material on the surface of the zona-free eggs, since zona-free eggs did not bind an 125I-labeled antibody directed against the ZP3 protein of the mouse zona pellucida. Pretreatment of either sperm or eggs with fucoidan (1 mg/ml) for 60 min prior to insemination caused only small decreases in sperm penetration levels, indicating that fucoidan exerted its major inhibitory effect on fusion only when present during insemination. Treatment of sperm, but not zona-free eggs, with fucosidase prior to insemination caused significant reductions in sperm penetration levels. Other glycosidic enzymes, including glucosidase, galactosidase, and N-acetylglucosaminidase, had no inhibitory effect on the sperm. These data suggest that an L-fucose component of the sperm surface is involved in sperm-egg fusion in the mouse.     

Identification of mouse trp homologs and lipid rafts from spermatogenic cells and sperm.

Intracellular Ca(2+) has an important regulatory role in the control of sperm motility, capacitation, and the acrosome reaction (AR). However, little is known about the molecular identity of the membrane systems that regulate Ca(2+) in sperm. In this report, we provide evidence for the expression of seven Drosophila transient receptor potential homolog genes (trp1-7) and three of their protein products (Trp1, Trp3 and Trp6) in mouse sperm. Allegedly some trps encode capacitative Ca(2+) channels. Immunoconfocal images showed that while Trp6 was present in the postacrosomal region and could be involved in sperm AR, expression of Trp1 and Trp3 was confined to the flagellum, suggesting that they may serve sperm to regulate important Ca(2+)-dependent events in addition to the AR. Likewise, one of these proteins (Trp1) co-immunolocalized with caveolin-1, a major component of caveolae, a subset of lipid rafts potentially important for signaling events and Ca(2+) flux. Furthermore, by using fluorescein-coupled cholera toxin B subunit, which specifically binds to the raft component ganglioside GM1, we identified caveolin- and Trp-independent lipid rafts residing in the plasma membrane of mature sperm. Notably, the distribution of GM1 changes drastically upon completion of the AR.     

Scorpion toxins that block T-type Ca2+ channels in spermatogenic cells inhibit the sperm acrosome reaction

The acrosome reaction (AR) is a Ca(2+)-dependent event required for sperm to fertilize the egg. The activation of T-type voltage-gated Ca(2+) channels plays a key role in the induction of this process. This report describes the actions of two toxins from the scorpion Parabuthus granulatus named kurtoxin-like I and II (KLI and KLII, respectively) on sperm Ca(2+) channels. Both toxins decrease T-type Ca(2+) channel activity in mouse spermatogenic cells and inhibit the AR in mature sperm. Saturating concentrations of the toxins inhibited at most approximately 70% of the whole-cell Ca(2+) current, suggesting the presence of a toxin-resistant component. In addition, both toxins inhibited approximately 60% of the AR, which is consistent with the participation of T-type Ca(2+) channels in the sperm AR.