Regulation of the starfish sperm acrosome reaction by cGMP, pH, cAMP and Ca2+

In the starfish, Asterias amurensis, three components in the jelly coat of eggs, namely acrosome reaction-inducing substance (ARIS), Co-ARIS and asterosap, act in concert on homologous spermatozoa to induce the acrosome reaction (AR). Molecular recognition between the sperm surface molecules and the egg jelly molecules must underlie signal transduction events triggering the AR. Asterosap is a sperm-activating molecule, which stimulates rapid synthesis of intracellular cGMP, pH and Ca(2+). This transient elevation of Ca(2+) level is caused by a K(+)-dependent Na(+)/Ca(2+) exchanger, and the increase of intracellular pH is sufficient for ARIS to induce the AR. The concerted action of ARIS and asterosap could induce elevate intracellular cAMP levels in starfish sperm and the sustained increase in [Ca(2+)], which is essential for the AR. The signaling pathway induced by these factors seems to be synergistically regulated to trigger the AR in starfish sperm.     

Physiological inducers of the acrosome reaction

This article reviews recent studies on physiological inducers of the acrosome reaction in starfish. Upon encountering the jelly coat of eggs, starfish sperm undergo the acrosome reaction in response to a cooperation of three jelly components: a sulfated glycoprotein named acrosome reaction-inducing substance (ARIS), a group of steroidal saponins named Co-ARIS, and an oligopeptide presumably having an activity to increase the intracellular pH of sperm. ARIS induces the acrosome reaction in high Ca²⁺ or high pH sea water. In normal sea water, both ARIS and Co-ARIS are required for the induction. In addition to ARIS and Co-ARIS, a third jelly component, the oligopeptide, is necessary to mimic the full capacity of the jelly coat to induce the acrosome reaction. ARIS and Co-ARIS cooperatively increase the intracellular Ca²⁺ by stimulating Ca²⁺ channels, while the oligopeptide increases the intracellular pH by stimulating Na⁺/H⁺ exchange systems. When sperm meet the eggs, both changes are simultaneously achieved in them and thus they undergo the acrosome reaction.     

Acrosome Reaction-Inducing Substance Purified from the Egg Jelly Inhibits the Jelly-Induced Acrosome Reaction in Starfish: An Apparent Contradiction

Previous studies indicated that two components of the egg jelly are required for induction of the acrosome reaction in starfish: a sulfated glycoprotein called acrosome reaction-inducing substance (ARIS) and a diffusible organic substance(s) called Co-ARIS. In the present study the sites of action of ARIS and Co-ARIS and their temporal relationships were examined. When sperm had been treated for a few minutes with ARIS, or a crude preparation of Co-ARIS (Fraction M₈), or inadequate amounts of jelly, or sufficient jelly in low Ca²⁺ sea water, they did not undergo the acrosome reaction when the deficiencies were corrected. Moreover, they became nonresponsive to the jelly. Pronase digest of ARIS (P-ARIS) but not of Fraction M₈ retained this capacity. A steroidal saponin purified as Co-ARIS did not have this capacity. This suggests the presence of a third jelly component, probably an oligopeptide(s), participating in induction of the acrosome reaction. Activation of Ca²⁺ -uptake seems to be at least one, if not the only, action site of ARIS and Co-ARIS, because ARIS, P-ARIS, and Fraction M₈ inhibited jelly-induced Ca²⁺ -uptake by sperm, and because the calcium ionophore A23187 by-passed the blockage by these components of the jelly-induced acrosome reaction.     

Egg jelly components responsible for histone degradation and acrosome reaction in the starfish, Asterina pectinifera.

In the starfish, Asterina pectinifera, egg jelly induces the degradation of sperm histones as well as the acrosome reaction. We have isolated histone degradation-inducing components from the egg jelly. The histone degradation and the acrosome reaction are induced by a co-operative action of ARIS, which is an extremely large, sulfated glycoprotein with diffusible substance(s) in the jelly. Co-ARIS I, a steroidal saponin of the jelly, is effective to induce both reactions in the presence of ARIS.     

Correlation Between the Molecular Structure and the Biological Activity of Co-ARIS, a Cofactor for Acrosome Reaction-Inducing Substance

Two components in the egg jelly are required for inducing the acrosome reaction in starfish; a sulfated glycoprotein called acrosome reaction-inducing substance (ARIS) and its cofactor called Co-ARIS. Three distinct molecules were isolated as the major Co-ARIS' and designated as Co-ARIS' I, II and III. Structural analysis of Co-ARIS' revealed that they are steroidal saponins comprising a sulfated steroid and a pentasaccharide chain. Co-ARIS' I and II differ only in the steroidal side chain. In the presence of ARIS, each Co-ARIS induced the acrosome reaction with a maximal effect at 100–200 μM (Co-ARIS I) or 25–50 μM (Co-ARIS' II and III). Mixtures of Co-ARIS' I, II and III were more effective than the individuals. The activity of Co-ARIS was considerably reduced by solvolytic desulfation but was not affected at all by periodate oxidation. Reduction by NaBH₄ decreased the activity of Co-ARIS I and enhanced that of Co-ARIS II. Treatment of Co-ARIS III with NaBH₄ did not affect the activity as anticipated from its structure. These results suggest that the sulfate moiety and the side chain of steroid are important for the activity of Co-ARIS. The saccharide chain, however, seems not necessarily to be strictly specified for the activity.     

Induction of the Acrosome Reaction in Starfish

In the starfish, Asterias amurensis , at least two distinct components of the egg jelly are required for inducing the acrosome reaction: a sulfated glycoprotein named acrosome reaction-inducing substance (ARIS) and a diffusible organic substance(s) named Co-ARIS. The following evidence suggested that ARIS and Co-ARIS cooperatively activate CA-channels of the sperm plasma membrane and eventually induce dramatic changes in sperm morphology, the acrosome reaction. 1) Pronase digest of ARIS (P-ARIS) and Co-ARIS, either as a pure or a crude preparation (Fraction M₈), were fully effective in combination for induction of the acrosome reaction in normal sea water, although they were not effective individually. P- ARIS alone induced the acrosome reaction fully in high Ca²⁺ sea water and markedly at high pHs, whereas Fraction M₈ alone did not induce the reaction even in these conditions. The reaction was not induced by increase in either the Ca²⁺ concentration or the pH of sea water, but was markedly induced in the absence of jelly components by raising both the pH and Ca²⁺ concentration together. 2) The ionophore A23187 induced the acrosome reaction appreciably when present alone and fully in the presence of monensin or Fraction M₈. Monesin alone was ineffective. 3) The jelly or a combination of ARIS and Fraction M₈ caused abrupt Ca²⁺ -uptake by the sperm. The Ca-channel blockers verapamil and diltiazem inhibited the jelly-induced acrosome reaction.     

Purification of Co-ARIS, a Cofactor for Acrosome Reaction-Inducing Substance, from the Egg Jelly of Starfish

In the starfish, Asterias amurensis , the acrosome reaction-inducing capacity of the egg jelly has been attributed to two components in the egg jelly: a sulfated glycoprotein (ARIS) and an unidentified low-molecular weight substance (Co-ARIS). In the process of purification of Co-ARIS, we found that Co-ARIS is not a single chemical entity but a series of closely related molecules. Co-ARIS was first group-separated by using octadecylsilane and anion-exchanger. Three major Co-ARIS' (I, II and III) were then purified by successive chromatographies using octadecylsilane and silica gel to the homogeneity in thin layer chromatography. Each of purified Co-ARIS', together with ARIS or the Pronase-digest of ARIS, induced the acrosome reaction at high ratios in normal seawater.     

Acrosome reaction in the cumulus oophorus revisited: involvement of a novel sperm-released factor NYD-SP8

Fertilization is a process involving multiple steps that lead to the final fusion of one sperm and the oocyte to form the zygote. One of the steps, acrosome reaction (AR), is an exocytosis process, during which the outer acrosome membrane fuses with the inner sperm membrane, leading to the release of acrosome enzymes that facilitate sperm penetration of the egg investments. Though AR has been investigated for decades, the initial steps of AR in vivo, however, remain largely unknown. A well elucidated model holds the view that AR occurs on the surface of the zona pellucida (ZP), which is triggered by binding of sperm with one of the ZP glycosylated protein, ZP3. However, this model fails to explain the large number of ‘falsely’ acrosome-reacted sperms found within the cumulus layer in many species examined. With the emerging evidence of cross-talk between sperm and cumulus cells, the potential significance of AR in the cumulus oophorus, the outer layer of the egg, has been gradually revealed. Here we review the acrosome status within the cumulus layer, the cross-talk between sperm and cumulus cells with the involvement of a novel sperm-released factor, NYD-SP8, and re-evaluate the importance and physiological significance of the AR in the cumulus in fertilization.     

Visualization of GM1 with cholera toxin B in live epididymal versus ejaculated bull, mouse, and human spermatozoa

The organization of membrane subdomains in mammalian sperm has recently generated controversy, with several reports describing widely differing localization patterns for the ganglioside GM1. Using the pentameric B subunit of cholera toxin (CTB), we found GM1 to be restricted to the plasma membrane overlying the acrosome in the heads of live murine sperm. Interestingly, CTB had minimal binding to live bovine and human sperm. To investigate whether this difference in GM1 localization was because of species differences or differences between collection from the epididymis (mouse) or an ejaculate (bull, human), we examined epididymal bovine and human sperm. We found that GM1 localized to the plasma membrane overlying the acrosome in sperm from these species. To determine whether some component of seminal plasma was interfering with the ability of CTB to access GM1, we incubated epididymal mouse sperm with fluid from murine seminal vesicles and epididymal bull sperm with bovine seminal plasma. This treatment largely abolished the ability of the CTB to bind to GM1, producing a fluorescence pattern similar to that reported for the human. The most abundant seminal plasma protein, PDC-109, was not responsible for this loss. As demonstration that the seminal plasma was not removing GM1, sperm exposed to seminal plasma were fixed before CTB addition, and again displayed fluorescence over the acrosome. These observations reconcile inconsistencies reported for the localization of GM1 in sperm of different species, and provide evidence for the segregation of GM1 to a stable subdomain in the plasma membrane overlying the acrosome.     

Acrosome reaction is subfamily specific in sea star fertilization

In the fertilization process of sea stars, sperm is activated to go through the acrosome reaction before cell fusion. We focused on induction of the acrosome reaction as a key process in fertilization. Six species of sea stars were used in this study: Asterias amurensis, Asterias rubens, Asterias forbesi, Aphelasterias japonica, Distolasterias nipon, and Asterina pectinifera. Acrosome reaction assays indicate that the acrosome reaction can be induced across species within Asteriinae subfamily. However, cross-fertilization assays indicate that sea stars have species specificity in fertilization. Therefore, steps after the acrosome reaction are responsible for the species specificity. To explain acrosome reaction subfamily specificity at the molecular level, the sugar components of egg jelly were examined and analyzed by principal component analysis. A. amurensis and A. forbesi belong to the same induction group of the acrosome reaction. D. nipon and An. pectinifera are in a unique group. Enzyme-linked immunosorbent assays indicate that Asteriinae subfamily share a common glycan structure, the Fragment 1 of Acrosome Reaction-Inducing Substance from A. amurensis. Fragment 1 plays an important role in the subfamily specificity of acrosome reaction induction. In addition, A. amurensis sperm activating peptide was recognized by sperm from the same superorder. These results demonstrate that the specificity of acrosome reaction induction is present at the subfamily level in sea stars.     

Guanylyl cyclase and cGMP-specific phosphodiesterase participate in the acrosome reaction of starfish sperm

In the starfish, Asterias amurensis, the cooperation of three components of the egg jelly, i.e. ARIS (acrosome reaction-inducing substance), Co-ARIS and asterosap, is responsible for inducing the acrosome reaction. Experimentally, ARIS and asterosap are sufficient for the induction. However, when sperm are treated only with asterosap, they become unresponsive to the egg jelly to undergo the reaction. In this study, we analysed the mechanism of the acrosome reaction, using sperm inactivation by asterosap as a clue. Asterosap causes a rapid and transient increase in intracellular cGMP through the activation of the asterosap receptor, a guanylyl cyclase, and causes an increase in intracellular Ca(2+). When sperm were pretreated with asterosap, the guanylyl cyclase seemed to be inactivated irreversibly by dephosphorylation. They were still responsive to ARIS but no longer to asterosap. However, in the presence of IBMX or zaprinast, inhibitors against phosphodiesterases (PDEs), they retained their capacity to undergo the acrosome reaction in response to the egg jelly or ARIS alone. IBMX and zaprinast suppressed the intracellular catabolism of cGMP, but not of cAMP. These results suggest that guanylyl cyclase and cGMP-specific, IBMX- and zaprinast-susceptible PDEs are involved in the regulation of the acrosome reaction.     

Novel conserved structural domains of acrosome reaction-inducing substance are widespread in invertebrates

In the starfish Asterias amurensis, acrosome reaction inducing substance (ARIS) is the main factor responsible for allowing sperm to recognize the egg jelly and begin the acrosome reaction (AR). ARIS is a large proteoglycan-like molecule, and its pentasaccharide repeat, Fragment 1 (Fr. 1), is responsible for inducing AR. Here, we investigated the primary structure of ARIS for the first time in order to improve our understanding of its functionality. Electrophoretic analysis revealed that ARIS is a complex of three proteins, all of which are modified by the Fr. 1 sugar chain. Sequencing indicated that there are two novel, conserved domains in all three ARIS proteins: ARIS N-terminus (AR-N) and ARIS C-terminus (AR-C) domains. We also found that other echinoderms possess ARIS proteins that are capable of inducing the AR for homologous sperm, indicating that ARIS proteins may be a ubiquitous component for echinoderm fertilization. Moreover, we identified ARIS-like genes from Ctenophora to Protochordata.     

The SLO3 sperm-specific potassium channel plays a vital role in male fertility

Here we show a unique example of male infertility conferred by a gene knockout of the sperm-specific, pH-dependent SLO3 potassium channel. In striking contrast to wild-type sperm which undergo membrane hyperpolarization during capacitation, we found that SLO3 mutant sperm undergo membrane depolarization. Several defects in SLO3 mutant sperm are evident under capacitating conditions, including impaired motility, a bent “hairpin” shape, and failure to undergo the acrosome reaction (AR). The failure of AR is rescued by valinomycin which hyperpolarizes mutant sperm. Thus SLO3 is the principal potassium channel responsible for capacitation-induced hyperpolarization, and membrane hyperpolarization is crucial to the AR.     

Biochemical Studies on the Acrosome Reaction of the Starfish, Asterias Amurensis II. Purification and Characterization of Acrosome Reaction-Inducing Substance

The acrosome reaction-inducing substance (ARIS) was purified from egg jelly of the starfish, Asterias amurensis. The purification procedure included elimination of neutral glycoproteins from the ARIS fraction by isoelectric pointprecipitation and subsequent gel filtrations on Sephadex G–50 and Bio-Gel A-50m columns. The final preparation of ARIS was homogeneous as judged by cellulose acetate electrophoresis of ARIS and by ion-exchange chromatography on DEAE-Sephadex A–25 of S-carboxymethylated ARIS. ARIS is a very large, sulfated glycoprotein containing fucose, galactose, galactosamine and glucosamine as sugar components. It requires diffusible cofactor (Co-ARIS) for full biological activity. A Pronase digest of ARIS retained its capacity to induce the acrosome reaction when Co-ARIS was added to the bioassay system. The physiological significance of the carbohydrate moiety of ARIS is discussed.     

Asterosap-induced elevation in intracellular pH is indispensable for ARIS-induced sustained increase in intracellular Ca2+ and following acrosome reaction in starfish spermatozoa

In the starfish, Asterias amurensis, the cooperation of three components of the egg jelly, namely ARIS (acrosome reaction-inducing substance), Co-ARIS and asterosap, is responsible for the induction of acrosome reaction. For the induction, ARIS alone is enough in high-Ca2+ or high-pH seawater, but, besides ARIS, the addition of either Co-ARIS or asterosap is required in normal seawater. Asterosap transiently increased both the intracellular pH (pHi) and Ca2+ ([Ca2+]i), while ARIS slightly elevated the basal level of [Ca2+]i. However, a sustained elevation of [Ca2+]i and acrosome reaction occurred if sperm were simultaneously treated with ARIS and asterosap. EGTA inhibited the sustained [Ca2+]i elevation and acrosome reaction. The sustained [Ca2+]i elevation and acrosome reaction were highly susceptible to SKF96365 and Ni2+, specific blockers of the store-operated Ca2+ channel (SOC). These results suggest that sustained [Ca2+]i elevation, mediated by the SOC-like channel, is a prerequisite for the acrosome reaction. In high-pH seawater, ARIS alone induced a prominent [Ca2+]i increase and acrosome reaction, which were similarly sensitive to SKF96365. The acrosome reaction was effectively induced by ARIS alone when pHi was artificially increased to more than 7.7. Asterosap increased pHi from 7.6 +/- 0.1 to 7.7 +/- 0.1. Furthermore, the sustained [Ca2+]i elevation and acrosome reaction, induced by a combination of ARIS and asterosap, were drastically inhibited by a slight reduction in pHi. Taking these results into account, we suggest that an asterosap-induced pHi elevation is required for triggering the ARIS-induced sustained [Ca2+]i elevation and consequent acrosome reaction.     

Effect of dilauroylphosphatidylcholine liposomes on motility, induction of the acrosome reaction, and subsequent egg penetration of ram epididymal sperm

The effects of dilauroylphosphatidylcholine (PC12) on ram epididymal sperm motility, acrosome reaction (AR) induction, plasma membrane permeability, mitochondrial function, and sperm penetration into zona-free hamster eggs were determined. PC12 (50 microM) induced cell motility in caput and cauda sperm, as measured by subjective estimation and automated motility analysis. Motion parameters of treated caput sperm approached those of control ejaculated sperm. Flow cytometric analysis revealed that membrane permeability to propidium iodide and mitochondrial uptake of rhodamine 123 changed during epididymal transit. PC12 induced the AR in sperm from all epididymal regions relative to control incubated sperm (caput 17% vs. control 8%; corpus 29% vs. control 13%; proximal cauda 48% vs. control 4%; distal cauda 51% vs. control 9%). After PC12 treatment, egg penetration by sperm was increased for sperm from the corpus (corpus 7% vs. control 0%) and cauda (proximal 48% vs. control 0%; distal 51% vs. control 0%), but not for caput sperm (caput 0% vs. control 0%). These studies establish that some sperm in each region of the epididymis possess the capacity for movement and the AR. Caput sperm, however, were unique in that they could not penetrate eggs. Additional maturational changes must occur in the caput and/or corpus epididymidis before penetration capacity can be expressed.     

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.     

Sperm plasma membrane breakdown during Drosophila fertilization requires sneaky, an acrosomal membrane protein

Fertilization typically involves membrane fusion between sperm and eggs. In Drosophila, however, sperm enter eggs with membranes intact. Consequently, sperm plasma membrane breakdown (PMBD) and subsequent events of sperm activation occur in the egg cytoplasm. We previously proposed that mutations in the sneaky (snky) gene result in male sterility due to failure in PMBD. Here we support this proposal by demonstrating persistence of a plasma membrane protein around the head of snky sperm after entry into the egg. We further show that snky is expressed in testes and encodes a predicted integral membrane protein with multiple transmembrane domains, a DC-STAMP-like domain, and a variant RING finger. Using a transgene that expresses an active Snky-Green fluorescent protein fusion (Snky-GFP), we show that the protein is localized to the acrosome, a membrane-bound vesicle located at the apical tip of sperm. Snky-GFP also allowed us to follow the fate of the protein and the acrosome during fertilization. In many animals, the acrosome is a secretory vesicle with exocytosis essential for sperm penetration through the egg coats. Surprisingly, we find that the Drosophila acrosome is a paternally inherited structure. We provide evidence that the acrosome induces changes in sperm plasma membrane, exclusive of exocytosis and through the action of the acrosomal membrane protein Snky. Existence of testis-expressed Snky-like genes in many animals, including humans, suggests conserved protein function. We relate the characteristics of Drosophila Snky, acrosome function and sperm PMBD to membrane fusion events that occur in other systems.     

Induction of the acrosome reaction in black tiger shrimp (Penaeus monodon) requires sperm trypsin-like enzyme activity

Trypsin-like enzymes in egg water (EW), a natural acrosome reaction (AR) inducer, are known for their importance in shrimp AR. In this report, we describe a unique phenomenon of the AR of black tiger shrimp (Penaeus monodon) sperm. It was completed within 45-60 sec and comprised only the acrosomal exocytosis and depolymerization of the sperm head anterior spike. We used peptidyl fluorogenic substrates to show the presence of trypsin-like enzymes in P. monodon EW and sperm, but minimal activities of chymotrypsin-like enzymes. In sperm, these trypsin-like enzymes existed both on the sperm surface and in the acrosome. The acrosomal enzyme was revealed as a 45-kDa band by fluorogenic substrate in-gel zymography. Although EW possessed high trypsin-like enzyme activities, they were not essential for the AR induction; EW pretreated with an irreversible trypsin inhibitor, or heat-inactivated EW (HI-EW), to abolish the trypsin-like activities could still induce the AR. The HI-EW-induced AR was inhibited by the presence of a membrane impermeant soybean trypsin inhibitor (SBTI) in the sperm suspension, indicating the significance of sperm-borne trypsin-like enzymes (on the surface and/or in the acrosome) in this AR process. However, pretreatment of sperm with SBTI followed by its removal from the suspension still allowed the AR to occur within 5 min of sperm exposure to HI-EW. Since trypsin-like activity of the SBTI-pretreated sperm surface at 5 min after SBTI removal was at the minimal level, our results suggest the importance of the acrosomal trypsin-like enzyme in the AR process.     

Phospholipase B Is Activated in Response to Sterol Removal and Stimulates Acrosome Exocytosis in Murine Sperm

Despite a strict requirement for sterol removal for sperm to undergo acrosome exocytosis (AE), the mechanisms by which changes in membrane sterols are transduced into changes in sperm fertilization competence are poorly understood. We have previously shown in live murine sperm that the plasma membrane overlying the acrosome (APM) contains several types of microdomains known as membrane rafts. When characterizing the membrane raft-associated proteomes, we identified phospholipase B (PLB), a calcium-independent enzyme exhibiting multiple activities. Here, we show that sperm surface PLB is activated in response to sterol removal. Both biochemical activity assays and immunoblots of subcellular fractions of sperm incubated with the sterol acceptor 2-hydroxypropyl-β-cyclodextrin (2-OHCD) confirmed the release of an active PLB fragment. Specific protease inhibitors prevented PLB activation, revealing a mechanistic requirement for proteolytic cleavage. Competitive inhibitors of PLB reduced the ability of sperm both to undergo AE and to fertilize oocytes in vitro, suggesting an important role in fertilization. This was reinforced by our finding that incubation either with protein concentrate released from 2-OHCD-treated sperm or with recombinant PLB peptide corresponding to the catalytic domain was able to induce AE in the absence of other stimuli. Together, these results lead us to propose a novel mechanism by which sterol removal promotes membrane fusogenicity and AE, helping confer fertilization competence. Importantly, this mechanism provides a basis for the newly emerging model of AE in which membrane fusions occur during capacitation/transit through the cumulus, prior to any physical contact between the sperm and the oocyte''s zona pellucida.