Specific binding of acrosome-reaction-inducing substance to the head of starfish spermatozoa

Acrosome reaction in starfish: signal molecules in the jelly coat and their receptors

Animal eggs are generally encased in one or more extra-cellular coats that protect the egg from biological, chemical and mechanical hazards. These coats contain some essential molecules for sperm to fertilise an appropriate egg, such as the specific ligand for sperm binding and the specific signal for induction of the acrosome reaction. In starfish, the outermost egg coat is a relatively thick gelatinous layer called the jelly coat. When starfish sperm encounter the jelly coat of homologous eggs, they undergo the acrosome reaction within a second or less (Dale et al., 1981; Ikadai & Hoshi, 1981; Sase et al., 1995). We have thus searched the jelly coat for the signal molecule(s) that triggers the acrosome reaction in the starfish, Asterias amurensis. It is known that three components in the jelly coat, namely acrosome reaction-inducing substance (ARIS), Co-ARIS and asterosap, act in concert on homologous spermatozoa to elicit the acrosome reaction immediately and efficiently (Hoshi et al., 1994,1999).

ARIS alone induces the acrosome reaction only in high calcium or high pH seawater. In normal seawater, besides ARIS, either Co-ARIS or asterosap is required for the induction. Without ARIS, no combination of Co-ARIS and asterosap can induce the acrosome reaction in normal, high calcium or high pH seawater. A mixture of ARIS and Co-ARIS increases the intracellular Ca2+ level, whereas asterosap increases the intra-cellular pH (Matsui et al., 1986a, b; Nishigaki et al., 1996). These events are prerequisites for the induction of the acrosome reaction. Indeed, the triad of ARIS, CoARIS and asterosap provides the best conditions for the induction of the acrosome reaction in normal sea-water (Hoshi et al., 1994, 1999).

Glycobiology of reproductive processes in marine animals: the state of the art

Glycobiology is the study of complex carbohydrates in biological systems and represents a developing field of science that has made huge advances in the last half century. In fact, it combines all branches of biomedical research, revealing the vast and diverse forms of carbohydrate structures that exist in nature. Advances in structure determination have enabled scientists to study the function of complex carbohydrates in more depth and to determine the role that they play in a wide range of biological processes. Glycobiology research in marine systems has primarily focused on reproduction, in particular for what concern the chemical communication between the gametes. The current status of marine glycobiology is primarily descriptive, devoted to characterizing marine glycoconjugates with potential biomedical and biotechnological applications. In this review, we describe the current status of the glycobiology in the reproductive processes from gametogenesis to fertilization and embryo development of marine animals.

Acrosome reaction-related steroidal saponin, Co-ARIS, from the starfish induces structural changes in microdomains

Cofactor for acrosome reaction-inducing substance (Co-ARIS) is a steroidal saponin from the starfish Asterias amurensis. Saponins exist in many plants and few animals as self-defensive chemicals, but Co-ARIS has been identified as a cofactor for inducing the acrosome reaction (AR). In A. amurensis, the AR is induced by the cooperative action of egg coat components (ARIS, Co-ARIS, and asterosap); however, the mechanism of action of Co-ARIS is obscure. In this study we elucidated the membrane dynamics involved in the action of Co-ARIS. We found that cholesterol specifically inhibited the Co-ARIS activity for AR induction and detected the binding of labeled compounds with sperm using radioisotope-labeled Co-ARIS. Co-ARIS treatment did not reduce the content of sperm sterols, however, the condition was changed and localization of GM1 ganglioside on the periacrosomal region disappeared. We then developed a caveola-breaking assay, a novel method to detect the effect of chemicals on microdomains of culture cell, and confirmed the disturbance of somatic cell caveolae in the presence of Co-ARIS. Finally, by atomic force microscopy observations and surface plasmon resonance measurements using an artificial membrane, we revealed that Co-ARIS colocalized with GM1 clusters on the microdomains. Through this study, we revealed a capacitation-like event for AR in starfish sperm.

Binding of sperm proacrosin/β-acrosin to zona pellucida glycoproteins is sulfate and stereodependent. Synthesis of a novel fertilization inhibitor

Specific binding of spermatozoa to the zona pellucida that surrounds mammalian eggs is a key step in the fertilization process. However, the sperm proteins that recognise zona pellucida receptors remain contentious despite longstanding research efforts to identify them. Here we present evidence that proacrosin, a tissue-specific protein found within the acrosomal vesicle of all mammalian spermatozoa, is a multifunctional protein that mediates binding of acrosome-reacted spermatozoa to zona glycoproteins via a stereospecific polysulfate recognition mechanism. Using sulfated versus non-sulfated forms of chemically defined compounds in binding assays employing native proteins in their normal cellular location or conjugated to FluoSpheres, we have attempted to identify the sulfation "code" required for recognition. Results show that protein conformation is important for specificity and that at least 2 sulfate groups are required to cross-link spatially separated docking sites on proacrosin. The consistently most effective inhibitory compounds were suramin and quercetin-3beta-d-glucoside sulfate. The results support our hypothesis that proacrosin is one of several proteins in the acrosomal matrix that retain acrosome reacted spermatozoa on the zona surface prior to penetration. They also establish, as a proof-of-principle, the feasibility of synthesising sulfated compounds of high specificity as antifertility agents for human or animal use.

Cyclic AMP-dependent PKA phosphorylates starfish sperm proteins during acrosome reaction

The induction of acrosome reaction (AR) happens when starfish spermatozoa encounter the egg jelly (EJ). This complex process involves different signal transduction pathways, such as elevation of cAMP and the activation of protein kinase A (PKA). The specific inhibitors of PKA (H89 and KT5720) have been shown to inhibit the EJ-induced Ca2+ elevation and AR. By using a Phospho-Ser/Thr PKA substrate antibody, we have detected an increased phosphorylation of 150, 200 and 220-kDa protein bands when starfish spermatozoa treated with EJ. The specific PKA inhibitors effectively inhibit phosphorylation of these proteins, suggesting an involvement of PKA on EJ-induced AR.

Gamete interaction: Is it species-specific?

Reproductive isolation is pivotal to maintain species separation and it can be achieved through a plethora of mechanisms. In addition, the development of barriers to gamete interaction may drive speciation. Such barriers to interspecific gamete interaction can be prezygotic or postzygotic. Considering the great diversity in animal species, it is easy to assume that regulation of the early steps of fertilization is critical to maintain species identity. One prezygotic mechanism that is often mentioned in the literature is that gamete interaction is limited to gametes of the same species. But do gametes of all animals interact in a species-specific way? Are gamete interactions completely species-specific or perhaps just species-restricted? In species in which species-restrictions have been described, is the interspecies barrier at one major step in the fertilization process or is it a combination of partially restricted steps that together lead to a block in interspecific fertilization? Are the mechanisms used to avoid interspecific crosses different between free-spawning organisms and those with internal fertilization? This review will address these questions, focusing on prezygotic barriers, and will describe what is known about the molecular biology that may account for species-limited gamete recognition and fertilization.

Characterization of the sperm receptor for acrosome reaction-inducing substance of the starfish, Asterias amurensis

Acrosome reaction-inducing substance (ARIS) in the jelly coat of starfish eggs is a highly sulfated proteoglycan-like molecule of an apparent molecular size over 10(4) kDa and plays a pivotal role in the induction of acrosome reaction in homologous spermatozoa. It is known in Asterias amurensis that ARIS binds to a restricted area of the anterior portion of sperm head, and that a glycan fragment of ARIS, named Fragment 1, consisting of 10 repeats or so of a pentasaccharide unit retains the biological activity of ARIS to an appreciable extent. In this report, we have shown the binding of Fragment 1, a relatively small pure glycan fragment of ARIS, to the putative ARIS receptor on the sperm surface by three independent methods. First, the specific binding of P-ARIS to isolated sperm membranes was monitored in real-time by using a surface plasmon resonance detector, namely a Biacore sensor system. The specific and quantitative binding of Fragment 1 to the intact sperm and to isolated sperm membranes was similarly monitored. Secondly, the binding of 125I-labeled Fragment 1 to the intact sperm was stoichiometrically measured, for which we had developed a unique procedure for radioiodination of saccharide chains. It is found that Fragment 1 competes with P-ARIS for the binding to ARIS-receptor, suggesting that Fragment 1 is a useful ligand in the search for ARIS receptor protein(s). Thirdly, the putative receptor molecules were specifically labeled by using Fragment 1 as a ligand for photoaffinity crosslink technique. Taking these results into account, we conclude that starfish sperm have the ARIS receptor, which consists most probably of 50 to 60 kDa proteins, of reasonably high affinity (for Fragment 1, Kd = 15 microM, Bmax = 8.4 x 10(4) per cell).

Ion channels in sperm physiology

Fertilization is a matter of life or death. In animals of sexual reproduction, the appropriate communication between mature and competent male and female gametes determines the generation of a new individual. Ion channels are key elements in the dialogue between sperm, its environment, and the egg. Components from the outer layer of the egg induce ion permeability changes in sperm that regulate sperm motility, chemotaxis, and the acrosome reaction. Sperm are tiny differentiated terminal cells unable to synthesize protein and difficult to study electrophysiologically. Thus understanding how sperm ion channels participate in fertilization requires combining planar bilayer techniques, in vivo measurements of membrane potential, intracellular Ca2+ and intracellular pH using fluorescent probes, patch-clamp recordings, and molecular cloning and heterologous expression. Spermatogenic cells are larger than sperm and synthesize the ion channels that will end up in mature sperm. Correlating the presence and cellular distribution of various ion channels with their functional status at different stages of spermatogenesis is contributing to understand their participation in differentiation and in sperm physiology. The multi-faceted approach being used to unravel sperm ion channel function and regulation is yielding valuable information about the finely orchestrated events that lead to sperm activation, induction of the acrosome reaction, and in the end to the miracle of life.

Structure and function of asterosaps, sperm-activating peptides from the jelly coat of starfish eggs

Jelly coat of starfish eggs has the capacity to activate homologous spermatozoa and induce the acrosome reaction. We have isolated 12 sperm-activating peptides (SAPs) from the egg jelly of the starfish, Asterias amurensis. Eleven SAPs were structurally identified by sequence analysis and electro-spray ionisation mass spectrometry. All of them are glutamine-rich tetratriacontapeptides with an intramolecular disulphide linkage between Cys8 and Cys32. They are much larger than sea urchin SAPs and do not show any significant sequence similarities to known proteins. Thus we have collectively named them asterosaps. The amino terminal region, where structural diversity of asterosaps is observed, is not important for their activity, whereas the disulphide linkage is essential. Asterosaps do not induce the acrosome reaction by themselves, but are able to induce the acrosome reaction in combination with an egg jelly glycoconjugate named ARIS. Furthermore, anti-asterosap rabbit antibody significantly decreased the acrosome reaction-inducing activity of the jelly solution and the activity was restored by addition of excess asterosap. These results support our hypothesis that the main physiological role of SAPs is the induction of the acrosome reaction in cooperation with two other jelly components, ARIS and Co-ARIS.

Estimation by radiation inactivation of the minimum functional size of acrosome-reaction-inducing substance (ARIS) in the starfish, Asterias amurensis

In the starfish Asterias amurensis, the jelly coat of the eggs contains a glycoprotein essential for the induction of the acrosome reaction in homologous spermatozoa that is termed the acrosome-reaction-inducing substance (ARIS). ARIS is a highly sulphated and fucose-rich glycoprotein of extremely high molecular mass (> 10(4) kDa). ARIS was irradiated with high-energy electrons in order to estimate the minimum size required for its biological activity. The minimum functional unit or target size of ARIS was estimated to be c. 14 kDa by target size analysis. ARIS was significantly disintegrated by the irradiation, yet the total sugar content was not apparently reduced. The binding of 125I-labelled ARIS to spermatozoa competed with that of irradiated ARIS, although the affinity of ARIS was much reduced after irradiation.

Ultrastructural localization of acrosome reaction-inducing substance (ARIS) on sperm of the starfish Asterias amurensis

Using colloidal gold tagged ligands we have identified the ultrastructural site of ARIS binding to intact and acrosome-reacted starfish sperm. In intact sperm, colloidal gold conjugated ARIS was specifically localized to a single domain (0.1-0.3 micron in diameter) on the plasma membrane. This site was located on the anterior-lateral aspect of the sperm head, that is, just peripheral to the region occupied by the acrosomal vesicle and periacrosomal components. When sperm were labeled with colloidal gold conjugated ARIS, washed to remove unbound label, and then induced to undergo the acrosome reaction, the labeled patch remained associated with the plasma membrane and was positioned just lateral to the acrosomal process. However, when sperm were suspended in labeled ARIS and induced to undergo the acrosome reaction, label was observed along the entire anterior aspect of the sperm head with the exception of the acrosomal process. Labeling along the entire anterior aspect of the sperm head in this case was deemed to be nonspecific and due to binding of colloidal gold tagged molecules to components formerly located within the acrosomal vesicle, as the same pattern was obtained using colloidal gold tagged bovine serum albumin. Quantitative and qualitative aspects of ARIS binding observed here by electron microscopy are in agreement with measured binding characteristics previously reported (Ushiyama et al., 1993a: Zygote 1:121-127; Ushiyama et al., 1993b: J Reprod Dev 39:53-54), and indicate that the site of labeled ARIS binding represents a specific plasma membrane domain occupied by ARIS receptors.