Egg sialoglycans increase intracellular pH and potentiate the acrosome reaction of sea urchin sperm

Sialylation: fate decision of mammalian sperm development, fertilization, and male fertility†

Sperm development, maturation, and successful fertilization within the female reproductive tract are intricate and orderly processes that involve protein translation and post-translational modifications. Among these modifications, sialylation plays a crucial role. Any disruptions occurring throughout the sperm's life cycle can result in male infertility, yet our current understanding of this process remains limited. Conventional semen analysis often fails to diagnose some infertility cases associated with sperm sialylation, emphasizing the need to comprehend and investigate the characteristics of sperm sialylation. This review reanalyzes the significance of sialylation in sperm development and fertilization and evaluates the impact of sialylation damage on male fertility under pathological conditions. Sialylation serves a vital role in the life journey of sperm, providing a negatively charged glycocalyx and enriching the molecular structure of the sperm surface, which is beneficial to sperm reversible recognition and immune interaction. These characteristics are particularly crucial during sperm maturation and fertilization within the female reproductive tract. Moreover, enhancing the understanding of the mechanism underlying sperm sialylation can promote the development of relevant clinical indicators for infertility detection and treatment.

Species-specific mechanisms during fertilization

The perpetuation and preservation of distinct species rely on mechanisms that ensure that only interactions between gametes of the same species can give rise to viable and fertile offspring. Species-specificity can act at various stages, ranging from physical/behavioral pre-copulatory mechanisms, to pre-zygotic incompatibility during fertilization, to post-zygotic hybrid incompatibility. Herein, we focus on our current knowledge of the molecular mechanisms responsible for species-specificity during fertilization. While still poorly understood, decades of research have led to the discovery of molecules implicated in species-specific gamete interactions, starting from initial sperm-egg attraction to the binding of sperm and egg. While many of these molecules have been described as species-specific in their mode of action, relatively few have been demonstrated as such with definitive evidence. Thus, we also raise remaining questions that need to be addressed in order to characterize gamete interaction molecules as species-specific.

Capacitation in Plant and Animal Fertilization

Sexual reproduction relies on the successful fusion of the sperm and egg cell. Despite the vast differences between plants and animals, there are similarities at a molecular level between plant and animal reproduction. While the molecular basis of fertilization has been extensively studied in plants, the process of capacitation has received little attention until recently. Recent research has started to uncover the molecular basis of plant capacitation. Furthermore, recent studies suggest that the key molecules in plants and animal fertilization are functionally conserved. Here, we review new insights for our understanding of capacitation of pollen tube and fertilization in plants and also propose that there are commonalities in the process of sexual reproduction between plants and animals.

A novel thyroglobulin-binding lectin from the brown alga Hizikia fusiformis and its antioxidant activities

A lectin (HFL) was isolated from the brown alga, Hizikia fusiformis, through ion exchange on cellulose DE52 and HPLC with a TSK-gel G4000PWXL column. SDS-PAGE showed that HFL had a molecular mass of 16.1 kDa. The HPLC (with a TSK-gel G4000PWXL column) indicated that HFL is a tetramer in its native state. The total carbohydrate content was 41%. Glucose, galactose and fucose were the monosaccharide units of HFL, and the normalized mol% values were 6, 14 and 80, respectively. HFL contains a large amount of the acidic amino acid, Asx. The β-elimination reaction suggested that the oligosaccharide and peptide moieties of HFL may belong to the N-glucosidic linkage. The amino acid sequences, of about five segments of HFL, were acquired by MALDI-TOF/TOF, and the sequences have no homology with other lectins. HFL was found to agglutinate sheep erythrocytes. The hemagglutination activity was inhibited by thyroglobulin, from bovine thyroid, but not by any of the monosaccharides tested. The lectin reaction was independent of the presence of the divalent cation Ca(2+). HFL showed free radical scavenging activity against hydroxyl, DPPH and ABTS(+) radicals.

Polysialic acid: biosynthesis, novel functions and applications

As an anti-adhesive, a reservoir for key biological molecules, and a modulator of signaling, polysialic acid (polySia) is critical for nervous system development and maintenance, promotes cancer metastasis, tissue regeneration and repair, and is implicated in psychiatric diseases. In this review, we focus on the biosynthesis and functions of mammalian polySia, and the use of polySia in therapeutic applications. PolySia modifies a small subset of mammalian glycoproteins, with the neural cell adhesion molecule, NCAM, serving as its major carrier. Studies show that mammalian polysialyltransferases employ a unique recognition mechanism to limit the addition of polySia to a select group of proteins. PolySia has long been considered an anti-adhesive molecule, and its impact on cell adhesion and signaling attributed directly to this property. However, recent studies have shown that polySia specifically binds neurotrophins, growth factors, and neurotransmitters and that this binding depends on chain length. This work highlights the importance of considering polySia quality and quantity, and not simply its presence or absence, as its various roles are explored. The capsular polySia of neuroinvasive bacteria allows these organisms to evade the host immune response. While this "stealth" characteristic has made meningitis vaccine development difficult, it has also made polySia a worthy replacement for polyetheylene glycol in the generation of therapeutic proteins with low immunogenicity and improved circulating half-lives. Bacterial polysialyltransferases are more promiscuous than the protein-specific mammalian enzymes, and new studies suggest that these enzymes have tremendous therapeutic potential, especially for strategies aimed at neural regeneration and tissue repair.

Sea urchin spermatozoa generate at least two reactive oxygen species; the type of reactive oxygen species changes under different conditions

Reactive oxygen species (ROS) cause oxidative stress and act as signal transduction molecules in many cells. Spermatozoa from several mammals generate ROS, which are involved in male infertility and signaling during capacitation. In the present study, we investigated ROS generation by sea urchin spermatozoa at the initiation of motility, during dilution with seawater, and following egg jelly treatment. In seawater containing an ROS indicator, 5-(and 6-)chloromethyl-2',7'-dichlorodihydrofluorescein diacetate (CM-H(2)DCFDA), fluorescence increased after the addition of spermatozoa. The ROS generation rate was dependent upon the dilution ratio and respiratory rate of the spermatozoa. Spermatozoa in sodium-free seawater did not increase fluorescence, but fluorescence did increase with the addition of NaCl. Sodium chloride also led to the initiation of sperm motility and respiration. Using the indicator MitoSOX Red, ROS generation was detected from spermatozoa exposed to egg jelly dissolved in seawater, but not in normal seawater. Moreover, the respiratory inhibitor antimycin A prevented CM-H(2)DCFDA-detectable ROS and increased MitoSox-detectable ROS at a higher concentration. These findings revealed that the ROS generated were of different species, possibly hydrogen peroxide (H(2)O(2)) and superoxide anion (O(-)(2)), and their detected levels were altered by egg jelly. We concluded that sea urchin spermatozoa generate at least two species of ROS depending on the physiological conditions to which they are exposed. It is possible that the major ROS from sea urchin spermatozoa changes during the course of fertilization.

Calcium Channels in the Development, Maturation, and Function of Spermatozoa

A proper dialogue between spermatozoa and the egg is essential for conception of a new individual in sexually reproducing animals. Ca2+ is crucial in orchestrating this unique event leading to a new life. No wonder that nature has devised different Ca2+-permeable channels and located them at distinct sites in spermatozoa so that they can help fertilize the egg. New tools to study sperm ionic currents, and image intracellular Ca2+ with better spatial and temporal resolution even in swimming spermatozoa, are revealing how sperm ion channels participate in fertilization. This review critically examines the involvement of Ca2+ channels in multiple signaling processes needed for spermatozoa to mature, travel towards the egg, and fertilize it. Remarkably, these tiny specialized cells can express exclusive channels like CatSper for Ca2+ and SLO3 for K+, which are attractive targets for contraception and for the discovery of novel signaling complexes. Learning more about fertilization is a matter of capital importance; societies face growing pressure to counteract rising male infertility rates, provide safe male gamete-based contraceptives, and preserve biodiversity through improved captive breeding and assisted conception initiatives.

Laboratory on sea urchin fertilization

Since about 1880, the eggs and sperm of sea urchins have been used for the study of fertilization, the metabolic activation of development and gene regulatory mechanisms governing embryogenesis. Sea urchin gametes are a favorite material for observations of the process of fertilization in advanced high school, community college, and university biology laboratory courses. This article is a laboratory handout, designed for the student to follow in learning about fertilization. In addition to observations of sperm-egg interaction, simple experiments are described that demonstrate some mechanisms involved in the process. The hope is that by making simple observations of fertilization, the student will gain an appreciation for the fact that successive generations of higher organisms are bridged by the fusion of egg and sperm, two very different single cells.

Seminal fluid from sea urchin (Lytechinus variegatus) contains complex sulfated polysaccharides linked to protein

The eggs of sea urchins are covered by a jelly coat, which contains high concentrations of sulfated polysaccharides. These carbohydrates show species-specificity in inducing the sperm acrosome reaction. Several studies about the egg jelly of sea urchins have been published, but there is no information about the composition of the seminal fluid of these echinoderms. Here we report for the first time the occurrence of complex sulfated polysaccharides in the seminal fluid of the sea urchin Lytechinus variegatus. These polysaccharides occur as three fractions that differ mostly in their carbohydrate/protein ratios. The native molecular masses of the polymers are very high (> or = 200 kDa) but, after digestion with papain the size decreases to approximately 8 kDa. All fractions have a similar carbohydrate composition, containing mostly galactose, glucosamine and mannose. The heterogeneous sulfated polysaccharides differ from vertebrate glycosaminoglycans and also from all previously described polysaccharides from invertebrates. The physiological role of the sulfated carbohydrates from seminal fluid is not yet determined. However, by analogy with the effects proposed for some glycoproteins found in vertebrate seminal fluid, it may be possible that the sulfated polysaccharides from invertebrate are also involved in fertilization process.

A unique 2-sulfated {beta}-galactan from the egg jelly of the sea urchin Glyptocidaris crenularis: conformation flexibility versus induction of the sperm acrosome reaction

Sulfated polysaccharides from the egg jelly of sea urchins act as species-specific inducers of the sperm acrosome reaction, which is a rare molecular mechanism of carbohydrate-induced signal-transduction event in animal cells. The sea urchin polysaccharides differ in monosaccharide composition (l-fucose or l-galactose), glycosylation, and sulfation sites, but they are always in the alpha-anomeric configuration. Herein, structural analysis of the polysaccharide from the sea urchin Glyptocidaris crenularis surprisingly revealed a unique sulfated beta-d-galactan composed by (3-beta-d-Galp-2(OSO(3))-1-->3-beta-d-Galp-1)(n) repeating units. Subsequently, we used the G. crenularis galactan to compare different 2-sulfated polysaccharides as inducers of the acrosome reaction using homologous and heterologous sperm. We also tested the effect of chemically over-sulfated galactans. Intriguingly, the anomeric configuration of the glycosidic linkage rather than the monosaccharide composition (galactose or fucose) is the preferential structural requirement for the effect of these polysaccharides on sea urchin fertilization. Nuclear magnetic resonance and molecular dynamics indicate that sulfated alpha-galactan or alpha-fucan have less dynamic structural behavior, exhibiting fewer conformational populations, with an almost exclusive conformational state with glycosidic dihedral angles Phi/Psi = -102 degrees /131 degrees . The preponderant conformer observed in the sulfated alpha-galactan or alpha-fucan is not observed among populations in the beta-form despite its more flexible structure in solution. Possibly, a proper spatial arrangement is required for interaction of the sea urchin-sulfated polysaccharides with the specific sperm receptor.

Structure, biology, evolution, and medical importance of sulfated fucans and galactans

Sulfated fucans and galactans are strongly anionic polysaccharides found in marine organisms. Their structures vary among species, but their major features are conserved among phyla. Sulfated fucans are found in marine brown algae and echinoderms, whereas sulfated galactans occur in red and green algae, marine angiosperms, tunicates (ascidians), and sea urchins. Polysaccharides with 3-linked, beta-galactose units are highly conserved in some taxonomic groups of marine organisms and show a strong tendency toward 4-sulfation in algae and marine angiosperms, and 2-sulfation in invertebrates. Marine algae mainly express sulfated polysaccharides with complex, heterogeneous structures, whereas marine invertebrates synthesize sulfated fucans and sulfated galactans with regular repetitive structures. These polysaccharides are structural components of the extracellular matrix. Sulfated fucans and galactans are involved in sea urchin fertilization acting as species-specific inducers of the sperm acrosome reaction. Because of this function the structural evolution of sulfated fucans could be a component in the speciation process. The algal and invertebrate polysaccharides are also potent anticoagulant agents of mammalian blood and represent a potential source of compounds for antithrombotic therapies.

Involvement of sperm proteases in the binding of sperm to the vitelline envelope in Xenopus laevis

Sperm binding to the vitelline envelope in dejellied Xenopus laevis eggs was effectively inhibited by inhibitors for trypsin (soybean trypsin inhibitor and p-toluenesulfonyl-L-lysine chloroethyl ketone) and aminopeptidase B (o-phenanthroline, bestatin, and arphamenine B). Likewise, synthetic 4-methylcoumaryl-7-amide (MCA) substrates (t-butoxycarbonyl-GlyArgArg-MCA, benzyloxycarbonyl-ArgArg-MCA, and Arg-MCA) inhibited binding. Consistently, when jellied eggs were inseminated in the presence of these substrates or inhibitors for proteases, fertilization was effectively blocked. The medium in which live sperm or the sperm membrane fraction were suspended exhibited hydrolyzing activities against the synthetic substrates mentioned above, and these activities were effectively inhibited by the protease inhibitors. Ultracentrifugal fractionation of the sperm suspension following induction of the acrosome reaction by a calcium ionophore, A23187, indicated that a considerable amount of the total tryptic and aminopeptidase B activity was released into the medium. On this occasion, part of the tryptic and aminopeptidase B activity was definitely estimated to be discharged in association with a vesiculated membrane, supporting the notion that the proteases involved in binding to the vitelline envelope are present on the sperm plasma membrane.

Sialylation of cell surface glycoconjugates is essential for osteoclastogenesis

Sialic acid, which is located at the end of the carbohydrate moiety of cell surface glycoconjugates, is involved in many biologic responses, such as intercellular reactions and virus-cell fusion, especially in hematopoietic cells. Here we provide experimental evidence that the sialic acid of cell surface glycoconjugates has a role in osteoclast differentiation. Lectin histochemical study demonstrated the existence of both alpha (2,3)-linked-sialic acid and alpha (2,6)-linked-sialic acid in mouse bone marrow-derived macrophages and in the RAW264.7 macrophage cell line, which are osteoclast precursors. Flow cytometric analysis of surface lectin staining revealed the kinetics of these sialic acids during osteoclastogenesis: alpha (2,3)-linked-sialic acid was abundantly expressed throughout osteoclastogenesis, whereas alpha (2,6)-linked-sialic acid levels declined at the terminal stage of osteoclast differentiation. To investigate the role of sialic acid in osteoclast differentiation, we performed an osteoclastogenesis assay with or without exogenous sialidase treatment. Desialylated cells formed TRAP-positive mononuclear cells, but did not become multinuclear cells despite the normal expression of osteoclast markers such as cathepsin K, integrin beta3, and nuclear factor-ATc1. Flow cytometric analysis also demonstrated that exogenous sialidase effectively removed alpha (2,6)-linked-sialic acid, but only slightly changed the alpha (2,3)-linked-sialic acid content, suggesting that alpha (2,6)-linked-sialic acid might be involved in osteoclast differentiation. Findings from knockdown analysis using small interfering RNA oligonucleotides against alpha 2,6-sialyltransferase support this idea: alpha (2,6)-linked-sialic acid-deficient cells markedly inhibit the formation of multinuclear osteoclasts. Our findings suggest that alpha (2,6)-linked-sialic acid of cell surface glycoconjugates has a role in osteoclast differentiation, possibly via its role in the cell-cell fusion process.

PKA activation in concert with ARIS and asterosap induces the acrosome reaction in starfish

The acrosome reaction (AR) is a fundamental event for fertilization, which is induced in concert with acrosome reaction-inducing substance (ARIS) and asterosap, both of which are components of starfish egg jelly (EJ). During the AR, a spermatozoon undergoes a series of physiological changes, such as in intracellular cGMP concentration ([cGMP]i), pHi and intracellular Ca2+ concentration ([Ca2+]i). Affinity purification of cGMP-binding protein resulted in the isolation of a regulatory subunit of the cAMP-dependent protein kinase A (PKA), suggesting the involvement of a cAMP-dependent pathway in the AR. By using a cAMP enzyme immunoassay, [cAMP]i was found to increase in starfish spermatozoa when stimulated with ARIS and asterosap. ARIS could also increase the [cAMP]i in the presence of high pH seawater. Pretreatment of spermatozoa with two specific and cell-permeable PKA inhibitors, H89 and KT5720, prevented the induction of the AR in a concentration-dependent manner. These results suggest that PKA activity participates in the induction of the AR with ARIS and asterosap. To investigate this, we have cloned a gene that encodes a regulatory subunit of PKA that had been identified in starfish spermatozoa.

Sperm channel diversity and functional multiplicity

Ion channels are extraordinarily efficient machines that move ions in diversely controlled manners, allowing cells to rapidly exchange information with the outside world and with other cells. Communication is the currency of fertilization, as it is of most fundamental cell signaling events. Ion channels are deeply involved in the dialogue between sperm, its surroundings, and the egg. How sperm swim, find the egg and fertilize it depend on ion permeability changes modulated by environmental cues and components of the egg outer layer. Different ion channels distinctly localized in these tiny, amazing cells perform specific decoding functions that shape the sophisticated behavior of sperm. It is not surprising that certain sperm ion channels are turning out to be unique. New strategies to characterize sperm ion transport have opened exciting possibilities to dissect sperm-egg signaling and unveil novel contraception targets.

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.

Cyclic GMP-specific phosphodiesterase-5 regulates motility of sea urchin spermatozoa

Motility, chemotaxis, and the acrosome reaction of animal sperm are all regulated by cyclic nucleotides and protein phosphorylation. One of the cyclic AMP-dependent protein kinase (PKA) substrates in sea urchin sperm is a member of the phosphodiesterase (PDE) family. The molecular identity and in vivo function of this PDE remained unknown. Here we cloned and characterized this sea urchin sperm PDE (suPDE5), which is an ortholog of human PDE5. The recombinant catalytic domain of suPDE5 hydrolyzes only cyclic GMP (cGMP) and the activity is pH-dependent. Phospho-suPDE5 localizes mainly to sperm flagella and the phosphorylation increases when sperm contact the jelly layer surrounding eggs. In vitro dephosphorylation of suPDE5 decreases its activity by approximately 50%. PDE5 inhibitors such as Viagra block the activity of suPDE5 and increase sperm motility. This is the first PDE5 protein to be discovered in animal sperm. The data are consistent with the hypothesis that suPDE5 regulates cGMP levels in sperm, which in turn modulate sperm motility.

Tandem mass spectrometry identifies proteins phosphorylated by cyclic AMP-dependent protein kinase when sea urchin sperm undergo the acrosome reaction

The exocytotic acrosome reaction (AR), which is required for fertilization, occurs when sea urchin sperm contact the egg jelly (EJ) layer. Among other physiological changes, increases in adenylyl cyclase activity, cAMP and cAMP-dependent protein kinase (PKA) activity occur coincident with the AR. By using inhibitors of PKA, a permeable analog of cAMP and the phosphodiesterase inhibitor IBMX, we show that PKA activity is required for AR induction by EJ. A minimum of six sperm proteins are phosphorylated by PKA upon exposure to EJ, as detected by a PKA substrate-specific antibody. The phosphorylation of these proteins and the percentage of acrosome reacted sperm can be regulated by PKA modulators. The fucose sulfate polymer (FSP), a major component of EJ, is the molecule that triggers sperm PKA activation. Extracellular Ca(2+) is required for PKA activation. Six sperm proteins phosphorylated by PKA were identified by tandem mass spectrometry (MS/MS) utilizing the emerging sea urchin genome. Based on their identities and localizations in sperm head and flagellum, the putative functions of these proteins in sperm physiology and AR induction are discussed.

Calcium Channels and Ca2+ Fluctuations in Sperm Physiology

Generating new life in animals by sexual reproduction depends on adequate communication between mature and competent male and female gametes. Ion channels are instrumental in the dialogue between sperm, its environment, and the egg. The ability of sperm to swim to the egg and fertilize it is modulated by ion permeability changes induced by environmental cues and components of the egg outer layer. Ca(2+) is probably the key messenger in this information exchange. It is therefore not surprising that different Ca(2+)-permeable channels are distinctly localized in these tiny specialized cells. New approaches to measure sperm currents, intracellular Ca(2+), membrane potential, and intracellular pH with fluorescent probes, patch-clamp recordings, sequence information, and heterologous expression are revealing how sperm channels participate in fertilization. Certain sperm ion channels are turning out to be unique, making them attractive targets for contraception and for the discovery of novel signaling complexes.

Defending the zygote: search for the ancestral animal block to polyspermy

Fertilization is the union of a single sperm and an egg, an event that results in a diploid embryo. Animals use many mechanisms to achieve this ratio; the most prevalent involves physically blocking the fusion of subsequent sperm. Selective pressures to maintain monospermy have resulted in an elaboration of diverse egg and sperm structures. The processes employed for monospermy are as diverse as the animals that result from this process. Yet, the fundamental molecular requirements for successful monospermic fertilization are similar, implying that animals may have a common ancestral block to polyspermy. Here, we explore this hypothesis, reviewing biochemical, molecular, and genetic discoveries that lend support to a common ancestral mechanism. We also consider the evolution of alternative or radical techniques, including physiological polyspermy, with respect to our ability to describe a parsimonious guide to fertilization.

Positive Selection in the Carbohydrate Recognition Domains of Sea Urchin Sperm Receptor for Egg Jelly (suREJ) Proteins

A wealth of evidence shows that protein-carbohydrate recognition mediates the steps of gamete interaction during fertilization. Carbohydrate-recognition domains (CRDs) comprise a large family of ancient protein modules of approximately 120 amino acids, having the same protein fold, that bind terminal sugar residues on glycoproteins and polysaccharides. Sea urchin sperm express three suREJ (sea urchin receptor for egg jelly) proteins on their plasma membranes. suREJ1 has two CRDs, whereas suREJ2 and suREJ3 both have one CRD. suREJ1 binds the fucose sulfate polymer (FSP) of egg jelly to induce the sperm acrosome reaction. The structure of FSP is species specific. Therefore, the suREJ1 CRDs could encode molecular recognition between sperm and egg underlying the species-specific induction of the acrosome reaction. The functions of suREJ2 and suREJ3 have not been explored, but suREJ3 is exclusively localized on the plasma membrane over the sperm acrosomal vesicle and is physically associated with sea urchin polycystin-2, a known cation channel. An evolutionary analysis of these four CRDs was performed for six sea urchin species. Phylogenetic analysis shows that these CRDs were already differentiated in the common ancestor of these six sea urchins. The CRD phylogeny agrees with previous work on these species based on one nuclear gene and several mitochondrial genes. Maximum likelihood shows that positive selection acts on these four CRDs. Threading the suREJ CRDs onto the prototypic CRD crystal structure shows that many of the sites under positive selection are on extended loops, which are involved in saccharide binding. This is the first demonstration of positive selection in CRDs and is another example of positive selection acting on the evolution of gamete-recognition proteins.

Ligands and receptors mediating signal transduction in sea urchin spermatozoa

Sea urchins have long been a model system for the study of fertilization. Much has been learned about how sea urchin sperm locate and fertilize the egg. Sperm and eggs are spawned simultaneously into the surrounding seawater. Sperm signaling pathways lead to downstream events that ensure fertilization. Upon spawning, sperm must acquire motility and then they must swim towards or respond to the egg in some way. Finally, they must undergo a terminal exocytotic event known as the acrosome reaction that allows the sperm to bind to the vitelline layer of the egg and then to fuse with the egg plasma membrane. Motility is stimulated by exposure to seawater, while later events are orchestrated by factors from the egg. The sperm signaling pathways are exquisitely tuned to bring the sperm to the egg, bind, and fuse the two cells as quickly as possible.

A third sea urchin sperm receptor for egg jelly module protein, suREJ2, concentrates in the plasma membrane over the sperm mitochondrion

Sea urchin spermatozoa are model cells for studying signal transduction events underlying flagellar motility and the acrosome reaction. We previously described the sea urchin sperm receptor for egg jelly 1 (suREJ1) which consists of 1450 amino acids, has one transmembrane segment and binds to the fucose sulfate polymer of egg jelly to induce the sperm acrosome reaction. We also cloned suREJ3 which consists of 2681 amino acids and has 11 putative transmembrane segments. Both these proteins localize to the plasma membrane over the acrosomal vesicle. While cloning suREJ1, we found suREJ2, which consists of 1472 amino acids, has two transmembrane segments and is present in the entire sperm plasma membrane, but is concentrated over the sperm mitochondrion. Experimental evidence suggests that, unlike suREJ1 and suREJ3, suREJ2 does not project extracellularly from the plasma membrane, but is an intracellular plasma membrane protein. All three sea urchin sperm REJ proteins possess a protein module of > 900 amino acids, termed 'the REJ module', that is shared by the human autosomal dominant polycystic kidney disease protein, polycystin-1, and PKDREJ, a testis-specific protein in mammals whose function is unknown. In the present study, we describe the sequence, domain structure and localization of suREJ2 and speculate on its possible function.

Store-operated calcium channels trigger exocytosis of the sea urchin sperm acrosomal vesicle

The acrosome reaction (AR) of sperm is a prerequisite for fusion with the egg. In sea urchins, the complete AR (CAR) consists of exocytosis of the acrosomal vesicle (AV) and polymerization of acrosomal actin to form the approximately 1 micro m long acrosomal process. The fucose sulfate polymer (FSP) of egg jelly stimulates Ca(2+) entry through two distinct Ca(2+) channels and induces the CAR. Here we report that the second channel is blocked by SKF96365 (SKF), an inhibitor of store-operated channels. SKF also blocks the thapsigargin (TG), trifluoperazine (TFP), and calmidizolium (CMZ) stimulated Ca(2+) entry into sperm. These data indicate that the second Ca(2+) channel is a store-operated channel (SOC) that may be regulated by calmodulin. The TG, TFP, and CMZ-induced intracellular Ca(2+) elevations are similar to those induced by FSP, but the sperm acrosomal process does not polymerize. An antibody to bindin, the major protein of the AV, showed that in a significant percentage of these drug-treated sperm, the AV had undergone exocytosis. When NH(4)Cl was added to increase intracellular pH, the TG-treated sperm polymerized actin to form the acrosomal process. We conclude that the second Ca(2+) channel of sea urchin sperm is a SOC that triggers AV exocytosis.

Structural requirements for species-specific induction of the sperm acrosome reaction by sea urchin egg sulfated fucan

The sulfated fucan (SF) of egg jelly induces the acrosome reaction (AR) of sea urchin sperm. Strongylocentrotus franciscanus (Sf) SF is sulfated only at the 2-position. Strongylocentrotus purpuratus (Sp) has two SF isotypes, each one being female specific. One is rich in sulfate at both the 2- and 4-positionS (SF-1), and the other is rich in sulfate at the 4-position, but not the 2-position (SF-2). Sf SF is poor at inducing the AR of Sp sperm, presumably due to lack of 4-sulfation. Sp SF-1 is better at inducing the AR of Sf sperm than Sp SF-2, hypothetically due to increased 2-sulfation. Chemical oversulfation of Sf SF increases the percentage of AR of Sp sperm, showing that 4-sulfation is important for recognition of SF by Sp sperm. Chemically oversulfated Sp SF-2 is better at inducing the Sf sperm AR, presumably because of increased 2-sulfation. The species, Strongylocentrotus drobachiensis (Sd), has an SF-2 that is exclusively 2-sulfated (like Sf), except the glycosidic linkage in Sd is alpha(1-->4), whereas in Sf it is alpha(1-->3). Sd SF-2 does not induce the AR of Sf sperm, showing the strict requirement for the alpha(1-->3) linkage in recognition between Sf sperm and SF. Egg jelly from Echinometra lucunter (El) contains sulfated galactan (SG) which differs from Sf SF only in that the monosaccharide is L-galactose, not L-fucose. This SG and Sf SF are equally potent in inducing the AR of Sf sperm, showing that modification at C6 of L-fucose is not important for proper recognition between SF and Sf sperm receptors. This system permits study of the structural basis for recognition between sulfated polysaccharide and receptors controlling signal transduction pathways in animal cells.

Egg fucose sulfate polymer, sialoglycan, and speract all trigger the sea urchin sperm acrosome reaction

Macromolecules surrounding eggs induce the acrosome reaction (AR) of spermatozoa. In sea urchins, three egg jelly (EJ) molecules: a fucose sulfate polymer (FSP), a sialoglycan (SG), and speract mediate ionic fluxes triggering the AR. SG and speract are noninductive without FSP. Speract's role in AR induction is controversial. Here we show that speract potentiates the FSP-induced AR at pH 7.0, approximately 1 pH unit lower than natural seawater. At pH 7.0, a mixture of FSP, SG, and speract produces the intracellular pH increase necessary for maximum AR induction. Each EJ component may mediate a distinct intracellular pH control mechanism, and all three may function synergistically to increase the intracellular pH permitting AR induction. Speract peptides are an ancient family. Although important for activating cyclic nucleotide-mediated pathways in today's seawater of pH approximately 8, speract may have been more important in AR induction in the paleo-ocean of pH approximately 7.

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).