Ligands and receptors mediating signal transduction in sea urchin spermatozoa

Pharmacological perturbation of peroxisome-proliferator-activated receptor gamma alters motility and mitochondrial function of bovine sperm

BACKGROUND

Sperm transit through the female reproductive relies upon maintenance of sperm motility. Peroxisome-proliferator-activated receptor gamma (PPARγ) is a ligand-activated nuclear transcription factor with roles in glucose metabolism and reproductive processes including placental function. PPARγ roles in the mammalian postejaculatory sperm function are incompletely defined.

OBJECTIVES

Determine expression, localization, and functions of PPARγ in postejaculatory bovine sperm.

MATERIALS AND METHODS

Frozen-thawed bovine sperm from three to four different bulls were pooled and subjected to immunofluorescence and western blot for detection and localization of PPARγ. Functions in sperm energetics were explored through the addition of pharmacological inhibition (GW; GW9662) and activation (Ros; Rosiglitazone) in the culture medium at 0 and 24 h under non-capacitating conditions. Samples were analyzed for sperm kinematics (CASA) and mitochondrial membrane potential (MMP; JC-1 fluorophore).

RESULTS

PPARγ was detected in bovine sperm and co-localized to the acrosome with re-localization to the equatorial region in acrosome-compromised sperm. The addition of Ros 50 µM for 24 h maintained superior total and progressive motility of sperm compared to vehicle control (VC-DMSO 0.01%). The PPARγ antagonist GW 1 µM was detrimental to both total and progressive motility. A challenge experiment (Ros + GW) partially rescued total and progressive motility phenotypes observed with GW incubation. GW-treated samples had a lower number of sperm with high MMP at 24 h compared to Ros or VC. The negative GW MMP phenotype was reversed with the addition of Ros + GW. Likewise, GW-treated samples had more sperm with low MMP compared to VC and Ros, and this phenotype was partially restored with Ros + GW.

CONCLUSION

PPARγ is expressed in post-ejaculatory bovine sperm with regulatory roles in sperm motility and MMP. These findings implicate PPARγ as a novel regulator of postejaculatory mammalian sperm energetics through non-canonical signaling mechanisms.

Morphology of male and female reproductive systems in the ground beetle Apotomus and the peculiar sperm ultrastructure of A. rufus (P. Rossi, 1790) (Coleoptera, Carabidae)

Relatively few studies have focused on evolutionary losses of sexually selected male traits. We use light and electron microscopy to study the male and female reproductive anatomy of Apotomus ground beetles (Coleoptera, Carabidae), a lineage that we reconstruct as likely having lost sperm conjugation, a putative sexually selected trait. We pay particular attention to the structure of the testes and spermatheca. Both of these organs share a strikingly similar shape-consisting of long blind canals arranged into several concentric overlapping rings measuring approximately 18 mm and 19.5 mm in total length, respectively. The similarity of these structures suggests a positive evolutionary correlation between female and male genital organs. Males are characterized by unifollicular testes with numerous germ cysts, which contain 64 sperm cells each, and we record a novel occurrence of sperm cyst "looping", a spermatogenic innovation previously only known from some fruit fly and Tenebrionid beetle sperm. The sperm are very long (about 2.7 mm) and include an extraordinarily long helicoidal acrosome, a short nucleus, and a long flagellum. These findings confirm the structural peculiarity of sperm, testis, and female reproductive tract (FRT) of Apotomus species relative to other ground beetles, which could possibly be the result of shifts in sexual selection.

Ocean acidification impacts sperm swimming performance and pHi in the New Zealand sea urchin Evechinus chloroticus

In sea urchins, spermatozoa are stored in the gonads in hypercapnic conditions (pH<7.0). During spawning, sperm are diluted in seawater of pH>8.0, and there is an alkalinization of the sperm's internal pH (pHi) through the release of CO2 and H+. Previous research has shown that when pHi is above 7.2-7.3, the dynein ATPase flagellar motors are activated, and the sperm become motile. It has been hypothesised that ocean acidification (OA), which decreases the pH of seawater, may have a narcotic effect on sea urchin sperm by impairing the ability to regulate pHi, resulting in decreased motility and swimming speed. Here we use data collected from the same individuals to test the relationship between pHi and sperm motility/performance in the New Zealand sea urchin Evechinus chloroticus (Valenciennes) under near- (2100) and far-future (2150) atmospheric pCO2 conditions (RCP 8.5: pH 7.77, 7.51). Decreasing seawater pH significantly negatively impacted the proportion of motile sperm), and four of the six computer-assisted sperm analysis (CASA) sperm performance measures. In control conditions, sperm had an activated pHi of 7.52. E. chloroticus sperm could not defend pHi. in future OA conditions; there was a stepped decrease in the pHi at pH 7.77, with no significant difference in mean pHi between pH 7.77 and 7.51. Paired measurements in the same males showed a positive relationship between pHi and sperm motility, but with a significant difference in the response between males. Differences in motility and sperm performance in OA conditions may impact fertilization success in a future ocean.

Cathepsin D in prawn reproductive system: its localization and function in actin degradation

Cathepsin D (CAT-D) is a well-known aspartic protease that serves a function as house-keeping lysosomal enzyme in all somatic cells. Its existence in reproductive tissues is highly variable, even in the somatic derived epithelial cells of reproductive tract. In Macrobrachium rosenbergii, existence of MrCAT-D and its translational product was detected in both somatic cells (Sertoli-like supporting cells) and developing spermatogenic cells as well as along accessory spermatic ducts. Specifically, MrCAT-D was localized onto the sperm surface rather than within the acrosomal matrix, as evident by similar staining pattern of anti-CAT-D on live and aldehyde fixed sperm. MrCAT-D in testicular extracts and sperm isolates showed active enzyme activities towards its specific fluorogenic substrate (MCA-Gly-Lys-Pro-Ile-Leu-Phe-Phe-Arg-Leu-Lys (Dnp)-D-Arg-NH₂). MrCAT-D also exerted its function towards hydrolyzing filamentous actin, the meshwork of which is shown to be localized at the junction between germ cells and supporting cells and spermatogonia in M. rosenbergii testicular epithelium. Together, we have localized MrCAT-D transcript and its translational product in both supporting and germ cells of testis and claimed its enzymatic function towards actin degradation, which may be related to sperm release from the epithelial cell interaction.

Slow evolution under purifying selection in the gamete recognition protein bindin of the sea urchin Diadema

Bindin is a sperm protein that mediates attachment and membrane fusion of gametes. The mode of bindin evolution varies across sea urchin genera studied to date. In three genera it evolves under positive selection, in four under mostly purifying selection, and in one, results have been mixed. We studied bindin evolution in the pantropical sea urchin Diadema, which split from other studied genera 250 million years ago. We found that Diadema bindin is structurally similar to that of other genera, but much longer (418 amino acids). In seven species of Diadema, bindin evolves under purifying selection, more slowly than in any other sea urchin genus. Only bindin of the recently rediscovered D. clarki shows evidence of positive selection. As D. clarki is sympatric with D. setosum and D. savignyi, positive selection could arise from avoidance of maladaptive hybridization. However, D. setosum and D. savignyi overlap in the Indo-West Pacific, yet their bindins show no evidence of positive selection, possibly because the two species spawn at different times. Bindin in the East Pacific D. mexicanum, the West Atlantic D. antillarum, the East Atlantic D. africanum, and the Indo-Pacific D. paucispinum also evolves slowly under purifying selection.

Nonspecific expression of fertilization genes in the crown-of-thorns Acanthaster cf. solaris: Unexpected evidence of hermaphroditism in a coral reef predator

The characterization of gene expression in gametes has advanced our understanding of the molecular basis for ecological variation in reproductive success and the evolution of reproductive isolation. These advances are especially significant for ecologically important keystone predators such as the coral-eating crown-of-thorns sea stars (COTS, Acanthaster) which are the most influential predator species in Indo-Pacific coral reef ecosystems and the focus of intensive management efforts. We used RNA-seq and transcriptome assemblies to characterize the expression of genes in mature COTS gonads. We described the sequence and domain organization of eight genes with sex-specific expression and well known functions in fertilization in other echinoderms. We found unexpected expression of genes in one ovary transcriptome that are characteristic of males and sperm, including genes that encode the sperm-specific guanylate cyclase receptor for an egg pheromone, and the sperm acrosomal protein bindin. In a reassembly of previously published RNA-seq data from COTS testes, we found a complementary pattern: strong expression of four genes that are otherwise well known to encode egg-specific fertilization proteins, including the egg receptor for bindin (EBR1) and the acrosome reaction-inducing substance in the egg coat (ARIS1, ARIS2, ARIS3). We also found histological evidence of both eggs and sperm developing in the same gonad in several COTS individuals from a parallel study. These results suggest the occurrence of hermaphrodites, and the potential for reproductive assurance via self-fertilization. Our findings have implications for management of COTS populations, especially in consideration of the large size and massive fecundity of these sea stars.

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.

Role of transient receptor potential channels in regulating spermatozoa functions: A mini-review

Flagellar navigation along the genital tract of male and female in spermatozoa is accomplished through a number of biological, physiological, biochemical, and electrophysiological alterations in spermatozoa. These alterations are highly precise, dynamic, and regulated through a number of ion channels along with their associated pathways. Beating of flagella along with intracellular metabolism of spermatozoa is associated with fluxing of Ca++ as well as release of Ca++ from different sources. Calcium fluxing through the spermatozoa is mediated through sperm-specific calcium channel and also through transient receptor potential (TRP) channels which are diversified multifamily of ion channels which are activated through a number of extracellular agents such as pH, temperature, chemicals, and pathogens. Research has shown the dynamic role of TRP channels in regulating sperm functions such as sperm chemotaxis, rheotaxis, thermotaxis, and eventually fertilization. Diversified forms of TRP and their involvement in regulation of sperm function opens new horizons of understanding of the sperm function and, in specific, issues related to infertility. This mini-review is an attempt to draw some insights into the action of TRP channels in regulating sperm fertility competence through both calcium-dependent and calcium-independent mechanisms.

Egg Coat Proteins Across Metazoan Evolution

All animal oocytes are surrounded by a glycoproteinaceous egg coat, a specialized extracellular matrix that serves both structural and species-specific roles during fertilization. Egg coat glycoproteins polymerize into the extracellular matrix of the egg coat using a conserved protein-protein interaction module-the zona pellucida (ZP) domain-common to both vertebrates and invertebrates, suggesting that the basic structural features of egg coats have been conserved across hundreds of millions of years of evolution. Egg coat proteins, as with other proteins involved in reproduction, are frequently found to be rapidly evolving. Given that gamete compatibility must be maintained for the fitness of sexually reproducing organisms, this finding is somewhat paradoxical and suggests a role for adaptive diversification in reproductive protein evolution. Here we review the structure and function of metazoan egg coat proteins, with an emphasis on the potential role their evolution has played in the creation and maintenance of species boundaries.

The Acrosome Reaction: A Historical Perspective

Acrosome reaction is often referred to as acrosomal exocytosis, but it differs significantly from normal exocytosis. While the vesicle membrane initially holding excreting molecules remains on the cell surface during exocytosis, the outer acrosomal membrane and plasma membrane are lost by forming vesicles during acrosome reaction. In this context, the latter process resembles a release of exosome. However, recent experimental data indicate that the most important roles of acrosome reaction lie not in the release of acrosomal contents (or "vesiculated" plasma and outer acrosomal membrane complexes) but rather in changes in sperm membrane. This review describes the mechanism of fertilization vis-a-vis sperm membrane change, with a brief historical overview of the half-century study of acrosome reaction.

Evolution of gamete attraction molecules: evidence for purifying selection in speract and its receptor, in the pantropical sea urchin Diadema

Many free-spawning marine invertebrates, such as sea urchins, lack any courtship or assortative mating behavior. Mate recognition in such cases occur at the gametic level, and molecules present on the sperm and egg are major determinants of species-specific fertilization. These molecules must also coevolve in relation to each other in order to preserve functional integrity. When sea urchins release their gametes in seawater, diffusible molecules from the egg, termed sperm-activating peptides, activate and attract the sperm to swim toward the egg, initiating a series of interactions between the gametes. Although the compositions and diversity of such sperm-activating peptides have been characterized in a variety of sea urchins, little is known about the evolution of their genes. Here we characterize the genes encoding the sperm-activating peptide of the egg (speract) and its receptor on the sperm, and examine their evolutionary dynamics in the sea urchin genus Diadema, in the interest of determining whether they are involved in reproductive isolation between the species. We found evidence of purifying selection on several codon sites in both molecules and of selectively neutral evolution in others. The diffusible speract peptide that activates sperm is invariant across species, indicating that Diadema egg peptides do not discriminate between con- and hetero-specific sperm at this stage of the process. Speract and its receptor do not contribute to reproductive isolation in Diadema.

The biological networks in studying cell signal transduction complexity: The examples of sperm capacitation and of endocannabinoid system

Cellular signal transduction is a complex phenomenon, which plays a central role in cell surviving and adaptation. The great amount of molecular data to date present in literature, together with the adoption of high throughput technologies, on the one hand, made available to scientists an enormous quantity of information, on the other hand, failed to provide a parallel increase in the understanding of biological events. In this context, a new discipline arose, the systems biology, aimed to manage the information with a computational modeling-based approach. In particular, the use of biological networks has allowed the making of huge progress in this field.

Here we discuss two possible application of the use of biological networks to explore cell signaling: the study of the architecture of signaling systems that cooperate in determining the acquisition of a complex cellular function (as it is the case of the process of activation of spermatozoa) and the organization of a single specific signaling systems expressed by different cells in different tissues (i.e. the endocannabinoid system). In both the cases we have found that the networks follow a scale free and small world topology, likely due to the evolutionary advantage of robustness against random damages, fastness and specific of information processing, and easy navigability.

NAADP and the two-pore channel protein 1 participate in the acrosome reaction in mammalian spermatozoa

A TPCN1 gene–deficient mouse strain is used to show that two convergent working NAADP-dependent pathways with nonoverlapping activation and self-inactivation profiles for distinct NAADP concentrations drive acrosomal exocytosis, by which TPC1 is central for the pathway activated by low-micromolar NAADP concentrations.

The functional relationship between the formation of hundreds of fusion pores during the acrosome reaction in spermatozoa and the mobilization of calcium from the acrosome has been determined only partially. Hence, the second messenger NAADP, promoting efflux of calcium from lysosome-like compartments and one of its potential molecular targets, the two-pore channel 1 (TPC1), were analyzed for its involvement in triggering the acrosome reaction using a TPCN1 gene–deficient mouse strain. The present study documents that TPC1 and NAADP-binding sites showed a colocalization at the acrosomal region and that treatment of spermatozoa with NAADP resulted in a loss of the acrosomal vesicle that showed typical properties described for TPCs: Registered responses were not detectable for its chemical analogue NADP and were blocked by the NAADP antagonist trans-Ned-19. In addition, two narrow bell-shaped dose-response curves were identified with maxima in either the nanomolar or low micromolar NAADP concentration range, where TPC1 was found to be responsible for activating the low affinity pathway. Our finding that two convergent NAADP-dependent pathways are operative in driving acrosomal exocytosis supports the concept that both NAADP-gated cascades match local NAADP concentrations with the efflux of acrosomal calcium, thereby ensuring complete fusion of the large acrosomal vesicle.

Uncovering the genetic basis for early isogamete differentiation: a case study of Ectocarpus siliculosus

BACKGROUND

The phenomenon of sexual reproduction characterizes nearly all eukaryotes, with anisogamy being the most prevalent form of gamete discrimination. Since dimorphic gametes most likely descend from equal-sized specialized germ cells, identifying the genetic bases of the early functional diversification in isogametes can provide better understanding of the evolution of sexual dimorphism. However, despite the potential importance to the evolutionary biology field, no comprehensive survey of the transcriptome profiling in isomorphic gametes has been reported hitherto.

RESULTS

Gamete differentiation on the genomic level was investigated using Ectocarpus siliculosus, a model organism for brown algal lineage which displays an isogamous sexual reproduction cycle. Transcriptome libraries of male and female gametes were generated using Next Generation Sequencing technology (SOLiD) and analyzed to identify differentially regulated genes and pathways with potential roles in fertilization and gamete specialization. Gamete transcriptomes showed a high level of complexity with a large portion of gender specific gene expression. Our results indicate that over 4,000 of expressed genes are differentially regulated between male and female, including sequences related to cell movement, carbohydrate and lipid metabolism, signaling, transport and RNA processing.

CONCLUSIONS

This first comprehensive transcriptomic study of protist isogametes describes considerable adaptation to distinct sexual roles, suggesting that functional anisogamy precedes morphological differentiation. Several sex-biased genes and pathways with a putative role in reproduction were identified, providing the basis for more detailed investigations of the mechanisms underlying evolution of mating types and sexual dimorphism.

Identification of aquaporins in eggs and early embryogenesis of the sea urchin Paracentrotus lividus

Sea urchins are echinoderms, marine invertebrates found at the base of the deutorostome lineage, which show separate sexes and are external spawners. In the sea urchin, efficient regulation of water homeostasis is essential for many biological processes such as cellular respiration, normal fertilization and correct embryo growth. In order to clarify some of these processes, the present study reports on the identification and function of aquaporin proteins in the sea urchin. Our results show, by immunoblot, immunoelectron microscopy and immunofluorescence analysis, the presence of aquaporin1- and aquaporin3-like proteins in virgin eggs and in early embryogenesis of Paracentrotus lividus and, by using known inhibitors of aquaporin functions, the functional and relevant role of aquaporin-3 in the fertilization process. AQP3 in particular seems to play a crucial role in high velocity water flux formations involved in the detachment of the vitelline layer during the slow block of polyspermy, while the presence of AQP1 and the increase of AQP3 in the first phase of the P. lividus developmental cycle, suggest their involvement in the appropriate homeostasis for embryo development.

Premature sperm activation and defective spermatogenesis caused by loss of spe-46 function in Caenorhabditis elegans

Given limited resources for motility, sperm cell activation must be precisely timed to ensure the greatest likelihood of fertilization. Like those of most species, the sperm of C. elegans become active only after encountering an external signaling molecule. Activation coincides with spermiogenesis, the final step in spermatogenesis, when the spherical spermatid undergoes wholesale reorganization to produce a pseudopod. Here, we describe a gene involved in sperm activation, spe-46. This gene was identified in a suppressor screen of spe-27(it132ts), a sperm-expressed gene whose product functions in the transduction of the spermatid activation signal. While spe-27(it132ts) worms are sterile at 25°C, the spe-46(hc197)I; spe-27(it132ts)IV double mutants regain partial fertility. Single nucleotide polymorphism mapping, whole genome sequencing, and transformation rescue were employed to identify the spe-46 coding sequence. It encodes a protein with seven predicted transmembrane domains but with no other predicted functional domains or homology outside of nematodes. Expression is limited to spermatogenic tissue, and a transcriptional GFP fusion shows expression corresponds with the onset of the pachytene stage of meiosis. The spe-46(hc197) mutation bypasses the need for the activation signal; mutant sperm activate prematurely without an activation signal in males, and mutant males are sterile. In an otherwise wild-type genome, the spe-46(hc197) mutation induces a sperm defective phenotype. In addition to premature activation, spe-46(hc197) sperm exhibit numerous defects including aneuploidy, vacuolization, protruding spikes, and precocious fusion of membranous organelles. Hemizygous worms [spe-46(hc197)/mnDf111] are effectively sterile. Thus, spe-46 appears to be involved in the regulation of spermatid activation during spermiogenesis, with the null phenotype being an absence of functional sperm and hypomorphic phenotypes being premature spermatid activation and numerous sperm cell defects.

Egg cell-secreted EC1 triggers sperm cell activation during double fertilization

Double fertilization is the defining characteristic of flowering plants. However, the molecular mechanisms regulating the fusion of one sperm with the egg and the second sperm with the central cell are largely unknown. We show that gamete interactions in Arabidopsis depend on small cysteine-rich EC1 (EGG CELL 1) proteins accumulating in storage vesicles of the egg cell. Upon sperm arrival, EC1-containing vesicles are exocytosed. The sperm endomembrane system responds to exogenously applied EC1 peptides by redistributing the potential gamete fusogen HAP2/GCS1 (HAPLESS 2/GENERATIVE CELL SPECIFIC 1) to the cell surface. Furthermore, fertilization studies with ec1 quintuple mutants show that successful male-female gamete interactions are necessary to prevent multiple-sperm cell delivery. Our findings provide evidence that mutual gamete activation, regulated exocytosis, and sperm plasma membrane modifications govern flowering plant gamete interactions.

Sex and speciation: Drosophila reproductive tract proteins- twenty five years later

The protein electrophoresis revolution, nearly fifty years ago, provided the first glimpse into the nature of molecular genetic variation within and between species and showed that the amount of genetic differences between newly arisen species was minimal. Twenty years later, 2D electrophoresis showed that, in contrast to general gene-enzyme variation, reproductive tract proteins were less polymorphic within species but highly diverged between species. The 2D results were interesting and revolutionary, but somewhat uninterpretable because, at the time, rapid evolution and selective sweeps were not yet part of the common vocabulary of evolutionary biologists. Since then, genomic studies of sex and reproduction-related (SRR) genes have grown rapidly into a large area of research in evolutionary biology and are shedding light on a number of phenomena. Here we review some of the major and current fields of research that have greatly contributed to our understanding of the evolutionary dynamics and importance of SRR genes and genetic systems in understanding reproductive biology and speciation.

Involvement of ABCB1 and ABCC1 transporters in sea urchin Echinometra lucunter fertilization

Fertilization is an ordered sequence of cellular interactions that promotes gamete fusion to form a new individual. Since the pioneering work of Oskar Hertwig conducted on sea urchins, echinoderms have contributed to the understanding of cellular and molecular aspects of the fertilization processes. Studies on sea urchin spermatozoa reported the involvement of a plasma membrane protein that belongs to the ABC proteins superfamily in the acrosome reaction. ABC transporters are expressed in membranes of eukaryotic and prokaryotic cells, and are associated with the transport of several compounds or ions across biomembranes. We aimed to investigate ABCB1 and ABCC1 transporter activity in sea urchin spermatozoa and their involvement in fertilization. Our results indicate that Echinometra lucunter spermatozoa exhibit a low intracellular calcein accumulation (18.5% stained cells); however, the ABC blockers reversin205, verapamil, and MK571 increased dye accumulation (93.0-96.6% stained cells). We also demonstrated that pharmacologically blocking ABCB1 and ABCC1 decreased spermatozoa fertilizing capacity (70% inhibition), and this phenotype was independent of extracellular calcium. These data suggest that functional spermatozoa ABCB1 and ABCC1 transporters are crucial for a successful fertilization. Additional studies must be performed to investigate the involvement of membrane lipid homeostasis in the fertilization process.

Allurin, an amphibian sperm chemoattractant having implications for mammalian sperm physiology

Eggs of many species are surrounded by extracellular coats that emit ligands to which conspecific sperm respond by undergoing chemotaxis and changes in metabolism, motility, and acrosomal status in preparation for fertilization. Here we review methods used to measure sperm chemotaxis and focus on recent studies of allurin, a 21-kDa protein belonging to the Cysteine-RIch Secretory Protein (CRISP) family that has chemoattraction activity for both amphibian and mammalian sperm. Allurin is unique in being the first extensively characterized Crisp protein found in the female reproductive tract and is the product of a newly discovered amphibian gene within a gene cluster that has been largely conserved in mammals. Study of its expression, function, and tertiary structure could lead to new insights in the role of Crisp proteins in sperm physiology.

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.

Assembly of the fluorescent acrosomal matrix and its fate in fertilization in the water strider, Aquarius remigis

Animal sperm show remarkable diversity in both morphology and molecular composition. Here we provide the first report of intense intrinsic fluorescence in an animal sperm. The sperm from a semi-aquatic insect, the water strider, Aquarius remigis, contains an intrinsically fluorescent molecule with properties consistent with those of flavin adenine dinucleotide (FAD), which appears first in the acrosomal vesicle of round spermatids and persists in the acrosome throughout spermiogenesis. Fluorescence recovery after photobleaching reveals that the fluorescent molecule exhibits unrestricted mobility in the acrosomal vesicle of round spermatids but is completely immobile in the acrosome of mature sperm. Fluorescence polarization microscopy shows a net alignment of the fluorescent molecules in the acrosome of the mature sperm but not in the acrosomal vesicle of round spermatids. These results suggest that acrosomal molecules are rearranged in the elongating acrosome and FAD is incorporated into the acrosomal matrix during its formation. Further, we followed the fate of the acrosomal matrix in fertilization utilizing the intrinsic fluorescence. The fluorescent acrosomal matrix was observed inside the fertilized egg and remained structurally intact even after gastrulation started. This observation suggests that FAD is not released from the acrosomal matrix during the fertilization process or early development and supports an idea that FAD is involved in the formation of the acrosomal matrix. The intrinsic fluorescence of the A. remigis acrosome will be a useful marker for following spermatogenesis and fertilization.

Fertilization in C. elegans requires an intact C-terminal RING finger in sperm protein SPE-42

Background

The C. elegans sperm protein SPE-42, a membrane protein of unknown structure and molecular function, is required for fertilization. Sperm from worms with spe-42 mutations appear normal but are unable to fertilize eggs. Sequence analysis revealed the presence of 8 conserved cysteine residues in the C-terminal cytoplasmic domain of this protein suggesting these residues form a zinc-coordinating RING finger structure.

Results

We made an in silico structural model of the SPE-42 RING finger domain based on primary sequence analysis and previously reported RING structures. To test the model, we created spe-42 transgenes coding for mutations in each of the 8 cysteine residues predicted to coordinate Zn⁺⁺ ions in the RING finger motif. Transgenes were crossed into a spe-42 null background and protein function was measured by counting progeny. We found that all 8 cysteines are required for protein function. We also showed that sequence differences between the C-terminal 29 and 30 amino acids in C. elegans and C. briggsae SPE-42 following the RING finger domain are not responsible for the failure of the C. briggsae SPE-42 homolog to rescue C. elegans spe-42 mutants.

Conclusions

The results suggest that a bona fide RING domain is present at the C-terminus of the SPE-42 protein and that this motif is required for sperm-egg interactions during C. elegans fertilization. Our structural model of the RING domain provides a starting point for further structure-function analysis of this critical region of the protein. The C-terminal domain swap experiment suggests that the incompatibility between the C. elegans and C. briggsae SPE-42 proteins is caused by small amino acid differences outside the C-terminal domain.

Lessons from nonmammalian species

There is abundant evidence for the presence of endogenous cannabinoid signaling systems in many nonmammalian species, including several classes of invertebrates. Interest in the study of these animals largely relates to their production of distinct and measurable specialized behaviors. The ability to alter these behaviors through manipulation of cannabinoid signaling has provided important insight into both the phylogenetic history and physiological relevance of this essential neuromodulatory system.This chapter presents a review of literature relevant to cannabinoid-altered behaviors in nonmammalian species from insects through advanced vocal learning avian species. Integration of findings supports a common role for endocannabinoid (ECB) modulation of ingestive and locomotor behaviors, with interesting contrasting agonist effects that distinguish vertebrate and invertebrate classes. Studies in amphibians and birds suggest that ECB signaling may function as a behavioral switch, allowing redirection from less- to more-essential behaviors in response to emergent environmental changes. Overall, the studies provide evidence for cannabinoid modulation of aggression, emesis, feeding behavior, locomotor activity, reproductive behaviors, vocal learning, sensory perception and stress responses.

Sulfated alpha-L-galactans from the sea urchin ovary: selective 6-desulfation as eggs are spawned

The sea urchin eggs are surrounded by a jelly coat, which contains sulfated polysaccharides with unique structures. These molecules are responsible for inducing the species-specific acrosome reaction, an obligatory event for the binding of sperm and fusion with the egg. The mechanism of biosynthesis of these sulfated polysaccharides is virtually unknown. The egg jelly of the sea urchin Echinometra lucunter contains a simple 2-sulfated, 3-linked alpha-L-galactan. Here, we pulse labeled the sea urchin ovary in vitro with (35)S-sulfate to follow the biosynthesis of the sulfated alpha-L-galactan. We found that the ovary contains a 2,6-disulfated, 3-linked alpha-L-galactan, which incorporates (35)S-sulfate more avidly than the 2-sulfated isoform. The 2,6-disulfated alpha-L-galactan was purified by anion exchange chromatography, analyzed by electrophoresis and characterized by 1D and 2D nuclear magnetic resonance spectra. We also investigated the location of the sulfated polysaccharides on the oocytes using histochemical procedures. The stain revealed high amounts of sulfated polysaccharide in mature oocytes and accessory cells. The amount of intracellular sulfated polysaccharides decreased as oocytes are spawned. We speculate that 2,6-disulfated galactan is initially synthesized in the ovary and that 6-sulfate ester is removed when the polysaccharide is secreted into the egg jelly. Similar events related to remodeling of sulfated polysaccharides have been reported in other biological systems.

Endogenous signaling pathways and chemical communication between sperm and egg

Red abalone (Haliotis rufescens) sperm detect a waterborne chemical cue released by conspecific eggs, and change their swimming behavior to increase the likelihood of fertilization success. Previously, we isolated the natural sperm attractant by bioassay-guided fractionation and high-performance liquid chromatography, and chemically identified it as the free-amino acid l-tryptophan (l-Trp). In the present study, levels of this ecologically meaningful compound were quantified in various abalone tissues, and in freshly spawned eggs. Tryptophan was the least abundant of 19 dissolved free amino acids (DFAAs) in ovary, testis, foot muscle, gill, stomach and hemolymph. As a proportion of the DFAA pool, however, Trp concentrations were significantly elevated in eggs (three- to seven-times higher) relative to all other sampled tissues. Natural rates of Trp release from eggs also were measured and correlated with fertility. Fertilization success peaked during an initial 30 min period (post-spawn), but decreased to nil over the next 50 min. Closely paralleling these events, Trp accumulated in seawater around freshly spawned eggs for the first 45 min (post-spawn) before decaying rapidly from solution. Older eggs stopped releasing Trp approximately when they became infertile, revealing a critical link between gamete physiology and chemical signaling. This apparent negative feedback loop did not arise from tryptophan oxidation, uptake by bacteria in seawater, or a degrading enzyme released by eggs. As a metabolic precursor critical to development of the larval nervous system, Trp could be an honest indicator of egg fitness for prospective sperm suitors. Our results suggest that endogenous signaling pathways have been co-opted for external communication between gametes, as an adaptation to increase reproductive success by promoting sperm navigation towards fertile eggs.

Purification and multimer formation of allurin, a sperm chemoattractant from Xenopus laevis egg jelly

Allurin, a sperm chemoattractant isolated from Xenopus laevis egg jelly, can be purified in one step from an extract of diffusible jelly proteins ("egg water") using a FPLC or HPLC anion exchange column and a multi-step NaCl gradient. Allurin homomultimers were detected by Western blotting with antibodies prepared against the purified protein or peptides within the protein. Allurin multimers were stable and resisted dissociation by SDS and beta-mercaptoethanol. Alkylation of allurin provided evidence for two free sulfhydryl groups but did not eliminate multimer formation, suggesting that intermolecular disulfide bond formation is not required for allurin aggregation. Concentration of egg water was accompanied by a reduction of chemoattractant activity that could not be fully accounted for by homomultimer formation. Rather, the presence of a multiphasic dose-activity curve upon partial purification and formation of hetero-allurin complexes during concentration suggested that egg water may contain allurin-binding proteins that reduce multimer formation and activity.

Regulation of sperm flagellar motility activation and chemotaxis caused by egg-derived substance(s) in sea cucumber

The sea cucumber Holothuria atra is a broadcast spawner. Among broadcast spawners, fertilization occurs by means of an egg-derived substance(s) that induces sperm flagellar motility activation and chemotaxis. Holothuria atra sperm were quiescent in seawater, but exhibited flagellar motility activation near eggs with chorion (intact eggs). In addition, they moved in a helical motion toward intact eggs as well as a capillary filled with the water layer of the egg extracts, suggesting that an egg-derived compound(s) causes motility activation and chemotaxis. Furthermore, demembranated sperm flagella were reactivated in high pH (> 7.8) solution without cAMP, and a phosphorylation assay using (gamma-32P)ATP showed that axonemal protein phosphorylation and dephosphorylation also occurred in a pH-dependent manner. These results suggest that the activation of sperm motility in holothurians is controlled by pH-sensitive changes in axonemal protein phosphorylation. Ca2+ concentration affected the swimming trajectory of demembranated sperm, indicating that Ca2+-binding proteins present at the flagella may be associated with regulation of flagellar waveform. Moreover, the phosphorylation states of several axonemal proteins were Ca2+-sensitive, indicating that Ca2+ impacts both kinase and phosphatase activities. In addition, in vivo sperm protein phosphorylation occurred after treatment with a water-soluble egg extract. Our results suggest that one or more egg-derived compounds activate motility and subsequent chemotactic behavior via Ca2+-sensitive flagellar protein phosphorylation.

Reassessing the role of protein-carbohydrate complementarity during sperm-egg interactions in the mouse

Despite years of intense study by many investigators, it may appear that we have made little progress towards a molecular understanding of mammalian sperm binding to the egg zona pellucida. An abundance of evidence derived from in vitro assays suggests that sperm-zona pellucida binding is dependent upon sperm recognition of specific glycan moieties on the zona pellucida glycoproteins. However, there is considerable disagreement regarding the identity of the zona pellucida sugars thought to mediate sperm binding, as well as disagreement over the identity of the sperm receptors themselves. Moreover, results from in vivo gene-targeting strategies fail to support a role for many, if not all, of the sperm receptors and their zona pellucida ligands implicated from in vitro assays. Nevertheless, a retrospective view of the literature suggests that some common principles are emerging regarding the molecular basis of mammalian sperm-zona binding, both with respect to the nature of the components that mediate binding, as well as the involvement of distinct receptor-ligand interactions, that involve both protein- and carbohydrate-dependent mechanisms of binding.

A polycystin-1 controls postcopulatory reproductive selection in mice

Pkdrej, a member of the polycystin-1 gene family, is expressed only in the male germ line. Male mice that are homozygous for a targeted mutation in the Pkdrej allele (Pkdrej(tm/tm)) are fertile in unrestricted mating trials, but exhibit lower reproductive success when competing with wild-type males in sequential mating trials and in artificial insemination of mixed-sperm populations. Following mating, sperm from Pkdrej(tm/tm) mice require >2 h longer than those of wild-type males to be detected within the egg/cumulus complex in the oviduct. Sperm from mice of both genotypes are able to capacitate in vitro. However, one of the component processes of capacitation, the ability to undergo a zona pellucida-evoked acrosome reaction, develops more slowly in sperm from Pkdrej(tm/tm) animals than in sperm from wild-type males. In contrast, a second component process of capacitation, the transition to hyperactivated flagellar motility, develops with a similar time course in both genotypes. These two behavioral consequences of capacitation, exocytotic competence and altered motility, are therefore differentially regulated. These data suggest that Pkdrej controls the timing of fertilization in vivo through effects on sperm transport and exocytotic competence and is a factor in postcopulatory sexual selection.

A novel function for the presenilin family member spe-4: inhibition of spermatid activation in Caenorhabditis elegans

Background

Sperm cells must regulate the timing and location of activation to maximize the likelihood of fertilization. Sperm from most species, including the nematode Caenorhabditis elegans, activate upon encountering an external signal. Activation for C. elegans sperm occurs as spermatids undergo spermiogenesis, a profound cellular reorganization that produces a pseudopod. Spermiogenesis is initiated by an activation signal that is transduced through a series of gene products. It is now clear that an inhibitory pathway also operates in spermatids, preventing their premature progression to spermatozoa and resulting in fine-scale control over the timing of activation. Here, we describe the involvement of a newly assigned member of the inhibitory pathway: spe-4, a homolog of the human presenilin gene PS1. The spe-4(hc196) allele investigated here was isolated as a suppressor of sterility of mutations in the spermiogenesis signal transduction gene spe-27.

Results

Through mapping, complementation tests, DNA sequencing, and transformation rescue, we determined that allele hc196 is a mutation in the spe-4 gene. Our data show that spe-4(hc196) is a bypass suppressor that eliminates the need for the spermiogenesis signal transduction. On its own, spe-4(hc196) has a recessive, temperature sensitive spermatogenesis-defective phenotype, with mutants exhibiting (i) defective spermatocytes, (ii) defective spermatids, (iii) premature spermatid activation, and (iv) spermatozoa defective in fertilization, in addition to a small number of functional sperm which appear normal microscopically.

Conclusion

A fraction of the sperm from spe-4(hc196) mutant males progress directly to functional spermatozoa without the need for an activation signal, suggesting that spe-4 plays a role in preventing spermatid activation. Another fraction of spermatozoa from spe-4(hc196) mutants are defective in fertilization. Therefore, prematurely activated spermatozoa may have several defects: we show that they may be defective in fertilization, and earlier work showed that they obstruct sperm transfer from males at mating. hc196 is a hypomorphic allele of spe-4, and its newly-discovered role inhibiting spermiogenesis may involve known proteolytic and/or calcium regulatory aspects of presenilin function, or it may involve yet-to-be discovered functions.

Xenopus tropicalis allurin: expression, purification, and characterization of a sperm chemoattractant that exhibits cross-species activity

Previously we reported the identification of the first vertebrate sperm chemoattractant, allurin, in the frog Xenopus laevis (Xl) and demonstrated that it was a member of the CRISP family of proteins. Here we report identification, purification, and characterization of Xenopus tropicalis (Xt) allurin, a homologous protein in X. tropicalis. "Egg water" as well as purified allurin from both species exhibit efficient cross-species sperm chemoattractant activity. Western blots show that Xt egg water contains a single anti-allurin cross-reactive protein whose molecular weight (20,497 Da by MALDI MS) agrees well with the molecular weight of the hypothetical gene product for a newly recognized "Crisp A" gene in the X. tropicalis genome. A recombinant form of the protein, expressed in 3T3 cells, exhibits chemoattraction for both Xt and Xl sperm and cross reacts with anti-allurin antibodies. Examination of Crisp protein expression in the Xt oviduct using RT-PCR showed that of five documented Xt Crisp genes (Crisps 2, 3, LD1, LD2 and A) only Crisp A was expressed. In contrast, Crisp 2, Crisp 3, Crisp LD1, and Crisp LD2, but not Crisp A, were all found to be expressed in the Xt testes while subsets of Crisp proteins where expressed in the Xt ovary. These data suggest that Crisp proteins in amphibians may play multiple roles in sperm production, maturation and guidance just as they are thought to in mammals indicating that Crisp protein involvement in reproduction may not be limited to mammals.

Rho-kinase (ROCK) in sea urchin sperm: its role in regulating the intracellular pH during the acrosome reaction

Sperm must undergo the acrosome reaction (AR) in order to fertilize the egg. In sea urchins, this reaction is triggered by the egg jelly (EJ) which, upon binding to its sperm receptor, induces increases in the ion permeability of the plasma membrane and changes in protein phosphorylation. Here, we demonstrated that the sperm expresses ROCK (approximately 135kDa), which is a serine/threonine protein kinase. ROCK localized, as RhoGTPase (Rho), in the acrosomal region, midpiece and flagellum. H-1152, a ROCK antagonist, inhibited the two cellular processes defining the AR: the acrosomal exocytosis and the actin polymerization. The ionophores nigericin and A23187 reversed the AR inhibition induced by H-1152, suggesting that ROCK functions at the level of the EJ-induced ion fluxes. Accordingly, H-1152 blocked 70% the intracellular alkalinization induced by EJ. These results indicate that EJ activates a Na+-H+ exchanger (NHE) in the sperm through a Rho/ROCK-dependent signaling pathway that culminates in the AR.

Particulate and soluble adenylyl cyclases participate in the sperm acrosome reaction

cAMP is important in sea urchin sperm signaling, yet the molecular nature of the adenylyl cyclases (ACs) involved remained unknown. These cells were recently shown to contain an ortholog of the mammalian soluble adenylyl cyclase (sAC). Here, we show that sAC is present in the sperm head and as in mammals is stimulated by bicarbonate. The acrosome reaction (AR), a process essential for fertilization, is influenced by the bicarbonate concentration in seawater. By using functional assays and immunofluorescence techniques we document that sea urchin sperm also express orthologs of multiple isoforms of transmembrane ACs (tmACs). Our findings employing selective inhibitors for each class of AC indicate that both sAC and tmACs participate in the sperm acrosome reaction.

Stimulation of the nitric oxide/cyclic guanosine monophosphate signaling pathway elicits human sperm chemotaxis in vitro

OBJECTIVE

To investigate whether nitric oxide (NO) may attract human spermatozoa via activation of the soluble guanylate cyclase (sGC)/cyclic guanosine monophosphate (cGMP) pathway.

DESIGN

Prospective study.

SETTING

Academic research institution.

PATIENT(S)

Seven normozoospermic patients belonging to couples presenting for infertility evaluation.

INTERVENTION(S)

Sperm samples were processed by the swim-up technique.

MAIN OUTCOME MEASURE(S)

Sperm chemotaxis detected by a choice device (specially designed three-well plexiglass chamber), intracellular level of cGMP (by radioimmunoassay), and sperm motility parameters (by computer-assisted sperm analysis).

RESULT(S)

After a 20-minute incubation, the NO donor S-nitrosoglutathione (GSNO) increased the synthesis of cGMP and exerted a significant chemoattraction on human spermatozoa. The GSNO-induced migration of sperm was inhibited by PTIO (a NO scavenger), ODQ (an sGC inhibitor), and Rp-8-Br-cGMPS (an inhibitor of cGMP-dependent protein kinases). The cell-permeating cGMP analog 8-Br-cGMP acted as a potent chemoattractant per se: this effect was inhibited by Rp-8-Br-cGMPS.

CONCLUSION(S)

These data suggest that NO may exert a chemoattractant effect on human spermatozoa and that the signal transduction involves the activation of sGC, the synthesis of cGMP, and the activation of cGMP-dependent protein kinases.

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.

[Ca2+]i signalling in sperm — making the most of what you've got

Thanks to a worrying decrease in male fertility, understanding how sperm 'work' is a matter both of interest and great importance. Sperm of all animals detect various environmental cues. The 'behavioural' and physiological responses of sperm must be specific, appropriate and correctly timed. Strangely, in a cell with few organelles and minimal cytoplasmic volume, internal Ca(2+) concentration, [Ca(2+)](i), regulates almost all these activities. How does such a simple cell achieve this - and is it as simple as it seems?