Endogenous signaling pathways and chemical communication between sperm and egg

Ocean Acidification, but Not Environmental Contaminants, Affects Fertilization Success and Sperm Motility in the Sea Urchin Paracentrotus lividus

Ocean acidification poses an increasing concern for broadcast spawning species that release gametes in the water column where fertilization occurs. Indeed, the functionality of gametes and their interactions may be negatively affected by reduced pH. Susceptibility to other environmental stressors, such as pollutants, may be also altered under acidified conditions, resulting in more detrimental effects. To verify this hypothesis, combined exposures to CO2-driven acidification and environmentally relevant concentrations (0.5 µg/L) of three contaminants (caffeine, diclofenac, and PFOS, all singularly or in mixture) were carried out to highlight potential negative effects on fertilization success and motility of sperm in the sea urchin Paracentrotus lividus. Our results showed a significant reduction in the percentage of fertilized eggs when sperm were pre-exposed to reduced pH (ambient pH minus 0.4 units) compared to that of controls (ambient, pH = 8.1). Sperm speed and motility also decreased when sperm were activated and then exposed at reduced pH. Conversely, at both pH values tested, no significant effect due to the contaminants, nor of their interaction with pH, was found on any of the biological endpoints considered.

The functions of CAP superfamily proteins in mammalian fertility and disease

BACKGROUND

Members of the cysteine-rich secretory proteins (CRISPS), antigen 5 (Ag5) and pathogenesis-related 1 (Pr-1) (CAP) superfamily of proteins are found across the bacterial, fungal, plant and animal kingdoms. Although many CAP superfamily proteins remain poorly characterized, over the past decade evidence has accumulated, which provides insights into the functional roles of these proteins in various processes, including fertilization, immune defence and subversion, pathogen virulence, venom toxicology and cancer biology.

OBJECTIVE AND RATIONALE

The aim of this article is to summarize the current state of knowledge on CAP superfamily proteins in mammalian fertility, organismal homeostasis and disease pathogenesis.

SEARCH METHODS

The scientific literature search was undertaken via PubMed database on all articles published prior to November 2019. Search terms were based on following keywords: 'CAP superfamily', 'CRISP', 'Cysteine-rich secretory proteins', 'Antigen 5', 'Pathogenesis-related 1', 'male fertility', 'CAP and CTL domain containing', 'CRISPLD1', 'CRISPLD2', 'bacterial SCP', 'ion channel regulator', 'CatSper', 'PI15', 'PI16', 'CLEC', 'PRY proteins', 'ASP proteins', 'spermatogenesis', 'epididymal maturation', 'capacitation' and 'snake CRISP'. In addition to that, reference lists of primary and review article were reviewed for additional relevant publications.

OUTCOMES

In this review, we discuss the breadth of knowledge on CAP superfamily proteins with regards to their protein structure, biological functions and emerging significance in reproduction, health and disease. We discuss the evolution of CAP superfamily proteins from their otherwise unembellished prokaryotic predecessors into the multi-domain and neofunctionalized members found in eukaryotic organisms today. At least in part because of the rapid evolution of these proteins, many inconsistencies in nomenclature exist within the literature. As such, and in part through the use of a maximum likelihood phylogenetic analysis of the vertebrate CRISP subfamily, we have attempted to clarify this confusion, thus allowing for a comparison of orthologous protein function between species. This framework also allows the prediction of functional relevance between species based on sequence and structural conservation.

WIDER IMPLICATIONS

This review generates a picture of critical roles for CAP proteins in ion channel regulation, sterol and lipid binding and protease inhibition, and as ligands involved in the induction of multiple cellular processes.

Cherchez la femme - impact of ocean acidification on the egg jelly coat and attractants for sperm

The impact of ocean acidification on marine invertebrate eggs and consequences for sperm chemotaxis are unknown. In the sea urchins Heliocidaris tuberculata and H. erythrogramma, with small (93µm) and large (393µm) eggs, respectively, we documented the effect of decreased pH on the egg jelly coat, an extracellular matrix that increases target size for sperm and contains sperm attracting molecules. In near future conditions (pH 7.8, 7.6) the jelly coat of H. tuberculata decreased by 11 and 21%, reducing egg target size by 9 and 17%, respectively. In contrast, the egg jelly coat of H. erythrogramma was not affected. The reduction in the jelly coat has implications for sperm chemotaxis in H. tuberculata. In the presence of decreased pH and egg chemicals, the sperm of this species increased their velocity, motility and linearity, behaviour that was opposite to that seen for sperm exposed to egg chemicals in ambient conditions. Egg chemistry appears to cause a reduction in sperm velocity where attractants guide them in the direction of the egg. Investigation of the effects of decreased pH on sperm isolated from egg chemistry does not provide an integrative assessment of the effects of ocean acidification on sperm function. Differences in the sensitivity of the jelly coat of the two species is likely associated with egg evolution in H. erythrogramma. We highlight important unappreciated impacts of ocean acidification on marine gamete functionality, and insights into potential winners and losers in a changing ocean, pointing to the advantage conveyed by evolution of large eggs.

Sperm chemotaxis promotes individual fertilization success in sea urchins

Reproductive success fundamentally shapes an organism's ecology and evolution, and gamete traits mediate fertilization, which is a critical juncture in reproduction. Individual male fertilization success is dependent on the ability of sperm from one male to outcompete the sperm of other males when searching for a conspecific egg. Sperm chemotaxis, the ability of sperm to navigate towards eggs using chemical signals, has been studied for over a century, but such studies have long assumed that this phenomenon improves individual male fitness without explicit evidence to support this claim. Here, we assess fertilization changes upon use of a chemoattractant-digesting peptidase and use a microfluidic device coupled with a fertilization assay to determine the effect of sperm chemotaxis on individual male fertilization success in the sea urchin Lytechinus pictus. We show that removing chemoattractant from the gametic environment decreases fertilization success. We further find that individual male differences in chemotaxis to a well-defined gradient of attractant correlate with individual male differences in fertilization success. These results demonstrate that sperm chemotaxis is an important contributor to individual reproductive success.

Lectin staining and flow cytometry reveals female-induced sperm acrosome reaction and surface carbohydrate reorganization

All cells are covered by glycans, an individually unique layer of oligo- and polysaccharides that are critical moderators of self-recognition and other cellular-level interactions (e.g. fertilization). The functional similarity between these processes suggests that gamete surface glycans may also have an important, but currently overlooked, role in sexual selection. Here we develop a user-friendly methodological approach designed to facilitate future tests of this possibility. Our proposed method is based on flow cytometric quantification of female-induced sperm acrosome reaction and sperm surface glycan modifications in the Mediterranean mussel Mytilus galloprovincialis. In this species, as with many other taxa, eggs release water-soluble factors that attract conspecific sperm (chemoattraction) and promote potentially measurable changes in sperm behavior and physiology. We demonstrate that flow cytometry is able to identify sperm from other seawater particles as well as accurately measure both acrosome reaction and structural modifications in sperm glycans. This methodological approach can increase our understanding of chemically-moderated gamete-level interactions and individual-specific gamete recognition in Mytilus sp. and other taxa with similar, easily identifiable acrosome structure. Our approach is also likely to be applicable to several other species, since carbohydrate-mediated cellular-level interactions between gametes are universal among externally and internally fertilizing species.

The physics of broadcast spawning in benthic invertebrates

Most benthic invertebrates broadcast their gametes into the sea, whereupon successful fertilization relies on the complex interaction between the physics of the surrounding fluid flow and the biological properties and behavior of eggs and sperm. We present a holistic overview of the impact of instantaneous flow processes on fertilization across a range of scales. At large scales, transport and stirring by the flow control the distribution of gametes. Although mean dilution of gametes by turbulence is deleterious to fertilization, a variety of instantaneous flow phenomena can aggregate gametes before dilution occurs. We argue that these instantaneous flow processes are key to fertilization efficiency. At small scales, sperm motility and taxis enhance contact rates between sperm and chemoattractant-releasing eggs. We argue that sperm motility is a biological adaptation that replaces molecular diffusion in conventional mixing processes and enables gametes to bridge the gap that remains after aggregation by the flow.

Sexual selection and the evolution of egg-sperm interactions in broadcast-spawning invertebrates

Many marine invertebrate taxa are broadcast spawners, where multiple individuals release their gametes into the water for external fertilization, often in the presence of gametes from heterospecifics. Consequently, sperm encounter the considerable challenges of locating and fertilizing eggs from conspecific females. To overcome these challenges, many taxa exhibit species-specific attraction of sperm toward eggs through chemical signals released from eggs (sperm chemotaxis) and species-specific gamete recognition proteins (GRPs) that mediate compatibility of gametes at fertilization. In this prospective review, we highlight these selective forces, but also emphasize the role that sexual selection, manifested through sperm competition, cryptic female choice, and evolutionary conflicts of interest between the sexes (sexual conflict), can also play in mediating the action of egg chemoattractants and GRPs, and thus individual reproductive fitness. Furthermore, we explore patterns of selection at the level of gametes (sperm phenotype, gamete plasticity, and egg traits) to identify putative traits targeted by sexual selection in these species. We conclude by emphasizing the excellent, but relatively untapped, potential of broadcast-spawning marine invertebrates as model systems to illuminate several areas of research in post-mating sexual selection.

Global change ecotoxicology: Identification of early life history bottlenecks in marine invertebrates, variable species responses and variable experimental approaches

Climate change is a threat to marine biota because increased atmospheric CO₂ is causing ocean warming, acidification, hypercapnia and decreased carbonate saturation. These stressors have toxic effects on invertebrate development. The persistence and success of populations requires all ontogenetic stages be completed successfully and, due to their sensitivity to environmental stressors, developmental stages may be a population bottleneck in a changing ocean. Global change ecotoxicology is being used to identify the marine invertebrate developmental stages vulnerable to climate change. This overview of research, and the methodologies used, shows that most studies focus on acidification, with few studies on ocean warming, despite a long history of research on developmental thermotolerance. The interactive effects of stressors are poorly studied. Experimental approaches differ among studies. Fertilization in many species exhibits a broad tolerance to warming and/or acidification, although different methodologies confound inter-study comparisons. Early development is susceptible to warming and most calcifying larvae are sensitive to acidification/increased pCO₂. In multistressor studies moderate warming diminishes the negative impact of acidification on calcification in some species. Development of non-calcifying larvae appears resilient to near-future ocean change. Although differences in species sensitivities to ocean change stressors undoubtedly reflect different tolerance levels, inconsistent handling of gametes, embryos and larvae probably influences different research outcomes. Due to the integrative 'developmental domino effect', life history responses will be influenced by the ontogenetic stage at which experimental incubations are initiated. Exposure to climate change stressors from early development (fertilization where possible) in multistressor experiments is needed to identify ontogenetic sensitivities and this will be facilitated by more consistent methodologies.

Assessing the potential for egg chemoattractants to mediate sexual selection in a broadcast spawning marine invertebrate

In numerous species, egg chemoattractants play a critical role in guiding sperm towards unfertilized eggs (sperm chemotaxis). Until now, the known functions of sperm chemotaxis include increasing the effective target size of eggs, thereby promoting sperm-egg encounters, and facilitating species recognition. Here, we report that in the broadcast spawning mussel, Mytilus galloprovincialis, egg chemoattractants may play an unforeseen role in sexual selection by enabling sperm to effectively 'choose' between the eggs of different conspecific females. In an initial experiment, we confirmed that sperm chemotaxis occurs in M. galloprovincialis by showing that sperm are attracted towards unfertilized eggs when given the choice of eggs or no eggs in a dichotomous chamber. We then conducted two cross-classified mating experiments, each comprising the same individual males and females crossed in identical male × female combinations, but under experimental conditions that offered sperm 'no-choice' (each fertilization trial took place in a Petri dish and involved a single male and female) or a 'choice' of a female's eggs (sperm were placed in the centre of a dichotomous choice chamber and allowed to choose eggs from different females). We show that male-by-female interactions characterized fertilization rates in both experiments, and that there was remarkable consistency between patterns of sperm migration in the egg-choice experiment and fertilization rates in the no-choice experiment. Thus, sperm appear to exploit chemical cues to preferentially swim towards eggs with which they are most compatible during direct sperm-to-egg encounters. These results reveal that sperm differentially select eggs on the basis of chemical cues, thus exposing the potential for egg chemoattractants to mediate mate choice for genetically compatible partners. Given the prevalence of sperm chemotaxis across diverse taxa, our findings may have broad implications for sexual selection in other mating systems.

Egg jelly proteins stimulate directed motility in Xenopus laevis sperm

Previously we have shown that extracts from Xenopus egg jelly (egg water) increase the passage of sperm through a porous membrane in a dose-dependent manner. Although this assay has shown that sperm accumulation occurs only in the presence of an egg water gradient, it has not revealed the dynamic features of how Xenopus sperm swim in such gradients. Here, we use video microscopic observations to trace sperm trajectories in a Zigmond chamber. Our results show that Xenopus sperm swim in linear and gently curving paths and only infrequently perform turns. In the presence of an egg water gradient, however, the percent of sperm swimming up the gradient axis and the net distance traveled by each sperm along this axis was increased significantly. There was no change in curvilinear velocity. Rather, the orientation of sperm travel was shifted to more closely match that of the gradient axis. In addition, using a porous filter assay, we demonstrate that the egg water protein allurin, in both purified and recombinant forms, stimulates directed motility of sperm. Finally, we use Oregon Green 488-conjugated allurin to show that this protein binds primarily to the sperm midpiece; binding of allurin to the entire head was observed in a minor subpopulation of sperm. Dose dependence of allurin binding occurred over the 0-1 µg/ml range and correlated well with previously published dose-dependent sperm attraction data. Binding was rapid with a half-time of about 10 sec. These data suggest that egg water proteins bind to sperm and modify sperm-orienting behavior.

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.

Mouse sperm exhibit chemotaxis to allurin, a truncated member of the cysteine-rich secretory protein family

Allurin, a 21 kDa protein isolated from egg jelly of the frog Xenopus laevis, has previously been demonstrated to attract frog sperm in two-chamber and microscopic assays. cDNA cloning and sequencing has shown that allurin is a truncated member of the Cysteine-Rich Secretory Protein (CRISP) family, whose members include mammalian sperm-binding proteins that have been postulated to play roles in spermatogenesis, sperm capacitation and sperm-egg binding in mammals. Here, we show that allurin is a chemoattractant for mouse sperm, as determined by a 2.5-fold stimulation of sperm passage across a porous membrane and by analysis of sperm trajectories within an allurin gradient as observed by time-lapse microscopy. Chemotaxis was accompanied by an overall change in trajectory from circular to linear thereby increasing sperm movement along the gradient axis. Allurin did not increase sperm velocity although it did produce a modest increase in flagellar beat frequency. Oregon Green 488-conjugated allurin was observed to bind to the sub-equatorial region of the mouse sperm head and to the midpiece of the flagellum. These findings demonstrate that sperm have retained the ability to bind and respond to truncated Crisp proteins over 300 million years of vertebrate evolution.

Sperm chemotaxis, fluid shear, and the evolution of sexual reproduction

Chemical communication is fundamental to sexual reproduction, but how sperm search for and find an egg remains enigmatic. For red abalone (Haliotis rufescens), a large marine snail, the relationship between chemical signaling and fluid motion largely determines fertilization success. Egg-derived attractant plumes are dynamic, changing their size and shape in response to unique combinations of physical and chemical environmental features. Attractant plumes that promote sexual reproduction, however, are limited to a precise set of hydrodynamic conditions. Performance-maximizing shears are those that most closely match flows in native spawning habitats. Under conditions in which reproductive success is chronically limited by sperm availability, gametes are under selection for mechanisms that increase sperm-egg encounter. Here, chemoattraction is found to provide a cheap evolutionary alternative for enhancing egg target size without enlarging cytoplasmic and/or cell volume. Because egg signaling and sperm response may be tuned to meet specific fluid-dynamic constraints, shear could act as a critical selective pressure that drives gamete evolution and determines fitness.

Swimming of Xenopus laevis sperm exhibits multiple gears and its duration is extended by egg jelly constituents

The motility of Xenopus sperm is initiated by the osmotic shock experienced when these cells are ejaculated into low-salinity pond water. Motility is brief and is required for the sperm to penetrate the jelly layers and fertilize the egg. In this study we demonstrate that extracts of egg jelly contain factors that extend the period of sperm motility as well as providing a chemoattractant activity as previously reported. Both activities are partially dependent on extracellular calcium. Time-lapse and video microscopy show that after activation of motility the number of motile sperm decreases rapidly, with a half-time of about 2 min. Addition of 10% v/v egg jelly extract ("egg water") increased the number of motile sperm 2-fold over controls at 20 s and about 4- to 10-fold over controls at 10 min after initiation of motility. Extension of motility lifetime was not mediated by a nonspecific protein or by allurin, the egg-water protein that has chemoattractant activity. The helical path of Xenopus sperm exhibited tight coupling between rotational and forward velocities in egg jelly, but coupling changed rapidly from moment to moment in low-salinity buffer. Our observations suggest that jelly-derived factors regulate both the longevity and directionality of sperm propulsion.

Sperm chemotaxis as revealed with live and synthetic eggs

Fertilization is one of the least understood fundamental biological processes. How sperm search for and find an egg remains enigmatic. Sperm attraction to egg-derived chemical cues may be significant evolutionarily for maintaining species barriers and important ecologically for increasing gamete encounters. New tools are needed, however, to resolve the functional consequences of these dissolved signal molecules. Freshly spawned eggs from red abalone (Haliotis rufescens) naturally release l-tryptophan, which stimulates chemotactic responses by conspecific sperm. Here, microspheres were manufactured to the approximate size and the same shape as female gametes and formulated to emit controlled doses of chemoattractant, imitating natural l-tryptophan release rates. When experimentally tested for effectiveness, male gametes did not distinguish between chemically impregnated mimics and live eggs, demonstrating that l-tryptophan alone is both necessary and sufficient to promote chemotaxis, and confirming the identity of a native sperm attractant. The techniques that we describe can be used to create synthetic eggs for most animal and plant species, including humans. Egg mimics increase the capacity for experimental manipulation and enable realistic studies of sperm behavior even in the absence of female gametes.