suREJ proteins: new signalling molecules in sea urchin spermatozoa

Gamete activation: basic knowledge and clinical applications

Background

The first clues to the process of gamete activation date back to nearly 60 years ago. The mutual activation of gametes is a crucial event during fertilization. In the testis and ovaries, spermatozoa and oocytes are in a state of meiotic and metabolic quiescence and require reciprocal signals in order to undergo functional changes that lead to competence for fertilization. First, the oocyte activates sperm by triggering motility, chemoattraction, binding and the acrosome reaction, culminating with the fusion of the two plasma membranes. At the end of this cascade of events, collectively known as sperm capacitation, sperm-induced oocyte activation occurs, generating electrical, morphological and metabolic modifications in the oocyte.

Objective and rationale

The aim of this review is to provide the current state of knowledge regarding the entire process of gamete activation in selected specific animal models that have contributed to our understanding of fertilization in mammals, including humans. Here we describe in detail the reciprocal induction of the two activation processes, the molecules involved and the mechanisms of cell interaction and signal transduction that ultimately result in successful embryo development and creation of a new individual.

Search methods

We carried out a literature survey with no restrictions on publication date (from the early 1950s to March 2016) using PubMed/Medline, Google Scholar and Web of Knowledge by utilizing common keywords applied in the field of fertilization and embryo development. We also screened the complete list of references published in the most recent research articles and relevant reviews published in English (both animal and human studies) on the topics investigated.

Outcomes

Literature on the principal animal models demonstrates that gamete activation is a pre-requisite for successful fertilization, and is a process common to all species studied to date. We provide a detailed description of the dramatic changes in gamete morphology and behavior, the regulatory molecules triggering gamete activation and the intracellular ions and second messengers involved in active metabolic pathways in different species. Recent scientific advances suggest that artificial gamete activation may represent a novel technique to improve human IVF outcomes, but this approach requires caution.

Wider implications

Although controversial, manipulation of gamete activation represents a promising tool for ameliorating the fertilization rate in assisted reproductive technologies. A better knowledge of mechanisms that transform the quiescent oocyte into a pluripotent cell may also provide new insights for the clinical use of stem cells.

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.

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.

Evidence of amino acid diversity-enhancing selection within humans and among primates at the candidate sperm-receptor gene PKDREJ

Sperm-egg interaction is a crucial step in fertilization, yet the identity of most interacting sperm-egg proteins that mediate this process remains elusive. Rapid evolution of some fertilization proteins has been observed in a number of species, including evidence of positive selection in the evolution of components of the mammalian egg coat. The rapid evolution of the egg-coat proteins could strongly select for changes on the sperm receptor, to maintain the interaction. Here, we present evidence that positive selection has driven the evolution of PKDREJ, a candidate sperm receptor of mammalian egg-coat proteins. We sequenced PKDREJ from a panel of 14 primates, including humans, and conducted a comparative maximum-likelihood analysis of nucleotide changes and found evidence of positive selection. An additional panel of 48 humans was surveyed for nucleotide polymorphisms at the PKDREJ locus. The regions predicted to have been subject to adaptive evolution among primates show several amino acid polymorphisms within humans. The distribution of polymorphisms suggests that balancing selection may maintain diverse PKDREJ alleles in some populations. It remains unknown whether there are functional differences associated with these diverse alleles, but their existence could have consequences for human fertility.

Functional characterization of PKDREJ, a male germ cell-restricted polycystin

Polycystin-1 regulates a number of cellular processes through the formation of complexes with the polycystin-2 ion channel or with other signal transduction proteins. Polycystin-1 is expressed in many tissues but other members of this gene family are distributed in a more restricted fashion. PKDREJ expression has been detected only in the mammalian testis, where it is restricted to the spermatogenic lineage and retained in mature sperm. However, the functional characteristics of this protein and its role in sperm biology are not well understood. In this study it is shown that PKDREJ can modulate G protein signaling and associates with several members of the polycystin-2 family. These interactions, as well as polycystin-2 association with TRPC channels, are consistent with a role of this protein in the regulation of the acrosome reaction and in other aspects of sperm physiology.

The versatile nature of the calcium-permeable cation channel TRPP2

TRPP2 is a member of the transient receptor potential (TRP) superfamily of cation channels, which is mutated in autosomal dominant polycystic kidney disease (ADPKD). TRPP2 is thought to function with polycystin 1-a large integral protein-as part of a multiprotein complex involved in transducing Ca(2+)-dependent information. TRPP2 has been implicated in various biological functions including cell proliferation, sperm fertilization, mating behaviour, mechanosensation and asymmetric gene expression. Although its function as a Ca(2+)-permeable cation channel is well established, its precise role in the plasma membrane, the endoplasmic reticulum and the cilium is controversial. Recent studies suggest that TRPP2 function is highly dependent on the subcellular compartment of expression, and is regulated by many interactions with adaptor proteins. This review summarizes the most pertinent evidence about the properties of TRPP2 channels, focusing on the compartment-specific functions of mammalian TRPP2.

Polycystic kidney disease and receptor for egg jelly is a plasma membrane protein of mouse sperm head

The mammalian polycystic kidney disease (PKD) gene family comprises eight members whose role in cell physiology is still poorly understood. Two of the founding members of the PKD family, PKD1 and PKD2, are responsible for the majority of cases of autosomal dominant polycystic kidney disease. The present study focuses on a PKD1 homologue, mouse polycystic kidney disease and receptor for egg jelly (PKDREJ) and its putative role in mammalian fertilization. To examine PKDREJ tissue distribution multiple-tissue Northern blot analysis was performed. We observed that PKDREJ expression is confined to mouse testis. A PKDREJ transcript was detected in spermatogenic cells by in situ hybridization with mouse testicular tissue. Upon heterologous expression PKDREJ was retained in intracellular membrane compartments and unlike PKD1 did not undergo cleavage in the G-protein-coupled receptor proteolytic site domain (GPS). Immunocytochemical experiments on isolated epididymal mouse spermatozoa using PKDREJ-specific polyclonal antibodies revealed that the protein is localized in the acrosomal region and on the inner aspect of the falciform-shaped head. To precisely characterize PKDREJ expression in the acrosomal region, transmission electron microscopy was performed. Immunogold labeling was only visible at the plasma membrane of the mouse sperm head. Collectively, these data suggest PKDREJ to be a sperm plasma membrane protein presumably contributing to transmembrane signaling in mammalian spermatozoa.

Polycystins: polymodal receptor/ion-channel cellular sensors

Transient receptor potential (TRP) channel proteins are divided into seven subgroups that are currently designated as TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPN (NOMP-C, from no mechanoreceptor potential-C), TRPA (ankyrin-like with transmembrane domains 1) and TRPP (polycystin). TRPC, TRPV and TRPM are related to canonical TRP proteins whereas TRPN, TRPA and TRPP (polycystin) are more divergent. Most TRP channels are linked to sensory stimuli, including phototransduction, thermosensation and mechanosensation. The TRPP subfamily was named after its founding member, polycystin kidney disease-2 (PKD2), a gene product mutated in many cases of autosomal dominant polycystic kidney disease (ADPKD). ADPKD is a major inherited nephropathy, affecting over 1:1,000 of the worldwide population, characterized by the progressive development of fluid-filled cysts from the tubules and collecting ducts of affected kidneys. Loss-of-function mutations in either polycystin-2, a non-selective cation channel, or polycystin-1 (PKD1), a large plasma membrane integral protein, give rise to ADPKD. PKD1 and PKD2 are thought to function together as part of a multiprotein receptor/ion-channel complex or independently and may be involved in transducing Ca(2+)-dependent mechanosensitive signals in response to cilia bending in renal epithelial cells and endodermally derived cells. Further information on the growing number and physiological properties of these TRP-polycystins is the basis of this review.

Polycystins, calcium signaling, and human diseases

Autosomal dominant polycystic kidney disease (ADPKD) is a major, inherited nephropathy affecting over 1:1000 of the worldwide population. It is a systemic condition with frequent hepatic and cardiovascular manifestations in addition to the progressive development of fluid-filled cysts from the tubules and collecting ducts of affected kidneys. The pathogenesis of cyst formation is currently thought to involve increased proliferation of epithelial cells, mild dedifferentiation, and fluid accumulation. In the past decade, study of ADPKD led to the discovery of a unique family of highly complex proteins, the polycystins. Loss-of-function mutations in either of two polycystin proteins, polycystin-1 or polycystin-2, give rise to ADPKD. These proteins are thought to function together as part of a multiprotein complex that may initiate Ca2+ signals, directing attention to the regulation of intracellular Ca2+ as a possible misstep that participates in cyst formation. Here we review what is known about the Ca2+ signaling functions of polycystin proteins and focus on findings that have significantly advanced our physiological insight. Special attention is paid to the recently discovered role of these proteins in the mechanotransduction of the renal primary cilium and the model it suggests.

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).

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

Sea urchin egg jelly (EJ) triggers sperm acrosome reaction (AR), an exocytotic event required for membrane fusion of the gametes. Purified fucose sulfate polymer (FSP) in EJ is one inducer of the AR. Binding of FSP to its receptor regulates opening of two distinct calcium channels and also elevates intracellular pH (pH(i)). EJ also contains sialic acid-rich glycans (sialoglycans (SG)) that were isolated by beta-elimination followed by DEAE chromatography. In the presence of limiting amounts of FSP, the SG fraction markedly potentiates the AR; however, by itself SG has no activity. The SG fraction increases the pH(i) of sperm without increasing intracellular Ca(2+). The SG-induced increase in pH(i) is not blocked by nifedipine or high K(+), whereas the FSP-induced pH(i) increase is sensitive to both these agents. Treatment of the SG fraction with neuraminidase or mild metaperiodate that specifically cleaves the glycerol side chain of sialic acid abolishes the AR potentiation and ability of SG to elevate pH(i). These data are the first to show that there are at least two pathways to induce sperm pH(i) increase and that egg surface sialic acid plays a role in triggering the sperm AR.

Ion transport in sperm signaling

Ion channels and transporters, key elements in sperm-egg signaling and environmental sensing, are essential for fertilization. External cues and components from the outer envelopes of the egg influence sperm ion permeability and behavior. Combining in vivo measurements of membrane potential, intracellular ions, and second messengers with new molecular approaches and reconstitution strategies are revealing how sperm ion channels participate in motility, sperm maturation, and the acrosome reaction. Sperm are tiny differentiated terminal cells unable to synthesize proteins and difficult to characterize electrophysiologically. Spermatogenic cells, the progenitors of sperm, have become useful tools for probing sperm ion channels since they are larger and molecular biology techniques can be applied. These complementary strategies are opening new avenues to determine how sperm ion channels function in gamete signaling.