Signal transduction at fertilization: The Ca2+ release pathway in echinoderms and other invertebrate deuterostomes

RNA interference as a next-generation control method for suppressing Varroa destructor reproduction in honey bee (Apis mellifera) hives

BACKGROUND

The Varroa mite (Varroa destructor) is considered to be the greatest threat to apiculture worldwide. RNA interference (RNAi) using double-stranded RNA (dsRNA) as a gene silencing mechanism has emerged as a next-generation strategy for mite control.

RESULTS

We explored the impact of a dsRNA biopesticide, named vadescana, designed to silence the calmodulin gene in Varroa, on mite fitness in mini-hives housed in a laboratory. Two dosages were tested: 2 g/L dsRNA and 8 g/L dsRNA. Vadescana appeared to have no effect on mite survival, however, mite fertility was substantially reduced. The majority of foundress mites exposed to vadescana failed to produce any offspring. No dose-dependent effect of vadescana was observed, as both the low and high doses inhibited mite reproduction equally well in the mini-hives and neither dose impacted pupal survival of the honey bee. Approximately 95% of bee pupae were alive at uncapping across all treatment groups.

CONCLUSION

These findings suggest that vadescana has significant potential as an effective alternative to conventional methods for Varroa control, with broader implications for the utilization of RNAi as a next-generation tool in the management of pest species. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Identification of SH2 Domain-Mediated Protein Interactions that Operate at Fertilization in the Sea Star Patiria miniata

The signaling mechanisms controlling internal calcium release at fertilization in animals are still largely unknown. Echinoderms, such as the sea star Patiria miniata, produce abundant and easily accessible sperm and eggs. In addition, eggs are naturally synchronized at the same cell cycle stage, collectively making these animals an attractive model to study the signaling proteins controlling fertilization. However, the lack of antibodies to identify proteins in this model system has slowed progress in identifying key signaling molecules. With the advances in mass spectrometry, we present a method for identifying tyrosine phosphorylated proteins binding to GST-tagged SH2 domains in sea star cell lysates for downstream mass spectrometry analysis.

Extracellular Ca2+ Is Required for Fertilization in the African Clawed Frog, Xenopus laevis

Background

The necessity of extracellular Ca²⁺ for fertilization and early embryonic development in the African clawed frog, Xenopus laevis, is controversial. Ca²⁺ entry into X. laevis sperm is reportedly required for the acrosome reaction, yet fertilization and embryonic development have been documented to occur in high concentrations of the Ca²⁺ chelator BAPTA. Here we sought to resolve this controversy.

Methodology/principal finding

Using the appearance of cleavage furrows as an indicator of embryonic development, we found that X. laevis eggs inseminated in a solution lacking added divalent cations developed normally. By contrast, eggs inseminated in millimolar concentrations of BAPTA or EGTA failed to develop. Transferring embryos to varying solutions after sperm addition, we found that extracellular Ca²⁺ is specifically required for events occurring within the first 30 minutes after sperm addition, but not after. We found that the fluorescently stained sperm were not able to penetrate the envelope of eggs inseminated in high BAPTA, whereas several had penetrated the vitelline envelope of eggs inseminated without a Ca²⁺ chelator, or with BAPTA and saturating CaCl₂. Together these results indicate that fertilization does not occur in high concentrations of Ca²⁺ chelators. Finally, we found that the jelly coat includes >5 mM of readily diffusible Ca²⁺.

Conclusions/Significance

Taken together, these data are consistent with requirement of extracellular Ca²⁺ for fertilization. Based on our findings, we hypothesize that the jelly coat surrounding the egg acts as a reserve of readily available Ca²⁺ ions to foster fertilization in changing extracellular milieu.

Transmembrane signal transduction in oocyte maturation and fertilization: focusing on Xenopus laevis as a model animal

Fertilization is a cell biological phenomenon of crucial importance for the birth of new life in a variety of multicellular and sexual reproduction species such as algae, animal and plants. Fertilization involves a sequence of events, in which the female gamete "egg" and the male gamete "spermatozoon (sperm)" develop, acquire their functions, meet and fuse with each other, to initiate embryonic and zygotic development. Here, it will be briefly reviewed how oocyte cytoplasmic components are orchestrated to undergo hormone-induced oocyte maturation and sperm-induced activation of development. I then review how sperm-egg membrane interaction/fusion and activation of development in the fertilized egg are accomplished and regulated through egg coat- or egg plasma membrane-associated components, highlighting recent findings and future directions in the studies using Xenopus laevis as a model experimental animal.

Biotinylation of oocyte cell surface proteins of the starfish Patiria miniata

Understanding the signal transduction processes that occur during oocyte maturation and fertilization requires knowledge of the constituent proteins from the cell surface to relevant intracellular compartments. To identify starfish oocyte and egg cell surface proteins, a biotinylation method was adapted from prior protocols using B cells, leukocytes, mouse oocytes, and sea urchin eggs (Cole et al. Mol Immunol 24:699-705, 1987; Flaherty and Swann NJ. Mol Reprod Dev 35:285-292, 1993; Haley and Wessel. Dev Biol 272:191-202, 2004; Hurley et al. J Immunol Methods 85:195-202, 1985). This method utilizes the water-soluble Sulfo-NHS-Biotin, which does not cross the egg plasma membrane. The process of biotinylation does not appear to have any effect on the process of oocyte maturation or fertilization. Furthermore, it can be used with either vitelline-intact or vitelline-free oocytes and allows the proteins to be visualized successfully through immunoblotting, immunoprecipitation, or by scanning confocal microscopy.

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.

Comparative biology of sperm factors and fertilization-induced calcium signals across the animal kingdom

Fertilization causes mature oocytes or eggs to increase their concentrations of intracellular calcium ions (Ca²⁺) in all animals that have been examined, and such Ca²⁺ elevations, in turn, provide key activating signals that are required for non-parthenogenetic development. Several lines of evidence indicate that the Ca²⁺ transients produced during fertilization in mammals and other taxa are triggered by soluble factors that sperm deliver into oocytes after gamete fusion. Thus, for a broad-based analysis of Ca²⁺ dynamics during fertilization in animals, this article begins by summarizing data on soluble sperm factors in non-mammalian species, and subsequently reviews various topics related to a sperm-specific phospholipase C, called PLCζ, which is believed to be the predominant activator of mammalian oocytes. After characterizing initiation processes that involve sperm factors or alternative triggering mechanisms, the spatiotemporal patterns of Ca²⁺ signals in fertilized oocytes or eggs are compared in a taxon-by-taxon manner, and broadly classified as either a single major transient or a series of repetitive oscillations. Both solitary and oscillatory types of fertilization-induced Ca²⁺ signals are typically propagated as global waves that depend on Ca²⁺ release from the endoplasmic reticulum in response to increased concentrations of inositol 1,4,5-trisphosphate (IP₃). Thus, for taxa where relevant data are available, upstream pathways that elevate intraoocytic IP3 levels during fertilization are described, while other less-common modes of producing Ca²⁺ transients are also examined. In addition, the importance of fertilization-induced Ca²⁺ signals for activating development is underscored by noting some major downstream effects of these signals in various animals.

Calcium and egg activation in Drosophila

In many animals, a rise in intracellular calcium levels is the trigger for egg activation, the process by which an arrested mature oocyte transitions to prepare for embryogenesis. In nearly all animals studied to date, this calcium rise, and thus egg activation, is triggered by the fertilizing sperm. However in the insects that have been examined, fertilization is not necessary to activate their oocytes. Rather, these insects' eggs activate as they transit through the female's reproductive tract, regardless of male contribution. Recent studies in Drosophila have shown that egg activation nevertheless requires calcium and that the downstream events and molecules of egg activation are also conserved, despite the difference in initial trigger. Genetic studies have uncovered essential roles for the calcium-dependent enzyme calcineurin and its regulator calcipressin, and have hinted at roles for calmodulin, in Drosophila egg activation. Physiological and in vitro studies have led to a model in which mechanical forces that impact the Drosophila oocyte as it moves through the reproductive tract triggers the influx of calcium from the external environment, thereby initiating egg activation. Future research will aim to test this model, as well as to determine the spatiotemporal dynamics of cytoplasmic calcium flux and mode of signal propagation in this unique system.

Dynamics of PLCγ and Src family kinase 1 interactions during nuclear envelope formation revealed by FRET-FLIM

The nuclear envelope (NE) breaks down and reforms during each mitotic cycle. A similar process happens to the sperm NE following fertilisation. The formation of the NE in both these circumstances involves endoplasmic reticulum membranes enveloping the chromatin, but PLCγ-dependent membrane fusion events are also essential. Here we demonstrate the activation of PLCγ by a Src family kinase (SFK1) during NE assembly. We show by time-resolved FRET for the first time the direct in vivo interaction and temporal regulation of PLCγ and SFK1 in sea urchins. As a prerequisite for protein activation, there is a rapid phosphorylation of PLCγ on its Y783 residue in response to GTP in vitro. This phosphorylation is dependent upon SFK activity; thus Y783 phosphorylation and NE assembly are susceptible to SFK inhibition. Y783 phosphorylation is also observed on the surface of the male pronucleus (MPN) in vivo during NE formation. Together the corroborative in vivo and in vitro data demonstrate the phosphorylation and activation of PLCγ by SFK1 during NE assembly. We discuss the potential generality of such a mechanism.

Rho-kinase in sea urchin eggs and embryos

The activation of sea urchin eggs at fertilization provides an ideal system for studying the molecular events involved in cellular activation. Rho GTPases, which are key signaling enzymes in eukaryotes, are involved in sustaining the activation of sea urchin eggs; however, their downstream effectors have not yet been characterized. In somatic cells, RhoA regulates a serine/threonine kinase known as Rho-kinase (ROCK). The activity of ROCK in early sea urchin development has been inferred, but not tested directly. A ROCK gene was identified in the sea urchin (Strongylocentrotus purpuratus) genome and the sequence of its cDNA determined. The sea urchin ROCK (SpROCK) sequence predicts a protein of 158 kDa with >72% and 45% identities with different protein orthologues of the kinase catalytic domain and the complete protein sequence, respectively. SpROCK mRNA levels are high in unfertilized eggs and decrease to 35% after 15 min postfertilization and remain low up to the 4 cell stage. Antibodies to the human ROCK-I kinase domain revealed SpROCK to be concentrated in the cortex of eggs and early embryos. Co-immunoprecipitation assays indicate that RhoA and SpROCK are physically associated. This association is destroyed by treatment with the C3 exoenzyme and with the ROCK antagonist H-1152. H-1152 also inhibited DNA synthesis in embryos. We conclude that the Rho-dependent signaling pathway, via SpROCK, is essential for early embryonic development.

Gamete membrane microdomains and their associated molecules in fertilization signaling

Fertilization is the fundamental system of biological reproduction in many organisms, including animals, plants, and algae. A growing body of knowledge has emerged to explain how fertilization and activation of development are accomplished. Studies on the molecular mechanisms of fertilization are in progress for a wide variety of multicellular organisms. In this review, we summarize recent findings and debates about the long-standing questions concerning fertilization: how egg and sperm become competent for their interaction with each other, how the binding and fusion of these gamete cells are made possible, and how the fertilized eggs initiate development to a newborn. We will focus on the structure and function of the membrane microdomains (MDs) of egg and sperm that may serve as a platform or signaling center for the aforementioned cellular functions. In particular, we provide evidence that MDs of eggs from the African clawed frog, Xenopus laevis, play a pivotal role in receiving extracellular signals from fertilizing sperm and then transmitting them to the egg cytoplasm, where the tyrosine kinase Src is present and responsible for the subsequent signaling events collectively called egg activation. The presence of a new signaling axis involving uroplakin III, an MD-associated transmembrane protein, and Src in this system will be highlighted and discussed.

Signal transduction of fertilization in frog eggs and anti-apoptotic mechanism in human cancer cells: common and specific functions of membrane microdomains

Membrane microdomains or lipid/membrane rafts are distinct areas on the plasma membranes, where a specific subset of lipids (e.g. cholesterol, sphingolipids) and proteins (e.g. glycosylphosphatidylinositol-anchored proteins, growth factor receptor/kinases) are getting together and functioning for several aspects of cellular functions. Our recent investigation has revealed that fertilization of African clawed frog, Xenopus laevis, requires cholesterol-dependent nature of egg membrane microdomains. Moreover, fertilization of Xenopus eggs involves proteolytic cleavage of the extracellular part and subsequent phosphorylation of a cytoplasmic tyrosine residue of uroplakin III, an egg membrane microdomain-associated protein. Protease activity toward uroplakin III seems to be derived from fertilizing sperm, while phosphorylation of uroplakin III seems to be catalyzed by the egg tyrosine kinase Src, whose activation is required for cytoplasmic rearrangement of fertilized eggs; so-called 'egg activation'. Therefore, it is assumed that uroplakin III serves an integral part of signal transduction in fertilization of Xenopus. Our more recent study on human cancer cells has revealed that a similar but distinct scheme of signal transduction operates in anti-apoptotic growth of cells. Namely, in human bladder carcinoma cells, cooperation of uroplakin III and Src, both of which localize to the membrane microdomains, allows cells to escape from apoptotic cell death and proliferate under culture conditions deprived of serum. In this review, I briefly introduce about biology of fertilization and cancer, and then present and discuss our experimental data on general importance and specific features of membrane microdomains in Xenopus fertilization and anti-apoptosis in human bladder carcinoma cells.

The roles of Ca2+, downstream protein kinases, and oscillatory signaling in regulating fertilization and the activation of development

Reviews in Developmental Biology have covered the pathways that generate the all-important intracellular calcium (Ca(2+)) signal at fertilization [Miyazaki, S., Shirakawa, H., Nakada, K., Honda, Y., 1993a. Essential role of the inositol 1,4,5-trisphosphate receptor/Ca(2+) release channel in Ca(2+) waves and Ca(2+) oscillations at fertilization of mammalian eggs. Dev. Biol. 158, 62-78; Runft, L., Jaffe, L., Mehlmann, L., 2002. Egg activation at fertilization: where it all begins. Dev. Biol. 245, 237-254] and the different temporal responses of Ca(2+) in many organisms [Stricker, S., 1999. Comparative biology of calcium signaling during fertilization and egg activation in animals. Dev. Biol. 211, 157-176]. Those reviews raise the importance of identifying how Ca(2+) causes the events of egg activation (EEA) and to what extent these temporal Ca(2+) responses encode developmental information. This review covers recent studies that have analyzed how these Ca(2+) signals are interpreted by specific proteins, and how these proteins regulate various EEA responsible for the onset of development. Many of these proteins are protein kinases (CaMKII, PKC, MPF, MAPK, MLCK) whose activity is directly or indirectly regulated by Ca(2+), and whose amount increases during late oocyte maturation. We cover biochemical progress in defining the signaling pathways between Ca(2+) and the EEA, as well as discuss how oscillatory or multiple Ca(2+) signals are likely to have specific advantages biochemically and/or developmentally. These emerging concepts are put into historical context, emphasizing that key contributions have come from many organisms. The intricate interdependence of Ca(2+), Ca(2+)-dependent proteins, and the EEA raise many new questions for future investigations that will provide insight into the extent to which fertilization-associated signaling has long-range implications for development. In addition, answers to these questions should be beneficial to establishing parameters of egg quality for human and animal IVF, as well as improving egg activation protocols for somatic cell nuclear transfer to generate stem cells and save endangered species.

Mechanical stimulation by osmotic and hydrostatic pressure activates Drosophila oocytes in vitro in a calcium-dependent manner

Embryogenesis in vertebrates and marine invertebrates begins when a mature oocyte is fertilized, resulting in a rise in intracellular calcium (Ca(2+)) that activates development. Insect eggs activate without fertilization via an unknown signal imparted to the egg during ovulation or egg laying. One hypothesis for the activating signal is that deformation of eggs as they pass through a tight orifice provides a mechanical stimulus to trigger activation. Ovulation could produce two forms of mechanical stimulus: external pressure resulting from the passage of oocytes from the ovary into the narrow oviducts, and osmotic pressure caused by hydration-induced swelling of the oocyte within the oviducts. Ovulation could also trigger activation by placing the oocyte in a new environment that contains an activating substance, such as a particular ion. Here, we provide the first evidence that Drosophila oocytes require Ca(2+) for activation, and that activation can be triggered in vitro by mechanical stimuli, specifically osmotic and hydrostatic pressure. Our results suggest that activation in Drosophila is triggered by a mechanosensitive process that allows external Ca(2+) to enter the oocyte and drive the events of activation. This will allow exploitation of Drosophila genetics to dissect molecular pathways involving Ca(2+) and the activation of development.

Characterization of a sperm factor for egg activation at fertilization of the newt Cynops pyrrhogaster

Eggs of the newt, Cynops pyrrhogaster, arrested at the second meiotic metaphase are activated by sperm at fertilization and then complete meiosis to initiate development. We highly purified a sperm factor for egg activation from a sperm extract with several chromatographies. The purified fraction containing only a 45 kDa protein induced egg activation accompanied by an intracellular Ca2+ increase when injected into unfertilized eggs. Although injection of mouse phospholipase C (PLC) zeta-mRNA caused a Ca2+ increase and egg activation, partial amino acid sequences of the 45 kDa protein were homologous to those of Xenopus citrate synthase, but not to PLCs. An anti-porcine citrate synthase antibody recognized the 45 kDa protein both in the purified fraction and in the sperm extract. Treatment with the anti-citrate synthase antibody reduced the egg-activation activity in the sperm extract. Injection of porcine citrate synthase or mRNA of Xenopus citrate synthase induced a Ca2+ increase and caused egg activation. A large amount of the 45 kDa protein was localized in two lines elongated from the neck to the middle piece of sperm. These results indicate that the 45 kDa protein is a major component of the sperm factor for egg activation at newt fertilization.

Phospholipid binding properties and functional characterization of a sea urchin phospholipase Cdelta in urchin and mouse eggs

We recently identified a novel phospholipase Cdelta isoform, PLC-deltasu, in sea urchin gametes, whose precise functional role during fertilization and early embryogenesis remains unknown. Here, we characterized the binding of the PLC-deltasu PH domain to different phosphatidylinositol (PI) phospholipids and studied changes in its localization during fertilization. The PLC-deltasu PH domain bound most strongly to PI(3,4)P(2) and PI(3,5)P(2) phospholipids, in contrast to the PLCdelta1 PH domain which bound predominantly to PI(4,5)P(2). A green fluorescent protein tagged PLC-deltasu PH domain localized to the plasma membrane and its localization increased at fertilization and following addition of a Ca(2+) ionophore. However, recombinant PLC-deltasu failed to cause Ca(2+) signals like those seen at fertilization, in mouse and sea urchin eggs. Our findings suggest that PLC-deltasu is unlikely to be directly involved in the process of egg activation but may play a role in mediating extracellular signals transmitted via the PI 3'-kinase pathway.

Characterization of Xenopus egg membrane microdomains containing uroplakin Ib/III complex: roles of their molecular interactions for subcellular localization and signal transduction

A single-transmembrane protein uroplakin III (UPIII) and its tetraspanin binding-partner uroplakin Ib (UPIb) are members of the UP proteins that were originally identified in mammalian urothelium. In Xenopus laevis eggs, these proteins: xUPIII and xUPIb, are components of the cholesterol-enriched membrane microdomains or "rafts" and involved in the sperm-egg membrane interaction and subsequent egg activation signaling via Src tyrosine kinase at fertilization. Here, we investigate whether the xUPIII-xUPIb complex is in close proximity to CD9, a tetraspanin that has been implicated in the sperm-egg fusion in the mouse and GM1, a ganglioside typically enriched in egg rafts. Preparation of the egg membrane microdomains using different non-ionic detergents (Brij 98 and Triton X-100), chemical cross-linking, co-immunoprecipitation, in vitro kinase assay and in vitro fertilization experiments demonstrated that GM1, but not CD9, is in association with the xUPIII-xUPIb complex and contributes to the sperm-dependent egg activation. Transfection experiments using HEK293 cells demonstrated that xUPIII and xUPIb localized efficiently to the cholesterol-dependent membrane microdomains when they were co-expressed, whereas co-expression of xUPIII and CD9, instead of xUPIb, did not show this effect. Furthermore, xUPIII and xUPIb were shown to suppress kinase activity of the wild type, but not a constitutively active form of, Xenopus Src protein co-expressed in HEK293 cells. These results provide novel insight into the molecular architecture of the egg membrane microdomains containing xUPIII, xUPIb and Src, which may contribute to the understanding of sperm-egg interaction and signaling during Xenopus fertilization.

A functional genomic and proteomic perspective of sea urchin calcium signaling and egg activation

The sea urchin egg has a rich history of contributions to our understanding of fundamental questions of egg activation at fertilization. Within seconds of sperm-egg interaction, calcium is released from the egg endoplasmic reticulum, launching the zygote into the mitotic cell cycle and the developmental program. The sequence of the Strongylocentrotus purpuratus genome offers unique opportunities to apply functional genomic and proteomic approaches to investigate the repertoire and regulation of Ca(2+) signaling and homeostasis modules present in the egg and zygote. The sea urchin "calcium toolkit" as predicted by the genome is described. Emphasis is on the Ca(2+) signaling modules operating during egg activation, but the Ca(2+) signaling repertoire has ramifications for later developmental events and adult physiology as well. Presented here are the mechanisms that control the initial release of Ca(2+) at fertilization and additional signaling components predicted by the genome and found to be expressed and operating in eggs at fertilization. The initial release of Ca(2+) serves to coordinate egg activation, which is largely a phenomenon of post-translational modifications, especially dynamic protein phosphorylation. Functional proteomics can now be used to identify the phosphoproteome in general and specific kinase targets in particular. This approach is described along with findings to date. Key outstanding questions regarding the activation of the developmental program are framed in the context of what has been learned from the genome and how this knowledge can be applied to functional studies.

Signal transduction pathways leading to Ca2+ release in a vertebrate model system: Lessons from Xenopus eggs

At fertilization, eggs unite with sperm to initiate developmental programs that give rise to development of the embryo. Defining the molecular mechanism of this fundamental process at the beginning of life has been a key question in cell and developmental biology. In this review, we examine sperm-induced signal transduction events that lead to release of intracellular Ca(2+), a pivotal trigger of developmental activation, during fertilization in Xenopus laevis. Recent data demonstrate that metabolism of inositol 1,4,5-trisphosphate (IP(3)), a second messenger for Ca(2+) release, is carefully regulated and involves phospholipase C (PLC) and the tyrosine kinase Src. Roles of other potential regulators in this pathway, such as phosphatidylinositol 3-kinase, heterotrimeric GTP-binding protein, phospholipase D (PLD) and phosphatidic acid (PA) are also discussed. Finally, we address roles of egg lipid/membrane microdomains or 'rafts' as a platform for the sperm-egg membrane interaction and subsequent signaling events of egg activation.