Egg activation at fertilization: where it all begins

Development of Recombinant PLC-Zeta Protein as a Therapeutic Intervention for the Clinical Treatment of Oocyte Activation Failure

The sperm-specific phospholipase C zeta (PLCζ) protein is widely considered as the predominant physiological stimulus for initiating the Ca2+ release responsible for oocyte activation during mammalian fertilization. The increasing number of genetic and clinical reports that directly link PLCζ defects and/or deficiencies with oocyte activation failure (OAF) necessitates the use of a powerful therapeutic intervention to overcome such cases of male factor infertility. Currently, in vitro fertilization (IVF) clinics treat OAF cases after intracytoplasmic sperm injection (ICSI) with Ca2+ ionophores. Despite their successful use, such chemical agents are unable to trigger the physiological pattern of Ca2+ oscillations. Moreover, the safety of these ionophores is not yet fully established. We have previously demonstrated that recombinant PLCζ protein can be successfully used to rescue failed oocyte activation, resulting in efficient blastocyst formation. Herein, we produced a maltose binding protein (MBP)-tagged recombinant human PLCζ protein capable of inducing Ca2+ oscillations in mouse oocytes similar to those observed at fertilization. Circular dichroism (CD) experiments revealed a stable, well-folded protein with a high helical content. Moreover, the recombinant protein could retain its enzymatic properties for at least up to 90 days after storage at -80 °C. Finally, a chick embryo model was employed and revealed that exposure of fertilized chicken eggs to MBP-PLCζ did not alter the embryonic viability when compared to the control, giving a first indication of its safety. Our data support the potential use of the MBP-PLCζ recombinant protein as an effective therapeutic tool but further studies are required prior to its use in a clinical setting.

Genetic mechanisms of fertilization failure and early embryonic arrest: a comprehensive review

BACKGROUND

Infertility and pregnancy loss are longstanding problems. Successful fertilization and high-quality embryos are prerequisites for an ongoing pregnancy. Studies have proven that every stage in the human reproductive process is regulated by multiple genes and any problem, at any step, may lead to fertilization failure (FF) or early embryonic arrest (EEA). Doctors can diagnose the pathogenic factors involved in FF and EEA by using genetic methods. With the progress in the development of new genetic technologies, such as single-cell RNA analysis and whole-exome sequencing, a new approach has opened up for us to directly study human germ cells and reproductive development. These findings will help us to identify the unique mechanism(s) that leads to FF and EEA in order to find potential treatments.

OBJECTIVE AND RATIONALE

The goal of this review is to compile current genetic knowledge related to FF and EEA, clarifying the mechanisms involved and providing clues for clinical diagnosis and treatment.

SEARCH METHODS

PubMed was used to search for relevant research articles and reviews, primarily focusing on English-language publications from January 1978 to June 2023. The search terms included fertilization failure, early embryonic arrest, genetic, epigenetic, whole-exome sequencing, DNA methylation, chromosome, non-coding RNA, and other related keywords. Additional studies were identified by searching reference lists. This review primarily focuses on research conducted in humans. However, it also incorporates relevant data from animal models when applicable. The results were presented descriptively, and individual study quality was not assessed.

OUTCOMES

A total of 233 relevant articles were included in the final review, from 3925 records identified initially. The review provides an overview of genetic factors and mechanisms involved in the human reproductive process. The genetic mutations and other genetic mechanisms of FF and EEA were systematically reviewed, for example, globozoospermia, oocyte activation failure, maternal effect gene mutations, zygotic genome activation abnormalities, chromosome abnormalities, and epigenetic abnormalities. Additionally, the review summarizes progress in treatments for different gene defects, offering new insights for clinical diagnosis and treatment.

WIDER IMPLICATIONS

The information provided in this review will facilitate the development of more accurate molecular screening tools for diagnosing infertility using genetic markers and networks in human reproductive development. The findings will also help guide clinical practice by identifying appropriate interventions based on specific gene mutations. For example, when an individual has obvious gene mutations related to FF, ICSI is recommended instead of IVF. However, in the case of genetic defects such as phospholipase C zeta1 (PLCZ1), actin-like7A (ACTL7A), actin-like 9 (ACTL9), and IQ motif-containing N (IQCN), ICSI may also fail to fertilize. We can consider artificial oocyte activation technology with ICSI to improve fertilization rate and reduce monetary and time costs. In the future, fertility is expected to be improved or restored by interfering with or supplementing the relevant genes.

Significant decrease of maternal mitochondria carryover using optimized spindle-chromosomal complex transfer

Mutations in mitochondrial DNA (mtDNA) contribute to a variety of serious multi-organ human diseases, which are strictly inherited from the maternal germline. However, there is currently no curative treatment. Attention has been focused on preventing the transmission of mitochondrial diseases through mitochondrial replacement (MR) therapy, but levels of mutant mtDNA can often unexpectedly undergo significant changes known as mitochondrial genetic drift. Here, we proposed a novel strategy to perform spindle-chromosomal complex transfer (SCCT) with maximal residue removal (MRR) in metaphase II (MII) oocytes, thus hopefully eliminated the transmission of mtDNA diseases. With the MRR procedure, we initially investigated the proportions of mtDNA copy numbers in isolated karyoplasts to those of individual oocytes. Spindle-chromosomal morphology and copy number variation (CNV) analysis also confirmed the safety of this method. Then, we reconstructed oocytes by MRR-SCCT, which well developed to blastocysts with minimal mtDNA residue and normal chromosomal copy numbers. Meanwhile, we optimized the manipulation order between intracytoplasmic sperm injection (ICSI) and SCC transfer and concluded that ICSI-then-transfer was conducive to avoid premature activation of reconstructed oocytes in favor of normal fertilization. Offspring of mice generated by embryos transplantation in vivo and embryonic stem cells derivation further presented evidences for competitive development competence and stable mtDNA carryover without genetic drift. Importantly, we also successfully accomplished SCCT in human MII oocytes resulting in tiny mtDNA residue and excellent embryo development through MRR manipulation. Taken together, our preclinical mouse and human models of the MRR-SCCT strategy not only demonstrated efficient residue removal but also high compatibility with normal embryo development, thus could potentially be served as a feasible clinical treatment to prevent the transmission of inherited mtDNA diseases.

Alternative signal pathways underly fertilization and egg activation in a fish with contrasting modes of spawning

BACKGROUND

The processes of fertilization and egg activation are vital for early embryogenesis. However, while the mechanisms associated with key events during these processes differ among species and modes of spawning, the signal pathways underlying these processes are opaque for many fishes, including economically important species.

RESULTS

We investigated phenotypic traits, ultrastructure and protein expression levels in the eggs of the topmouth culter (Culter alburnus), a protected and economically important freshwater fish that exhibits two spawning modes, producing semi-buoyant eggs and adhesive eggs. Unfertilized eggs of C. alburnus were examined, as well as eggs at fertilization and 30 min post fertilization. Our results showed that in semi-buoyant eggs, energy metabolism was activated at fertilization, followed by elevated protein expression of cytoskeleton and extracellular matrix (ECM)-receptor interactions that resulted in rapid egg swelling; a recognized adaptation for lotic habitats. In contrast, in adhesive eggs fertilization initiated the process of sperm-egg fusion and blocking of polyspermy, followed by enhanced protein expression of lipid metabolism and the formation of egg envelope adhesion and hardening, which are adaptive in lentic habitats.

CONCLUSION

Our findings indicate that alternative signal pathways differ between modes of spawning and timing during the key processes of fertilization and egg activation, providing new insights into the molecular mechanisms involved in adaptive early embryonic development in teleost fishes.

Current Approaches to and the Application of Intracytoplasmic Sperm Injection (ICSI) for Avian Genome Editing

Poultry are one of the most valuable resources for human society. They are also recognized as a powerful experimental animal for basic research on embryogenesis. Demands for the supply of low-allergen eggs and bioreactors have increased with the development of programmable genome editing technology. The CRISPR/Cas9 system has recently been used to produce transgenic animals and various animals in the agricultural industry and has also been successfully adopted for the modification of chicken and quail genomes. In this review, we describe the successful establishment of genome-edited lines combined with germline chimera production systems mediated by primordial germ cells and by viral infection in poultry. The avian intracytoplasmic sperm injection (ICSI) system that we previously established and recent advances in ICSI for genome editing are also summarized.

Xenopus laevis lack the critical sperm factor PLCζ

Fertilization of eggs from the African clawed frog Xenopus laevis is characterized by an increase in cytosolic calcium, a phenomenon that is also observed in other vertebrates such as mammals and birds. During fertilization in mammals and birds, the transfer of the soluble PLCζ from sperm into the egg is thought to trigger the release of calcium from the endoplasmic reticulum (ER). Injecting sperm extracts into eggs reproduces this effect, reinforcing the hypothesis that a sperm factor is responsible for calcium release and egg activation. Remarkably, this occurs even when sperm extracts from X. laevis are injected into mouse eggs, suggesting that mammals and X. laevis share a sperm factor. However, X. laevis lacks an annotated PLCZ1 gene, which encodes the PLCζ enzyme. In this study, we attempted to determine whether sperm from X. laevis express an unannotated PLCZ1 ortholog. We identified PLCZ1 orthologs in 11 amphibian species, including 5 that had not been previously characterized, but did not find any in either X. laevis or the closely related Xenopus tropicalis. Additionally, we performed RNA sequencing on testes obtained from adult X. laevis males and did not identify potential PLCZ1 orthologs in our dataset or in previously collected ones. These findings suggest that PLCZ1 may have been lost in the Xenopus lineage and raise the question of how fertilization triggers calcium release and egg activation in these species.

Changes in cortical endoplasmic reticulum clusters in the fertilized mouse oocyte†

Oocytes from many invertebrate and vertebrate species exhibit unique endoplasmic reticulum (ER) specializations (cortical ER clusters), which are thought to be essential for egg activation. In examination of cortical ER clusters, we observed that they were tethered to previously unreported fenestrae within the cortical actin layer. Furthermore, studies demonstrated that sperm preferentially bind to the plasma membrane overlying the fenestrae, establishing close proximity to underlying ER clusters. Moreover, following sperm-oocyte fusion, cortical ER clusters undergo a previously unrecognized global change in volume and shape that persists through sperm incorporation, before dispersing at the pronuclear stage. These changes did not occur in oocytes from females mated with Izumo1 -/- males. In addition to these global changes, highly localized ER modifications were noted at the sperm binding site as cortical ER clusters surround the sperm head during incorporation, then form a diffuse cloud surrounding the decondensing sperm nucleus. This study provides the first evidence that cortical ER clusters interact with the fertilizing sperm, indirectly through a previous unknown lattice work of actin fenestrae, and then directly during sperm incorporation. These observations raise the possibility that oocyte ER cluster-sperm interactions provide a competitive advantage to the oocyte, which may not occur during assisted reproductive technologies such as intracytoplasmic sperm injection.

Species-Specific Gamete Interaction during Sea Urchin Fertilization: Roles of the Egg Jelly and Vitelline Layer

In sea urchins, the sequence of the cellular and molecular events characterizing the fertilization process has been intensively studied. We have learned that to activate the egg, the fertilizing sperm must undergo morphological modifications (the acrosome reaction, AR) upon reaching the outer gelatinous layer enveloping the egg (egg jelly), which triggers the polymerization of F-actin on the sperm head to form the acrosomal process. The AR exposes bindin, an adhesive sperm protein essential for the species-specific interaction with the cognate receptor on the egg vitelline layer. To investigate the specific roles of the egg jelly and vitelline layer at fertilization of sea urchin eggs, Paracentrotus lividus eggs were incubated in acidic seawater, which removes the egg jelly, i.e., experimental conditions that should prevent the occurrence of the AR, and inseminated in the same medium. At variance with the prevailing view, our results have shown that these dejellied P. lividus eggs can still interact with sperm in acidic seawater, albeit with altered fertilization responses. In particular, the eggs deprived of the vitelline layer reacted with multiple sperm but with altered Ca2+ signals. The results have provided experimental evidence that the plasma membrane, and not the vitelline layer, is where the specific recognition between gametes occurs. The vitelline layer works in unfertilized eggs to prevent polyspermy.

Regulation of the Actin Cytoskeleton-Linked Ca2+ Signaling by Intracellular pH in Fertilized Eggs of Sea Urchin

In sea urchin, the immediate contact of the acrosome-reacted sperm with the egg surface triggers a series of structural and ionic changes in the egg cortex. Within one minute after sperm fuses with the egg plasma membrane, the cell membrane potential changes with the concurrent increases in intracellular Ca²⁺ levels. The consequent exocytosis of the cortical granules induces separation of the vitelline layer from the egg plasma membrane. While these cortical changes are presumed to prevent the fusion of additional sperm, the subsequent late phase (between 1 and 4 min after fertilization) is characterized by reorganization of the egg cortex and microvilli (elongation) and by the metabolic shift to activate de novo protein and DNA syntheses. The latter biosynthetic events are crucial for embryonic development. Previous studies suggested that the early phase of fertilization was not a prerequisite for these changes in the second phase since the increase in the intracellular pH induced by the exposure of unfertilized sea urchin eggs to ammonia seawater could start metabolic egg activation in the absence of the cortical granule exocytosis. In the present study, we have demonstrated that the incubation of unfertilized eggs in ammonia seawater induced considerable elongations of microvilli (containing actin filaments) as a consequence of the intracellular pH increase, which increased the egg’s receptivity to sperm and made the eggs polyspermic at fertilization despite the elevation of the fertilization envelope (FE). These eggs also displayed compromised Ca²⁺ signals at fertilization, as the amplitude of the cortical flash was significantly reduced and the elevated intracellular Ca²⁺ level declined much faster. These results have also highlighted the importance of the increased internal pH in regulating Ca²⁺ signaling and the microvillar actin cytoskeleton during the late phase of the fertilization process.

Gene mutations associated with fertilization failure after in vitro fertilization/intracytoplasmic sperm injection

Fertilization failure during assisted reproductive technologies (ART) is often unpredictable, as this failure is encountered only after in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) have been performed. The etiology of fertilization failure remains elusive. More and more mutations of genes are found to be involved in human fertilization failure in infertile patients as high throughput sequencing techniques are becoming widely applied. In this review, the mutations of nine important genes expressed in sperm or oocytes, PLCZ1, ACTL7A, ACTL9, DNAH17, WEE2, TUBB8, NLRP5, ZP2, and TLE6, were summarized and discussed. These abnormalities mainly have shown Mendelian patterns of inheritance, including dominant and recessive inheritance, although de novo mutations were present in some cases. The review revealed the crucial roles of each reported gene in the fertilization process and summarized all known mutations and their corresponding phenotypes. The review suggested the mutations might become promising targets for precision treatments in reproductive medicine. Moreover, our work will provide some helpful clues for genetic counseling, risk prediction, and optimizing clinical treatments for human infertility by supplying the useful and timely information on the genetic causes leading to fertilization failure.

Developmental series of gene expression clarifies maternal mRNA provisioning and maternal-to-zygotic transition in a reef-building coral

BACKGROUND

Maternal mRNA provisioning of oocytes regulates early embryogenesis. Maternal transcripts are degraded as zygotic genome activation (ZGA) intensifies, a phenomenon known as the maternal-to-zygotic transition (MZT). Here, we examine gene expression over nine developmental stages in the Pacific rice coral, Montipora capitata, from eggs and embryos at 1, 4, 9, 14, 22, and 36 h-post-fertilization (hpf), as well as swimming larvae (9d), and adult colonies.

RESULTS

Weighted Gene Coexpression Network Analysis revealed four expression peaks, identifying the maternal complement, two waves of the MZT, and adult expression. Gene ontology enrichment revealed maternal mRNAs are dominated by cell division, methylation, biosynthesis, metabolism, and protein/RNA processing and transport functions. The first MZT wave occurs from ~4-14 hpf and is enriched in terms related to biosynthesis, methylation, cell division, and transcription. In contrast, functional enrichment in the second MZT wave, or ZGA, from 22 hpf-9dpf, includes ion/peptide transport and cell signaling. Finally, adult expression is enriched for functions related to signaling, metabolism, and ion/peptide transport. Our proposed MZT timing is further supported by expression of enzymes involved in zygotic transcriptional repression (Kaiso) and activation (Sox2), which peak at 14 hpf and 22 hpf, respectively. Further, DNA methylation writing (DNMT3a) and removing (TET1) enzymes peak and remain stable past ~4 hpf, suggesting that methylome programming occurs before 4 hpf.

CONCLUSIONS

Our high-resolution insight into the coral maternal mRNA and MZT provides essential baseline information to understand parental carryover effects and the sensitivity of developmental success under increasing environmental stress.

Breeding of Cav2.3 deficient mice reveals Mendelian inheritance in contrast to complex inheritance in Cav3.2 null mutant breeding

High voltage-activated Ca_v2.3 R-type Ca²⁺ channels and low voltage-activated Ca_v3.2 T-type Ca²⁺ channels were reported to be involved in numerous physiological and pathophysiological processes. Many of these findings are based on studies in Ca_v2.3 and Ca_v3.2 deficient mice. Recently, it has been proposed that inbreeding of Ca_v2.3 and Ca_v3.2 deficient mice exhibits significant deviation from Mendelian inheritance and might be an indication for potential prenatal lethality in these lines. In our study, we analyzed 926 offspring from Ca_v3.2 breedings and 1142 offspring from Ca_v2.3 breedings. Our results demonstrate that breeding of Ca_v2.3 deficient mice shows typical Mendelian inheritance and that there is no indication of prenatal lethality. In contrast, Ca_v3.2 breeding exhibits a complex inheritance pattern. It might be speculated that the differences in inheritance, particularly for Ca_v2.3 breeding, are related to other factors, such as genetic specificities of the mutant lines, compensatory mechanisms and altered sperm activity.

Fertilization and development of Arbacia lixula eggs are affected by osmolality conditions

The sea urchin Arbacia lixula coexist with Paracentrotus lividus in the Mediterranean, but the two sea urchin species are quite different from each other. Concerning the female gamete, A. lixula eggs are much darker than those of P. lividus due to the characteristic pigmentation. Upon insemination, the fertilization envelope formed by A. lixula eggs is remarkably thinner than that of P. livius eggs, which implies that the cortical organization of the eggs in the two species may be quite different. In this communication, we examined the phenotypic plasticity of A. lixula eggs in the changing osmolality. The plasma membrane, cortical actin cytoskeleton and vesicles are extensively altered in the eggs exposed to 40% seawater for 15 min. When fertilized, the Ca²⁺ response in these eggs was significantly compromised and the sperm often failed to enter the eggs. Remarkably, the pattern of the Ca²⁺ response was restored when these eggs were transferring back to the natural seawater before fertilization, while the actin cytoskeleton partially reverted to the original state. Nonetheless, these eggs restored in seawater failed to regain the innate sperm receptivity that allows only one sperm to enter in natural seawater. Thus, the ability to guide monospermic fertilization is lost by water entry into the eggs, and the eggs incorporated either multiple or no sperm. On the other hand, eggs briefly exposed to hypertonic seawater exhibited no evident morphological anomaly. Nonetheless, the monospermic eggs that experienced a brief exposure (15 min) to hypertonic seawater prior to fertilization in natural seawater displayed a subtly altered sperm-induced Ca²⁺ response and morpho-functional anomaly around the pluteus stage. Our results suggest that A. lixula eggs attain only a limited extent of cytological plasticity, and that the osmolality shock affects the physical nature of the egg surface which in turn affects the developmental programming.

Mechanisms of doxorubicin-mediated reproductive toxicity - A review

The anticancer drug doxorubicin has been associated with several adverse side-effects including reproductive toxicity in both genders. The current review has complied the mechanisms of doxorubicin induced reproductive toxicity. The articles cited in the review were searched using Google Scholar, PubMed, Scopus, Science Direct. Doxorubicin treatment has been found to cause a decrease in testicular mass along with histopathological deformities, oligospermia and abnormalities in sperm morphology. Apart from severely affecting the normal physiological role of both Leydig cells and Sertoli cells, doxorubicin also causes chromosome abnormalities and affects DNA methylase enzyme. Testicular lipid metabolism has been found to be negatively affected by doxorubicin treatment resulting in altered profile of sphingolipids glycerophospholipids and neutral lipids. Dysregulation of 3β-hydroxysteroid dehydrogenase (3β-HSD) and 17β- hydroxysteroid dehydrogenase (17β-HSD) are strongly linked to testicular exposure to doxorubicin. Further, oxidative stress along with endoplasmic reticulum stress are also found to aggravate the male reproductive functioning in doxorubicin treated conditions. Several antioxidant enzymes such as superoxide dismutase, catalase, glutathione peroxidase (GPx) are downregulated by doxorubicin. It also disturbs the hormones of the hypothalamic-pituitary-gonadal (HPG)-axis including testosterone, luteinizing hormone, follicle stimulating hormone etc. In females, the drug disturbs folliculogenesis and oogenesis leading to failure of ovulation and uterine cycle. In rodent model the drug shortens pro-estrous and estrous phases. It was also found that doxorubicin causes mitochondrial dysfunction in oocytes with impaired calcium signaling along with ER stress. The goal of the present review is to comprehends various pathways due to which doxorubicin treatment promotes toxicity in male and female reproductive system.

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.

Supplementation with embryo chicken egg extract improves exercise performance and exerts anti-fatigue effects via AMPK/mTOR signalling pathway in mice

BACKGROUND

Embryo chicken egg is a nutritional supplement that has been used to enhance physical fitness and promote wound healing according to traditional Chinese medicine for many years. In this study, we evaluated the effects of embryo chicken egg extract (ECE) on the exercise performance and fatigue in mice and the underlying mechanisms.

RESULTS

The results indicated that ECE can prolong the exhaustive swimming time, decrease lactic acid, blood urea nitrogen, creatine kinase, and malondialdehyde levels, and increase superoxide dismutase, glutathione peroxidase, and glycogen levels. Additionally, ECE can also regulate the balance of oxidative stress via the adenosine monophosphate activated protein kinase/mammalian target of rapamycin signalling pathway.

CONCLUSION

Taken together, these results showed that ECE can improve exercise performance and reduce physical fatigue in mice, which indicates that ECE can be used as a potential supplement to reduce physical fatigue. © 2020 Society of Chemical Industry.

Electrofusion Stimulation Is an Independent Factor of Chromosome Abnormality in Mice Oocytes Reconstructed via Spindle Transfer

Oocytes reconstructed by spindle transfer (ST) are prone to chromosome abnormality, which is speculated to be caused by mechanical interference or premature activation, the mechanism is controversial. In this study, C57BL/6N oocytes were used as the model, and electrofusion ST was performed under normal conditions, Ca²⁺ free, and at room temperature, respectively. The effect of enucleation and electrofusion stimulation on MPF activity, spindle morphology, γ-tubulin localization and chromosome arrangement was compared. We found that electrofusion stimulation could induce premature chromosome separation and abnormal spindle morphology and assembly by decreasing the MPF activity, leading to premature activation, and thus resulting in chromosome abnormality in oocytes reconstructed via ST. Electrofusion stimulation was an independent factor of chromosome abnormality in oocytes reconstructed via ST, and was not related to enucleation, fusion status, temperature, or Ca²⁺. The electrofusion stimulation number should be minimized, with no more than 2 times being appropriate. As the electrofusion stimulation number increased, several typical abnormalities in chromosome arrangement and spindle assembly occurred. Although blastocyst culture could eliminate embryos with chromosomal abnormalities, it would significantly decrease the number of normal embryos and reduce the availability of embryos. The optimum operating condition for electrofusion ST was the 37°C group without Ca²⁺.

Inositol-1,4,5-Trisphosphate Receptor-1 and -3 and Ryanodine Receptor-3 May Increase Ooplasmic Ca²⁺ During Quail Egg Activation

We previously reported that egg activation in Japanese quail is driven by two distinct types of intracellular Ca²⁺ ([Ca²⁺]i): transient elevations in [Ca²⁺]i induced by phospholipase Czeta 1 (PLCZ1) and long-lasting spiral-like Ca²⁺ oscillations by citrate synthase (CS) and aconitate hydratase 2 (ACO2). Although the blockade of inositol 1,4,5-trisphosphate receptors (ITPRs) before microinjections of PLCZ1, CS, and ACO2 cRNAs only prevented transient increases in [Ca²⁺]i, a microinjection of an agonist of ryanodine receptors (RYRs) induced spiral-like Ca²⁺ oscillations, indicating the involvement of both ITPRs and RYRs in these events. In this study, we investigated the isoforms of ITPRs and RYRs responsible for the expression of the two types of [Ca²⁺]i increases. RT-PCR and western blot analyses revealed that ITPR1, ITPR3, and RYR3 were expressed in ovulated eggs. These proteins were degraded 3 h after the microinjection of PLCZ1, CS, and ACO2 cRNAs, which is the time at which egg activation was complete. However, degradation of ITPR1 and ITPR3, but not RYR3, was initiated 30 min after a single injection of PLCZ1 cRNA, corresponding to the time of the initial Ca²⁺ wave termination. In contrast, RYR3 degradation was observed 3 h after the microinjection of CS and ACO2 cRNAs. These results indicate that ITPRs and RYR3 differentially mediate in creases in [Ca²⁺]i during egg activation in Japanese quail, and that downregulation of ITPRs and RYR3-mediated events terminate the initial Ca²⁺ wave and spiral-like Ca²⁺ oscillations, respectively.

Phospholipase Cζ (PLCζ) versus postacrosomal sheath WW domain-binding protein (PAWP): Which molecule will survive as a sperm factor?

During mammalian fertilization, sperm is fused with the oocyte's membrane, triggering the resumption of meiosis from the metaphase II arrest, the extrusion of the second polar body, and the exocytosis of cortical granules; these events are collectively called 'oocyte activation.' In all species studied to date, the transient rise in the cytosolic level of calcium (in particular, the repeated calcium increases called 'calcium oscillations' in mammals) is required for these events. Researchers have focused on identifying the factor(s) that can induce calcium oscillations during fertilization. Sperm‐specific phospholipase C, i.e., PLC zeta (PLCζ), is a strong candidate of the factor(s), and several research groups using different species obtained evidence that PLCζ is a sperm factor that can induce calcium oscillations during fertilization. However, postacrosomal sheath Tryptophan‐Tryptophan (WW)—domain‐binding protein (PAWP) was recently shown to have a pivotal role in inducing calcium oscillations in some species. In this review, we focus on PLCζ and PAWP as sperm factors, and we discuss this controversy: Which of these two molecules survives as a sperm factor?

Effects of Salinity and pH of Seawater on the Reproduction of the Sea Urchin Paracentrotus lividus

AbstractFertilization and early development are usually the most vulnerable stages in the life of marine animals, and the biological processes during this period are highly sensitive to the environment. In nature, sea urchin gametes are shed in seawater, where they undergo external fertilization and embryonic development. In a laboratory, it is possible to follow the exact morphological and biochemical changes taking place in the fertilized eggs and the developing embryos. Thus, observation of successful fertilization and the subsequent embryonic development of sea urchin eggs can be used as a convenient biosensor to assess the quality of the marine environment. In this paper, we have examined how salinity and pH changes affect the normal fertilization process and the following development of Paracentrotus lividus. The results of our studies using confocal microscopy, scanning and transmission electron microscopy, and time-lapse Ca²⁺ image recording indicated that both dilution and acidification of seawater have subtle but detrimental effects on many aspects of the fertilization process. They include Ca²⁺ signaling and coordinated actin cytoskeletal changes, leading to a significantly reduced rate of successful fertilization and, eventually, to abnormal or delayed embryonic development.

Zinc protection of fertilized eggs is an ancient feature of sexual reproduction in animals

One of the earliest and most prevalent barriers to successful reproduction is polyspermy, or fertilization of an egg by multiple sperm. To prevent these supernumerary fertilizations, eggs have evolved multiple mechanisms. It has recently been proposed that zinc released by mammalian eggs at fertilization may block additional sperm from entering. Here, we demonstrate that eggs from amphibia and teleost fish also release zinc. Using Xenopus laevis as a model, we document that zinc reversibly blocks fertilization. Finally, we demonstrate that extracellular zinc similarly disrupts early embryonic development in eggs from diverse phyla, including Cnidaria, Echinodermata, and Chordata. Our study reveals that a fundamental strategy protecting human eggs from fertilization by multiple sperm may have evolved more than 650 million years ago.

Label-free proteomic analysis reveals the differentiation between unfertilized and fertilized Beijing-You chicken eggs

Egg fertilization is a dynamic process, including varieties of biochemical changes. To better understand the molecular mechanisms during the egg embryo development, the objective of this study was to quantify protein expression changes between fertilized and unfertilized Beijing-You chicken eggs using label-free liquid chromatography-tandem mass spectrometry method. The results showed that a total of 1241 proteins were identified from fertilized and unfertilized eggs, 229 proteins were observed difference in fertilized eggs (p < 0.05) compared with that in unfertilized eggs. The expressions of 86 proteins were up-regulated and 48 proteins were down-regulated in fertilized eggs. STRING database analysis and Gene Ontology analysis results showed that these differentially expressed proteins significantly interacted and were involved in lipid transport and inflammatory response biological processes. The mRNA and protein expression levels of most differentially expressed proteins Apolipoprotein B, Fibrinogen alpha chain, Transferrin receptor protein 1, Phospholipid transfer protein and Vimentin were validated by RT-PCR and western blot. These results could provide possible novel insights for the molecular mechanism of egg fertilization.

Reactive oxygen species in reproduction: harmful, essential or both?

The process of embryonic development is crucial and radically influences preimplantation embryo competence. It involves oocyte maturation, fertilization, cell division and blastulation and is characterized by different key phases that have major influences on embryo quality. Each stage of the process of preimplantation embryonic development is led by important signalling pathways that include very many regulatory molecules, such as primary and secondary messengers. Many studies, both in vivo and in vitro, have shown the importance of the contribution of reactive oxygen species (ROS) as important second messengers in embryo development. ROS may originate from embryo metabolism and/or oocyte/embryo surroundings, and their effect on embryonic development is highly variable, depending on the needs of the embryo at each stage of development and on their environment (in vivo or under in vitro culture conditions). Other studies have also shown the deleterious effects of ROS in embryo development, when cellular tissue production overwhelms antioxidant production, leading to oxidative stress. This stress is known to be the cause of many cellular alterations, such as protein, lipid, and DNA damage. Considering that the same ROS level can have a deleterious effect on the fertilizing oocyte or embryo at certain stages, and a positive effect at another stage of the development process, further studies need to be carried out to determine the rate of ROS that benefits the embryo and from what rate it starts to be harmful, this measured at each key phase of embryonic development.

Cellular and molecular aspects of oocyte maturation and fertilization: a perspective from the actin cytoskeleton

ABSTRACT

Much of the scientific knowledge on oocyte maturation, fertilization, and embryonic development has come from the experiments using gametes of marine organisms that reproduce by external fertilization. In particular, echinoderm eggs have enabled the study of structural and biochemical changes related to meiotic maturation and fertilization owing to the abundant availability of large and transparent oocytes and eggs. Thus, in vitro studies of oocyte maturation and sperm-induced egg activation in starfish are carried out under experimental conditions that resemble those occurring in nature. During the maturation process, immature oocytes of starfish are released from the prophase of the first meiotic division, and acquire the competence to be fertilized through a highly programmed sequence of morphological and physiological changes at the oocyte surface. In addition, the changes in the cortical and nuclear regions are essential for normal and monospermic fertilization. This review summarizes the current state of research on the cortical actin cytoskeleton in mediating structural and physiological changes during oocyte maturation and sperm and egg activation in starfish and sea urchin. The common denominator in these studies with echinoderms is that exquisite rearrangements of the egg cortical actin filaments play pivotal roles in gamete interactions, Ca2+ signaling, exocytosis of cortical granules, and control of monospermic fertilization. In this review, we also compare findings from studies using invertebrate eggs with what is known about the contributions made by the actin cytoskeleton in mammalian eggs. Since the cortical actin cytoskeleton affects microvillar morphology, movement, and positioning of organelles and vesicles, and the topography of the egg surface, these changes have impacts on the fertilization process, as has been suggested by recent morphological studies on starfish oocytes and eggs using scanning electron microscopy. Drawing the parallelism between vitelline layer of echinoderm eggs and the zona pellucida of mammalian eggs, we also discuss the importance of the egg surface in mediating monospermic fertilization.

GRAPHICAL ABSTRACT

Toward the understanding of biology of oocyte life cycle in Xenopus Laevis: No oocytes left behind

BACKGROUND

For the past more than 25 years, we have been focusing on the developmental and reproductive biology of the female gametes, oocytes, and eggs, of the African clawed frog Xenopus laevis.

METHODS

The events associated with the life cycle of these cells can be classified into the four main categories: first, oogenesis and cell growth in the ovary during the first meiotic arrest; second, maturation and ovulation that occur simultaneously and result in the acquisition of fertilization competence and the second meiotic arrest; third, fertilization, that is sperm-induced transition from egg to zygote; and fourth, egg death after spontaneous activation in the absence of fertilizing sperm.

MAIN FINDINGS

Our studies have demonstrated that signal transduction system involving tyrosine kinase Src and other oocyte/egg membrane-associated molecules such as uroplakin III and some other cytoplasmic proteins such as mitogen-activated protein kinase (MAPK) play important roles for successful ovulation, maturation, fertilization, and initiation of embryonic development.

CONCLUSION

We summarize recent advances in understanding cellular and molecular mechanisms underlying life cycle events of the oocytes and eggs. Our further intention is to discuss and predict potentially promising impact of the recent findings on the challenges facing reproductive biology and medicine, as well as societal contexts.

Ion channels and signaling pathways used in the fast polyspermy block

Fertilization of an egg by multiple sperms, polyspermy, is lethal to most sexually reproducing species. To combat the entry of additional sperm into already fertilized eggs, organisms have developed various polyspermy blocks. One such barrier, the fast polyspermy block, uses a fertilization-activated depolarization of the egg membrane to electrically inhibit supernumerary sperm from entering the egg. The fast block is commonly used by eggs of oviparous animals with external fertilization. In this review, we discuss the history of the fast block discovery, as well as general features shared by all organisms that use this polyspermy block. Given the diversity of habitats of external fertilizers, the fine details of the fast block-signaling pathways differ drastically between species, including the identity of the depolarizing ions. We highlight the known molecular mediators of these signaling pathways in amphibians and echinoderms, with a fine focus on ion channels that signal these fertilization-evoked depolarizations. We also discuss the investigation for a fast polyspermy block in mammals and teleost fish, and we outline potential fast block triggers. Since the first electrical recordings made on eggs in the 1950s, the fields of developmental biology and electrophysiology have substantially matured, and yet we are only now beginning to discern the intricate molecular mechanisms regulating the fast block to polyspermy.

Protein phosphatase 2A is essential to maintain meiotic arrest, and to prevent Ca2+ burst at spawning and eventual parthenogenesis in the larvacean Oikopleura dioica

Unfertilized eggs of most animals are arrested at a certain point in the meiotic cell cycles. Reinitiation of meiosis and the start of embryogenesis are triggered by fertilization. This arrest is essential for preventing parthenogenetic activation and for promoting proper initiation of development by fertilization. In the larvacean Oikopleura dioica, which is a simple model organism for studies of chordate development, the unfertilized egg is arrested at metaphase of meiosis I. We show here that protein phosphatase 2A (PP2A) is essential for maintenance of meiotic arrest after spawning of oocytes. Knockdown (KD) of the maternal PP2A catalytic subunit, which was found in functional screening of maternal factors, caused unfertilized eggs to spontaneously release polar bodies after spawning, and then start pseudo-cleavages without fertilization, namely, parthenogenesis. Parthenogenetic embryos failed to undergo proper mitosis and cytokinesis because of lack of a centrosome, which is to be brought into the egg by a sperm. Activation of the KD oocytes was triggered by possible rise of ambient and intracellular pH upon their release from the gonad into seawater at spawning. Live recording of intracellular calcium level of the KD oocytes indicated that the pH rise caused an aberrant Ca²⁺ burst, which mimicked the Ca²⁺ burst that occurs at fertilization. Then, the aberrant Ca²⁺ burst triggered meiosis resumption through Calcium/calmodulin-dependent protein kinase (CaMK II). Therefore, PP2A is essential for maintenance of meiotic arrest and prevention of parthenogenesis by suppressing the aberrant Ca²⁺ burst at spawning.

Sodium-mediated fast electrical depolarization does not prevent polyspermic fertilization in Paracentrotus lividus eggs

During sea urchins fertilization, the activating spermatozoon triggers a series of physiological changes that transforms the quiescent egg into a dynamic zygote. It has been suggested that several of these egg activation events, e.g. sperm-induced plasma membrane depolarization and the Ca2+-linked cortical reaction, play additional roles to prevent the entry of supernumerary spermatozoa. In particular, the abrupt shift in egg membrane potential at fertilization, which is sustained by a Na+ influx, has been considered as a fast mechanism to block polyspermy. To test the relevance of the Na+-mediated fast electrical block to polyspermy, we fertilized sea urchin eggs in artificial seawater with a low concentration of Na+; nearly all the eggs were still monospermic, as judged by the number of Hoechst 33422-stained sperm. When fertilized in normal seawater, eggs that were pre-incubated in the low Na+ medium exhibited impaired elevation of the fertilization envelope. Nevertheless, these eggs manifested entry of a single spermatozoon, suggesting that the fertilization envelope was not the primary determinant of the block to polyspermy. Furthermore, we showed that the abnormal cleavage patterns displayed by eggs pre-incubated in low Na+, which were often considered a hallmark of polyspermy, were due to the alterations in the cortical actin filaments dynamics following fertilization, and not to the formation of multipolar spindles associated with supernumerary sperm centrosomes. Hence, our results suggested that Paracentrotus lividus eggs do not utilize Na+ to rapidly prevent additional spermatozoa from entering the egg, at variance with the hypothesis of an electrical fast block to polyspermy.

Cycling through mammalian meiosis: B-type cyclins in oocytes

B-type cyclins in association with Cdk1 mediate key steps of mitosis and meiosis, by phosphorylating a plethora of substrates. Progression through the meiotic cell cycle requires the execution of two cell divisions named meiosis I and II without intervening S-phase, to obtain haploid gametes. These two divisions are highly asymmetric in the large oocyte. Chromosome segregation in meiosis I and sister chromatid segregation in meiosis II requires the sharp, switch-like inactivation of Cdk1 activity, which is brought about by degradation of B-type cyclins and counteracting phosphatases. Importantly and contrary to mitosis, inactivation of Cdk1 must not allow S-phase to take place at exit from meiosis I. Here, we describe recent studies on the regulation of translation and degradation of B-type cyclins in mouse oocytes, and how far their roles are redundant or specific, with a special focus on the recently discovered oocyte-specific role of cyclin B3.

Gap junction-dependent coordination of intercellular calcium signalling in the developing appendicularian tunicate Oikopleura dioica

We characterized spontaneous Ca2+ signals in Oikopleura dioica embryos from pre-fertilization to gastrula stages following injection of GCaMP6 mRNA into unfertilized eggs. The unfertilized egg exhibited regular, transient elevations in intracellular Ca2+ concentration with an average duration of 4-6 s and an average frequency of about 1 every 2.5 min. Fertilization was accompanied by a longer Ca2+ transient that lasted several minutes. Thereafter, regular Ca2+ transients were reinstated that spread within seconds among blastomeres and gradually increased in duration (by about 50%) and decreased in frequency (by about 20%) by gastrulation. Peak amplitudes also exhibited a dynamic, with a transitory drop occurring at about the 4-cell stage and a subsequent rise. Each peak was preceded by about 15 s by a smaller and shorter Ca2+ increase (about 5% of the main peak amplitude, average duration 3 s), which we term the "minipeak". By gastrulation, Ca2+ transients exhibited a stereotyped initiation site on either side of the 32-64-cell embryo, likely in the nascent muscle precursor cells, and spread thereafter symmetrically in a stereotyped spatial pattern that engaged blastomeres giving rise to all the major tissue lineages. The rapid spread of the transients relative to the intertransient interval created a coordinated wave that, on a coarse time scale, could be considered an approximate synchronization. Treatment with the divalent cations Ni2+ or Cd2+ gradually diminished peak amplitudes, had only moderate effects on wave frequency, but markedly disrupted wave synchronization and normal development. The T-type Ca2+ channel blocker mibefradil similarly disrupted normal development, and eliminated the minipeaks, but did not affect wave synchronization. To assess the role of gap junctions in calcium wave spread and coordination, we first characterized the expression of two Oikopleura connexins, Od-CxA and Od-CxB, both of which are expressed during pre-gastrulation and gastrula stages, and then co-injected double-stranded inhibitory RNAs together with CGaMP6 to suppress connexin expression. Connexin mRNA knockdown led to a gradual increase in Ca2+ transient peak width, a decrease of interpeak interval and a marked disruption of wave synchronization. As seen with divalent cations and mibefradil, this desynchronization was accompanied by a disruption of normal development.

Ion Channel Function During Oocyte Maturation and Fertilization

The proper maturation of both male and female gametes is essential for supporting fertilization and the early embryonic divisions. In the ovary, immature fully-grown oocytes that are arrested in prophase I of meiosis I are not able to support fertilization. Acquiring fertilization competence requires resumption of meiosis which encompasses the remodeling of multiple signaling pathways and the reorganization of cellular organelles. Collectively, this differentiation endows the egg with the ability to activate at fertilization and to promote the egg-to-embryo transition. Oocyte maturation is associated with changes in the electrical properties of the plasma membrane and alterations in the function and distribution of ion channels. Therefore, variations on the pattern of expression, distribution, and function of ion channels and transporters during oocyte maturation are fundamental to reproductive success. Ion channels and transporters are important in regulating membrane potential, but also in the case of calcium (Ca2+), they play a critical role in modulating intracellular signaling pathways. In the context of fertilization, Ca2+ has been shown to be the universal activator of development at fertilization, playing a central role in early events associated with egg activation and the egg-to-embryo transition. These early events include the block of polyspermy, the completion of meiosis and the transition to the embryonic mitotic divisions. In this review, we discuss the role of ion channels during oocyte maturation, fertilization and early embryonic development. We will describe how ion channel studies in Xenopus oocytes, an extensively studied model of oocyte maturation, translate into a greater understanding of the role of ion channels in mammalian oocyte physiology.

Single Ca2+ transients vs oscillatory Ca2+ signaling for assisted oocyte activation: limitations and benefits

Oocyte activation is a calcium (Ca²⁺)-dependent process that has been investigated in depth, in particular, regarding its impact on assisted reproduction technology (ART). Following a standard model of signal transduction, Ca²⁺ drives the meiotic progression upon fertilization in all species studied to date. However, Ca²⁺ changes during oocyte activation are species specific, and they can be classified in two modalities based on the pattern defined by the Ca²⁺ signature: a single Ca²⁺ transient (e.g. amphibians) or repetitive Ca²⁺ transients called Ca²⁺ oscillations (e.g. mammals). Interestingly, assisted oocyte activation (AOA) methods have highlighted the ability of mammalian oocytes to respond to single Ca²⁺ transients with normal embryonic development. In this regard, there is evidence supporting that cellular events during the process of oocyte activation are initiated by different number of Ca²⁺ oscillations. Moreover, it was proposed that oocyte activation and subsequent embryonic development are dependent on the total summation of the Ca²⁺ peaks, rather than to a specific frequency pattern of Ca²⁺ oscillations. The present review aims to demonstrate the complexity of mammalian oocyte activation by describing the series of Ca²⁺-linked physiological events involved in mediating the egg-to-embryo transition. Furthermore, mechanisms of AOA and the limitations and benefits associated with the application of different activation agents are discussed.

The role and mechanism of action of sperm PLC-zeta in mammalian fertilisation

At mammalian fertilisation, the fundamental stimulus that triggers oocyte (egg) activation and initiation of early embryonic development is an acute rise of the intracellular-free calcium (Ca²⁺) concentration inside the egg cytoplasm. This essential Ca²⁺ increase comprises a characteristic series of repetitive Ca²⁺ oscillations, starting soon after sperm-egg fusion. Over the last 15 years, accumulating scientific and clinical evidence supports the notion that the physiological stimulus that precedes the cytosolic Ca²⁺ oscillations is a novel, testis-specific phospholipase C (PLC) isoform, known as PLC-zeta (PLCζ). Sperm PLCζ catalyses the hydrolysis of phosphatidylinositol 4,5-bisphosphate triggering cytosolic Ca²⁺ oscillations through the inositol 1,4,5-trisphosphate signalling pathway. PLCζ is the smallest known mammalian PLC isoform with the most elementary domain organisation. However, relative to somatic PLCs, the PLCζ isoform possesses a unique potency in stimulating Ca²⁺ oscillations in eggs that is attributed to its novel biochemical characteristics. In this review, we discuss the latest developments that have begun to unravel the vital role of PLCζ at mammalian fertilisation and decipher its unique mechanism of action within the fertilising egg. We also postulate the significant potential diagnostic and therapeutic capacity of PLCζ in alleviating certain types of male infertility.

Sperm-oocyte contact induces outside-in signaling via PYK2 activation

Fertilization is a multi-step process that begins with plasma membrane interactions that enable sperm - oocyte binding followed by fusion of the sperm and oocyte plasma membranes. Once membrane fusion has occurred, sperm incorporation involves actin remodeling events within the oocyte cortex that allow the sperm head to penetrate the cortical actin layer and gain access to the ooplasm. Despite the significance for reproduction, the control mechanisms involved in gamete binding, fusion, and sperm incorporation are poorly understood. While it is known that proline - rich tyrosine kinase 2 (PYK2 or PTK2b) kinase activity plays an important role in fertilization, its specific function has not been addressed. The present study made use of a zona-free mouse oocyte fertilization assay to investigate the relationship between PYK2 activity and sperm - oocyte binding and fusion, as well as localized changes in actin polymerization and sperm incorporation. In this assay, the majority of bound sperm had no apparent effect on the oocyte and only a few became incorporated into the ooplasm. However, a subset of bound sperm were associated with a localized response in which PYK2 was recruited to the oocyte cortex where it frequently co-localized with a ring or disk of f-actin. The frequency of sperm-oocyte binding sites that exhibited this actin response was reduced in pyk2-/- oocytes and the pyk2-/- oocytes proved less efficient at incorporating sperm, indicating that this protein kinase may have an important role in sperm incorporation. The response of PYK2 to sperm-oocyte interaction appeared unrelated to gamete fusion since PYK2 was recruited to sperm - binding sites under conditions where sperm - oocyte fusion was prevented and since PYK2 suppression or ablation did not prevent sperm - oocyte fusion. While a direct correlation between the PYK2 response in the oocyte and the successful incorporation of individual bound sperm remains to be established, these findings suggest a model in which the oocyte is not a passive participant in fertilization, but instead responds to sperm contact by localized PYK2 signaling that promotes actin remodeling events required to physically incorporate the sperm head into the ooplasm.

Signaling-Mediated Regulation of Meiotic Prophase I and Transition During Oogenesis

Generation of healthy oocytes requires coordinated regulation of multiple cellular events and signaling pathways. Oocytes undergo a unique developmental growth and differentiation pattern interspersed with long periods of arrest. Oocytes from almost all species arrest in prophase I of oogenesis that allows for long period of growth and differentiation essential for normal oocyte development. Depending on species, oocytes that transit from prophase I to meiosis I also arrest at meiosis I for fairly long periods of time and then undergo a second arrest at meiosis II that is completed upon fertilization. While there are species-specific differences in C. elegans, D. melanogaster, and mammalian oocytes in stages of prophase I, meiosis I, or meiosis II arrest, in all cases cell signaling pathways coordinate the developmental events controlling oocyte growth and differentiation to regulate these crucial phases of transition. In particular, the ERK MAP kinase signaling pathway, cyclic AMP second messengers, and the cell cycle regulators CDK1/cyclin B are key signaling pathways that seem evolutionarily conserved in their control of oocyte growth and meiotic maturation across species. Here, I identify the common themes and differences in the regulation of key meiotic events during oocyte growth and maturation.

Oocyte viability and cortical activation under different salt solutions in Prochilodus lineatus (Teleostei: Prochilodontidae)

This study aimed to evaluate the effect of five salt solutions in the maintenance of morphological features of cortical alveolus, hydration and fertilization capacity of Prochilodus lineatus oocytes. For this purpose, five saline solutions were tested: Ringer's solution, Ringer's lactate solution, Hank's balanced salt solution (HBSS), Hank's balanced salt solution without calcium (HBSS without calcium) and solution for salmonid eggs. Oocytes were maintained for 2 hr in saline solution with controlled temperature subsequently evaluated for hydration, cortical activation and fertilization ability. In the evaluation of the fertilization ability, two controls were used: C1-fertilized oocytes after extrusion-and C2-oocytes kept in ovarian fluid and fertilized after 2 hr. There was a significant reduction in the viability of oocytes C2 (28.8% ± 12.9%) compared to C1 (65.3% ± 26.7%), and no significant differences were found between treatments HBSS and HBSS without calcium and C2. Only HBSS and HBSS without calcium maintained the non-activated state of the gametes, with a fertilization rate of 16.4% ± 6.7% and 5.6% ± 2.3%, respectively; however, they did not extend the viability of oocytes, such that they continued to undergo degradation during the storage period, similar to oocytes retained only in ovarian fluid.

Effects of Simulated Mobile Phone Electromagnetic Radiation on Fertilization and Embryo Development

This study investigated the effects of 935-MHz electromagnetic radiation (ER) on fertilization and subsequent embryonic development in mice. Ovulating mice were irradiated at three ER intensities for 4 h/day (d) or 2 h/d for three consecutive days; the ova were then harvested for in vitro fertilization to observe the 6-h fertilization rate (6-FR), 72-h morula rate (72-MR), and 110-h blastula rate (110-BR). Compared with the control group, the 6-FR, 72-MR, and 110-BR were decreased in the low ER intensity group, but the differences were not significant; in the mid- and high-intensity ER groups, 72-MR and 110-BR in the 4 h/d and 2 h/d subgroups were decreased, showing significant differences compared with the control group. Moreover, the comparison between 4 h/d and 2 h/d subgroups showed significant differences. Mid- and high-intensity ER at 935 MHz can reduce the fertilization rate in mice, and reduce the blastulation rate, thus reducing the possibility of embryo implantation.

Imaging early embryonic calcium activity with GCaMP6s transgenic zebrafish

Intracellular Ca²⁺ signaling regulates cellular activities during embryogenesis and in adult organisms. We generated stable Tg[βactin2:GCaMP6s]^stl351 and Tg[ubi:GCaMP6s]^stl352 transgenic lines that combine the ubiquitously-expressed Ca²⁺ indicator GCaMP6s with the transparent characteristics of zebrafish embryos to achieve superior in vivo Ca²⁺ imaging. Using the Tg[βactin2:GCaMP6s]^stl351 line featuring strong GCaMP6s expression from cleavage through gastrula stages, we detected higher frequency of Ca²⁺ transients in the superficial blastomeres during the blastula stages preceding the midblastula transition. Additionally, GCaMP6s also revealed that dorsal-biased Ca²⁺ signaling that follows the midblastula transition persisted longer during gastrulation, compared with earlier studies. We observed that dorsal-biased Ca²⁺ signaling is diminished in ventralized ichabod/β-catenin2 mutant embryos and ectopically induced in embryos dorsalized by excess β-catenin. During gastrulation, we directly visualized Ca²⁺ signaling in the dorsal forerunner cells, which form in a Nodal signaling dependent manner and later give rise to the laterality organ. We found that excess Nodal increases the number and the duration of Ca²⁺ transients specifically in the dorsal forerunner cells. The GCaMP6s transgenic lines described here enable unprecedented visualization of dynamic Ca²⁺ events from embryogenesis through adulthood, augmenting the zebrafish toolbox.

Male infertility-linked point mutation reveals a vital binding role for the C2 domain of sperm PLCζ

Sperm-specific phospholipase C zeta (PLCζ) is widely considered to be the physiological stimulus that evokes intracellular calcium (Ca²⁺) oscillations that are essential for the initiation of egg activation during mammalian fertilisation. A recent genetic study reported a male infertility case that was directly associated with a point mutation in the PLCζ C2 domain, where an isoleucine residue had been substituted with a phenylalanine (I489F). Here, we have analysed the effect of this mutation on the in vivo Ca²⁺ oscillation-inducing activity and the in vitro biochemical properties of human PLCζ. Microinjection of cRNA or recombinant protein corresponding to PLCζ^I489F mutant at physiological concentrations completely failed to cause Ca²⁺ oscillations and trigger development. However, this infertile phenotype could be effectively rescued by microinjection of relatively high (non-physiological) amounts of recombinant mutant PLCζ^I489F protein, leading to Ca²⁺ oscillations and egg activation. Our in vitro biochemical analysis suggested that the PLCζ^I489F mutant displayed similar enzymatic properties, but dramatically reduced binding to PI(3)P and PI(5)P-containing liposomes compared with wild-type PLCζ. Our findings highlight the importance of PLCζ at fertilisation and the vital role of the C2 domain in PLCζ function, possibly due to its novel binding characteristics.

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.

Sperm Navigation Mechanisms in the Female Reproductive Tract

Fertilization, the union of an oocyte and a sperm, is a fundamental process that restores the diploid genome and initiates embryonic development. For the sperm, fertilization is the end of a long journey, one that starts in the male testis before transitioning to the female reproductive tract's convoluted tubule architecture. Historically, motile sperm were thought to complete this journey using luck and numbers. A different picture of sperm has emerged recently as cells that integrate complex sensory information for navigation. Chemical, physical, and thermal cues have been proposed to help guide sperm to the waiting oocyte. Molecular mechanisms are being delineated in animal models and humans, revealing common features, as well as important differences. Exposure to pheromones and nutritional signals can modulate guidance mechanisms, indirectly impacting sperm motility performance and fertility. These studies highlight the importance of sensory information and signal transduction in fertilization.

The sperm phospholipase C-ζ and Ca2+ signalling at fertilization in mammals

A series of intracellular oscillations in the free cytosolic Ca(2+) concentration is responsible for activating mammalian eggs at fertilization, thus initiating embryo development. It has been proposed that the sperm causes these Ca(2+) oscillations after membrane fusion by delivering a soluble protein into the egg cytoplasm. We previously identified sperm-specific phospholipase C (PLC)-ζ as a protein that can trigger the same pattern of Ca(2+) oscillations in eggs seen at fertilization. PLCζ appears to be the elusive sperm factor mediating egg activation in mammals. It has potential therapeutic use in infertility treatments to improve the rate of egg activation and early embryo development after intra-cytoplasmic sperm injection. A stable form of recombinant human PLCζ could be a prototype for use in such in vitro fertilization (IVF) treatments. We do not yet understand exactly how PLCζ causes inositol 1,4,5-trisphosphate (InsP3) production in eggs. Sperm PLCζ is distinct among mammalian PI-specific PLCs in that it is far more potent in triggering Ca(2+) oscillations in eggs than other PLCs, but it lacks a PH domain that would otherwise be considered essential for binding to the phosphatidylinositol 4,5-bisphosphate (PIP2) substrate. PLCζ is also unusual in that it does not appear to interact with or hydrolyse plasma membrane PIP2. We consider how other regions of PLCζ may mediate its binding to PIP2 in eggs and how interaction of PLCζ with egg-specific factors could enable the hydrolysis of internal sources of PIP2.

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.

Arabinogalactan proteins and arabinan pectins abound in the specialized matrices surrounding female gametes of the fern Ceratopteris richardii

Both male and female gametes of archegoniates are highly specialized cells surrounded by an extraprotoplasmic matrix rich in AGPs, which are speculated to facilitate development and gamete fusion through Ca 2+) oscillations. An additional layer, the egg envelope, forms around the egg periphery, except at the fertilization pore, and contains arabinose-rich polymers that presumably impart flexibility for the rapidly growing zygote and embryo. The abundant AGPs and arabinan pectins associated with the eggs of C. richardii not only are integral to development, fertilization, and early embryogenesis, but also may be involved in desiccation tolerance important to the survival of the reproductive gametophyte. A defining feature of gametogenesis in archegoniates is the deposition of a special matrix outside of the plasmalemma of both egg and sperm cells that displaces the primary cell wall away from the protoplasm. It is within this matrix that gamete differentiation occurs. In leptosporangiate ferns, maturation of the egg cell involves the deposition of a second specialized wall, the so-called egg envelope that surrounds the cell except at the fertilization pore, a narrow site where gamete fusion takes place. We provide the first conclusive evidence of the macromolecular constituents in the unique structures surrounding fern egg cells before and after fertilization. To test the hypotheses that the egg extracellular matrix contains arabinogalactan proteins (AGPs) as does the sperm cell matrix, and that cell wall polysaccharides, especially pectins, are components of the egg envelope, we examined the expression patterns of AGPs and cell wall constituents during oogenesis in Ceratopteris richardii. Utilizing histochemical stains for callose, cellulose and AGPs coupled with immunogold localizations employing a suite of monoclonal antibodies to cell wall components (JIM13, JIM8, LM2, LM5, LM6, LM19, LM20 and anticallose), we demonstrate that AGPs, but not pectins, are abundant in the matrix around egg cells and degrading neck canal and ventral canal cells during archegonial development. A striking finding is that both AGPs and (1,5)-α-L-arabinan pectin epitopes are principle components of the egg envelope before and after fertilization, suggesting that they are important in both egg maturation and gamete fusion.

First report on facultative parthenogenetic activation of eggs in sterlet sturgeon, Acipenser ruthenus

This study reported facultative parthenogenetic cleavage development of sterlet sturgeon Acipenser ruthenus eggs and quantified the percentage of parthenogenetically developed eggs in relation to the fertilization ability of different females. When eggs were activated in freshwater, 5.1-13.7% of eggs developed parthenogenetically, while among those activated eggs 3.6-9.4% developed to 2 cells, 0.4-4.5% developed to 4 cells, and 0-0.8% developed to 8 cells. The mean percentage of fertilized and parthenogenetically activated eggs among the females was negatively correlated (R(2)=0.71, p=0.036), which indicates that parthenogenetic activation rate of sterlet eggs depends on the quality of eggs in terms of fertilization rate.

Rab3A, Rab27A, and Rab35 regulate different events during mouse oocyte meiotic maturation and activation

Rab family members play important roles in membrane trafficking, cell growth, and differentiation. Almost all components of the cell endomembrane system, the nucleus, and the plasma membrane are closely related to RAB proteins. In this study, we investigated the distribution and functions of three members of the Rab family, Rab3A, Rab27A, and Rab35, in mouse oocyte meiotic maturation and activation. The three Rab family members showed different localization patterns in oocytes. Microinjection of siRNA, antibody injection, or inhibitor treatment showed that (1) Rab3A regulates peripheral spindle and cortical granule (CG) migration, polarity establishment, and asymmetric division; (2) Rab27A regulates CG exocytosis following MII-stage oocyte activation; and (3) Rab35 plays an important role in spindle organization and morphology maintenance, and thus meiotic nuclear maturation. These results show that Rab proteins play important roles in mouse oocyte meiotic maturation and activation and that different members exert different distinct functions.