Parthenogenetic activation of mouse eggs by microinjection of a truncated c-kit tyrosine kinase present in spermatozoa

Applications of advances in mRNA-based platforms as therapeutics and diagnostics in reproductive technologies

The recent COVID-19 pandemic led to many drastic changes in not only society, law, economics, but also in science and medicine, marking for the first time when drug regulatory authorities cleared for use mRNA-based vaccines in the fight against this outbreak. However, while indeed representing a novel application of such technology in the context of vaccination medicine, introducing RNA into cells to produce resultant molecules (proteins, antibodies, etc.) is not a novel principle. It has been common practice to introduce/inject mRNA into oocytes and embryos to inhibit, induce, and identify several factors in a research context, while such aspects have also been proposed as potential therapeutic and diagnostic applications to combat infertility in humans. Herein, we describe key areas where mRNA-based platforms have thus far represented potential areas of clinical applications, describing the advantages and limitations of such applications. Finally, we also discuss how recent advances in mRNA-based platforms, driven by the recent pandemic, may stand to benefit the treatment of infertility in humans. We also present brief future directions as to how we could utilise recent and current advancements to enhance RNA therapeutics within reproductive biology, specifically with relation to oocyte and embryo delivery.

The Therapeutic and Diagnostic Potential of Phospholipase C Zeta, Oocyte Activation, and Calcium in Treating Human Infertility

Oocyte activation, a fundamental event during mammalian fertilisation, is initiated by concerted intracellular patterns of calcium (Ca2+) release, termed Ca2+ oscillations, predominantly driven by testis-specific phospholipase C zeta (PLCζ). Ca2+ exerts a pivotal role in not just regulating oocyte activation and driving fertilisation, but also in influencing the quality of embryogenesis. In humans, a failure of Ca2+ release, or defects in related mechanisms, have been reported to result in infertility. Furthermore, mutations in the PLCζ gene and abnormalities in sperm PLCζ protein and RNA, have been strongly associated with forms of male infertility where oocyte activation is deficient. Concurrently, specific patterns and profiles of PLCζ in human sperm have been linked to parameters of semen quality, suggesting the potential for PLCζ as a powerful target for both therapeutics and diagnostics of human fertility. However, further to PLCζ and given the strong role played by Ca2+ in fertilisation, targets down- and up-stream of this process may also present a significantly similar level of promise. Herein, we systematically summarise recent advancements and controversies in the field to update expanding clinical associations between Ca2+-release, PLCζ, oocyte activation and human fertility. We discuss how such associations may potentially underlie defective embryogenesis and recurrent implantation failure following fertility treatments, alongside potential diagnostic and therapeutic avenues presented by oocyte activation for the diagnosis and treatment of human infertility.

OUP accepted manuscript

BACKGROUND

Oocyte activation deficiency (OAD) is attributed to the majority of cases underlying failure of ICSI cycles, the standard treatment for male factor infertility. Oocyte activation encompasses a series of concerted events, triggered by sperm-specific phospholipase C zeta (PLCζ), which elicits increases in free cytoplasmic calcium (Ca²⁺) in spatially and temporally specific oscillations. Defects in this specific pattern of Ca²⁺ release are directly attributable to most cases of OAD. Ca²⁺ release can be clinically mediated via assisted oocyte activation (AOA), a combination of mechanical, electrical and/or chemical stimuli which artificially promote an increase in the levels of intra-cytoplasmic Ca²⁺. However, concerns regarding safety and efficacy underlie potential risks that must be addressed before such methods can be safely widely used.

OBJECTIVE AND RATIONALE

Recent advances in current AOA techniques warrant a review of the safety and efficacy of these practices, to determine the extent to which AOA may be implemented in the clinic. Importantly, the primary challenges to obtaining data on the safety and efficacy of AOA must be determined. Such questions require urgent attention before widespread clinical utilization of such protocols can be advocated.

SEARCH METHODS

A literature review was performed using databases including PubMed, Web of Science, Medline, etc. using AOA, OAD, calcium ionophores, ICSI, PLCζ, oocyte activation, failed fertilization and fertilization failure as keywords. Relevant articles published until June 2019 were analysed and included in the review, with an emphasis on studies assessing large-scale efficacy and safety.

OUTCOMES

Contradictory studies on the safety and efficacy of AOA do not yet allow for the establishment of AOA as standard practice in the clinic. Heterogeneity in study methodology, inconsistent sample inclusion criteria, non-standardized outcome assessments, restricted sample size and animal model limitations render AOA strictly experimental. The main scientific concern impeding AOA utilization in the clinic is the non-physiological method of Ca²⁺ release mediated by most AOA agents, coupled with a lack of holistic understanding regarding the physiological mechanism(s) underlying Ca²⁺ release at oocyte activation.

LIMITATIONS, REASONS FOR CAUTION

The number of studies with clinical relevance using AOA remains significantly low. A much wider range of studies examining outcomes using multiple AOA agents are required.

WIDER IMPLICATIONS

In addition to addressing the five main challenges of studies assessing AOA safety and efficacy, more standardized, large-scale, multi-centre studies of AOA, as well as long-term follow-up studies of children born from AOA, would provide evidence for establishing AOA as a treatment for infertility. The delivery of an activating agent that can more accurately recapitulate physiological fertilization, such as recombinant PLCζ, is a promising prospect for the future of AOA. Further to PLCζ, many other avenues of physiological oocyte activation also require urgent investigation to assess other potential physiological avenues of AOA.

STUDY FUNDING/COMPETING INTERESTS

D.G. was supported by Stanford University’s Bing Overseas Study Program. J.K. was supported by a Healthcare Research Fellowship Award (HF-14-16) made by Health and Care Research Wales (HCRW), alongside a National Science, Technology, and Innovation plan (NSTIP) project grant (15-MED4186-20) awarded by the King Abdulaziz City for Science and Technology (KACST). The authors have no competing interests to declare.

Phospholipase C-zeta levels are not correlated with fertilisation rates in infertile couples

In mammals, 'oocyte activation' is triggered by certain proteins, one of which is phospholipase C-zeta. Recent evidence suggests that low expression of phospholipase C-zeta might be associated with male infertility, while a limited number of studies claimed the opposite. This study was designed to test whether quantity of phospholipase C-zeta and in vitro fertilisation rates are correlated or not, assessed by flow cytometry. Semen samples from 43 infertile couples were analysed for the percentage and mean fluorescent intensity (MFI) of phospholipase C-zeta protein. Results were confirmed by immunofluorescent labelling. Patients with a fertilisation rate of 40% or lower were involved in the low fertilisation group, while the high fertilization group consisted of patients with a fertilisation rate of 60% and higher. Quantitative analyses by flow cytometry showed no significant difference among the low fertilisation and high fertilisation groups when phospholipase C-zeta ratio or MFI was considered. No correlation was found between pregnancy rates and phospholipase C-zeta quantity. None of the total fertilisation failure cases were lack of phospholipase C-zeta. In fact, fertilisation was possible even when phospholipase C-zeta levels were very low. Thus, we concluded that phospholipase C-zeta quantity cannot be considered as a diagnostic tool for male infertility.

The paternal toolbox for embryo development and health

The sperm is essential for reconstitution of embryonic diploidy and highly specialized developmental functions. Immediately after gamete fusion, the sperm-borne PLC-zeta triggers activation, generating intracellular free Ca2+ oscillations. Mutations in the PLC-zeta encoding gene are associated with the absence of this factor in mature sperm and inability to achieve fertilization. Sperm play also a role in the greater game of the choreography of fertilization. In the human, the sperm centrioles are introduced into the oocyte environment with gamete fusion. They interact with the oocyte cytoskeletal apparatus to form a functional pair of centrosomes and ultimately regulate pronuclear juxtaposition in preparation for the first cleavage. As a consequence, the fidelity of chromosome segregation during the first cell divisions depends on the function of sperm centrioles. Sperm DNA integrity is essential for embryo development and health. Damaged DNA does not impact on the sperm fertilization ability following ICSI. However, detrimental effects emerge at pre- and post-implantation stages. Sperm-specific epigenetic factors also play an active role in the regulation of embryonic development, as shown by correlations between reduced embryo morphological quality and incorrect chromatin packaging during spermiogenesis or abnormal methylation of sperm CpG islands. This functional landscape demonstrates that the contribution of the sperm to development goes far beyond its well-established role in fertilization. Clinical studies confirm this view and indicate sperm function as a crucial aspect of research to increase the efficacy of assisted reproduction treatments.

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?

Ca2+ Signaling and Homeostasis in Mammalian Oocytes and Eggs

Changes in the intracellular concentration of calcium ([Ca2+]i) represent a vital signaling mechanism enabling communication between and among cells as well as with the environment. Cells have developed a sophisticated set of molecules, "the Ca2+ toolkit," to adapt [Ca2+]i changes to specific cellular functions. Mammalian oocytes and eggs, the subject of this review, are not an exception, and in fact the initiation of embryo devolvement in all species is entirely dependent on distinct [Ca2+]i responses. Here, we review the components of the Ca2+ toolkit present in mammalian oocytes and eggs, the regulatory mechanisms that allow these cells to accumulate Ca2+ in the endoplasmic reticulum, release it, and maintain basal and stable cytoplasmic concentrations. We also discuss electrophysiological and genetic studies that have uncovered Ca2+ influx channels in oocytes and eggs, and we analyze evidence supporting the role of a sperm-specific phospholipase C isoform as the trigger of Ca2+ oscillations during mammalian fertilization including its implication in fertility.

Two novel ligand-independent variants of the VEGFR-1 receptor are expressed in human testis and spermatozoa, one of them with the ability to activate SRC proto-oncogene tyrosine kinases

The vascular endothelial growth factor receptor 1 (VEGFR-1) family of receptors is preferentially expressed in endothelial cells, with the full-length and mostly the soluble (sVEGFR-1) isoforms being the most expressed ones. Surprisingly, cancer cells (MDA-MB-231) express, instead, alternative intracellular VEGFR-1 variants. We wondered if these variants, that are no longer dependent on ligands for activation, were expressed in a physiological context, specifically in spermatogenic cells, and whether their expression was maintained in spermatozoa and required for human fertility. By interrogating a human library of mature testis cDNA, we characterized two new truncated intracellular variants different from the ones previously described in cancer cells. The new isoforms were transcribed from alternative transcription start sites (aTSS) located respectively in intron-19 (i₁₉VEGFR-1) and intron-28 (i₂₈VEGFR-1) of the VEGFR-1 gene (GenBank accession numbers JF509744 and JF509745) and expressed in mature testis and spermatozoa. In this paper, we describe the characterization of these isoforms by RT-PCR, northern blot, and western blot, their preferential expression in human mature testis and spermatozoa, and the elements that punctuate their proximal promoters and suggest cues for their expression in spermatogenic cells. Mechanistically, we show that i₁₉VEGFR-1 has a strong ability to phosphorylate and activate SRC proto-oncogene non-receptor tyrosine kinases and a significant bias toward a decrease in expression in patients considered infertile by WHO criteria.

KIT as a therapeutic target for non-oncological diseases

KIT is a receptor tyrosine kinase that after binding to its ligand stem cell factor activates signaling cascades linked to biological processes such as proliferation, differentiation, migration and cell survival. Based on studies performed on SCF and/or KIT mutant animals that presented anemia, sterility, and/or pigmentation disorders, KIT signaling was mainly considered to be involved in the regulation of hematopoiesis, gametogenesis, and melanogenesis. More recently, novel animal models and ameliorated cellular and molecular techniques have led to the discovery of a widen repertoire of tissue compartments and functions that are being modulated by KIT. This is the case for the lung, heart, nervous system, gastrointestinal tract, pancreas, kidney, liver, and bone. For this reason, the tyrosine kinase inhibitors that were originally developed for the treatment of hemato-oncological diseases are being currently investigated for the treatment of non-oncological disorders such as asthma, rheumatoid arthritis, and alzheimer's disease, among others. The beneficial effects of some of these tyrosine kinase inhibitors have been proven to depend on KIT inhibition. This review will focus on KIT expression and regulation in healthy and pathologic conditions other than cancer. Moreover, advances in the development of anti-KIT therapies, including tyrosine kinase inhibitors, and their application will be discussed.

The eggstraordinary story of how life begins

More than 15 years have elapsed since the identification of phospholipase C ζ1 (PLCζ) from a genomic search for mouse testis/sperm-specific PLCs. This molecule was proposed to represent the sperm factor responsible for the initiation of calcium (Ca²⁺ ) oscillations required for egg activation and embryo development in mammals. Supporting evidence for this role emerged from studies documenting its expression in all mammals and other vertebrate species, the physiological Ca²⁺ rises induced by injection of its messenger RNA into mammalian and nonmammalian eggs, and the lack of expression in infertile males that fail intracytoplasmic sperm injection. In the last year, genetic animal models have added support to its role as the long sought-after sperm factor. In this review, we highlight the findings that demonstrated the role of Ca²⁺ as the universal signal of egg activation and the experimental buildup that culminated with the identification of PLCζ as the soluble sperm factor. We also discuss the structural-functional properties that make PLCζ especially suited to evoke oscillations in eggs. Lastly, we examine unresolved aspects of the function and regulation of PLCζ and whether or not it is the only sperm factor in mammalian sperm.

Sperm-derived factors enhance the in vitro developmental potential of haploid parthenotes

Parthenotes are characterized by poor in vitro developmental potential either due to the ploidy status or the absence of paternal factors. In the present study, we demonstrate the beneficial role of sperm-derived factors (SDF) on the in vitro development of mouse parthenotes. Mature (MII) oocytes collected from superovulated Swiss albino mice were activated using strontium chloride (SrCl2) in the presence or absence of various concentrations of SDF in M16 medium. The presence of SDF in activation medium did not have any significant influence on the activation rate. However, a significant increase in the developmental potential of the embryos and increased blastocyst rate (P < 0.01) was observed at 50 µg/ml concentration. Furthermore, the activated oocytes from this group exhibited early cleavage and cortical distribution of cortical granules that was similar to that of normally fertilized zygotes. Culturing 2-cell stage parthenotes in the presence of SDF significantly improved the developmental potential (P < 0.05) indicating that they also play a significant role in embryo development. In conclusion, artificial activation of oocytes with SDF can improve the developmental potential of parthenotes in vitro.

Phospholipase C zeta and calcium oscillations at fertilisation: The evidence, applications, and further questions

Oocyte activation is a fundamental event at mammalian fertilisation, initiated by a series of characteristic calcium (Ca²⁺) oscillations in mammals. This characteristic pattern of Ca²⁺ release is induced in a species-specific manner by a sperm-specific enzyme termed phospholipase C zeta (PLCζ). Reduction or absence of functional PLCζ within sperm underlies male factor infertility in humans, due to mutational inactivation or abrogation of PLCζ protein expression. Underlying such clinical implications, a significant body of evidence has now been accumulated that has characterised the unique biochemical and biophysical properties of this enzyme, further aiding the unique clinical opportunities presented. Herein, we present and discuss evidence accrued over the past decade and a half that serves to support the identity of PLCζ as the mammalian sperm factor. Furthermore, we also discuss the potential novel avenues that have yet to be examined regarding PLCζ mechanism of action in both the oocyte, and the sperm. Finally, we discuss the advances that have been made regarding the clinical therapeutic and diagnostic applications of PLCζ in potentially treating male infertility as a result of oocyte activation deficiency (OAD), and also possibly more general cases of male subfertility.

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.

An Orchestrated Intron Retention Program in Meiosis Controls Timely Usage of Transcripts during Germ Cell Differentiation

Global transcriptome reprogramming during spermatogenesis ensures timely expression of factors in each phase of male germ cell differentiation. Spermatocytes and spermatids require particularly extensive reprogramming of gene expression to switch from mitosis to meiosis and to support gamete morphogenesis. Here, we uncovered an extensive alternative splicing program during this transmeiotic differentiation. Notably, intron retention was largely the most enriched pattern, with spermatocytes showing generally higher levels of retention compared with spermatids. Retained introns are characterized by weak splice sites and are enriched in genes with strong relevance for gamete function. Meiotic intron-retaining transcripts (IRTs) were exclusively localized in the nucleus. However, differently from other developmentally regulated IRTs, they are stable RNAs, showing longer half-life than properly spliced transcripts. Strikingly, fate-mapping experiments revealed that IRTs are recruited onto polyribosomes days after synthesis. These studies reveal an unexpected function for regulated intron retention in modulation of the timely expression of select transcripts during spermatogenesis.

BCAS2 is involved in alternative mRNA splicing in spermatogonia and the transition to meiosis

Breast cancer amplified sequence 2 (BCAS2) is involved in multiple biological processes, including pre-mRNA splicing. However, the physiological roles of BCAS2 are still largely unclear. Here we report that BCAS2 is specifically enriched in spermatogonia of mouse testes. Conditional disruption of Bcas2 in male germ cells impairs spermatogenesis and leads to male mouse infertility. Although the spermatogonia appear grossly normal, spermatocytes in meiosis prophase I and meiosis events (recombination and synapsis) are rarely observed in the BCAS2-depleted testis. In BCAS2 null testis, 245 genes are altered in alternative splicing forms; at least three spermatogenesis-related genes (Dazl, Ehmt2 and Hmga1) can be verified. In addition, disruption of Bcas2 results in a significant decrease of the full-length form and an increase of the short form (lacking exon 8) of DAZL protein. Altogether, our results suggest that BCAS2 regulates alternative splicing in spermatogonia and the transition to meiosis initiation, and male fertility.

Breast cancer amplified sequence 2 (BCAS2) is involved in pre-mRNA splicing but its physiological role is unclear. Here, the authors find BCAS2 enriched in mice spermatogonia in the testes, and BCAS2 deletion in germ cells alters alternative splicing of spermatogenesis-related genes, causing male infertility.

The anaphase-promoting complex initiates zygote division in Arabidopsis through degradation of cyclin B1

As the start of a new life cycle, activation of the first division of the zygote is a critical event in both plants and animals. Because the zygote in plants is difficult to access, our understanding of how this process is achieved remains poor. Here we report genetic and cell biological analyses of the zygote-arrest 1 (zyg1) mutant in Arabidopsis, which showed zygote-lethal and over-accumulation of cyclin B1 D-box-GUS in ovules. Map-based cloning showed that ZYG1 encodes the anaphase-promoting complex/cyclosome (APC/C) subunit 11 (APC11). Live-cell imaging studies showed that APC11 is expressed in both egg and sperm cells, in zygotes and during early embryogenesis. Using a GFP-APC11 fusion construct that fully complements zyg1, we showed that GFP-APC11 expression persisted throughout the mitotic cell cycle, and localized to cell plates during cytokinesis. Expression of non-degradable cyclin B1 in the zygote, or mutations of either APC1 or APC4, also led to a zyg1-like phenotype. Biochemical studies showed that APC11 has self-ubiquitination activity and is able to ubiquitinate cyclin B1 and promote degradation of cyclin B1. These results together suggest that APC/C-mediated degradation of cyclin B1 in Arabidopsis is critical for initiating the first division of the zygote.

Signal transduction in mammalian oocytes during fertilization

Mammalian embryo development begins when the fertilizing sperm triggers a series of elevations in the oocyte's intracellular free Ca(2+) concentration. The elevations are the result of repeated release and re-uptake of Ca(2+) stored in the smooth endoplasmic reticulum. Ca(2+) release is primarily mediated by the phosphoinositide signaling system of the oocyte. The system is stimulated when the sperm causes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG); IP3 then binds its receptor on the surface of the endoplasmic reticulum that induces Ca(2+) release. The manner in which the sperm generates IP3, the Ca(2+) mobilizing second messenger, has been the subject of extensive research for a long time. The sperm factor hypothesis has eventually gained general acceptance, according to which it is a molecule from the sperm that diffuses into the ooplasm and stimulates the phosphoinositide cascade. Much evidence now indicates that the sperm-derived factor is phospholipase C-zeta (PLCζ) that cleaves PIP2 and generates IP3, eventually leading to oocyte activation. A recent addition to the candidate sperm factor list is the post-acrosomal sheath WW domain-binding protein (PAWP), whose role at fertilization is currently under debate. Ca(2+) influx across the plasma membrane is also important as, in the absence of extracellular Ca(2+), the oscillations run down prematurely. In pig oocytes, the influx that sustains the oscillations seems to be regulated by the filling status of the stores, whereas in the mouse other mechanisms might be involved. This work summarizes the current understanding of Ca(2+) signaling in mammalian oocytes.

Egg Activation at Fertilization by a Soluble Sperm Protein

The most fundamental unresolved issue of fertilization is to define how the sperm activates the egg to begin embryo development. Egg activation at fertilization in all species thus far examined is caused by some form of transient increase in the cytoplasmic free Ca2+ concentration. What has not been clear, however, is precisely how the sperm triggers the large changes in Ca2+ observed within the egg cytoplasm. Here, we review the studies indicating that the fertilizing sperm stimulates a cytosolic Ca2+ increase in the egg specifically by delivering a soluble factor that diffuses into the cytosolic space of the egg upon gamete membrane fusion. Evidence is primarily considered in species of eggs where the sperm has been shown to elicit a cytosolic Ca2+ increase by initiating Ca2+ release from intracellular Ca2+ stores. We suggest that our best understanding of these signaling events is in mammals, where the sperm triggers a prolonged series of intracellular Ca2+ oscillations. The strongest empirical studies to date suggest that mammalian sperm-triggered Ca2+ oscillations are caused by the introduction of a sperm-specific protein, called phospholipase C-zeta (PLCζ) that generates inositol trisphosphate within the egg. We will discuss the role and mechanism of action of PLCζ in detail at a molecular and cellular level. We will also consider some of the evidence that a soluble sperm protein might be involved in egg activation in nonmammalian species.

Initiation and organization of events during the first cell cycle in mammals: applications in cloning

The technology of cloning involves transplanting a diploid nucleus into a mature oocyte cytoplast. The cytoplast is then activated to initiate the first cell cycle of development as a nuclear transplant embryo. Initiation and regulation of events during the first cell cycle are, therefore, critical for proper reprogramming of the donor nucleus and development as a cloned embryo. Activation is normally induced by the sperm and is mediated by a series of intracellular free calcium ([Ca(2+)](i)) oscillations that last for several hours. Although it is not known precisely how the sperm induces activation, current evidence favors the delivery, by the sperm, of a soluble protein factor that causes the production of IP3. IP3 acts to open a Ca(2+) channel in the endoplasmic reticulum and release Ca(2+) into the cytosol. A variety of methods have been used to duplicate or replace the sperm-induced [Ca(2+)](i) increase to cause activation in nuclear transplant embryos. It has been found that treatments that cause a single transient [Ca(2+)](i) activate some oocytes with the level of activation increasing as the oocyte ages. Attempts have been made to extend the period of time over which [Ca(2+)](i) oscillations occur. This has been successful in increasing activation rates of less mature oocytes but the techniques are still cumbersome. An alternative method, that has been very successful, is the combination of a treatment that elevates [Ca(2+)](i) and a treatment that maintains low levels of maturation promoting factor for several hours after the initial [Ca(2+)](i) elevation. The sperm also contributes the centrosome that organizes microtubules during the first cell cycle. One current hypothesis for regulation of sperm centrosomal activity consists of a dephosphorylation of sperm connecting piece proteins following sperm entry into the oocyte and activation of the oocyte. Dephosphorylation of these proteins results in the disassembly of the connecting piece and assembly of a functional centrosome. In nuclear transfer, centrosomal components are contributed by the donor cell. If the cell is fused to the cytoplast before centriole replication then a single aster forms. If the cell is fused after centriole replication then two asters form. In either case and even in parthenogenetic oocytes, which do not have centrioles, the first cell cycle progresses to metaphase. However, progress is slow and some defects are observed in the assembly of chromosomes into a metaphase plate.

Novel signalling mechanism and clinical applications of sperm-specific PLCζ

Egg activation is the first step of embryonic development and in mammals is triggered by a series of cytoplasmic calcium (Ca2+) oscillations. Sperm–egg fusion initiates these Ca2+ oscillations by introducing a sperm-specific protein factor into the egg cytoplasm. Substantial evidence indicates that this protein is a sperm-specific phospholipase C (PLC), termed PLC-zeta (PLCζ). PLCζ stimulates cytoplasmic Ca2+ oscillations matching those at fertilization triggering early embryonic development in several mammalian species. Structurally, PLCζ is comprised of four EF-hands, a C2 domain, and X and Y catalytic domains. PLCζ is an unusual PLC since it lacks a pleckstrin homology (PH) domain. It is also distinctive in that its X–Y linker is not involved in auto-inhibition of catalytic activity, but instead binds to phosphatidylinositol 4,5-bisphosphate (PIP2). Moreover, relative to other PLC isoforms, PLCζ possesses unique potency in stimulating Ca2+ oscillations in eggs, although it does not appear to bind to plasma membrane PIP2. In contrast, PLCζ appears to interact with intracellular vesicles in eggs that contain PIP2. I discuss the recent advances in our knowledge of the intriguing biochemical and physiological properties of sperm PLCζ and postulate potential roles for PLCζ in terms of clinical diagnosis and therapy for certain forms of male infertility.

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.

An oncofetal and developmental perspective on testicular germ cell cancer

Germ cell tumors (GCTs) represent a diverse group of tumors presumably originating from (early fetal) developing germ cells. Most frequent are the testicular germ cell cancers (TGCC). Overall, TGCC is the most frequent malignancy in Caucasian males (20-40 years) and remains an important cause of (treatment related) mortality in these young men. The strong association between the phenotype of TGCC stem cell components and their totipotent ancestor (fetal primordial germ cell or gonocyte) makes these tumors highly relevant from an onco-fetal point of view. This review subsequently discusses the evidence for the early embryonic origin of TGCCs, followed by an overview of the crucial association between TGCC pathogenesis, genetics, environmental exposure and the (fetal) testicular micro-environment (genvironment). This culminates in an evaluation of three genvironmentally modulated hallmarks of TGCC directly related to the oncofetal pathogenesis of TGCC: (1) maintenance of pluripotency, (2) cell cycle control/cisplatin sensitivity and (3) regulation of proliferation/migration/apoptosis by KIT-KITL mediated receptor tyrosine kinase signaling. Briefly, TGCC exhibit identifiable stem cell components (seminoma and embryonal carcinoma) and progenitors that show large and consistent similarities to primordial/embryonic germ cells, their presumed totipotent cells of origin. TGCC pathogenesis depends crucially on a complex interaction of genetic and (micro-)environmental, i.e. genvironmental risk factors that have only been partly elucidated despite significant effort. TGCC stem cell components also show a high degree of similarity with embryonic stem/germ cells (ES) in the regulation of pluripotency and cell cycle control, directly related to their exquisite sensitivity to DNA damaging agents (e.g. cisplatin). Of note, (ES specific) micro-RNAs play a pivotal role in the crossover between cell cycle control, pluripotency and chemosensitivity. Moreover, multiple consistent observations reported TGCC to be associated with KIT-KITL mediated receptor tyrosine kinase signaling, a pathway crucially implicated in proliferation, migration and survival during embryogenesis including germ cell development. In conclusion, TGCCs are a fascinating model for onco-fetal developmental processes especially with regard to studying cell cycle control, pluripotency maintenance and KIT-KITL signaling. The knowledge presented here contributes to better understanding of the molecular characteristics of TGCC pathogenesis, translating to identification of at risk individuals and enhanced quality of care for TGCC patients (diagnosis, treatment and follow-up).

Paternal RNA contributions in the Caenorhabditis elegans zygote

Development of the early embryo is thought to be mainly driven by maternal gene products and post-transcriptional gene regulation. Here, we used metabolic labeling to show that RNA can be transferred by sperm into the oocyte upon fertilization. To identify genes with paternal expression in the embryo, we performed crosses of males and females from divergent Caenorhabditis elegans strains. RNA sequencing of mRNAs and small RNAs in the 1-cell hybrid embryo revealed that about one hundred sixty paternal mRNAs are reproducibly expressed in the embryo and that about half of all assayed endogenous siRNAs and piRNAs are also of paternal origin. Together, our results suggest an unexplored paternal contribution to early development.

Sperm-induced Ca2+ release during egg activation in mammals

This review discusses the role that the sperm-specific phospholipase C zeta (PLCζ) is proposed to play during the fertilization of mammalian eggs. At fertilization, the sperm initiates development by causing a series of oscillations in cytosolic concentrations of calcium [Ca(2)] within the egg. PLCζ mimics the sperm at fertilization, causing the same pattern of Ca(2+) release as seen at fertilization. Introducing PLCζ into mouse eggs also mimics a number of other features of the way in which the fertilizing sperm triggers Ca(2+) oscillations. We discuss the localization of PLCζ within the egg and present a hypothesis about the localization of PLCζ within the sperm before the initiation of fertilization.

c-kit expression profile and regulatory factors during spermatogonial stem cell differentiation

Background

It has been proven that c-kit is crucial for proliferation, migration, survival and maturation of spermatogenic cells. A periodic expression of c-kit is observed from primordial germ cells (PGCs) to spermatogenetic stem cells (SSCs), However, the expression profile of c-kit during the entire spermatogenesis process is still unclear. This study aims to reveal and compare c-kit expression profiles in the SSCs before and after the anticipated differentiation, as well as to examine its relationship with retinoic acid (RA) stimulation.

Results

We have found that there are more than 4 transcripts of c-kit expressed in the cell lines and in the testes. The transcripts can be divided into short and long categories. The long transcripts include the full-length canonical c-kit transcript and the 3′ end short transcript. Short transcripts include the 3.4 kb short transcript and several truncated transcripts (1.9-3.2 kb). In addition, the 3.4 kb transcript (starting from intron 9 and covering exons 10 ~ 21) is discovered to be specifically expressed in the spermatogonia. The extracellular domain of Kit is obtained in the spermatogonia stage, but the intracellular domain (50 kDa) is constantly expressed in both SSCs and spermatogonia. The c-kit expression profiles in the testis and the spermatogonial stem cell lines vary after RA stimulation. The wave-like changes of the quantitative expression pattern of c-kit (increase initially and decrease afterwards) during the induction process are similar to that of the in vivo male germ cell development process.

Conclusions

There are dynamic transcription and translation changes of c-kit before and after SSCs’ anticipated differentiation and most importantly, RA is a significant upstream regulatory factor for c-kit expression.

Sperm PLCζ: from structure to Ca2+ oscillations, egg activation and therapeutic potential

Significant evidence now supports the assertion that cytosolic calcium oscillations during fertilization in mammalian eggs are mediated by a testis-specific phospholipase C (PLC), termed PLC-zeta (PLCζ) that is released into the egg following gamete fusion. Herein, we describe the current paradigm of PLCζ in this fundamental biological process, summarizing recent important advances in our knowledge of the biochemical and physiological properties of this enzyme. We describe the data suggesting that PLCζ has distinct features amongst PLCs enabling the hydrolysis of its substrate, phosphatidylinositol 4,5-bisphosphate (PIP2) at low Ca(2+) levels. PLCζ appears to be unique in its ability to target PIP2 that is present on intracellular vesicles. We also discuss evidence that PLCζ may be a significant factor in human fertility with potential therapeutic capacity.

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.

Protein-tyrosine kinase signaling in the biological functions associated with sperm

In sexual reproduction, two gamete cells (i.e., egg and sperm) fuse (fertilization) to create a newborn with a genetic identity distinct from those of the parents. In the course of these developmental processes, a variety of signal transduction events occur simultaneously in each of the two gametes, as well as in the fertilized egg/zygote/early embryo. In particular, a growing body of knowledge suggests that the tyrosine kinase Src and/or other protein-tyrosine kinases are important elements that facilitate successful implementation of the aforementioned processes in many animal species. In this paper, we summarize recent findings on the roles of protein-tyrosine phosphorylation in many sperm-related processes (from spermatogenesis to epididymal maturation, capacitation, acrosomal exocytosis, and fertilization).

Stem Cell Factor Receptor/c-Kit: From Basic Science to Clinical Implications

Stem cell factor (SCF) is a dimeric molecule that exerts its biological functions by binding to and activating the receptor tyrosine kinase c-Kit. Activation of c-Kit leads to its autophosphorylation and initiation of signal transduction. Signaling proteins are recruited to activated c-Kit by certain interaction domains (e.g., SH2 and PTB) that specifically bind to phosphorylated tyrosine residues in the intracellular region of c-Kit. Activation of c-Kit signaling has been found to mediate cell survival, migration, and proliferation depending on the cell type. Signaling from c-Kit is crucial for normal hematopoiesis, pigmentation, fertility, gut movement, and some aspects of the nervous system. Deregulated c-Kit kinase activity has been found in a number of pathological conditions, including cancer and allergy. The observation that gain-of-function mutations in c-Kit can promote tumor formation and progression has stimulated the development of therapeutics agents targeting this receptor, e.g., the clinically used inhibitor imatinib mesylate. Also other clinically used multiselective kinase inhibitors, for instance, sorafenib and sunitinib, have c-Kit included in their range of targets. Furthermore, loss-of-function mutations in c-Kit have been observed and shown to give rise to a condition called piebaldism. This review provides a summary of our current knowledge regarding structural and functional aspects of c-Kit signaling both under normal and pathological conditions, as well as advances in the development of low-molecular-weight molecules inhibiting c-Kit function.

Oocyte activation and phospholipase C zeta (PLCζ): diagnostic and therapeutic implications for assisted reproductive technology

Infertility affects one in seven couples globally and has recently been classified as a disease by the World Health Organisation (WHO). While in-vitro fertilisation (IVF) offers effective treatment for many infertile couples, cases exhibiting severe male infertility (19-57%) often remain difficult, if not impossible to treat. In such cases, intracytoplasmic sperm injection (ICSI), a technique in which a single sperm is microinjected into the oocyte, is implemented. However, 1-5% of ICSI cycles still fail to fertilise, affecting over 1000 couples per year in the UK alone. Pregnancy and delivery rates for IVF and ICSI rarely exceed 30% and 23% respectively. It is therefore imperative that Assisted Reproductive Technology (ART) protocols are constantly modified by associated research programmes, in order to provide patients with the best chances of conception. Prior to fertilisation, mature oocytes are arrested in the metaphase stage of the second meiotic division (MII), which must be alleviated to allow the cell cycle, and subsequent embryogenesis, to proceed. Alleviation occurs through a series of concurrent events, collectively termed 'oocyte activation'. In mammals, oocytes are activated by a series of intracellular calcium (Ca2+) oscillations following gamete fusion. Recent evidence implicates a sperm-specific phospholipase C, PLCzeta (PLCζ), introduced into the oocyte following membrane fusion as the factor responsible. This review summarises our current understanding of oocyte activation failure in human males, and describes recent advances in our knowledge linking certain cases of male infertility with defects in PLCζ expression and activity. Systematic literature searches were performed using PubMed and the ISI-Web of Knowledge. Databases compiled by the United Nations and World Health Organisation databases (UNWHO), and the Human Fertilization and Embryology Authority (HFEA) were also scrutinised. It is clear that PLCζ plays a fundamental role in the activation of mammalian oocytes, and that genetic, molecular, or biochemical perturbation of this key enzyme is strongly linked to human infertility where oocyte activation is deficient. Consequently, there is significant scope for our understanding of PLCζ to be translated to the ART clinic, both as a novel therapeutic agent with which to rescue oocyte activation deficiency (OAD), or as a prognostic/diagnostic biomarker of oocyte activation ability in target sperm samples.

Starting a new life: sperm PLC-zeta mobilizes the Ca2+ signal that induces egg activation and embryo development: an essential phospholipase C with implications for male infertility

We have discovered that a single sperm protein, phospholipase C-zeta (PLCζ), can stimulate intracellular Ca(2+) signalling in the unfertilized oocyte ('egg') culminating in the initiation of embryonic development. Upon fertilization by a spermatozoon, the earliest observed signalling event in the dormant egg is a large, transient increase in free Ca(2+) concentration. The fertilized egg responds to the intracellular Ca(2+) rise by completing meiosis. In mammalian eggs, the Ca(2+) signal is delivered as a train of long-lasting cytoplasmic Ca(2+) oscillations that begin soon after gamete fusion and persist beyond the completion of meiosis. Sperm PLCζ effects Ca(2+) release from egg intracellular stores by hydrolyzing the membrane lipid PIP(2) and consequent stimulation of the inositol 1,4,5-trisphosphate (InsP(3) ) receptor Ca(2+) -signalling pathway, leading to egg activation and early embryogenesis. Recent advances have refined our understanding of how PLCζ induces Ca(2+) oscillations in the egg and also suggest its potential dysfunction as a cause of male infertility.

c-kit and its related genes in spermatogonial differentiation

Spermatogenesis is the process of production of male gametes from SSCs. The SSCs are the stem cells that differentiate into male gametes in the testis. in the mean time, the Spg are remarkable for their potential multiple trans-differentiations, which make them greatly invaluable for clinical applications. However, the molecular mechanism controlling differentiation of the Spg is still not clear. Among the discovered spermatogenesis-related genes, c-kit seems to be expressed first by the Spgs thus may play a central role in switching on the differentiation process. Expression of Kit and the activation of the Kit/Kitl pathway coincide with the start of differentiation of Spgs. Several genes have been discovered to be related to the Kit/Kitl pathway. in this review, we have summarized the recent discoveries of c-kit and the Kit/Kitl pathway-related genes in the spermatogenic cells during different stages of spermatogenesis.

The integrin-binding motif RGDS induces protein tyrosine phosphorylation without activation in Bufo arenarum (Amphibia) oocytes

Integrins are cell adhesion molecules that are thought to be involved in sperm–oocyte interaction. Nevertheless, their function in mammalian fertilization is still controversial, as different species behave differently. In amphibians, their role is mainly supported byXenopus laevisstudies, where RGDS peptide induces oocyte activation. We recently provided evidence suggesting the presence and involvement of integrins in the interaction of the oocyte plasma membrane (PM) with sperm in the amphibianBufo arenarum. In order to understand the role of integrin homologs in oocytes and their possible contribution to egg activation mechanisms, we examined the presence of integrin subunits and the effect of RGDS peptide on oocytes and during fertilization. Western blot studies detected integrin subunits α5, αV and β1 in oocytes. In sperm, we could detect only the αV integrin subunit. We found that RGDS peptide was unable to elicit egg activation or MAPK dephosphorylation, but can induce reversible inhibition of fertilization. A similar partial inhibition was produced by an anti-β1 integrin antibody. Using an anti-phosphotyrosine antibody we found major changes in phosphotyrosine-containing proteins in egg extracts minutes after fertilization. Cytosol and PMs isolated from oocytes and fertilized eggs showed additional fertilization-induced phosphorylated proteins. Some of these were also present in cytosol and PMs from RGDS-treated oocytes (partially mimicking fertilization). These findings suggest thatB. arenarumfertilization involves integrins (e.g. β1 subunit) as adhesion proteins. Our data support the view that RGDS-binding receptors may function as signaling receptors inB. arenarumoocytes, but integrin engagement by RGDS is not sufficient for oocyte activation.

Preservation of sperm within the mouse cauda epididymidis in salt or sugars at room temperature

The development of preservation techniques for male gametes at room temperature might allow us to store them in a simple and cost-effective manner. In this study, we studied the use of pure salt or sugar to preserve the whole cauda epididymidis, because it is known that food can be preserved in this way at room temperature for long periods. Mouse epididymides were placed directly in powdered salt (NaCl) or sugars (glucose or raffinose) for 1 day to 1 year at room temperature. Spermatozoa were recovered from the preserved organs after being rehydrated with medium and then isolated sperm heads were microinjected into fresh oocytes. Importantly, the oocyte activation capacity of spermatozoa was maintained after epididymal storage in NaCl for 1 year, whereas most untreated spermatozoa failed to activate oocytes within 1 month of storage. Pronuclear morphology, the rate of extrusion of a second polar body and the methylation status of histone H3 lysine 9 (H3K9me3) in those zygotes were similar to those of zygotes fertilized with fresh spermatozoa. However, the developmental ability of the zygotes decreased within 1 day of sperm storage. This effect led to nuclear fragmentation at the 2-cell embryo stage, irrespective of the storage method used. Thus, although the preserved sperm failed to allow embryo development, their oocyte activation factors were maintained by salt storage of the epididymis for up to 1 year at room temperature.

Impact of marine drugs on animal reproductive processes

The discovery and description of bioactive substances from natural sources has been a research topic for the last 50 years. In this respect, marine animals have been used to extract many new compounds exerting different actions. Reproduction is a complex process whose main steps are the production and maturation of gametes, their activation, the fertilisation and the beginning of development. In the literature it has been shown that many substances extracted from marine organisms may have profound influence on the reproductive behaviour, function and reproductive strategies and survival of species. However, despite the central importance of reproduction and thus the maintenance of species, there are still few studies on how reproductive mechanisms are impacted by marine bioactive drugs. At present, studies in either marine and terrestrial animals have been particularly important in identifying what specific fine reproductive mechanisms are affected by marine-derived substances. In this review we describe the main steps of the biology of reproduction and the impact of substances from marine environment and organisms on the reproductive processes.

Control of KIT signalling in male germ cells: what can we learn from other systems?

The KIT ligand (KITL)/KIT-signalling system is among several pathways known to be essential for fertility. In the postnatal testis, the KIT/KITL interaction is crucial for spermatogonial proliferation, differentiation, survival and subsequent entry into meiosis. Hence, identification of endogenous factors that regulate KIT synthesis is important for understanding the triggers driving germ cell maturation. Although limited information is available regarding local factors in the testicular microenvironment that modulate KIT synthesis at the onset of spermatogenesis, knowledge from other systems could be used as a basis for identifying how KIT function is regulated in germ cells. This review describes the known regulators of KIT, including transcription factors implicated in KIT promoter regulation. In addition, specific downstream outcomes in biological processes that KIT orchestrates are addressed. These are discussed in relationship to current knowledge of mammalian germ cell development.

KIT variants in bovine ovarian cells and corpus luteum

We report the presence of KIT variants in granulosa and thecal cells of the follicle and endothelial and steroidogenic cells of the corpus luteum. Transcripts of both full-length splice variants, KIT and KITA, were ubiquitously detected in all cell types, in contrast to transcripts for truncated KIT. RT-PCR with exon-intron-specific primers suggested that KIT transcripts retained intron sequences. We used domain-specific KIT antibodies to identify truncated KIT proteins in cell conditioned media and lysates. These proteins represented soluble KIT and a so far disregarded intracellular KIT fragment, and were ubiquitously present. In contrast, glycosylated variants of full-length KIT were predominantly detected in thecal and endothelial cells. All KIT variants were encountered again in COS-7 cells transfected with a vector containing KITA. Phorbol 12-myristate-13-acetate treatment induced levels of truncated KITs, and this effect was repressed by the metalloproteinase inhibitor TAPI-1. Our findings show that ectodomain cleavage of full-length KIT generates an intracellular KIT. Our experiments suggest that replenishing full-length KIT differs among various ovarian cell types.

Human sperm devoid of PLC, zeta 1 fail to induce Ca(2+) release and are unable to initiate the first step of embryo development

Egg activation, which is the first step in the initiation of embryo development, involves both completion of meiosis and progression into mitotic cycles. In mammals, the fertilizing sperm delivers the activating signal, which consists of oscillations in free cytosolic Ca(2+) concentration ([Ca(2+)](i)). Intracytoplasmic sperm injection (ICSI) is a technique that in vitro fertilization clinics use to treat a myriad of male factor infertility cases. Importantly, some patients who repeatedly fail ICSI also fail to induce egg activation and are, therefore, sterile. Here, we have found that sperm from patients who repeatedly failed ICSI were unable to induce [Ca(2+)](i) oscillations in mouse eggs. We have also shown that PLC, zeta 1 (PLCZ1), the sperm protein thought to induce [Ca(2+)](i) oscillations, was localized to the equatorial region of wild-type sperm heads but was undetectable in sperm from patients who had failed ICSI. The absence of PLCZ1 in these patients was further confirmed by Western blot, although genomic sequencing failed to reveal conclusive PLCZ1 mutations. Using mouse eggs, we reproduced the failure of sperm from these patients to induce egg activation and rescued it by injection of mouse Plcz1 mRNA. Together, our results indicate that the inability of human sperm to initiate [Ca(2+)](i) oscillations leads to failure of egg activation and sterility and that abnormal PLCZ1 expression underlies this functional defect.

Expression of EGFL7 in primordial germ cells and in adult ovaries and testes

We have previously reported the isolation and characterization of a novel endothelial-restricted gene, Egfl7, that encodes a secreted protein of about 30-kDa. We and others demonstrated that Egfl7 is highly expressed by endothelial cells during embryonic development and becomes down-regulated in the adult vasculature. In the present paper, we show that during mouse embryonic development, Egfl7 is also expressed by primordial germ cells (PGC). Expression is down-regulated when PGCs differentiate into pro-spermatogonia and oogonia, and by 15.5 dpc Egfl7 can no longer be detected in the germ line of both sexes. Notably, Egfl7 is again transiently up-regulated in germ cells of the adult testis. In contrast, expression in the ovary remains limited to the vascular endothelium. Our results provide the first evidence of a non-endothelial expression of EGFL7 and suggest distinctive roles for Egfl7 in vascular development and germ cell differentiation.