Role of Fyn kinase in oocyte developmental potential

Extracellular vesicles secreted by cumulus cells contain microRNAs that are potential regulatory factors of mouse oocyte developmental competence

The role of cumulus cells (CCs) in the acquisition of oocyte developmental competence is not yet fully understood. In a previous study, we matured cumulus-denuded fully-grown mouse oocytes to metaphase II (MII) on a feeder layer of CCs (FL-CCs) isolated from developmentally competent (FL-SN-CCs) or incompetent (FL-NSN-CCs) SN (surrounded nucleolus) or NSN (not surrounding nucleolus) oocytes, respectively. We observed that oocytes cultured on the former could develop into blastocysts, while those matured on the latter arrested at the 2-cell stage. To investigate the CC factors contributing to oocyte developmental competence, here we focused on the CCs' release into the medium of extracellular vesicles (EVs) and on their miRNA content. We found that, during the 15-h transition to MII, both FL-SN-CCs and FL-NSN-CCs release EVs that can be detected, by confocal microscopy, inside the zona pellucida (ZP) or the ooplasm. The majority of EVs are <200 nm in size, which is compatible with their ability to cross the ZP. Next-generation sequencing of the miRNome of FL-SN-CC versus FL-NSN-CC EVs highlighted 74 differentially expressed miRNAs, with 43 up- and 31 down-regulated. Although most of these miRNAs do not have known roles in the ovary, in silico functional analysis showed that seven of these miRNAs regulate 71 target genes with specific roles in meiosis resumption (N = 24), follicle growth (N = 23), fertilization (N = 1), and the acquisition of oocyte developmental competence (N = 23). Overall, our results indicate CC EVs as emerging candidates of the CC-to-oocyte communication axis and uncover a group of miRNAs as potential regulatory factors.

Screening Novel Per- and Polyfluoroalkyl Substances in Human Blood Based on Nontarget Analysis and Underestimated Potential Health Risks

Nontarget analysis has gained prominence in screening novel perfluoroalkyl and polyfluoroalkyl substances (PFASs) in the environment, yet remaining limited in human biological matrices. In this study, 155 whole blood samples were collected from the general population in Shijiazhuang City, China. By nontarget analysis, 31 legacy and novel PFASs were assigned with the confidence level of 3 or above. For the first time, 11 PFASs were identified in human blood, including C1 and C3 perfluoroalkyl sulfonic acids (PFSAs), C4 ether PFSA, C8 ether perfluoroalkyl carboxylic acid (ether PFCA), C4-5 unsaturated perfluoroalkyl alcohols, C9-10 carboxylic acid-perfluoroalkyl sulfonamides (CA-PFSMs), and C1 perfluoroalkyl sulfonamide. It is surprising that the targeted PFASs were the highest in the suburban population which was impacted by industrial emission, while the novel PFASs identified by nontarget analysis, such as C1 PFSA and C9-11 CA-PFSMs, were the highest in the rural population who often drank contaminated groundwater. Combining the toxicity prediction results of the bioaccumulation potential, lethality to rats, and binding affinity to target proteins, C3 PFSA, C4 and C7 ether PFSAs, and C9-11 CA-PFSMs exhibit great health risks. These findings emphasize the necessity of broadening nontarget analysis in assessing the PFAS exposure risks, particularly in rural populations.

Signaling Proteins Recruited to the Sperm Binding Site: Role of β-Catenin and Rho A

Sperm interaction with the oocyte plasma membrane triggers a localized response in the mouse oocyte that leads to remodeling of oocyte surface as well as the underlying cortical actin layer. The recent demonstration that PTK2B is recruited and activated at the sperm binding site raised the possibility that multiple signaling events may be activated during this stage of fertilization. The present study demonstrated that β-catenin and Rho A were recruited to the cortex underlying bound/fused sperm. To determine whether sperm-oocyte contact was sufficient to initiate β-catenin recruitment, Cd9-null, and PTK2b-null oocytes were tested for the ability to recruit β-catenin to sperm binding sites. Both Cd9 and Ptk2b ablation reduced β-catenin recruitment raising the possibility that PTK2B may act downstream of CD9 in the response to sperm binding/fusion. Further immunofluorescence study revealed that β-catenin co-localized with f-actin in the interstitial regions between actin layer fenestrae. Rho A, in contrast, was arranged underneath the actin layer in both the fenestra and the interstitial regions suggesting that they may play different roles in the oocyte.

Targeted whole exome sequencing and Drosophila modelling to unveil the molecular basis of primary ovarian insufficiency

STUDY QUESTION

Can a targeted whole exome sequencing (WES) on a cohort of women showing a primary ovarian insufficiency (POI) phenotype at a young age, combined with a study of copy number variations, identify variants in candidate genes confirming their deleterious effect on ovarian function?

SUMMARY ANSWER

This integrated approach has proved effective in identifying novel candidate genes unveiling mechanisms involved in POI pathogenesis.

WHAT IS KNOWN ALREADY

POI, a condition occurring in 1% of women under 40 years of age, affects women’s fertility leading to a premature loss of ovarian reserve. The genetic causes of POI are highly heterogeneous and several determinants contributing to its prominent oligogenic inheritance pattern still need to be elucidated.

STUDY DESIGN, SIZE, DURATION

WES screening for pathogenic variants of 41 Italian women with non-syndromic primary and early secondary amenorrhoea occurring before age 25 was replicated on another 60 POI patients, including 35 French and 25 American women, to reveal statistically significant shared variants.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The Italian POI patients’ DNA were processed by targeted WES including 542 RefSeq genes expressed or functioning during distinct reproductive or ovarian processes (e.g. DNA repair, meiosis, oocyte maturation, folliculogenesis and menopause). Extremely rare variants were filtered and selected by means of a Fisher Exact test using several publicly available datasets. A case-control Burden test was applied to highlight the most significant genes using two ad-hoc control female cohorts. To support the obtained data, the identified genes were screened on a novel cohort of 60 Caucasian POI patients and the same case-control analysis was carried out. Comparative analysis of the human identified genes was performed on mouse and Drosophila melanogaster by analysing the orthologous genes in their ovarian phenotype, and two of the selected genes were fruit fly modelled to explore their role in fertility.

MAIN RESULTS AND THE ROLE OF CHANCE

The filtering steps applied to search for extremely rare pathogenic variants in the Italian cohort revealed 64 validated single-nucleotide variants/Indels in 59 genes in 30 out of 41 screened women. Burden test analysis highlighted 13 ovarian genes as being the most enriched and significant. To validate these findings, filtering steps and Burden analysis on the second cohort of Caucasian patients yielded 11 significantly enriched genes. Among them, AFP, DMRT3, MOV10, FYN and MYC were significant in both patient cohorts and hence were considered strong candidates for POI. Mouse and Drosophila comparative analysis evaluated a conserved role through the evolution of several candidates, and functional studies using a Drosophila model, when applicable, supported the conserved role of the MOV10 armitage and DMRT3 dmrt93B orthologues in female fertility.

LARGE SCALE DATA

The datasets for the Italian cohort generated during the current study are publicly available at ClinVar database (http://www.ncbi.nlm.nih.gov/clinvar/): accession numbers SCV001364312 to SCV001364375.

LIMITATIONS, REASONS FOR CAUTION

This is a targeted WES analysis hunting variants in candidate genes previously identified by different genomic approaches. For most of the investigated sporadic cases, we could not track the parental inheritance, due to unavailability of the parents’ DNA samples; in addition, we might have overlooked additional rare variants in novel candidate POI genes extracted from the exome data. On the contrary, we might have considered some inherited variants whose clinical significance is uncertain and might not be causative for the patients’ phenotype. Additionally, as regards the Drosophila model, it will be extremely important in the future to have more mutants or RNAi strains available for each candidate gene in order to validate their role in POI pathogenesis.

WIDER IMPLICATIONS OF THE FINDINGS

The genomic, statistical, comparative and functional approaches integrated in our study convincingly support the extremely heterogeneous oligogenic nature of POI, and confirm the maintenance across the evolution of some key genes safeguarding fertility and successful reproduction. Two principal classes of genes were identified: (i) genes primarily involved in meiosis, namely in synaptonemal complex formation, asymmetric division and oocyte maturation and (ii) genes safeguarding cell maintenance (piRNA and DNA repair pathways).

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by Italian Ministry of Health grants ‘Ricerca Corrente’ (08C621_2016 and 08C924_2019) provided to IRCCS Istituto Auxologico Italiano, and by ‘Piano Sostegno alla Ricerca’ (PSR2020_FINELLI_LINEA_B) provided by the University of Milan; M.P.B. was supported by Telethon-Italy (grant number GG14181). There are no conflicts of interest.

Contrasting effects of the Toll-like receptor 4 in determining ovarian follicle endowment and fertility in female adult mice

Toll-like receptor 4 (TLR4) is best known for its role in bacteria-produced lipopolysaccharide recognition. Regarding female reproduction, TLR4 is expressed by murine cumulus cells and participates in ovulation and in cumulus-oocyte complex (COC) expansion, maternal-fetal interaction and preterm labour. Despite these facts, the role of TLR4 in ovarian physiology is not fully understood. Therefore, the aim of the present study was to investigate the effects of TLR4 genetic ablation on mice folliculogenesis and female fertility, through analysis of reproductive crosses, ovarian responsiveness and follicular quantification in TLR4-/- (n = 94) and C57BL/6 mice [wild type (WT), n = 102]. TLR4-deficient pairs showed a reduced number of pups per litter (P = 0.037) compared with WT. TLR4-/- mice presented more primordial, primary, secondary and antral follicles (P < 0.001), however there was no difference in estrous cyclicity (P > 0.05). A lower (P = 0.006) number of COC was recovered from TLR4-/- mice oviducts after superovulation, and in heterozygous pairs, TLR4-/- females also showed a reduction in the pregnancy rate and in the number of fetuses per uterus (P = 0.007) when compared with WT. Altogether, these data suggest that TLR4 plays a role in the regulation of murine folliculogenesis and in determining ovarian endowment. TLR4 deficiency may affect ovulation and pregnancy rates, potentially decreasing fertility, therefore the potential side effects of its blockade have to be carefully investigated.

Auxin-inducible protein degradation as a novel approach for protein depletion and reverse genetic discoveries in mammalian oocytes†

The disruption of protein expression is a major approach used for investigating protein function in mammalian oocytes. This is often achieved with RNAi/morpholino-mediated knockdown or gene knockout, leading to long-term loss of proteins of interest. However, these methods have noteworthy limitations, including (a) slow protein turnover can prohibit use of these approaches; (b) essential roles in early events precludes characterization of functions in subsequent events; (c) extended protein loss can allow time for compensatory mechanisms and other unanticipated events that confound interpretation of results. The work presented here examines the use of auxin-inducible degradation, a powerful new approach that overcomes these limitations through the depletion of one's protein of interest through controllable ubiquitin-mediated degradation. This method has been employed in yeast and mammalian cell lines, and here we demonstrate the utility of auxin-inducible degradation in mouse oocytes at multiple stages of meiosis, through degradation of exogenously expressed EGFP. We also evaluate important parameters for experimental design for use of this system in oocytes. This study thus expands the toolkit of researchers in oocyte biology, establishing the use of this unique and versatile approach for depleting proteins in oocytes, and providing researchers with valuable information to make use of this system.

Genome-wide association study provides strong evidence of genes affecting the reproductive performance of Nellore beef cows

Reproductive traits are economically important for beef cattle production; however, these traits are still a bottleneck in indicine cattle since these animals typically reach puberty at older ages when compared to taurine breeds. In addition, reproductive traits are complex phenotypes, i.e., they are controlled by both the environment and many small-effect genes involved in different pathways. In this study, we conducted genome-wide association study (GWAS) and functional analyses to identify important genes and pathways associated with heifer rebreeding (HR) and with the number of calvings at 53 months of age (NC53) in Nellore cows. A total of 142,878 and 244,311 phenotypes for HR and NC53, respectively, and 2,925 animals genotyped with the Illumina Bovine HD panel (Illumina^®, San Diego, CA, USA) were used in GWAS applying the weighted single-step GBLUP (WssGBLUP) method. Several genes associated with reproductive events were detected in the 20 most important 1Mb windows for both traits. Significant pathways for HR and NC53 were associated with lipid metabolism and immune processes, respectively. MHC class II genes, detected on chromosome 23 (window 25-26Mb) for NC53, were significantly associated with pregnancy success of Nellore cows. These genes have been proved previously to be associated with reproductive traits such as mate choice in other breeds and species. Our results suggest that genes associated with the reproductive traits HR and NC53 may be involved in embryo development in mammalian species. Furthermore, some genes associated with mate choice may affect pregnancy success in Nellore cattle.

Regulation of GVBD in mouse oocytes by miR-125a-3p and Fyn kinase through modulation of actin filaments

Meiotically arrested oocytes are characterized by the presence of the nuclear structure known as germinal-vesicle (GV), the breakdown of which (GVBD) is associated with resumption of meiosis. Fyn is a pivotal factor in resumption of the first meiotic division; its inhibition markedly decreases the fraction of oocytes undergoing GVBD. Here, we reveal that in mouse oocytes Fyn is post-transcriptionally regulated by miR-125a-3p. We demonstrate that in oocytes resuming meiosis miR-125a-3p and Fyn exhibit a reciprocal expression pattern; miR-125a-3p decreases alongside with an increase in Fyn expression. Microinjection of miR-125a-3p inhibits GVBD, an effect that is markedly reduced by Fyn over-expression, and impairs the organization of the actin rim surrounding the nucleus. Lower rate of GVBD is also observed in oocytes exposed to cytochalasin-D or blebbistatin, which interfere with actin polymerization and contractility of actin bundles, respectively. By down-regulating Fyn in HEK-293T cells, miR-125a-3p reduces the interaction between actin and A-type lamins, which constitute the nuclear-lamina. Our findings suggest a mechanism, by which a decrease in miR-125a-3p during oocyte maturation facilitates GVBD by allowing Fyn up-regulation and the resulting stabilization of the interaction between actin and A-type lamins.

LynA regulates an inflammation-sensitive signaling checkpoint in macrophages

Clustering of receptors associated with immunoreceptor tyrosine-based activation motifs (ITAMs) initiates the macrophage antimicrobial response. ITAM receptors engage Src-family tyrosine kinases (SFKs) to initiate phagocytosis and macrophage activation. Macrophages also encounter nonpathogenic molecules that cluster receptors weakly and must tune their sensitivity to avoid inappropriate responses. To investigate this response threshold, we compared signaling in the presence and absence of receptor clustering using a small-molecule inhibitor of Csk, which increased SFK activation and produced robust membrane-proximal signaling. Surprisingly, receptor-independent SFK activation led to a downstream signaling blockade associated with rapid degradation of the SFK LynA. Inflammatory priming of macrophages upregulated LynA and promoted receptor-independent signaling. In contrast, clustering the hemi-ITAM receptor Dectin-1 induced signaling that did not require LynA or inflammatory priming. Together, the basal-state signaling checkpoint regulated by LynA expression and degradation and the signaling reorganization initiated by receptor clustering allow cells to discriminate optimally between pathogens and nonpathogens.

MAPK3/1 (ERK1/2) and Myosin Light Chain Kinase in Mammalian Eggs Affect Myosin-II Function and Regulate the Metaphase II State in a Calcium- and Zinc-Dependent Manner

Vertebrate eggs are arrested at metaphase of meiosis II, a state classically known as cytostatic factor arrest. Maintenance of this arrest until the time of fertilization and then fertilization-induced exit from metaphase II are crucial for reproductive success. Another key aspect of this meiotic arrest and exit is regulation of the metaphase II spindle, which must be appropriately localized adjacent to the egg cortex during metaphase II and then progress into successful asymmetric cytokinesis to produce the second polar body. This study examined the mitogen-activated protein kinases MAPK3 and MAPK1 (also known as ERK1/2) as regulators of these two related aspects of mammalian egg biology, specifically testing whether this MAPK pathway affected myosin-II function and whether myosin-II perturbation would produce some of the same effects as MAPK pathway perturbation. Inhibition of the MEK1/2-MAPK pathway with U0126 leads to reduced levels of phosphorylated myosin-regulatory light chain (pMRLC) and causes a reduction in cortical tension, effects that are mimicked by treatment with the myosin light chain kinase (MLCK) inhibitor ML-7. These data indicate that one mechanism by which the MAPK pathway acts in eggs is by affecting myosin-II function. We further show that MAPK or MLCK inhibition induces loss of normal cortical spindle localization or parthenogenetic egg activation. This parthenogenesis is dependent on cytosolic and extracellular calcium and can be rescued by hyperloading eggs with zinc, suggesting that these effects of inhibition of MLCK or the MAPK pathway are linked with dysregulation of ion homeostasis.

Recurrent triploidy due to a failure to complete maternal meiosis II: whole-exome sequencing reveals candidate variants

Triploidy is a relatively common cause of miscarriage; however, recurrent triploidy has rarely been reported. A healthy 34-year-old woman was ascertained because of 18 consecutive miscarriages with triploidy found in all 5 karyotyped losses. Molecular results in a sixth loss were also consistent with triploidy. Genotyping of markers near the centromere on multiple chromosomes suggested that all six triploid conceptuses occurred as a result of failure to complete meiosis II (MII). The proband's mother had also experienced recurrent miscarriage, with a total of 18 miscarriages. Based on the hypothesis that an inherited autosomal-dominant maternal predisposition would explain the phenotype, whole-exome sequencing of the proband and her parents was undertaken to identify potential candidate variants. After filtering for quality and rarity, potentially damaging variants shared between the proband and her mother were identified in 47 genes. Variants in genes coding for proteins implicated in oocyte maturation, oocyte activation or polar body extrusion were then prioritized. Eight of the most promising candidate variants were confirmed by Sanger sequencing. These included a novel change in the PLCD4 gene, and a rare variant in the OSBPL5 gene, which have been implicated in oocyte activation upon fertilization and completion of MII. Several variants in genes coding proteins playing a role in oocyte maturation and early embryonic development were also identified. The genes identified may be candidates for the study in other women experiencing recurrent triploidy or recurrent IVF failure.

Post-ovulatory aging of oocytes disrupts kinase signaling pathways and lysosome biogenesis

Post-ovulatory aging of oocytes results in the progressive loss of fertilization and developmental competence. This degradation of oocyte quality has been the object of numerous investigations, primarily focused on individual signaling pathways which provide limited insight into the status of global signaling events. The purpose of the present investigation was to comprehensively assess broad patterns of signaling pathway activity during in vitro aging as an initial step in defining control points that can be targeted to prevent the reduction in oocyte quality during prolonged culture. An antibody microarray-based phospho-proteome analysis performed on oocytes before and after eight hours of culture revealed significant changes in the abundance or activation state of 43 proteins that function in a wide variety of protein kinase-mediated signaling pathways. Several of the most significantly affected kinases were studied by Western blot and confocal immunofluorescence to corroborate the array results. Prolonged culture resulted in global changes in the abundance and activity of protein kinases that regulate the response to calcium, stress, and cell-cycle control. Examination of intracellular structures revealed a previously unrecognized increase in the abundance of large autophogagic lysosomes, which correlates with changes in protein kinase pathways. These results provide insight into the stresses experienced by oocytes during culture and the diversity of responses that results from them. The observed increase in autophagy-related activity, together with the disruptions in calcium signaling, cell-cycle, and stress-response pathways, have the potential to negatively impact oocyte quality by interfering with the normal sequence of biochemical changes that constitute egg activation following fertilization.

PTK2b function during fertilization of the mouse oocyte

Fertilization triggers rapid changes in intracellular free calcium that serve to activate multiple signaling events critical to the initiation of successful development. Among the pathways downstream of the fertilization-induced calcium transient is the calcium-calmodulin dependent protein tyrosine kinase PTK2b or PYK2 kinase. PTK2b plays an important role in fertilization of the zebrafish oocyte and the objective of the present study was to establish whether PTK2b also functions in mammalian fertilization. PTK2b was activated during the first few hours after fertilization of the mouse oocyte during the period when anaphase resumption was underway and prior to the pronuclear stage. Suppression of PTK2b kinase activity in oocytes blocked sperm incorporation and egg activation although sperm-oocyte binding was not affected. Oocytes that failed to incorporate sperm after inhibitor treatment showed no evidence of a calcium transient and no evidence of anaphase resumption suggesting that egg activation did not occur. The results indicate that PTK2b functions during the sperm-egg fusion process or during the physical incorporation of sperm into the egg cytoplasm and is therefore critical for successful development.

SRC-family tyrosine kinases in oogenesis, oocyte maturation and fertilization: an evolutionary perspective

The oocyte is a highly specialized cell poised to respond to fertilization with a unique set of actions needed to recognize and incorporate a single sperm, complete meiosis, reprogram maternal and paternal genomes and assemble them into a unique zygotic genome, and finally initiate the mitotic cell cycle. Oocytes accomplish this diverse series of events through an array of signal transduction pathway components that include a characteristic collection of protein tyrosine kinases. The src-family protein kinases (SFKs) figure importantly in this signaling array and oocytes characteristically express certain SFKs at high levels to provide for the unique actions that the oocyte must perform. The SFKs typically exhibit a distinct pattern of subcellular localization in oocytes and perform critical functions in different subcellular compartments at different steps during oocyte maturation and fertilization. While many aspects of SFK signaling are conserved among oocytes from different species, significant differences exist in the extent to which src-family-mediated pathways are used by oocytes from species that fertilize externally vs those which are fertilized internally. The observation that several oocyte functions which require SFK signaling appear to represent common points of failure during assisted reproductive techniques in humans, highlights the importance of these signaling pathways for human reproductive health.

Molecular mechanisms of asymmetric division in oocytes

In contrast to symmetric division in mitosis, mammalian oocyte maturation is characterized by asymmetric cell division that produces a large egg and a small polar body. The asymmetry results from oocyte polarization, which includes spindle positioning, migration, and cortical reorganization, and this process is critical for fertilization and the retention of maternal components for early embryo development. Although actin dynamics are involved in this process, the molecular mechanism underlying this remained unclear until the use of confocal microscopy and live cell imaging became widespread in recent years. Information obtained through a PubMed database search of all articles published in English between 2000 and 2012 that included the phrases "oocyte, actin, spindle migration," "oocyte, actin, polar body," or "oocyte, actin, asymmetric division" was reviewed. The actin nucleation factor actin-related protein 2/3 complex and its nucleation-promoting factors, formins and Spire, and regulators such as small GTPases, partitioning-defective/protein kinase C, Fyn, microRNAs, cis-Golgi apparatus components, myosin/myosin light-chain kinase, spindle stability regulators, and spindle assembly checkpoint regulators, play critical roles in asymmetric cell division in oocytes. This review summarizes recent findings on these actin-related regulators in mammalian oocyte asymmetric division and outlines a complete signaling pathway.

C-Src and c-Yes are two unlikely partners of spermatogenesis and their roles in blood-testis barrier dynamics

Src family kinases (SFKs), in particular c-Src and c-Yes, are nonreceptor protein tyrosine kinases that mediate integrin signaling at focal adhesion complex at the cell-extracellular matrix interface to regulate cell adhesion, cell cycle progression, cell survival, proliferation and differentiation, most notably in cancer cells during tumorigenesis and metastasis. Interestingly, recent studies have shown that these two proto-oncogenes are integrated components of the stem cell niche and the cell-cell actin-based anchoring junction known as ectoplasmic specialization (ES) at the: (1) Sertoli cell-spermatid interface known as apical ES and (2) Sertoli-Sertoli cell interface known as basal ES which together with tight junctions (TJ), gap junctions and desmosomes constitute the blood-testis barrier (BTB). At the stem cell niche, these SFKs regulate spermatogonial stem cell (SSC) renewal to maintain the proper population of SSC/spermatogonia for spermatogenesis. At the apical ES and the BTB, c-Src and c-Yes confer cell adhesion either by maintaining the proper phosphorylation status of integral membrane proteins at the site which in turn regulates protein-protein interactions between integral membrane proteins and their adaptors, or by facilitating androgen action on spermatogenesis via a nongenomic pathway which also modulates cell adhesion in the seminiferous epithelium. Herein, we critically evaluate recent findings in the field regarding the roles of these two unlikely partners of spermatogenesis. We also propose a hypothetical model on the mechanistic functions of c-Src and c-Yes in spermatogenesis so that functional experiments can be designed in future studies.

Intersecting roles of protein tyrosine kinase and calcium signaling during fertilization

The oocyte is a highly specialized cell that must respond to fertilization with a preprogrammed series of signal transduction events that establish a block to polyspermy, trigger resumption of the cell cycle and execution of a developmental program. The fertilization-induced calcium transient is a key signal that initiates the process of oocyte activation and studies over the last several years have examined the signaling pathways that act upstream and downstream of this calcium transient. Protein tyrosine kinase signaling was found to be an important component of the upstream pathways that stimulated calcium release at fertilization in oocytes from animals that fertilize externally, but a similar pathway has not been found in mammals which fertilize internally. The following review will examine the diversity of signaling in oocytes from marine invertebrates, amphibians, fish and mammals in an attempt to understand the basis for the observed differences. In addition to the pathways upstream of the fertilization-induced calcium transient, recent studies are beginning to unravel the role of protein tyrosine kinase signaling downstream of the calcium transient. The PYK2 kinase was found to respond to fertilization in the zebrafish system and seems to represent a novel component of the response of the oocyte to fertilization. The potential impact of impaired PTK signaling in oocyte quality will also be discussed.

PYK2: a calcium-sensitive protein tyrosine kinase activated in response to fertilization of the zebrafish oocyte

Fertilization begins with binding and fusion of a sperm with the oocyte, a process that triggers a high amplitude calcium transient which propagates through the oocyte and stimulates a series of preprogrammed signal transduction events critical for zygote development. Identification of the pathways downstream of this calcium transient remains an important step in understanding the basis of zygote quality. The present study demonstrates that the calcium-calmodulin sensitive protein tyrosine kinase PYK2 is a target of the fertilization-induced calcium transient in the zebrafish oocyte and that it plays an important role in actin-mediated events critical for sperm incorporation. At fertilization, PYK2 was activated initially at the site of sperm-oocyte interaction and was closely associated with actin filaments forming the fertilization cone. Later PYK2 activation was evident throughout the entire oocyte cortex, however activation was most intense over the animal hemisphere. Fertilization-induced PYK2 activation could be blocked by suppressing calcium transients in the ooplasm via injection of BAPTA as a calcium chelator. PYK2 activation could be artificially induced in unfertilized oocytes by injection of IP3 at concentrations sufficient to induce calcium release. Functionally, suppression of PYK2 activity by chemical inhibition or by injection of a dominant-negative construct encoding the N-terminal ERM domain of PKY2 inhibited formation of an organized fertilization cone and reduced the frequency of successful sperm incorporation. Together, the above findings support a model in which PYK2 responds to the fertilization-induced calcium transient by promoting reorganization of the cortical actin cytoskeleton to form the fertilization cone.

Role of FYN kinase in spermatogenesis: defects characteristic of Fyn-null sperm in mice

FYN kinase is highly expressed in the testis and has been implicated in testis and sperm function, yet specific roles for this kinase in testis somatic and germ cells have not been defined. The purpose of the present investigation was to identify aspects of spermatogenesis, spermiation, or sperm fertilizing capacity that required FYN for normal reproductive function. Matings between Fyn-null males and wild-type females resulted in normal litter sizes, despite the fact that Fyn-null males exhibited reduced epididymal size and sperm count. Morphological analysis revealed a high frequency of abnormal sperm morphology among Fyn-null sperm, and artificial insemination competition studies demonstrated that Fyn-null sperm possessed reduced fertilizing capacity. Fyn-null sperm exhibited nearly normal motility during capacitation in vitro but reduced ability to undergo the acrosome reaction and fertilize oocytes. The typical pattern of capacitation-induced protein tyrosine phosphorylation was slightly modified in Fyn-null sperm, with reduced abundance of several minor phosphoproteins. These findings are consistent with a model in which FYN kinase plays an important role in proper shaping of the head and acrosome within the testis and possibly an additional role in the sperm acrosome reaction, events required for development of full fertilizing capacity in sperm.

Sperm-egg interaction

A crucial step of fertilization is the sperm-egg interaction that allows the two gametes to fuse and create the zygote. In the mouse, CD9 on the egg and IZUMO1 on the sperm stand out as critical players, as Cd9(-/-) and Izumo1(-/-) mice are healthy but infertile or severely subfertile due to defective sperm-egg interaction. Moreover, work on several nonmammalian organisms has identified some of the most intriguing candidates implicated in sperm-egg interaction. Understanding of gamete membrane interactions is advancing through characterization of in vivo and in vitro fertilization phenotypes, including insights from less robust phenotypes that highlight potential supporting (albeit not absolutely essential) players. An emerging theme is that there are varied roles for gamete molecules that participate in sperm-egg interactions. Such roles include not only functioning as fusogens, or as adhesion molecules for the opposite gamete, but also functioning through interactions in cis with other proteins to regulate membrane order and functionality.

The spatial and mechanical challenges of female meiosis

Recent work shows that cytokinesis and other cellular morphogenesis events are tuned by an interplay among biochemical signals, cell shape, and cellular mechanics. In cytokinesis, this includes cross-talk between the cortical cytoskeleton and the mitotic spindle in coordination with cell cycle control, resulting in characteristic changes in cellular morphology and mechanics through metaphase and cytokinesis. The changes in cellular mechanics affect not just overall cell shape, but also mitotic spindle morphology and function. This review will address how these principles apply to oocytes undergoing the asymmetric cell divisions of meiosis I and II. The biochemical signals that regulate cell cycle timing during meiotic maturation and egg activation are crucial for temporal control of meiosis. Spatial control of the meiotic divisions is also important, ensuring that the chromosomes are segregated evenly and that meiotic division is clearly asymmetric, yielding two daughter cells - oocyte and polar body - with enormous volume differences. In contrast to mitotic cells, the oocyte does not undergo overt changes in cell shape with its progression through meiosis, but instead maintains a relatively round morphology with the exception of very localized changes at the time of polar body emission. Placement of the metaphase-I and -II spindles at the oocyte periphery is clearly important for normal polar body emission, although this is likely not the only control element. Here, consideration is given to how cellular mechanics could contribute to successful mammalian female meiosis, ultimately affecting egg quality and competence to form a healthy embryo.

Protein tyrosine kinase signaling during oocyte maturation and fertilization

The oocyte is a highly specialized cell capable of accumulating and storing energy supplies as well as maternal transcripts and pre-positioned signal transduction components needed for zygotic development, undergoing meiosis under control of paracrine signals from the follicle, fusing with a single sperm during fertilization, and zygotic development. The oocyte accomplishes this diverse series of events by establishing an array of signal transduction pathway components that include a select collection of protein tyrosine kinases (PTKs) that are expressed at levels significantly higher than most other cell types. This array of PTKs includes cytosolic kinases such as SRC-family PTKs (FYN and YES), and FAK kinases, as well as FER. These kinases typically exhibit distinct patterns of localization and in some cases are translocated from one subcellular compartment to another during meiosis. Significant differences exist in the extent to which PTK-mediated pathways are used by oocytes from species that fertilize externally versus internally. The PTK activation profiles as well as calcium signaling pattern seems to correlate with the extent to which a rapid block to polyspermy is required by the biology of each species. Suppression of each of the SRC-family PTKs as well as FER kinase results in failure of meiotic maturation or zygote development, indicating that these PTKs are important for oocyte quality and developmental potential. Future studies will hopefully reveal the extent to which these factors impact clinical assisted reproductive techniques in domestic animals and humans.