Program of early development in the mammal: changes in the patterns and absolute rates of tubulin and total protein synthesis during oocyte growth in the mouse

Mitochondrial dysfunction in the offspring of obese mothers and it's transmission through damaged oocyte mitochondria: Integration of mechanisms

In vivo and in vitro studies demonstrate that mitochondria in the oocyte, are susceptible to damage by suboptimal pre/pregnancy conditions, such as obesity. These suboptimal conditions have been shown to induce mitochondrial dysfunction (MD) in multiple tissues of the offspring, suggesting that mitochondria of oocytes that pass from mother to offspring, can carry information that can programme mitochondrial and metabolic dysfunction of the next generation. They also suggest that transmission of MD could increase the risk of obesity and other metabolic diseases in the population inter- and trans-generationally. In this review, we examined whether MD observed in offspring tissues of high energetic demand, is the result of the transmission of damaged mitochondria from the oocytes of obese mothers to the offspring. The contribution of genome-independent mechanisms (namely mitophagy) in this transmission were also explored. Finally, potential interventions aimed at improving oocyte/embryo health were investigated, to see if they may provide an opportunity to halter the generational effects of MD.

Fertility loss: negative effects of environmental toxicants on oogenesis

There has been a global decline in fertility rates, with ovulatory disorders emerging as the leading cause, contributing to a global lifetime infertility prevalence of 17.5%. Formation of the primordial follicle pool during early and further development of oocytes after puberty is crucial in determining female fertility and reproductive quality. However, the increasing exposure to environmental toxins (through occupational exposure and ubiquitous chemicals) in daily life is a growing concern; these toxins have been identified as significant risk factors for oogenesis in women. In light of this concern, this review aims to enhance our understanding of female reproductive system diseases and their implications. Specifically, we summarized and categorized the environmental toxins that can affect oogenesis. Here, we provide an overview of oogenesis, highlighting specific stages that may be susceptible to the influence of environmental toxins. Furthermore, we discuss the genetic and molecular mechanisms by which various environmental toxins, including metals, cigarette smoke, and agricultural and industrial toxins, affect female oogenesis. Raising awareness about the potential risks associated with toxin exposure is crucial. However, further research is needed to fully comprehend the mechanisms underlying these effects, including the identification of biomarkers to assess exposure levels and predict reproductive outcomes. By providing a comprehensive overview, this review aims to contribute to a better understanding of the impact of environmental toxins on female oogenesis and guide future research in this field.

Transcriptome analysis of sheep follicular development during prerecruitment, dominant, and mature stages after FSH superstimulation

Sheep is usually a monovular animal; superovulation technology is used to increase the number of offspring per individual and shorten generation intervals. To date, mature FSH superstimulatory treatments have been successfully used in sheep breeding, but much remains unknown about genes, pathways, and biological functions involved in follicular development. Therefore, in this study, we performed transcriptome profiling of small follicles (SFs; 2-2.5 mm), medium follicles (MFs; 3.5-4.5 mm), and large follicles (LFs; > 6 mm) in Mongolian ewes after FSH superstimulation. Furthermore, we identified differentially expressed genes and performed Kyoto Encyclopedia of Genes and Genomes pathway and Gene Ontology enrichment analyses in 3 separate pairwise comparisons. We found that ovarian steroidogenesis was significantly enriched in the SFs versus MFs analysis; the associated genes, cytochrome P450 family 19 (CYP19) and Hydroxy-delta-5-steroid dehydrogenase 3 beta- and steroid delta-isomerase 1 (HSD3B1), were significantly upregulated. Moreover, proline metabolism, glutathione metabolism, and PPAR signaling pathways were significantly enriched in the LFs versus SFs analysis; the associated genes, glutamate-cysteine ligase modifier subunit (GCLM) and cystathionine gamma-lyase (CTH), were significantly upregulated, whereas peroxisome proliferator-activated receptor gamma (PPARγ) was significantly downregulated. In summary, our study provides basic data and possible biological direction to further explore the molecular mechanism of sheep follicular development after FSH superstimulation.

Reconstitution of the oocyte transcriptional network with transcription factors

During female germline development, oocytes become a highly specialized cell type and form a maternal cytoplasmic store of crucial factors. Oocyte growth is triggered at the transition from primordial to primary follicle and is accompanied by dynamic changes in gene expression¹, but the gene regulatory network that controls oocyte growth remains unknown. Here we identify a set of transcription factors that are sufficient to trigger oocyte growth. By investigation of the changes in gene expression and functional screening using an in vitro mouse oocyte development system, we identified eight transcription factors, each of which was essential for the transition from primordial to primary follicle. Notably, enforced expression of these transcription factors swiftly converted pluripotent stem cells into oocyte-like cells that were competent for fertilization and subsequent cleavage. These transcription-factor-induced oocyte-like cells were formed without specification of primordial germ cells, epigenetic reprogramming or meiosis, and demonstrate that oocyte growth and lineage-specific de novo DNA methylation are separable from the preceding epigenetic reprogramming in primordial germ cells. This study identifies a core set of transcription factors for orchestrating oocyte growth, and provides an alternative source of ooplasm, which is a unique material for reproductive biology and medicine.

The oocyte-to-embryo transition in mouse: past, present, and future

The oocyte-to-embryo transition (OET) arguably initiates with formation of a primordial follicle and culminates with reprogramming of gene expression during the course of zygotic genome activation. This transition results in converting a highly differentiated cell, i.e. oocyte, to undifferentiated cells, i.e. initial blastomeres of a preimplantation embryo. A plethora of changes occur during the OET and include, but are not limited to, changes in transcription, chromatin structure, and protein synthesis; accumulation of macromolecules and organelles that will comprise the oocyte's maternal contribution to the early embryo; sequential acquisition of meiotic and developmental competence to name but a few. This review will focus on transcriptional and post-transcriptional changes that occur during OET in mouse because such changes are likely the major driving force for OET. We often take a historical and personal perspective, and highlight how advances in experimental methods often catalyzed conceptual advances in understanding the molecular bases for OET. We also point out questions that remain open and therefore represent topics of interest for future investigation.

Quantitative Microinjection of Morpholino Antisense Oligonucleotides into Mouse Oocytes to Examine Gene Function in Meiosis-I

Specific protein depletion is a powerful approach for assessing individual gene function in cellular processes, and has been extensively employed in recent years in mammalian oocyte meiosis-I. Conditional knockout mice and RNA interference (RNAi) methods such as siRNA or dsRNA microinjection are among several approaches to have been applied in this system over the past decade. RNAi by microinjection of Morpholino antisense Oligonucleotides (MO), in particular, has proven highly popular and tractable in many studies, since MOs have high specificity of interaction, low cell toxicity, and are more stable than other microinjected RNAi molecules. Here, we describe a method of MO microinjection into the mouse germinal vesicle-stage (GV) oocyte followed by a simple immunofluorescence approach for examination of gene function in meiosis-I.

Identification of a human subcortical maternal complex

Maternal effect genes play essential roles in early embryonic development. However, the mechanisms by which maternal effect genes regulate mammalian early embryonic development remain largely unknown. Recently, we identified a subcortical maternal complex (SCMC) that is composed of at least four proteins encoded by Mater, Floped, Tle6 and Filia and is critical for mouse preimplantation development. The present study demonstrates that human SCMC homologous genes (NLRP5, OOEP, TLE6 and KHDC3L) are specifically expressed in the oocytes of human fetal ovaries. The proteins of this complex co-localize in the subcortex of human oocytes and early embryos. Furthermore, the SCMC proteins physically interact with each other when they are co-expressed in cell lines. These results indicate that human NLRP5, OOEP, TLE6 and KHDC3L function as a complex in the oocytes and early embryos of Homo sapiens. Considering the important roles of the SCMC in mouse early embryogenesis, the characterization of the human SCMC will provide a basis for investigating human early embryonic development and will have clinical implications in human female infertility or recurrent spontaneous abortion.

Molecular mechanisms underlying the activation of mammalian primordial follicles

In humans and other mammalian species, the pool of resting primordial follicles serves as the source of developing follicles and fertilizable ova for the entire length of female reproductive life. One question that has intrigued biologists is: what are the mechanisms controlling the activation of dormant primordial follicles. Studies from previous decades have laid a solid, but yet incomplete, foundation. In recent years, molecular mechanisms underlying follicular activation have become more evident, mainly through the use of genetically modified mouse models. As hypothesized in the 1990s, the pool of primordial follicles is now known to be maintained in a dormant state by various forms of inhibitory machinery, which are provided by several inhibitory signals and molecules. Several recently reported mutant mouse models have shown that a synergistic and coordinated suppression of follicular activation provided by multiple inhibitory molecules is necessary to preserve the dormant follicular pool. Loss of function of any of the inhibitory molecules for follicular activation, including PTEN (phosphatase and tensin homolog deleted on chromosome 10), Foxo3a, p27, and Foxl2, leads to premature and irreversible activation of the primordial follicle pool. Such global activation of the primordial follicle pool leads to the exhaustion of the resting follicle reserve, resulting in premature ovarian failure in mice. In this review, we summarize both historical and recent results on mammalian primordial follicular activation and focus on the up-to-date knowledge of molecular networks controlling this important physiological event. We believe that information obtained from mutant mouse models may also reflect the molecular machinery responsible for follicular activation in humans. These advances may provide a better understanding of human ovarian physiology and pathophysiology for future clinical applications.

Control of mammalian oocyte growth and early follicular development by the oocyte PI3 kinase pathway: new roles for an old timer

A large amount of information has accumulated over the past decade on how gonadotropins, steroid hormones and growth factors regulate development of the mammalian ovarian follicle. Moreover, the bi-directional communication between mammalian oocytes and their surrounding somatic (granulosa) cells has also been shown to be crucial for this process. The intra-ovarian factors, or more specifically, the intra-oocyte signaling pathways that control oocyte growth and early follicular development are largely unknown, however. Based on both in vitro studies and in vivo functional studies using gene-modified mouse models, this review focuses on the key features of the phosphatidylinositol 3 kinase (PI3K) pathway in growing mouse oocytes and on the novel functions of the oocyte PI3K pathway in controlling mammalian oocyte growth and follicular development that have come to light only recently. We propose that the PI3K pathway in the oocyte, which is activated by granulosa cell-produced Kit ligand (KL) via the oocyte-surface receptor Kit, may serve as an intra-oocyte network that regulates both oocyte growth and the early development of ovarian follicles.

Comparative protein expression profiling in human cumulus cells in relation to oocyte fertilization and ovarian stimulation protocol

Comparative profiling was performed on proteins synthesized in human cumulus cells (CC) from individual oocytes recovered after two different ovarian stimulation protocols for classical IVF (cIVF). Using high-resolution two-dimensional protein electrophoresis after metabolic labelling with [35S]-methionine, protein expression was profiled in CC of metaphase II oocytes obtained after two different ovarian stimulation protocols (rFSH versus human menopausal gonadotrophin). Analysis was done on CC from two cIVF cycles in the same patient and then extended to CC from individual oocytes from two groups of patients. CC from single oocytes have robust levels of protein expression into 600-800 protein spots. Comparison of CC protein expression from oocytes obtained from the same patient but after two different stimulation protocols shows that the type of hormonal treatment influences CC protein expression. In contrast, CC from oocytes obtained under the same stimulation protocol but with different fertilization outcome show a high profile similarity with differences in only a few spots. Comparison of two groups of patients indicates that dissimilarities in protein pattern between patients become very high, even when comparing the same stimulation protocol and oocyte fertilization outcome. Thus protein expression profiling of human CC may provide a correlation between the synthesis of specific cumulus proteins and maturity and fecundity.

Transcript profiling during mouse oocyte development and the effect of gonadotropin priming and development in vitro

The molecular basis for acquisition of meiotic and developmental competence, the two main outcomes of oocyte development and essential for producing an egg capable of being fertilized and supporting development to term, is largely unknown. Using microarrays, we characterized global changes in gene expression in oocytes derived from primordial, primary, secondary, small antral, and large antral follicles and used Expression Analysis Systematic Explorer (EASE) to identify biological and molecular processes that accompany these transitions and likely underpin acquisition of meiotic and developmental competence. The greatest degree of change in gene expression occurs during the primordial to primary follicle transition. Of particular interest is that specific chromosomes display significant changes in their overall transcriptional activity and that in some cases these changes are largely confined to specific regions on these chromosomes. We also examined the transcript profile of oocytes that developed in vitro, as well as following eCG priming. Remarkably, the expression profiles only differed by 4% and 2% from oocytes that developed in vivo when compared to oocytes that developed in vitro from either primordial or secondary follicles, respectively. About 1% of the genes were commonly mis-expressed, and EASE analysis revealed there is an over-representation of genes involved in transcription. Developmental competence of oocytes obtained from eCG-primed mice was substantially improved when compared to oocytes obtained from unprimed mice, and this correlated with decreased expression of genes implicated in basal transcription.

Detection of classical swine fever virus in the ovaries of experimentally infected sows

Six sows were infected intranasally with a Korean isolate of classical swine fever virus (CSFV). The distribution of virus in ovarian tissues was then assessed for 21 days by in-situ hybridization and immunohistochemistry. CSFV was detected in the ovaries between 7 and 21 days post-inoculation (dpi) by both methods, but the labelling was particularly intense and widespread at 7 dpi. CSFV nucleic acid and antigen were located almost exclusively within the cytoplasm of cells shown by haematoxylin and eosin staining to be macrophages, which were numerous in atretic follicles. Small numbers of CSFV nucleic acid-positive cells with distinctly round morphology and oval nuclei, resembling monocytes, were also observed in the blood vessels of sows at 7 and 14 dpi. CSFV nucleic acid and antigen were not observed in primordial, primary or secondary follicles from infected sows at 7, 14 or 21 dpi. The results suggest that CSFV replicates in circulating peripheral monocytes and gains access to ovarian tissues from the bloodstream, and that this contributes to the distribution of CSFV in macrophages throughout the atretic follicles.

The molecular basis of oocyte growth and development

The mammalian oocyte is formed early in foetal life and may remain suspended at diplotene of the first meiotic prophase until several decades later. When, following follicular recruitment, the oocyte enters a growth phase where it increases in volume, and undergoes replication and redistribution of the cytoplasmic organelles. These modifications, including secretion of the zona pellucida, reflect a period of intensive RNA synthesis as the oocyte grows and accumulates the molecular program for embryogenesis. Throughout oocyte development follicle cell support is fundamental to provide the germ line cell with nutrients and growth regulators to ensure progression through the protracted growth phase. Conversely, the oocyte actively promotes growth and differentiation of the follicular cells. Finally, re-initiation of meiosis in mature oocytes results in the production of haploid gametes which are capable of supporting early embryo development. Our present knowledge of the molecular biology of mammalian oogenesis is discussed in this review.

Immunohistochemical evidence for the localization of bovine viral diarrhea virus, a single-stranded RNA virus, in ovarian oocytes in the cow

Bovine viral diarrhea virus (BVDV) is a single-stranded RNA virus responsible for enteric disease and reproductive failure in cattle. The virus can pass vertically from cow to fetus, causing abortion, birth of malformed calves, and calves born with persistent and life-long infections. In this study, we investigated the tropism of BVDV in ovarian tissue from persistently infected animals. Three heifers persistently infected with BVDV were euthanatized and their ovaries were recovered. A specimen of each ovary was taken (n = 6) for virus isolation, and the remaining ovarian tissue was stored at -70 C. Cryosections (6 microm) cut from each ovary were analyzed for the presence of BVDV antigens by indirect immunofluorescence. The immunofluorescent analysis employed two monoclonal antibodies, WB103 and WB162, previously raised against the nonstructural protein NS3 and the envelop glycoprotein E2, respectively. High titers (6.97 +/- 0.17 log10 tissue culture infective dose50/ml) of BVDV were recovered from 6/6 ovarian samples; NS3 and E2 were widely distributed within the ovarian stroma, the cumulus cell population, and the oocytes maturing in primordial, primary, and secondary follicles. Overall, 362/1,939 (18.7%) of the oocytes contained BVDV antigens, and there was no significant (P > 0.05) difference in the proportion of BVDV-infected oocytes recorded within the primordial (227/1,247, 18.2%), primary (122/630, 19.4%), and secondary (13/62, 21.0%) follicle populations. Although the developmental potential of the infected oocytes could not be established in the present study, we conclude that bovine oocyte and the cumulus cells are susceptible to BVDV infection.

Differential expression of ZPC in the bovine ovary, oocyte, and embryo

Using nonradioactive in situ hybridization (ISH), the mRNA encoding the zona glycoprotein bZPC was localized in bovine ovaries, oocytes, and embryos. In the ovary, the distribution of the mRNA was correlated with the developmental stage of the follicle. Whereas in primordial and primary follicles the mRNA was predominantly seen in the oocyte, it was found in both the oocyte and the follicle cells of secondary and tertiary follicles. In 2-day-old embryos produced by in vitro fertilization (IVF), no mRNA encoding ZPC could be demonstrated. Immunoblotting using monospecific polyclonal antibodies against porcine ZPC revealed a distinct band at a molecular weight of 47 kD in the ovarian cortex of cows, calves, and fetuses as well as in bovine follicle cells. Immunohistochemistry using the ZPC antibody displayed a strong signal in the zona pellucida of bovine oocytes and 2- to 6-day-old embryos as well as in the follicle cells. Our results show that during follicular development bovine ZPC is synthesized by the oocyte of the primary follicle and by both the oocyte and the follicle cells of the secondary and tertiary follicle. After fertilization, the synthesis of the zona protein is finished.

Growth and development of the mammalian oocyte

The oocyte is not only the rarest and the largest cell in the body, but it also has one of the most remarkable life histories. Formed in the fetal ovary and suspended at diplotene of meiosis, it may wait for years before beginning to grow, and not until this process is complete can it resume meiosis and undergo fertilisation. Major changes in the number, morphology and distribution of cytoplasmic organelles occur during growth, and a molecular program for embryogenesis is formed. Specific yolk proteins are absent and much of the RNA and some of the protein are degraded by the cleavage stage. The zona pellucida has been intensively studied, but knowledge of oocyte-specific genes is otherwise surprisingly patchy given the significance of this cell type and the expansion of reproductive technology. Finally, it is now clear that oocytes are not mere passengers which depend on granulosa cells for nutrition and regulation but actively promote the growth and differentiation of their follicles.

Mutant female mice carrying a single mZP3 allele produce eggs with a thin zona pellucida, but reproduce normally

The mouse egg zona pellucida (ZP) is composed of three glycoproteins, called mZP1-3. Disruption of the mZP3 gene by targeted mutagenesis yields mice that are homozygous (mZP3-/-) for the null mutation; although the mutant mice are viable, females are infertile and their eggs lack a ZP. On the other hand, females heterozygous (mZP3+/-) for the mutation are fertile and their eggs have a ZP. Here, we examined fully grown oocytes from mZP3+/- females and found that, although they have a ZP, it is less than half the width (approximately 2.7 microns; volume, approximately 56 pl) of the ZP of oocytes from wild-type (mZP3+/+) mice (approximately 6.2 microns; volume, approximately 145 pl). Oocyte ZP were purified from ovarian homogenates by gradient centrifugation. Immunostaining of purified ZP on Western gels permitted an estimate to be made of the relative amounts of mZP3 and mZP2 present in the ZP of oocytes from mZP3+/+ and mZP3+/- mice. We found that the ZP from mZP3+/- mice contained, on average, 55 +/- 15% of the mZP3 and 44 +/- 8% of the mZP2 present in the ZP of mZP3+/+ mice; a result quite consistent with the observed widths and calculated volumes of the ZP. Despite the presence of a relatively thin ZP surrounding their eggs, reproduction of female mZP3+/- mice was indistinguishable from female mZP3+/+ mice. These results strongly suggest that, when a single mZP3 allele is present, approximately half the wild-type amount of mZP3 and approximately half the wild-type amount of mZP2 is assembled into a ZP. While this produces a relatively thin ZP, it apparently has no affect on reproduction. Furthermore, these results are consistent with the current molecular model for ZP structure.

Regulation of the acquisition of meiotic competence in the mouse: changes in the subcellular localization of cdc2, cyclin B1, cdc25C and wee1, and in the concentration of these proteins and their transcripts

During their development, mammalian oocytes acquire the ability to resume meiosis. We demonstrate that the concentration of p34cdc2 increases during the acquisition of meiotic competence, as determined by immunoblotting, whereas the concentration of cyclin B1 decreases. Laser-scanning confocal microscopy corroborated these changes and furthermore indicate that an increase occurs in the nuclear concentration of each protein. Results of immunoblotting experiments demonstrate that associated with the acquisition of meiotic competence is an increase in the concentration of cdc25C, an activator of p34cdc2/cyclin B kinase, and a decrease in wee1, an inhibitor of cdc2/cyclin B kinase. These changes were again corroborated by laser-scanning confocal microscopy, which also indicates that an increase in the nuclear concentration of wee1 occurs. The concentration of the transcripts encoding these proteins, however, is essentially similar in meiotically incompetent and competent oocytes. Thus, these changes in protein concentration that occur during oocyte development likely reflect changes in the translational efficiency of their mRNAs. Consistent with this is that the relative rate of synthesis of p34cdc2 in meiotically competent oocytes is approximately 3 times greater than that in meiotically incompetent oocytes, whereas the stability of newly synthesized p34cdc2 is essentially the same in each cell type.

Mammalian oocyte growth in vitro is stimulated by soluble factor(s) produced by preantral granulosa cells and by Sertoli cells

We have evaluated the possibility that mouse oocyte growth in vitro could be achieved under the influence of soluble compound(s) released by different somatic cell types. For this purpose, zona-free denuded oocytes from 12-day-old mice were cultured on monolayers of NIH-3T3 fibroblasts, which are able to establish gap junctional communications with them, in the presence or absence of media conditioned by preantral granulosa cells or by Sertoli cells, plated at increasing concentrations from 0.3-1 x 10(6) ml-1 cells. After 3 days, no increase in vitellus diameter was recorded from fibroblast-coupled oocytes maintained in culture medium or in the presence of media conditioned by 0.3 x 10(6) ml-1 Sertoli cells. By contrast, increasing proportions of coupled oocytes grew, provided the continuous presence of media conditioned by 0.5 or 1 x 10(5) ml-1 Sertoli cells, or by 0.3, 0.5, and 1 x 10(5) ml-1 preantral granulosa cells. Since the ligand of c-kit, the growth factor KL, promotes the growth in vitro of oocytes cultured in follicles from 8-day-old mice, an antibody against mouse KL was used to evaluate whether in our culture conditions KL might also be responsible for the growth of oocytes from 12-day-old mice. No inhibition of growth was evident in oocytes cultured directly on preantral granulosa or Sertoli-cell monolayers. Furthermore, the growth of fibroblast-coupled oocytes cultured in media conditioned by preantral granulosa cells was not significantly affected by the presence of this antibody during culture. By contrast, a high percentage of oocytes cultured on fibroblasts in the presence of media conditioned by Sertoli cells showed a significant inhibition of growth and no metabolic cooperativity. It was concluded that, besides KL, other bioactive factor(s) released by either preantral granulosa or Sertoli cells can induce a significant stimulation of mouse oocyte growth in vitro.

Localization of the mRNA encoding the zona protein ZP3 alpha in the porcine ovary, oocyte and embryo by non-radioactive in situ hybridization

The mRNA of the zona pellucida glycoprotein ZP3 alpha was localized in frozen sections of pig ovaries, isolated oocytes and early embryos by in situ hybridization using biotinylated oligonucleotide probes. In follicles, the distribution of mRNA for ZP3 alpha was correlated with the developmental stage: in primordial and primary follicles, the mRNA was shown to be predominantly localized in the oocyte. In secondary follicles, mRNA was found in both the oocyte and follicle cells. In tertiary and preovulatory follicles, the follicle cells showed distinct staining, whereas the oocyte was labelled weakly. In the early embryo, i.e. 2 days after fertilization, mRNA for ZP3 alpha could not be demonstrated. Our results suggest that, in the pig, the zona pellucida protein ZP3 alpha is synthesized by the oocyte and the follicle cells in sequence. After fertilization, synthesis of ZP3 alpha is terminated.

Association between p34cdc2 levels and meiotic arrest in pig oocytes during early growth

The molecules involved in determining meiotic competence were determined in porcine oocytes isolated from preantral and antral follicles of different sizes. Oocytes isolated from preantral follicles had a mean diameter of 78 microns, contained diffuse filamentous chromatin in the germinal vesicle and were incapable of progressing from the G2 to the M phase of the cycle even after 72 h in culture. Oocytes from early antral follicles had a mean diameter of 105 microns, showed a filamentous chromatin configuration and about half resumed meiosis but arrested at metaphase I (MI) when cultured. Oocytes from mid-antral (3-4 mm) and large antral follicles (5-6 mm) had mean oocyte diameters of 115 and 119 microns respectively, contained condensed chromatin around the nucleolus and progressed to metaphase II (MII) in 48% and 93% of instances respectively. Analysis of p34cdc2, the catalytic subunit of maturation promoting factor (MPF), by immunoblotting indicates that the inability of small (78 microns) oocytes to resume meiosis is due, at least in part, to inadequate levels of the catalytic subunit of MPF. On the other hand, the inability of intermediate-sized (105 microns) oocytes from antral follicles to complete the first meiotic division by progressing beyond MI appears not to be limited by levels of p34cdc2, which are maximal by this stage. We postulate that an inadequacy of molecules other than p34cdc2 limits progression of MI to MII; the acquisition of these molecules during the final stages of growth may be correlated with the formation of the perinucleolar chromatin rim in the germinal vesicle.

Mitogen-activated protein (MAP) kinase during the acquisition of meiotic competence by growing oocytes of the mouse

During the growth phase of oogenesis, oocytes acquire the ability to undergo meiotic maturation. Although the molecular basis of this meiotic competence is unknown, specific differences in microtubular organization exist between incompetent and competent mammalian oocytes. Mitogen-activated protein (MAP) kinase has been implicated in microtubular regulation and is present in fully grown competent oocytes of mice, suggesting a possible role for this protein in the acquisition of meiotic competence. We report that the MAP kinase species, p42ERK2 and p44ERK1, were detectable by immunoblotting in incompetent oocytes at the early stages of oocyte growth and throughout subsequent growth and acquisition of competence. In partially competent oocytes, which can enter metaphase but cannot complete the first meiotic division, both p42ERK2 and p44ERK1 became phosphorylated, as judged by retarded electrophoretic mobility, and a morphologically normal spindle was assembled. In incompetent oocytes, which cannot enter metaphase, p42ERK2 and p44ERK1 remained nonphosphorylated. When these oocytes were treated with okadaic acid, an inhibitor of protein phosphatases 1 and 2A, a portion of them entered metaphase and the slow-migrating phosphorylated forms of p42ERK2 and p44ERK1 were observed. These phosphorylated forms appeared more rapidly, relative to the time of entry into metaphase, than during maturation of fully competent oocytes. The remaining incompetent oocytes, which did not enter metaphase during okadaic acid treatment, also did not generate slow-migrating p42ERK2 and p44ERK1. These results suggest that the acquisition of meiotic competence during oocyte growth is not linked to the de novo appearance of p42ERK2 or p44ERK1, that the failure of partially competent oocytes to complete meiosis I reflects a defect acting downstream or independently of MAP kinase phosphorylation, and that the ability of meiotically incompetent oocytes to generate phosphorylated forms of p42ERK2 and p44ERK1 in response to okadaic acid is linked to the ability to enter metaphase.

Morulae at compaction and the pattern of protein synthesis in mouse embryos

Compaction of mouse embryos at the 8-cell stage causes a drastic change in cell form and in cell-to-cell contacts in 3-4 h. We have studied the effect of inhibitors of transcription (alpha-amanitin), DNA replication (aphidicolin) and compaction (cytochalasin D, EGTA, alpha-lactalbumin and Con A) on the pattern of protein synthesis using gel electrophoresis. Our results show that the pattern of protein synthesis is regulated principally by passage through S phase during each early cell cycle rather than by de novo transcription, while changes induced in cell form or contacts do not alter the pattern significantly.

Gene expression during oogenesis in mice

A mouse egg is the end-product of oogenesis--a process initiated during fetal development and completed months later at the time of sperm-egg fusion. Oogenesis includes many important events. Among these are formation of female germ cells, initiation and completion of meiosis, and establishment of a maternal store of materials to support fertilization and preimplantation development. The latter takes place largely during germ cell growth in sexually mature females and involves extensive gene expression. Ribonucleic acid and protein accumulate to unusually high levels during this relatively short phase of oogenesis. Recent studies have demonstrated that establishment of a maternal store of materials in the growing mouse egg is both transcriptionally and translationally regulated. Specific examples of both types of regulation are presented here in the context of gene expression during oogenesis in mice.

Synthesis and developmental regulation of an egg specific mouse protein translated from maternal mRNA

Proteins synthesized by DDK mice embryos were analyzed by 2D electrophoresis and a new egg-specific polypeptide, D14, was identified. The protein is characterized by its high rate of synthesis and electrophoretic properties (MW 36,500, pl greater than 8). The synthesis of D14 is strictly developmentally regulated: starting in the maturing oocyte in the few hours following germinal vesicle breakdown (GVBD), it remains high over the first cell cycle and decreases abruptly during the two-cell stage. The arrest of D14 synthesis is triggered by egg activation and does not directly depend on transcription by the zygotic genome. Nevertheless, drugs that perturb the onset of zygotic transcription concomitantly inhibit D14 arrest of synthesis. D14 is present in both cytoplasmic and nuclear compartments at the two-cell stage; it is very stable and remains detectable at least until the eight-cell stage in the preimplantation embryo. Embryos of wild strains of mice synthesized either D14 or a D14 related polypeptide at a rate comparable to that of DDK embryos, which was at least ten times greater than that found in other laboratory strains. Both the developmental regulation and the genetic variability in its rate of synthesis make D14 an interesting polypeptide for the study of regulation of maternal information in the very early stages of mouse embryo development.

An upstream region of the mouse ZP3 gene directs expression of firefly luciferase specifically to growing oocytes in transgenic mice

The gene encoding the mouse egg primary receptor for sperm, a zona pellucida glycoprotein called ZP3, is expressed exclusively in growing oocytes within ovaries of sexually immature and mature female mice. We have constructed a transgene in which 6.5 kilobases of ZP3 gene 5'-flanking sequence is fused to the coding region of the firefly luciferase gene, and we have generated four independent lines of transgenic mice. In these animals, the transgene is expressed exclusively in ovaries. Furthermore, within ovaries, expression is confined to growing oocytes, and luciferase activity can be detected by assaying individual, isolated oocytes. The pattern of firefly luciferase expression during oocyte growth is similar to that observed in previous studies of ZP3 expression during oogenesis in mice. Observations reported here strongly suggest that cis-acting elements present in the ZP3 gene 5'-flanking region regulate oocyte-specific and, therefore, sex-specific expression of the sperm receptor gene during mouse development. They also suggest that such elements can be used to direct expression of cloned genes specifically to oocytes of transgenic mice and to evaluate the effects of such expression on various aspects of early mammalian development.

Developmental change in follicular cell-enhanced amino acid uptake into mouse oocytes that depends on intact gap junctions and transport system Gly

Uptake of L-alanine, L-lysine, and choline into both preantral and antral mouse oocytes was enhanced by follicular cells. Follicular cells also enhanced glycine uptake into oocytes at the preantral stage of development, but no effect of these cells was observed at the antral stage. Glycine uptake was predominantly Na+ dependent and inhibited almost completely by 10 mM sarcosine, moderately by proline and its analog pipecolate, and poorly or not at all by other amino acids. By these criteria, glycine transport was mainly via system Gly in follicular cells and the oolemma at both the preantral and antral stages. Moreover, an increase in glycine transport via the oolemma between the preantral and antral stages was more than threefold larger than was the increase in transport of alanine or lysine. This relatively large increase in glycine-specific transport in the oolemma appears to obscure the ability of follicular cells to enhance glycine uptake into antral oocytes. In contrast to other amino acids, leucine uptake into oocytes was not enhanced by follicular cells unless 14 other amino acids were also present at their concentrations in mouse serum. An inhibitor of gap junctional communication, 18-alpha-glycyrrhetinic acid, abolished follicular cell-enhanced uptake of glycine and choline into preantral oocytes. Therefore, the extent to which follicular cells enhance uptake of a particular amino acid into oocytes depends on at least three physiologically important variables. Namely, enhancement may depend on the stage of follicular development, the presence of other amino acids in the environment, and gap junctional communication.

Genes for extracellular-matrix-degrading metalloproteinases and their inhibitor, TIMP, are expressed during early mammalian development

Extracellular matrix (ECM) remodeling accompanies cell migration, cell-cell interactions, embryo expansion, uterine implantation, and tissue invasion during mammalian embryogenesis. We have found that mouse embryos secrete functional ECM-degrading metalloproteinases, including collagenase and stromelysin, that are inhibitable by the tissue inhibitor of metalloproteinases (TIMP) and that are regulated during peri-implantation development and endoderm differentiation. mRNA transcripts for collagenase, stromelysin, and TIMP were detected as maternal transcripts in the unfertilized egg, were present at the zygote and cleavage stages, and increased at the blastocyst stage and with endoderm differentiation. These data suggest that metalloproteinases function in cell-ECM interactions during growth, development, and implantation of mammalian embryos.

Oocyte-specific expression and developmental regulation of ZP3, the sperm receptor of the mouse zona pellucida

The mouse zona pellucida is composed of three sulfated glycoproteins, encoded by the oocyte genome, that have important biological functions in preimplantation development. One of the zona gene products, ZP3, functions as the sperm receptor at fertilization. Our present data demonstrate that the ZP3 gene is transcribed in oocytes where its expression is developmentally regulated. Resting primordial oocytes do not express ZP3 mRNA, but these transcripts rapidly accumulate in growing oocytes so that they represent 0.1-0.2% of the total poly(A+) RNA. As oocytes complete their growth and undergo meiotic maturation, the abundance of ZP3 transcripts falls off dramatically; ovulated eggs contain less than 15% of peak levels. The oocyte-specific accumulation of ZP3 transcripts serves as an attractive system for further studies of factors that modulate developmentally regulated genes during mammalian oogenesis.

Morphological differentiation of the growing oocyte of Ctenomys torquatus (Rodentia, Octodontidae)

The ultrastructural changes observed during the growth phase of oocytes of Ctenomys torquatus (Rodentia, Octodontidae) are reported. Interest was particularly centered on the transformation and/or distribution of the components of the endoplasmic reticulum. According to the observations made it is suggested that the endoplasmic reticulum stores some kind of material which may support early stages of development.

Changes in total RNA, polyadenylated RNA, and actin mRNA during meiotic maturation of mouse oocytes

The total RNA content of mouse oocytes, as measured by ethidium bromide fluorescence, was found to decrease by 19% during meiotic maturation (ovulated eggs contain 19% less RNA than full-grown oocytes). Consistent with these results, prelabeled stable RNA of full-grown oocytes decreased by about 20% during in vitro maturation. Polyadenylated RNA represented 19% of total prelabeled RNA in full-grown oocytes and 10% in oocytes matured in vitro, confirming previous results on in vivo prepared material. To distinguish between deadenylation and degradation for one mRNA, the amount and state of adenylation of actin mRNA was examined using Northern blots of oocyte RNA probed with a nick-translated beta-actin cloned chicken cDNA. The results showed that the amount of actin mRNA remained similar during maturation, but its molecular weight decreased slightly. Experiments in which RNA was treated with oligo(dT) and RNase H demonstrated that the actin mRNA was deadenylated during maturation, when actin synthesis is known to decline. These results indicate that the previously defined loss of bulk RNA and changes in the state of adenylation of mRNA during the first 11/2 days of embryogenesis actually begin during the 12 hr of meiotic maturation preceding fertilization.

Gene expression during oogenesis and oocyte development in mammals

Mouse oocytes progress through early meiotic prophase during fetal life and reach the diplotene stage by birth. During prepubertal and reproductive life, oocytes are continuously selected to grow from the pool of small primordial oocytes. Growing oocytes reach full size in 2 weeks, and full-grown oocytes are present in rapidly enlarging follicles for about 5 days before meiotic maturation and ovulation. RNA synthesis during early meiotic prophase, as estimated from [3H]uridine incorporation followed by autoradiography and from electron microscopic analysis of nuclear components, proceeds at a moderate rate throughout except for a brief period in early pachytene when synthesis is low or absent. RNA synthesis continues in primordial oocytes at a moderate rate. Incorporation studies, electron microscopic analyses, and particularly measurements of ongoing RNA polymerase activity (completion of initiated chains as analysed in tissue sections) indicate a distinctly increased rate of synthesis during oocyte growth over that of primordial oocytes, followed by a decline in full-grown oocytes. During growth, this rate increases severalfold. The absolute rate of synthesis of heterogeneous nuclear RNA (using rRNA as a standard) during mid-growth is very rapid, but nevertheless still much lower than that in typical lampbrush chromosomes. Most of the hnRNA turns over with a half-life of about 20 min, as is typical in somatic cells. Newly synthesized mRNA-like RNA enters the cytoplasm at about one-half the rate of rRNA, and about one-third of the ribosomes and one-fourth of the mRNA appear in polysomes. In full-grown oocytes, the rate of synthesis falls distinctly, but a significant level of synthesis continues until it essentially ceases at breakdown of the germinal vesicle. During meiotic prophase, chromosomes are most compact at pachytene and unfold lateral projections as RNA synthesis increases in late pachytene-early diplotene. In primordial oocytes, the diplotene state of chromosomes is obvious in most mammals, but in rodents the chromosomes are more evenly dispersed and are said to be in a dictyate state, although they are still presumably in a diplotene configuration. The chromosome core, which is present in leptotene through early diplotene stages, apparently disappears in the dictyate stage.(ABSTRACT TRUNCATED AT 400 WORDS)

Enzymatic dissection of the functions of the mouse egg's receptor for sperm

During the course of sperm-egg interaction in mice, zona pellucida glycoprotein ZP3 (approximately equal to 80 kDa) serves as both receptor for sperm (J. D. Bleil and P. M. Wassarman, 1980c, Cell 20, 873-882) and inducer of the acrosome reaction (J. D. Bleil and P. M. Wassarman, 1983, Dev. Biol. 95, 317-324). In this investigation, small ZP3 glycopeptides (approximately equal to 1.5-6 kDa), obtained by extensive digestion of the purified glycoprotein with insoluble Pronase, were assayed for both sperm receptor and acrosome reaction-inducing activities. While ZP3 glycopeptides were virtually as effective as intact ZP3 in inhibiting binding of sperm to eggs in vitro ("receptor activity"), unlike intact ZP3, they failed to induce sperm to undergo the acrosome reaction. The latter was determined by indirect immunofluorescence using a monoclonal antibody directed against the acrosomal cap region of sperm. These results suggest that the sperm receptor activity of ZP3 is dependent only on its carbohydrate components, whereas acrosome reaction-inducing activity is dependent on the polypeptide chain of ZP3 as well.

Relationship between rates of synthesis and intracellular distribution of ribosomal proteins during oogenesis in the mouse

Ribosomal proteins are synthesized continuously in nonequimolar amounts during oogenesis in the mouse (M. J. LaMarca and P. M. Wassarman, 1979, Develop. Biol. 73, 103), even though ribosomal proteins are found in equimolar amounts in ribosomes. In this report, the distribution of newly synthesized ribosomal proteins between the cytoplasm and germinal vesicle (nucleus) of fully grown mouse oocytes has been examined. As compared to total newly synthesized protein, ribosomal proteins were found to be highly concentrated in the oocyte's germinal vesicle. Furthermore, an inverse relationship was found between rates of synthesis of individual ribosomal proteins and percentages of newly synthesized ribosomal proteins associated with germinal vesicles. As a result of this relationship, the amounts of newly synthesized ribosomal proteins associated with germinal vesicles approximated an equimolar situation. Even in the presence of actinomycin D, oocytes continued to synthesize ribosomal proteins which were found associated with germinal vesicles in amounts similar to those observed in the absence of the drug. These results suggest that, although synthesis of ribosomal proteins by mouse oocytes is not coordinately regulated, a post-translational mechanism exists for adjusting the stoichiometry of these proteins within the oocyte's germinal vesicle; this mechanism apparently is not dependent upon concomitant ribosomal-RNA synthesis.

Regulation of mouse oocyte growth: probable nutritional role for intercellular communication between follicle cells and oocytes in oocyte growth

Intercellular communication, as determined by two different assay procedures, was established in vitro between mouse oocytes free of adhering follicle cells and monolayers of either follicle or 3T3 cells. Both of these cell types are known to be able to form homologous gap junctions, and follicle cells naturally form heterologous gap junctions with oocytes in vivo. Monolayers of L cells that are communication deficient did not establish intercellular communication with oocytes as determined by the two different assays for intercellular communication. The diameter of oocytes cultured for 4 days in medium or on monolayers of L cells decreased markedly, 9.7 and 13.1 micron, respectively. In contrast, oocytes cultured for 4 days on follicle cell monolayers increased on the average about 4.7 micron in diameter. Oocytes cultured for 4 days on monolayers of 3T3 cells decreased slightly in diameter, i.e., 2.1 micron. Results from these experiments support a nutritional role for intercellular communication between follicle cells and oocytes in oocyte growth.

Half-lives and relative amounts of stored and polysomal ribosomes and poly(A) + RNA in mouse oocytes

Growing mouse oocytes were labeled in vitro with [3H]uridine and chased for 2 or for 7 days to estimate the relative amounts of RNA appearing in different fractions and to follow their turnover. Oocytes were lysed and thoroughly dispersed in the presence of 1% DOC, and centrifuged on sucrose gradients to separate polysomes from smaller components not engaged in translation. After the short chase, one-third of the labeled ribosomes appeared in EDTA-sensitive polysomes. The proportion of ribosomes in both fractions remained stable during the long chase, demonstrating no net flow from one fraction to the other. When gradient fractions were analyzed by poly(U) Sepharose chromatography, it was found that about 20% of the labeled poly(A)+ RNA appeared in polysomes after the short chase. The half-lives of stored and translated mRNA were followed relative to stable rRNA during the long chase. Stored mRNA was completely stable, but translated mRNA turned over with a t1/2 of about 6 days. Other methods for separating stored from translated components were not successful, including sedimentation of putative large complexes (fibrillar lattices) containing stored components, or chromatography of lysates on oligo(dT)-cellulose. Results presented here combined with our previous results demonstrate that, during meiotic maturation, the percent of labeled stable RNA which is polyadenylated declines from 19 to 10%, suggesting deadenylation or degradation of half of the accumulated maternal mRNA.

Oocyte specific regulation of PGK-1 isozyme activity in female germ cells of the mouse

The expression of electrophoretic variant forms of the X-linked enzyme phosphoglycerate kinase (PGK-1) was examined during the ontogeny of the female germ-line followingX-chromosome reactivation. Non-growing oocytes from foetal and neonatal ovaries of heterozygous females show a higher PGK-1A isozyme activity, a reflection of prior non-randomX-inactivation favouring activity of theX-chromosome carrying thePgk-1aallele andXceclocus. On oocyte growth, the total PGK-1 enzyme activity increases 40–50 fold and the pattern of PGK-1 isozyme expression changes giving an electrophoretic pattern now skewed in favour of the PGK-1B isozymic form. Activity of the PGK-1B isozyme exceeds that of PGK-1A in all growing oocytes with a total activity greater than 0·075 nmol h−1oocyte−1. Mice homozygous for eitherPgk-1allele show similar PGK-1 specific activities in somatic tissues where oneX-chromosome is active, but oocytes ofPgk-lbhomozygotes show a higher specific activity compared to those ofPgk-1ahomozygotes. Thus increased activity of the PGK-1B isozyme relative to the PGK-1A isozyme is specific to growing oocytes.

Regulation of mouse oocyte meiotic maturation: implication of a decrease in oocyte cAMP and protein dephosphorylation in commitment to resume meiosis

Mouse oocytes are reversibly inhibited from resuming meiotic maturation in vitro by cAMP phosphodiesterase inhibitors such as 3-isobutyl-1-methyl xanthine (IBMX) and cAMP analogs such as dibutyryl cAMP (dbcAMP). Oocytes cultured in IBMX-containing medium were transferred to and cultured in IBMX-free medium for various periods of time prior to their return to either IBMX- or dbcAMP-containing medium. Results from these experiments defined a period of time in which oocytes became committed to resuming meiosis. Forskolin, which elevated the intracellular oocyte cAMP concentration, transiently inhibited oocytes from resuming meiosis. Levels of cAMP were determined in oocytes incubated in medium that allows resumption of meiosis. The level of oocyte cAMP decreased significantly during the time in which oocytes become committed to resuming meiosis. This decrease in oocyte cAMP was not observed in oocytes inhibited from resuming meiosis by IBMX. In addition, cAMP levels were determined in preovulatory antral follicles, cumulus cell-oocyte complexes, and oocytes during gonadotropin-induced resumption of meiosis in vivo. A decrease in oocyte cAMP preceded resumption of meiosis as manifested by germinal vesicle breakdown (GVBD). This decrease apparently occurred before or during a period of time in which follicle and cumulus cell cAMP were increasing. Associated with commitment to resume meiosis was a characteristic set of changes in oocyte phosphoprotein metabolism that preceded GVBD. These changes are, to date, some of the first reported biochemical changes that precede GVBD. Results from these experiments are discussed in terms of a possible role cAMP may play in regulation of resumption of meiosis in mammals.

Expression of creatine kinase isoenzyme during oogenesis and embryogenesis in the mouse

Creatine kinase activity was discovered in the growing mouse oocyte and in the preimplantation embryo. Changes in the enzyme activity during the growth and maturation of the egg and during the development of the embryo up to the blastocyst stage were determined. Close similarity of the protein to the brain-type isoenzyme of creatine kinase was established immunochemically. The kinetic parameters of the brain-type isoenzyme (M. R. Iyengar, C. E. Fluellen, and C. W. L. Iyengar, 1982, J. Muscle Cell Motil. 3, 231-246) and the pattern of development-associated changes in activity suggest a possible role for creatine kinase in maintaining the reported high ATP/ADP ratio (L. Ginsberg and N. Hillman, 1975, J. Reprod. Fertil. 43, 83-90), which is essential for the biosynthetic activities of the embryo.

Biosynthesis of the major zona pellucida glycoprotein secreted by oocytes during mammalian oogenesis

An antiserum directed specifically against ZP2, the major glycoprotein of the mouse egg's extracellular coat (zona pellucida), has been used to immunoprecipitate intracellular precursors of ZP2 that were synthesized by growing mouse oocytes cultured in vitro. Pulse-chase experiments revealed that the immediate precursor of mature, 120 kilodalton (kd) ZP2 is a 91 kd species that unlike mature ZP2, is sensitive to digestion by endo-beta-N-acetylglucosaminidase H (Endo H) and is converted by the endoglycosidase into an 81 kd species. An 81 kd species is only found intracellularly when growing oocytes are cultured in the presence of tunicamycin. These results suggest that ZP2 is synthesized as an 81 kd polypeptide chain that is first "core"-glycosylated at asparagine residues with high-mannose-type oligosaccharides, giving rise to a 91 kd intermediate (Endo-H-sensitive), and then processed to complex-type oligosaccharides prior to secretion as mature, 120 kd ZP2 (Endo H-insensitive). Furthermore, electrophoretic analyses of mature ZP2, ZP2 precursor (91 kd) and Endo H-treated ZP2 precursor (81 kd) suggest that there are six N-linked oligosaccharides per molecule and that the extreme heterogeneity of mature ZP2 is a consequence of the oligosaccharides and not the polypeptide chain itself.

rRNA accumulation and protein synthetic patterns in growing mouse oocytes

The rRNA contents of mouse primordial oocytes, three stages of growing oocytes, full-grown oocytes, and ovulated ova have been measured by hybridization of RNA samples to excess 3H-DNA complementary to rRNA. Since it was known from previous work that rRNA is stable, the results when plotted against days of oocyte growth indicated that rRNA was synthesized at a constant rate over the first 9 days of growth and about 1.5 times faster in the last 5 days. The maximum value of 0.3 ng per oocyte was attained by about 14 days of growth in oocytes 59 micrometers in diameter, well below the maximum diameter of 77 micrometers for full-grown oocytes. The stability of proteins synthesized in mid-growth phase oocytes was measured by labeling for 5 h with 35S-methionine and then following the decline of incorporated label during a 48h chase; 40% of the label decayed with a half-life of 11 h. and 60% was apparently stable. The two-dimensional electrophoretic patterns of labeled proteins synthesized by growing and full-grown oocytes were compared. The principal change was the appearance or great increase in intensity of several spots in full-grown oocytes as compared to growing oocytes. Egg proteins separated on a two-dimensional gel were visualized by silver staining. The cytoskeletal proteins actin, tubulin, and putative intermediate filament protein, as well as putative lactate dehydrogenase, were synthesized in growing and full-grown oocytes, and accumulated to form a significant portion of bulk egg protein.